JP7748017B2 - steel plate - Google Patents

Description

This disclosure relates to steel sheets.

Steel plate with a high C content (high carbon steel plate) is sometimes used as a material for mechanical parts such as automobile parts and industrial machinery parts. When these mechanical parts are manufactured using steel plate as a material, the manufacturing method is as follows: The steel plate is cold worked to create an intermediate product in the shape of the mechanical part. The intermediate product is then quenched and tempered. High-strength mechanical parts are manufactured through these manufacturing processes. In order to achieve high strength in the mechanical parts after quenching, the steel plate must have excellent hardenability when quenched during the mechanical part manufacturing process. Furthermore, the steel plate is cold worked before quenching. Therefore, the steel plate must not only have excellent hardenability but also excellent cold workability.

Patent documents 1 and 2 propose steel sheets that have excellent hardenability during quenching and excellent cold workability when used as materials for mechanical parts.

The steel sheet disclosed in Patent Document 1 contains, by mass%, 0.20 to 0.40% C, 0.10% or less Si, 0.50% or less Mn, 0.03% or less P, 0.010% or less S, 0.10% or less sol. Al, 0.0050% or less N, and 0.0005 to 0.0050% B. It also contains 0.002 to 0.030% in total of one or more of Sb, Sn, Bi, Ge, Te, and Se, with the balance being Fe and unavoidable impurities. In this steel sheet, the proportion of dissolved B in the B content is 70% or more. Furthermore, the microstructure is composed of ferrite and cementite. Furthermore, the cementite density within the ferrite grains is 0.08 particles/μm² or ^less . Patent Document 1 describes that total elongation can be increased by keeping the cementite density of ferrite grains low.

The steel sheet disclosed in Patent Document 2 contains, by mass%, C: 0.10% to 0.33%, Si: 0.01% to 0.50%, Mn: 0.40% to 1.25%, P: 0.03% to 0.01%, S: 0.01% to 0.10%, N: 0.01% to 0.50%, and Cr: 0.50% to 1.50%, with the balance being Fe and unavoidable impurities. It also has a microstructure containing ferrite and carbides. The volume ratio of ferrite and carbides to the entire microstructure is 90% or more, and the volume ratio of pro-eutectoid ferrite to the entire microstructure is 20% to 80%. The Mn concentration in the carbides is 0.10 mass% or more and 0.50 mass% or less, and the ratio of the number of carbides having a particle size of 1 μm or more to the total number of carbides is 30% or more and 60% or less. Patent Document 2 describes that by reducing the Mn concentration in the carbides, the carbides become more easily dissolved during quenching, and as a result, hardenability is improved.

International Publication No. 2015/146173 International Publication No. 2020/175665

Among machine parts, bearings for automobile parts and knitting needles for textile machinery parts are required to have high strength and excellent wear resistance. Therefore, the steel plate used as the material must have further improved hardenability. Therefore, steel plate with a C content of more than 0.90% is sometimes used for high-strength machine parts. Even steel plate with such a high C content is required to have excellent cold workability.

The objective of this disclosure is to provide a steel sheet that can achieve both excellent hardenability and excellent cold workability even when the C content exceeds 0.90%.

The steel sheet according to the present disclosure has a chemical composition, in mass%, of C: more than 0.90 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.60%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%, with the balance being Fe and impurities. In the microstructure of the steel sheet, the total area ratio of ferrite and cementite particles is 95% or more, and the average grain size of ferrite is 15.0 μm or less. The Cr concentration [Cr] _θ in mass% in the cementite particles is 7.90% or less, and the Mo concentration [Mo] _θ in mass% in the cementite particles is 1.50% or less. The cementite particles have an average particle size of 1.50 μm or less and a maximum particle size of 5.00 μm or less. Among the cementite particles, cementite particles having an aspect ratio of 3.0 or less are considered to be spherical cementite particles, and the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 75% or more.

The steel sheet according to the present disclosure has excellent hardenability and excellent cold workability.

The inventors conducted research into steel sheets with excellent hardenability and excellent cold workability. As a result, the inventors obtained the following findings:

First, the inventors investigated the improvement of hardenability and cold workability in steel sheets with a C content of more than 0.90% from the perspective of chemical composition. As a result, the inventors of the present invention believed that a chemical composition consisting of, in mass%, C: more than 0.90 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.60%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and the balance being Fe and impurities, would be able to achieve both improved hardenability and improved cold workability. Therefore, the inventors of the present invention further investigated the microstructure of steel sheets having the above-mentioned chemical composition.

The inventors first investigated means for improving hardenability in the microstructure of steel sheet. The microstructure of steel sheet having the above-mentioned chemical composition is substantially composed of ferrite and cementite particles. When manufacturing mechanical parts using steel sheet as a raw material, in order to improve the hardenability of the steel sheet during quenching in the manufacturing process of the mechanical parts, it is preferable that the cementite particles in the steel sheet are easily dissolved during quenching. In order to improve the solid solubility of cementite particles during quenching, it is preferable that the particle size of the cementite particles is small. In the case of steel sheet having the above-mentioned chemical composition, hardenability is improved if the average particle size of the cementite particles is 1.50 μm or less.

Furthermore, as a result of the inventors' investigations, it was found that the Mn concentration in cementite particles does not affect the solid solution of cementite particles during quenching. However, the inventors discovered that the Cr and Mo concentrations in cementite particles have a significant effect on the solid solution of cementite particles during quenching. Specifically, if the Cr and Mo concentrations in cementite particles are high, the cementite particles become less likely to dissolve during quenching.

Based on the above findings, the inventors conducted further studies and found that when the Cr concentration [Cr] _θ in the cementite particles is 7.90% or less and the Mo concentration [Mo] _θ is 1.50% or less, the cementite particles are more likely to form a solid solution during heat treatment, and the hardenability of the steel sheet is improved.

The inventors also investigated means for improving the cold workability of steel sheet microstructures. To improve the cold workability of steel sheet, it is effective to set the average particle size of cementite particles to 1.50 μm or less and further increase the spheroidization rate of the cementite particles. Furthermore, the average particle size of ferrite also affects cold workability. Therefore, in the steel sheet of this embodiment, the average particle size of cementite particles is set to 1.50 μm or less, the spheroidization rate of cementite particles is set to 75% or more, and the average particle size of ferrite is set to 15.0 μm or less. If the average particle size of ferrite is 15.0 μm or less, the grain boundary area of ferrite increases. In this case, grain boundary diffusion accelerates the dissolution of cementite particles during quenching. Therefore, if the average particle size of ferrite is 15.0 μm or less, not only cold workability but also hardenability is improved.

However, even when a steel sheet has the above-mentioned chemical composition, the average particle size of cementite particles is 1.50 μm or less, the Cr concentration [Cr] _θ in the cementite particles is 7.90% or less, the Mo concentration [Mo] _θ is 1.50% or less, the spheroidization rate is 75% or more, and the average particle size of ferrite is 15.0 μm or less, there are cases where sufficient cold workability is still not obtained. Therefore, the present inventors conducted further studies. As a result, the present inventors have obtained the following findings.

If the C content exceeds 0.90%, the size of the cementite particles that form during the cooling process of hot working is likely to vary. Even if the average particle size of cementite particles is 1.50 μm or less, if the size of the cementite particles varies and coarse cementite is present, the coarse cementite will become the starting point for cracks during cold working. As a result, the cold workability of the steel sheet will be reduced.

Based on the above findings, the inventors conducted further research. As a result, the inventors discovered that if the maximum particle size of the cementite particles in the above-mentioned steel sheet is 5.00 μm or less, the deterioration of cold workability caused by coarse cementite can be suppressed, and both excellent hardenability and excellent cold workability can be achieved.

The steel plate of this embodiment, which was completed based on the above findings, has the following configuration.

A steel sheet having a first configuration has a chemical composition, in mass%, of C: more than 0.90 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.60%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%, with the balance being Fe and impurities. In the microstructure of the steel sheet, the total area ratio of ferrite and cementite particles is 95% or more, and the average grain size of ferrite is 15.0 μm or less. The Cr concentration [Cr] _θ in mass% in the cementite particles is 7.90% or less, and the Mo concentration [Mo] _θ in mass% in the cementite particles is 1.50% or less. The cementite particles have an average particle size of 1.50 μm or less and a maximum particle size of 5.00 μm or less. Among the cementite particles, those having an aspect ratio of 3.0 or less are considered spherical cementite particles, and the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 75% or more.

The steel plate of the second configuration is a steel plate of the first configuration, and its chemical composition contains, in mass %, one or more elements selected from the group consisting of Mo: 0.001 to 0.500%, Ni: 0.0001 to 1.000%, B: 0.0001 to 0.0100%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, and Ti: 0.001 to 0.150%.

The steel sheet of this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[Features of the steel sheet according to this embodiment]
The steel sheet of this embodiment satisfies the following features 1 to 7.
(Feature 1)
The chemical composition, in mass%, is C: over 0.90 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.60%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%, with the balance being Fe and impurities.
(Feature 2)
In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more.
(Feature 3)
The average grain size of the ferrite is 15.0 μm or less.
(Feature 4)
The Cr concentration [Cr] _θ in mass % in the cementite particles is 7.90% or less, and the Mo concentration [Mo] _θ in mass % in the cementite particles is 1.50% or less.
(Feature 5)
The average particle size of the cementite particles is 1.50 μm or less.
(Feature 6)
The maximum particle size of the cementite particles is 5.00 μm or less.
(Feature 7)
Among the cementite particles, cementite particles having an aspect ratio of 3.0 or less are considered spherical cementite particles, and the spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 75% or more.
Features 1 to 7 will be explained below.

[(Feature 1) Chemical composition]
The chemical composition of the steel sheet of this embodiment contains the following elements.

C: more than 0.90 to 1.30%
Carbon (C) improves the hardenability of steel sheet. As a result, when quenching is performed in a process for manufacturing a mechanical component using steel sheet as a raw material, the strength of the mechanical component is increased. Furthermore, C causes undissolved cementite particles to remain after quenching and tempering in a process for manufacturing a mechanical component using steel sheet as a raw material, thereby improving the wear resistance of the mechanical component. If the C content is 0.90% or less, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the C content exceeds 1.30%, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet will be reduced, and further, the toughness of machine parts manufactured using the steel sheet will be reduced.
Therefore, the C content is more than 0.90% to 1.30%.
The lower limit of the C content is preferably 0.91%, more preferably 0.93%, and even more preferably 0.95%.
The upper limit of the C content is preferably 1.28%, more preferably 1.25%, and even more preferably 1.20%.

Si: 0.01~0.50%
Silicon (Si) deoxidizes steel during the steelmaking process of steel sheet production. Si also increases the temper softening resistance of steel sheet when tempered in the process of manufacturing mechanical parts using the steel sheet as a raw material. If the Si content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Si content exceeds 0.50%, the strength of the steel sheet becomes excessively high due to solid solution strengthening, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Si content is 0.01 to 0.50%.
The lower limit of the Si content is preferably 0.02%, more preferably 0.05%, and even more preferably 0.10%.
The upper limit of the Si content is preferably 0.48%, more preferably 0.44%, and even more preferably 0.40%.

Mn: 0.20-1.30%
Manganese (Mn) improves the hardenability of steel sheets depending on the amount of manganese dissolved in austenite during heating in the quenching process in the manufacturing of mechanical parts using steel sheets as raw materials. As a result, the strength of mechanical products is increased. If the Mn content is less than 0.20%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Mn content exceeds 1.30%, the strength of the steel sheet becomes excessively high due to solid solution strengthening, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Mn content is 0.20 to 1.30%.
The lower limit of the Mn content is preferably 0.25%, more preferably 0.30%, and even more preferably 0.35%.
The upper limit of the Mn content is preferably 1.25%, more preferably 1.20%, and even more preferably 1.15%.

P: 0.100% or less Phosphorus (P) is an impurity, and the P content is more than 0%. If the P content exceeds 0.100%, the toughness of the steel plate decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the P content is 0.100% or less.
The P content is preferably as low as possible. However, excessive reduction in the P content significantly increases the production cost. Therefore, in consideration of industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
The upper limit of the P content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.050%.

S: 0.100% or less Sulfur (S) is an impurity, and the S content is greater than 0%. If the S content exceeds 0.100%, S generates excessive sulfides. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced.
Therefore, the S content is 0.100% or less.
The S content is preferably as low as possible. However, excessive reduction in the S content significantly increases the production cost. Therefore, in consideration of industrial production, the lower limit of the S content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
The upper limit of the S content is preferably 0.090%, more preferably 0.080%, even more preferably 0.050%, even more preferably 0.030%, and still more preferably 0.025%.

Al: 0.100% or less Aluminum (Al) is an impurity, and the Al content exceeds 0%. Al bonds with N to form AlN. AlN refines austenite grains during heating in the quenching process in the process of manufacturing mechanical parts using steel sheet as a raw material. Refinement of austenite grains reduces the hardenability of the steel sheet. If the Al content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, austenite grains are excessively refined during heating in the quenching process, significantly reducing the hardenability of the steel sheet.
Therefore, the Al content is 0.100% or less.
The lower limit of the Al content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.
The upper limit of the Al content is preferably 0.090%, more preferably 0.080%, even more preferably 0.070%, and still more preferably 0.050%.
In the chemical composition of the steel sheet of this embodiment, the Al content means the acid-soluble Al (sol. Al) content.

Cr:0.01~1.60%
Chromium (Cr) enhances the hardenability of steel sheets depending on the amount of chromium dissolved in austenite during heating in the quenching process of manufacturing mechanical parts from steel sheets. As a result, the strength of the mechanical parts is increased. If the Cr content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Cr content exceeds 1.60%, it becomes difficult to maintain the Cr concentration [Cr] _θ in the cementite particles at 7.90% or less. In this case, the dissolution of cementite particles slows down during heating in the quenching process in the process of manufacturing mechanical parts using the steel sheet as a raw material. As a result, the hardenability deteriorates. Furthermore, the strength of the steel sheet becomes excessively high. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet deteriorates.
Therefore, the Cr content is 0.01 to 1.60%.
The lower limit of the Cr content is preferably 0.02%, more preferably 0.03%, even more preferably 0.05%, even more preferably 0.10%, even more preferably 0.20%, even more preferably 0.30%, even more preferably 0.40%, even more preferably more than 0.50%, even more preferably 0.52%, and even more preferably 0.55%.
The upper limit of the Cr content is preferably 1.57%, more preferably 1.55%, and even more preferably 1.50%.

N: 0.0150% or less Nitrogen (N) is an impurity, and the N content exceeds 0%. N bonds with Al to form AlN. AlN refines austenite grains during heating in the quenching process in the process of manufacturing mechanical parts using steel sheet as a raw material. Refinement of austenite grains reduces the hardenability of the steel sheet. If the N content exceeds 0.0150%, even if the contents of other elements are within the ranges of this embodiment, the austenite grains are excessively refined during heating in quenching, significantly reducing the hardenability of the steel sheet.
Therefore, the N content is 0.0150% or less.
The lower limit of the N content is preferably 0.0001%, and more preferably 0.0005%.
The upper limit of the N content is preferably 0.0140%, more preferably 0.0130%, and even more preferably 0.0125%.

The remainder of the chemical composition of the steel sheet of this embodiment consists of Fe and impurities. Here, the term "impurities" in the chemical composition refers to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during industrial production of the steel sheet, and are acceptable within a range that does not adversely affect the steel sheet of this embodiment.

The impurities may include the following elements in the amounts listed below.
One or more elements selected from the group consisting of Cu: 0 to 0.15%, W: 0 to 0.15%, Ta: 0 to 0.15%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.050%, As: 0 to 0.050%, Mg: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, and Ca: 0 to 0.050%. All of these elements are tramp elements, and are impurities in the steel sheet of this embodiment.

[Optional Elements]
The chemical composition of the steel sheet of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo: 0-0.500%, Ni: 0-1.000%, B: 0-0.0100%, V: 0-0.500%, Nb: 0-0.500%, and Ti: 0-0.150%. All of these elements are optional elements. These optional elements will be described below.

[Group 1 (Mo, Ni and B)]
The chemical composition of the steel sheet of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo, Ni, and B. These elements are all optional elements and may not be contained. When contained, Mo, Ni, and B improve the hardenability of the steel sheet.

Mo: 0~0.500%
Molybdenum (Mo) is an optional element and may not be contained, that is, the Mo content may be 0%.
When Mo is contained, that is, when the Mo content is more than 0%, Mo dissolved in austenite during heating in the quenching process in the process of manufacturing mechanical parts using the steel sheet as a raw material improves the hardenability of the steel sheet. Therefore, the strength of the mechanical product increases. Mo also increases the temper softening resistance of the steel sheet. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent.
However, if the Mo content exceeds 0.500%, the strength of the steel sheet becomes excessively high even if the contents of other elements are within the ranges of this embodiment, and therefore the cold workability of the steel sheet deteriorates.
Therefore, the Mo content is 0 to 0.500%.
The lower limit of the Mo content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, even more preferably 0.005%, and still more preferably 0.010%.
The upper limit of the Mo content is preferably 0.450%, more preferably 0.400%, even more preferably 0.350%, and still more preferably 0.300%.

Ni: 0-1.000%
Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%.
When Ni is contained, that is, when the Ni content is more than 0%, Ni dissolved in austenite during heating in the quenching process in the process of manufacturing mechanical parts using the steel sheet as a raw material improves the hardenability of the steel sheet. Therefore, the strength of the mechanical parts increases. Ni also increases the temper softening resistance of the steel sheet. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent.
However, if the Ni content exceeds 1.000%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel sheet becomes excessively high, and therefore the cold workability of the steel sheet deteriorates.
Therefore, the Ni content is 0 to 1.000%.
The lower limit of the Ni content is preferably more than 0%, more preferably 0.001%, even more preferably 0.005%, even more preferably 0.007%, and even more preferably 0.010%.
The upper limit of the Ni content is preferably 0.950%, more preferably 0.900%, even more preferably 0.800%, even more preferably 0.700%, and still more preferably 0.600%.

B: 0-0.0100%
Boron (B) is an optional element and may not be contained, that is, the B content may be 0%.
When B is contained, that is, when the B content is more than 0%, B dissolved in austenite during heating in the quenching step in the process of manufacturing mechanical parts using the steel sheet as a raw material improves the hardenability of the steel sheet, thereby increasing the strength of the mechanical parts. Even if even a small amount of B is contained, the above effect can be obtained to some extent.
However, if the B content exceeds 0.0100%, B compounds are formed even if the contents of other elements are within the ranges of this embodiment. In this case, sufficient hardenability cannot be obtained. Furthermore, the cold workability of the steel sheet is reduced.
Therefore, the B content is 0 to 0.0100%.
The lower limit of the B content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and still more preferably 0.0005%.
The upper limit of the B content is preferably 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and still more preferably 0.0050%.

[Group 2: V, Nb, and Ti]
The chemical composition of the steel sheet according to this embodiment may further contain one or more elements selected from the group consisting of V, Nb, and Ti, instead of a portion of Fe. These elements are optional and may not be contained. When contained, V, Nb, and Ti form carbides and suppress coarsening of austenite grains during heating in the quenching step in the process of manufacturing mechanical components using the steel sheet as a raw material. Therefore, the toughness of the mechanical components is improved.

V: 0-0.500%
Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%.
When V is contained, that is, when the V content is more than 0%, V forms carbides and suppresses the coarsening of austenite grains during heating in the quenching process of manufacturing mechanical parts from steel plates. Therefore, the toughness of the mechanical parts is improved. Even if even a small amount of V is contained, the above effect can be obtained to some extent.
However, if the V content exceeds 0.500%, excessive carbides are formed, which causes precipitation strengthening of the steel sheet, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment.
Therefore, the V content is 0 to 0.500%.
The lower limit of the V content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, and still more preferably 0.005%.
The upper limit of the V content is preferably 0.480%, more preferably 0.450%, and even more preferably 0.400%.

Nb: 0-0.500%
Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%.
When Nb is contained, that is, when the Nb content is more than 0%, Nb forms carbides and suppresses the coarsening of austenite grains during heating in the quenching process of manufacturing mechanical parts using steel plate as a raw material. This improves the toughness of the mechanical parts. Nb also bonds with N to suppress the formation of nitrides by solute B. This improves the hardenability of the steel plate due to solute B. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent.
However, if the Nb content exceeds 0.500%, excessive carbides are formed, which causes precipitation strengthening of the steel sheet, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Nb content is 0 to 0.500%.
The lower limit of the Nb content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, and still more preferably 0.005%.
The upper limit of the Nb content is preferably 0.480%, more preferably 0.460%, even more preferably 0.430%, even more preferably 0.400%, even more preferably 0.350%, and even more preferably 0.300%.

Ti: 0-0.150%
Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%.
When Ti is contained, that is, when the Ti content is more than 0%, Ti forms carbides and suppresses the coarsening of austenite grains during heating in the quenching process of manufacturing mechanical parts using steel plate as a raw material. This improves the toughness of the mechanical parts. Ti also bonds with N to suppress the formation of nitrides by solute B. This improves the hardenability of the steel plate due to the solute B. Even if Ti is contained even a small amount, the above effects can be obtained to some extent.
However, if the Ti content exceeds 0.150%, excessive carbides are formed, which leads to precipitation strengthening of the steel sheet, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Ti content is 0 to 0.150%.
The lower limit of the Ti content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, even more preferably 0.005%, and still more preferably 0.010%.
The upper limit of the Ti content is preferably 0.145%, more preferably 0.130%, even more preferably 0.120%, even more preferably 0.100%, and still more preferably 0.080%.

[Method for measuring the chemical composition of steel sheet]
The chemical composition of the steel sheet of this embodiment can be measured by a known elemental analysis method. Specifically, chips are collected from the interior of the steel sheet to a depth of 0.1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a known inert gas fusion-thermal conductivity method.

[(Feature 2) Microstructure]
In the microstructure of the steel sheet of this embodiment, the total area ratio of ferrite and cementite particles is 95% or more, that is, the microstructure of the steel sheet of this embodiment is substantially composed of ferrite and cementite particles.

In the microstructure, the remaining structure other than ferrite and cementite particles is, for example, one or more types selected from the group consisting of bainite, martensite, and pearlite.

Preferably, the total area ratio of ferrite and cementite particles in the microstructure is 96% or more, more preferably 97% or more, even more preferably 98% or more, and even more preferably 99% or more. The microstructure may be a structure consisting of ferrite and cementite particles.

If the total area ratio of ferrite and cementite particles is 95% or more, the steel plate will have sufficient hardenability and cold workability, provided that Features 1 and 3 to 7 are met.

[Method for measuring the total area ratio of ferrite and cementite particles in a microstructure]
The total area ratio of ferrite and cementite particles in the microstructure can be measured by the following method.
A test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate. The surface of the test piece parallel to the rolling direction and thickness direction (i.e., the surface 15 mm in the rolling direction and thickness direction) is used as the observation surface. The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is etched using 3% nitric acid alcohol (nital etchant). Of the etched observation surface, five observation fields at a depth of 1/4 of the plate thickness from the steel plate surface in the thickness direction are randomly selected and secondary electron images are observed using a scanning electron microscope (SEM) at 1000x magnification. Each observation field is a rectangle measuring 100 μm in the thickness direction and 120 μm in the rolling direction. The observation field is selected so that the center position of the observation field in the thickness direction is 1/4 of the plate thickness from the steel plate surface in the thickness direction.

In the observation field, ferrite and cementite particles exhibit different contrast and morphology from other structures (bainite, martensite, pearlite, etc.). Therefore, ferrite and cementite particles within the observation field are identified based on their contrast and morphology.

The total area ratio (%) of ferrite and cementite particles is calculated based on the total area of ferrite and cementite particles in the five observation fields and the total area of the five observation fields (100 μm x 120 μm x 5).

[(Feature 3) Average grain size of ferrite]
Furthermore, in the steel sheet of this embodiment, the average grain size of ferrite is 15.0 μm or less.
If the average grain size of ferrite exceeds 15.0 μm, the grain boundary area becomes small, and the effect of promoting the dissolution of cementite particles by grain boundary diffusion cannot be obtained. In this case, sufficient hardenability cannot be obtained in the hardening process when manufacturing mechanical parts using the steel sheet as a raw material. As a result, the strength of the mechanical parts manufactured using the steel sheet as a raw material decreases.

When the average grain size of ferrite is 15.0 μm or less, the grain boundary area of ferrite is sufficiently large, and cementite particles dissolve quickly during quenching. As a result, hardenability is improved, provided that Features 1, 2, and 4 to 7 are met. Furthermore, when the average grain size of ferrite is 15.0 μm or less, the cold workability of the steel sheet is improved.

The upper limit of the average grain size of ferrite is preferably 14.5 μm, more preferably 14.0 μm, even more preferably 13.5 μm, and still more preferably 13.0 μm.
The lower limit of the average particle size of ferrite is preferably 3.0 μm, more preferably 3.5 μm, and even more preferably 4.0 μm.

[Method for measuring average grain size of ferrite]
The average grain size of ferrite can be measured by the following method.
A test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate. The cross section of the surface of the test piece parallel to the rolling direction and thickness direction (i.e., the surface 15 mm in the rolling direction and thickness direction) is used as the observation surface. The observation surface of the test piece is mirror-polished. After mirror-polishing, etching is performed with a 3% nital etching solution. The average grain size of ferrite is determined on the etched observation surface using the following method. The ferrite grain size number is determined by the intercept method in accordance with JIS G 0551:2020. The magnification of the optical microscope is selected so that the number of ferrite grains cut by a single line segment is at least 10 in one field of view. After selecting the magnification, the cut lengths are determined for five fields of view. The ferrite grain size number is determined from the arithmetic mean of the cut lengths of the five fields of view. The average grain size of ferrite (μm) is determined from the obtained grain size number. The average grain size of ferrite is determined by rounding the obtained value to one decimal place.

[(Feature 4) Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in cementite particles]
In the steel sheet of this embodiment, the Cr concentration [Cr] _θ in mass % in the cementite particles is 7.90% or less, and the Mo concentration [Mo] _θ in mass % in the cementite particles is 1.50% or less.

When the Cr concentration [Cr] _θ or the Mo concentration [Mo] _θ in the cementite particles of a steel sheet is high, the cementite particles do not dissolve sufficiently during heating in the quenching step of manufacturing a mechanical component using the steel sheet as a raw material. In this case, the hardenability of the steel sheet is reduced. As a result, the mechanical component manufactured using the steel sheet as a raw material does not have sufficient strength.

When the Cr concentration [Cr] _θ in the cementite particles of the steel sheet is 7.90% or less and the Mo concentration [Mo] _θ in the cementite particles is 1.50% or less, the Cr concentration [Cr] _θ and the Mo concentration [Mo] _θ in the cementite particles are sufficiently low. Therefore, on the premise that Features 1 to 3 and Features 5 to 7 are satisfied, the cementite particles are sufficiently dissolved during heating in the above-described quenching step, and the hardenability of the steel sheet is improved.

The upper limit of the Cr concentration [Cr] _θ in the cementite particles is preferably 7.80%, more preferably 7.70%, even more preferably 7.60%, even more preferably 7.50%, even more preferably 7.20%, even more preferably 7.00%, even more preferably 6.50%, even more preferably 5.00%, even more preferably 3.00%, and even more preferably 2.50%.
The upper limit of the Mo concentration [Mo] _θ in the cementite grains is preferably 1.40%, more preferably 1.30%, and even more preferably 1.20%.
The Cr concentration [Cr] _θ in the cementite particles is equal to or greater than the Cr content in the chemical composition of the steel sheet, and the Mo concentration [Mo] _θ in the cementite particles is equal to or greater than the Mo content in the chemical composition of the steel sheet.

[Method for measuring Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in cementite particles]
The Cr concentration [Cr] _θ and the Mo concentration [Mo] _θ in the cementite particles can be measured by the following method.
A test piece measuring 10 mm x 10 mm x plate thickness is taken from the center of the steel plate width.

Constant-current electrolysis is performed on the test specimen using a 10% AA-based solution (a solution containing 10% by volume of acetylacetone and a methanol solution containing 1% by mass of tetramethylammonium chloride).

Specifically, the above-mentioned 10% AA-based solution was prepared. Then, using the 10% AA-based solution, the test piece was subjected to constant-current electrolysis at room temperature while maintaining a current density of 20 mA/ ^cm2 . After constant-current electrolysis, the test piece was immersed in an alcohol solution and then subjected to ultrasonic cleaning.

The 10% AA solution used for constant current electrolysis and the alcohol solution used for subsequent ultrasonic cleaning are suction filtered through a filter with a mesh size of 0.2 μm to extract the residue.

The extracted residue is subjected to chemical elemental analysis. Specifically, the residue is dissolved in acid to obtain a solution. Chemical elemental analysis using inductively coupled plasma optical emission spectroscopy (ICP-OES) is performed on the solution to obtain the Cr mass and Mo mass in the residue. Based on the obtained Cr mass in the residue, the Mo mass in the residue, and the total mass of the residue, the Cr concentration [Cr] _θ (mass%) and the Mo concentration [Mo] _θ (mass%) in the residue are calculated. The Cr concentration [Cr] _θ (mass%) in the residue is calculated by rounding the obtained value to the nearest tenths place. The Mo concentration [Mo] _θ (mass%) in the residue is calculated by rounding the obtained value to the nearest tenths place.

The obtained residue is substantially composed of cementite particles. That is, the amount of particles other than cementite particles (inclusions and carbides other than cementite particles) in the residue is negligibly small. Therefore, the Cr concentration and Mo concentration in the residue are considered to be the Cr concentration [Cr] _θ (mass%) and the Mo concentration [Mo] _θ (mass%) in the cementite particles.

[(Feature 5) Average particle size of cementite particles]
In the steel sheet of this embodiment, the average particle size of the cementite particles is 1.50 μm or less.
If the average particle size of the cementite particles exceeds 1.50 μm, the cementite particles do not dissolve sufficiently during heating in the quenching step in the process of manufacturing a mechanical component using the steel sheet as a raw material. In this case, the hardenability of the steel sheet is reduced. As a result, the mechanical component manufactured using the steel sheet as a raw material does not have sufficient strength.

If the average particle size of the cementite particles is 1.50 μm or less, the cementite particles are sufficiently small. In this case, the cementite particles dissolve sufficiently during heating in the above-mentioned quenching process. As a result, the hardenability of the steel sheet is improved, provided that Features 1 to 4, 6, and 7 are satisfied. Furthermore, this contributes to the spheroidization of the cementite, improving the cold workability of the steel sheet.

The upper limit of the average particle size of the cementite particles is preferably 1.45 μm, more preferably 1.40 μm, even more preferably 1.35 μm, and still more preferably 1.30 μm.
To improve hardenability, a smaller average particle size of cementite particles is preferable. However, if the average particle size of cementite particles is too small, the hardness of the steel sheet becomes too high due to precipitation strengthening. In this case, the cold workability of the steel sheet deteriorates. Therefore, the preferred lower limit of the average particle size of cementite particles is 0.05 μm, more preferably 0.10 μm, even more preferably 0.15 μm, even more preferably 0.20 μm, even more preferably 0.25 μm, and even more preferably 0.30 μm.

[Method for measuring the average particle size of cementite particles]
The average particle size of the cementite particles can be determined by the following method.
A test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate. The surface of the test piece parallel to the rolling direction and thickness direction (i.e., the surface 15 mm in the rolling direction and thickness direction) is defined as the observation surface.

The observation surface is etched using picral solution. Secondary electron images are taken of five random observation fields located at a depth of 1/4 of the plate thickness from the surface of the etched observation surface. Specifically, a scanning electron microscope (SEM) is used to observe the five observation fields at a magnification of 2000x, and the above-mentioned secondary electron images are taken. Each observation field is a rectangle measuring 50 μm in the plate thickness direction and 60 μm in the rolling direction. The observation field is selected so that the center position of the observation field in the plate thickness direction is located at a depth of 1/4 of the plate thickness.

In each secondary electron image, cementite particles are identified based on the contrast. Of the identified cementite particles, those that are entirely contained within the observation field of view are taken as the measurement target. In other words, cementite particles that partially extend beyond the observation field of view are excluded from the measurement target. The area of each cementite particle to be measured is determined, and the circle-equivalent diameter of each cementite particle is calculated based on the area. The calculated circle-equivalent diameter is taken as the particle diameter of the cementite particle. The particle diameter is determined using well-known image processing software.
The arithmetic mean value of the particle diameters of the cementite particles obtained in the five observation fields is defined as the average particle diameter (μm) of the cementite particles.

[(Feature 6) Maximum particle size of cementite particles]
In the steel sheet of this embodiment, the maximum particle size of the cementite particles is 5.00 μm or less.
If the maximum particle size of cementite particles exceeds 5.00 μm, during the manufacturing process of mechanical parts using the steel sheet as a material, the coarse cementite particles in the steel sheet become the origin of cracks during cold working of the steel sheet. As a result, sufficient cold workability cannot be obtained. Furthermore, the Cr concentration [Cr] _θ and the Mo concentration [Mo] _θ in the cementite particles tend to become concentrated. As a result, the hardenability of the steel sheet is reduced, and mechanical parts manufactured using the steel sheet as a material cannot obtain sufficient strength.

When the maximum particle size of cementite particles is 5.00 μm or less, the remaining coarse cementite particles are suppressed after the steel plate is quenched. As a result, the hardenability and cold workability of the steel plate are improved in mechanical parts manufactured using the steel plate, provided that Features 1 to 5 and 7 are satisfied.

The preferred upper limit of the maximum particle size of cementite particles is 4.90 μm, more preferably 4.80 μm, even more preferably 4.70 μm, and even more preferably 4.60 μm.

[Method for measuring maximum particle size of cementite particles]
The maximum particle size of the cementite particles can be determined by the above-mentioned [Method for measuring the average particle size of cementite particles]. Specifically, the maximum particle size of the cementite particles obtained by the [Method for measuring the average particle size of cementite particles] is defined as the maximum particle size (μm) of the cementite particles.

[(Feature 7) Spheroidization rate]
In the steel sheet of this embodiment, among the plurality of cementite particles, cementite particles having an aspect ratio of 3.0 or less are considered to be spherical cementite particles. The spheroidization rate, which is the ratio of the total number of spherical cementite particles to the total number of the plurality of cementite particles, is 75% or more.

If the spheroidization rate is 75% or more, the steel sheet can obtain significantly excellent cold workability, provided that Features 1 to 6 are satisfied. Therefore, the steel sheet of this embodiment has a spheroidization rate of 75% or more.
A higher spheroidization rate is preferable. The lower limit of the spheroidization rate is preferably 78%, more preferably 80%, even more preferably 83%, and still more preferably 85%.
A preferred upper limit of the spheroidization rate is 100%. However, if the spheroidization rate is excessively increased, the production cost increases significantly. Therefore, in consideration of industrial production, the upper limit of the spheroidization rate is, for example, 95%, e.g., 90%.

[Method for measuring spheroidization rate]
The spheroidization rate can be measured by the following method.
The aspect ratio is determined for each of the multiple cementite particles measured in five observation fields using the above-mentioned "Method for Measuring the Average Particle Diameter of Cementite Particles." Specifically, the contour line of the cementite particle is sandwiched between two parallel line segments. The maximum distance between the two parallel line segments sandwiching the cementite particle is defined as the major axis (μm). Furthermore, when the contour line of the cementite particle is sandwiched between two line segments parallel to the major axis, the distance between the two line segments (i.e., the width in the direction perpendicular to the major axis) is defined as the minor axis (μm).

The aspect ratio (= major axis/minor axis) of each cementite particle is calculated based on the obtained major axis (μm) and minor axis (μm). Of all cementite particles in the five observation fields, cementite particles with an aspect ratio of 3.0 or less are identified as "spheroidal cementite particles." The ratio of the total number of spherical cementite particles to the total number of multiple cementite particles being measured is defined as the spheroidization rate (%). The spheroidization rate is calculated as an integer value obtained by rounding the calculated value to one decimal place.

[Effects of the steel sheet according to this embodiment]
The steel sheet of this embodiment, which satisfies the above-described features 1 to 7, has sufficient hardenability during quenching in the manufacturing process when a mechanical part is manufactured using the steel sheet. Furthermore, the steel sheet of this embodiment has sufficient cold workability.

[About hardenability]
In the steel sheet of this embodiment, the fact that sufficient hardenability is obtained means the following evaluation.

[Hardenability evaluation method]
(A _c1 transformation point measurement)
A test piece having a width of 10 mm, a length of 80 mm, and a thickness of the plate thickness is taken from the center of the plate width of the steel plate of this embodiment. The width direction of the test piece is parallel to the longitudinal direction of the steel plate. If the plate thickness exceeds 3 mm, the thickness of the test piece is adjusted to 3 mm by grinding. The thermal expansion coefficient during heating is measured using a plate Formaster testing machine. The A _c1 transformation point is determined from the obtained thermal expansion coefficient.

(Basic hardness measurement)
A plate-shaped test piece is taken from the widthwise center of the steel plate, and the shape of the plate-shaped test piece is 15 mm in the rolling direction of the steel plate, 30 mm in the width direction, and plate thickness.
The plate test specimen is heated to the A _c1 transformation point + 100°C for 30 minutes using a salt bath. The plate test specimen is then immersed in oil at 60°C in an oil tank for quenching. The quenched plate test specimen is cut into two equal halves in the plate width direction. The cut surfaces are mirror-polished. Vickers hardness tests in accordance with JIS Z2244:2020 are performed at three arbitrary points in the center of the plate thickness on the polished cut surfaces. The test force is 98 N. The arithmetic mean value of the obtained Vickers hardness is defined as the basic quenched hardness HD0 (HV). The basic quenched hardness HD0 is an integer value obtained by rounding the obtained value to one decimal place.

(Hardenability evaluation)
A plate-shaped test piece is taken from the widthwise center of the steel plate, and the shape of the plate-shaped test piece is 15 mm in the rolling direction of the steel plate, 30 mm in the width direction, and plate thickness.
The plate test specimen is immersed in a salt bath at 80°C above the _Ac1 transformation point for 10 minutes. The plate test specimen is then removed from the salt bath and immersed in oil at 60°C in an oil tank for quenching. The quenched plate test specimen is cut into two equal halves in the plate width direction. The cut surfaces are mirror-polished. Vickers hardness tests in accordance with JIS Z2244:2020 are performed at three arbitrary points in the center of the plate thickness on the polished cut surfaces. The test force is 98 N. The arithmetic mean value of the obtained Vickers hardness is defined as the quenched hardness HD1 (HV). The quenched hardness HD1 is an integer value obtained by rounding the obtained value to one decimal place.
If the obtained hardness HD1 is 95% or more of the basic hardness HD0, the steel plate is determined to have sufficient hardenability.

[Cold workability]
In the steel sheet of this embodiment, the fact that sufficient cold workability is obtained means the following evaluation.

[Cold workability evaluation method]
A JIS No. 5 plate test piece specified in JIS Z2241:2022 is taken from the center of the steel plate width. The test piece thickness is 1 mm. The test piece thickness is adjusted by grinding, if necessary. A V-notch is formed at the longitudinal center position of the parallel part of the test piece. The V-notch opening angle is 45° and the V-notch depth is 2 mm. The depth direction of the V-notch corresponds to the width direction of the parallel part of the test piece. The gauge length is 5 mm, including the V-notch portion. The longitudinal direction of the test piece is parallel to the rolling direction of the steel plate.

A fracture elongation test is conducted using test specimens at room temperature in air. The butt elongation after fracture is measured, and the resulting butt elongation (fracture elongation) (%) is defined as the notch elongation (%). If the resulting notch elongation is 8.0% or greater, the steel plate is deemed to have sufficient cold workability.

[Steel plate applications]
The steel sheet of this embodiment is suitable as a material for machine parts for automobiles. Examples of machine parts for automobiles include automobile bearings, springs, washers, etc. The steel sheet of this embodiment is suitable as a material for machine parts for textile machinery. Examples of machine parts for textile machinery include knitting needles, etc. The steel sheet of this embodiment can be widely used in applications requiring excellent hardenability and excellent cold workability.

[Method of manufacturing steel sheets]
An example of a method for manufacturing the steel sheet of this embodiment will be described. The method for manufacturing the steel sheet described below is one example for manufacturing the steel sheet of this embodiment. Therefore, the steel sheet having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the method for manufacturing the steel sheet of this embodiment.

An example of the method for manufacturing the steel sheet according to this embodiment includes the following steps.
(Step 1) Material preparation step (Step 2) Hot rolling step (Step 3) Cold rolling step (Step 4) Cold-rolled sheet annealing step In this embodiment, the annealing step is not performed after the hot rolling step and before the cold rolling step.

The main manufacturing conditions in the above steps 1 to 4 are as follows:
(Condition 1) Average cooling rate CR1 from the finish rolling temperature to the intermediate temperature MT in process 2: 8.5°C/sec or more (Condition 2) Intermediate temperature MT in process 2: CT+80°C or less and A _c1 transformation point or less (Condition 3) Coiling temperature CT in process 2: 600 to 700°C
(Condition 4) No hot-rolled sheet annealing is performed. (Condition 5) Cold rolling rate RR in process 3: 20 to 60%.
(Condition 6) Annealing temperature T1 in step 4: 550 to 750°C
(Condition 7) Holding time t1 in step 4: 10 to 60 hours

Each process is explained below.

[(Process 1) Material preparation process]
In the material preparation step, a material satisfying Feature 1 is prepared. The material is produced, for example, by the following method: Molten steel having a chemical composition satisfying Feature 1 is produced. A slab is produced using the molten steel by a well-known continuous casting method.

[(Step 2) Hot rolling step]
In the hot rolling process, a prepared material (slab) is hot-rolled to produce a steel plate. The hot rolling process includes a rough rolling process in which the material is rough-rolled to produce a rough bar (intermediate steel plate), and a finish rolling process in which the rough bar is finish-rolled to produce a steel plate.

In the rough rolling process, the material (slab) is heated in a heating furnace. The heated material is rolled using a rough rolling mill to produce a rough bar. The heating temperature of the material in the rough rolling process is, for example, 1050 to 1300°C. The material is left in the heating furnace for 30 minutes or more, preferably 60 minutes or more. There is no particular upper limit to the time spent in the furnace, but it is, for example, 300 minutes.

In the finish rolling process, the rough bar is further rolled (finish rolling) using a finish rolling mill to produce steel plate. The finish rolling mill includes multiple stands arranged in a row. Each stand is equipped with a pair of work rolls. The surface temperature of the steel plate at the outlet of the final stand of the finish rolling mill that reduces the steel plate is defined as the finish rolling temperature (°C). In this embodiment, the finish rolling temperature is 800 to 950°C. After finish rolling, the hot-rolled steel plate is transported to the coiler on a run-out table and then wound by the coiler to form a coil. Cooling after finish rolling is carried out in two stages. The average cooling rate CR1 in the first stage of cooling, the intermediate temperature MT which is the temperature at which the first stage of cooling is switched to the second stage of cooling, and the coiling temperature CT will be described later. The coiled steel plate is cooled to room temperature.

[(Step 3) Cold rolling step]
In the cold rolling process, the steel sheet after the hot rolling process is subjected to cold rolling using a cold rolling mill. The cold rolling mill is, for example, a reverse rolling mill consisting of a single rolling stand, and the rolling stand includes a pair of work rolls.

In the cold rolling process, cold rolling is carried out using the above-mentioned reverse rolling mill to produce cold-rolled steel sheets. The cold reduction ratio (RR) in the cold rolling process will be described later.

In this embodiment, the hot-rolled steel sheet is subjected to a cold rolling process without being annealed after the hot rolling process. In other words, the annealing process is not performed after the hot rolling process but before the cold rolling process. In this manufacturing method, strain is accumulated in the steel sheet in the hot rolling process and the cold rolling process, and then the annealing process is performed. As a result, ferrite grains of appropriate size, cementite grains of appropriate size, and appropriate concentrations of Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in the cementite grains are obtained.

[(Step 4) Cold-rolled sheet annealing step]
In the cold-rolled sheet annealing step, the cold-rolled steel sheet after the cold rolling step is subjected to annealing treatment. In the annealing step, the annealing temperature T1 and the holding time t1 at the annealing temperature T1 are adjusted to adjust the degree of ferrite recrystallization and cementite particle precipitation.

[Conditions 1 to 7]
In the above-described steps 1 to 4, the following conditions 1 to 7 are satisfied.
(Condition 1) Average cooling rate CR1 from the finish rolling temperature to the intermediate temperature MT in process 2: 8.5°C/sec or more (Condition 2) Intermediate temperature MT in process 2: CT+80°C or less and A _c1 transformation point or less (Condition 3) Coiling temperature CT in process 2: 600 to 700°C
(Condition 4) No hot-rolled sheet annealing is performed. (Condition 5) Cold rolling rate RR in process 3: 20 to 60%.
(Condition 6) Annealing temperature T1 in step 4: 550 to 750°C
(Condition 7) Holding time t1 in step 4: 10 to 60 hours Each condition will be explained below.

[(Condition 1) Average cooling rate CR1]
In the cooling stage of the hot rolling process, the average cooling rate (°C/sec) from the finish rolling temperature to the intermediate temperature MT is defined as the average cooling rate CR1 (°C/sec). The steel sheet of this embodiment is a so-called hypereutectoid steel. Therefore, during cooling from the finish rolling temperature, the steel sheet passes through a temperature range where pro-eutectoid cementite is likely to form (pro-eutectoid cementite formation temperature range).
If the cooling rate in the pro-eutectoid cementite formation temperature range is too slow, coarse pro-eutectoid cementite is formed. The formed coarse pro-eutectoid cementite tends to remain coarse even after the cold-rolled sheet annealing process. As a result, the maximum particle size of cementite particles in the steel sheet may exceed 5.00 μm.

To suppress the formation of coarse pro-eutectoid cementite, the cooling rate in the pro-eutectoid cementite formation temperature range must be sufficiently fast. If the average cooling rate CR1 between the finish rolling temperature and the intermediate temperature MT is 8.5°C/sec or higher, the residence time of the steel sheet in the pro-eutectoid cementite formation temperature range is sufficiently short. Therefore, the formation of coarse pro-eutectoid cementite can be suppressed. As a result, the average particle size of the cementite particles is 1.50 μm or less, and the maximum particle size of the cementite particles is 5.00 μm or less.

The upper limit of the average cooling rate CR1 is not particularly limited. However, due to equipment constraints, the upper limit of the average cooling rate CR1 is, for example, 50.0°C/sec.

[(Condition 2) Intermediate Temperature MT]
In the cooling stage of the hot rolling process, the temperature between the finish rolling temperature and the coiling temperature CT, at which cooling at the average cooling rate CR1 is stopped, is defined as the intermediate temperature MT (°C). If the intermediate temperature MT is higher than CT + 80°C or higher than the A _c1 transformation point, the pro-eutectoid cementite formation temperature range may exist in the hot-rolled steel sheet even in the temperature range below the intermediate temperature MT. Therefore, pro-eutectoid cementite is likely to form below the intermediate temperature MT. In this case, the maximum particle size of cementite particles in the steel sheet may exceed 5.00 μm. If the intermediate temperature MT is lower than CT + 80°C and lower than the A _c1 transformation point, the pro-eutectoid cementite formation temperature range can be cooled at the average cooling rate CR1. Therefore, the formation of pro-eutectoid cementite can be sufficiently suppressed, and the maximum particle size of cementite particles in the steel sheet can be set to 5.00 μm or less. In this embodiment, the intermediate temperature MT is higher than the coiling temperature CT. Furthermore, when the intermediate temperature MT satisfies condition 2, the average cooling rate from the intermediate temperature MT to the coiling temperature CT is slower than the average cooling rate CR1.

[(Condition 3) Regarding the coiling temperature CT]
In the hot rolling process, the coiling temperature CT affects the spheroidization rate of cementite particles and the Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in the cementite particles. If the coiling temperature CT is less than 600°C, the hardness of the hot-rolled steel sheet becomes excessively high, making it impossible to carry out the subsequent cold rolling process.

On the other hand, if the coiling temperature CT exceeds 700°C, the Cr concentration [Cr] _θ in the cementite particles exceeds 7.90%, and the Mo concentration [Mo] _θ exceeds 1.50%. Furthermore, the spheroidization of cementite becomes insufficient, and the spheroidization rate of the cementite particles becomes less than 75%. Furthermore, the maximum particle size of the cementite particles in the steel sheet may exceed 5.00 µm.

If the coiling temperature CT is 600 to 700°C, steel sheets that satisfy characteristics 1 to 7 can be produced, provided that other conditions are met.

[(Condition 4) Hot-rolled sheet annealing]
In the manufacturing process of this embodiment, hot-rolled sheet annealing is not performed between the hot rolling process and the cold rolling process. As described above, in this manufacturing method, strain is accumulated in the steel sheet during the hot rolling process and the cold rolling process, and then the annealing process is performed. This results in appropriately sized ferrite grains, appropriately sized cementite particles, and appropriate concentrations of Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in the cementite particles. The strain in the hot-rolled steel sheet is smaller than that in the cold-rolled steel sheet. Therefore, when hot-rolled sheet annealing is performed to spheroidize the cementite in the hot-rolled steel sheet, it takes a long time for spheroidization. In this case, the total annealing heating time, which is the heating time for the hot-rolled sheet annealing and the heating time for the subsequent cold-rolled sheet annealing process, becomes excessively long. As a result, the ferrite grains and cementite particles become coarse, and the Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in the cementite particles become excessive.

[(Condition 5) Cold Rolling Rate RR]
In the cold rolling process, the cold rolling reduction rate RR is defined by the following formula:
Cold rolling rate RR (%) = (1 - (thickness of cold-rolled steel sheet after cold rolling process / thickness of hot-rolled steel sheet before cold rolling process)) × 100

If the cold rolling rate RR is 20% or more, sufficient strain is introduced into the steel sheet. In this case, spheroidization of cementite is promoted in the subsequent annealing process, and the spheroidization rate becomes 75% or more. On the other hand, if the cold rolling rate RR is less than 20%, the strain introduced into the steel sheet is insufficient. In this case, ferrite recrystallization does not progress, the steel sheet becomes hard, and the cold workability deteriorates. In this case, it becomes difficult to measure the ferrite grain size.
If the cold rolling rate RR exceeds 60%, cold rolling becomes difficult. Therefore, the cold rolling rate RR is 60% or less.

If the cold rolling ratio RR is 20 to 60%, steel sheets that satisfy characteristics 1 to 7 can be produced, provided that other conditions are met.

[(Condition 6) Annealing Temperature T1]
In the cold-rolled sheet annealing process, the annealing temperature T1 adjusts the spheroidization rate of cementite particles in the steel sheet. If the annealing temperature T1 is less than 550°C, the spheroidization of cementite becomes insufficient, and the spheroidization rate of cementite particles becomes less than 75%. Furthermore, ferrite recrystallization does not proceed, the steel sheet becomes hard, and the cold workability deteriorates.

On the other hand, if the annealing temperature T1 exceeds 750°C, the annealing temperature is too high. In this case, the Cr concentration [Cr] _θ in the cementite particles exceeds 7.90%, or the Mo concentration [Mo] _θ exceeds 1.50%. Furthermore, the spheroidization of the cementite becomes insufficient, and the spheroidization rate of the cementite particles becomes less than 75%.

If the annealing temperature T1 is 550 to 750°C, steel sheets satisfying characteristics 1 to 7 can be produced, provided that other conditions are met.

[(Condition 7) Holding time t1 at annealing temperature T1]
In the cold-rolled sheet annealing process, the holding time t1 at the annealing temperature T1 affects the spheroidization rate of cementite particles and the Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in the cementite particles. Specifically, if the holding time t1 is less than 10 hours, the cementite is not sufficiently spheroidized.

On the other hand, if the holding time t1 exceeds 60 hours, the holding time is too long, and in this case, Cr and Mo are concentrated in the cementite particles, so that the Cr concentration [Cr] _θ in the cementite particles exceeds 7.90% and the Mo concentration [Mo] _θ exceeds 1.50%.

If the holding time t1 is 10 to 60 hours, steel plates satisfying characteristics 1 to 7 can be produced, provided that other conditions are met.

The above manufacturing process allows for the production of steel plates that satisfy features 1 to 7.

The effects of the steel sheet of this embodiment will be explained in more detail below using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel sheet of this embodiment. Therefore, the steel sheet of this embodiment is not limited to this one example of conditions.

Steel plates were manufactured having the chemical compositions shown in Tables 1A and 1B.

Specifically, molten steel was continuously cast to produce slabs. The slabs were then subjected to a hot rolling process. Specifically, the slabs were heated at 1050-1300°C for 240 minutes. The heated slabs were rolled in a roughing mill to produce rough bars. The rough bars were then rolled in a finishing mill to produce steel plates. The finish rolling temperature for each test number was 800-950°C. The hot-rolled steel plates after finish rolling were wound into coils. The coiled steel plates were allowed to cool to room temperature. The average cooling rate CR1, intermediate temperature MT, and coiling temperature CT during the hot rolling process for each test number are shown in the "CR1 (°C/sec)," "intermediate temperature MT (°C)," and "coiling temperature CT (°C)" columns of Table 2 (Table 2A and Table 2B). The average cooling rate CR2 from the intermediate temperature MT to the coiling temperature CT was less than the average cooling rate CR1 for all test numbers.

For test numbers 1 to 71, the hot-rolled steel sheets after the hot rolling process were subjected to a cold rolling process without hot-rolled sheet annealing. The cold-rolling reduction ratio RR in the cold rolling process is shown in the "Cold-rolling reduction ratio RR (%)" column of Table 2 (Table 2A and Table 2B). The cold-rolled steel sheets after cold rolling were subjected to a cold-rolled sheet annealing process. The annealing temperature T1 and holding time t1 are shown in the "T1 (°C)" and "t1 (hours)" columns of Table 2 (Table 2A and Table 2B). In the cold-rolled sheet annealing process, the steel sheets were furnace-cooled after holding time t1. For test number 72, hot-rolled sheet annealing was performed after the hot rolling process but before the cold rolling process. The annealing temperature (°C) and holding time (hours) at the annealing temperature in the hot-rolled sheet annealing process are shown in the "Annealing temperature (°C)" and "Annealing time (hours)" columns of Table 2 (Table 2A and Table 2B). Steel plates were manufactured through the above manufacturing process.

[Evaluation test]
The following tests were carried out on the manufactured steel plates with each test number.
(Test 1) Chemical composition measurement test (Test 2) Total area ratio measurement test of ferrite and cementite particles (Test 3) Average ferrite particle size measurement test (Test 4) Average particle size measurement test of cementite particles (Test 5) Maximum particle size measurement test of cementite particles (Test 6) Measurement test of spheroidization rate of cementite particles (Test 7) Measurement test of Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in cementite particles (Test 8) Hardenability evaluation test (Test 9) Cold workability evaluation test Tests 1 to 9 will be described below.

[(Test 1) Chemical composition measurement test]
The chemical compositions of the steel sheets of each steel number were measured based on the method described above in [Method for measuring the chemical composition of steel sheets]. As a result, the chemical compositions of the steel sheets of each steel number were as shown in Tables 1A and 1B.

[(Test 2) Total Area Ratio Measurement Test of Ferrite and Cementite Particles]
The total area ratio of ferrite and cementite particles for each test number was determined based on the method described in the above-mentioned [Method for measuring the total area ratio of ferrite and cementite particles in a microstructure]. The obtained total area ratios of ferrite and cementite particles are shown in the "Total area ratio of ferrite and cementite particles (%)" column of Table 3 (Table 3A and Table 3B). In the "Total area ratio of ferrite and cementite particles (%)" column of Table 3 (Table 3A and Table 3B), "≧95" indicates that the total area ratio of ferrite and cementite particles was 95% or more. Note that in all test numbers, when a remainder other than ferrite and cementite particles was present in the microstructure, the remainder was one or more types selected from the group consisting of bainite, martensite, and pearlite.

[(Test 3) Ferrite average grain size measurement test]
The average ferrite grain size (μm) of the steel sheet of each test number was determined based on the method described in the above [Method for measuring the average ferrite grain size]. The obtained average ferrite grain sizes are shown in the "Ferrite grain size (μm)" column of Table 3 (Table 3A and Table 3B).

[(Test 4) Average particle size measurement test for cementite particles]
The average particle size (μm) of cementite particles in the steel sheets of each test number was determined based on the method described in the above-mentioned [Method for measuring the average particle size of cementite particles]. The obtained average particle sizes of cementite particles are shown in the "Cementite particle size (μm)" column in Table 3 (Table 3A and Table 3B).

[(Test 5) Maximum particle size measurement test for cementite particles]
The maximum particle size (μm) of cementite particles in the steel sheet having each test number was determined based on the method described in the above-mentioned [Method for measuring maximum particle size of cementite particles]. If the obtained maximum particle size of cementite particles was 5.00 μm or less, it was marked with "E" (Excellent) in the "Maximum cementite particle size ≦ 5 μm" column in Table 3 (Table 3A and Table 3B). On the other hand, if the maximum particle size of cementite particles exceeded 5.00 μm, it was marked with "NA" (Not Accepted) in the "Maximum cementite particle size ≦ 5 μm" column in Table 3 (Table 3A and Table 3B).

[(Test 6) Measurement test of spheroidization rate of cementite particles]
The spheroidization ratio (%) of cementite particles of the steel plate having each test number was determined based on the method described in the above-mentioned [Method for measuring spheroidization ratio]. The obtained spheroidization ratios are shown in the "Spheroidization ratio (%)" column of Table 3 (Table 3A and Table 3B).

[(Test 7) Measurement test of Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in cementite particles]
The Cr concentration [Cr] _θ (mass%) and Mo concentration [Mo] _θ (mass%) in the cementite particles of the steel sheet of each test number were determined based on the method described in the above-mentioned [Method for measuring Cr concentration [Cr] _θ and Mo concentration [Mo] _θ in cementite particles]. The obtained Cr concentration [Cr] _θ and Mo concentration [Mo] _θ are shown in the columns "[Cr] _θ (mass%)" and "[Mo] _θ (mass%)" in Table 3 (Table 3A and Table 3B).

[(Test 8) Hardenability Evaluation Test]
The hardenability of the steel plates of each test number was evaluated at the time of quenching based on the method described in the above-mentioned [Hardenability Evaluation Method]. The obtained hardenability HD1 is shown in the "Hardenability HD1 (HV)" column of Table 3 (Table 3A and Table 3B). The "Hardenability Lower Limit (HV)" in Table 3 indicates the value of "Basic Hardenability HD0 (HV)" in Table 1B x 0.95. The A _c1 transformation point (°C) is shown in "A _c1 Point (°C)" in Table 1B. If the hardenability HD1 was equal to or greater than the hardenability lower limit, it was determined that sufficient hardenability was obtained (indicated by "E" in the "Hardenability Evaluation" column of Table 3 (Table 3A and Table 3B)). On the other hand, if the hardenability HD1 was less than the hardenability lower limit, it was determined that sufficient hardenability was not obtained (indicated by "NA" in the "Hardenability Evaluation" column of Table 3 (Table 3A and Table 3B)).

[(Test 9) Cold workability evaluation test]
The cold workability of the steel sheets of each test number was evaluated based on the method described in the above-mentioned [Cold workability evaluation method]. The notch elongation obtained is shown in the "Notch elongation (%)" column of Table 3 (Table 3A and Table 3B). If the notch elongation obtained was 8.0% or more, it was determined that sufficient cold workability was obtained. On the other hand, if the notch elongation was less than 8.0%, it was determined that sufficient cold workability was not obtained.

[Evaluation results]
With reference to Table 1A, Table 1B, Table 2 (Table 2A and Table 2B), and Table 3 (Table 3A and Table 3B), in test numbers 1 to 46, the chemical composition was appropriate and conditions 1 to 7 of the manufacturing conditions were satisfied. Therefore, the steel sheets with these test numbers satisfied features 1 to 7. As a result, sufficient hardenability and sufficient cold workability were obtained.

On the other hand, in test number 47, the C content was too high. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not achieved.

In test number 48, the Si content was too high. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not achieved.

In test number 49, the Mn content was too low. As a result, sufficient hardenability was not achieved.

In test number 50, the Mn content was too high. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not achieved.

In test numbers 51 and 52, the Cr content was too high, so the Cr concentration [Cr] _θ in mass% in the cementite particles exceeded 7.90%, and sufficient hardenability was not obtained.

In test numbers 53 and 54, the Mo content was too high, so the Mo concentration [Mo] _θ in mass % in the cementite particles exceeded 1.50%, and sufficient hardenability was not obtained.

In test numbers 55 and 56, the intermediate temperature MT exceeded the coiling temperature CT + 80°C. As a result, the maximum particle size of the cementite particles exceeded 5.00 μm. In addition, in test number 56, the spheroidization rate of the cementite particles was low. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not achieved.

In test numbers 57 and 58, although the chemical composition was appropriate, the average cooling rate CR1 was too slow. As a result, the maximum particle size of cementite particles exceeded 5.00 μm. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not achieved.

In Test No. 59, the intermediate temperature MT was higher than the A _cl transformation point. Therefore, the maximum particle size of cementite particles exceeded 5.00 µm. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not obtained.

In Test Nos. 60 and 61, although the chemical composition was appropriate, the coiling temperature CT was too high. As a result, the maximum particle size of the cementite particles exceeded 5.00 μm. In addition, the Cr concentration [Cr] _θ in the cementite particles was too high, and further, the spheroidization rate of the cementite particles was low. As a result, sufficient hardenability and sufficient cold workability were not obtained.

In test numbers 62 and 63, although the chemical composition was appropriate, the cold rolling rate RR was too low. As a result, ferrite recrystallization did not progress and was not completed. As a result, the ferrite grain size could not be measured (shown as "-" in the "Ferrite grain size (μm)" column in Table 3B). As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not achieved.

Test Nos. 64 and 65 had appropriate chemical compositions, but the annealing temperature T1 in the cold-rolled sheet annealing process was too low. As a result, the spheroidization rate of cementite particles was low, at less than 75%. Furthermore, ferrite recrystallization was not complete, making it impossible to measure the ferrite grain size (shown as "-" in the "Ferrite grain size (μm)" column in Table 3B). As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not achieved.

In Test Nos. 66 and 67, although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing step was too high. As a result, the Cr concentration [Cr] _θ in the cementite particles was too high, and further, the spheroidization rate of the cementite particles was low. As a result, sufficient hardenability and sufficient cold workability were not obtained.

In test numbers 68 and 69, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing process was too short. As a result, the spheroidization rate of cementite particles was too low, at less than 75%. As a result, the notch elongation was less than 8.0%, and sufficient cold workability was not achieved.

In the test numbers 70 and 71, although the chemical composition was appropriate, the holding time t1 in the cold-rolled sheet annealing process was too long. Therefore, the Cr concentration [Cr] _θ in the cementite particles was too high. As a result, sufficient hardenability was not obtained.

In Test No. 72, hot-rolled sheet annealing was performed after the hot rolling process but before the cold rolling process. Therefore, the ferrite grain size and cementite grain size were too large. Furthermore, the Cr concentration [Cr] _θ in the cementite grains was too high. As a result, sufficient hardenability was not obtained.

The above describes embodiments of the present disclosure. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

Claims (2)

A steel plate,
The chemical composition is in mass %:
C: more than 0.90 to 1.30%,
Si: 0.01 to 0.50%,
Mn: 0.20-1.30%,
P: 0.100% or less,
S: 0.100% or less,
Al: 0.100% or less,
Cr: 0.01-1.60%,
N: 0.0150% or less,
Mo: 0-0.500%,
Ni: 0-1.000%,
B: 0 to 0.0100%,
V: 0 to 0.500%,
Nb: 0 to 0.500%, and
Ti: 0 to 0.150%;
The balance is Fe and impurities.
In the microstructure, the total area ratio of ferrite and cementite particles is 95% or more,
The average particle size of the ferrite is 15.0 μm or less,
The Cr concentration [Cr] _θ in mass% in the cementite particles is 7.90% or less, and the Mo concentration [Mo] _θ in mass% in the cementite particles is 1.50% or less,
The average particle size of the cementite particles is 1.50 μm or less,
The maximum particle size of the cementite particles is 5.00 μm or less,
Among the cementite particles, the cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles, and a spheroidization rate, which is the ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 75% or more.
steel plate. The steel sheet according to claim 1,
The chemical composition is in mass %:
Mo: 0.001-0.500%,
Ni: 0.0001 to 1.000%,
B: 0.0001 to 0.0100%,
V: 0.001-0.500%,
Nb: 0.001 to 0.500%, and
Ti: 0.001 to 0.150%;
steel plate.