JP7799191B2 - case hardened steel - Google Patents

Description

The present invention relates to case-hardening steel.

Machine structural parts such as automotive parts, industrial machinery parts, and construction machinery parts may be surface hardened to improve fatigue strength, for example.

Of the various surface hardening treatments, carburizing is often used on mechanical structural parts such as those mentioned above. Carburizing is a process in which parts are heated in the austenite temperature range in a carburizing atmosphere, then quenched and tempered. A hardened layer is formed on the surface of carburized parts. This hardened layer not only provides high fatigue strength, but also high toughness because the core of the part is quenched and tempered as well.

In response to recent demands for high fatigue strength, alloying elements are being added to increase the strength of the core of parts. However, adding alloying elements to increase the strength of the core of parts reduces the toughness of the parts. For this reason, there is a demand for case-hardening steel that has a good balance of strength and toughness.

As a case-hardening steel for obtaining a mechanical structural part to be used after carburizing, for example, Patent Document 1 proposes a case-hardening steel containing, in mass%, C: 0.2 to 0.3%, Si: 0.6% or less, Mn: 0.95 to 2.2%, P: 0.03% or less, S: 0.03% or less, Cr: 0.1 to 1.8, Al: 0.06% or less, N: 0.02% or less, and O: 0.003% or less, and satisfying the following formulas (1) and (2):
-163×[C]+43.1×[Si]-55.2×[Mn]+32.6×[Cu]-30.0×[Ni]-47.8×[Cr]+104×[Mo]+412×[V]+677 ≦540…(1)
40.4×[C]+1.31×[Si]+18.7×[Mn]+8.37×[Cu]+5.33×[Ni]+5.57×[Cr]+11.8×[Mo]-51.1×[V]-17.8≧15 (2)
However, the element symbol in brackets [ ] indicates the content (mass %).

JP 2012-1774 A

Patent Document 1 discloses that by appropriately controlling the chemical composition and transformation start temperature, it is possible to create case-hardened steel with extremely small heat treatment distortion. However, Patent Document 1 does not specifically consider strength and toughness.

The present invention was made in light of the above-mentioned current situation, and aims to obtain case-hardened steel that has high strength and excellent toughness after carburizing treatment.

The gist of the present invention is as follows:

(1) A case-hardening steel according to one aspect of the present invention has a chemical composition, in mass%,
C: 0.17-0.35%,
Si: 0.28-0.60%,
Mn: 1.03 to 1.45%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.050%,
Cr: 1.05-1.50%,
N: 0.020% or less,
Mo: 0.03-0.15%,
Contains Ni: 0.03 to 0.19% and O: 0.0030% or less, with the balance being Fe and impurities;
A case-hardening steel characterized by satisfying the following (1) and having a Cr concentration in cementite of 1.30 mass% or more.
2.20≦1.5×Cr+4.5×Mo≦2.90 ... (1)
In the above formula (1), the element symbols indicate the content of each element in mass %.
(2) A case-hardening steel according to another embodiment of the present invention has a chemical composition, in mass%,
C: 0.17-0.35%,
Si: 0.28-0.60%,
Mn: 1.03 to 1.45%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.050%,
Cr: 1.05-1.50%,
N: 0.020% or less,
Mo: 0.03-0.15%,
Contains Ni: 0.03 to 0.19% and O: 0.0030% or less, and further contains one or more selected from the group consisting of the following Group A, Group B, and Group C:
the balance being Fe and impurities;
The following condition (1) is satisfied, and the Cr concentration in cementite is 1.30 mass% or more.
2.20≦1.5×Cr+4.5×Mo≦2.90 ... (1)
In the above formula (1), the element symbols indicate the content of each element in mass %.
[Group A]
Ti: 0.005% or less,
V: 0.010% or less,
One or more selected from the group consisting of Nb: 0.005% or less and B: 0.0015% or less [Group B]
One or two selected from the group consisting of Cu: 0.50% or less and Sn: 0.100% or less [Group C]
(3) One or two selected from the group consisting of Ca: 0.0050% or less and Mg: 0.0050% or less. (2) The case-hardened steel described in (2) above may have a chemical composition containing the A group in mass%.
(4) The case-hardened steel according to (2) above may have a chemical composition containing, in mass %, the B group.
(5) The case-hardened steel according to (2) above may have a chemical composition containing, in mass %, the C group.

The above aspect of the present invention makes it possible to provide case-hardened steel that has high strength and excellent toughness after carburizing.

The requirements for the case-hardened steel according to this embodiment are explained in detail below. Note that in this embodiment, case-hardened steel refers to steel that is machined into a part shape as needed, then carburized before use (for example, as a mechanical structural part).

<Chemical composition>
First, the chemical composition of the case-hardened steel according to this embodiment will be described. The numerical ranges described below, separated by "to", include the lower and upper limits. Values indicated as "less than" and "greater than" do not include the values in the numerical range. All "%" in the chemical composition means "mass %."

The case-hardened steel according to this embodiment has a chemical composition, in mass percent, of C: 0.17-0.35%, Si: 0.28-0.60%, Mn: 1.03-1.45%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.050%, Cr: 1.05-1.50%, N: 0.020% or less, Mo: 0.03-0.15%, Ni: 0.03-0.19%, O: 0.0030% or less, and the balance: Fe and impurities. Each element is explained below.

C: 0.17-0.35%
Carbon (C) increases the strength of machine structural parts (hereinafter sometimes simply referred to as "parts") after carburizing. To obtain the desired strength in the parts, the C content is set to 0.17% or more. The C content is preferably 0.19% or more.
However, if the C content exceeds 0.35%, the strength of the core of the part becomes too high, resulting in a deterioration in toughness. Therefore, the C content is set to 0.35% or less. The C content is preferably 0.32% or less.

Si: 0.28-0.60%
Silicon (Si) improves the hardenability of case-hardened steel. In order to improve the hardenability of case-hardened steel and obtain the desired strength in parts, the Si content is set to 0.28% or more. The Si content is preferably 0.30% or more.
However, if the Si content exceeds 0.60%, the toughness of the part deteriorates. Therefore, the Si content is set to 0.60% or less, and preferably 0.55% or less.

Mn: 1.03-1.45%
Manganese (Mn) increases the strength of parts. If the Mn content is less than 1.03%, the desired strength cannot be obtained in parts. Therefore, the Mn content is set to 1.03% or more. The Mn content is preferably 1.10% or more.
On the other hand, if the Mn content exceeds 1.45%, the toughness of the part deteriorates. Therefore, the Mn content is set to 1.45% or less, and preferably 1.40% or less.

P: 0.050% or less Phosphorus (P) is inevitably contained in steel. P is prone to segregation in steel, causing localized reduction in ductility. In particular, if the P content exceeds 0.050%, the localized reduction in ductility becomes significant. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.030% or less.
The lower limit of the P content is not particularly limited, but may be more than 0% or 0.002% or more.

S: 0.050% or less Sulfur (S) is inevitably contained in steel. S combines with Mn in steel to form MnS, which may cause cracking during hot forging. If the S content exceeds 0.050%, cracking becomes more pronounced. Therefore, the S content is set to 0.050% or less. The S content is preferably 0.040% or less.
The lower limit of the S content is not particularly limited, but may be more than 0% or 0.005% or more.

Al: 0.005-0.050%
Aluminum (Al) is an element effective as a deoxidizer. If the Al content is less than 0.005%, deoxidation will be insufficient. Therefore, the Al content is set to 0.005% or more. The Al content is preferably 0.010% or more.
On the other hand, if the Al content exceeds 0.050%, coarse oxides are formed, which deteriorates the toughness of the part. Therefore, the Al content is set to 0.050% or less, and preferably 0.040% or less.

Cr: 1.05-1.50%
Chromium (Cr) improves the hardenability of case-hardened steel, increases the strength of the core of the part after carburizing, and stabilizes it by dissolving in carbides, thereby increasing the toughness of the core of the part. If the Cr content is less than 1.05%, the above effects cannot be obtained. Therefore, the Cr content is set to 1.05% or more. The Cr content is preferably 1.10% or more.
On the other hand, if the Cr content exceeds 1.50%, not only will the solid solution in carbides become saturated, but the toughness of the part will also deteriorate. Therefore, the Cr content is set to 1.50% or less, and preferably 1.30% or less.

N: 0.020% or less Nitrogen (N) is inevitably contained in steel. Nitrogen may also combine with Ti in the steel to form TiN, thereby degrading the toughness of the core of the part. If the N content exceeds 0.020%, the toughness of the core of the part will be significantly degraded. Therefore, the N content is set to 0.020% or less. The N content is preferably 0.018% or less, and more preferably 0.015% or less.
The lower limit of the N content is not particularly limited, but may be more than 0% or 0.005% or more.

Mo: 0.03~0.15%
Molybdenum (Mo) improves the hardenability of case-hardened steel, increases the strength of the core of the part after carburizing, and stabilizes it by dissolving in carbides, thereby increasing the toughness of the core of the part. If the Mo content is less than 0.03%, the above effects cannot be obtained. Therefore, the Mo content is set to 0.03% or more. The Mo content is preferably 0.05% or more.
On the other hand, if the Mo content exceeds 0.15%, not only does solid solution in carbides become saturated, but the toughness of the part deteriorates. Therefore, the Mo content is set to 0.15% or less, and preferably 0.13% or less.

Ni: 0.03-0.19%
Nickel (Ni) increases the toughness of parts. If the Ni content is less than 0.03%, this effect cannot be obtained. Therefore, the Ni content is set to 0.03% or more. The Ni content is preferably 0.05% or more.
On the other hand, if the Ni content exceeds 0.19%, the toughness of the part deteriorates. Therefore, the Ni content is set to 0.19% or less, and preferably 0.18% or less.

O: 0.0030% or less If the O content in steel is too high, it forms coarse oxides that become the starting point of fracture, deteriorating the toughness of the part. Therefore, the O content is set to 0.0030% or less. The O content is preferably 0.0020% or less, and more preferably 0.0015% or less.
The lower limit of the O content is not particularly limited, but may be 0% or more, or may be more than 0% or 0.0005% or more.

2.20≦1.5×Cr+4.5×Mo≦2.90 ... (1)
In the steel material for carbonitriding of this embodiment, it is important to control the contents of alloying elements in order to preferably control the Cr concentration in cementite. In particular, it is important to control the Cr content and the Mo content, which stabilizes cementite.
If the value of the middle part of the above formula (1) is 2.20 or more, the Cr concentration in cementite can be controlled favorably. As a result, high toughness can be obtained in the part. Therefore, the value of the middle part of the formula (1) is set to 2.20 or more. Preferably, it is set to 2.30 or more or 2.35 or more.
On the other hand, if the value of the middle part of formula (1) exceeds 2.90, not only will the above effect saturate, but the toughness of the part will deteriorate due to solid solution strengthening by Cr and Mo. Therefore, the value of the middle part of formula (1) is set to 2.90 or less, and preferably 2.80 or less or 2.70 or less.

The balance of the chemical composition of the case-hardened steel according to this embodiment may be Fe and impurities. In this embodiment, impurities include elements that are mixed in from raw materials such as ore or scrap, or the manufacturing environment, during the industrial production of steel materials, and are acceptable within a range that does not impair the properties of the case-hardened steel according to this embodiment.

The chemical composition of the case-hardened steel according to this embodiment may further contain, as an optional element, one or more elements selected from the group consisting of the following groups A, B, and C, instead of a portion of Fe. When the following optional elements are not contained, the content is 0%.
[Group A]
Ti: 0.005% or less,
V: 0.010% or less,
One or more selected from the group consisting of Nb: 0.005% or less and B: 0.0015% or less [Group B]
One or two selected from the group consisting of Cu: 0.50% or less and Sn: 0.100% or less [Group C]
One or two selected from the group consisting of Ca: 0.0050% or less and Mg: 0.0050% or less

Ti: 0.005% or less Titanium (Ti) combines with N in steel to form TiN, but if the Ti content exceeds 0.005%, coarse TiN may be formed, which may deteriorate the toughness of the core of the part. Therefore, the Ti content is set to 0.005% or less.
Since it is preferable that Ti is not contained, the Ti content may be 0%.

V: 0.010% or less V is an element that improves the strength of steel materials through solid solution strengthening. V may be added as needed to achieve this effect. However, if the V content exceeds 0.010%, a large amount of carbonitrides may precipitate, which may deteriorate the toughness of the part. Therefore, the V content is preferably 0.010% or less.
The V content may be 0%.

Nb: 0.005% or less Niobium (Nb) combines with N in steel to form NbN, but if the Nb content exceeds 0.005%, coarse NbN may be formed, which may deteriorate the toughness of the core of the part. Therefore, the Nb content is set to 0.005% or less.
It is preferable that Nb is not contained, and therefore the Nb content may be 0%.

B: 0.0015% or less B is an element that bonds with N in steel to form nitrides during induction hardening. If the B content exceeds 0.0015%, coarse nitrides are formed, which may deteriorate the toughness of parts after induction hardening. Therefore, the B content is preferably 0.0015% or less.
Since it is preferable that B is not contained, the B content may be 0%.

Cu: 0.50% or less Copper (Cu) increases the strength of parts. To achieve this effect, Cu may be added as needed. To ensure the above effect, the Cu content is preferably 0.02% or more.
However, if the Cu content exceeds 0.50%, the toughness of the part may be deteriorated. Therefore, if Cu is contained, the Cu content is set to 0.50% or less, and preferably 0.45% or less.

Sn: 0.100% or less Sn is an element that suppresses coarsening of crystal grains and improves the strength of steel. Sn may be added as needed to achieve this effect. However, if the Sn content exceeds 0.100%, the steel becomes embrittled and prone to fracture during hot rolling. Therefore, the Sn content is preferably 0.100% or less.
The Sn content may be 0%.

Ca: 0.0050% or less Ca is an element that fixes S in steel as spherical CaS and suppresses the formation of elongated inclusions such as MnS, thereby improving the formability of the steel material. Ca may be added as needed to achieve this effect. However, even if the Ca content exceeds 0.0050%, the above effect saturates. Therefore, the Ca content is preferably 0.0050% or less.
The Ca content may be 0%.

Mg: 0.0050% or less Mg has the effect of improving the formability of steel by adjusting the shape of inclusions in steel to a preferred shape. Mg may be added as needed to achieve this effect. However, if the Mg content exceeds 0.0050%, excessive inclusions may be generated in the steel, which may actually reduce the formability of the steel. Therefore, the Mg content is preferably 0.0050% or less.
The Mg content may be 0%.

The chemical composition of the case-hardened steel may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.
The content of each element is determined by rounding off the measured value to the nearest significant digit based on the significant digits specified in this embodiment, to obtain the value of the content of each element specified in this embodiment down to the least significant digit.

Cr concentration in cementite: 1.30% by mass or more In the steel material for carbonitriding according to this embodiment, the Cr concentration in cementite is set to 1.30% by mass or more. By setting the Cr concentration in cementite to 1.30% by mass or more, high toughness can be exhibited in the core of the part after carbonitriding. Although the details of the mechanism by which increasing the Cr concentration in cementite improves toughness after carbonitriding are unknown, the inventors speculate that the distribution of solute Cr in austenite during heating during quenching, etc., has an effect.
However, if the Cr concentration in cementite is too high, the above effect saturates, so the upper limit may be set to 3.00 mass % or less.

Next, a method for manufacturing the case-hardening steel according to the present embodiment will be described. The manufacturing method is not particularly limited, but the following method can be mentioned, for example.
First, a steel having the above-described chemical composition is melted and a slab is produced. The produced slab is then bloomed to produce a steel billet. The resulting slab is then hot-rolled to obtain a steel material for carbonitriding. In this process, in order to fully dissolve Cr in austenite, the heating temperature of the slab is preferably 1050°C or higher, and the heating time is preferably 1 hour or longer. Furthermore, in order to ensure sufficient time for Cr to concentrate in cementite, the cementite transformation temperature is preferably high. Therefore, it is preferable that the finish processing temperature in the hot rolling be 850°C or higher, the area reduction rate in the finish processing be 30% or higher, and the average cooling rate after the finish processing be 0.5°C/s or higher.

Next, an example of the present invention will be described. However, the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the objectives of the present invention.

A 150 kg ingot with the chemical composition shown in Tables 1A and 1B was melted using a vacuum melting furnace. The resulting ingot was heated to 1,200°C for 1.5 hours, then hot forged at a finish processing temperature of 900°C with a finish processing area reduction of 40%. After finish processing, the average cooling rate from finish processing to 300°C was 1.0°C/s, yielding a 35 mm diameter round steel bar.

A test piece for microstructure observation was taken so that a depth of 1/2 the cross-sectional radius of the steel bar (8.75 mm from the circumferential surface) could be observed on a cross section parallel to the longitudinal direction including the center line of the obtained round steel bar. When the structure of this test piece was observed with an optical microscope using nital corrosion, all of the test pieces were ferrite-pearlite structures. Furthermore, the microstructure was observed with a scanning electron microscope (SEM), and the Cr content in the cementite was quantitatively analyzed using an energy dispersive X-ray spectrometer (EDS) attached to the SEM. Observation with the SEM was performed at a magnification of 2000x, and five fields of view were observed. The white portion observed in the pearlite structure (a lamellar structure of ferrite and cementite) in the secondary electron image observation field was identified as cementite, and EDS analysis was performed on the cementite to obtain the Cr concentration in the cementite. The acceleration voltage during EDS analysis was 20 kV, and one point in each field of view was measured. The average value of the Cr concentration in cementite obtained from the measurements of a total of five points was taken as the measured value.
The measurement times obtained are shown in Tables 1A and 1B.

Round steel bars were machined into test pieces with a diameter of 30 mm and subjected to an austenitizing treatment by heating at 850°C for 30 minutes. They were then quenched by immersion in oil at room temperature, and subsequently tempered by heating at 160°C for 60 minutes to obtain round bar test pieces. These round bar test pieces simulate the core of a part when case-hardened steel is carburized.

Tensile test specimens and impact test specimens were taken from the obtained round steel bars, centered at the 1/2 radius position. The tensile test specimens were No. 14A bar-shaped test specimens conforming to JIS Z 2241:2011, with a parallel portion length of 35 mm, a parallel portion diameter of 5 mm, and a grip portion diameter of 10 mm. Five of these bar-shaped test specimens were subjected to a tensile test conforming to JIS Z 2241:2011 to determine the average tensile strength TS (GPa).
The impact test specimens were U-notch test specimens with a length of 55 mm, a cross section of 10 mm square, a notch depth of 2 mm, and a notch bottom radius of 1 mm, in accordance with JIS Z 2242:2018. The notch was machined on the surface close to the front surface of the round bar. Five of these U-notch test specimens were used to perform a Charpy impact test at room temperature in accordance with JIS Z 2242:2018, and the average absorbed energy (J/cm ² ) was determined.
Table 2 shows the test results.

When the obtained absorbed energy was 100 J/ ^cm2 or more, the obtained tensile strength was 1.05 GPa or more, and the product of the absorbed energy and the tensile strength TS was 125 J/ ^cm2 ·GPa or more, it was determined that high strength and excellent toughness were obtained after the carburizing treatment.
If either one of the conditions was not satisfied, it was determined that high strength and excellent toughness could not be obtained after carburizing treatment.

Looking at Tables 1A, 1B, and 2, it can be seen that the case-hardened steel according to the present invention exhibited high strength and excellent toughness after carburizing. On the other hand, the case-hardened steel according to the comparative example did not exhibit high strength or excellent toughness.

Claims (5)

The chemical composition, in mass%, is
C: 0.17-0.35%,
Si: 0.28-0.60%,
Mn: 1.03 to 1.45%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.050%,
Cr: 1.05-1.50%,
N: 0.020% or less,
Mo: 0.03-0.15%,
Contains Ni: 0.03 to 0.19% and O: 0.0030% or less, with the balance being Fe and impurities;
A case-hardening steel characterized by satisfying the following (1) and having a Cr concentration in cementite of 1.30 mass% or more.
2.20≦1.5×Cr+4.5×Mo≦2.90 ... (1)
In the above formula (1), the element symbols indicate the content of each element in mass %. The chemical composition, in mass%, is
C: 0.17-0.35%,
Si: 0.28-0.60%,
Mn: 1.03 to 1.45%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.050%,
Cr: 1.05-1.50%,
N: 0.020% or less,
Mo: 0.03-0.15%,
Contains Ni: 0.03 to 0.19% and O: 0.0030% or less, and further contains one or more selected from the group consisting of the following Group A, Group B, and Group C:
the balance being Fe and impurities;
A case-hardening steel characterized by satisfying the following (1) and having a Cr concentration in cementite of 1.30 mass% or more.
2.20≦1.5×Cr+4.5×Mo≦2.90 ... (1)
In the above formula (1), the element symbols indicate the content of each element in mass %.
[Group A]
Ti: 0.005% or less,
V: 0.010% or less,
One or more selected from the group consisting of Nb: 0.005% or less and B: 0.0015% or less [Group B]
One or two selected from the group consisting of Cu: 0.50% or less and Sn: 0.100% or less [Group C]
One or two selected from the group consisting of Ca: 0.0050% or less and Mg: 0.0050% or less The case-hardening steel according to claim 2, having a chemical composition containing, by mass %, the Group A elements. The case-hardening steel according to claim 2, having a chemical composition containing, by mass %, the B group. The case-hardening steel according to claim 2, having a chemical composition containing, by mass %, the C group.