JP7705065B2 - hot rolled steel plate - Google Patents

Description

The present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet having high strength, as well as excellent fatigue strength, toughness and ductility.

In recent years, the use of high-strength steel plates in automotive components has been actively considered in order to improve the durability and collision safety of automobiles. However, increasing the strength of steel plates generally results in a decrease in toughness. Therefore, in the development of high-strength steel plates, it is important to increase the strength without degrading the material properties. In particular, for high-strength steel plates used in automotive components, it is important to ensure the fatigue durability of the parts. When processed into parts, cracks propagate from the punched surfaces, etc., and the fatigue durability of the parts is not necessarily improved even when high-strength steel plates are used.

In response to this, Patent Document 1 proposes a high-strength hot-rolled steel sheet with excellent bending workability, in which the metal structure has a surface region in which the ferrite phase is the main phase and an internal region in which the bainite phase is the main phase, and the proportion of the surface region in the thickness direction of the steel sheet is 1.0 to 5.0% of the total sheet thickness on both the front and back sides of the steel sheet.

Patent Document 2 proposes a high-strength hot-rolled steel sheet with excellent workability, which has a central portion mainly made of bainite and a surface layer mainly made of polygonal ferrite, the surface layer being formed in an area extending to a depth of at least 0.2 mm from both surfaces of the steel sheet.

Patent document 3 proposes a high-strength steel plate with excellent bendability, in which the average Vickers hardness and standard deviation of hardness from the surface layer to the 1/2 plate thickness position are kept low.

Patent Document 4 proposes a hot-rolled steel sheet that has improved fatigue properties and surface machinability by controlling the area fraction of martensite and Vickers hardness within a predetermined range for each depth direction of the sheet thickness.

International Publication No. 2014/171057 Japanese Patent Application Publication No. 2001-279378 International Publication No. 2018/151331 Japanese Patent Application Publication No. 2017-186634

However, in the hot-rolled steel sheets described in Patent Documents 1 to 3, the surface layer is softened with ferrite as the main phase, leaving room for further improvement in fatigue properties.

In addition, in the invention described in Patent Document 4, the surface layer is softened, leaving room for further improvement in fatigue strength. Furthermore, because precipitation strengthening is performed within the plate thickness, dislocation motion in ferrite is inhibited, and from this perspective, there is room for further improvement in toughness.

In recent years, against a background of demands for further weight reduction in automobiles and increasingly complex shapes of parts, there is a demand for high-strength hot-rolled steel sheets having even higher fatigue strength and toughness.
The present invention has been made in view of the above problems, and aims to provide a hot-rolled steel sheet having high strength, excellent fatigue strength and toughness. Another object of the present invention is to provide a hot-rolled steel sheet having the above-mentioned properties and, in addition, excellent ductility, which is a property generally required for a hot-rolled steel sheet to be used for an automobile member.

Precipitation-strengthened structures inhibit dislocation motion and therefore have excellent fatigue strength. For this reason, precipitation-strengthened structures are often used in automobile suspension parts. On the other hand, when dislocation motion is suppressed, plastic deformation is less likely to occur, and impact properties (especially toughness) deteriorate. Therefore, it is presumed that fatigue strength and impact properties are in a contradictory relationship. In order to improve both fatigue strength and toughness, the inventors analyzed in detail the deformation mechanisms of fatigue strength and impact strength. As a result, the inventors considered that the metal structure and hardness of the surface region of the hot-rolled steel sheet have a large effect on fatigue strength, and the metal structure and hardness of the internal region of the hot-rolled steel sheet have a large effect on crack propagation.

The gist of the present invention made based on the above findings is as follows.
(1) A hot-rolled steel sheet according to one embodiment of the present invention has a chemical composition, in mass%,
C: 0.02-0.30%,
Si: 0.10-2.00%,
Mn: 0.5-3.0%,
sol. Al: 0.10 to 1.00%,
Ti: 0.06-0.20%,
P: 0.1000% or less,
S: 0.0100% or less,
N: 0.0100% or less,
Nb: 0 to 0.100%,
Ca: 0-0.0060%,
Mo: 0 to 0.50%,
Cr: 0-1.00%,
V: 0 to 0.40%,
Ni: 0 to 0.40%,
Cu: 0 to 0.40%,
B: 0 to 0.0020% and Sn: 0 to 0.20%;
The balance is Fe and impurities,
The metal structure of the internal region contains, in terms of area ratio, one or both of martensite and bainite in total of 40 to 80%, ferrite in total of 20 to 60%, and the area ratio of the remaining structure is less than 5%,
a ratio αs/αc of a ferrite area ratio αs of the surface layer region to a ferrite area ratio αc of the inner region is 1.15 to 2.50;
a ratio βs/βc of a total area ratio βs of martensite and bainite in the surface layer region to a total area ratio βc of the martensite and bainite in the internal region is 0.30 to 0.90;
a hardness difference ratio (1-Hvs/Hvc) between a Vickers hardness Hvs of the surface region and a Vickers hardness Hvc of the inner region is 0.20 or less;
The tensile strength is 980 MPa or more.
(2) In the hot-rolled steel sheet described in (1) above, the chemical composition is, in mass%,
Nb: 0.010-0.100%,
Ca: 0.0005-0.0060%,
Mo: 0.02-0.50%,
Cr: 0.02-1.00%,
V: 0.01-0.40%,
Ni: 0.01 to 0.40%,
Cu: 0.01-0.40%,
B: 0.0001 to 0.0020%, and Sn: 0.01 to 0.20%
may contain one or more selected from the group consisting of:

According to the above-mentioned aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, as well as excellent fatigue strength, toughness and ductility. This hot-rolled steel sheet is of great industrial value because it can reduce the weight and improve the durability of vehicle bodies such as automobiles.

A hot-rolled steel sheet according to one embodiment of the present invention (hereinafter, sometimes referred to as the hot-rolled steel sheet according to this embodiment) will be described. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present invention.

Each of the constituent elements of the present invention will be described in detail below. First, the reasons for limiting the chemical composition of the hot-rolled steel sheet according to the present embodiment will be described.
In the following, the numerical ranges described with "to" include the lower and upper limits. Numerical values indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages relating to chemical compositions are mass % unless otherwise specified.

The chemical composition of the hot-rolled steel sheet according to this embodiment is, in mass%, C: 0.02-0.30%, Si: 0.10-2.00%, Mn: 0.5-3.0%, sol. Al: 0.10-1.00%, Ti: 0.06-0.20%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0100% or less, and the balance: Fe and impurities. Each element is described in detail below.

<C: 0.02-0.30%>
C is an important element for improving the strength of a hot-rolled steel sheet. In order to obtain a desired strength, the C content is set to 0.02% or more, and preferably 0.04% or more.
On the other hand, if the C content exceeds 0.30%, the toughness of the hot-rolled steel sheet deteriorates, so the C content is set to 0.30% or less, preferably 0.20% or less.

<Si: 0.10-2.00%>
Silicon is an element that has the effect of suppressing the formation of carbides during ferrite transformation and improving the toughness of the hot-rolled steel sheet. To obtain this effect, the silicon content is set to 0.10% or more, preferably 0.20% or more or 0.50% or more.
On the other hand, if the Si content exceeds 2.00%, the toughness of the hot-rolled steel sheet deteriorates, so the Si content is set to 2.00% or less, preferably 1.50% or less.

<Mn: 0.5-3.0%>
Mn is an element effective in improving the hardenability and strength of the hot-rolled steel sheet by solid solution strengthening. To obtain this effect, the Mn content is set to 0.5% or more, and preferably 1.0% or more.
On the other hand, if the Mn content exceeds 3.0%, MnS, which is detrimental to toughness and fatigue strength, is formed, so the Mn content is set to 3.0% or less, preferably 2.5% or less or 2.0% or less.

<Sol. Al: 0.10 to 1.00%>
Al is an important element for controlling ferrite transformation. To obtain this effect, the sol. Al content is set to 0.10% or more, preferably 0.15% or more or 0.20% or more.
On the other hand, if the sol. Al content exceeds 1.00%, alumina precipitates in clusters, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the sol. Al content is set to 1.00% or less, preferably 0.80% or less or 0.50% or less.
Incidentally, sol. Al means acid-soluble Al, and indicates solute Al present in the steel in a solid solution state.

<Ti: 0.06-0.20%>
Ti is an element that strengthens ferrite by precipitation and is also an important element for controlling ferrite transformation to obtain a desired amount of ferrite. In order to obtain excellent fatigue strength by controlling precipitation strengthening and ferrite transformation, the Ti content is set to 0.06% or more, preferably 0.08% or more.
On the other hand, if the Ti content exceeds 0.20%, inclusions due to TiN are formed, and the toughness of the hot-rolled steel sheet is deteriorated. Therefore, the Ti content is set to 0.20% or less, preferably 0.16% or less or 0.13% or less.

<P: 0.1000% or less>
P is an impurity, and the lower the P content, the better. In particular, if the P content exceeds 0.1000%, the workability and weldability of the hot-rolled steel sheet are significantly reduced, and the fatigue strength is also reduced. Therefore, the P content is set to 0.1000% or less. Preferably, it is 0.0500% or less or 0.0200% or less.
There is no particular need to specify a lower limit for the P content, but from the viewpoint of refining costs, it is preferably set to 0.0010% or more.

<S: 0.0100% or less>
S is an impurity, and the lower the S content, the better. In particular, if the S content exceeds 0.0100%, a large amount of inclusions such as MnS, which are harmful to the isotropy of toughness, are generated. Therefore, the S content is set to 0.0100% or less. When better toughness is required, the S content is preferably set to 0.0060% or less. More preferably, it is set to 0.0050% or less.
There is no particular need to specify a lower limit for the S content, but from the viewpoint of refining costs, it is preferably set to 0.0001% or more.

<N: 0.0100% or less>
N is an impurity. If the N content exceeds 0.0100%, coarse Ti nitrides are formed in the high temperature range, which deteriorates the toughness of the hot-rolled steel sheet. Therefore, the N content is set to 0.0100% or less. Preferably, it is 0.0060% or less or 0.0050% or less.
There is no need to specify a lower limit for the N content, but from the viewpoint of refining costs, it is preferable to set the lower limit to 0.0001% or more.

The hot-rolled steel sheet according to this embodiment may contain the above chemical components, with the balance being Fe and impurities. In this embodiment, impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment, and/or substances that are acceptable to the extent that they do not adversely affect the hot-rolled steel sheet according to this embodiment.

Although not essential for providing the desired characteristics, the following optional elements may be included to reduce manufacturing variations and further improve the strength of the hot-rolled steel sheet. However, since it is not essential to include these elements, the lower limit of the content of these elements is 0%. Note that if the content of each optional element is less than the lower limit of the content explained below, it can be considered to be an impurity.

<Nb: 0.010-0.100%>
Nb is an element that has the effect of increasing the strength of a hot-rolled steel sheet by refining the crystal grain size of the hot-rolled steel sheet and by precipitation strengthening of NbC. In order to reliably obtain this effect, the Nb content is preferably 0.010% or more.
On the other hand, if the Nb content exceeds 0.100%, the above effect is saturated. Therefore, even if Nb is contained, the Nb content is set to 0.100% or less, preferably 0.060% or less.

<Ca: 0.0005-0.0060%>
Ca is an element that disperses a large number of fine oxides during deoxidation of molten steel and has the effect of refining the structure of the hot-rolled steel sheet. Ca is also an element that fixes S in the steel as spherical CaS, suppresses the generation of elongated inclusions such as MnS, and improves the hole expandability of the hot-rolled steel sheet. To reliably obtain these effects, it is preferable that the Ca content is 0.0005% or more.
On the other hand, even if the Ca content exceeds 0.0060%, the above effect is saturated. Therefore, even if Ca is contained, the Ca content is set to 0.0060% or less, and preferably 0.0040% or less.

<Mo: 0.02-0.50%>
Mo is an element effective in strengthening the precipitation of ferrite. To reliably obtain this effect, the Mo content is preferably 0.02% or more, and more preferably 0.10% or more.
On the other hand, if the Mo content is excessive, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, even if Mo is contained, the Mo content is set to 0.50% or less, and preferably 0.30% or less.

<Cr: 0.02-1.00%>
Cr is an element effective in improving the strength of a hot-rolled steel sheet. To reliably obtain this effect, the Cr content is preferably 0.02% or more, and more preferably 0.10% or more.
On the other hand, if the Cr content is excessive, the ductility of the hot-rolled steel sheet decreases, so even if Cr is contained, the Cr content is set to 1.00% or less, and preferably 0.80% or less.

<V:0.01~0.40%>
V improves the strength of the hot-rolled steel sheet by strengthening through precipitation, strengthening through grain refinement by inhibiting the growth of ferrite crystal grains, and strengthening through dislocation strengthening by inhibiting recrystallization. To reliably obtain these effects, the V content is preferably 0.01% or more.
On the other hand, if the V content is excessive, a large amount of carbonitrides precipitates, which reduces the formability of the hot-rolled steel sheet, so the V content is set to 0.40% or less, and preferably 0.20% or less.

<Ni: 0.01-0.40%>
Ni suppresses phase transformation at high temperatures and improves the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, the Ni content is preferably 0.01% or more.
On the other hand, if the Ni content is excessive, the weldability of the hot-rolled steel sheet is reduced, so the Ni content is set to 0.40% or less, and preferably 0.20% or less.

<Cu: 0.01-0.40%>
Cu exists in the form of fine particles in steel and improves the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, the Cu content is preferably 0.01% or more.
On the other hand, if the Cu content is excessive, the weldability of the hot-rolled steel sheet is reduced, so the Cu content is set to 0.40% or less, and preferably 0.20% or less.

<B: 0.0001 to 0.0020%>
B suppresses phase transformation at high temperatures and improves the strength of the hot-rolled steel sheet. To reliably obtain this effect, the B content is preferably 0.0001% or more.
On the other hand, if the B content is excessive, B precipitates are formed and the strength of the hot-rolled steel sheet is reduced, so the B content is set to 0.0020% or less, and preferably 0.0005% or less.

<Sn: 0.01-0.20%>
Sn is an element that suppresses the coarsening of crystal grains and improves the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, the Sn content is preferably 0.01% or more.
On the other hand, if the Sn content is excessive, the steel becomes embrittled and easily breaks during rolling, so the Sn content is set to 0.20% or less, preferably 0.10% or less.

The chemical composition of the above-mentioned hot-rolled steel sheet may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Sol. Al may be measured by ICP-AES using the filtrate after thermally decomposing the sample with acid. C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.

Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
In the hot-rolled steel sheet according to this embodiment, the metal structure of the internal region contains, in terms of area ratio, one or both of martensite and bainite in total at 40 to 80%, and ferrite at 20 to 60%, with the area ratio of the remaining structure being less than 5%, and the ratio αs/αc of the ferrite area ratio αs of the surface region to the ferrite area ratio αc of the internal region is 1.15 to 2.50, and the hardness difference ratio (1-Hvs/Hvc) between the Vickers hardness Hvs of the surface region and the Vickers hardness Hvc of the internal region is 0.20 or less.

The internal region refers to the region from 1/8 of the plate thickness from the surface of the hot-rolled steel plate to 3/8 of the plate thickness from the surface, with the center at 1/4 of the plate thickness from the surface. The surface layer region refers to the region from the surface of the hot-rolled steel plate to a depth of 20 μm from the surface.

A structure mainly composed of martensite and bainite has a fine structure and excellent toughness. Although the mechanism is largely unknown, it is known that steel having a structure mainly composed of martensite and bainite has inferior fatigue strength compared to precipitation hardened steel and dual phase (DP) steel of ferrite and martensite. On the other hand, in precipitation hardened steel and DP steel, the high-speed dislocation movement in ferrite is hindered, so that fatigue strength and toughness are inferior. Conventionally, in automotive parts, the steel plate structure was created according to the required characteristics, but as further strength is increased, it is becoming difficult to obtain both high fatigue strength and toughness. Therefore, unlike conventional technology, the hot-rolled steel sheet according to this embodiment utilizes a composite structure of ferrite and martensite with excellent fatigue strength and precipitation hardening by increasing the amount of ferrite in the surface layer region, and utilizes a metal structure mainly composed of one or two types of martensite and bainite with excellent toughness in the inner region. This makes it possible to obtain a high strength of 980 MPa or more, as well as excellent fatigue strength, toughness and ductility.

Metal structure of the internal region The metal structure of the internal region of a hot-rolled steel sheet has a large effect on the toughness of the hot-rolled steel sheet. Therefore, the metal structure of the internal region is mainly a low-temperature transformation structure. The low-temperature transformation structure refers to martensite and bainite. If the total area ratio of these structures is less than 40%, the toughness of the hot-rolled steel sheet becomes inferior. Therefore, the total area ratio of martensite and bainite is set to 40% or more. It is preferably 45% or more, and more preferably 50% or more.

On the other hand, if the total area ratio of martensite and bainite exceeds 80%, the difference in hardness with the metal structure of the surface layer region becomes large, and the fatigue strength of the hot-rolled steel sheet becomes inferior. Therefore, the total area ratio of martensite and bainite is set to 80% or less. It is preferably 75% or less, and more preferably 70% or less.

In this embodiment, when the metal structure of the internal region contains only one of martensite or bainite, the content of only one of martensite or bainite needs to be within the above-mentioned range, and when the metal structure of the internal region contains both martensite and bainite, the total content of both martensite and bainite needs to be within the above-mentioned range.

If the area ratio of ferrite in the metal structure of the internal region is less than 20%, the difference in hardness with the metal structure of the surface region will be large, resulting in inferior fatigue strength of the hot-rolled steel sheet. Therefore, the area ratio of ferrite is set to 20% or more. It is preferably 25% or more, and more preferably 30% or more.

On the other hand, if the area ratio of ferrite exceeds 60%, the precipitation-strengthened ferrite grains may not relieve strain and may not ensure workability, resulting in a deterioration in the toughness of the hot-rolled steel sheet. Therefore, the area ratio of ferrite is set to 60% or less. It is preferably 55% or less, and more preferably 50% or less.

The metal structure of the internal region has a residual structure of less than 5% by area. The residual structure is one or more of pearlite and retained austenite. The residual structure is preferably less than 3%, more preferably 2.5% or less, and even more preferably 2% or less.

Metal structure of surface region In the metal structure of the surface region of the hot-rolled steel sheet, if the ratio αs/αc of the ferrite area ratio αs of the surface region to the ferrite area ratio αc of the inner region is less than 1.15, the suppression of dislocation motion in ferrite becomes insufficient, and the fatigue strength of the hot-rolled steel sheet becomes inferior. Therefore, αs/αc is set to 1.15 or more. It is preferably 1.20 or more or 1.30 or more, and more preferably 1.50 or more.

On the other hand, if αs/αc exceeds 2.50, carbon is concentrated in the thickness direction during ferrite transformation, and the difference in hardness with the metal structure of the inner region becomes large, resulting in inferior toughness and/or fatigue strength of the hot-rolled steel sheet. Therefore, αs/αc is set to 2.50 or less. It is preferably 2.20 or less, and more preferably 2.00 or less.

In the metal structure of the surface region of a hot-rolled steel sheet, the ratio βs/βc of the total area ratio βs of martensite and bainite in the surface region to the total area ratio βc of martensite and bainite in the internal region is preferably 0.30 to 0.90. When βs/βc is 0.90 or less, dislocation motion in martensite and bainite is sufficiently suppressed, and the fatigue strength of the hot-rolled steel sheet is increased. βs/βc is more preferably 0.85 or less, and even more preferably 0.80 or less.

On the other hand, by making βs/βc 0.30 or more, the concentration of carbon in the thickness direction during the transformation of martensite and bainite is suppressed, which leads to an increase in the difference in hardness with the metal structure of the inner region, and the toughness and fatigue strength of the hot-rolled steel sheet are improved. βs/βc is more preferably 0.40 or more, even more preferably 0.45 or more, and even more preferably 0.50 or more.

The metal structure of the surface region may contain 30 to 80% ferrite by area ratio. The metal structure of the surface region may also contain one or more of bainite, martensite, pearlite, and retained austenite in a total of 20 to 70% by area ratio as the remaining structure other than ferrite.

A method for measuring metal structure: A sample is cut out from a hot-rolled steel sheet so that a plate thickness cross section perpendicular to the surface can be observed. The plate thickness cross section of this sample is polished using silicon carbide paper of #600 to #1500, and then finished to a mirror surface using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water, and then subjected to nital etching. Next, at any position in the longitudinal direction of the sample cross section, photographs of multiple fields of view are taken using a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL). An evenly spaced grid is drawn on the photograph, and the structure at the grid point is identified. The number of grid points corresponding to each structure is obtained, and divided by the total number of grid points to obtain the area ratio of each structure. The greater the total number of grid points, the more accurately the area ratio can be obtained. In this embodiment, the grid interval is 2 μm x 2 μm, and the total number of grid points is 1500 points.

Areas where cementite precipitates in a lamellar form within the grains are judged to be pearlite. Areas where the brightness is low and no substructure is visible are judged to be ferrite. Areas where the brightness is high and the substructure is not revealed by etching are judged to be martensite and retained austenite. Areas that do not fall into any of the above categories are judged to be bainite. The area fraction of martensite is obtained by subtracting the area fraction of retained austenite determined by the EBSD analysis described below from the area fractions of martensite and retained austenite determined from the photographs.

A sample is cut out from the same position as the above measurement so that a plate thickness cross section perpendicular to the surface can be observed. After polishing the plate thickness cross section of this sample using silicon carbide paper of #600 to #1500, a liquid in which diamond powder of grain size 1 to 6 μm is dispersed in a dilution liquid such as alcohol or pure water is used to finish it into a mirror surface. Next, the sample is polished for 8 minutes using colloidal silica that does not contain an alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample. At any position in the longitudinal direction of the sample cross section, measurement is performed by electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information. For the measurement, an EBSD device consisting of a thermal field emission scanning electron microscope (JSM-7001F made by JEOL) and an EBSD detector (DVC5 type detector made by TSL) is used. At this time, the degree of vacuum in the EBSD device is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. The obtained crystal orientation information is used to calculate the area ratio of the retained austenite using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. Note that a crystal structure of fcc is determined to be retained austenite.

By performing each of the above measurements in the region from 1/8 of the plate thickness depth from the surface to 3/8 of the plate thickness depth from the surface, and in the region from the surface of the hot-rolled steel plate to a depth of 20 μm from the surface, the area ratio of the metal structure in each of the internal region and surface region is obtained.

Hardness difference ratio between the Vickers hardness of the surface layer region and the Vickers hardness of the inner region: 0.20 or less If the hardness difference ratio between the Vickers hardness Hvs of the surface layer region and the Vickers hardness Hvc of the inner region (1-Hvs/Hvc) exceeds 0.20, the surface layer region becomes soft and the fatigue strength of the hot-rolled steel sheet becomes inferior. Therefore, the hardness difference ratio between Hvs and Hvc (1-Hvs/Hvc) is set to 0.20 or less. It is preferably 0.15 or less, and more preferably 0.10 or less.
The smaller the ratio of the hardness difference between Hvs and Hvc, (1-Hvs/Hvc), the more preferable it is. From the viewpoint of production, however, it may be set to -0.10 or more, 0.00 or more, or 0.01 or more.

Vickers hardness measurement method A test piece is cut out from a hot-rolled steel plate so that a plate thickness cross section perpendicular to the surface can be observed. After polishing the plate thickness cross section of the test piece using silicon carbide paper of #600 to #1500, a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a dilution liquid such as alcohol or pure water is used to finish it to a mirror surface. This plate thickness cross section is the measurement surface. Using a micro Vickers hardness tester, the Vickers hardness is measured at intervals of 3 times or more of the indentation in the region of the measurement surface from 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface with a load of 1 kgf. A total of 20 points are measured, and the average value is calculated to obtain the Vickers hardness Hvc of the metal structure of the internal region. Similarly, the Vickers hardness is measured in the region of the measurement surface from the surface to a depth of 20 μm from the surface, and the average value of the 20 points is calculated to obtain the Vickers hardness Hvs of the metal structure of the surface layer region. The obtained Hvs and Hvc are used to calculate (1-Hvs/Hvc) to obtain the hardness difference ratio of Vickers hardness.

The hot-rolled steel sheet according to this embodiment has a tensile (maximum) strength of 980 MPa or more. Preferably, it is 1000 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts are limited, and the contribution to the weight reduction of the vehicle body is small. There is no need to particularly limit the upper limit, but from the viewpoint of suppressing die wear, it may be 1500 MPa or less or 1300 MPa or less.
In addition, the hot-rolled steel sheet according to this embodiment may have a total elongation of 10% or more, an absorbed energy at -20°C of 80 J/ ^cm2 or more, and a fatigue limit ratio (fatigue strength/tensile strength) of 0.48 or more.

The tensile strength and total elongation are evaluated by conducting a tensile test in accordance with JIS Z 2241: 2011. The test piece is a No. 5 test piece of JIS Z 2241: 2011. The tensile test piece is taken from a quarter portion from the end in the sheet width direction, and the direction perpendicular to the rolling direction is the longitudinal direction.
For toughness, first, a V-notch test piece of 2.5 mm subsize as specified in JIS Z 2242:2018 is taken from a position close to the position where the test piece used in the tensile test was taken. Using this test piece, a Charpy impact test with a C-direction notch is performed at -20°C to measure the absorbed energy. For hot-rolled steel sheets with a thickness of less than 2.5 mm, the test is performed over the entire thickness.
The fatigue strength is measured using a Schenk type plane bending fatigue tester in accordance with JIS Z 2275:1978. The stress load during the measurement is set to a test speed of 30 Hz in both directions, and the fatigue strength is measured at 107 cycles. The fatigue strength at 107 cycles is then divided by the tensile strength measured by the above-mentioned tensile test to calculate the fatigue limit ratio (fatigue strength/tensile strength).

The thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the hot-rolled steel sheet is less than 1.2 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 1.2 mm or more. It is preferably 1.4 mm or more. On the other hand, if the thickness exceeds 8.0 mm, it may be difficult to obtain the above-mentioned metal structure after hot rolling. Therefore, the thickness may be 8.0 mm or less. It is preferably 6.0 mm or less.

The hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., to form a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of electroplating layers include electrogalvanizing and electrogalvanizing Zn-Ni alloy plating. Examples of hot-dip plating layers include hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. The coating weight is not particularly limited and may be the same as in the past. In addition, it is also possible to further improve corrosion resistance by performing an appropriate chemical conversion treatment (for example, application of a silicate-based chromium-free chemical conversion treatment liquid and drying) after plating.

The hot-rolled steel sheet according to this embodiment has the above-mentioned chemical composition and metal structure, and thus has the above-mentioned effects, regardless of the manufacturing method. However, the manufacturing method described below is preferable because it allows the hot-rolled steel sheet according to this embodiment to be obtained stably.

In a preferred manufacturing method for the hot-rolled steel sheet according to this embodiment, bending is performed during finish rolling of the hot rolling to impart strain to the surface region and promote ferrite transformation in the surface region. Precipitation-strengthened ferrite is crystallized in the surface region, and then quenched, so that martensite and bainite are generated in addition to ferrite in the internal region. This makes it possible to reduce the difference in hardness between the precipitation-strengthened surface region and the internal region where low-temperature transformation structure has been generated without precipitation strengthening.

Hot rolling The heating temperature of the slab has a significant effect on the solution treatment and elimination of elemental segregation. By setting the heating temperature of the slab to 1100°C or higher, it is possible to prevent the solution treatment and elimination of elemental segregation from becoming insufficient, and as a result, it is possible to suppress the deterioration of the tensile properties and toughness of the product. In addition, by setting the heating temperature of the slab to 1350°C or lower, the effects of the solution treatment and elimination of elemental segregation can be saturated. Therefore, the heating temperature of the slab is preferably 1100 to 1350°C. More preferably, it is 1150 to 1300°C.
In this embodiment, the temperature of the slab and the temperature of the steel plate refer to the surface temperature of the slab and the surface temperature of the steel plate.

In the finish rolling, the slab is passed through a rolling stand for finish rolling several times in succession. In the finish rolling, it is preferable that the temperature of the hot rolled steel sheet after the final pass (finishing temperature) is Ar ₃ point or higher, and the rolling reduction of the final pass is 12 to 45%.
The temperature of the hot-rolled steel sheet after the final pass is the lowest temperature in the finish rolling in which the steel sheet is rolled by multiple stands. The reduction ratio after the final pass can be expressed as {(t ₀ -t ₁ )/t ₀ }×100(%), where t ₀ is the inlet thickness before the final pass and t ₁ is the outlet thickness after the final pass. The Ar ₃ point is expressed by the following formula (1).

Ar ₃ points = 901-325×C+33×Si-92×Mn+287×P+40×sol. Al...Formula (1)
Each element symbol in the above formula (1) indicates the content (mass%) of each element. When the element is not contained, 0 is substituted.

By setting the temperature of the hot-rolled steel sheet after the final pass of finish rolling (finishing temperature) to the Ar ₃ point or higher, the generation of ferrite during finish rolling can be suppressed, and as a result, the desired metal structure and characteristics can be obtained.

By setting the reduction rate of the final pass of finish rolling to 12% or more, recrystallization can be promoted during finish rolling, the metal structure of the internal and surface regions can be favorably controlled, and excellent fatigue strength can be obtained. Furthermore, by setting the reduction rate of the final pass to 45% or less, an increase in the load on the rolling stand and deterioration of the shape of the hot-rolled steel sheet after finish rolling can be suppressed. Therefore, it is preferable that the reduction rate of the final pass in finish rolling is 12 to 45%. More preferably, it is 15 to 45%.

It is preferable to apply a strain of 0.002 to 0.020 to the surface layer region (the region from the surface to a depth of 20 μm from the surface) of the hot-rolled steel sheet by performing bending between the final pass of the finish rolling and the pass immediately before it. By setting the strain during bending to 0.002 or more, it is possible to create a desired metal structure in the surface layer region. Therefore, it is preferable that the strain during bending is 0.002 or more. It is more preferable that the strain is 0.003 or more or 0.004 or more.
In addition, by setting the strain during bending to 0.020 or less, buckling during finish rolling is likely to occur, and loss of manufacturing stability can be suppressed. In addition, by setting the strain during bending to 0.020 or less, the metal structure of the surface layer region and the inner region can be favorably controlled. Therefore, it is preferable that the strain during bending is 0.020 or less. More preferably, it is 0.015 or less or 0.010 or less.

The bending process is carried out between stands by using rolls to push up the steel sheet from below, and the strain during bending can be controlled by adjusting the amount of pushing up and the bending angle by the diameter of the rolls.
For example, when bending is performed by pushing up a steel sheet from below with rolls between stands, the amount of strain during bending can be calculated by the following formula (2).

Strain amount = 1.5 × (plate thickness) × (push-up amount) / (diameter of the push-up device tip) ² ... Equation (2)

It is preferable that the time that elapses from the completion of finish rolling to the start of cooling is 1.6 seconds or less. By setting the time that elapses from the completion of finish rolling to the start of cooling to 1.6 seconds or less, the recovery of distortion caused by bending and rolling can be suppressed, and the metal structure of the surface region can be favorably controlled.

After the finish rolling is completed, it is preferable to perform primary cooling at an average cooling rate of 40°C/sec or more to a temperature range of 600 to 750°C, and then air-cooling for 2 to 6 seconds. Generally, the cooling rate during air-cooling is 2 to 10°C/sec.
By setting the cooling stop temperature at an average cooling rate of 40° C./sec or more in the temperature range of 600 to 750° C. and then performing air cooling, the ferrite transformation can be promoted and the desired amount of ferrite can be obtained.

After air cooling, it is preferable to perform secondary cooling at an average cooling rate of 60°C/s or more to a temperature range of 200°C or less, and then wind it into a coil. By setting the average cooling rate to a temperature range of 200°C or less at 60°C/s or more, martensitic transformation can be promoted and the desired amount of martensite and bainite can be obtained.

Here, the average cooling rate is defined as the temperature drop of the steel plate from the start to the end of cooling divided by the time required from the start to the end of cooling.

In addition, cooling equipment includes equipment that does not have an air-cooling section along the way and equipment that has one or more air-cooling sections along the way. Either type of cooling equipment may be used in this embodiment. Even when cooling equipment that has an air-cooling section is used, it is sufficient that the average cooling rate from the start to the end of cooling is within the above-mentioned range.

Since the hot-rolled steel sheet is coiled immediately after the secondary cooling, the coiling temperature is approximately equal to the cooling stop temperature of the secondary cooling. By setting the coiling temperature at 200°C or less, it is possible to suppress the formation of large amounts of polygonal ferrite or bainite, and to obtain the desired metal structure and characteristics.

After coiling, the hot-rolled steel sheet may be subjected to temper rolling in the usual manner, or may be pickled to remove scale formed on the surface. Alternatively, the sheet may be plated with the above-mentioned hot-dip galvanizing or electrolytic galvanizing, or may be subjected to a chemical conversion treatment.

According to the above manufacturing method, it is possible to stably manufacture hot-rolled steel sheets having the above-mentioned metal structure. Therefore, it is possible to stably manufacture hot-rolled steel sheets having high strength, as well as excellent fatigue strength and toughness.

Next, the effect of one aspect of the present invention will be explained in more detail using an example. However, the conditions in the example are an example of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this example of conditions. 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 object of the present invention.

Steels having the chemical compositions shown in Table 1 were melted and continuously cast into slabs having thicknesses of 240 to 300 mm. The resulting slabs were used to obtain hot-rolled steel sheets shown in Tables 4 and 5 under the manufacturing conditions shown in Tables 2 and 3.
The bending process was performed by pushing up the steel sheet from below between the stands with rolls. The amount of strain during bending was controlled by adjusting the bending angle by the amount of pushing up and the diameter of the rolls. The amount of strain during bending was calculated using the above formula (2).

The area fraction of the metal structure in the internal region and surface region, Vickers hardness, tensile strength, total elongation, absorbed energy at -20°C, and fatigue limit ratio of the obtained hot-rolled steel sheets were determined using the methods described above. The measurement results are shown in Tables 4 and 5.

Method for evaluating the properties of hot-rolled steel sheets When the tensile strength TS was 980 MPa or more, the hot-rolled steel sheets were judged to be excellent in strength and to pass the test. On the other hand, when the tensile strength TS was less than 980 MPa, the hot-rolled steel sheets were judged to be not excellent in strength and to fail the test.

When the total elongation was 10% or more, the hot-rolled steel sheet was judged to be excellent in ductility and to pass the test. On the other hand, when the total elongation was less than 10%, the hot-rolled steel sheet was judged to be not excellent in ductility and to fail the test.
When the absorbed energy at -20°C was 80 J/ ^cm2 or more, the hot-rolled steel sheet was judged to have excellent toughness and passed the test. On the other hand, when the absorbed energy at -20°C was less than 80 J/ ^cm2 , the hot-rolled steel sheet was judged to have poor toughness and failed the test.
When the fatigue limit ratio was 0.48 or more, the hot-rolled steel sheet was judged to have excellent fatigue strength and to pass the test. On the other hand, when the fatigue limit ratio was less than 0.48, the hot-rolled steel sheet was judged to have poor fatigue strength and to fail the test.

From Tables 4 and 5, it can be seen that the hot-rolled steel sheets according to the examples of the present invention have high strength as well as excellent toughness, fatigue strength and ductility.
On the other hand, it is seen that the heat-rolled steel sheets according to the comparative examples are inferior in at least one of strength, toughness, and fatigue strength.

According to the above-mentioned aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength as well as excellent fatigue strength, toughness and ductility. This hot-rolled steel sheet makes it possible to reduce the weight of the body of an automobile or the like, to integrally mold parts, and to shorten the processing steps, thereby improving fuel efficiency and reducing manufacturing costs, and therefore has high industrial value.

Claims (2)

The chemical composition, in mass%, is
C: 0.02-0.30%,
Si: 0.10-2.00%,
Mn: 0.5-3.0%,
sol. Al: 0.10 to 1.00%,
Ti: 0.06-0.20%,
P: 0.1000% or less,
S: 0.0100% or less,
N: 0.0100% or less,
Nb: 0 to 0.100%,
Ca: 0-0.0060%,
Mo: 0 to 0.50%,
Cr: 0-1.00%,
V: 0 to 0.40%,
Ni: 0 to 0.40%,
Cu: 0 to 0.40%,
B: 0 to 0.0020% and Sn: 0 to 0.20%;
The balance is Fe and impurities,
The metal structure of the internal region contains, in terms of area ratio, one or both of martensite and bainite in total of 40 to 80%, ferrite in total of 20 to 60%, and the area ratio of the remaining structure is less than 5%,
a ratio of a ferrite area ratio αs of the surface region to a ferrite area ratio αc of the inner region, αs/αc, is 1.15 to 2.50;
a ratio βs/βc of a total area ratio βs of martensite and bainite in the surface layer region to a total area ratio βc of the martensite and bainite in the internal region is 0.30 to 0.90;
a hardness difference ratio (1-Hvs/Hvc) between a Vickers hardness Hvs of the surface region and a Vickers hardness Hvc of the inner region is 0.20 or less;
A hot-rolled steel sheet having a tensile strength of 980 MPa or more. The chemical composition, in mass%,
Nb: 0.010-0.100%,
Ca: 0.0005-0.0060%,
Mo: 0.02-0.50%,
Cr: 0.02-1.00%,
V: 0.01-0.40%,
Ni: 0.01 to 0.40%,
Cu: 0.01-0.40%,
The hot-rolled steel sheet according to claim 1, further comprising one or more selected from the group consisting of B: 0.0001 to 0.0020% and Sn: 0.01 to 0.20%.