WO2024150706A1 - Hot-rolled steel sheet - Google Patents

Abstract

The present invention provides a hot-rolled steel sheet having excellent productivity and excellent cold rolling properties.　This hot-rolled steel sheet has a prescribed chemical composition in which the carbon content is 0.20-0.70 mass%. The metal structure at a depth position of 1/4 of the sheet thickness comprises at least 20% by area of ferrite, at least 40% by area of pearlite, and 0-10% by area of the remainder composition. Ferrite crystal grains measured by electron backscatter diffraction satisfy a prescribed relationship.

Description

Hot-rolled steel sheets

The present invention relates to a hot-rolled steel sheet.
The present invention claims priority based on Japanese Patent Application No. 2023-003920, filed in Japan on January 13, 2023, the contents of which are incorporated herein by reference.

The so-called high carbon steel used in gears in various machines, automobile transmission parts, seat recliners, etc., is high carbon cold rolled steel sheet, which is manufactured through cold rolling. This steel sheet is cold worked into the desired shape, and then quenched as necessary to ensure the desired hardness, to become the required part.

The strength (hardness) and abrasion resistance ultimately required for high carbon cold rolled steel sheets as components are adjusted by the chemical composition of the steel sheet and the heat treatment conditions, basically the large amount of carbon contained and the final quenching treatment. However, the problem of reduced cold rolling properties of hot rolled steel sheets due to the large amount of carbon contained is a problem, and for steel sheets immediately before cold rolling in the manufacturing process of high carbon cold rolled steel sheets (hot rolled steel sheets), what is required is elongation, i.e., excellent cold rolling properties, rather than strength (hardness).

As a steel sheet having excellent cold rolling properties, Patent Document 1 discloses a high carbon hot rolled steel sheet having a composition containing, by mass%, C: 0.10 to 0.33%, Si: 0.15 to 0.35%, Mn: 0.5 to 0.9%, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0065% or less, Cr: 0.90 to 1.5%, with the balance being Fe and unavoidable impurities, a microstructure having ferrite and cementite, and further, a cementite density of 0.25 pieces/μm ² or less, a hardness of HV 110 to 160, and a total elongation of 40% or more.

International Publication No. 2018/155254

However, the high-carbon hot-rolled steel sheet in Patent Document 1 has been subjected to spheroidizing annealing, and although it has excellent cold rolling properties, the increase in the number of processes due to spheroidizing annealing poses a problem. Therefore, there is a demand for a hot-rolled steel sheet with higher productivity than the high-carbon hot-rolled steel sheet in Patent Document 1.

The present invention was made in consideration of the above circumstances, and aims to provide a hot-rolled steel sheet with excellent productivity and cold rolling properties.

The inventors conducted detailed studies on the cold rolling properties of hot-rolled steel sheets, looking at the chemical composition, metal structure, and manufacturing conditions. The inventors discovered that by increasing the reduction rate in the early stages of hot rolling and rolling under light pressure in the final stage, the ferrite fraction of the resulting hot-rolled steel sheet increases and the grain size distribution changes, improving the cold rolling properties.

The present invention has been made in view of the above findings.
<1> The hot-rolled steel sheet of aspect 1 of the present invention has, in mass%, C: 0.20 to 0.70%, Si: 0.010 to 0.300%, Mn: 0.3 to 2.0%, Al: 0.001 to 0.100%, N: 0.0010 to 0.0100%, P: 0.008% to 0.030%, S: 0.010% or less, O: 0.0025% or less, Cr: 1.500% or less, B: 0.010% or less, Nb: 0.50% or less, Mo: 0.50% or less, V: 0.50% or less, Ti: 0.3000% or less, Cu: 0.500% or less, W: 0.500% or less, Ta: 0.500% or less, Ni: 0.500% or less, Mg: 0.003% The steel sheet has a chemical composition containing 0.003% or less Ca, 0.030% or less Y, 0.030% or less Zr, 0.030% or less La, 0.030% or less Ce, 0.030% or less Sn, 0.030% or less Sb, and 0.030% or less As, with the balance being Fe and impurities, and at a depth position of 1/4 of the sheet thickness, the metal structure is composed of 20 area % or more ferrite, 40 area % or more pearlite, and a balance structure of 0 area % to 10 area %, and the balance structure contains at least one of bainite and martensite, and the ferrite structure measured by electron backscatter diffraction method is Among the crystal grains, ferrite crystal grains excluding 5% of the total number of the ferrite crystal grains from the maximum crystal grain size side and 5% of the total number of the ferrite crystal grains from the minimum crystal grain size side of the ferrite crystal grains are treated as evaluated ferrite crystal grains, the minimum value of the crystal grain size of the evaluated ferrite crystal grains is set as a first grain size, the maximum value of the crystal grain size of the evaluated ferrite crystal grains is set as a second grain size, a crystal grain size obtained by adding 1/3 of the difference between the second grain size and the first grain size to the first grain size is set as a third grain size, and a combination of the second grain size and the first grain size is set as a third grain size. When the crystal grain size plus 2/3 of the difference between the first grain size and the third grain size is set as the fourth grain size, a range equal to or greater than the first grain size and equal to or less than the third grain size is set as the first grain size range, a range greater than the third grain size and equal to or less than the fourth grain size is set as the second grain size range, and a range greater than the fourth grain size and equal to or less than the second grain size is set as the third grain size range, the number of the evaluated ferrite crystal grains in the first grain size range is 2.5 to 3.0 times the number of the evaluated ferrite crystal grains in the second grain size range, and the number of the evaluated ferrite crystal grains in the third grain size range is 2.0 to 2.5 times the number of the evaluated ferrite crystal grains in the second grain size range.
<2> According to a second aspect of the present invention, in the hot-rolled steel sheet of the first aspect, the average grain size of the evaluated ferrite grains in the first grain size range may be 3 μm to 20 μm.
<3> According to a third aspect of the present invention, in the hot-rolled steel sheet of the first or second aspect, the average grain size of the evaluated ferrite grains in the third grain size range may be 80 μm to 120 μm.
<4> A fourth aspect of the present invention may be such that, in the hot-rolled steel sheet according to any one of the first to third aspects, the Vickers hardness Hv at a depth position of ¼ of the sheet thickness may be 160 or less.
<5> A fifth aspect of the present invention is the hot-rolled steel sheet according to any one of the first to fourth aspects, in which the total elongation may be 40% or more.

The above aspect of the present invention makes it possible to provide a hot-rolled steel sheet with excellent productivity and cold rolling properties.

The chemical composition and metal structure of the hot-rolled steel sheet according to one embodiment of the present invention (hereinafter, sometimes simply referred to as the steel sheet according to this embodiment), as well as the rolling conditions in the manufacturing method for producing the steel sheet, are described in detail below.

<Chemical composition>
First, the chemical composition of the steel sheet according to this embodiment will be described. Unless otherwise specified, "%" indicating the content of each element in the chemical composition means mass%. In this specification, a numerical range expressed using "to" means a range including the numerical values written before and after "to" as the lower and upper limits.

[C: 0.20-0.70%]
C (carbon) is an essential element for increasing the strength of steel sheets. If the C content is less than 0.20%, the effect of improving the cold rolling property by controlling the structure cannot be sufficiently obtained. The C content is set to 0.20% or more, and preferably 0.25% or more.
On the other hand, if the C content exceeds 0.70%, the weldability and cold rolling property are deteriorated. Therefore, the C content is set to 0.70% or less, and preferably 0.60% or less. .
If the C content is within the above range, it is possible to ensure the usual level of tensile properties required for a high carbon cold rolled steel sheet after cold rolling and heat treatment.

[Si:0.010-0.300%]
Silicon (Si) is a solid solution strengthening element and is generally contained in high carbon cold rolled steel sheets to increase the strength of the steel sheets. The Si content is set to 0.010% or more, and preferably to 0.100% or more.
On the other hand, excessive Si content leads to embrittlement of the steel sheet, and it becomes difficult to ensure sufficient cold rolling properties even if microstructural control is applied. Therefore, the Si content is set to 0.300% or less. The Si content is preferably 0.150% or less.
If the Si content is within the above range, it is possible to ensure the usual level of tensile properties required for a high carbon cold rolled steel sheet after cold rolling and heat treatment.

[Mn: 0.3 to 2.0%]
Mn has the effect of improving the hardenability of steel and is an element that is generally contained in high carbon cold rolled steel sheets. If the Mn content is less than 0.3%, the effect of improving cold rolling properties by structure control is not achieved. Therefore, the Mn content is set to 0.3% or more, and preferably 1.0% or more.
On the other hand, if the Mn content exceeds 2.0%, the formation of ferrite in the hot-rolled steel sheet is suppressed, and the desired cold rolling properties cannot be obtained. Therefore, the Mn content is set to 2.0% or less. The content is preferably 1.5% or less.
If the Mn content is within the above range, it is possible to ensure the usual level of tensile properties required for a high carbon cold rolled steel sheet after cold rolling and heat treatment.

[Al: 0.001-0.100%]
Al is an element that has a deoxidizing effect on steel. Therefore, Al may be contained in the steel. In order to obtain the above effect, the Al content is preferably 0.001% or more. Al Content is preferably 0.005% or more.
On the other hand, if Al is contained in excess, not only does the above effect saturate, resulting in an increase in cost, but also the transformation temperature of the steel rises, increasing the load during hot rolling. The Al content is 100% or less. The Al content is preferably 0.090% or less. The Al content means the so-called total Al (T-Al) content.

[N:0.0010-0.0100%]
N is an element that forms coarse nitrides in the steel sheet and deteriorates the cold rolling properties of the steel sheet. If the N content exceeds 0.0100%, the above deterioration becomes significant. The N content is 0.0100% or less. The N content may be 0.0090% or less, 0.0080% or less, or 0.0070% or less.
On the other hand, if the N content is less than 0.0010%, the manufacturing cost increases significantly. The N content is set to 0.0010% or more. The N content may be set to 0.0020% or more.

[P: 0.008% to 0.030%]
P is an element contained in steel as an impurity, and segregates at grain boundaries to embrittle the steel and deteriorate the cold rolling properties. For this reason, the P content is set to 0.030% or less. The P content is preferably 0.020% or less, and more preferably 0.010% or less.
The lower the P content, the better, but taking into consideration the time and cost required for removing P, the P content is set to 0.008% or more.

[S: 0.010% or less]
S is an element contained in steel as an impurity and forms sulfide-based inclusions that deteriorate cold rolling properties, so the S content is set to 0.010% or less. The S content is preferably 0.009% or less, and more preferably 0.007% or less. The lower the S content, the better, and 0% is acceptable. However, taking into consideration the time and cost required for removing S, a content of 0.009% or less is preferable. It may be 0.001% or more.

[O: 0.0025% or less]
O is an element that forms coarse oxides in steel and deteriorates cold rolling properties. If the O content exceeds 0.0025%, the cold rolling properties tend to deteriorate significantly. Therefore, the O content is set to 0.0025% or less. The O content may be 0.0020% or less, or 0.0015% or less.
The O content is preferably small. However, it is economically undesirable to set the O content to less than 0.0001% because of excessively high costs. For this reason, the O content is set to 0.0001% or more. The O content may be set to 0.0010% or more.

The steel plate according to this embodiment may contain the above elements, with the remainder being Fe and impurities. Here, impurities are elements that are mixed in due to various factors in the manufacturing process and raw materials such as ores and scraps when industrially manufacturing steel, and their presence is permitted to the extent that they do not impair the properties of the steel plate according to this embodiment. Impurities also include elements that are not intentionally added to the steel plate according to this embodiment.

The steel plate according to this embodiment may further contain one or more elements (optional elements) from the following: Cr, B, Nb, Mo, V, Ti, Cu, W, Ta, Ni, Mg, Ca, Y, Zr, La, Ce, Sn, Sb, and As. These elements do not necessarily have to be contained, so the lower limit is 0%.

[Cr: 1.500% or less]
Cr is an element that is effective in increasing the hardenability and strength of steel sheets, and is generally used in high carbon cold rolled steel sheets. Therefore, Cr may be contained in the steel. In order to obtain the effect of improving the rollability, the Cr content is preferably 0.001% or more.
On the other hand, if the Cr content exceeds 1.500%, Cr segregates in the center of the steel sheet to form coarse Cr carbides, which may deteriorate the cold rolling properties. 1.500% or less.

[B: 0.010% or less]
B suppresses the formation of ferrite and pearlite during the cooling process from austenite, promotes the formation of low-temperature transformation structures such as bainite or martensite, and is an element beneficial for increasing the strength of high-carbon cold-rolled steel sheets. In order to obtain the above-mentioned effects of B, the B content is preferably 0.001% or more. On the other hand, the formation of coarse B oxides and borides in the steel is prevented. This can lead to the formation of voids during cold rolling, which can deteriorate the cold rolling properties of the steel sheet. It inhibits the effect of improving rollability, so the B content is set to 0.010% or less.

[Nb: 0.50% or less]
Nb is an element effective for controlling the morphology of carbides, and is also an element effective for improving the toughness of steel plate because its addition refines the structure. Therefore, Nb may be contained in steel. In order to obtain the above effect due to Nb, the Nb content is preferably 0.01% or more. On the other hand, if Nb is added excessively, a large number of coarse Nb carbides are precipitated, which may cause voids during cold rolling. This may become the starting point and deteriorate the cold rolling property of the steel sheet. Therefore, the Nb content is set to 0.50% or less.

[Mo: 0.50% or less]
Mo is an element effective in strengthening high carbon cold rolled steel sheets. Therefore, Mo may be contained in the steel. In order to increase the strength of high carbon cold rolled steel sheets by Mo, the Mo content should be 0.05 or less. It is preferable that the Mo content is 0.01% or more. On the other hand, if added in excess, the cost increases and coarse Mo carbides are formed, which may deteriorate the cold rolling properties of the steel sheet. For this reason, the Mo content is set to 0.01% or more. . 50% or less.

[V: 0.50% or less]
V is an element effective in controlling the morphology of carbides, and is also effective in improving the toughness of steel plates because it refines the structure when V is added. Therefore, V may be contained in steel. In order to obtain the effect of V, the V content is preferably 0.01% or more. On the other hand, if V is added in excess, a large number of fine V carbides are precipitated, which increases the strength of the steel sheet and significantly reduces the ductility. Therefore, the V content is set to 0.50% or less.

[Ti: 0.3000% or less]
Ti is an important element for controlling the morphology of carbides, and a large amount of Ti promotes an increase in the strength of ferrite, so it is an element contained in high carbon cold rolled steel sheets. However, if the Ti content is 0.0001% or more, the effect of improving the strength of ferrite can be obtained. On the other hand, excessive addition of Ti causes coarse Ti oxides or Ti carbo-nitrides to exist in the steel. Therefore, the Ti content is set to 0.3000% or less.

[Cu: 0.500% or less]
Cu is an element that contributes to improving the strength of the steel sheet, and is contained in high carbon cold rolled steel sheet. Therefore, Cu may be contained in the steel. In order to obtain the above effect, the Cu content is It is preferable that the Cu content is 0.001% or more. However, if the Cu content is too high, red shortness may be caused, and the productivity in hot rolling may be reduced. Furthermore, if the Cu content is too high, There is a risk of causing a deterioration in cold rolling property due to the formation of coarse inclusions. Therefore, the Cu content is set to 0.500% or less, and preferably 0.300% or less.

[W: 0.500% or less]
W is a carbide-forming element and is an effective element for increasing the strength of steel sheets, and is contained in high carbon cold rolled steel sheets. Therefore, W may be contained in steel. In the present invention, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more. The W content is further preferably 0.010% or more. .
On the other hand, if W is contained in an excessive amount, the effect saturates and the cost increases. Therefore, when W is contained, the W content is set to 0.500% or less. The W content is set to 0.400% or less. It is preferable that:

[Ta: 0.500% or less]
Ta is an element effective for controlling the morphology of carbides and improving the strength of steel sheets, and is contained in high carbon cold rolled steel sheets. Therefore, Ta may be contained in steel. In order to obtain the above effect, it is preferable that the Ta content is 0.001% or more. On the other hand, if the Ta content is too high, a large number of fine Ta carbides are precipitated, which may lead to a decrease in the ductility of the steel sheet and a decrease in the cold rolling property of the steel sheet. For this reason, the Ta content is set to 0.500% or less. It is more preferable that the Ta content is 0.300% or less. It is even more preferable that the Ta content is 0.200% or less.

[Ni:0.500%]
Ni is an element effective in improving the strength of steel sheets, and is contained in high carbon cold rolled steel sheets. Therefore, Ni may be contained in steel. In order to obtain the above effect, the Ni content is The Ni content is preferably 0.001% or more. The Ni content is more preferably 0.010% or more. On the other hand, if the Ni content is too high, the ductility of the steel sheet decreases, and the cold rolling properties are deteriorated. Therefore, the Ni content is set to 0.500% or less, and preferably 0.400% or less.

[Mg: 0.003% or less]
Mg is an element that controls the morphology of sulfides and oxides and contributes to improving the bendability of the steel sheet, and is contained in high carbon cold rolled steel sheet, so Mg may be contained in the steel. In order to obtain the above effects, the Mg content is preferably 0.001% or more. However, if the Mg content is too high, there is a risk of causing a decrease in cold rolling property due to the formation of coarse inclusions. For this reason, the Mg content is set to 0.003% or less, and preferably to 0.002% or less.

[Ca: 0.003% or less]
Ca is an element that can control the morphology of sulfides with a small amount of Ca. Therefore, Ca may be contained in the steel. In order to obtain the above effect, the Ca content should be 0.001% or more. However, if the Ca content is too high, coarse Ca oxides may be generated, and the Ca oxides may become the starting points for cracks during cold working, resulting in deterioration of cold rolling properties. Therefore, the Ca content is set to 0.003% or less, and preferably 0.002% or less.

[Y: 0.030% or less]
Y is an element that effectively controls the morphology of sulfides even when the content is small. Therefore, Y may be contained in the steel. In order to obtain the above effect, the Y content However, if the Y content is too high, coarse Y oxides are generated, which may deteriorate the cold rolling property and the fracture resistance. The content is set to 0.030% or less. The Y content is preferably set to 0.020% or less.

[Zr: 0.030% or less]
Zr is an element that can control the morphology of sulfides with a small amount. Therefore, Zr may be contained in the steel. In order to obtain the above effect, the Zr content is preferably 0.001% or more. However, if the Zr content is too high, coarse Zr oxides are generated, which may deteriorate the cold rolling property. Therefore, the Zr content is set to 0.030% or less. is preferably 0.020% or less.

[La: 0.030% or less]
La is an element that effectively controls the morphology of sulfides even when the content is small. Therefore, La may be contained in the steel. In order to obtain the above effect, the La content The La content is preferably 0.001% or more. However, if the La content is too high, coarse La oxides are generated, which may deteriorate the cold rolling property and the fracture resistance. The La content is set to 0.030% or less. The La content is preferably set to 0.020% or less.

[Ce: 0.030% or less]
Ce is an element that effectively controls the morphology of sulfides even when the content is small. Therefore, Ce may be contained in the steel. In order to obtain the above effect, the Ce content The Ce content is preferably 0.001% or more. However, if the Ce content is too high, coarse Ce oxides are generated, which may deteriorate the cold rolling property and the fracture resistance. The Ce content is set to 0.030% or less. The Ce content is preferably set to 0.020% or less.

[Sn: 0.030% or less]
Sn is an element that may be contained in a steel sheet when scrap is used as a raw material for the steel sheet. In addition, Sn may cause a decrease in the cold rolling property of the steel sheet due to embrittlement of ferrite. For this reason, The lower the Sn content, the better. The Sn content is set to 0.030% or less. The Sn content is preferably set to 0.020% or less. However, it is not recommended to reduce the Sn content to less than 0.001%. However, this is not preferable because it leads to an excessive increase in refining costs. Therefore, the Sn content may be set to 0.001% or more.

[Sb: 0.030% or less]
Like Sn, Sb is an element that can be contained in steel sheets when scrap is used as the raw material for the steel sheets. Sb strongly segregates at grain boundaries, embrittling the grain boundaries, reducing ductility, and further increasing the cooling time. This may result in a decrease in hot rolling property. Therefore, the smaller the Sb content, the better. The Sb content is set to 0.030% or less. The Sb content is preferably set to 0.020% or less. Reducing the Sb content to less than 0.001% is not preferable because it leads to an excessive increase in refining costs. Therefore, the Sb content may be set to 0.001% or more.

[As: 0 to 0.030%]
Like Sn and Sb, As is an element that can be contained in steel sheets when scrap is used as the raw material for the steel sheets. As strongly segregates at grain boundaries and may cause a decrease in cold rolling properties. Therefore, the smaller the As content, the better. The As content is set to 0.030% or less. The As content is preferably set to 0.020% or less. The As content is set to less than 0.001%. However, it is not preferable to reduce the As content to 0.001% or more because this leads to an excessive increase in refining costs.

The chemical composition of the steel sheet according to this embodiment can be determined by the following method.
The chemical composition of the steel sheet described above may be measured by a general chemical composition. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

<Metal structure at a depth of 1/4 of the plate thickness>
Next, the metal structure at a depth position of 1/4 of the plate thickness of the steel plate according to this embodiment (a position 1/4 of the plate thickness from the surface along the plate thickness direction) will be described.
In the description of the metal structure of the steel sheet according to the present embodiment, the structure fraction is expressed as an area fraction. Therefore, unless otherwise specified, "%" in the description of the metal structure represents "area %".

The steel plate according to this embodiment has a metal structure at a depth position of 1/4 of the plate thickness (a position 1/4 of the plate thickness from the surface along the plate thickness direction) that is composed of 20 area % or more of ferrite, 40 area % or more of pearlite, and a remaining structure between 0 area % and 10 area %.

(Ferrite: 20% by area or more)
Ferrite is a relatively soft phase in a high carbon steel sheet such as that of the present invention, and when it is mixed with a hard phase such as martensite, it improves the ductility of the steel sheet and improves the cold rolling properties. Therefore, at a depth position of 1/4 of the sheet thickness, ferrite is 20 area % or more. Ferrite is preferably 35 area % or more. The upper limit of ferrite is not particularly limited, but in a high carbon steel sheet such as that of the present invention, it is, for example, 60 area % or less.

(Perlite: 40% or more by area)
Pearlite is a structure that contains a large amount of cementite, and consumes C (carbon) in the steel, which contributes to increasing strength, and softens the steel. Therefore, at a depth position of 1/4 of the plate thickness, pearlite is 40 area % or more. More preferably, pearlite is 50 area % or more.
The upper limit of pearlite is not particularly limited, and is, for example, 80 area % or less.

(Remainder structure)
The remaining structure is 0 area% or more and 10 area% or less. The remaining structure refers to the remaining structure obtained by removing pearlite and ferrite from the metal structure. The remaining structure includes at least one of bainite and martensite. The remaining structure is preferably a structure mainly composed of at least one of bainite and martensite. Here, "a structure mainly composed of at least one of bainite and martensite" means that the total of bainite and martensite is 60 area% or more with respect to the total area of the remaining structure. More preferably, the total of bainite and martensite is 80 area% or more with respect to the total area of the remaining structure. The upper limit of the total of bainite and martensite with respect to the total area of the remaining structure may be 100 area%. Since bainite is softer than martensite, it is preferable to control so that bainite is formed instead of martensite in the steel plate according to the present embodiment in which ductility is emphasized over strength. Here, martensite refers to fresh martensite and tempered martensite, but in the steel plate according to the present embodiment in which ductility is more important than strength, it is preferable to control so that tempered martensite is formed instead of fresh martensite. For example, bainite is 5 area% or more, preferably 7 area% or more. Martensite is, for example, 5 area% or less, preferably 3 area% or less.

Next, we will explain how to identify each metal structure at a depth of 1/4 of the plate thickness and calculate the area ratio.

Identification of each metal structure and calculation of its area and area ratio can be performed by EBSD (Electron Back Scattering Diffraction), X-ray measurement, corrosion using Nital reagent or Lepera solution, and by observing the steel plate cross section parallel to the rolling direction and perpendicular to the plate surface at a magnification of 1,000 to 50,000 times using a scanning electron microscope. When measuring the area ratio of any structure, measurements are taken at three locations and the average value is calculated. In this case, in the present invention, software such as "OIM DataCollectionTM (ver.7)" made by TSL Solutions Co., Ltd. is used as the crystal orientation data acquisition software.

The area and area ratio of ferrite can be measured by the following method. That is, the observation surface is finished by colloidal silica polishing or electrolytic polishing, and measurements are made at intervals (pitch) of 0.2 μm in a square region (square with a side length of 1/8 to 3/8 along the plate thickness direction) centered at 1/4 of the plate thickness from the surface of the steel plate and ranging from 1/8 to 3/8 of the plate thickness) using an EBSD attached to a scanning electron microscope. The sample preparation conditions are within the range of conditions recommended in the Japan Society for Materials Science standard "Crystal orientation misorientation measurement standard for material evaluation using the electron backscatter diffraction (EBSD) method". The value of the local misorientation average (Grain Average Misorientation: GAM) is calculated from the measurement data. Then, the area and area ratio are measured for areas where the local misorientation average value is less than 0.5° as ferrite. Here, the boundary with a crystal orientation misorientation of 15° or more is determined to be a grain boundary, and the area surrounded by this grain boundary is considered to be a crystal grain. The local misorientation average is calculated by calculating the misorientation between adjacent measurement points and averaging it for all measurement points within the grain.

The area and area ratio of bainite are measured by taking a sample from a cross section of the steel plate parallel to the rolling direction, polishing the observed surface, etching the surface with nital solution, and observing a square region of 1/8 to 3/8 of the plate thickness centered at 1/4 of the plate thickness with a field emission scanning electron microscope (FE-SEM), and calculating the area ratio using known image analysis software. Note that the area ratio can be calculated using, for example, "ImageJ" as the image analysis software. Here, "ImageJ" is an open source, public domain image processing software that is widely used by those skilled in the art.
In observation with an FE-SEM, for example, the structure in the above-mentioned square observation region is classified as follows. Bainite is a collection of lath-shaped crystal grains, and the lath structure is a region that does not contain iron-based carbides with a major axis of 20 nm or more, or, if the lath structure contains iron-based carbides with a major axis of 20 nm or more, the carbides belong to a single variant, that is, a group of iron-based carbides elongated in the same direction. Here, the group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by within 5°.

As with bainite, the area ratio of martensite is measured by taking a sample from a cross section of the steel plate parallel to the rolling direction, polishing the observation surface, etching it with nital solution, and observing a square region of 1/8 to 3/8 of the plate thickness centered at 1/4 of the plate thickness with a field emission scanning electron microscope (FE-SEM), and calculating the area ratio using known image analysis software. The area ratio can be calculated using, for example, "ImageJ" as the image analysis software. "ImageJ" is an open source, public domain image processing software that is widely used by those skilled in the art. Martensite has a high dislocation density and has substructures such as blocks and packets within the grains, so it can be distinguished from other metal structures by electron channeling contrast images taken with a scanning electron microscope.

The pearlite area ratio can be measured by taking a sample from a cross section of the steel plate parallel to the rolling direction, polishing the observation surface, corroding it with Nital reagent, and observing a square region in the range of 1/8 to 3/8 thickness, centered at 1/4 of the plate thickness from the surface of the steel plate, using a secondary electron image taken with a scanning electron microscope. Regions in the secondary electron image where the bright and dark contrasts are lamellar are judged to be pearlite, and the area ratio is calculated using the image analysis software "ImageJ" mentioned above. Judging pearlite based on the contrast of secondary electron images is generally performed by those skilled in the art, and can be easily determined by those skilled in the art. If the total area ratio of each structure obtained by the above evaluation method is different from 100%, the area ratio of each structure is calculated by multiplying the area ratio of each structure by 100/(total area ratio of each structure).

(Evaluated ferrite grains)
In the steel sheet of this embodiment, the distribution state of the ferrite crystal grains in the sheet thickness direction is set within a predetermined range in order to ensure cold rolling properties. Specifically, fine ferrite crystal grains are relatively increased, and coarse ferrite crystal grains are relatively increased. The reason why cold rolling properties can be ensured by setting the distribution state of the ferrite crystal grains in the sheet thickness direction within a predetermined range is not clear, but the inventor speculates as follows. The mixture of fine and coarse grains causes a distribution of strain applied to each crystal grain. When the distribution is present, it is presumed that strain can be released from grains with high strain to grains with low strain during rolling (strain dispersion), and greater cold rolling properties can be ensured than in the case of a normal distribution. In order to evaluate the distribution state of the grain size of the ferrite crystal grains more precisely, the ferrite grains excluding 5% of the smallest grain size side and 5% of the largest grain size side of all ferrite grains are used as the ferrite grains for evaluation, and the distribution state of the grain size is evaluated.
Specifically, in the ferrite grains measured at a depth position of 1/4 of the plate thickness by the electron backscatter diffraction (EBSD) method described above, ferrite grains excluding 5% of the total number of ferrite grains from the maximum grain size side and 5% of the total number of ferrite grains from the minimum grain size side of the ferrite grains are regarded as the evaluated ferrite grains. In this case, if the number of n grains from the maximum grain size side or the minimum grain size side is less than 5% of the total number and the number of n+1 grains is more than 5% of the total number, n+1 grains are excluded. Here, the minimum value of the grain size of the evaluated ferrite grains is set as the first grain size, the maximum value of the grain size of the evaluated ferrite grains is set as the second grain size, the grain size obtained by adding 1/3 of the difference between the second grain size and the first grain size to the first grain size is set as the third grain size, and the grain size obtained by adding 2/3 of the difference between the second grain size and the first grain size to the first grain size is regarded as the fourth grain size. In addition, the range of the first particle size or more and the third particle size or less is set as the first particle size range, the range of the third particle size or more and the fourth particle size or less is set as the second particle size range, and the range of the fourth particle size or more and the second particle size or less is set as the third particle size range.

The number of evaluated ferrite grains in the first grain size range of the steel plate according to this embodiment is 2.5 to 3.0 times the number of evaluated ferrite grains in the second grain size range. In addition, the number of evaluated ferrite grains in the third grain size range of the steel plate according to this embodiment is 2.0 to 2.5 times the number of evaluated ferrite grains in the second grain size range. In this way, by making the number of evaluated ferrite grains in the first grain size range 2.5 to 3.0 times the number of evaluated ferrite grains in the second grain size range and the number of evaluated ferrite grains in the third grain size range 2.0 to 2.5 times the number of evaluated ferrite grains in the second grain size range, the evaluated ferrite grains in each grain size range have a clear distribution, which makes the strain distribution remarkable and improves the cold rolling property of the steel plate. It is preferable that the number of evaluated ferrite grains in the first grain size range of the steel plate according to this embodiment is 2.6 to 3.0 times the number of evaluated ferrite grains in the second grain size range. In the steel plate of this embodiment, the number of evaluated ferrite grains in the third grain size range is preferably 2.2 to 2.5 times the number of evaluated ferrite grains in the second grain size range.

In the steel plate according to this embodiment, the number of evaluated ferrite grains in the first grain size range is preferably 1.1 to 1.4 times the number of evaluated ferrite grains in the third grain size range.
More preferably, the number of evaluated ferrite grains in the first grain size range of the steel plate according to this embodiment is 1.2 to 1.4 times the number of evaluated ferrite grains in the third grain size range.
The number of evaluated ferrite grains in the first grain size range of the steel plate according to this embodiment is 1.1 to 1.4 times the number of evaluated ferrite grains in the third grain size range, thereby further improving the cold rolling properties of the steel plate.

The average grain size of the evaluated ferrite grains in the first grain size range is preferably 3 μm to 20 μm. The more preferable average grain size of the evaluated ferrite grains in the first grain size range is 3 μm to 10 μm. By having the average grain size of the evaluated ferrite grains in the first grain size range be 3 μm to 20 μm, the fine grains can accumulate a lot of strain, and the strain can be efficiently distributed toward the larger grains, thereby further improving the cold rolling properties of the steel sheet.

It is preferable that the average grain size of the evaluated ferrite grains in the second grain size range is more than 20 μm and less than 80 μm. A more preferable average grain size of the evaluated ferrite grains in the second grain size range is 30 μm to 70 μm, and more preferably 40 μm to 60 μm. By having the average grain size of the evaluated ferrite grains in the second grain size range be 30 μm to 70 μm, the cold rolling properties of the steel sheet can be further improved.

The average grain size of the evaluated ferrite grains in the third grain size range is preferably 80 μm to 120 μm. The more preferable average grain size of the evaluated ferrite grains in the third grain size range is 100 μm to 120 μm. By having the average grain size of the evaluated ferrite grains in the third grain size range be 80 μm to 120 μm, the cold rolling properties of the steel sheet can be further improved.

Next, we will explain how to measure ferrite grains at a depth of 1/4 of the plate thickness.

Ferrite can be identified and the grain size calculated by observing the steel plate cross section parallel to the rolling direction and perpendicular to the plate surface at a magnification of 1,000 to 50,000 times using EBSD (Electron Back Scattering Diffraction) and a scanning electron microscope. In this case, the present invention uses software such as "OIM DataCollectionTM (ver. 7)" manufactured by TSL Solutions Co., Ltd. as software for acquiring crystal orientation data.

The observation surface is finished by colloidal silica polishing or electrolytic polishing, and measurements are taken at intervals (pitch) of 0.2 μm in a square region (square with sides of 1/8 to 3/8 of the thickness along the plate thickness direction) centered at 1/4 of the plate thickness from the surface of the steel plate, at 0.2 μm intervals (pitch). The sample preparation conditions are within the range recommended in the Japan Society for Materials Science standard "Crystal orientation misorientation measurement standard for material evaluation using electron backscatter diffraction (EBSD) method". The local misorientation average (Grain Average Misorientation: GAM) value is calculated from the measurement data. Areas where the local misorientation average value is less than 0.5° are considered to be ferrite crystal grains. Here, boundaries where the crystal orientation misorientation is 15° or more are considered to be grain boundaries, and the areas surrounded by these grain boundaries are considered to be crystal grains. The average local orientation difference is calculated by calculating the orientation difference between adjacent measurement points and averaging it for all measurement points within the crystal grain. The crystal grain size is measured for the obtained ferrite crystal grain. The crystal grain size is the circle equivalent diameter. Here, the circle equivalent diameter of a crystal grain means the diameter of a circle having an area equal to the area of the crystal grain.

<Vickers hardness at a depth of 1/4 of the plate thickness>
From the viewpoint of cold rolling properties, the Vickers hardness Hv at a depth position of 1/4 of the plate thickness of the steel plate according to this embodiment is preferably 160 or less. More preferably, the Vickers hardness Hv at a depth position of 1/4 of the plate thickness is 150 or less. By having the Vickers hardness Hv at a depth position of 1/4 of the plate thickness of 160 or less, the cold rolling properties can be further improved.

The Vickers hardness at a depth of 1/4 of the plate thickness of the steel plate can be measured by the following procedure. First, the plate thickness cross section parallel to the rolling direction and plate thickness direction of the steel plate is mechanically polished to a mirror finish. On this polished surface, the Vickers hardness (HV) is measured at 12 points on a straight line parallel to the rolling direction at a distance (depth) of 1/4 of the plate thickness from the surface of the steel plate toward the inside of the plate thickness, with an indentation load of 20 gf. The Vickers hardness at a depth of 1/4 of the plate thickness of the steel plate is determined by averaging the Vickers hardness of 10 points excluding the lowest and highest values from these 12 measured points. The distance between each measurement point is preferably at least four times the distance of the indentation. The distance at least four times the distance of the indentation referred to here is the distance obtained by multiplying the diagonal length of the indentation made by the diamond indenter by a value of at least four times when measuring the Vickers hardness.

[Total elongation is 40% or more]
From the viewpoint of cold rolling properties, the total elongation of the steel sheet according to the present embodiment is preferably 40% or more, and more preferably 50% or more.

Total elongation can be determined by taking a JIS No. 5 tensile test piece from the steel plate in the direction perpendicular to the rolling direction and plate thickness direction, and conducting a tensile test in accordance with JIS Z 2241:2011.

<Thickness>
The thickness of the steel plate according to the present embodiment is not limited and may be, for example, 1.5 mm to 5.0 mm.

<Production Method>
The steel sheet according to this embodiment can be manufactured by a manufacturing method including the following steps (I) to (IV).
(I) A heating step in which a slab having the above-mentioned composition is heated to 1100°C or more and less than 1350°C. (II) A finish rolling step in which the slab after the heating step is passed through a plurality of rolling stands in succession to be rolled, in which σ determined by the following formula (1) satisfies 40≦σ≦80 in each of the first four rolling stands and 5≦σ≦10 in the last stand. (III) A cooling step in which cooling is started at an average cooling rate of 30°C/s or more within 0.2 to 2.0 seconds after the end of the finish rolling step, and cooled to a temperature range of 550°C to 650°C. (IV) A winding step in which, after the cooling step, the coiling temperature is in the temperature range of 550°C to 650°C. Each step will be described below. Each temperature below refers to the surface temperature of the slab or steel plate.

[Heating process]
In the heating step, it is preferable to heat a slab having the same chemical composition as the steel plate according to the present embodiment described above to 1100°C or more and less than 1350°C. If the heating temperature is less than 1100°C, the homogenization of the material is likely to be insufficient. In addition, if the heating temperature is 1350°C or more, it becomes difficult for the number of evaluated ferrite grains in the first grain size range, the number of evaluated ferrite grains in the second grain size range, and the number of evaluated ferrite grains in the third grain size range to satisfy the predetermined relationship.

[Finish rolling process]
In the finish rolling step, the slab after the heating step is subjected to rough rolling as necessary, and then passed through multiple rolling stands in succession for rolling. In the method for producing a steel sheet according to this embodiment, it is preferable to roll the slab after the heating step so that σ (unit: kgf/ ^mm2 ) defined by the following formula (1) satisfies 40≦σ≦80 in each of the first four rolling stands, and satisfies 5≦σ≦10 in the final rolling stand.
σ=exp(0.753+3000/T)×ε ^0.21 ×ε' ^0.13 ...(1)
Here, T is the temperature (K) immediately before entering the stand, ε is the equivalent plastic strain, and ε' is the strain rate (/s).

By setting σ, determined by the following formula (1), to 40 or more in each of the first four rolling stands, it is possible to increase the amount of strain accumulated in the steel sheet during and immediately after processing, and to appropriately refine the crystal grains formed by recrystallization during and after processing. In addition, by controlling the rolling conditions of the final rolling stand described below, it becomes easier for the number of evaluated ferrite grains in the first grain size range, the number of evaluated ferrite grains in the second grain size range, and the number of evaluated ferrite grains in the third grain size range after austenite → ferrite transformation to satisfy the specified relationship.

In addition to the difficulty of rolling in each of the first four rolling stands so that σ, as determined by the above formula (1), exceeds 80, rolling to make σ exceed 80 results in an excessively fine structure, and even if the rolling conditions of the final rolling stand described below are controlled, it becomes difficult for the number of evaluated ferrite grains in the first grain size range, the number of evaluated ferrite grains in the second grain size range, and the number of evaluated ferrite grains in the third grain size range after austenite → ferrite transformation to satisfy the specified relationship.

In the finish rolling process, it is preferable to perform rolling so that 5≦σ≦10 is satisfied in the last rolling stand. By performing rolling so that σ, determined by the following formula (1), is satisfied 40≦σ≦80 in each of the first four rolling stands, and 5≦σ≦10 is satisfied in the last rolling stand, a difference occurs in the amount of strain introduced into each crystal grain. This causes selective crystal grain growth during the coiling process, and a mixed grain structure of ferrite crystal grains, which is characteristic of the present invention, is formed after the austenite → ferrite transformation. In addition, by rolling so that 5≦σ≦10 is satisfied in the last rolling stand, some crystal grains grow sufficiently and become coarse, so that the Vickers hardness Hv at a depth position of 1/4 of the plate thickness can be reduced to 160 or less in the steel plate after the austenite → ferrite transformation.

It is preferable that the finish rolling start temperature is 1000°C or higher. This makes it easier to accumulate appropriate strain in each of the first four rolling stands. If the finish rolling start temperature is 1000°C or higher, it makes it easier to accumulate appropriate strain in the austenite grains during and immediately after processing in each of the first four rolling stands.

The interpass time between each rolling stand in the finishing rolling process is preferably 10.0 seconds or less. This makes it easier to accumulate appropriate strain in the austenite grains during and immediately after processing. There is no particular need to set a lower limit, and it is possible to make it as short as possible, but considering a practical equipment configuration, the lower limit is about 0.1 seconds. Furthermore, by setting the interpass time between each rolling stand in the finishing rolling process to 0.2 seconds or more and 3.0 seconds or less, it becomes easier to accumulate appropriate strain. The average interpass time between each rolling stand is preferably 0.2 seconds or more and 3.0 seconds or less.

It is preferable that the temperature at the exit of the final rolling stand in the finishing rolling process be 850°C or higher and 1000°C or lower. By setting the temperature at the exit of the final rolling stand to 850°C or higher and 1000°C or lower, it becomes easier to accumulate appropriate strain in the austenite grains during and immediately after processing in the final rolling stand.

It is preferable to set the cumulative reduction rate in the finish rolling process to 60% or more. By setting the cumulative reduction rate to 60% or more, it becomes easier to accumulate appropriate strain in the austenite grains during and immediately after processing.

"Cooling process"
In the cooling step, it is preferable to start cooling at an average cooling rate of 30° C./s or more between 0.2 and 2.0 seconds after the end of the finish rolling step. By cooling at 30° C./s or more between 0.2 and 2.0 seconds, the growth of austenite grains can be stopped. This makes it easier for the number of evaluated ferrite grains in the first grain size range to be 2.5 to 3.0 times the number of evaluated ferrite grains in the second grain size range after austenite-to-ferrite transformation, and for the number of evaluated ferrite grains in the third grain size range to be 2.0 to 2.5 times the number of evaluated ferrite grains in the second grain size range. The average cooling rate can be calculated by dividing the difference between the temperature at the start of cooling and the temperature at the end of cooling by the time from the start of cooling to the end of cooling.

In the cooling process, it is preferable to cool to a temperature range of 550°C to 650°C (cooling stop temperature range). By cooling to a temperature range of 550°C to 650°C, the formation of martensite and bainite can be suppressed and the formation of pearlite and ferrite can be promoted.

"Winding process"
In the coiling process, after the cooling process, the coiling temperature is in the temperature range of 550 to 650° C. By coiling in the temperature range of 550 to 650° C., the formation of martensite and bainite can be suppressed and the formation of pearlite and ferrite can be promoted.

The present invention will now be described more specifically with reference to examples.
Slabs having the chemical compositions shown in Tables 1 and 2 were cast. The cast slabs were heated under the conditions in Table 3 and subjected to finish rolling. After finish rolling, the slabs were cooled and coiled under the conditions in Tables 2A and 2B. The average interpass time between each rolling stand in Table 3 is the average value of the interpass times between each rolling stand. The interpass time between each rolling stand in each Example was 0.2 seconds or more and 3.0 seconds or less.

　Test pieces for SEM observation were taken from the obtained hot-rolled steel sheet as described above, and the cross section of the sheet thickness parallel to the rolling direction was polished. The metal structure was then observed at a depth of 1/4 of the sheet thickness using the method described above, and the area ratios of ferrite, pearlite, bainite, and martensite at a depth of 1/4 of the sheet thickness were obtained. The results obtained are shown in Table 5. Similarly, the grain size of ferrite grains was measured using the method described above, and the number N1 and average grain size of ferrite grains evaluated in the first grain size range, the number N2 and average grain size of ferrite grains evaluated in the second grain size range, and the number N3 and average grain size of ferrite grains evaluated in the third grain size range were evaluated. The results obtained are shown in Table 5. Table 6 shows the ratio (N1/N2) of the number N1 of ferrite grains evaluated in the first grain size range to the number N2 of ferrite grains evaluated in the second grain size range, the ratio (N3/N2) of the number N3 of ferrite grains evaluated in the third grain size range to the number N2 of ferrite grains evaluated in the second grain size range, the ratio (N1/N3) of the number N1 of ferrite grains evaluated in the first grain size range to the number N2 of ferrite grains evaluated in the third grain size range, and the Vickers hardness.

The total elongation was determined by taking JIS No. 5 tensile test pieces from the hot-rolled steel sheet in a direction perpendicular to the rolling direction and conducting a tensile test in accordance with JIS Z 2241:2011. The results are shown in Table 6.

In addition, the cold rolling properties of the above hot-rolled steel sheets were evaluated for 90° bending workability using a 90° V-block test. When the thickness of the steel sheet is t and the inside minimum bending radius of the punch is R, a 90° punch with a curvature of R/t = 1 was used to press the test piece into a 90° die, after which the test piece was removed and the outside of the bend was visually observed. If cracks were found as a result of the visual observation, they were marked with an X, and if no abnormalities were found, an ◯. A gap with a maximum width of 1 mm or more was defined as a "crack". The results are shown in Table 6.

As can be seen from Table 6, all of the steels of the present invention had excellent cold rolling properties. In addition, the steels of the present invention had excellent cold rolling properties without spheroidizing annealing, and therefore had excellent productivity.

The hot-rolled steel sheet disclosed herein has excellent cold rolling properties and is therefore highly applicable in industry.

Claims (5)

1. In mass percent,
   C: 0.20-0.70%,
   Si: 0.010-0.300%,
   Mn: 0.3-2.0%,
   Al: 0.001-0.100%,
   N: 0.0010-0.0100%,
   P: 0.008% to 0.030%,
   S: 0.010% or less,
   O: 0.0025% or less,
   Cr: 1.500% or less,
   B: 0.010% or less,
   Nb: 0.50% or less,
   Mo: 0.50% or less,
   V: 0.50% or less,
   Ti: 0.3000% or less,
   Cu: 0.500% or less,
   W: 0.500% or less,
   Ta: 0.500% or less,
   Ni: 0.500% or less,
   Mg: 0.003% or less,
   Ca: 0.003% or less,
   Y: 0.030% or less,
   Zr: 0.030% or less,
   La: 0.030% or less,
   Ce: 0.030% or less,
   Sn: 0.030% or less,
   Sb: 0.030% or less, and
   As: 0.030% or less, the balance being Fe and impurities, and the metal structure at a depth position of 1/4 of the plate thickness is
   20% or more by area of ferrite;
   40% or more by area of pearlite;
   The remaining structure is 0 area % or more and 10 area % or less,
   the remaining structure includes at least one of bainite and martensite,
   In the ferrite grains measured by the electron backscatter diffraction method, ferrite grains excluding 5% of the total number of the ferrite grains from the maximum grain size side and 5% of the total number of the ferrite grains from the minimum grain size side of the ferrite grains are treated as evaluation ferrite grains;
   A minimum value of the grain size of the evaluated ferrite grains is set as a first grain size;
   The maximum value of the grain size of the evaluated ferrite grains is set as a second grain size;
   A crystal grain size obtained by adding 1/3 of the difference between the second grain size and the first grain size to the first grain size is set as a third grain size;
   A fourth grain size is set to a crystal grain size obtained by adding 2/3 of the difference between the second grain size and the first grain size to the first grain size;
   A range of the first particle size range is set to be equal to or larger than the first particle size and equal to or smaller than the third particle size,
   A range exceeding the third particle size and equal to or smaller than the fourth particle size is set as a second particle size range;
   When the range exceeding the fourth particle size and equal to or less than the second particle size is set as the third particle size range,
   the number of the evaluated ferrite grains in the first grain size range is 2.5 times or more and 3.0 times or less the number of the evaluated ferrite grains in the second grain size range,
   A hot-rolled steel sheet, wherein the number of the evaluated ferrite grains in the third grain size range is 2.0 to 2.5 times the number of the evaluated ferrite grains in the second grain size range.
2. The hot-rolled steel sheet according to claim 1, wherein the average grain size of the evaluated ferrite grains in the first grain size range is 3 μm to 20 μm.
3. The hot-rolled steel sheet according to claim 1, wherein the average grain size of the evaluated ferrite grains in the third grain size range is 80 μm to 120 μm.
4. The hot-rolled steel sheet according to claim 1, in which the Vickers hardness Hv is 160 or less at a depth position of 1/4 of the sheet thickness.
5. Hot-rolled steel sheet according to any one of claims 1 to 4, having a total elongation of 40% or more.