Steel plate and its manufacturing method

Abstract

This steel sheet has a composition on a mass basis containing C: 0.70-1.30%, Si: 0.01-0.50%, Mn: 0.05-1.30%, P: 0.100% or less, S: 0.1000% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0-1.20%, Ni: 0-2.800%, Mo: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, Ti: 0-0.150%, and B: 0-0.0100%, with the remainder being Fe and impurities. It has a metallic structure containing ferrite with an average grain size of 15.0 μm or more, and cementite with an average particle size of more than 0.40 μm and less than or equal to 0.75 μm. The cementite in question has a surface area ratio of 85% or more of spheroidized cementite with an aspect ratio of 3.0 or less relative to all cementite.

Description

Technical Field

[0001] The present invention relates to a steel sheet and a method for manufacturing the same.

Background Art

[0002] High-carbon steel sheets (hereinafter abbreviated as "steel sheets") are used as materials for structural parts and mechanical parts used in various machines and devices such as automobiles. After being processed into a predetermined shape, these steel sheets are heat-treated (quenched and tempered) such as quenching and tempering to become various parts. Therefore, the steel sheets are required to be excellent in workability and hardenability. In particular, in heat treatment (quenching and tempering), rapid heating such as high-frequency heating may be used, so further improvement in hardenability is desired. As a steel sheet used for various parts as described above, for example, in Patent Document 1, in terms of weight ratio, C: 0.75 to 1.00%, Si: 0.05 to 0.35%, Mn: 0.10 to 0.60%, P: 0.02% or less, S: 0.01% or less, Cr: 0.50 to 1.00%, Ni: 0.50 to 2.00%, Al: 0.10% or less, O: 0.0015% or less, Mo: 0.5% or less, and the balance is substantially composed of Fe and inevitable alloy components, and a high-carbon thin steel sheet characterized by a ferrite structure in which cementite having an average particle size of 0.5 to 2.0 μm is dispersed has been proposed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Although the high-carbon thin steel sheet described in Patent Document 1 controls the average particle size range of cementite particles to 0.5 to 2.0 μm in order to achieve both precision punching processability and resistance to rolling fatigue after heat treatment (tempering), controlling only the average particle size of cementite particles may not sufficiently improve at least one of processability and hardenability.

[0005] This invention was made to solve the above-mentioned problems, and aims to provide a steel sheet with excellent workability and hardenability, and a method for manufacturing the same. [Means for solving the problem]

[0006] The inventors of this invention conducted intensive research to solve the above problems and found that the workability and hardenability of steel sheets can be improved by controlling the composition and microstructure (particularly the average grain size of ferrite and the average particle size of cementite, as well as the area ratio of spheroidized cementite). Furthermore, the inventors found that a steel sheet having the above properties can be obtained by cold rolling and annealing a hot-rolled steel sheet having a predetermined composition under predetermined conditions. The present invention was completed against this background.

[0007] In other words, the present invention has a composition by mass containing C: 0.70~1.30%, Si: 0.01~0.50%, Mn: 0.05~1.30%, P: 0.100% or less, S: 0.1000% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0~1.20%, Ni: 0~2.800%, Mo: 0~0.500%, V: 0~0.500%, Nb: 0~0.500%, Ti: 0~0.150%, and B: 0~0.0100%, with the remainder being Fe and impurities. The metal structure has ferrite with an average grain size of 15.0 μm or more, and cementite with an average particle diameter of more than 0.40 μm and less than or equal to 0.75 μm. The cementite in question is a steel sheet in which the area ratio of spheroidized cementite with an aspect ratio of 3.0 or less to all cementite is 85% or more.

[0008] Furthermore, the present invention relates to a cold rolling process to obtain a cold-rolled steel sheet by cold rolling a hot-rolled steel sheet containing, by mass, C: 0.70~1.30%, Si: 0.01~0.50%, Mn: 0.05~1.30%, P: 0.100% or less, S: 0.1000% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0~1.20%, Ni: 0~2.800%, Mo: 0~0.500%, V: 0~0.500%, Nb: 0~0.500%, Ti: 0~0.150%, and B: 0~0.0100%, with the remainder being Fe and impurities, at a total rolling rate of 15~50%. The cold-rolled steel sheet is annealed in one step at a uniform temperature of 720 to 780°C. Includes, The present invention relates to a method for manufacturing a steel sheet, wherein the annealing process involves heating from room temperature to the soaking temperature at an average heating rate of 70°C / h or more, and after annealing, cooling in the temperature range from 710°C to 300°C at an average cooling rate of 10°C / h or less. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a steel sheet with excellent workability and hardenability, and a method for manufacturing the same. [Modes for carrying out the invention]

[0010] The embodiments of the present invention will be described in detail below. The present invention is not limited to the embodiments described below, and it should be understood that modifications, improvements, etc., made to the embodiments described below, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the invention, also fall within the scope of the present invention.

[0011] In this specification, a numerical range indicated by "~" means a range that includes the numbers before and after "~" as the lower and upper limits, unless otherwise specified. In this specification, a numerical range indicated by "greater than" or "less than" means a range that does not include the number as the lower or upper limit. Furthermore, with respect to the numerical ranges described stepwise in this specification, the upper limit of one stepwise numerical range may be replaced with the upper limit of another stepwise numerical range or the value shown in the examples. Similarly, with respect to the numerical ranges described stepwise in this specification, the lower limit of one stepwise numerical range may be replaced with the lower limit of another stepwise numerical range or the value shown in the examples. Moreover, a numerical range may be formed by combining any of the upper and lower limits described in this specification. In this specification, unless otherwise specified, the "%" notation for composition refers to "mass%".

[0012] <Steel plate> The steel sheet according to the embodiment of the present invention has a composition containing C: 0.70-1.30%, Si: 0.01-0.50%, Mn: 0.05-1.30%, P: 0.100% or less, S: 0.1000% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0-1.20%, Ni: 0-2.800%, Mo: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, Ti: 0-0.150%, and B: 0-0.0100%, with the remainder being Fe and impurities. Herein, in this specification, "steel plate" means a plate-shaped (including strip-shaped) material formed from steel. Furthermore, in this specification, "impurities" refers to components that are mixed in during the industrial manufacturing of steel sheets due to various factors in the raw materials such as ore and scrap, and in the manufacturing process, and which are acceptable as long as they do not adversely affect the present invention. Examples of impurities include Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, Ca, and the like. Regarding the content of each element, "containing xx% or less" means that it contains xx% or less, but also more than 0% (especially above the impurity level). The details of the above composition will be explained below.

[0013] (C: 0.70~1.30%) Carbon (C) is an element necessary to ensure hardness after heat treatment (tempering). If the C content is low, much of the cementite will dissolve during annealing, resulting in fewer nuclei for precipitation of spheroidized cementite during cooling. As a result, the area ratio of spheroidized cementite decreases, and workability deteriorates. Therefore, the C content should be 0.70% or more, preferably 0.72% or more, more preferably 0.74% or more, and even more preferably more than 0.80%. On the other hand, if the C content is high, cementite (especially grain boundary cementite) will not dissolve easily during annealing, and the ferrite grains will not coarse sufficiently. As a result, the average crystal grain size of ferrite will decrease while the average particle size of cementite will increase, leading to decreased workability and hardenability. Therefore, the C content should be 1.30% or less, preferably 1.28% or less, and more preferably 1.25% or less.

[0014] (Si: 0.01~0.50%) Si is an element necessary for deoxidation. To obtain this effect sufficiently, the Si content should be 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the Si content is too high, cementite will not dissolve well during heat treatment (tempering), and the hardenability will decrease. Also, the ferrite grains will not coarse sufficiently during annealing, resulting in a smaller average grain size of ferrite and thus reduced workability. Therefore, the Si content should be 0.50% or less, preferably 0.48% or less, and more preferably 0.45% or less.

[0015] (Mn: 0.05~1.30%) Mn is an element necessary to improve the hardenability of the steel plate. If the Mn content is too low, the hardenability will decrease. Therefore, the Mn content should be 0.05% or more, preferably 0.10% or more, and more preferably 0.15% or more. On the other hand, if the Mn content is too high, it becomes difficult for cementite to dissolve during heat treatment (quenching and tempering), resulting in a decrease in hardenability. Also, the average crystal grain size of ferrite becomes smaller, leading to a decrease in workability. In addition, the steel plate becomes high-strength due to solid solution strengthening, resulting in a decrease in workability. For this reason, the Mn content should be 1.30% or less, preferably 1.28% or less, and more preferably 1.25% or less.

[0016] (P: 0.100% or less) The lower the P content, the more preferable it is. If it is too much, properties such as toughness after heat treatment (quenching and tempering) will deteriorate. For this reason, the P content should be 0.100% or less, preferably 0.098% or less, and more preferably 0.095% or less. On the other hand, since the lower the P content, the more preferable it is, the lower limit is not particularly limited. However, excessive reduction of the P content causes an increase in cost. Therefore, the P content can be, for example, 0.001% or more.

[0017] (S: 0.1000% or less) S forms MnS and easily becomes a fracture initiation point, reducing the workability of the steel plate. For this reason, the S content should be 0.1000% or less, preferably 0.0980% or less, and more preferably 0.0950% or less. On the other hand, since the lower the S content, the more preferable it is, the lower limit is not particularly limited. However, excessive reduction of the S content causes an increase in cost. Therefore, the S content can be, for example, 0.0001% or more.

[0018] (Al: 0.100% or less) Al is an element used for deoxidation. However, if the Al content is too high, inclusions will increase and the workability of the steel sheet will decrease. Therefore, the Al content is 0.100% or less, preferably 0.095% or less, more preferably 0.090% or less. On the other hand, the Al content may be low and the lower limit is not particularly limited. However, from the perspective of obtaining the above effects of Al, the Al content can be, for example, 0.001% or more, 0.003% or more, or 0.005% or more.

[0019] (N: 0.0150% or less) N is an element that forms AlN and suppresses excessive coarsening of crystal grains during heat treatment (quenching and tempering) due to the pinning effect. However, if the N content is too high, the effect will saturate and lead to a decrease in toughness. Therefore, the N content is 0.0150% or less, preferably 0.0140% or less, more preferably 0.0130% or less. On the other hand, the N content may be low and the lower limit is not particularly limited. However, excessive reduction of the N content causes an increase in cost, so the N content can be, for example, 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0020] (Cr: 0 - 1.20%) Cr is an element that improves the strength after heat treatment (quenching and tempering), but it is not an essential element, and the Cr content may be 0%. However, if the Cr content is too high, it becomes difficult for cementite to dissolve during heat treatment (quenching and tempering), resulting in a decrease in hardenability. Also, the average crystal grain size of ferrite becomes small, and the workability decreases. Therefore, the Cr content is 1.20% or less, preferably 1.10% or less, more preferably 1.00% or less. Thus, the Cr content can be 0 - 1.20%. On the other hand, the lower limit value of the Cr content is not particularly limited, but from the perspective of obtaining the above effects of Cr, it can be, for example, 0.01% or more, 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more. Thus, from the perspective of obtaining the effects of Cr, the Cr content can be in the range having the above upper and lower limit values, for example, 0.01 - 1.20%.

[0021] (Ni: 0~2.800%) Ni is an element that dissolves in steel to improve its strength without impairing its toughness, but it is not an essential element, and the Ni content may be 0%. However, Ni is an expensive element, and excessive amounts will increase costs. For this reason, the Ni content should be 2,800% or less, preferably 2,750% or less, and more preferably 2,700% or less. Therefore, the Ni content can be 0 to 2,800%. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of obtaining the above-mentioned effects of Ni, it can be, for example, 0.001% or more, 0.010% or more, or 0.050% or more. Therefore, from the viewpoint of obtaining the effects of Ni, the Ni content can be within the range of the above-mentioned upper and lower limits, for example, 0.001 to 2,800%.

[0022] (Mo: 0~0.500%) Mo is an element that improves strength after heat treatment (tempering), but it is not an essential element, and the Mo content may be 0%. However, if the Mo content is too high, cementite will not dissolve easily during heat treatment (tempering), reducing hardenability. Also, the average grain size of ferrite will decrease, reducing workability. For this reason, the Mo content should be 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less. Therefore, the Mo content can be 0 to 0.500%. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of obtaining the above-mentioned effects of Mo, it can be, for example, 0.001% or more, 0.005% or more, or 0.010% or more. Therefore, from the viewpoint of obtaining the effects of Mo, the Mo content can be within the range having the above-mentioned upper and lower limits, for example, 0.001 to 0.500%.

[0023] (V:0~0.500%, Nb:0~0.500%, Ti:0~0.150%) V, Nb, and Ti are all elements that improve strength after heat treatment (tempering) by carbide precipitation, but they are not particularly essential elements, and their content may be 0%. However, if the content of these elements is too high, excessive carbides will be formed, and the workability of the steel sheet will decrease. For this reason, the V content should be 0.500% or less, preferably 0.480% or less, more preferably 0.460% or less; the Nb content should be 0.500% or less, preferably 0.480% or less, more preferably 0.460% or less; and the Ti content should be 0.150% or less, preferably 0.148% or less, more preferably 0.145% or less, and even more preferably 0.100% or less. Therefore, the content of both V and Nb can be 0 to 0.500%, and the Ti content can be 0 to 0.150%. On the other hand, the lower limit of the content of these elements is not particularly limited, but from the viewpoint of obtaining the above-mentioned effects of these elements, it can be, for example, 0.001% or more, 0.003% or more, or 0.005% or more. Therefore, from the viewpoint of obtaining the effects of these elements, the content of these elements can be within the range having the above-mentioned upper and lower limits, for example, the content of V and Nb can both be 0.001 to 0.500%, and the Ti content can be 0.001 to 0.150%.

[0024] (B: 0~0.0100%) B is an element that segregates at grain boundaries and improves toughness after heat treatment (tempering), but it is not an essential element, and the B content may be 0%. However, if the B content is too high, its effect will saturate, and the raw material cost will increase. For this reason, the B content should be 0.0100% or less, preferably 0.0090% or less, more preferably 0.0080% or less, and even more preferably 0.0050% or less. Therefore, the B content can be 0 to 0.0100%. On the other hand, the lower limit of the B content is not particularly limited, but from the viewpoint of obtaining the above-mentioned effect of B, it can be, for example, 0.0001% or more, 0.0003% or more, or 0.0005% or more. Therefore, from the viewpoint of obtaining the effect of B, the B content can be within the range having the above-mentioned upper and lower limits, for example, 0.0001 to 0.0100%.

[0025] (Cu: less than 0.15%, W: less than 0.15%, Ta: less than 0.15%, Sn: less than 0.050%, Sb: less than 0.050%, Co: less than 0.050%, As: less than 0.050%, Mg: less than 0.050%, Y: less than 0.050%, Zr: less than 0.050%, La: less than 0.050%, Ce: less than 0.050%, and Ca: less than 0.050%) Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca are impurities and do not necessarily have to be present in the steel sheet. These elements may be present as impurities individually or in pairs or combinations. The content of Cu, W, and Ta is all between 0 and 0.15%, preferably between 0.01 and 0.14%. The content of Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca is all between 0 and 0.050%, preferably between 0.001 and 0.045%. The composition of the steel plate described above can be determined by the following method. Test specimens are taken from steel plates, and measurements are performed on these specimens using common methods such as ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using the combustion-infrared absorption method, while N can be measured using the inert gas fusion-thermal conductivity method.

[0026] Next, the metallographic structure of a steel sheet according to an embodiment of the present invention will be described. The steel sheet according to the embodiment of the present invention has a metallic structure containing ferrite with an average grain size of 15.0 μm or more, and cementite with an average particle diameter of more than 0.40 μm and less than or equal to 0.75 μm. By controlling the average grain size of ferrite to 15.0 μm or more, the steel sheet becomes softer, thereby improving its workability. From the viewpoint of stably ensuring this effect, the average grain size of ferrite is preferably 16.0 μm or more, more preferably 17.0 μm or more. The upper limit of the average grain size of ferrite is not particularly limited, but is preferably 100.0 μm or less, more preferably 80.0 μm or less, and even more preferably 60.0 μm or less. By controlling the average particle size of cementite to be between 0.40 μm and 0.75 μm, solid solution of cementite is promoted during heat treatment (tempering), and austenite coarsens, thereby improving hardenability. From the viewpoint of stably ensuring this effect, the average grain size of cementite is preferably 0.45 to 0.70 μm, more preferably 0.50 to 0.69 μm.

[0027] Here, the average grain size of ferrite is determined in accordance with JIS G0551:2020. Specifically, the L-section (cross section) of a test specimen cut from a steel plate, parallel to the rolling direction and thickness direction, is polished, and then immersed in a 3% nital etching solution to reveal the microstructure. In this cross section, the microstructure is observed at five arbitrary locations using a scanning electron microscope (SEM), with the center of the field of view being at a position 1 / 4 of the plate thickness (1 / 4 depth position) from the surface of the steel plate. The magnification is set to 500 to 3000 times depending on the size of the crystal grains. The average crystal grain size at each location is determined from the obtained microstructure images using the sectioning method. The average value of the measurement results at the five locations is taken as the average crystal grain size of ferrite. The average particle size of cementite can be determined by the following method. Specifically, the L-shaped cross-section of a test specimen cut from a steel plate is polished, and then immersed in a picral etching solution to reveal the microstructure. In this cross-section, the microstructure is observed at five arbitrary locations using a scanning electron microscope (SEM), with the 1 / 4 depth position as the center of the field of view. The magnification is set to 500 to 3000x depending on the size of the cementite grains. In the obtained microstructure images, the equivalent circular diameter of each cementite grain is determined using image analysis software, and the average value of the equivalent circular diameters of all cementite grains is calculated. The average particle size of the cementite is the average of the measurement results from the five locations.

[0028] The cementite has an area ratio of spheroidized cementite with an aspect ratio of 3.0 or less relative to the total cementite, with an area ratio of 85% or more. By controlling the area ratio of spheroidized cementite with an aspect ratio of 3.0 or less to 85% or more, the workability of the steel sheet can be improved. From the viewpoint of stably ensuring this effect, the area ratio of spheroidized cementite with an aspect ratio of 3.0 or less is preferably 86% or more, more preferably 87% or more. There is no particular upper limit to the area ratio of spheroidized cementite, but it is preferably 100% or less, more preferably 99% or less. The area percentage of spheroidized cementite with an aspect ratio of 3.0 or less is determined by observing the tissue in the same manner as measuring the average particle size of cementite, and then using image analysis software to calculate the area percentage of spheroidized cementite (the area percentage of spheroidized cementite with an aspect ratio of 3.0 or less relative to all cementite in the measurement area) as a percentage in the obtained tissue photograph. The area percentage of spheroidized cementite is the average of the measurement results from five locations. The aspect ratio of cementite refers to the ratio of the longest diameter to the shortest diameter of cementite in the tissue photograph (longest diameter / shortest diameter).

[0029] Because the steel sheet according to the embodiment of the present invention has the above-described composition and metal structure, it can have the following characteristics.

[0030] (Vickers hardness: 165Hv or less) The steel sheet according to the embodiment of the present invention preferably has a Vickers hardness of 165 Hv or less, more preferably 163 Hv or less, and even more preferably 161 Hv or less or 155 Hv or less. A Vickers hardness within this range indicates excellent workability. Vickers hardness is determined by grinding the cross-section (L-section) parallel to the rolling direction and thickness direction of a test specimen cut from a steel plate, and then performing a Vickers hardness test in accordance with JIS Z2244:2009 at a depth of 1 / 4. In the Vickers hardness test, the measurement load is 1 kgf (9.807 N), the load holding time is 10 seconds, and measurements are taken at three arbitrary locations, with the average value of these measurements being taken as the measurement result.

[0031] (Ferrite fraction after heating to 850°C at 100°C / s followed by oil cooling: 2% or less) In the embodiment of the present invention, it is preferable that the ferrite fraction in the metal structure after heating to 850°C at 100°C / s and then oil-cooling is 2% or less by area fraction. With a ferrite fraction within this range, it can be said that the hardenability is excellent even when rapid heating such as high-frequency heating is used. The ferrite fraction is determined by using a steel plate that has been heated to 850°C at 100°C / s and then cooled in oil at 60°C without being held at the heating temperature. Microstructure observation is performed in the same manner as the measurement of the average crystal grain size of ferrite described above, and the area ratio of ferrite (the area ratio of ferrite to the measurement area) is calculated using image analysis software from the resulting microstructure photograph. The ferrite fraction is the average value of the measurement results from five locations.

[0032] The steel sheet according to the embodiment of the present invention may be any of the following: hot-rolled steel sheet, hot-rolled annealed steel sheet, cold-rolled steel sheet, or cold-rolled annealed steel sheet, but cold-rolled annealed steel sheet is preferred. The thickness of the steel sheet is not particularly limited, but for example, it can be 10.0 mm or less, 8.0 mm or less, or 6.0 mm or less.

[0033] <Method of manufacturing steel plates> The method for manufacturing steel sheets according to the embodiment of the present invention is not particularly limited as long as it can produce steel sheets having the above-described characteristics. The following describes an example of a method for manufacturing steel sheets according to an embodiment of the present invention.

[0034] In the steel sheet (high-carbon steel sheet) according to the embodiment of the present invention, since it contains ferrite (α) and cementite (θ), it is important to coarse and soften the ferrite grains in order to improve its workability. However, grain growth of ferrite grains is suppressed by the pinning effect of cementite, so it becomes more difficult to coarse them as the carbon content increases. Therefore, in the annealing process, the material is heated uniformly in the two-phase regions of austenite (γ) and cementite (θ), and cementite is dissolved to form coarse austenite grains. Then, it is slowly cooled to transform the coarse austenite grains into coarse ferrite grains, thereby coarsening the ferrite grains. In addition, by adjusting the cold rolling and annealing conditions to appropriately control the cementite precipitation state, the hardenability is improved while suppressing a decrease in workability.

[0035] Specifically, the steel sheet according to the embodiment of the present invention includes a cold rolling step of obtaining a cold-rolled steel sheet by cold-rolling a hot-rolled steel sheet having the composition described above with a total rolling ratio of 15 to 50%, and an annealing step of annealing the cold-rolled steel sheet in one stage at a soaking temperature of 720 to 780°C. The annealing step is carried out by heating from room temperature to the soaking temperature at an average heating rate of 70°C / h or more, and then cooling in the temperature range of 710°C to 300°C at an average cooling rate of 10°C / h or less after annealing.

[0036] (Cold rolling process) The cold rolling process is a process in which hot-rolled steel sheets are cold-rolled to obtain cold-rolled steel sheets with a total rolling ratio of 15-50%. By setting the total rolling ratio to 15% or more, coarse austenite can be formed in the annealing process, making it possible to coarseen the ferrite grains and thus improving workability. Conversely, by setting the total rolling ratio to 50% or less, the manufacturability of the steel sheets can be improved. From the viewpoint of stably obtaining these effects, a total rolling ratio of 20-45% is preferable. The total rolling ratio can be expressed as (1-t2 / t1)×100(%), where t1 is the thickness of the sheet before the first cold rolling and t2 is the thickness of the sheet after the last cold rolling. The method for manufacturing hot-rolled steel sheets used in the cold-rolling process is not particularly limited and can be manufactured by conventional methods. Specifically, it can be obtained by hot-rolling a slab having the composition described above. Pickling may also be performed after hot-rolling. The conditions for these processes are not particularly limited and can be adjusted as appropriate depending on the composition and other factors.

[0037] (Annealing process) The annealing process is a one-stage annealing process for cold-rolled steel sheets obtained in the cold-rolling process. One-stage annealing means annealing performed at a single soaking temperature (one soaking cycle). The annealing process is carried out at a soaking temperature of 720-780°C. By setting the soaking temperature to 720°C or higher, cementite can be sufficiently dissolved, forming coarse austenite grains and enabling the coarsening of ferrite grains, thereby improving processability. Conversely, by setting the soaking temperature to 780°C or lower, cementite that acts as precipitation nuclei remains, making it easier for spheroidized cementite to precipitate during cooling, thus improving processability. The soaking time (the time spent at the soaking temperature) is not particularly limited, but is, for example, 3 to 15 hours. The atmosphere during the annealing process is not particularly limited, but can be carried out in a nitrogen-reducing atmosphere, for example. Similarly, the dew point during annealing is not particularly limited, but can be, for example, -60 to -5°C.

[0038] Furthermore, the annealing process involves heating from room temperature to the soaking temperature at an average heating rate of 70°C / h or higher. By heating at such an average heating rate, the solid solution of cementite can be promoted, making it easier to obtain the desired metallic structure. From the viewpoint of stably ensuring this effect, the average heating rate from room temperature to the soaking temperature is preferably 73°C / h or higher, and more preferably 75°C / h or higher. The upper limit of the average heating rate from room temperature to the soaking temperature is not particularly limited, but for example, it can be 120°C / h or lower.

[0039] Cooling after annealing (soaking) is performed in the temperature range of 710°C to 300°C at an average cooling rate of 10°C / h or less. By keeping the average cooling rate from 710°C to 300°C at 10°C / h or less, precipitation of spheroidized cementite becomes possible, thereby improving processability. From the viewpoint of stably ensuring such an effect, it is preferable that the average cooling rate from 710°C to 300°C be 9°C / h or less. The lower limit of the average cooling rate from 710°C to 300°C is not particularly limited, but can be, for example, 2°C / h or more. The cooling rate in the temperature range above 710°C and below 300°C is not particularly limited. It is preferable to cool after annealing to 100°C or less. [Examples]

[0040] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0041] After producing slabs with the composition shown in Table 1 (the remainder being Fe and impurities other than the elements shown in Table 1) by continuous casting, hot-rolled steel sheets were obtained by hot-rolling the slabs. Next, a cold-rolling process and a one-stage annealing process were carried out under the conditions shown in Tables 2-1 and 2-2 to obtain cold-rolled annealed steel sheets with a thickness of 2.5 mm. The soaking time for the annealing process was 5 hours in all cases, the furnace atmosphere was a nitrogen-reducing atmosphere in all cases, and the dew point was -60 to -5°C in all cases.

[0042] [Table 1]

[0043] [Table 2-1]

[0044] [Table 2-2]

[0045] The cold-rolled and annealed steel sheets obtained in the above examples were evaluated as follows.

[0046] (Average crystal grain size of ferrite and average particle size of cementite) The average grain size of ferrite and the average particle size of cementite were measured according to the method described above. The test specimens were 15 mm in the rolling direction, 10 mm in the width direction, and 2.5 mm in thickness.

[0047] (Area percentage of spheroidized cementite with an aspect ratio of 3.0 or less relative to all cementite) Following the method described above, the area percentage of spheroidized cementite with an aspect ratio of 3.0 or less relative to all cementite was determined. The test specimens were 15 mm in the rolling direction, 10 mm in the width direction, and 2.5 mm in thickness.

[0048] (Vickers hardness) The Vickers hardness was measured according to the method described above. In this evaluation, a Vickers hardness of 165 Hv or less indicates good machinability, while a Vickers hardness exceeding 165 Hv indicates insufficient machinability.

[0049] (Hardenability) To evaluate hardenability, the ferrite fraction was determined after heating the steel plate to 850°C at 100°C / s and then cooling it in oil at 60°C. The ferrite fraction was determined according to the method described above. The test specimens used for microstructure observation were 15 mm in the rolling direction, 10 mm in the plate width direction, and 2.5 mm in thickness. In this evaluation, materials with a ferrite fraction of 2% or less by area are marked with ○ (good hardenability), and materials with a ferrite fraction of more than 2% by area are marked with × (insufficient hardenability).

[0050] The evaluation results are shown in Tables 3-1 and 3-2. In Tables 3-1 and 3-2, etc., "area ratio of spheroidized cementite with an aspect ratio of 3.0 or less to all cementite" is abbreviated as "area ratio of spheroidized cementite".

[0051] [Table 3-1]

[0052] [Table 3-2]

[0053] As shown in Table 3-1, the cold-rolled and annealed steel sheets of Examples 1 to 35 had appropriate steel sheet composition and microstructure, resulting in good evaluation results and confirming good workability and hardenability. In contrast, as shown in Table 3-2, the cold-rolled and annealed steel sheet of Comparative Example 1 had too much carbon content, resulting in a small average grain size of ferrite and a large average particle size of cementite. Consequently, both its workability and hardenability were insufficient. This is thought to be due to insufficient solid solution of cementite during annealing. In Comparative Example 2, the cold-rolled and annealed steel sheet had too little carbon content, resulting in a reduced area ratio of spheroidized cementite. Consequently, its workability was insufficient. This is thought to be because much of the cementite dissolved during annealing, leaving fewer nuclei for precipitation of spheroidized cementite during cooling. In Comparative Example 3, the cold-rolled and annealed steel sheet had an excessively high Si content, which made it difficult for cementite to dissolve during heat treatment, resulting in reduced hardenability. Furthermore, the average grain size of ferrite was small, leading to insufficient workability.

[0054] The cold-rolled and annealed steel sheet in Comparative Example 4 had too little Mn content, resulting in reduced hardenability. In Comparative Example 5, the cold-rolled and annealed steel sheet had an excessively high Mn content, resulting in high strength through solid solution strengthening and reduced workability. Furthermore, the average grain size of ferrite decreased, further reducing workability. Additionally, cementite did not dissolve easily during heat treatment, leading to reduced hardenability. In Comparative Example 6, the cold-rolled and annealed steel sheet had an excessively high Cr content, which made it difficult for cementite to dissolve during heat treatment, resulting in reduced hardenability. Furthermore, the average grain size of ferrite was small, leading to insufficient workability. In Comparative Example 7, the cold-rolled and annealed steel sheet had an excessively high Mo content, which made it difficult for cementite to dissolve during heat treatment, resulting in reduced hardenability. Furthermore, the average grain size of ferrite was small, leading to insufficient workability. In Comparative Examples 8 and 9, the cold-rolled and annealed steel sheets had a low total rolling ratio in the cold-rolling process, resulting in a small average grain size of ferrite. Consequently, their workability was insufficient. In Comparative Examples 10 and 11, the cold-rolled and annealed steel sheets had a small average grain size of ferrite because the average heating rate from room temperature to the soaking temperature was too slow during the annealing process. As a result, the workability was insufficient.

[0055] In Comparative Example 12, the cold-rolled and annealed steel sheet had a low soaking temperature during the annealing process, resulting in a small average grain size of ferrite. Consequently, its workability was insufficient. In Comparative Example 13, the cold-rolled and annealed steel sheet had a low area ratio of spheroidized cementite because the soaking temperature during the annealing process was too high. As a result, its workability was insufficient. In Comparative Example 14, the cold-rolled annealed steel sheet had an average heating rate that was too slow from room temperature to the soaking temperature during the annealing process, and the soaking temperature was too high, resulting in a large average particle size of cementite. Consequently, its hardenability was insufficient. In Comparative Examples 15 and 16, the cold-rolled and annealed steel sheets had a low area ratio of spheroidized cementite because the average cooling rate from 710°C to 300°C during the annealing process was too fast. As a result, their workability was insufficient.

[0056] As can be seen from the above results, the present invention provides a steel sheet with excellent workability and hardenability, and a method for manufacturing the same.

[0057] Therefore, the present invention can provide a steel sheet with excellent workability and hardenability and a method for manufacturing the same by adopting the following embodiments [1] to [9].

[0058] [1] The composition, by mass, contains C: 0.70-1.30%, Si: 0.01-0.50%, Mn: 0.05-1.30%, P: 0.100% or less, S: 0.1000% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0-1.20%, Ni: 0-2.800%, Mo: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, Ti: 0-0.150%, and B: 0-0.0100%, with the remainder being Fe and impurities. The metal structure has ferrite with an average grain size of 15.0 μm or more, and cementite with an average particle diameter of more than 0.40 μm and less than or equal to 0.75 μm. The cementite is a steel plate in which the area ratio of spheroidized cementite with an aspect ratio of 3.0 or less to all cementite is 85% or more. [2] The steel sheet according to [1], comprising one or more selected from the group consisting of Cr: 0.01 to 1.20%, Ni: 0.001 to 2.800%, Mo: 0.001 to 0.500%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, Ti: 0.001 to 0.150%, and B: 0.0001 to 0.0100% by mass. [3] The steel sheet according to [1] or [2], wherein the impurities include one or more selected from the group consisting of Cu: 0 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 0.050% by mass. [4] A cold-rolled and annealed steel sheet, as described in any one of [1] to [3]. [5] A steel plate described in any one of [1] to [4], having a Vickers hardness of 165 Hv or less. [6] A steel sheet as described in any one of [1] to [5], wherein the ferrite fraction after heating to 850°C at 100°C / s and then oil cooling is 2% or less.

[0059] [7] A cold rolling process to obtain a cold-rolled steel sheet by cold rolling a hot-rolled steel sheet containing, by mass, C: 0.70~1.30%, Si: 0.01~0.50%, Mn: 0.05~1.30%, P: 0.100% or less, S: 0.1000% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0~1.20%, Ni: 0~2.800%, Mo: 0~0.500%, V: 0~0.500%, Nb: 0~0.500%, Ti: 0~0.150%, and B: 0~0.0100%, with the remainder being Fe and impurities, at a total rolling rate of 15~50%, The cold-rolled steel sheet is annealed in one step at a uniform temperature of 720 to 780°C. Includes, The annealing step involves heating from room temperature to the soaking temperature at an average heating rate of 70°C / h or more, and after annealing, cooling in the temperature range from 710°C to 300°C at an average cooling rate of 10°C / h or less, in a method for manufacturing steel sheets. [8] The method for manufacturing a steel sheet according to [7], wherein the hot-rolled steel sheet comprises one or more selected from the group consisting of Cr: 0.01 to 1.20%, Ni: 0.001 to 2.800%, Mo: 0.001 to 0.500%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, Ti: 0.001 to 0.150%, and B: 0.0001 to 0.0100% by mass. [9] A method for manufacturing a steel sheet according to [7] or [8], wherein the impurity comprises one or more selected from the group consisting of Cu: 0 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 0.050% by mass.

Claims

1. The composition, by mass, contains C: 0.70-1.30%, Si: 0.01-0.50%, Mn: 0.05-1.30%, P: 0.100% or less, S: 0.1000% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0-1.20%, Ni: 0-2.800%, Mo: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, Ti: 0-0.150%, and B: 0-0.0100%, with the remainder being Fe and impurities. The metal structure has ferrite with an average grain size of 15.0 μm or more, and cementite with an average particle diameter of more than 0.40 μm and less than or equal to 0.75 μm. The cementite is a steel plate in which the area ratio of spheroidized cementite with an aspect ratio of 3.0 or less to all cementite is 85% or more.

2. The steel sheet according to claim 1, comprising, by mass, one or more elements selected from the group consisting of Cr: 0.01 to 1.20%, Ni: 0.001 to 2.800%, Mo: 0.001 to 0.500%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, Ti: 0.001 to 0.150%, and B: 0.0001 to 0.0100%.

3. The steel sheet according to claim 1 or 2, wherein the impurities include one or more selected from the group consisting of Cu: 0 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 0.050% by mass.

4. The steel sheet according to claim 1 or 2, which is a cold-rolled and annealed steel sheet.

5. The steel plate according to claim 1 or 2, wherein the Vickers hardness is 165 Hv or less.

6. The steel sheet according to claim 1 or 2, wherein the ferrite fraction after heating to 850°C at 100°C / s and then oil-cooling is 2% or less.

7. A cold rolling process to obtain a cold-rolled steel sheet by cold rolling a hot-rolled steel sheet containing, by mass, C: 0.70-1.30%, Si: 0.01-0.50%, Mn: 0.05-1.30%, P: 0.100% or less, S: 0.1000% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0-1.20%, Ni: 0-2.800%, Mo: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, Ti: 0-0.150%, and B: 0-0.0100%, with the remainder being Fe and impurities, at a total rolling rate of 15-50%. The cold-rolled steel sheet is annealed in one step at a soaking temperature of 720 to 780°C. Includes, The annealing step involves heating from room temperature to the soaking temperature at an average heating rate of 70°C / h or more, and after annealing, cooling in the temperature range from 710°C to 300°C at an average cooling rate of 10°C / h or less, in a method for manufacturing steel sheets.

8. The method for manufacturing a steel sheet according to claim 7, wherein the hot-rolled steel sheet contains one or more selected from the group consisting of Cr: 0.01 to 1.20%, Ni: 0.001 to 2.800%, Mo: 0.001 to 0.500%, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, Ti: 0.001 to 0.150%, and B: 0.0001 to 0.0100% by mass.

9. The method for manufacturing a steel sheet according to claim 7 or 8, wherein the impurities include one or more selected from the group consisting of Cu: 0 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 0.050% by mass.

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