Steel sheet and production method therefor

Abstract

Provided is a steel sheet having a composition which contains, in terms of mass, 0.70-1.30% C, 0.01-0.50% Si, 0.05-1.30% Mn, up to 0.100% P, up to 0.100% S, up to 0.100% Al, up to 0.0150% N, 0-1.20% Cr, 0-2.800% Ni, 0-0.500% Mo, 0-0.500% V, 0-0.500% Nb, 0-0.150% Ti, and 0-0.0100% B, the remainder comprising Fe and impurities, and having a metallographic structure which includes ferrite having an average crystal-grain diameter of 15.0 μm or larger and cementite having an average grain diameter of 0.70-1.20 μm. In the cementite, the areal proportion of spheroidized cementite having aspect ratios of 3.0 or less is 85% or higher with respect to the whole cementite and the ratio of the number of intragranular cementite grains to the total number of cementite grains is 0.70 or higher.

Description

Steel Plate and Method for Producing the Same

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

[0002] High-carbon steel plates (hereinafter abbreviated as "steel plates") are used as materials for structural parts and mechanical parts used in various machines and devices such as automobiles. After this steel plate is processed into a predetermined shape, it is heat-treated (quenched and tempered) such as quenching and tempering to become various parts. Therefore, the steel plate is required to have excellent workability and hardenability and high toughness after heat treatment (quenching and tempering). As a steel plate used for various parts as described above, for example, in Patent Document 1, by 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 plate characterized by a ferrite structure in which cementite with an average particle size of 0.5 to 2.0 μm is dispersed has been proposed.

[0003] Japanese Patent Laid-Open No. 9-87805

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

[0005] The present invention has been made to solve the above problems, and an object thereof is to provide a steel plate excellent in workability and hardenability and having high toughness after heat treatment (quenching and tempering) and a method for producing the same.

[0006] The inventors of this invention conducted intensive research to solve the above problems and found that by controlling the composition and microstructure of the steel sheet (particularly the average grain size of ferrite and the average particle size of cementite, as well as the specific precipitation state of cementite), it is possible to improve workability, hardenability, and toughness after heat treatment (tempering). 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 on a mass basis containing C: 0.70 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.05 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0 to 1.20%, Ni: 0 to 2.800%, Mo: 0 to 0.500%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and B: 0 to 0.0100%, with the remainder being Fe and impurities, and 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 0.70 to 1.20 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, and the ratio of the number of intragranular cementite to the total number of cementite is 0.70 or more.

[0008] Furthermore, the present invention includes a cold rolling step of obtaining a cold-rolled steel sheet by cold rolling a hot-rolled steel sheet containing, by mass, C: 0.70 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.05 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0 to 1.20%, Ni: 0 to 2.800%, Mo: 0 to 0.500%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and B: 0 to 0.0100%, with the remainder being Fe and impurities, at a total rolling rate 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 invention relates to a method for manufacturing a steel sheet, wherein the annealing process involves heating from room temperature to 710°C at an average heating rate of 30°C / h or more, from 710°C to the soaking temperature at an average heating rate of 5°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.

[0009] According to the present invention, it is possible to provide a steel sheet with excellent workability and hardenability, and high toughness after heat treatment (tempering), as well as a method for manufacturing the same.

[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, numerical ranges expressed using "~" mean a range that includes the numbers before and after "~" as the lower and upper limits, respectively, unless otherwise specified. Furthermore, in this specification, numerical ranges preceded by "greater than" or "less than" mean a range that does not include the number as the lower or upper limit. Also, for numerical ranges described in steps in this specification, the upper limit of one step may be replaced with the upper limit of another step described numerical range or the value shown in the example. Similarly, for numerical ranges described in steps in this specification, the lower limit of one step may be replaced with the lower limit of another step described numerical range or the value shown in the example. Furthermore, a numerical range may be a combination of any of the upper and lower limits described in this specification. In this specification, "%" in relation to composition means "mass%" unless otherwise specified.

[0012] <Steel Sheet> The steel sheet according to the embodiment of the present invention has a composition containing C: 0.70 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.05 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0 to 1.20%, Ni: 0 to 2.800%, Mo: 0 to 0.500%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and B: 0 to 0.0100%, with the remainder being Fe and impurities. Hereinafter, "steel sheet" means a plate-like (including strip-like) material formed from steel. Furthermore, in this specification, "impurities" refer 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 the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. Examples of impurities include Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, Ca, etc. Regarding the content of each element, "containing xx% or less" means that it is xx% or less, but contains more than 0% (especially above the impurity level). The details of the above composition will be explained below.

[0013] (C: 0.70-1.30%) 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, and there will be fewer nuclei for precipitation of spheroidized cementite during cooling. As a result, the area ratio of spheroidized cementite decreases, and workability and toughness after heat treatment (tempering) tend to decrease. 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 grain size of ferrite will decrease while the average particle size of cementite will increase, and the ratio of the number of intragranular cementite particles to the total number of cementite particles will also decrease, thus reducing workability and toughness after heat treatment (tempering). 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, the ferrite grains will not coarse sufficiently during annealing, resulting in a smaller average grain size of ferrite and a smaller ratio of the number of cementite grains within each grain to the total number of cementite grains. This reduces workability and toughness after heat treatment (tempering). In addition, cementite will not dissolve easily during heat treatment (tempering), reducing hardenability. 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 steel sheets. If the Mn content is too low, the hardenability will decrease, so 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, the steel sheet will become high in strength due to solid solution strengthening, and the workability will decrease. Also, the average grain size of ferrite will decrease, further reducing workability. Furthermore, cementite will not be able to solid-solve during heat treatment (tempering), and the hardenability will decrease. 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) A lower P content is preferable, as too much P will reduce properties such as toughness after heat treatment (tempering). 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 a lower P content is preferable, there is no particular lower limit. However, excessive reduction of the P content will lead to increased costs, so the P content can be, for example, 0.001% or more.

[0017] (S: 0.100% or less) S easily forms MnS, which acts as a fracture initiation point and reduces the workability of the steel sheet. For this reason, the S content should be 0.100% or less, preferably 0.098% or less, and more preferably 0.095% or less. On the other hand, a lower S content is preferable, so there is no particular lower limit. However, excessive reduction of the S content leads to increased costs, so the S content can be, for example, 0.001% or more.

[0018] (Al: 0.100% or less) Al is an element used for deoxidation. However, if the Al content is too high, the number of inclusions increases and the workability of the steel sheet decreases. For this reason, the Al content should be 0.100% or less, preferably 0.095% or less, and more preferably 0.090% or less. On the other hand, the Al content may be low, and there is no particular lower limit. However, from the viewpoint 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 grain coarsening during heat treatment (tempering) due to its pinning effect. However, if the N content is too high, the effect saturates, leading to a decrease in toughness. For this reason, the N content should be 0.0150% or less, preferably 0.0140% or less, and more preferably 0.0130% or less. On the other hand, the N content may be low, and there is no particular lower limit. However, excessive reduction of the N content leads to increased costs, 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 strength after heat treatment (tempering), but it is not an essential element, and the Cr content may be 0%. However, if the Cr content is too high, cementite will not dissolve easily during heat treatment (tempering), and hardenability will decrease. Also, the average grain size of ferrite will decrease, and workability will decrease. For this reason, the Cr content should be 1.20% or less, preferably 1.10% or less, and more preferably 1.00% or less. Therefore, the Cr content can be 0-1.20%. On the other hand, the lower limit of the Cr content is not particularly limited, but from the viewpoint 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. Therefore, from the viewpoint of obtaining the effects of Cr, the Cr content can be in the range having the above upper and lower limits, for example, 0.01-1.20%.

[0021] (Ni: 0-2.800%) Ni is an element that dissolves in steel and improves strength without impairing 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 0-2.800%. 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 having the above-mentioned upper and lower limits, for example, 0.001-2.800%.

[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), and hardenability will decrease. Also, the average grain size of ferrite will decrease, and the ratio of the number of cementite grains to the total number of cementite grains will also decrease, resulting in decreased workability and toughness after heat treatment (tempering). 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 0-0.500%. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of obtaining the above 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 of 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 the content of these elements may be 0%. However, if the content of these elements is too high, excessive carbides will be generated, 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 V and Nb can both 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, for example, it can be 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 to 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 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 in the range having the above upper and lower limits, for example, 0.0001 to 0.0100%.

[0025] (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%) Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca are impurities and do not necessarily have to be included in the steel sheet. These elements may be included as impurities individually or in pairs or more. The content of Cu, W, and Ta is 0 to less than 0.15%, preferably 0.01 to 0.14%. The content of Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca is 0 to less than 0.050%, preferably 0.001 to 0.045%. The composition of the steel sheet described above can be determined by the following method. A test piece is taken from the steel sheet, and this test piece is measured by a general method such as ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.

[0026] Next, the microstructure of the steel sheet according to the embodiment of the present invention will be described. The steel sheet according to the embodiment of the present invention has a microstructure that includes ferrite with an average grain size of 15.0 μm or more and cementite with an average particle diameter of 0.70 to 1.20 μm. By controlling the average grain size of ferrite to 15.0 μm or more, the steel sheet becomes softer, thereby improving 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 diameter of cementite to 0.70 to 1.20 μm, the occurrence of cracks originating from cementite is suppressed, and the austenite is less likely to coarseen during heat treatment (tempering), thereby improving toughness after heat treatment (tempering). From the viewpoint of stably ensuring this effect, the average particle size of cementite is preferably 0.75 to 1.18 μm, more preferably 0.76 to 1.17 μm, and even more preferably 0.80 to 1.16 μm.

[0027] Here, the average grain size of ferrite is determined in accordance with JIS G0551:2020. Specifically, after polishing the cross section (L section) parallel to the rolling direction and thickness direction of the test piece cut from the steel plate, it is 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 in the thickness direction. At this time, the magnification is set to 500 to 3000 times depending on the size of the crystal grains. The average grain size at each location is determined from the obtained microstructure photographs using the sectioning method. The average value of the measurement results at the five locations is taken as the average grain size of ferrite. The average particle size of cementite is determined by the following method. Specifically, the microstructure is observed at five arbitrary locations using a scanning electron microscope (SEM) in the same manner as when determining the average grain size of ferrite. At this time, the magnification is set to 500 to 3000 times depending on the size of the cementite grains. In the obtained tissue 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 five locations.

[0028] The cementite is characterized by having an area ratio of spheroidized cementite with an aspect ratio of 3.0 or less relative to all cementite particles, 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 and its toughness after heat treatment (tempering) 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 ratio of spheroidized cementite with an aspect ratio of 3.0 or less is determined by observing the microstructure in the same manner as measuring the average particle size of cementite, and then using image analysis software to calculate the area ratio of spheroidized cementite (the area ratio of spheroidized cementite with an aspect ratio of 3.0 or less relative to all cementite particles in the measurement area) as a percentage in the obtained microstructure 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] Cementite is defined as having a ratio of intragranular cementite particles to the total number of cementite particles of 0.70 or higher. By controlling the ratio of intragranular cementite particles to the total number of cementite particles to 0.70 or higher, austenite is refined during heat treatment (tempering), thereby improving toughness after heat treatment (tempering). The ratio of intragranular cementite particles to the total number of cementite particles is determined by observing the microstructure in the same manner as measuring the average particle size, and then using image analysis software to calculate the ratio of intragranular cementite particles (the ratio of the number of intragranular cementite particles to the total number of cementite particles in the measurement area) in the obtained microstructure photograph. The ratio of intragranular cementite particles is the average value of the measurement results from five locations. Intragranular cementite refers to cementite particles in the microstructure photograph whose interface with the matrix phase is not in contact with a ferrite grain boundary.

[0030] 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.

[0031] (Vickers hardness: 155 Hv or less) The steel sheet according to the embodiment of the present invention preferably has a Vickers hardness of 155 Hv or less, more preferably 153 Hv or less, and even more preferably 150 Hv or less. A Vickers hardness within this range can be said to have excellent workability. The Vickers hardness is determined by polishing a cross section (L section) parallel to the rolling direction and thickness direction of a test piece cut from the steel sheet, 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 any three locations, with the average value of these measurements being taken as the measurement result.

[0032] (Ferrite fraction when heated at 830°C for 5 minutes and then oil-cooled at 60°C: 2% or less) In the embodiment of the present invention, it is preferable that the ferrite fraction in the metal structure when heated at 830°C for 5 minutes and then oil-cooled at 60°C is 2% or less in area. A ferrite fraction within this range indicates excellent hardenability. The ferrite fraction is determined by using a steel sheet obtained by heating at 830°C for 5 minutes and then cooling in oil at 60°C, observing the structure in the same manner as the measurement of the average crystal grain size of ferrite described above, and calculating the area ratio of ferrite (the area ratio of ferrite in the measurement area) using image analysis software in the obtained microstructure photograph. The ferrite fraction is the average value of the measurement results from five locations.

[0033] (Charpy impact value at 80°C after heating at 830°C for 5 minutes, oil cooling at 60°C, and tempering at 400°C for 15 minutes: 25 J / cm²) 2 (The above) The steel plate according to the embodiment of the present invention was heated at 830°C for 5 minutes, then oil-cooled at 60°C (corresponding to "quenching treatment"), and then tempered at 400°C for 15 minutes, resulting in a Charpy impact value of 25 J / cm² at 80°C. 2It is preferable that the values ​​are as described above. If the Charpy impact value is within this range, it can be said that the toughness after heat treatment (tempering) is excellent. The Charpy impact value is the value obtained by dividing the Charpy impact absorption energy by the cross-sectional area of ​​the test piece. The Charpy impact value is determined by taking a V-notch test piece with a width of 10 mm and a length of 55 mm from a steel plate obtained by heating at 830°C for 5 minutes, cooling in oil at an oil temperature of 60°C, and then tempering at 400°C for 15 minutes, and performing a Charpy impact test. The Charpy impact test is performed in accordance with JIS Z2242:2018, and the V-notch test piece is taken so that the length direction is parallel to the rolling direction. In addition, the V-notch test piece has a V-notch in the center of the length direction with an opening angle of 45°, a depth of 2 mm, and a notch width of 8 mm below the notch.

[0034] 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.

[0035] <Method for Manufacturing Steel Sheets> 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. An example of a method for manufacturing steel sheets according to the embodiment of the present invention will be described below.

[0036] 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 difficult to coarse them as the carbon content increases. Therefore, in the annealing process, the steel is heated uniformly in the two-phase region of austenite (γ) and cementite (θ), cementite is dissolved to form coarse austenite grains, and then the steel is slowly cooled to transform the coarse austenite grains into coarse ferrite grains, thereby coarsening the ferrite grains. Furthermore, by adjusting the cold rolling and annealing conditions to appropriately control the cementite precipitation state, the toughness after heat treatment (tempering) is improved while suppressing a decrease in workability.

[0037] Specifically, the steel sheet according to the embodiment of the present invention includes a cold rolling step of cold rolling a hot-rolled steel sheet having the composition described above at a total rolling rate of 15 to 50% to obtain a cold-rolled steel sheet, 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 involves heating from room temperature to 710°C at an average heating rate of 30°C / h or more, from 710°C to the soaking temperature at an average heating rate of 5°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.

[0038] (Cold Rolling Process) The cold rolling process is a process in which a hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet with a total rolling reduction of 15 to 50%. By setting the total rolling reduction to 15% or more, the formation of coarse austenite is promoted in the annealing process, and ferrite grains become coarser, thereby improving workability. In addition, the ratio of the number of cementite particles within each grain to the total number of cementite particles becomes larger, which improves toughness after heat treatment (tempering). On the other hand, by setting the total rolling reduction to 50% or less, the manufacturability of the steel sheet can be improved. From the viewpoint of stably obtaining these effects, a total rolling reduction of 20 to 45% is preferable. The total reduction 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 manufacturing method of the hot-rolled steel sheet 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.

[0039] (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 to 780°C. By setting the soaking temperature to 720°C or higher, cementite can be sufficiently dissolved, making it easier to obtain the desired metallic structure. By setting the soaking temperature to 780°C or lower, cementite that serves as a precipitation nucleation site remains, allowing for the precipitation of spheroidized cementite during cooling, thereby improving workability and toughness after heat treatment (tempering). The soaking time (holding time at the soaking temperature) is not particularly limited, but for example, it is 3 to 15 hours. The atmosphere of the annealing process is not particularly limited, but for example, it can be carried out in a nitrogen-reducing atmosphere. The dew point during annealing is also not particularly limited, but for example, it can be -60 to -5°C.

[0040] Furthermore, the annealing process involves heating from room temperature to 710°C at an average heating rate of 30°C / h or more, and from 710°C to the soaking temperature at an average heating rate of 5°C / h or more. By heating from room temperature to 710°C at an average heating rate of 30°C / h or more, and from 710°C to the soaking temperature at an average heating rate of 5°C / h or more, the solid solution of cementite can be promoted, making it easier to obtain the desired metal structure. From the viewpoint of stably ensuring such effects, it is preferable that the average heating rate from room temperature to 710°C be 35°C / h or more, and the average heating rate from 710°C to the soaking temperature be 6°C / h or more. The upper limit of the average heating rate from room temperature to 710°C is not particularly limited, but can be, for example, 100°C / h or less. Similarly, the upper limit of the average heating rate from 710°C to the soaking temperature is not particularly limited, but can be, for example, 20°C / h or less. In the annealing process, performing a soaking treatment once or more during the heating process to a temperature range of 720°C or higher, i.e., performing two or more stages of annealing, is undesirable because it can suppress the solid solution of cementite, making it impossible to obtain the desired metallic structure.

[0041] After annealing (soaking), cooling is carried out at an average cooling rate of 10°C / h or less in the temperature range from 710°C to 300°C. By setting the average cooling rate from 710°C to 300°C to 10°C / h or less, precipitation of spheroidized cementite becomes possible, so that the workability and toughness after heat treatment (quenching and tempering) can be improved. From the viewpoint of stably ensuring such an effect, the average cooling rate from 710°C to 300°C is preferably 9°C / h or less. The lower limit of the average cooling rate from 710°C to 300°C is not particularly limited, but for example, it can be set to 2°C / h or more. The cooling rate in the temperature range above 710°C and below 300°C is not particularly limited. Cooling after annealing is preferably carried out down to 100°C or less.

[0042] Hereinafter, the content of the present invention will be described in detail with reference to examples, but the present invention is not construed as being limited thereto.

[0043] After producing a slab having the composition shown in Table 1 (the balance is Fe and impurities other than the elements shown in Table 1) by the continuous casting method, a hot-rolled steel sheet was obtained by hot rolling the slab. Next, a cold-rolled annealed steel sheet with a thickness of 2.5 mm was obtained by performing a cold rolling process and an annealing process by single-stage annealing under the conditions shown in Tables 2-1 and 2-2. The soaking time in the annealing process was 5 hours in all cases. Also, a cold-rolled annealed steel sheet with a thickness of 2.5 mm was obtained by performing a cold rolling process and an annealing process by two-stage annealing under the conditions shown in Table 2-3. In the two-stage annealing, the soaking temperature in the first stage (the first time) was 710°C, the soaking time in the first stage was 15 to 20 hours, the soaking temperature in the second stage (the second time) was 730 to 740°C, and the soaking time in the second stage was 5 hours. The atmosphere in the annealing furnace was a nitrogen-reducing atmosphere in all cases, and the dew point was -60 to -5°C in all cases.

[0044]

[0045]

[0046]

[0047]

[0048] The following evaluations were performed on the cold-rolled annealed steel sheets obtained in the above examples.

[0049] (Average grain size of ferrite and average particle diameter of cementite) The average grain size of ferrite and the average particle diameter 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.

[0050] (Area ratio of spheroidized cementite with an aspect ratio of 3.0 or less relative to all cementite) The area ratio of spheroidized cementite with an aspect ratio of 3.0 or less relative to all cementite was determined according to the method described above. The test specimens were 15 mm in the rolling direction, 10 mm in the plate width direction, and 2.5 mm in thickness.

[0051] (Ratio of intragranular cementite to total cementite) The ratio of intragranular cementite to total cementite was determined according to the method described above. The test specimen was 15 mm in the rolling direction × 10 mm in the plate width direction × 2.5 mm in thickness. In this evaluation, a ratio of intragranular cementite to total cementite of 0.70 or higher is indicated by ○, and a ratio of intragranular cementite to total cementite of less than 0.70 is indicated by ×.

[0052] (Vickers hardness) The Vickers hardness was measured according to the method described above. In this evaluation, a Vickers hardness of 155 Hv or less indicates good machinability, while a Vickers hardness exceeding 155 Hv indicates insufficient machinability.

[0053] (Hardenability) To evaluate hardenability, the ferrite fraction was determined after heating the steel plate at 830°C for 5 minutes and then cooling it in oil at 60°C. The ferrite fraction was determined according to the method described above. The test specimen for microstructure observation was 15 mm in the rolling direction × 10 mm in the plate width direction × 2.5 mm in thickness. In this evaluation, a ferrite fraction of 2% or less in area ratio was indicated by ○ (good hardenability), and a ferrite fraction exceeding 2% in area ratio was indicated by × (insufficient hardenability).

[0054] (Toughness) As an evaluation of toughness, the Charpy impact value at 80°C was measured after heating the steel sheet at 830°C for 5 minutes, oil cooling at 60°C, and tempering at 400°C for 15 minutes. The Charpy impact value was determined by taking a V-notch test specimen from the cold-rolled and annealed steel sheet, heating it at 830°C for 5 minutes, oil cooling at 60°C, followed by tempering at 400°C for 15 minutes, and then performing a Charpy impact test at 80°C in accordance with JIS Z2242:2018. The V-notch test specimen was taken so that its length direction was parallel to the rolling direction. In this evaluation, the Charpy impact value was 25 J / cm 2 Those meeting the above criteria were marked with ○ (good toughness), and the Charpy impact value was 25 J / cm². 2 Values ​​below a certain threshold are indicated by × (insufficient toughness).

[0055] The evaluation results described above 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," and "ratio of the number of intragranular cementites to the total number of cementites" is abbreviated as "ratio of intragranular cementites."

[0056]

[0057]

[0058] As shown in Table 3-1, the cold-rolled annealed steel sheets of Examples 1 to 36 had appropriate steel sheet composition and metal structure, resulting in good evaluation results and confirming good workability, hardenability, and toughness. In contrast, as shown in Table 3-2, the cold-rolled 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, as well as a decrease in the ratio of intragranular cementite. Therefore, its workability and toughness were insufficient. This is thought to be because the solid solution of cementite was insufficient during annealing. The cold-rolled annealed steel sheet of Comparative Example 2 had too little carbon content, resulting in a decrease in the area ratio of spheroidized cementite. Therefore, its workability and toughness were insufficient. This is thought to be because a large amount of cementite dissolved during annealing, resulting in fewer nuclei for precipitation of spheroidized cementite during cooling. The cold-rolled and annealed steel sheet of Comparative Example 3 had too much Si content, resulting in a small average grain size of ferrite and a reduced ratio of cementite within grains. Consequently, its workability and toughness were insufficient. This is thought to be because the cementite did not dissolve easily during annealing. Furthermore, the cementite did not dissolve easily during heat treatment, leading to reduced hardenability.

[0059] In Comparative Example 4, the cold-rolled annealed steel sheet had too little Mn content, resulting in reduced hardenability. In Comparative Example 5, the cold-rolled annealed steel sheet had too much Mn content, leading to high strength through solid solution strengthening and reduced workability. The average grain size of ferrite also decreased, further reducing workability. Additionally, cementite did not dissolve easily during heat treatment, resulting in reduced hardenability. In Comparative Example 6, the cold-rolled annealed steel sheet had too much Cr content, making it difficult for cementite to dissolve easily during heat treatment, resulting in reduced hardenability. The average grain size of ferrite also decreased, resulting in insufficient workability. In Comparative Example 7, the cold-rolled annealed steel sheet had too much Mo content, making it difficult for cementite to dissolve easily during heat treatment, resulting in reduced hardenability. The average grain size of ferrite also decreased, and the ratio of cementite within grains also decreased, resulting in insufficient workability and toughness. In Comparative Examples 8 and 9, the cold-rolled and annealed steel sheets had a low total rolling rate in the cold rolling process, resulting in a small average grain size of ferrite and a reduced ratio of intragranular cementite. Consequently, their workability and toughness were insufficient. In Comparative Example 10, the cold-rolled and annealed steel sheet had a low average grain size of ferrite and a reduced ratio of intragranular cementite because the average heating rate from room temperature to 710°C in the annealing process was too slow. Consequently, their workability and toughness were insufficient.

[0060] In Comparative Example 11, the average heating rate from 710°C to the soaking temperature in the annealing process was too slow, resulting in a small average grain size of ferrite and a reduced ratio of intragranular cementite. Consequently, the workability and toughness were insufficient. In Comparative Example 12, the soaking temperature in the annealing process was too low, resulting in a small average grain size of ferrite and a reduced ratio of intragranular cementite. Consequently, the workability and toughness were insufficient. In Comparative Example 13, the soaking temperature in the annealing process was too high, resulting in a low area ratio of spheroidized cementite. Consequently, the workability and toughness were insufficient. In Comparative Example 14, the total rolling rate in the cold rolling process and the soaking temperature in the annealing process were too low, resulting in small average grain sizes of ferrite and cementite, as well as a low area ratio of spheroidized cementite and a low ratio of intragranular cementite. Therefore, the workability and toughness were insufficient. In Comparative Examples 15 and 16, the average cooling rate from 710°C to 300°C in the annealing process was too fast, resulting in a low area ratio of spheroidized cementite. Therefore, the workability and toughness were insufficient. In Comparative Example 17, two-stage annealing was performed, which resulted in a decrease in the average grain size of ferrite and the average particle size of cementite, as well as a decrease in the ratio of intragranular cementite. Therefore, the workability and toughness were insufficient. In Comparative Example 18, the carbon content was too low and two-stage annealing was performed, resulting in a decrease in the average grain size of ferrite and a decrease in the ratio of intragranular cementite. Therefore, the workability and toughness were insufficient.

[0061] As can be seen from the above results, the present invention provides a steel sheet with excellent workability and hardenability, and high toughness after heat treatment (tempering), as well as a method for manufacturing the same.

[0062] Therefore, by adopting the following embodiments [1] to

[10] , the present invention can provide a steel sheet with excellent workability and hardenability, and high toughness after heat treatment (tempering), as well as a method for manufacturing the same.

[0063] [1] The composition is such that, by mass, it contains C: 0.70-1.30%, Si: 0.01-0.50%, Mn: 0.05-1.30%, P: 0.100% or less, S: 0.100% 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, and 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 0.70-1.20 μm. [1] The cementite is a steel sheet in which the area ratio of spheroidized cementite having an aspect ratio of 3.0 or less to all cementite is 85% or more, and the ratio of the number of intragranular cementite to the total number of cementite is 0.70 or more. [2] The steel sheet according to [1], comprising, by mass, 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%. [3] The steel sheet according to [1] or [2], 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. [4] The steel sheet according to any one of [1] to [3], which is a cold-rolled and annealed steel sheet. [5] A steel sheet according to any one of [1] to [4], wherein the Vickers hardness is 155 Hv or less. [6] A steel sheet according to any one of [1] to [5], wherein the ferrite fraction is 2% or less after heating at 830°C for 5 minutes and then oil-cooling at 60°C. [7] A Charpy impact value at 80°C is 25 J / cm after heating at 830°C for 5 minutes, oil-cooling at 60°C, and then tempering at 400°C for 15 minutes. 2The steel plate described in any one of [1] to [6] above.

[0064] [8] A cold rolling step to obtain a cold-rolled steel sheet by cold rolling a hot-rolled steel sheet containing, on a mass basis, C: 0.70 to 1.30%, Si: 0.01 to 0.50%, Mn: 0.05 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, N: 0.0150% or less, Cr: 0 to 1.20%, Ni: 0 to 2.800%, Mo: 0 to 0.500%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, and B: 0 to 0.0100%, with the remainder being Fe and impurities, at a total rolling rate of 15 to 50%; and an annealing step to anneal the cold-rolled steel sheet in one stage at a soaking temperature of 720 to 780°C. A method for manufacturing a steel sheet, wherein the annealing step involves heating from room temperature to 710°C at an average heating rate of 30°C / h or more, from 710°C to the soaking temperature at an average heating rate of 5°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. [9] A method for manufacturing a steel sheet according to [8], 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.

[10] The method for manufacturing a steel sheet according to [8] or [9], 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: 0.100% 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, and 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 0.70-1.20 μm. The cementite 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, and the ratio of the number of intragranular cementite to the total number of cementite is 0.70 or more.

2. The steel sheet according to claim 1, comprising 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% by mass.

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. A steel sheet according to any one of claims 1 to 3, which is a cold-rolled and annealed steel sheet.

5. A steel plate according to any one of claims 1 to 4, wherein the Vickers hardness is 155 Hv or less.

6. The steel sheet according to any one of claims 1 to 5, wherein the ferrite fraction after heating at 830°C for 5 minutes and then oil-cooling at 60°C is 2% or less.

7. After heating at 830°C for 5 minutes, oil cooling at 60°C, and tempering at 400°C for 15 minutes, the Charpy impact value at 80°C was 25 J / cm². 2 The steel plate according to any one of claims 1 to 6.

8. The process comprises a cold rolling step 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: 0.100% 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%, and an annealing step to anneal the cold-rolled steel sheet in one stage at a soaking temperature of 720-780°C. The annealing step involves heating from room temperature to 710°C at an average heating rate of 30°C / h or more, from 710°C to the soaking temperature at an average heating rate of 5°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.

9. The method for manufacturing a steel sheet according to claim 8, 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.

10. The method for manufacturing a steel sheet according to claim 8 or 9, 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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