Steel sheet and method for manufacturing same

Abstract

The purpose of the present invention is to provide a steel sheet having high strength, high ductility, and low yield elongation, and a method for manufacturing the same. A steel sheet having a component composition containing, in mass%, 0.03% to 0.15% inclusive of C, 0.05% or less of Si, 0.10% to 0.60% inclusive of Mn, 0.025% or less of P, 0.020% or less of S, 0.20% or less of Al, 0.0001% to 0.0200% inclusive of N, 0.005% to 0.030% inclusive of Nb, the remainder being Fe and unavoidable impurities, 0.5% to 10.0% inclusive of martensite in terms of areal fraction, 5% to 30% inclusive of pearlite, bainite and granular cementite in terms of areal fraction, and the remainder being ferrite, among the granular cementite, granular cementite having a maximum particle diameter of 3 [mu] m or less is 1.0% or more in area fraction with respect to the overall structure.

Description

Technical Field

[0001] This invention relates to a high-strength, high-ductility, low-yield-elongation steel plate particularly suitable for use as a tank steel plate, and a method for manufacturing the same. Background Technology

[0002] In recent years, there has been a demand for lightweight tanks based on thinner steel plates, aimed at reducing CO2 emissions during tank transportation. However, thinner walls reduce tank strength, making high-strength steel plates essential.

[0003] As a high-strength steel sheet for cans, DR (Double Reduce) material is known. DR material is a steel sheet for cans that has been cold-rolled and annealed before being cold-rolled again to achieve high strength. However, there are problems: due to its low elongation, its workability is low, and the need for another cold rolling after annealing increases manufacturing costs. To solve these problems, there is a need to develop SR (Single Reduce) material with the same strength and high ductility as DR material.

[0004] Patent Document 1 discloses a steel plate having the following composition: by mass % containing C: 0.03% to 0.13%, Si: 0.05% to 0.01%, Mn: 0.01% to 0.6%, P: 0.025% to 0.020%, S: 0.01% to 0.20%, N: 0.0001% to 0.02%, Ti: 0.005% to 0.02%, and B: 0.0005% to 0.02%, with the balance being iron and unavoidable impurities, and by area percentage containing 84.0% or more ferrite, 0.5% to 10.0% martensite, and 0.1% to 10.0% bainite.

[0005] Patent document 2 discloses a steel plate for tanks, which has the following composition by mass: C: 0.085% to 0.130%, Si: 0.04% or less, Mn: 0.10% to 0.60%, P: 0.02% or less, S: more than 0.010% and less than 0.020%, Al: 0.02% to 0.10%, N: 0.0005% to 0.0040%, Nb: 0.007% or more. The steel plate for this tank has the following composition: B content (mass%) ≤ 0.030%, B content ≥ 0.0010% ≤ 0.0050%, B / N ratio ≥ 0.80, balance Fe and unavoidable impurities, and has a ferrite structure containing ≥ 1.0% pearlite by area fraction. The yield stress of the steel plate is ≥ 500 MPa, the tensile strength is ≥ 550 MPa, the uniform elongation is ≥ 10%, and the yield elongation is ≤ 5.0%.

[0006] Existing technical documents Patent documents Patent Document 1: International Publication No. 2021 / 167023 Patent Document 2: International Publication No. 2020 / 105406 Summary of the Invention

[0007] The problem that the invention aims to solve The following problems can be identified with respect to the aforementioned prior art. In the technology described in Patent Document 1, a yield elongation of up to 10% is allowed. When performing complex processing on the tank body, wrinkles caused by tensile strain may occur. In addition, since the lower limit of the upper yield stress is 400 MPa and the lower limit of the tensile strength is 500 MPa, there is a problem that the tank body strength is insufficient when the steel plate is made thinner.

[0008] While the technology described in Patent Document 2 can produce high-strength steel plates with a yield stress of 500 MPa or more and a tensile strength of 550 MPa or more, it suffers from the problem of wrinkles caused by tensile strain due to the allowable yield elongation of up to 5.0%. Furthermore, to ensure the strength of the steel plate when used in the main body of a tank, a certain Rockwell surface hardness is required, but neither Patent Document 1 nor Patent Document 2 mentions any hardness.

[0009] The purpose of this invention is to provide a steel plate with high strength, high ductility and low yield elongation that solves the aforementioned problems, and a method for manufacturing the same.

[0010] Methods for solving problems The present invention was made to solve the above-mentioned problems, and its main purpose is as follows.

[0011] [1] A steel plate having the following composition, wherein the aforementioned composition contains, by mass percent: C: 0.03% to 0.15% Si: below 0.05% Mn: 0.10% to 0.60% P: below 0.025% S: Below 0.020% Al: below 0.20% N: 0.0001% to 0.0200% Nb: 0.005% to 0.030% The balance of the aforementioned components is Fe and unavoidable impurities. Martensite comprises 0.5% to 10.0% by area fraction, pearlite, bainite, and granular cementite comprise 5% to 30% by area fraction, and the balance is ferrite. Among the aforementioned granular cementite, the granular cementite with a maximum particle size of less than 3 μm accounts for more than 1.0% of the total microstructure in terms of area fraction.

[0012] [2] The steel plate as described in [1], wherein, in addition to the aforementioned composition, it contains, by mass percent, one or more of the following components: Ni: below 0.15% Mo: 0.050% or less, Cr: less than 0.10% Ti: below 0.02% B: Below 0.02% V: Below 0.02%.

[0013] [3] The steel plate as described in [1] or [2], wherein, in addition to the aforementioned composition, it also contains Sn: less than 0.020% by mass%.

[0014] [4] The steel plate described in any one of [1] to [3] has a yield stress of 450 MPa or more, a tensile strength of 550 MPa or more, an HR30T of 68 or more, an elongation at break of 10% or more, and an elongation at yield of 4.5% or less.

[0015] [5] A method for manufacturing a steel plate, which is a method for manufacturing a steel plate as described in any one of [1] to [4] above, comprising: The heating process involves heating the steel raw material with the aforementioned composition at a temperature above 1150°C; The hot rolling process involves hot rolling the steel raw material after the aforementioned heating process at a final rolling temperature of 800°C to 950°C and a winding temperature of 450°C to 700°C, followed by pickling. The cold rolling process involves cold rolling the hot-rolled sheet after the aforementioned hot rolling process at a rolling yield of 80% or higher; and In the annealing process, the cold-rolled sheet after the aforementioned cold rolling process is held at an annealing temperature of 700°C to 900°C for 5 to 90 seconds, and then cooled at an average cooling rate of 50°C / s to a cooling stop temperature range of 600°C or below.

[0016] Invention Effects This invention enables the manufacture of steel plates with high strength, high ductility, and low yield elongation. According to this invention, the steel plates for tanks can be further thinned, thus achieving a reduction in CO2 emissions during tank transportation due to tank lightweighting. Furthermore, since the formation of wrinkles caused by tensile strain is suppressed, more complex processing of the tank body is possible. Detailed Implementation

[0017] The composition, microstructure, mechanical properties, and manufacturing conditions of this invention are described. In the description of the composition, % indicates mass percentage. Furthermore, cases with excellent tensile strength, yield stress, and HR30T are referred to as high strength.

[0018] C: 0.03% to 0.15% Carbon (C) is an element that helps improve yield stress, tensile strength, and HR30T. Additionally, it reduces yield elongation by forming pearlite, bainite, martensite, and granular cementite. If the C content is below 0.03%, the proportions of pearlite, bainite, martensite, and granular cementite decrease, leading to a reduction in yield stress, tensile strength, and HR30T; therefore, the C content needs to be 0.03% or higher. The C content is preferably 0.06% or higher, more preferably 0.09% or higher. The C content is further preferably 0.10% or higher. On the other hand, if the C content exceeds 0.15%, the elongation at break decreases, and the yield elongation increases due to the increase in dissolved C. Therefore, the C content needs to be 0.15% or lower. The C content is preferably 0.14% or lower. To produce a steel sheet with high strength, high ductility, and low yield elongation, the C content is more preferably 0.13% or lower. The C content is further preferably 0.12% or lower.

[0019] Si: below 0.05% Si is an element that helps improve yield stress and tensile strength, but if the content exceeds 0.05%, corrosion resistance decreases. Therefore, the Si content needs to be below 0.05%, preferably below 0.04%, more preferably below 0.03%, and even more preferably below 0.02%. The lower limit is not particularly limited, but for the purpose of increasing the strength of the steel plate, the Si content is preferably above 0.01%.

[0020] Mn: 0.10% to 0.60% Mn is an element that improves hardenability and promotes the formation of pearlite, bainite, and martensite. Furthermore, it is known to contribute to improving yield stress, tensile strength, and HR30T through solid solution strengthening. If the Mn content is less than 0.10%, pearlite, bainite, and martensite cannot be sufficiently formed, resulting in decreased yield stress, tensile strength, and HR30T; therefore, the Mn content is 0.10% or more. The Mn content is preferably 0.12% or more, more preferably 0.15% or more. Additionally, to ensure sufficient strength, the Mn content is further preferably 0.30% or more. On the other hand, if the Mn content exceeds 0.60%, the elongation at break decreases; therefore, the Mn content is 0.60% or less. Preferably, it is 0.58% or less. More preferably, the Mn content is 0.57% or less, further preferably 0.56% or less, and most preferably 0.55% or less.

[0021] P: below 0.025% Phosphorus (P) reduces ductility through grain boundary segregation and hardening of the steel sheet; therefore, the P content is 0.025% or less. The P content is preferably 0.022% or less. More preferably, it is 0.021% or less, even more preferably 0.020% or less, and most preferably 0.019% or less. On the other hand, P helps to improve the yield stress and tensile strength of the steel sheet; therefore, it is preferable to contain 0.001% or more. The P content is more preferably 0.003% or more.

[0022] S: below 0.020% Sulfur (S) forms sulfides such as MnS and TiS in steel, reducing its ductility; therefore, the S content is 0.020% or less. The S content is preferably 0.018% or less. More preferably, it is 0.017% or less, even more preferably 0.016% or less, and most preferably 0.015% or less. While there is no particular limitation on the lower limit, it is preferably 0.005% or more to reduce manufacturing workload. The S content is more preferably 0.006% or more.

[0023] Al: below 0.20% Al is an element included to remove oxygen from steel. Furthermore, by forming AlN in the steel, the amount of dissolved nitrogen decreases, thus reducing the yield elongation. Therefore, there is no lower limit, but it is preferable to contain 0.02% or more. The Al content is more preferably 0.03% or more. On the other hand, if the Al content exceeds 0.20%, excessive alumina is formed, reducing ductility; therefore, the Al content is 0.20% or less. The Al content is preferably 0.15% or less, more preferably 0.12% or less. To simultaneously achieve high strength and high ductility, the Al content is further preferably 0.09% or less. The most preferably Al content is 0.08% or less. It should be noted that the Al mentioned here refers to the total Al content.

[0024] N: 0.0001% to 0.0200% Nitrogen (N) is an element that helps to improve yield stress and tensile strength through solid solution strengthening. Therefore, the N content is 0.0001% or more. The N content is preferably 0.0003% or more, more preferably 0.0005% or more, and even more preferably 0.0008% or more. The N content is most preferably 0.0010% or more. On the other hand, if the N content exceeds 0.0200%, the yield elongation increases due to solid solution N, therefore the N content is 0.0200% or less. The N content is preferably 0.0150% or less, more preferably 0.0100% or less. To simultaneously achieve high strength and low yield elongation, the N content is further preferably 0.0040% or less. The N content is most preferably 0.0035% or less.

[0025] Nb: 0.005% to 0.030% Nitrogen (Nb) is an element that contributes to precipitation strengthening and grain refinement strengthening by forming fine NbC particles in steel. Therefore, the Nb content is 0.005% or more. The Nb content is preferably 0.006% or more, more preferably 0.007% or more. To simultaneously achieve high strength and high ductility, the Nb content is further preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.030%, sufficient ductility is difficult to ensure due to the increased recrystallization temperature. Therefore, the Nb content is 0.030% or less. To simultaneously achieve high strength and high ductility, the Nb content is preferably 0.028% or less. The Nb content is more preferably 0.026% or less, more preferably 0.024% or less, and most preferably 0.022% or less.

[0026] The steel plate of the present invention preferably contains, in addition to the above-mentioned composition, one or more elements selected from the following (Ni: less than 0.15%, Mo: less than 0.050%, Cr: less than 0.10%, Ti: less than 0.02%, B: less than 0.02%, V: less than 0.02%).

[0027] Ni: less than 0.15%, Mo: less than 0.050%, Cr: less than 0.10% Ni, Mo, and Cr are elements that promote the formation of pearlite, bainite, and martensite by improving hardenability. On the other hand, excessive addition of these elements reduces ductility. To simultaneously achieve sufficient hardenability and high strength and high ductility, when Ni is present, the Ni content is 0.15% or less. The Ni content is preferably 0.14% or less, more preferably 0.13% or less. When Mo is present, the Mo content is 0.050% or less. The Mo content is preferably 0.048% or less, more preferably 0.047% or less, and even more preferably 0.046% or less. When Cr is present, the Cr content is 0.10% or less. The Cr content is preferably 0.09% or less, more preferably 0.08% or less. There is no particular limitation on the lower limit, but the Ni content is preferably 0.02% or more, more preferably 0.07% or more, and even more preferably 0.12% or more. The Mo content is preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. The Cr content is preferably 0.04% or more, more preferably 0.05% or more, and even more preferably 0.06% or more.

[0028] Ti: below 0.02%, B: below 0.02% Ti contributes to a reduction in yield elongation by reducing dissolved nitrogen (N) in steel through the formation of TiN. Furthermore, since TiN preferentially forms over BN, the presence of both Ti and B ensures sufficient B dissolution, thus improving hardenability. On the other hand, if both Ti and B content exceed 0.02% and B content exceed 0.02%, sufficient ductility becomes difficult to guarantee. To simultaneously achieve improved hardenability and high ductility due to B, the Ti content is 0.02% or less when Ti is present. More preferably, the Ti content is 0.018% or less, and even more preferably, 0.016% or less. When B is present, the B content is 0.02% or less. Preferably, the B content is 0.018% or less, and more preferably 0.016% or less. While there is no particular limitation on the lower limit, the Ti content is preferably 0.005% or more, and more preferably 0.008% or more. The B content is preferably 0.0005% or more, and more preferably 0.002% or more.

[0029] V: Below 0.02% V helps promote the formation of pearlite, bainite, and martensite by improving solid solution strengthening, precipitation strengthening, and hardenability, thereby increasing yield stress, tensile strength, and HR30T. On the other hand, if the V content exceeds 0.02%, ductility decreases with increasing strength. To achieve both high strength and high ductility, the V content is 0.02% or less when V is present. The V content is preferably 0.018% or less. More preferably, it is 0.017% or less, further preferably 0.016% or less, and most preferably 0.015% or less. There is no particular limitation on the lower limit, but the V content is preferably 0.004% or more. More preferably, it is 0.006% or more, and further preferably 0.008% or more.

[0030] In addition, the present invention preferably contains the following elements (Sn: less than 0.020%) in addition to the above-mentioned components.

[0031] Sn: below 0.020% Sn is an element that helps to increase the strength of steel plates through solid solution strengthening. When Sn is present, the Sn content is 0.020% or less. The Sn content is preferably 0.018% or less, more preferably 0.015% or less, and preferably 0.012% or less. The Sn content is preferably 0.001% or more. The Sn content is more preferably 0.002% or more, and even more preferably 0.003% or more.

[0032] According to one embodiment of the present invention, the steel plate has the following composition: containing the above-mentioned components, with the balance being Fe and unavoidable impurities. Examples of unavoidable impurities include Cu, Ca, O, H, Co, W, Zn, Pb, As, Sb, Bi, etc.

[0033] The microstructure of the steel plate in this invention will be described.

[0034] Martensite area fraction: 0.5% to 10.0% By including martensitic structure, tensile strength and HR30T are improved. To ensure sufficient strength, the area fraction of martensite is 0.5% or more. The area fraction of martensite is preferably 1.0% or more. The area fraction of martensite is more preferably 1.1% or more, and even more preferably 1.2% or more. On the other hand, if the area fraction of martensite exceeds 10.0%, the ductility decreases, therefore the area fraction of martensite is 10.0% or less. The area fraction of martensite is preferably 8.0% or less. The area fraction of martensite is more preferably 7.9% or less, even more preferably 7.7% or less, and most preferably 7.5% or less.

[0035] The sum of the area fractions of pearlite, bainite, and granular cementite: 5% to 30% Pearlite, bainite, and granular cementite not only contribute to improving yield stress, tensile strength, and HR30T, but also reduce yield elongation. To simultaneously achieve high strength and low yield elongation, the sum of the area fractions of pearlite, bainite, and granular cementite is 5% or more. Preferably, the sum of the area fractions of pearlite, bainite, and granular cementite is 10% or more. More preferably, the sum of the area fractions of pearlite, bainite, and granular cementite is 11% or more, and even more preferably 12% or more. On the other hand, if the sum of the area fractions of pearlite, bainite, and granular cementite exceeds 30%, ductility decreases; therefore, the sum of the area fractions of pearlite, bainite, and granular cementite is 30% or less. Preferably, the sum of the area fractions of pearlite, bainite, and granular cementite is 25% or less. More preferably, the sum of the area fractions of pearlite, bainite, and granular cementite is 23% or less. The sum of the area fractions of pearlite, bainite, and granular cementite is further preferably 20% or less.

[0036] The balance is ferrite. To obtain a steel sheet with high ductility, the allowance is set as ferrite. The area fraction of ferrite is preferably 69.5% or more. More preferably, it is 69.8% or more, and even more preferably 70.0% or more. Furthermore, the area fraction of ferrite is preferably 85.0% or less. More preferably, it is 84.8% or less, and even more preferably 84.5% or less.

[0037] Among the aforementioned granular cementite, the granular cementite with a maximum particle size of less than 3 μm constitutes more than 1.0% of the overall microstructure in terms of area fraction. Granular cementite with a maximum particle size of 3 μm or less suppresses the increase in yield elongation caused by excess solid solution C. To produce steel sheets with low yield elongation, the aforementioned granular cementite, in terms of area fraction relative to the overall microstructure, is 1.0% or more. Preferably, the aforementioned granular cementite, in terms of area fraction relative to the overall microstructure, is 1.1% or more, more preferably 1.2% or more, and even more preferably 1.3% or more. On the other hand, if coarse granular cementite is present, the yield stress and tensile strength decrease; therefore, the area fraction of fine granular cementite is important, and the area fraction of granular cementite with a maximum particle size of 3 μm or less needs to be limited. There is no particular upper limit, but granular cementite with a maximum particle size of 3 μm or less, in terms of area fraction, is preferably 5.0% or less. More preferably, granular cementite with a maximum particle size of 3 μm or less, in terms of area fraction, is 5.0% or less, even more preferably 4.5% or less, and most preferably 4.0% or less.

[0038] Furthermore, it is even more preferable to limit the area fraction of granular cementite with a maximum particle size of 2 μm or less, preferably 0.80% or more in terms of area fraction. More preferably, it is 0.82% or more in terms of area fraction. Additionally, it is preferably 0.85% or less in terms of area fraction, more preferably 0.83% or less in terms of area fraction.

[0039] The mechanical properties of the steel plate in this invention will be described.

[0040] Yield stress: ≥450MPa, tensile strength: ≥550MPa, HR30T: ≥68, elongation at break: ≥10%, yield elongation: ≤4.5% When thinning the steel plate used for the tank body, the yield stress (yield strength) of the steel plate needs to be 450 MPa or more to maintain sufficient tank strength. Additionally, the tensile strength needs to be 550 MPa or more. Furthermore, the HR30T needs to be 68 or more. It should be noted that the yield stress is preferably 480 MPa or more, more preferably 490 MPa or more, and even more preferably 500 MPa or more. Furthermore, the tensile strength is preferably 570 MPa or more, more preferably 580 MPa or more, and even more preferably 590 MPa or more. Furthermore, the HR30T is preferably 70 or more, more preferably 70.5 or more, and even more preferably 71 or more. To ensure workability when using the steel plate for the tank body, the elongation at break needs to be 10% or more, more preferably 12% or more. The elongation at break is further preferably 12.5% ​​or more, and most preferably 13% or more. There is no particular upper limit, but the yield stress is preferably 700 MPa or less, more preferably 690 MPa or less, and even more preferably 680 MPa or less. The tensile strength is preferably 800 MPa or less, more preferably 790 MPa or less, and even more preferably 780 MPa or less. The HR30T is preferably 80 or less, more preferably 79.5 or less, and even more preferably 79 or less. The elongation at break is preferably 25% or less. The elongation at break is more preferably 24% or less, and even more preferably 23% or less.

[0041] Furthermore, to suppress wrinkles caused by tensile strain during can manufacturing and processing, the yield elongation needs to be 4.5% or less. The yield elongation is preferably 4.0% or less. More preferably, it is 3.5% or less. Even more preferably, it is 3.0% or less. While there is no particular limitation on the lower limit, the yield elongation is preferably 1.0% or more. More preferably, it is 1.1% or more.

[0042] The manufacturing method of the steel plate in this invention will be described.

[0043] The method for manufacturing steel plates in this invention is characterized by comprising: a heating step, wherein steel raw materials having the above-mentioned composition are heated at 1150°C or higher; a hot rolling step, wherein the steel after the heating step is hot rolled at a final rolling temperature of 800°C or higher and 950°C or lower, and a winding temperature of 450°C or higher and 700°C or lower, and then pickled; a cold rolling step, wherein the hot-rolled plate after the hot rolling step is cold-rolled at a rolling rate of 80% or higher; and an annealing step, wherein the cold-rolled plate after the cold rolling step is held at an annealing temperature of 700°C or higher and 900°C or lower for 5 seconds or higher and 90 seconds, and then cooled to a temperature range of 600°C or lower.

[0044] Heating temperature: above 1150℃ At low heating temperatures during the heating process, there is a possibility of forming coarse nitrides such as AlN, which reduces the strength and ductility of the steel sheet. Therefore, the heating temperature is 1150°C or higher. Preferably, the heating temperature is 1170°C or higher. More preferably, it is 1200°C or higher. Even more preferably, it is 1230°C or higher, and most preferably, it is 1250°C or higher. While there is no upper limit to the heating temperature, from a manufacturing cost perspective, it is preferably 1300°C or lower. More preferably, it is 1280°C or lower.

[0045] Final rolling temperature: 800℃ or higher, 950℃ or lower If the final rolling temperature in the hot rolling process exceeds 950°C, the ferrite grain size of the hot-rolled sheet becomes coarse. This coarse ferrite grain size also affects the steel sheet after subsequent processes, making it difficult to ensure sufficient strength. Therefore, the final rolling temperature is 950°C or lower. The final rolling temperature is preferably 930°C or lower, more preferably 900°C or lower. The final rolling temperature is further preferably 895°C or lower, and most preferably 890°C or lower. On the other hand, when the final rolling temperature is below 800°C, rolling occurs in a two-phase region of ferrite and austenite. Therefore, the strength of the steel sheet decreases due to the formation of coarse ferrite grains and the precipitation of coarse Nb carbides during hot rolling. Therefore, the final rolling temperature in the hot rolling process is 800°C or higher. It should be noted that the final rolling temperature is preferably 820°C or higher, more preferably 850°C or higher.

[0046] Winding temperature: 450℃ to 700℃ If the winding temperature exceeds 700°C, the ferrite grain size becomes coarser, reducing the strength of the steel sheet. Furthermore, the formation of coarse alloy carbides is promoted, preventing the cementite from fully dissolving during the annealing process, resulting in the retention of coarse granular cementite and a decrease in the area fraction of granular cementite with a maximum grain size of 3 μm or less. Additionally, the reduced area fraction of martensite, pearlite, and bainite contributes to both decreased strength and increased yield elongation. Therefore, the winding temperature is 700°C or lower. A winding temperature of 670°C or lower is preferred, more preferably 650°C or lower. A winding temperature of 640°C or lower is further preferred, and most preferably 630°C or lower. On the other hand, if the winding temperature is below 450°C, the precipitation of alloy carbides such as Nb carbides decreases, leading to reduced strength. Furthermore, the reduced area fraction of granular cementite with a maximum grain size of 3 μm or less results in an increased yield elongation. Therefore, the winding temperature is 450°C or higher. The winding temperature is preferably 480°C or higher, more preferably 500°C or higher. The winding temperature is further preferably 510°C or higher, and most preferably 520°C or higher. After winding, the surface is acid-washed with an aqueous solution of H₂SO₄, HCl, H₃PO₄, etc., for the purpose of removing oxide scale.

[0047] Rolling rate in cold rolling: over 80% Cold rolling is performed after the aforementioned hot rolling process. This cold rolling process refines the ferrite grain size, increasing yield stress and tensile strength. To ensure sufficient yield stress and tensile strength, the rolling ratio is 80% or more. It should be noted that the rolling ratio is preferably 82% or more, more preferably 85% or more. The rolling ratio is further preferably 86% or more, and most preferably 88% or more. There is no upper limit to the rolling ratio, but to ensure sufficient ductility, it is preferably 95% or less. Furthermore, the rolling ratio is more preferably 93% or less.

[0048] Annealing temperature: 700℃ to 900℃; Holding time: 5s to 90s; Cooling stop temperature: below 600℃; Average cooling rate until reaching below 600℃ (cooling stop temperature range): 50℃ / s or higher. Annealing is performed after the aforementioned cold rolling process. To achieve high strength, high ductility, and low yield elongation by promoting the formation of pearlite, bainite, and martensite, the annealing temperature is 700°C or higher. The annealing temperature is preferably 720°C or higher. More preferably, it is 730°C or higher, even more preferably 735°C or higher, and most preferably 740°C or higher. On the other hand, if the annealing temperature exceeds 900°C, the yield stress and tensile strength decrease due to the coarsening of ferrite grain size and fine precipitates that contribute to precipitation strengthening. Therefore, the annealing temperature is 900°C or lower. It should be noted that the annealing temperature is preferably 850°C or lower, more preferably 830°C or lower, and even more preferably 800°C or lower.

[0049] When the holding time at the annealing temperature is less than 5 seconds, the formation of pearlite, bainite, and martensite becomes insufficient, and the desired steel sheet properties are not obtained. Therefore, the holding time is 5 seconds or more. Preferably, the holding time is 6 seconds or more, more preferably 7 seconds or more, further preferably 8 seconds or more, and most preferably 10 seconds or more. On the other hand, when the holding time is longer than 90 seconds, the ferrite grain size becomes coarser, and the yield stress and tensile strength decrease. Therefore, the holding time at the annealing temperature is 90 seconds or less. Preferably, the holding time is 85 seconds or less, more preferably 80 seconds or less, further preferably 75 seconds or less, and most preferably 70 seconds or less.

[0050] If the cooling stop temperature after annealing exceeds 600°C, martensite formation becomes insufficient, and tensile strength decreases. Therefore, the cooling stop temperature is below 600°C. The cooling stop temperature is preferably below 595°C, more preferably below 590°C, further preferably below 585°C, and most preferably below 580°C. On the other hand, even if the cooling stop temperature is below 300°C, the effect on the martensite area fraction is small, and a significant improvement in steel sheet properties cannot be expected. Therefore, from the viewpoint of manufacturing cost, the cooling stop temperature is preferably above 300°C. The cooling stop temperature is more preferably above 310°C, further preferably above 320°C, and most preferably above 330°C. It should be noted that, in order to transform the untransformed austenite into bainite and martensite, the temperature range after cooling stop can be maintained at 150°C or higher. The aforementioned holding temperature range is preferably above 170°C, more preferably above 200°C. Alternatively, the temperature range can be maintained below 600°C. The aforementioned holding temperature range is preferably below 590°C, more preferably below 580°C. To suppress the tempering of martensite, the holding time after cooling is stopped within a temperature range of 150°C to 600°C (whichever is the aforementioned preferred temperature range) is preferably 300 s or less. More preferably, the holding time is 280 s or less, further preferably 260 s or less, and most preferably 240 s or less. Furthermore, the holding time is preferably 200 s or more, and more preferably 220 s or more.

[0051] When the average cooling rate up to the cooling stop temperature is less than 50°C / s, martensite formation becomes insufficient, resulting in reduced strength. Furthermore, the formation of coarse granular cementite is promoted, thus reducing the area fraction of granular cementite with a maximum particle size of 3 μm or less. Therefore, the average cooling rate after annealing is 50°C / s or more. The average cooling rate after annealing is preferably 55°C / s or more, more preferably 60°C / s or more. To produce a high-strength steel sheet with low yield elongation by increasing the martensite area fraction, the average cooling rate after annealing is further preferably 80°C / s or more. The average cooling rate after annealing is most preferably 85°C / s or more. While there is no particular upper limit, to reduce manufacturing load, the average cooling rate is preferably 200°C / s or less. The average cooling rate after annealing is more preferably 195°C / s or less, and even more preferably 190°C / s or less.

[0052] Temper rolling can also be performed after the aforementioned annealing process. Temper rolling increases yield stress and reduces yield elongation; therefore, it is preferable to perform temper rolling with a reduction rate of 0.5% or more. A reduction rate of 0.6% or more is more preferred, further preferred to be 0.8% or more, and most preferably 0.9% or more is more preferred. On the other hand, increasing the reduction rate in temper rolling reduces ductility; therefore, a reduction rate of 10% or less is preferable. A reduction rate of 8% or less is more preferred, further preferred to be 6% or less, and most preferably 5% or less is more preferred.

[0053] Example The following are embodiments of the present invention. The present invention is not limited to the embodiments shown herein.

[0054] Steel containing the composition of steel grades No. 1 to 29 shown in Table 1, with the balance being Fe and unavoidable impurities, was smelted and cast to obtain steel billets. The steel billets obtained here were heated, hot-rolled, cold-rolled, and annealed under the conditions shown in Table 2, and then subjected to quenching and tempering rolling with a reduction of 1% to obtain steel plates No. 1 to 39.

[0055] JIS 5 tensile test specimens, with the rolling direction as the tensile direction, and 30 mm square test specimens for Rockwell surface hardness determination were collected from the aforementioned steel plate and subjected to aging heat treatment at 210°C for 10 minutes in a thermostat. Tensile tests were performed on the test specimens according to JIS Z 2241, and the yield stress, tensile strength, elongation at break, and yield elongation were evaluated. Furthermore, HR30T was determined by measuring the Rockwell surface hardness of the plate using HR15T and converting it using the conversion table in JIS G 3303 (2017).

[0056] Table 3 shows the evaluation results for yield stress, tensile strength, elongation at break, elongation at yield, and HR30T.

[0057] The following steps were followed to observe the microstructure of the steel plate. After collecting test pieces from the steel plate, the section parallel to the rolling direction was ground and etched with nitric acid and ethanol to reveal the microstructure. Samples for microstructure observation were collected. A scanning electron microscope (SEM) was used to observe the microstructure at 3000x magnification at a position half the plate thickness in the thickness direction. Images of the microstructure in three randomly selected fields of view were taken. Table 3 shows the area fractions of granular cementite, martensite, and the sum of the area fractions of pearlite, bainite, and granular cementite in the SEM images measured using image processing software. Here, martensite is defined as the phase present at ferrite grain boundaries, grain boundary triplets, and with a relatively smooth surface in the second phase. The sum of the fractions of pearlite, bainite, and granular cementite is set as the difference between the overall fraction of the second phase and the fraction of martensite. It should be noted that the area fractions shown in Table 3 are the average values ​​of the three fields of view. In addition, the determination of granular cementite was performed using the image processing software Image-J, and the size was measured using the scale of the SEM image.

[0058] The inventive examples in Table 3 all have the following characteristics: yield stress of 450 MPa or more, tensile strength of 550 MPa or more, HR30T of 68 or more, elongation at break of 10% or more, and yield elongation of 4.5% or less. Therefore, it can be said that the inventive examples are steel plates with high strength, high ductility, and low yield elongation, suitable as materials for cans. On the other hand, in comparative examples where one or more of the composition, area fraction of the steel plate structure, or manufacturing conditions are outside the scope of the invention, any one of the yield stress, tensile strength, HR30T, elongation at break, or yield elongation is outside the scope of the invention.

[0059] [Table 1] [Table 2] [Table 3]

Claims

1. A steel sheet having a composition consisting of, in mass %: C: 0.03% or more and 0.15% or less, Si: 0.05% or less, Mn: 0.10% or more and 0.60% or less, P: 0.025% or less, S: 0.020% or less, Al: 0.20% or less, N: 0.0001% or more and 0.0200% or less, Nb: 0.005% or more and 0.030% or less, the balance being Fe and unavoidable impurities, martensite being 0.5% or more and 10.0% or less in area fraction, the sum of pearlite, bainite and granular cementite being 5% or more and 30% or less in area fraction, and the balance being ferrite, of the granular cementite, a granular cementite having a maximum particle diameter of 3 μm or less is 1.0% or more in area fraction relative to the entire structure.

2. The steel sheet according to claim 1, wherein, in addition to the composition, one or more selected from the group consisting of, in mass %: Ni: 0.15% or less, Mo: 0.050% or less, Cr: 0.10% or less, Ti: 0.02% or less, B: 0.02% or less, V: 0.02% or less.

3. The steel sheet according to claim 1 or 2, wherein, in addition to the composition, Sn: 0.020% or less in mass %.

4. The steel sheet according to any one of claims 1 to 3, having a yield stress of 450 MPa or more, a tensile strength of 550 MPa or more, an HR30T of 68 or more, an elongation at break of 10% or more, and a yield elongation of 4.5% or less.

5. A method of producing a steel sheet according to any one of claims 1 to 4, comprising: a heating step of heating a steel material having the composition at 1150°C or more; a hot rolling step of hot rolling the steel material after the heating step at a finish rolling temperature of 800°C or more and 950°C or less and a coiling temperature of 450°C or more and 700°C or less, and performing pickling; a cold rolling step of cold rolling a hot rolled sheet after the hot rolling step at a rolling rate of 80% or more; and an annealing step of holding a cold rolled sheet after the cold rolling step at an annealing temperature of 700°C or more and 900°C or less for 5 s or more and 90 s or less, and cooling to a cooling stop temperature range of 600°C or less at an average cooling rate of 50°C / s or more. ​

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