STEEL PLATES AND METHODS OF STEEL PLATE MANUFACTURING
Patent Information
- Application Number
- VN1202601334
- Authority / Receiving Office
- VN · VN
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2023-08-31
- Filing Date
- 2024-08-27
- Publication Date
- 2026-07-15
Abstract
Description
Steel plate and its manufacturing method
[0001] The present invention relates to a steel sheet having high strength, high ductility and low yield point elongation, which is particularly suitable as a steel sheet for cans, and a method for producing the same.
[0002] In recent years, CO emissions during can transport have increased ２ To reduce CO2 emissions, there is a need to reduce the weight of can bodies by reducing the gauge of steel sheets used for cans. However, as the strength of can bodies decreases as the gauge is reduced, it is essential to increase the strength of steel sheets.
[0003] DR (Double Reduce) material is known as a high-strength steel sheet for cans. DR material is a steel sheet for cans that has been strengthened by cold rolling, annealing, and then cold rolling again. However, DR material has problems such as low workability due to small elongation, and high manufacturing costs due to the need to cold roll again after annealing. To solve these problems, there is a need to develop SR (Single Reduce) material that has the same strength as DR material but high ductility.
[0004] Patent Document 1 proposes a steel plate containing, by mass%, C: 0.03% or more and 0.13% or less, Si: 0.05% or less, Mn: 0.01% or more and 0.6% or less, P: 0.025% or less, S: 0.020% or less, Al: 0.01% or more and 0.20% or less, N: 0.0001% or more and 0.02% or less, Ti: 0.005% or more and 0.02% or less, and B: 0.0005% or more and 0.02% or less, with the balance being iron and unavoidable impurities, and containing, by area ratio, 84.0% or more of ferrite, 0.5% or more and 10.0% or less of martensite, and 0.1% or more and 10.0% or less of bainite.
[0005] Patent Document 2 describes, in mass %, C: 0.085% or more and 0.130% or less, Si: 0.04% or less, Mn: 0.10% or more and 0.60% or less, P: 0.02% or less, S: more than 0.010% and 0.0 20% or less, Al: 0.02% or more and 0.10% or less, N: 0.0005% or more and 0.0040% or less, Nb: 0.007% or more and 0.030% or less, B: 0.0010% or more and 0.0050% or less The present invention proposes a steel sheet for cans, which contains the following: a steel sheet for cans containing the above elements, in which B / N, which is the ratio of the B content (mass%) to the N content (mass%), is 0.80 or more, with the balance being Fe and unavoidable impurities; a steel sheet for cans having a ferrite structure containing pearlite in an area fraction of 1.0% or more; and a steel sheet for cans having a yield stress of 500 MPa or more, a tensile strength of 550 MPa or more, a uniform elongation of 10% or more, and a yield elongation of 5.0% or less.
[0006] International Publication No. 2021 / 167023 International Publication No. 2020 / 105406
[0007] The above-mentioned conventional techniques have the following problems. The technique described in Patent Document 1 allows a yield elongation of up to 10%, which may cause wrinkles due to stretcher strain when the can body is subjected to complex processing. In addition, the lower limit of the upper yield stress is 400 MPa and the lower limit of the tensile strength is 500 MPa, which poses a problem that the can body strength is insufficient when the steel sheet is gauged down.
[0008] The technology described in Patent Document 2 can produce a high-strength steel sheet having a yield stress of 500 MPa or more and a tensile strength of 550 MPa or more, but since a yield elongation of up to 5.0% is permitted, there is a problem of wrinkles occurring due to stretcher strain. Furthermore, in order to ensure the strength when the steel sheet is used for the body of a can, a certain level of Rockwell superficial hardness is required, but neither Patent Document 1 nor Patent Document 2 describes hardness.
[0009] An object of the present invention is to provide a steel sheet having high strength, high ductility and low yield point elongation, which solves the above problems, and a method for manufacturing the same.
[0010] The present invention has been made to solve the above problems, and the gist of the present invention is as follows.
[0011] [1] A steel plate having a chemical composition containing, by 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, with the balance being Fe and unavoidable impurities, wherein martensite has an area fraction of 0.5% or more and 10.0% or less, the sum of pearlite, bainite, and granular cementite has an area fraction of 5% or more and 30% or less, and the balance is ferrite, and among the granular cementite, granular cementite having a maximum grain size of 3 μm or less accounts for 1.0% or more of the area fraction of the entire structure. [2] The steel sheet according to [1], which contains, in addition to the chemical composition, one or more selected from 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, and V: 0.02% or less, by mass %. [3] The steel sheet according to [1] or [2], which contains, in addition to the chemical composition, Sn: 0.020% or less, by mass %. [4] The steel sheet according to any of [1] to [3], which 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 a yield elongation of 4.5% or less. [5] A method for producing a steel sheet according to any one of [1] to [4] above, comprising: a heating step of heating a steel material having the chemical composition at 1150°C or higher; a hot rolling step of hot rolling the steel material after the heating step under conditions of a finishing temperature of 800°C or higher and 950°C or lower and a coiling temperature of 450°C or higher and 700°C or lower, followed by pickling; a cold rolling step of cold rolling the hot-rolled sheet after the hot rolling step under conditions of a rolling ratio of 80% or higher; and an annealing step of holding the cold-rolled sheet after the cold rolling step at an annealing temperature of 700°C or higher and 900°C or lower for 5 seconds or higher and 90 seconds or lower, and then cooling it at an average cooling rate of 50°C / s or higher to a cooling stop temperature range of 600°C or lower.
[0012] The present invention has made it possible to manufacture steel sheets with high strength, high ductility and low yield point elongation. The present invention makes it possible to further reduce the gauge of steel sheets for cans, thereby reducing CO2 emissions during can body transportation due to lighter can bodies. ２Furthermore, the occurrence of wrinkles due to stretcher strain is suppressed, allowing for more complex processing of the can body.
[0013] The component composition, structure, mechanical properties, and manufacturing conditions of the present invention will be described. In the component composition description, % means mass %. Furthermore, when the tensile strength, yield stress, and HR30T are excellent, it is called high strength.
[0014] C: 0.03% or more and 0.15% or less C is an element that contributes to improving yield stress, tensile strength, and HR30T. It also plays a role in reducing yield elongation by forming pearlite, bainite, martensite, and granular cementite. If the C content is less than 0.03%, the fraction of pearlite, bainite, martensite, and granular cementite decreases, resulting in reduced yield stress, tensile strength, and HR30T. Therefore, the C content must be 0.03% or more. The C content is preferably 0.06% or more, and more preferably 0.09% or more. The C content is further preferably 0.10% or more. On the other hand, if the C content exceeds 0.15%, the fracture elongation decreases, and the yield elongation increases due to the increased amount of solute C. Therefore, the C content must be 0.15% or less. The C content is preferably 0.14% or less. In order to obtain a steel sheet with high strength, high ductility and low yield point elongation, the C content is more preferably 0.13% or less, and even more preferably 0.12% or less.
[0015] Si: 0.05% or less Si is an element that contributes to improving yield stress and tensile strength, but if the content exceeds 0.05%, corrosion resistance decreases. Therefore, the Si content must be 0.05% or less, preferably 0.04% or less, more preferably 0.03% or less, and even more preferably 0.02% or less. There is no need to particularly limit the lower limit, but in order to increase the strength of the steel sheet, the Si content is preferably 0.01% or more.
[0016] Mn: 0.10% or more and 0.60% or less Mn is an element that improves hardenability and promotes the formation of pearlite, bainite, and martensite. Furthermore, it is known that Mn contributes 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 are not sufficiently formed, resulting in reduced yield stress, tensile strength, and HR30T. Therefore, the Mn content is set to 0.10% or more. The Mn content is preferably set to 0.12% or more, and more preferably set to 0.15% or more. Furthermore, to ensure sufficient strength, the Mn content is further preferably set to 0.30% or more. On the other hand, if the Mn content exceeds 0.60%, the elongation at break decreases, so the Mn content is set to 0.60% or less. It is preferably set to 0.58% or less. The Mn content is more preferably 0.57% or less, further preferably 0.56% or less, and most preferably 0.55% or less.
[0017] P: 0.025% or less Since P reduces ductility due to grain boundary segregation and hardening of the steel sheet, the P content is set to 0.025% or less. The P content is preferably set to 0.022% or less. The P content is more preferably set to 0.021% or less, even more preferably set to 0.020% or less, and most preferably set to 0.019% or less. On the other hand, since P contributes to improving the yield stress and tensile strength of the steel sheet, it is preferable to contain 0.001% or more. The P content is more preferably set to 0.003% or more.
[0018] S: 0.020% or less S forms sulfides such as MnS and TiS in steel, reducing ductility, so the S content is set to 0.020% or less. The S content is preferably set to 0.018% or less. The S content is more preferably set to 0.017% or less, even more preferably set to 0.016% or less, and most preferably set to 0.015% or less. There is no particular lower limit, but in order to reduce the manufacturing load, it is preferably set to 0.005% or more. The S content is more preferably set to 0.006% or more.
[0019] Al: 0.20% or less Al is an element contained to remove oxygen from steel. It also forms AlN in steel, thereby reducing solute N and lowering yield point elongation. Therefore, although there is no lower limit, a content of 0.02% or more is preferable. The Al content is more preferably 0.03% or more. On the other hand, if the Al content exceeds 0.20%, excessive alumina is generated, reducing ductility. Therefore, the Al content is set to 0.20% or less. The Al content is preferably 0.15% or less, and more preferably 0.12% or less. To achieve both high strength and high ductility, the Al content is even more preferably 0.09% or less. The Al content is most preferably 0.08% or less. Note that Al here refers to the total Al content.
[0020] N: 0.0001% or more and 0.0200% or less N is an element that contributes to improving yield stress and tensile strength through solid solution strengthening. Therefore, the N content is set to 0.0001% or more. The N content is preferably set to 0.0003% or more, more preferably set to 0.0005% or more, and even more preferably set to 0.0008% or more. The N content is most preferably set to 0.0010% or more. On the other hand, if the N content exceeds 0.0200%, the yield elongation increases due to the solid solution N, so the N content is set to 0.0200% or less. The N content is preferably set to 0.0150% or less, and more preferably set to 0.0100% or less. To achieve both high strength and low yield elongation, the N content is more preferably set to 0.0040% or less. The N content is most preferably set to 0.0035% or less.
[0021] Nb: 0.005% or more and 0.030% or less Nb is an element that contributes to precipitation strengthening and grain refinement strengthening by forming fine NbC in steel. Therefore, the Nb content is set to 0.005% or more. The Nb content is preferably set to 0.006% or more, and more preferably set to 0.007% or more. In order to achieve both high strength and high ductility, the Nb content is further preferably set to 0.010% or more. On the other hand, if the Nb content exceeds 0.030%, it becomes difficult to ensure sufficient ductility due to an increase in the recrystallization temperature. Therefore, the Nb content is set to 0.030% or less. In order to achieve both high strength and high ductility, the Nb content is preferably set to 0.028% or less. The Nb content is more preferably set to 0.026% or less, even more preferably set to 0.024% or less, and most preferably set to 0.022% or less.
[0022] In addition to the above-mentioned chemical composition, the steel sheet of the present invention preferably contains one or more elements selected from the following (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).
[0023] Ni: 0.15% or less, Mo: 0.050% or less, Cr: 0.10% or less. Ni, Mo, and Cr are elements that improve hardenability and promote the formation of pearlite, bainite, and martensite. On the other hand, excessive addition of these elements reduces ductility. In order to achieve sufficient hardenability, high strength, and high ductility at the same time, when Ni is contained, the Ni content is set to 0.15% or less. The Ni content is preferably set to 0.14% or less, and more preferably set to 0.13% or less. When Mo is contained, the Mo content is set to 0.050% or less. The Mo content is preferably set to 0.048% or less, more preferably set to 0.047% or less, and even more preferably set to 0.046% or less. When Cr is contained, the Cr content is set to 0.10% or less. The Cr content is preferably set to 0.09% or less, and more preferably set to 0.08% or less. Although there is no particular lower limit, 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.
[0024] Ti: 0.02% or less, B: 0.02% or less. Ti contributes to reducing yield point elongation by forming TiN in steel and reducing solute N. Furthermore, because TiN is formed preferentially over BN, containing both Ti and B ensures sufficient solute B and improves hardenability. On the other hand, if the Ti content exceeds 0.02% and the B content exceeds 0.02%, it becomes difficult to ensure sufficient ductility. To achieve both improved hardenability due to B and high ductility, when Ti is contained, the Ti content is set to 0.02% or less. The Ti content is more preferably set to 0.018% or less, and even more preferably set to 0.016% or less. When B is contained, the B content is set to 0.02% or less. The B content is preferably set to 0.018% or less, and even more preferably set to 0.016% or less. Although there is no particular lower limit, the Ti content is preferably 0.005% or more, more preferably 0.008% or more, and the B content is preferably 0.0005% or more, more preferably 0.002% or more.
[0025] V: 0.02% or less V contributes to improving yield stress, tensile strength, and HR30T by promoting the formation of pearlite, bainite, and martensite through solid solution strengthening, precipitation strengthening, and improved hardenability. On the other hand, if the V content exceeds 0.02%, ductility decreases as strength increases. In order to achieve both high strength and high ductility, when V is contained, the V content is set to 0.02% or less. The V content is preferably set to 0.018% or less. The V content is more preferably set to 0.017% or less, even more preferably set to 0.016% or less, and most preferably set to 0.015% or less. Although there is no particular lower limit, the V content is preferably set to 0.004% or more. The V content is more preferably set to 0.006% or more, and even more preferably set to 0.008% or more.
[0026] Furthermore, in the present invention, in addition to the above-mentioned component composition, it is preferable to contain the following element (Sn: 0.020% or less).
[0027] Sn: 0.020% or less Sn is an element that contributes to increasing the strength of steel sheet through solid solution strengthening. When Sn is contained, the Sn content is set to 0.020% or less. The Sn content is preferably set to 0.018% or less, more preferably set to 0.015% or less, and preferably set to 0.012% or less. The Sn content is preferably set to 0.001% or more. The Sn content is more preferably set to 0.002% or more, and even more preferably set to 0.003% or more.
[0028] A steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance being Fe and unavoidable impurities, such as Cu, Ca, O, H, Co, W, Zn, Pb, As, Sb, and Bi.
[0029] The structure of the steel sheet in the present invention will be described.
[0030] Area fraction of martensite: 0.5% or more and 10.0% or less The inclusion of a martensite structure improves tensile strength and HR30T. To ensure sufficient strength, the area fraction of martensite is set to 0.5% or more. The area fraction of martensite is preferably set to 1.0% or more. The area fraction of martensite is more preferably set to 1.1% or more, and even more preferably set to 1.2% or more. On the other hand, if the area fraction of martensite exceeds 10.0%, ductility decreases, so the area fraction of martensite is set to 10.0% or less. The area fraction of martensite is preferably set to 8.0% or less. The area fraction of martensite is more preferably set to 7.9% or less, even more preferably set to 7.7% or less, and most preferably set to 7.5% or less.
[0031] Sum of area fractions of pearlite, bainite, and granular cementite: 5% or more and 30% or less Pearlite, bainite, and granular cementite contribute to improving yield stress, tensile strength, and HR30T, as well as reducing yield elongation. To achieve both high strength and low yield elongation, the sum of the area fractions of pearlite, bainite, and granular cementite is set to 5% or more. The sum of the area fractions of pearlite, bainite, and granular cementite is preferably set to 10% or more. The sum of the area fractions of pearlite, bainite, and granular cementite is more preferably set to 11% or more, and even more preferably set to 12% or more. On the other hand, if the sum of the area fractions of pearlite, bainite, and granular cementite exceeds 30%, ductility decreases, so the sum of the area fractions of pearlite, bainite, and granular cementite is set to 30% or less. The sum of the area fractions of pearlite, bainite, and granular cementite is preferably set to 25% or less. The sum of the area fractions of pearlite, bainite, and granular cementite is more preferably 23% or less, and even more preferably 20% or less.
[0032] The remainder is ferrite. To obtain a steel sheet with high ductility, the remainder is ferrite. The area fraction of ferrite is preferably 69.5% or more. The area fraction of ferrite is more preferably 69.8% or more, and even more preferably 70.0% or more. Furthermore, the area fraction of ferrite is preferably 85.0% or less. The area fraction of ferrite is more preferably 84.8% or less, and even more preferably 84.5% or less.
[0033] Among the granular cementite, granular cementite with a maximum grain size of 3 μm or less accounts for 1.0% or more in area fraction relative to the entire structure. The granular cementite with a maximum grain size of 3 μm or less suppresses an increase in yield elongation due to excessive solute C. To obtain a steel sheet with low yield elongation, the granular cementite has an area fraction of 1.0% or more relative to the entire structure. The granular cementite has an area fraction of preferably 1.1% or more, more preferably 1.2% or more, and even more preferably 1.3% or more relative to the entire structure. On the other hand, the presence of coarse granular cementite reduces yield stress and tensile strength, so the area fraction of fine granular cementite is important, and it is necessary to limit the area fraction of granular cementite with a maximum grain size of 3 μm or less. Although there is no particular upper limit, the area fraction of granular cementite with a maximum grain size of 3 μm or less is preferably 5.0% or less. The area fraction of granular cementite having a maximum grain size of 3 μm or less is more preferably 5.0% or less, even more preferably 4.5% or less, and most preferably 4.0% or less.
[0034] It is further preferable to limit the area fraction of granular cementite having a maximum particle size of 2 μm or less, and the area fraction of granular cementite having a maximum particle size of 2 μm or less is preferably 0.80% or more. The area fraction of granular cementite having a maximum particle size of 2 μm or less is more preferably 0.82% or more. The area fraction of granular cementite having a maximum particle size of 2 μm or less is preferably 0.85% or less, and more preferably 0.83% or less.
[0035] The mechanical properties of the steel sheet according to the present invention will be described.
[0036] Yield stress: 450 MPa or more, tensile strength: 550 MPa or more, HR30T: 68 or more, elongation at break: 10% or more, yield elongation: 4.5% or less In order to maintain sufficient can body strength when the steel sheet used for the can body is gauged down, the yield stress (yield strength) of the steel sheet must be 450 MPa or more. The tensile strength must also be 550 MPa or more. The HR30T must also be 68 or more. The yield stress is preferably 480 MPa or more, more preferably 490 MPa or more, and even more preferably 500 MPa or more. The tensile strength is preferably 570 MPa or more, more preferably 580 MPa or more, and even more preferably 590 MPa or more. The HR30T is preferably 70 or more, more preferably 70.5 or more, and even more preferably 71 or more. In order to ensure workability when used for can bodies, the steel sheet must have a breaking elongation of 10% or more, and more preferably 12% or more. The breaking elongation is more preferably 12.5% or more, and most preferably 13% or more. There are no particular upper limits, 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 breaking elongation is preferably 25% or less. The breaking elongation is more preferably 24% or less, and even more preferably 23% or less. Furthermore, in order to prevent wrinkles from occurring due to stretcher strain during can making and when the can body is processed, the yield point elongation must be 4.5% or less. The yield point elongation is preferably 4.0% or less. The yield point elongation is more preferably 3.5% or less. The yield point elongation is even more preferably 3.0% or less. Although there is no particular lower limit, the yield point elongation is preferably 1.0% or more. The yield point elongation is more preferably 1.1% or more.
[0037] The method for producing a steel sheet according to the present invention will be described.
[0038] The method for producing a steel sheet according to the present invention is characterized by comprising: a heating step of heating a steel material having the above-described chemical composition at 1150°C or higher; a hot rolling step of hot rolling the steel after the heating step under conditions of a finishing temperature of 800°C or higher and 950°C or lower and a coiling temperature of 450°C or higher and 700°C or lower, followed by pickling; a cold rolling step of cold rolling the hot-rolled sheet after the hot rolling step under conditions of a rolling ratio of 80% or higher; and an annealing step of holding the cold-rolled sheet after the cold rolling step at an annealing temperature of 700°C or higher and 900°C or lower for 5 seconds or higher and 90 seconds or lower, and then cooling it to a temperature range of 600°C or lower.
[0039] Heating temperature: 1150°C or higher If the heating temperature in the heating step is low, coarse nitrides such as AlN may be formed, which may reduce the strength and ductility of the steel sheet, so the heating temperature is set to 1150°C or higher. The heating temperature is preferably 1170°C or higher. The heating temperature is more preferably 1200°C or higher. The heating temperature is further preferably 1230°C or higher, and most preferably 1250°C or higher. There is no upper limit to the heating temperature, but from the viewpoint of manufacturing costs, it is preferably 1300°C or lower. The heating temperature is more preferably 1280°C or lower.
[0040] Finishing temperature: 800°C or higher and 950°C or lower If the finishing temperature in the hot rolling process exceeds 950°C, the ferrite grain size of the hot-rolled sheet will become coarse, and the ferrite grain size will also become coarse in the steel sheet that has undergone subsequent processes, making it difficult to ensure sufficient strength. Therefore, the finishing temperature is set to 950°C or lower. The finishing temperature is preferably set to 930°C or lower, and more preferably to 900°C or lower. The finishing temperature is further preferably set to 895°C or lower, and most preferably to 890°C or lower. On the other hand, if the finishing temperature is lower than 800°C, rolling will occur in the two-phase region of ferrite and austenite, resulting in the generation of coarse ferrite grains, and coarse Nb carbides will precipitate during hot rolling, thereby reducing the strength of the steel sheet. Therefore, the finishing temperature in the hot rolling process is set to 800°C or higher. Note that the finishing temperature is preferably set to 820°C or higher, and more preferably to 850°C or higher.
[0041] Coiling temperature: 450°C or higher and 700°C or lower. When the coiling temperature exceeds 700°C, the ferrite grain size becomes coarse, resulting in a decrease in the strength of the steel sheet. Furthermore, the formation of coarse alloy carbides is promoted, which prevents cementite from being sufficiently dissolved during the annealing process, leaving coarse granular cementite. This reduces the area fraction of granular cementite with a maximum grain size of 3 μm or less. Furthermore, the area fractions of martensite, pearlite, and bainite decrease, resulting in a decrease in strength and an increase in yield point elongation. Therefore, the coiling temperature is set to 700°C or lower. The coiling temperature is preferably set to 670°C or lower, and more preferably to 650°C or lower. The coiling temperature is more preferably set to 640°C or lower, and most preferably to 630°C or lower. On the other hand, when the coiling temperature is lower than 450°C, the amount of precipitation of alloy carbides such as Nb carbide decreases, resulting in a decrease in strength. Furthermore, the area fraction of granular cementite having a maximum particle size of 3 μm or less decreases, and therefore the yield point elongation increases. Therefore, the coiling temperature is set to 450°C or higher. The coiling temperature is preferably set to 480°C or higher, and more preferably set to 500°C or higher. The coiling temperature is further preferably set to 510°C or higher, and most preferably set to 520°C or higher. After coiling, the wire is heated by H ２ SO ４ , HCl, H ３ P.O. ４ The acid is picked with an aqueous solution such as
[0042] Reduction ratio in cold rolling: 80% or more Cold rolling is performed after the hot rolling process. This cold rolling process refines the ferrite grain size, increasing the yield stress and tensile strength. In order to ensure sufficient yield stress and tensile strength, the rolling ratio is set to 80% or more. The rolling ratio is preferably set to 82% or more, and more preferably to 85% or more. The rolling ratio is further preferably set to 86% or more, and most preferably to 88% or more. There is no upper limit to the rolling ratio, but it is preferably set to 95% or less in order to ensure sufficient ductility. Furthermore, the rolling ratio is more preferably set to 93% or less.
[0043] Annealing temperature: 700°C or higher and 900°C or lower, holding time: 5 seconds or higher and 90 seconds or lower, cooling stop temperature: 600°C or lower, average cooling rate to 600°C or lower (cooling stop temperature range): 50°C / s or higher. Annealing is performed after the cold rolling process. The annealing temperature is 700°C or higher to promote the formation of pearlite, bainite, and martensite, thereby achieving high strength, high ductility, and low yield elongation. The annealing temperature is preferably 720°C or higher. The annealing temperature is more preferably 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 ferrite grain size and fine precipitates that contribute to precipitation strengthening will coarsen, resulting in a decrease in yield stress and tensile strength. Therefore, the annealing temperature is 900°C or lower. The annealing temperature is preferably 850°C or lower, more preferably 830°C or lower, and even more preferably 800°C or lower.
[0044] If the holding time at the annealing temperature is less than 5 seconds, the formation of pearlite, bainite, and martensite will be insufficient, and the required steel sheet properties will not be obtained. Therefore, the holding time is set to 5 seconds or more. The holding time is preferably set to 6 seconds or more, more preferably set to 7 seconds or more, even more preferably set to 8 seconds or more, and most preferably set to 10 seconds or more. On the other hand, if the holding time is longer than 90 seconds, the ferrite grain size will coarsen, and the yield stress and tensile strength will decrease. Therefore, the holding time at the annealing temperature is set to 90 seconds or less. The holding time is preferably set to 85 seconds or less, more preferably set to 80 seconds or less, even more preferably set to 75 seconds or less, and most preferably set to 70 seconds or less.
[0045] If the cooling stop temperature after annealing exceeds 600°C, martensite formation becomes insufficient and tensile strength decreases, so the cooling stop temperature is set to 600°C or lower. The cooling stop temperature is preferably set to 595°C or lower, more preferably 590°C or lower, even more preferably 585°C or lower, and most preferably 580°C or lower. On the other hand, even if the cooling stop temperature is set to less than 300°C, the effect on the area fraction of martensite is small and a significant improvement in steel sheet properties is not expected. Therefore, from the viewpoint of manufacturing costs, the cooling stop temperature is preferably set to 300°C or higher. The cooling stop temperature is more preferably set to 310°C or higher, even more preferably 320°C or higher, and most preferably 330°C or higher. Note that, in order to transform untransformed austenite into bainite or martensite, the cooling may be held in a temperature range of 150°C or higher after cooling is stopped. The holding temperature range is preferably 170°C or higher, more preferably 200°C or higher. The cooling may also be held in a temperature range of 600°C or lower. The holding temperature range is preferably 590°C or less, and more preferably 580°C or less. In order to suppress tempering of martensite, the holding time in the temperature range of 150°C or more and 600°C or less (if it is the preferred temperature range) after cooling is stopped is preferably 300 seconds or less. The holding time is more preferably 280 seconds or less, even more preferably 260 seconds or less, and most preferably 240 seconds or less. The holding time is preferably 200 seconds or more, and more preferably 220 seconds or more.
[0046] If the average cooling rate to the cooling stop temperature is less than 50°C / s, martensite formation will be insufficient, resulting in a decrease in strength. Furthermore, the formation of coarse granular cementite will be promoted, resulting in a decrease in the area fraction of granular cementite with a maximum grain size of 3 μm or less. Therefore, the average cooling rate after annealing is set to 50°C / s or more. The average cooling rate after annealing is preferably set to 55°C / s or more, and more preferably set to 60°C / s or more. In order to obtain a steel sheet with high strength and low yield elongation by increasing the area fraction of martensite, the average cooling rate after annealing is more preferably set to 80°C / s or more. The average cooling rate after annealing is most preferably set to 85°C / s or more. Although the upper limit is not particularly limited, the average cooling rate is preferably set to 200°C / s or less to reduce the manufacturing load. The average cooling rate after annealing is more preferably set to 195°C / s or less, and even more preferably set to 190°C / s or less.
[0047] Temper rolling may be performed after the annealing step. Temper rolling can increase the yield stress and reduce the yield elongation, so it is preferable to perform temper rolling with a reduction of 0.5% or more. The reduction is more preferably 0.6% or more, even more preferably 0.8% or more, and most preferably 0.9% or more. On the other hand, since a large reduction in temper rolling reduces ductility, the reduction is preferably 10% or less. The reduction is more preferably 8% or less, even more preferably 6% or less, and most preferably 5% or less.
[0048] Examples of the present invention are shown below, but the present invention is not limited to the examples shown here.
[0049] Steel slabs were obtained by melting and casting steels containing the components of steel types No. 1 to 29 shown in Table 1, with the balance being Fe and unavoidable impurities. The steel slabs thus obtained were heated, hot rolled, cold rolled, and annealed under the conditions shown in Table 2, and then temper rolled at a reduction ratio of 1%, to obtain steel plates No. 1 to 39.
[0050] From the steel sheets, JIS No. 5 tensile test pieces with the tensile direction along the rolling direction and 30 mm square test pieces used for measuring Rockwell superficial hardness were taken and subjected to aging heat treatment at 210°C for 10 minutes in an incubator. Tensile tests in accordance with JIS Z 2241 were carried out on the tensile test pieces to evaluate the yield stress, tensile strength, fracture elongation, and yield elongation. HR30T was determined by measuring the Rockwell superficial hardness of the sheet surface at HR15T and converting it using the conversion table of JIS G 3303 (2017).
[0051] Table 3 shows the evaluation results of yield stress, tensile strength, elongation at break, yield elongation and HR30T.
[0052] The steel sheet microstructure was observed according to the following procedure. After a test specimen was taken from the steel sheet, the cross section parallel to the rolling direction was polished and subjected to nital etching to reveal the microstructure, and a sample for microstructure observation was taken. A scanning electron microscope (SEM) was used to observe the steel sheet at a position halfway through the thickness direction at 3000x magnification, and images of the microstructure were taken from three randomly selected fields of view. Table 3 shows the area fraction of granular cementite, the area fraction of 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 was defined as the second phase present at ferrite grain boundaries and grain boundary triple junctions and having a relatively smooth surface. The combined fraction of pearlite, bainite, and granular cementite was calculated by subtracting the martensite fraction from the fraction of the entire second phase. The area fractions shown in Table 3 are the average values of the three fields of view. Granular cementite was identified using image processing software Image-J, and its size was measured using the scale bar used when photographing the SEM image.
[0053] All of the inventive examples in Table 3 have a yield stress of 450 MPa or more, a tensile strength of 550 MPa or more, an HR30T of 68 or more, a fracture elongation of 10% or more, and a yield elongation of 4.5% or less. Therefore, the inventive examples can be said to be steel sheets with high strength, high ductility, and low yield elongation, suitable for use as can materials. On the other hand, the comparative examples, in which one or more of the chemical composition, the area fraction of the steel sheet structure, and the manufacturing conditions were outside the ranges of the invention, had any of the yield stress, tensile strength, HR30T, fracture elongation, and yield elongation outside the ranges of the invention.
[0054]
[0055]
[0056]
Claims
1. A steel plate having a composition containing, by 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, with the balance being Fe and unavoidable impurities, wherein the area fraction of martensite is 0.5% or more and 10.0% or less, the sum of pearlite, bainite and granular cementite is 5% or more and 30% or less in area fraction, and the balance is ferrite, and among the granular cementite, granular cementite having a maximum grain size of 3 μm or less accounts for 1.0% or more of the area fraction of the entire structure.
2. The steel plate according to claim 1, containing, in addition to the above-mentioned chemical composition, one or more selected from, by 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, and V: 0.02% or less.
3. The steel sheet according to claim 1 or 2, further comprising, in addition to the above-mentioned chemical composition, the following mass percent: Sn: 0.020% or less.
4. A steel plate 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, a fracture elongation of 10% or more, and a yield elongation of 4.5% or less.
5. A method for producing a steel sheet according to any one of claims 1 to 4, comprising: a heating step of heating a steel material having the above-mentioned chemical composition at 1150°C or higher; a hot rolling step of hot rolling and pickling the steel material after the heating step under conditions of a finishing temperature of 800°C to 950°C and a coiling temperature of 450°C to 700°C; a cold rolling step of cold rolling the hot rolled sheet after the hot rolling step under conditions of a rolling ratio of 80% or higher; and an annealing step of holding the cold rolled sheet after the cold rolling step at an annealing temperature of 700°C to 900°C for 5 s to 90 s, and then cooling it at an average cooling rate of 50°C / s or higher to a cooling stop temperature range of 600°C or lower.