Steel plate and method for manufacturing the same

Abstract

An object 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. 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, and the balance consists of Fe and unavoidable impurities, and has a component composition, martensite has an area fraction of 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, the balance is ferrite, and among the granular cementite, the granular cementite having a maximum particle size of 3 μm or less is 1.0% or more in area fraction with respect to the entire structure.

Description

Technical Field

[0001] The present invention relates to a steel sheet having high strength, high ductility, and low yield elongation, which is particularly suitable as a steel sheet for cans, and a method for manufacturing the same.

Background Art

[0002] In recent years, in order to reduce the CO 2 emission during can body transportation, weight reduction of the can body by reducing the gauge of the steel sheet for cans has been demanded. Since the strength of the can body decreases as the gauge is reduced, it is essential to increase the strength of the steel sheet.

[0003] As a high-strength steel sheet for cans, DR (Double Reduce) material is known. The DR material is a steel sheet for cans that has been strengthened by performing cold rolling and annealing and then performing cold rolling again. However, since it has low elongation and thus low workability, and since it is necessary to perform cold rolling again after annealing, there is a problem that the manufacturing cost becomes high. In order to solve these problems, development of an SR (Single Reduce) material having the same strength as the DR material and high ductility has been demanded.

[0004] Patent Document 1 proposes a steel sheet 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 inevitable impurities, and containing, by area ratio, 84.0% or more ferrite, 0.5% or more and 10.0% or less martensite, and 0.1% or more and 10.0% or less bainite.

[0005] Patent Document 2 discloses a steel sheet for cans having a component composition containing, by 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 0.020% 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, with B / N, which is the ratio of the content of B (by mass%) to the content of N (by mass%), being 0.80 or more, and the balance being Fe and unavoidable impurities, and having a ferrite structure containing pearlite with an area fraction of 1.0% or more, 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.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] The following problems exist in the above prior art. In the technology described in Patent Document 1, a yield elongation up to 10% is allowed, and wrinkles may occur due to stretcher strain when the can body is subjected to complex processing. Also, 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 can body strength becomes insufficient when the steel sheet is gauged down.

[0008] In the technique described in Patent Document 2, although a high-strength steel sheet with a yield stress of 500 MPa or more and a tensile strength of 550 MPa or more can be obtained, there is a problem of the occurrence of wrinkles caused by stretcher strain because a yield elongation of up to 5.0% is allowed. Further, in order to ensure the strength when the steel sheet is used for the body of a can, a certain Rockwell superficial hardness or more is required, but neither Patent Document 1 nor Patent Document 2 has a description regarding hardness.

[0009] An object of the present invention is to provide a steel sheet having high strength, high ductility, and low yield elongation, which solves the above problems, and a method for manufacturing the same.

Means for Solving the Problems

[0010] The present invention has been made to solve the above problems, and the gist thereof is as follows.

[0011] [1] 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 are contained, and the balance consists of Fe and unavoidable impurities, 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, a steel sheet in which the granular cementite having a maximum particle size of 3 μm or less has an area fraction of 1.0% or more with respect to the entire structure. [2] In addition to the above component composition, 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, V: containing one or more selected from 0.02% or less, the steel sheet according to [1]. [3] In addition to the above component composition, containing Sn: 0.020% or less in mass%, the steel sheet according to [1] or [2]. [4] The steel sheet according to any one of [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 for manufacturing the steel sheet according to any one of [1] to [4], a heating step of heating a steel material having the above component composition at 1150°C or higher, a hot rolling step of performing hot rolling on the steel material after the heating step under the 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, and then pickling, a cold rolling step of performing cold rolling on the hot rolled sheet after the hot rolling step under the condition of a rolling reduction of 80% or more, 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 s or more and 90 s or less, and then cooling to a cooling stop temperature range of 600°C or lower at an average cooling rate of 50°C / s or more.

Advantages of the Invention

[0012] The present invention enables the production of a steel sheet having high strength, high ductility, and low yield elongation. According to the present invention, further gauge reduction of the steel sheet for cans becomes possible, so that CO emissions during transportation of the can body can be reduced due to the weight reduction of the can body. 2 In addition, since the generation of wrinkles caused by stretcher strain is suppressed, it becomes possible to perform more complex processing on the can body.

Embodiments for Carrying Out the Invention

[0013] The component composition, structure, mechanical properties, and manufacturing conditions of the present invention will be described. % in the description of the component composition means mass%. In addition, a case where the tensile strength, yield stress, and HR30T are excellent is referred to as high strength.

[0014] C: Above 0.03% and below 0.15% C is an element that contributes to the improvement of yield stress, tensile strength, and HR30T. Also, it plays a role in reducing the yield elongation by forming pearlite, bainite, martensite, and granular cementite. When the C content is less than 0.03%, the fractions of pearlite, bainite, martensite, and granular cementite decrease, and the yield stress, tensile strength, and HR30T decrease. Therefore, the C content needs to be 0.03% or more. It is preferably 0.06% or more, more preferably 0.09% or more, and even more preferably 0.10% or more. On the other hand, when the C content exceeds 0.15%, the elongation at fracture decreases, and the yield elongation increases due to the increase in solid-solution C. Therefore, the C content needs to be 0.15% or less. It is preferably 0.14% or less, more preferably 0.13% or less, and even more preferably 0.12% or less in order to obtain a steel sheet with high strength, high ductility, and low yield elongation.

[0015] Si: 0.05% or less Si is an element that contributes to the improvement of yield stress and tensile strength, but when the content exceeds 0.05%, the corrosion resistance decreases. Therefore, the Si content needs to be 0.05% or less, preferably 0.04% or less, more preferably 0.03% or less, and even more preferably 0.02% or less. The lower limit does not need to be particularly limited, but for the purpose of increasing 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 also known to contribute to the improvement of yield stress, tensile strength, and HR30T by solid solution strengthening. When the Mn content is less than 0.10%, pearlite, bainite, and martensite are not sufficiently formed, and the yield stress, tensile strength, and HR30T decrease. Therefore, the Mn content should be 0.10% or more. The Mn content is preferably 0.12% or more, and more preferably 0.15% or more. Also, to ensure sufficient strength, the Mn content is more preferably 0.30% or more. On the other hand, when the Mn content exceeds 0.60%, the elongation at break decreases. Therefore, the Mn content should be 0.60% or less. It is preferably 0.58% or less. The Mn content is more preferably 0.57% or less, even more preferably 0.56% or less, and most preferably 0.55% or less.

[0017] P: 0.025% or less P reduces ductility due to grain boundary segregation and hardening of the steel sheet. Therefore, the P content should be 0.025% or less. The P content is preferably 0.022% or less. The P content is more preferably 0.021% or less, even more preferably 0.020% or less, and most preferably 0.019% or less. On the other hand, since P contributes to the improvement of the yield stress and tensile strength of the steel sheet, it is preferably contained at 0.001% or more. The P content is more preferably 0.003% or more.

[0018] S: 0.020% or less S forms sulfides such as MnS and TiS in the steel, reducing ductility. Therefore, the S content should be 0.020% or less. The S content is preferably 0.018% or less. The S content is more preferably 0.017% or less, even more preferably 0.016% or less, and most preferably 0.015% or less. The lower limit is not particularly limited, but for the purpose of reducing manufacturing load, it is preferably 0.005% or more. The S content is more preferably 0.006% or more.

[0019] Al: Below 0.20% Al is an element contained to remove oxygen in steel. Also, by forming AlN in steel, the dissolved N is reduced, and the yield elongation is decreased. Therefore, although there is no lower limit, it is preferably contained at 0.02% or more. The Al content is more preferably 0.03% or more. On the other hand, when the Al content exceeds 0.20%, excessive alumina is generated and the ductility decreases, so the Al content is 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 amount.

[0020] N: 0.0001% or more and 0.0200% or less N is an element that contributes to the improvement of yield stress and tensile strength by 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, even more preferably 0.0008% or more. The N content is most preferably 0.0010% or more. On the other hand, when the N content exceeds 0.0200%, the yield elongation increases due to dissolved N, so the N content is 0.0200% or less. The N content is preferably 0.0150% or less, and more preferably 0.0100% or less. To achieve both high strength and low yield elongation, the N content is even more preferably 0.0040% or less. The N content is most preferably 0.0035% or less.

[0021] Nb: 0.005% or more and 0.030% or less Nb contributes to precipitation strengthening and grain refinement strengthening by forming fine NbC in steel. Therefore, the Nb content should be 0.005% or more. The Nb content is preferably 0.006% or more, and more preferably 0.007% or more. To achieve both high strength and high ductility, the Nb content is more preferably 0.010% or more. On the other hand, when the Nb content exceeds 0.030%, it becomes difficult to ensure sufficient ductility due to the increase in the recrystallization temperature. Therefore, the Nb content should be 0.030% or less. To achieve both high strength and high ductility, the Nb content is preferably 0.028% or less. The Nb content is more preferably 0.026% or less, even more preferably 0.024% or less, and most preferably 0.022% or less.

[0022] In addition to the above component composition, the steel sheet in 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 promote the formation of pearlite, bainite, and martensite by improving hardenability. On the other hand, excessive addition of these elements reduces ductility. To achieve both sufficient hardenability and high strength and high ductility, when Ni is contained, the Ni content should be 0.15% or less. It is preferably 0.14% or less, and more preferably 0.13% or less. When Mo is contained, the Mo content should be 0.050% or less. It is preferably 0.048% or less, more preferably 0.047% or less, and even more preferably 0.046% or less. When Cr is contained, the Cr content should be 0.10% or less. It is preferably 0.09% or less, and more preferably 0.08% or less. The lower limit is not particularly limited, 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.

[0024] Ti: 0.02% or less, B: 0.02% or less Ti forms TiN in steel to reduce the dissolved N, contributing to the reduction of yield elongation. Also, since TiN is preferentially formed over BN, ensuring sufficient dissolved B by containing both Ti and B improves hardenability. On the other hand, when the Ti content exceeds 0.02% and the B content exceeds 0.02%, it becomes difficult to ensure sufficient ductility. To achieve both the improvement of hardenability by B and high ductility, when Ti is contained, the Ti content should be 0.02% or less. It is more preferable that the Ti content is 0.018% or less, and even more preferable that the Ti content is 0.016% or less. When B is contained, the B content should be 0.02% or less. It is preferable that the B content is 0.018% or less, and more preferable that it is 0.016% or less. The lower limit is not particularly limited, but 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.

[0025] V: 0.02% or less V contributes to the improvement of yield stress, tensile strength, and HR30T by solid solution strengthening, precipitation strengthening, and promoting the formation of pearlite, bainite, and martensite due to the improvement of hardenability. On the other hand, when the V content exceeds 0.02%, the ductility decreases with the increase in strength. To achieve both high strength and high ductility, when V is contained, the V content should be 0.02% or less. It is preferable that the V content is 0.018% or less. It is more preferable that the V content is 0.017% or less, even more preferable that the V content is 0.016% or less, and most preferable that the V content is 0.015% or less. The lower limit is not particularly limited, but the V content is preferably 0.004% or more. It is more preferable that the V content is 0.006% or more, and even more preferable that the V content is 0.008% or more.

[0026] Furthermore, in addition to the above component composition, the present invention preferably contains the following element (Sn: 0.020% or less).

[0027] Sn: 0.020% or less Sn is an element that contributes to the strengthening of the steel sheet by solid solution strengthening. When Sn is contained, the Sn content shall be 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.

[0028] The steel sheet according to an embodiment of the present invention contains the above components and has a component composition in which the balance consists of Fe and unavoidable impurities. Here, examples of the unavoidable impurities include Cu, Ca, O, H, Co, W, Zn, Pb, As, Sb, Bi, etc.

[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 By including a martensite structure, the tensile strength and HR30T are improved. To ensure sufficient strength, the area fraction of martensite shall be 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, when the area fraction of martensite exceeds 10.0%, the ductility decreases, so the area fraction of martensite shall be 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.

[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 the improvement of yield stress, tensile strength, and HR30T, and in addition, reduce yield elongation. In order to achieve both high strength and low yield elongation, the sum of the area fractions of pearlite, bainite, and granular cementite shall be 5% or more. The sum of the area fractions of pearlite, bainite, and granular cementite is preferably 10% or more. The sum of the area fractions of pearlite, bainite, and granular cementite is more preferably 11% or more, and even more preferably 12% or more. On the other hand, when the sum of the area fractions of pearlite, bainite, and granular cementite exceeds 30%, the ductility decreases. Therefore, the sum of the area fractions of pearlite, bainite, and granular cementite shall be 30% or less. The sum of the area fractions of pearlite, bainite, and granular cementite is preferably 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 balance is ferrite To obtain a steel sheet with high ductility, the balance 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. Also, 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] The granular cementite with a maximum particle size of 3 μm or less in the granular cementite has an area fraction of 1.0% or more with respect to the entire structure Granular cementite with a maximum particle size of 3 μm or less suppresses the increase in yield elongation due to excessive dissolved C. In order to obtain a steel sheet with low yield elongation, the granular cementite in the entire structure should have an area fraction of 1.0% or more. Preferably, the area fraction of the granular cementite in the entire structure is 1.1% or more, more preferably 1.2% or more, and even more preferably 1.3% or more. On the other hand, the presence of coarse granular cementite reduces the yield stress and tensile strength. Therefore, the area fraction of fine granular cementite is important, and it is necessary to limit the area ratio of granular cementite with a maximum particle size of 3 μm or less. The upper limit is not particularly limited, but preferably, the granular cementite with a maximum particle size of 3 μm or less has an area fraction of 5.0% or less. More preferably, the granular cementite with a maximum particle size of 3 μm or less has an area fraction of 5.0% or less, even more preferably 4.5% or less, and most preferably 4.0% or less.

[0034] Furthermore, it is more preferable to limit the area fraction of granular cementite with a maximum particle size of 2 μm or less. Preferably, the granular cementite with a maximum particle size of 2 μm or less has an area fraction of 0.80% or more. More preferably, the granular cementite with a maximum particle size of 2 μm or less has an area fraction of 0.82% or more. Also, preferably, the granular cementite with a maximum particle size of 2 μm or less has an area fraction of 0.85% or less, and more preferably 0.83% or less.

[0035] The mechanical properties of the steel sheet in 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 gauge of the steel sheet used for the can body is reduced, it is necessary to set the yield stress (yield strength) of the steel sheet to 450 MPa or more. Also, the tensile strength needs to be 550 MPa or more. Also, HR30T needs to be 68 or more. Note that it is preferable to set the yield stress to 480 MPa or more, more preferably 490 MPa or more, and even more preferably 500 MPa or more. Also, it is preferable to set the tensile strength to 570 MPa or more, more preferably 580 MPa or more, and even more preferably 590 MPa or more. Also, it is preferable to set HR30T to 70 or more, more preferably 70.5 or more, and even more preferably 71 or more. In order to ensure the workability when the steel sheet is used for the can body, the elongation at break needs to be 10% or more, and more preferably 12% or more. The elongation at break is even more preferably 12.5% or more, and most preferably 13% or more. The upper limit is not particularly limited, but it is preferable to set the yield stress to 700 MPa or less, more preferably 690 MPa or less, and even more preferably 680 MPa or less. It is preferable to set the tensile strength to 800 MPa or less, more preferably 790 MPa or less, and even more preferably 780 MPa or less. It is preferable to set HR30T to 80 or less, more preferably HR30T to 79.5 or less, and even more preferably HR30T to 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. Also, in order to suppress the generation of wrinkles caused by stretcher strain during can manufacturing and when processing the can body, it is necessary to set the yield elongation to 4.5% or less. The yield elongation is preferably 4.0% or less. The yield elongation is more preferably 3.5% or less. The yield elongation is even more preferably 3.0% or less. The lower limit is not particularly limited, but the yield elongation is preferably 1.0% or more. The yield elongation is more preferably 1.1% or more.

[0037] The manufacturing method of the steel sheet in the present invention will be described.

[0038] The method for manufacturing a steel sheet according to the present invention includes a heating step of heating a steel material having the above-described component composition at 1150°C or higher, a hot rolling step of performing hot rolling on the steel after the heating step under the 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, pickling the steel, a cold rolling step of performing cold rolling on the hot-rolled sheet after the hot rolling step under the condition of a rolling reduction of 80% or higher, and an annealing step of annealing the cold-rolled sheet after the cold rolling step by holding it at an annealing temperature of 700°C or higher and 900°C or lower for 5 s or longer and 90 s or shorter and then cooling it to a temperature range of 600°C or lower.

[0039] Heating temperature: 1150°C or higher When 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. Therefore, 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 even more preferably 1230°C or higher, and most preferably 1250°C or higher. Although the upper limit of the heating temperature is not limited, it is preferably 1300°C or lower from the viewpoint of manufacturing cost. The heating temperature is more preferably 1280°C or lower.

[0040] Finishing temperature: 800°C or higher and 950°C or lower When the finishing temperature in the hot rolling process exceeds 950°C, the ferrite grain size of the hot rolled sheet becomes coarser, and the ferrite grain size of the steel sheet after subsequent processes also becomes coarser, making it difficult to ensure sufficient strength. Therefore, the finishing temperature should be 950°C or lower. It is preferably 930°C or lower, and more preferably 900°C or lower. It is even more preferably 895°C or lower, and most preferably 890°C or lower. On the other hand, when the finishing temperature is less than 800°C, rolling occurs in the two-phase region of ferrite and austenite, resulting in coarse ferrite grains, and the strength of the steel sheet decreases due to factors such as the precipitation of coarse Nb carbides during hot rolling. Therefore, the finishing temperature in the hot rolling process should be 800°C or higher. It is preferably 820°C or higher, and more preferably 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 coarser and the strength of the steel sheet decreases. Furthermore, the formation of coarse alloy carbides is promoted, so that cementite cannot be sufficiently dissolved in the annealing process and coarse granular cementite remains, reducing the area fraction of granular cementite with a maximum grain size of 3μm or less. Furthermore, the area fractions of martensite, pearlite, and bainite decrease, leading to a decrease in strength and an increase in yield elongation. Therefore, the coiling temperature should be 700°C or lower. It is preferably 670°C or lower, and more preferably 650°C or lower. It is even more preferably 640°C or lower, and most preferably 630°C or lower. On the other hand, when the coiling temperature is less than 450°C, the amount of precipitation of alloy carbides such as Nb carbides decreases, resulting in a decrease in strength. Furthermore, the area fraction of granular cementite with a maximum grain size of 3μm or less decreases, leading to an increase in yield elongation. Therefore, the coiling temperature should be 450°C or higher. It is preferably 480°C or higher, and more preferably 500°C or higher. It is even more preferably 510°C or higher, and most preferably 520°C or higher. After coiling, for the purpose of removing scale, H 2 SO4 , HCl, H 3 PO 4 Perform pickling in an aqueous solution such as this.

[0042] Rolling ratio in cold rolling: 80% or more After the hot rolling process described above, perform cold rolling. By this cold rolling process, the ferrite grain size is refined and the yield stress and tensile strength increase. To ensure sufficient yield stress and tensile strength, the rolling ratio is set to 80% or more. Note 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. The upper limit of the rolling ratio is not limited, but it is preferably 95% or less to ensure sufficient ductility. Also, the rolling ratio is more preferably 93% or less.

[0043] Annealing temperature: 700°C or more and 900°C or less, holding time: 5 s or more and 90 s or less, cooling stop temperature: 600°C or less, average cooling rate up to 600°C or less (cooling stop temperature range): 50°C / s or more Perform annealing after the cold rolling process described above. 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 more. The annealing temperature is preferably 720°C or more. The annealing temperature is more preferably 730°C or more, further preferably 735°C or more, and most preferably 740°C or more. On the other hand, when the annealing temperature exceeds 900°C, the yield stress and tensile strength decrease due to the coarsening of the ferrite grain size and the fine precipitates contributing to precipitation strengthening. Therefore, the annealing temperature is 900°C or less. Note that the annealing temperature is preferably 850°C or less, more preferably 830°C or less, and further preferably 800°C or less.

[0044] When the holding time at the annealing temperature is less than 5 s, the formation of pearlite, bainite, and martensite becomes insufficient, and the required steel plate properties cannot be obtained. Therefore, the holding time should be 5 s or more. The holding time is preferably 6 s or more, more preferably 7 s or more, even more preferably 8 s or more, and most preferably 10 s or more. On the other hand, when the holding time is longer than 90 s, the ferrite grain size becomes coarse, and the yield stress and tensile strength decrease. Therefore, the holding time at the annealing temperature should be 90 s or less. The holding time is preferably 85 s or less, more preferably 80 s or less, even more preferably 75 s or less, and most preferably 70 s or less.

[0045] When the cooling stop temperature after annealing holding exceeds 600°C, the formation of martensite becomes insufficient and the tensile strength decreases. Therefore, the cooling stop temperature should be 600°C or lower. It is preferably 595°C or lower, more preferably 590°C or lower, still more preferably 585°C or lower, and most preferably 580°C or lower. On the other hand, even if the cooling stop temperature is less than 300°C, the influence on the area fraction of martensite is small, and a significant improvement in the steel plate properties is not expected. Therefore, from the perspective of manufacturing cost, the cooling stop temperature is preferably 300°C or higher. It is more preferably 310°C or higher, still more preferably 320°C or higher, and most preferably 330°C or higher. In addition, in order to transform the untransformed austenite into bainite or martensite, it may be held in a temperature range of 150°C or higher after cooling stop. The holding temperature range is preferably 170°C or higher, more preferably 200°C or higher. Also, it may be held in a temperature range of 600°C or lower. The holding temperature range is preferably 590°C or lower, more preferably 580°C or lower. In order to suppress the tempering of martensite, the holding time for holding in a temperature range of 150°C or higher and 600°C or lower (the temperature range if it is the preferred temperature range) after cooling stop is preferably 300 s or less. The holding time is more preferably 280 s or less, still more preferably 260 s or less, and most preferably 240 s or less. Also, the holding time is preferably 200 s or more, more preferably 220 s or more.

[0046] When the average cooling rate until the cooling stop temperature is less than 50°C / s, the formation of martensite becomes insufficient, resulting in a decrease in strength. Furthermore, since the formation of coarse granular cementite is promoted, the area fraction of granular cementite with a maximum particle size of 3 μm or less decreases. Therefore, the average cooling rate after annealing should be 50°C / s or more. It is preferably 55°C / s or more, and more preferably 60°C / s or more. In order to obtain a high-strength and low yield elongation steel sheet by increasing the area fraction of martensite, the average cooling rate after annealing is more preferably 80°C / s or more. Most preferably, the average cooling rate after annealing is 85°C / s or more. The upper limit is not particularly limited, but for reducing the manufacturing load, the average cooling rate is preferably 200°C / s or less. It is more preferably 195°C / s or less, and even more preferably 190°C / s or less.

[0047] It is also possible to perform temper rolling after the annealing process. By performing temper rolling, the yield stress can be increased and the yield elongation can be reduced. Therefore, it is preferable to perform temper rolling with a reduction ratio of 0.5% or more. The reduction ratio 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, when the reduction ratio in temper rolling is increased, the ductility decreases. Therefore, the reduction ratio is preferably 10% or less. The reduction ratio is more preferably 8% or less, even more preferably 6% or less, and most preferably 5% or less.

Examples

[0048] Examples of the present invention are shown below. The present invention is not limited to the examples shown here.

[0049] A steel slab was obtained by melting and casting a steel containing the components of Steel Grades No. 1 to 29 shown in Table 1, with the balance being Fe and inevitable impurities. The steel slab thus obtained was heated, hot-rolled, cold-rolled, and annealed under the conditions shown in Table 2, and then temper rolling with a reduction ratio of 1% was performed to obtain Steel Sheets No. 1 to 39.

[0050] From the above steel plate, a JIS No. 5 tensile test piece with the direction along the rolling direction as the tensile direction and a 30 mm square test piece used for measuring Rockwell superficial hardness were taken, and age heat treatment was carried out at 210 °C for 10 minutes in a thermostat. For the tensile test piece, a tensile test was carried out in accordance with JIS Z 2241, and the yield stress, tensile strength, elongation at break and yield elongation were evaluated. Also, HR30T was obtained by measuring the Rockwell superficial hardness of the plate surface with 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] Observation of the steel plate structure was carried out according to the following procedure. After taking a test piece from the steel plate, a cross-section parallel to the rolling direction was polished, nitrided to reveal the structure, and a sample for structure observation was taken. The position at 1 / 2 of the plate thickness in the plate thickness direction was observed at a magnification of 3000 times using a scanning electron microscope (SEM), and the structures of three randomly selected fields of view were photographed. 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 measured using image processing software in the SEM image. Here, among the second phases, those present at ferrite grain boundaries and grain boundary triple points and having a relatively smooth surface were defined as martensite. The fraction obtained by summing pearlite, bainite and granular cementite was taken as the difference obtained by subtracting the martensite fraction from the fraction of the entire second phase. Note that the area fractions shown in Table 3 are the average values of three fields of view. Also, the identification of granular cementite was carried out using the image processing software Image-J, and the size measurement was carried out using the scale bar at the time of SEM image photography.

[0053] In the invention examples shown in Table 3, all of them have 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. Therefore, it can be said that the invention examples are steel sheets suitable as materials for cans, having high strength, high ductility, and low yield elongation. On the other hand, in the comparative examples where one or more of the component composition, area fraction of the steel sheet structure, and manufacturing conditions are outside the scope of the invention, any one of the yield stress, tensile strength, HR30T, elongation at break, and yield elongation is outside the scope of the invention.

[0054]

Table 1

[0055]

Table 2

[0056]

Table 3

Claims

1. 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, with the balance being Fe and unavoidable impurities; The area fraction of martensite is 0.5% or more and 10.0% or less, the sum of the area fraction of pearlite, bainite, and granular cementite is 5% or more and 30% or less, and the remainder is ferrite, The steel plate has a structure in which the area fraction of the granular cementite having a maximum grain size of 3 μm or less is 1.0% or more of the entire structure.

2. In addition to the above component composition, 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, The steel sheet according to claim 1, further comprising at least one selected from the group consisting of: V: 0.02% or less;

3. In addition to the above component composition, in mass%, The steel sheet according to claim 1, containing Sn: 0.020% or less.

4. In addition to the component composition, in mass %, The steel sheet according to claim 2, containing Sn: 0.020% or less.

5. 5. The steel plate according to claim 1, wherein the steel plate has 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.

6. The method for producing a steel sheet according to any one of claims 1 to 4, a heating step of heating a steel material having the above-mentioned composition at 1150°C or higher; a hot rolling process in which the steel material after the heating process is hot rolled 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, and then pickled; A method for producing a steel sheet, comprising: a cold rolling step in which the hot-rolled sheet after the hot rolling step is cold-rolled at a rolling ratio of 80% or more; and an annealing step in which the cold-rolled sheet after the cold rolling step is held at an annealing temperature of 700°C or more and 900°C or less for 5 seconds or more and 90 seconds or less, and then cooled to a cooling stop temperature range of 600°C or less at an average cooling rate of 50°C / s or more.

7. A method for manufacturing the steel plate according to claim 5, a heating step of heating a steel material having the above-mentioned composition at 1150°C or higher; a hot rolling process in which the steel material after the heating process is hot rolled 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, and then pickled; A method for producing a steel sheet, comprising: a cold rolling step in which the hot-rolled sheet after the hot rolling step is cold-rolled at a rolling ratio of 80% or more; and an annealing step in which the cold-rolled sheet after the cold rolling step is held at an annealing temperature of 700°C or more and 900°C or less for 5 seconds or more and 90 seconds or less, and then cooled to a cooling stop temperature range of 600°C or less at an average cooling rate of 50°C / s or more.