STEEL PLATES AND METHODS OF STEEL PLATE PRODUCTION

Abstract

The invention aims to propose a steel plate and a method for producing a steel plate; this steel plate has superior ductility and expansion properties and also has a yield strength (YP) of 450 MPa or greater.The steel plate has a chemical composition containing, in % by mass, C: 0.05% or greater and 0.13% or less, Si: 0.01% or greater and 0.04% or less, Mn: 0.1% or greater and 0.6% or less, P: 0.02% or less, S: 0.02% or less, Al: 0.01% or greater and 0.10% or less, N: 0.0005% or greater and 0.0040% or less, Ti: 0.005% or greater and 0.030% or less, Nb: 0.005% or greater and 0.030% or less, Mo: 0.01% or greater and 0.05% or less, and B: 0.0005% or greater and 0.0050% or less, with the remainder being iron and unavoidable impurities, in which the ferrite structure is present as a microstructure with an area ratio of 90% or greater, and, in the total amount of Ti and Nb, the total amount of Ti and Nb present as carbonitrite is 0.005% or greater by mass percentage.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a high-strength steel sheet for can use, which has excellent can expandability, a yield stress (YP) of 450 MPa or more, and an elongation of 20% or more, and a method for producing the same.

[0002] In recent years, efforts have been made to reduce the thickness of steel sheets for cans by increasing their strength in order to reduce costs. Specifically, the application of high-strength steel sheets with a YP of 450 MPa or more to cans has been considered. When such high strength is required, DR (double-reduced) materials, which are subjected to a second cold rolling after annealing, are sometimes used instead of SR (single-reduced) materials, which have been conventionally used in can steel sheets. However, although high strength can be achieved by performing the second cold rolling, ductility is reduced, making it impossible to use for three-piece irregular cans, which require expandability. In addition, the two-time rolling process results in higher costs than SR materials.

[0003] In response to such demands, for example, Patent Document 1 describes a composition of elements, in mass %, as follows: 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 0.005% or less, The present invention discloses a steel sheet for cans having a high strength and excellent formability, which has a component composition consisting of Fe and unavoidable impurities, a B / N ratio of the B content (mass%) to the N content (mass%) of 0.80 or more, and a ferrite structure containing pearlite at an area fraction of 1.0% or more, and which has 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.

[0004] Patent Document 2 discloses a steel sheet for cans having small yield point elongation, high ductility, and high strength, in which a steel containing, as a component composition, in mass %, C: 0.03 to 0.13%, Si: 0.03% or less, Mn: 0.3 to 0.6%, P: 0.02% or less, Al: 0.1% or less, N: 0.012% or less, and further one or more of Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, and B: 0.0005 to 0.005% is hot-rolled, cooled at a cooling rate of 40°C / s or less, and coiled at 550°C or higher, thereby promoting cementite precipitation after cold-rolling and recrystallization annealing, and as a result, the yield point elongation is set to 5% or less.

[0005] International Publication No. 2020 / 105406 Japanese Patent Application Laid-Open No. 2008-274332

[0006] However, in Patent Document 1, the lower limit of elongation is 10%, and ductility at the lower limit level is insufficient, making it difficult to use for irregular-shaped cans that require can expandability.

[0007] Furthermore, although Patent Document 2 mentions strength and elongation, it does not evaluate can expandability, and the relative merits of can expandability are unclear.

[0008] In view of the above-mentioned problems associated with the prior art, an object of the present invention is to provide a steel sheet having excellent ductility and expandability and a yield strength (YP) of 450 MPa or more, and a method for producing the same.

[0009] Here, excellent ductility in the present invention means that the elongation (EL) is 20% or more.

[0010] The inventors conducted extensive research to solve the above-mentioned problems. As a result, they found that by adjusting the steel composition, the amount of ferrite in the metal structure, and the amount of Nb and Ti precipitated as carbonitrides in the ferrite, a high-strength steel sheet with excellent ductility and expandability and a YP of 450 MPa or more can be obtained. By controlling the amount of Ti and Nb precipitated as carbonitrides, the strength of ferrite can be increased and the difference in strength between ferrite and other structures can be reduced. This makes it possible to suppress cracking during can expansion, resulting in a high-strength steel sheet that is optimal for the can body of a three-piece irregular can that is expanded. Furthermore, they also found that the amounts of Ti and Nb precipitated as carbonitrides can be adjusted by strictly controlling the manufacturing conditions, such as the coiling temperature in the hot rolling process and the heating rate in the annealing process.

[0011] The present invention has been made to solve the above-mentioned problems, and is summarized as follows: [1] A steel sheet containing, by mass%, C: 0.05% to 0.13%, Si: 0.01% to 0.04%, Mn: 0.1% to 0.6%, P: 0.02% or less, S: 0.02% or less, Al: 0.01% to 0.10%, N: 0.0005% to 0.0040%, Ti: 0.005% to 0.030%, Nb: 0.005% to 0.030%, Mo: 0.01% to 0.05%, and B: 0.0005% to 0.0050%, with the balance consisting of iron and unavoidable impurities, and having a ferrite structure with an area ratio of 90% or more as a microstructure, [2] A steel sheet according to [1], further comprising, in addition to the above-mentioned chemical composition, one or two selected from the group consisting of Cr: 0.005% or more and 0.100% or less, and Ni: 0.005% or more and 0.150% or less, in mass %. [3] A method for manufacturing a steel sheet, comprising: a hot rolling step of hot rolling a slab having the chemical composition according to [1] or [2] above under conditions of a finishing temperature of 800°C or more and 950°C or less and a coiling temperature of 550°C or more and 750°C or less; a cold rolling step of cold rolling the hot-rolled sheet that has undergone the hot rolling step at a rolling reduction of 85% or more; an annealing step of heating the cold-rolled sheet that has undergone the cold rolling step at an average heating rate of 5 to 40°C / sec to an annealing temperature of 640°C or more and 760°C or less, holding the cold-rolled sheet at the annealing temperature, and then cooling the cold-rolled sheet at an average cooling rate of 15°C / sec or more to a cooling stop temperature that is lower than the annealing temperature and is 450°C or more and 650°C or less; and a temper rolling step of temper rolling the annealed sheet that has undergone the annealing step at a temper rolling reduction of 0.5% or more and 5.0% or less.

[0012] According to the present invention, it is possible to provide a steel sheet having a yield strength of 450 MPa or more and excellent can expandability and ductility. Because the steel sheet of the present invention has excellent ductility, it is suitable for use as a steel sheet for cans that are formed into complex shapes, for example, as a three-piece irregular can that is subjected to can expanding processing. Furthermore, by applying parts manufactured according to the present invention to cans, further improvements in strength and weight reduction will be achieved, which will greatly contribute to the development of the industry.

[0013] The chemical composition of the steel sheet of the present invention, the appropriate range of the structure, and the reasons for limiting the range will be explained below. Note that "%" representing the chemical composition below means "mass %" unless otherwise specified. In addition, a steel sheet that is excellent in both ductility and expandability may also be simply referred to as having excellent workability.

[0014] C: 0.05% or more and 0.13% or less C is an element that contributes to strength and increases the strength of steel by dissolving in solid solution or precipitating as carbides in steel. To utilize these effects to obtain the desired yield strength, a C content of 0.05% or more is necessary. Therefore, the C content is set to 0.05% or more. Preferably, the C content is set to 0.06% or more, and more preferably, 0.08% or more. On the other hand, a C content exceeding 0.13% may result in a decrease in ductility due to an increase in yield strength and may impair weldability. Therefore, the C content is set to 0.13% or less. Preferably, the C content is set to 0.12% or less, more preferably, 0.11% or less, and even more preferably, 0.10% or less.

[0015] Si: 0.01% or more and 0.04% or less Si contributes to increasing the strength of steel through solid solution strengthening. In order to utilize these effects to obtain the desired yield strength, it is necessary to include 0.01% or more of Si. Therefore, the Si content is set to 0.01% or more. On the other hand, a content exceeding 0.04% may cause serious problems with corrosion resistance and surface properties. Therefore, the Si content is set to 0.04% or less. Preferably, it is 0.03% or less, and more preferably, it is 0.02% or less.

[0016] Mn: 0.1% or more and 0.6% or less Mn is an element that contributes to strength, and dissolves in steel to contribute to high strength. In order to obtain the strength targeted by the present invention, it is necessary to include 0.1% or more. Therefore, the Mn content is set to 0.1% or more. It is preferably 0.2% or more, more preferably 0.3% or more, and even more preferably 0.4% or more. On the other hand, a content exceeding 0.6% leads to a decrease in ductility. Therefore, the Mn content is set to 0.6% or less.

[0017] P: 0.02% or less P is inevitably mixed into steel and is an effective element for strengthening steel, so in this case it is preferable to include it at 0.001% or more. The P content is more preferably 0.002% or more, and even more preferably 0.004% or more. On the other hand, since P reduces weldability, the P content is set to 0.02% or less. Preferably, it is 0.018% or less. More preferably, it is 0.017% or less, and even more preferably, it is 0.016% or less.

[0018] S: 0.02% or less S is inevitably mixed into steel and forms coarse inclusions such as MnS, significantly reducing local ductility. Therefore, the S content is set to 0.02% or less. Preferably, it is set to 0.018% or less. Note that setting the S content to less than 0.001% would incur excessive costs for refining the steel. Therefore, the S content is preferably set to 0.001% or more. The S content is more preferably set to 0.003% or more, and even more preferably set to 0.005% or more.

[0019] Al: 0.01% or more and 0.10% or less Al acts as a deoxidizer, and to obtain this effect, a content of 0.01% or more is necessary. Therefore, the Al content is set to 0.01% or more. Preferably, it is set to 0.03% or more. On the other hand, if a large amount of Al is added, especially if the amount of Al exceeds 0.10%, the manufacturing cost becomes excessive. Therefore, the Al content is set to 0.10% or less. Preferably, it is set to 0.08% or less. More preferably, it is set to 0.07% or less.

[0020] N: 0.0005% or more and 0.0040% or less N combines with carbonitride-forming elements such as Al, Nb, and Ti to form precipitates, contributing to improved strength. To achieve this effect, a N content of 0.0005% or more is necessary. Preferably, the N content is 0.0006% or more. More preferably, the N content is 0.0007% or more, and even more preferably, 0.0010% or more. On the other hand, if N is contained in a large amount exceeding 0.0040%, aging resistance will be reduced. For this reason, the N content is set to 0.0040% or less. Preferably, the N content is 0.0035% or less. More preferably, the N content is 0.0034% or less, even more preferably, the N content is 0.0033% or less, and most preferably, the N content is 0.0032% or less.

[0021] Ti: 0.005% or more and 0.030% or less Ti is one of the important additive elements in the present invention because it combines with C and N to form carbonitrides and contributes to improving strength. It also suppresses the formation of BN and enhances the strength-improving effect of B segregation at grain boundaries. To achieve this effect, a Ti content of 0.005% or more is necessary. Therefore, the Ti content is set to 0.005% or more. Preferably, the Ti content is set to 0.010% or more. The Ti content is more preferably 0.011% or more, and even more preferably 0.012% or more. On the other hand, a Ti content exceeding 0.030% leads to a decrease in ductility. Therefore, the Ti content is set to 0.030% or less. Preferably, the Ti content is set to 0.025% or less. More preferably, the Ti content is set to 0.024% or less, even more preferably 0.023% or less, and most preferably 0.022% or less.

[0022] Nb: 0.005% or more and 0.030% or less Nb is one of the important additive elements in the present invention, as it combines with C and N to form carbonitrides, contributing to improved strength. To achieve this effect, a content of 0.005% or more is necessary. Therefore, the Nb content is set to 0.005% or more. Preferably, it is set to 0.010% or more. More preferably, it is set to 0.011% or more, and even more preferably, it is set to 0.012% or more. On the other hand, if Nb is contained in a large amount exceeding 0.030%, it will cause a decrease in ductility. Therefore, the Nb content is set to 0.030% or less. Preferably, it is set to 0.025% or less. More preferably, it is set to 0.024% or less, and even more preferably, it is set to 0.023% or less.

[0023] Mo: 0.01% or more and 0.05% or less Mo contributes to improving strength by dissolving in steel or precipitating as carbides. To obtain this effect, a content of 0.01% or more is necessary. Therefore, the Mo content is set to 0.01% or more. Preferably, it is set to 0.02% or more. More preferably, it is set to 0.021% or more. On the other hand, Mo is a very expensive element, and excessive inclusion leads to increased costs. Therefore, the Mo content is set to 0.05% or less. More preferably, it is set to 0.047% or less, and even more preferably, it is set to 0.045% or less.

[0024] B: 0.0005% or more and 0.0050% or less B contributes to improving strength by segregating at grain boundaries. To obtain this effect, a B content of 0.0005% or more is necessary. Therefore, the B content is set to 0.0005% or more. Preferably, it is 0.0010% or more. More preferably, it is 0.0012% or more, even more preferably, it is 0.0013% or more, and most preferably, it is 0.0014% or more. On the other hand, even if B is added in a large amount exceeding 0.0050%, the effect saturates. Therefore, the B content is set to 0.0050% or less. Preferably, it is 0.0040% or less. More preferably, it is 0.0038% or less, even more preferably, it is 0.0037% or less, and most preferably, it is 0.0036% or less.

[0025] The steel sheet of the present invention can obtain the desired properties with the above-mentioned essential elements, but in addition to the above-mentioned essential elements, the following elements can be contained as necessary.

[0026] Cr: 0.005% or more and 0.100% or less, Ni: 0.005% or more and 0.150% or less. Cr and Ni have the effect of improving hardenability and are therefore useful as strengthening elements for steel. To effectively exert this effect, it is preferable that Cr and Ni are each contained in an amount of 0.005% or more. Therefore, when Cr and Ni are contained, the Cr content is 0.005% or more and the Ni content is 0.005% or more. Preferably, the Cr content is 0.010% or more and the Ni content is 0.010% or more. More preferably, the Cr content is 0.011% or more and the Ni content is 0.011% or more, and even more preferably, the Cr content is 0.012% or more and the Ni content is 0.012% or more. On the other hand, Cr and Ni are expensive elements, and if their respective upper limits are exceeded, further improvement in the effect cannot be expected, so when Cr and Ni are contained, the Cr content is set to 0.100% or less and the Ni content is set to 0.150% or less. Preferably, the Cr content is 0.090% or less and the Ni content is 0.130% or less, and more preferably, the Cr content is 0.085% or less and the Ni content is 0.100% or less.

[0027] A steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance including Fe and unavoidable impurities. The unavoidable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are allowed to be present to a degree that does not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap. Examples of the unavoidable impurities include Ca, O, H, V, Sn, Co, W, Zn, Pb, As, Sb, and Bi.

[0028] Next, the microstructure, which is an important requirement of the steel sheet of the present invention, will be described. Note that the area ratio below is the area ratio relative to the entire steel sheet structure.

[0029] Ferrite: 90% or more in area fraction Ferrite is generated during cooling after annealing and contributes to improving the ductility of steel. If the area fraction of ferrite is less than 90%, it becomes difficult to ensure the desired ductility. Therefore, the area fraction of ferrite is set to 90% or more. It is preferably 91% or more, more preferably 95% or more, and even more preferably 96% or more. There is no particular upper limit, but the area fraction of ferrite is preferably 99% or less, and more preferably 98% or less.

[0030] Total of Ti and Nb Precipitated as Carbonitrides: 0.005% or More by Mass Ti and Nb precipitate as carbonitrides in ferrite, increasing the strength of ferrite and reducing the difference in strength between ferrite and phases other than ferrite, thereby suppressing cracking during can-expanding and achieving excellent can-expandability. If the total of Ti and Nb precipitated as carbonitrides is less than 0.005%, ferrite is not strengthened, resulting in a large difference in strength between ferrite and phases other than ferrite, resulting in poor can-expandability. It also becomes difficult to achieve the desired yield strength. Therefore, the total amount of Ti and Nb precipitated as carbonitrides is set to 0.005% or more of the total Ti and Nb content. The total amount of Ti and Nb precipitated as carbonitrides is preferably 0.010% or more, more preferably 0.015% or more, even more preferably 0.017% or more, and most preferably 0.020% or more. Although there is no particular upper limit, it is preferably 0.040% or less, more preferably 0.039% or less, even more preferably 0.038% or less, and most preferably 0.037% or less. The carbonitrides in the present invention include carbides and nitrides in addition to composite carbonitrides, and the desired effects can be achieved in either form.

[0031] The amounts of Ti and Nb obtained by subtracting the amounts of Ti and Nb contained as carbonitrides from the total amounts of Ti and Nb contained in the steel sheet are the amounts of dissolved Ti and dissolved Nb.

[0032] In the metal microstructure, the balance other than the ferrite does not need to be particularly limited. For example, the balance may include retained austenite, cementite, pearlite, bainite, martensite, etc. The balance is preferably 5% or less, more preferably 3% or less, and the lower limit may be 0%.

[0033] Furthermore, the thickness of the steel plate of the present invention is not particularly limited, but the thickness is preferably 0.10 mm or more, more preferably 0.12 mm or more, and even more preferably 0.13 mm or more. The thickness is preferably 0.60 mm or less, more preferably 0.50 mm or less, and even more preferably 0.40 mm or less.

[0034] The method for producing a steel sheet of the present invention is characterized in that a slab having the above-described chemical composition is hot-rolled at a finishing temperature of 800°C to 950°C and a coiling temperature of 550°C to 750°C (hot rolling step), then cold-rolled at a reduction of 85% or more (cold rolling step), heated to an annealing temperature at an average heating rate of 5 to 40°C / sec (S), held at an annealing temperature of 640°C to 760°C, and then cooled at an average cooling rate of 15°C / sec or more to a temperature range lower than the annealing temperature and of 450°C to 650°C (annealing step), and temper-rolled at a temper-rolling reduction of 0.5% to 5.0% (temper rolling step).

[0035] Hot Rolling Process Finishing Temperature: 800°C or More and 950°C or Less If the finishing temperature of hot rolling exceeds 950°C, the ferrite structure after hot rolling will become coarse, and the ferrite structure after annealing will also become coarse, resulting in a decrease in yield strength. For this reason, the finishing temperature is set to 950°C or less. The finishing temperature is preferably 930°C or less, more preferably 900°C or less, and even more preferably 890°C or less. Furthermore, if the finishing temperature is less than 800°C, rolling will occur in the two-phase region of ferrite and austenite, and coarse grains will be generated in the steel sheet surface layer, resulting in a decrease in yield strength. Therefore, the finish rolling temperature is set to 800°C or more. Preferably, it is 810°C or more, more preferably 850°C or more, and even more preferably 855°C or more. Note that the finishing temperature in the present invention refers to the finish rolling end temperature in finish rolling.

[0036] Coiling temperature: 550°C or higher and 750°C or lower In the present invention, excellent can expandability is achieved by controlling the coiling temperature and the rate of temperature rise up to the annealing temperature to obtain desired amounts of carbonitrides containing Nb and Ti. To achieve this effect, it is important to control the coiling temperature to 550°C or higher and 750°C or lower. If the coiling temperature exceeds 750°C, sufficient carbonitrides are not formed during coiling, making it difficult to ensure can expandability. In addition, scale is generated in the surface layer, making surface defects more likely to occur. For this reason, the coiling temperature is set to 750°C or lower. Preferably, it is 720°C or lower. More preferably, it is 710°C or lower, and even more preferably, it is 700°C or lower. If the coiling temperature is less than 550°C, sufficient carbonitrides are not formed during coiling, making it difficult to ensure can expandability. Therefore, the coiling temperature is set to 550°C or higher. Preferably, it is 600°C or higher. More preferably, it is 610°C or higher, and even more preferably, it is 620°C or higher. A hot-rolled sheet is obtained through the above steps.

[0037] Cold Rolling Process Reduction in Cold Rolling: 85% or More By controlling the reduction in cold rolling, the crystal grains during annealing become finer, and the martensite formed during cooling after annealing can be made finer. To achieve this effect, the reduction must be 85% or more. Preferably, the reduction is 87% or more. The reduction is more preferably 88% or more, and even more preferably 89% or more. On the other hand, if the reduction exceeds 95%, the rolling load increases significantly, increasing the load on the rolling mill. Therefore, the reduction is preferably 95% or less. The reduction is more preferably 94% or less, and even more preferably 93% or less. A cold-rolled sheet is obtained through the above steps.

[0038] Annealing Process Average Heating Rate to Annealing Temperature: 5 to 40°C / sec In the present invention, the heating rate to the annealing temperature is controlled along with the coiling temperature to obtain the desired amount of carbonitrides containing Ti and Nb, thereby achieving excellent can expandability. To achieve this effect, it is important to control the average heating rate to the annealing temperature to 5 to 40°C / sec. If the average heating rate is less than 5°C / sec, the carbonitrides obtained by hot rolling will dissolve during heating, and a sufficient amount of carbonitrides will not be obtained after annealing. This will result in a failure to strengthen the ferrite, increasing the difference in strength between ferrite and phases other than ferrite, resulting in poor can expandability. It will also be difficult to ensure the desired yield strength. Therefore, the average heating rate is set to 5°C / sec or higher. Preferably, it is 6°C / sec or higher, more preferably 7°C / sec or higher, and even more preferably 8°C / sec or higher. Most preferably, it is 10°C / sec or higher. Furthermore, if the average heating rate exceeds 40°C / sec, ferrite recrystallization does not occur sufficiently, and excessive unrecrystallized grains remain, resulting in increased strength and decreased ductility. Therefore, the average heating rate to the annealing temperature is set to 40°C / sec or less. It is preferably 38°C / sec or less, more preferably 37°C / sec or less, and even more preferably 36°C / sec or less.

[0039] Annealing temperature: 640°C or higher and 760°C or lower If the annealing temperature is lower than 640°C, ferrite recrystallization does not occur sufficiently, and excessive unrecrystallized grains remain, resulting in increased strength and decreased ductility. Therefore, the annealing temperature is set to 640°C or higher. Preferably, it is set to 660°C or higher. More preferably, it is set to 665°C or higher, and even more preferably, it is set to 670°C or higher. On the other hand, if the annealing temperature exceeds 760°C, carbonitrides containing Nb and Ti formed during hot rolling dissolve, making it impossible to obtain the desired amount of carbonitrides after annealing. Therefore, the annealing temperature is set to 760°C or lower. Preferably, it is set to 740°C or lower. More preferably, it is set to 735°C or lower, and even more preferably, it is set to 730°C or lower.

[0040] Average cooling rate: 15°C / sec or more If the average cooling rate from the annealing temperature to the cooling stop temperature is less than 15°C / sec, the desired amount of carbonitrides containing Ti and Nb cannot be obtained, resulting in poor can expandability. Therefore, the cooling rate is set to 15°C / sec or more. Preferably, it is set to 20°C / sec or more. More preferably, it is set to 23°C / sec or more, even more preferably, it is set to 24°C / sec or more, and most preferably, it is set to 25°C / sec or more. This cooling can be performed by gas cooling, mist cooling, roll cooling, water cooling, or a combination of two or more of these. There is no particular upper limit, but in order to maintain a good shape of the steel sheet after cooling, it is preferably set to 150°C / sec or less. More preferably, it is set to 147°C / sec or less, even more preferably, it is set to 146°C / sec or less, and most preferably, it is set to 145°C / sec or less.

[0041] Cooling stop temperature: Lower than the annealing temperature, and 450°C to 650°C. By setting the cooling stop temperature after annealing lower than the annealing temperature, and 450°C to 650°C, the desired amount of carbonitrides containing Ti and Nb can be obtained. If the cooling stop temperature exceeds 650°C, the desired amount of carbonitrides containing Ti and Nb cannot be obtained, and the can expandability becomes poor. Therefore, the cooling stop temperature is set to 650°C or lower. Preferably, it is set to 600°C or lower. More preferably, it is set to 595°C or lower, and even more preferably, it is set to 590°C or lower. On the other hand, if the cooling stop temperature is lower than 450°C, the desired amount of ferrite cannot be obtained, and ductility decreases. Therefore, the cooling stop temperature after annealing is set to 450°C or higher. Preferably, it is set to 500°C or higher. More preferably, it is set to 505°C or higher, and even more preferably, it is set to 510°C or higher. An annealed sheet is obtained through the above process. It is preferable to cool from the annealing temperature to the cooling stop temperature by at least 40°C. That is, the annealing temperature is preferably 40° C. or more higher than the cooling stop temperature, and the steel is preferably cooled from the annealing temperature by 40° C. or more to the cooling stop temperature.

[0042] Temper rolling ratio: 0.5% or more and 5.0% or less Temper rolling not only adjusts surface roughness and corrects shape, but also introduces strain into the annealed sheet, thereby improving yield strength. To achieve this effect, the temper rolling ratio needs to be 0.5% or more. Therefore, the temper rolling ratio is set to 0.5% or more. The temper rolling ratio is preferably set to 0.6% or more, more preferably 0.7% or more, even more preferably 0.8% or more, and most preferably 0.9% or more. On the other hand, if the temper rolling ratio exceeds 5.0%, excessive strain is introduced into the steel sheet, resulting in reduced elongation. Therefore, the temper rolling ratio is set to 5.0% or less, preferably 3.0% or less. The temper rolling ratio is more preferably set to 2.5% or less, even more preferably 2.3% or less, and most preferably 2.0% or less. The high-strength steel sheet of the present invention is obtained through the above steps.

[0043] EXAMPLES Hereinafter, examples of the steel sheet and the manufacturing method thereof according to the present invention will be described, but the present invention is not limited to the examples shown here.

[0044] Steels having the chemical compositions shown in Table 1 were melted and produced into sheet bar slabs with a thickness of 20 mm. These sheet bar slabs were hot-rolled under the conditions shown in Table 2. The resulting hot-rolled sheets were pickled with hydrochloric acid and cold-rolled at the reduction ratios shown in Table 2 to produce cold-rolled sheets with a thickness of 0.2 mm.

[0045] Next, the cold-rolled steel sheets were subjected to heating, annealing and holding, cooling, and temper rolling under the heat treatment conditions shown in Table 2 to obtain steel sheets (also referred to as steel sheets) as products.

[0046] The steel sheets thus obtained were subjected to the following investigations into their structure and mechanical properties. The results are shown in Table 2.

[0047] The area ratio of ferrite in the entire structure of the steel sheet was investigated by observing the surface of the product steel sheet at 1 / 2 the thickness position in the rolling direction cross section with a scanning electron microscope (SEM) after etching with nital. Observation was carried out at three randomly selected visual fields. Using a cross-sectional structural photograph at a magnification of 2000 times, binarization was performed using image processing software (Photoshop, manufactured by Adobe), and the area occupied by ferrite present in an arbitrarily set square area of ​​50 μm × 50 μm was determined, and the average value over the three visual fields was calculated, which was taken as the area ratio of ferrite. Black areas observed as blocky shapes were determined to be ferrite.

[0048] The amounts of Ti and Nb precipitated as carbonitrides were measured by extraction analysis. The samples were subjected to electrolytic extraction using a 10% AA-based electrolyte, and the amounts of Ti and Nb precipitated as carbonitrides were determined by ICP after mixed acid decomposition. A 0.2 μm filter mesh was used.

[0049] Mechanical Properties Mechanical properties (yield strength YP, elongation EL) were evaluated by conducting a tensile test in accordance with JIS Z 2241 using a JIS No. 5 test piece described in JIS Z 2241, with the rolling direction as the longitudinal direction (tensile direction). In the present invention, yield strength (YP) refers to the upper yield point if there is an upper yield point, or 0.2% proof stress if there is no upper yield point. The tensile speed was 10 mm / min.

[0050] Can expansion property: A rectangular blank was taken from the product steel sheet so that the rolling direction was the longitudinal direction of the test piece, and it was welded into a cylindrical shape and subjected to can expansion processing so that the can diameter was expanded by a maximum of 15%. In the part that had been subjected to the 15% can expansion processing, those that were visually free of cracks were rated as pass (◯), and those that had even one crack were rated as fail (×).

[0051]

[0052]

[0053] The high-strength steel sheets of the present invention had YP of 450 MPa or more and El of 20% or more, and were found to be excellent in can expandability. In contrast, the steel sheets of the comparative examples, which were outside the range of the present invention, did not have a satisfactory level in any of YP, EL, and can expandability, and were significantly inferior in at least one of strength, elongation, and can expandability compared to the steel sheets of the present invention.

Claims

1. A steel sheet having a composition comprising, by mass%, C: 0.05% or more and 0.13% or less, Si: 0.01% or more and 0.04% or less, Mn: 0.1% or more and 0.6% or less, P: 0.02% or less, S: 0.02% or less, Al: 0.01% or more and 0.10% or less, N: 0.0005% or more and 0.0040% or less, Ti: 0.005% or more and 0.030% or less, Nb: 0.005% or more and 0.030% or less, Mo: 0.01% or more and 0.05% or less, and B: 0.0005% or more and 0.0050% or less, with the remainder being iron and unavoidable impurities; and having a ferrite structure of 90% or more in area ratio as a microstructure; A steel plate, wherein the sum of Ti and Nb present as carbonitrides is 0.005% or more by mass% in the total Ti and Nb contents.

2. The steel plate according to claim 1, further comprising, in addition to the above-mentioned chemical composition, one or two selected from the following, by mass%, Cr: 0.005% or more and 0.100% or less, Ni: 0.005% or more and 0.150% or less.

3. A method for manufacturing steel sheet, comprising: a hot rolling process in which a slab having the chemical composition according to claim 1 or 2 is hot rolled under conditions of a finishing temperature of 800°C to 950°C and a coiling temperature of 550°C to 750°C; a cold rolling process in which the hot rolled sheet that has undergone the hot rolling process is cold rolled at a reduction rate of 85% or more; an annealing process in which the cold rolled sheet that has undergone the cold rolling process is heated to an annealing temperature of 640°C to 760°C at an average heating rate of 5 to 40°C / sec, and after holding at the annealing temperature, is cooled at an average cooling rate of 15°C / sec or more to a cooling stop temperature that is lower than the annealing temperature and is 450°C to 650°C; and a temper rolling process in which the annealed sheet that has undergone the annealing process is temper rolled at a temper rolling reduction rate of 0.5% to 5.0%.