High-strength cold-rolled steel sheets and the same methods for manufacturing them.

Abstract

DEPCT63 High-strength cold-rolled steel sheets with a high yield ratio and malleability are provided. Good practices and methods for producing high-strength cold-rolled steel sheets. High-strength cold-rolled steel sheets have a component composition that includes, Based on a percentage by mass, C: 0.12% or more and 0.25% or less. Si: less than 0.5%, Mn: 2.0% or more, and 3.0% or less, P: 0.05% or less. That, S: 0.005% or less, Al: 0.01% or more and 0.10% or less, and N: 0.010% or less, the remainder being Fe and minor impurities, percentage area. All of the martensite and martensite from the refractory process match 20% or more. And 90% or less, the percentage of ferrite area that corresponds to 10% or less, The percentage of the area of ​​the benite that matches 10% or more and 80% or less, percentage. The area of ​​the martensite-austenite component in benite is 1% or more and 10%. Or less, the percentage area of ​​cementite with an average grain size of 1 micrometer. Even less in benite, with values ​​of 0.1% or more and 5.0% or less. -----------------------------------------------------------

Description

High-strength cold-rolled steel sheet and method for manufacturing the same.

[0001] This invention relates to a high-strength cold-rolled steel sheet used in automobile parts and the like, and a method for manufacturing the same. More specifically, this invention relates to a high-strength cold-rolled steel sheet having a high yield ratio and excellent bendability, and a method for manufacturing the same.

[0002] In recent years, improving the fuel efficiency of automobiles has become a crucial issue from the standpoint of protecting the global environment. Accordingly, there has been a surge in efforts to reduce the weight of the vehicle body by increasing the strength of the steel plates used and making them thinner. Furthermore, in addition to increasing the strength of the steel plates used in the vehicle body, safety is required for the components used in the vehicle's frame, and a high yield ratio is advantageous for improving collision characteristics. Therefore, the development of steel plates with a high yield ratio is desired.

[0003] In response to these demands, composite steel structures of bainite and martensite, which suppress ferrite formation, have been developed. However, as steel sheets become stronger, their formability decreases, making processing difficult, and there is room for improvement. High-strength steel sheets are often formed mainly by bending, and improving formability requires improving bendability. While typical bending tests are conducted at speeds of around 10 mm / second, actual automotive steel sheets are formed at even higher speeds, and cracks may occur during high-speed forming even if no cracks occur in tests at normal speeds. Therefore, in the case of automotive steel sheets, it is necessary to evaluate their properties using high-speed bending tests. Thus, there is a demand for steel sheets that possess high strength, a high yield ratio, and good bendability, especially good bendability under high-speed deformation.

[0004] For example, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and a yield ratio of 0.65 or more, and a method for manufacturing the same. The component composition of the steel sheet in Patent Document 1 is, by mass%, C: 0.11% or more and 0.20% or less, Si: less than 0.50%, Mn: 2.2% or more and 3.5% or less, P: 0.03% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.006% or less, B: 0.0002% or more and 0.0030% or less, with the remainder being Fe and unavoidable impurities. Furthermore, the steel structure, in terms of area percentage, consists of less than 20% ferrite phase (including 0%), 50% or less bainite phase (including 0%), 50% or more martensite phase (including 100%), 70% or more auto-tempered martensite contained in the martensite phase (including 100%), and 2% or less retained austenite (including 0%). However, although the steel structure consists of bainite and martensite, and high strength and a high yield ratio are obtained by finely dispersing carbides within the auto-tempered martensite grains, the strength difference with bainite has become large, resulting in a R / t of 2.0 or more and a deterioration in bendability.

[0005] Patent Document 2 describes a material whose composition, in mass%, is as follows: C: 0.05% to 0.15%, Si: 0.01% to 1.00%, Mn: 2.2% to 3.5%, P: 0.001% to 0.050%, S: 0.010%, sol. Al: 0.005% to 0.100%, N: 0.0001% to 0.0060%, further containing one or more selected from Nb: 0.01% to 0.10%, Ti: 0.01% to 0.10%, with the remainder being Fe and unavoidable impurities. The steel structure consists of a ferrite phase with an area ratio of 5% to 80% and a martensite phase with an area ratio of 20% to 70% at the 1 / 2 position in the thickness direction of the steel sheet. A high-strength hot-dip galvanized steel sheet is disclosed, characterized by having a site phase and a bainite phase of 0% to 25% in area ratio, a hardness at a position 5 μm from the surface in the thickness direction of the steel sheet being 80% or less of the hardness at the 1 / 2 position in the thickness direction, and a hardness at a position 15 μm from the surface in the thickness direction of the steel sheet being 90% or more of the hardness at the 1 / 2 position in the thickness direction, a tensile strength of 780 MPa or more, and a durability ratio of 0.42 or more. The steel sheet described in Patent Document 2 also has a steel structure of bainite and martensite as described above, and is high strength and has excellent bendability, but the disclosed example contains 27% or more ferrite, so the yield ratio is low, less than 0.65.

[0006] Patent No. 5958666 Patent No. 5958659

[0007] As described above, there are no high-strength steel sheets with a high yield ratio that also exhibit excellent bendability. Therefore, the object of the present invention is to provide a high-strength cold-rolled steel sheet with a high yield ratio and excellent bendability, and a method for manufacturing the same. Specifically, "high strength" means a tensile strength of 780 MPa or more, "high yield ratio" means a yield ratio of 0.65 or more, and "excellent bendability" means a bendability (R / t) of 1.5 or less.

[0008] The inventors of this invention have diligently conducted research to solve the above problems. As a result, by optimizing the component composition and manufacturing conditions, they have succeeded in creating a composite structure of bainite and martensite with suppressed ferrite formation, and further controlling the size and amount of island-like martensite (MA) and cementite in the bainite, thereby producing a high-strength cold-rolled steel sheet with a high yield ratio and excellent bendability. The gist of their findings is as follows.

[0009] [1] A high-strength cold-rolled steel sheet having a composition in mass%, containing C: 0.12% or more and 0.25% or less, Si: less than 0.5%, Mn: 2.0% or more and 3.0%, P: 0.05% or less, S: 0.005% or less, Al: 0.01% or more and 0.10% or less, with the remainder being Fe and unavoidable impurities, and satisfying in terms of area ratio that the total of martensite and tempered martensite is 20% or more and 90% or less, ferrite is 10% or less, and bainite is 10% or more and 80% or less, with an area ratio of island martensite (MA) in bainite of 1% or more and 10% or less, and an area ratio of cementite with an average particle size of 1 μm or less in bainite of 0.1% or more and 5.0% or less.

[0010] [2] The high-strength cold-rolled steel sheet according to [1], wherein the component composition further contains B: 0.005% or less by mass.

[0011] [3] The component composition further contains, by mass%, one or more elements selected from Nb: 0.005% or more and 0.1%, Ti: 0.005% or more and 0.1%, and V: 0.005% or more and 0.3%. High-strength cold-rolled steel sheet according to [1] or [2].

[0012] [4] The component composition further contains, by mass%, one or more elements selected from Mo: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, and Ni: 1.0% or less, according to any one of [1] to [3].

[0013] [5] The high-strength cold-rolled steel sheet according to any one of [1] to [4], characterized in that the component composition further contains one or two elements selected from Ca: 0.0005% or more and 0.01% or less by mass, and REM: 0.0005% or more and 0.01% or less.

[0014] [6] A high-strength cold-rolled steel sheet according to any one of [1] to [5], having a plating layer on its surface.

[0015] [7] The high-strength cold-rolled steel sheet according to [6], wherein the plating layer is a hot-dip galvanized (GI) layer, an alloyed hot-dip galvanized (GA) layer, or a Zn-Al plating layer.

[0016] A hot rolling step in which a slab having the component composition described in any of [8] [1] to [5] is rolled under conditions where the finish rolling end temperature is 800°C or more and 1000°C or less, cooled under conditions where the average cooling rate is 20°C / second or more, and wound under conditions where the winding temperature is 550°C or more and 700°C or less; a cold rolling step in which the hot-rolled steel sheet obtained in the hot rolling step is cold-rolled under conditions where the reduction ratio is 20% or more; and the cold-rolled steel sheet obtained in the cold rolling step is Ac ３ Primary heating involves heating to a temperature range of 15°C to 900°C, Ac ３ Primary annealing is performed by holding the material in a temperature range of 10°C to 900°C for 10 to 1200 seconds, followed by primary cooling at an average cooling rate of 3°C / second or more to a cooling stop temperature of 100°C or less, and then Ac ３ Secondary heating, which involves heating to a temperature range above 1°C but below 900°C, Ac ３ A method for manufacturing a high-strength cold-rolled steel sheet, comprising: a secondary annealing step of holding the sheet in a temperature range above 900°C for 10 seconds or more and 1200 seconds or less; a secondary cooling step of cooling to a cooling stop temperature of 350°C to 600°C at an average cooling rate of 10°C / second or more after the secondary annealing step; a tertiary holding step of holding the sheet in a temperature range of 350°C to 600°C for 1 second or more and 1200 seconds or less after the secondary annealing step; and a tertiary cooling step of cooling at an average cooling rate of 5°C / second or more.

[0017] [9] A method for manufacturing a high-strength cold-rolled steel sheet according to [8], further comprising a plating step of applying a plating treatment to the surface after the annealing step.

[0018] The steel structure of the present invention is mainly composed of bainite and martensite, with suppressed ferrite formation, and further, the amount of MA in the bainite and the size and amount of cementite are adjusted. By adopting the component composition of the present invention and this steel structure, it is possible to provide high-strength cold-rolled steel sheets with a high yield ratio and excellent bendability.

[0019] In particular, the high-strength cold-rolled steel sheet of the present invention is suitable as a steel sheet for automobiles. Applying the high-strength cold-rolled steel sheet of the present invention to automobile structural members makes it possible to achieve both weight reduction and improved collision safety in automobiles. In other words, the present invention leads to higher performance automobile bodies.

[0020] This figure shows an example of the tissue after measuring the area ratio. This figure shows an example of the tissue after two-stage etching.

[0021] Embodiments of the present invention will be described below. However, the present invention is not limited to the embodiments described below.

[0022] First, the component composition of the Odaka High-Strength Cold-Rolled Steel Sheet of the present invention will be explained. In the following explanation, the unit "%" for component content means "mass%".

[0023] C: 0.12% to 0.25% Carbon (C) significantly affects the strength of steel sheets, so it is necessary to include a certain amount. Also, since C is an element that lowers the transformation point, adding C allows austenite annealing to be performed at a lower temperature, which is very effective in suppressing ferrite formation. If the C content is less than 0.12%, the desired tensile strength cannot be reached even if the manufacturing conditions are optimized. Therefore, the C content should be 0.12% or more. Preferably, it is 0.14% or more, more preferably 0.16% or more, and even more preferably 0.18% or more. On the other hand, if the C content is greater than 0.25%, in the temperature range of 350°C to 600°C during annealing, the cementite in bainite becomes coarser, making it easier for voids to form and thus reducing bendability. Therefore, the C content should be 0.25% or less. Preferably, it is 0.23% or less, more preferably 0.22% or less, and even more preferably 0.20% or less.

[0024] Si: Less than 0.5% Si is an element that promotes ferrite formation. If the Si content is 0.5% or more, it becomes difficult to keep the ferrite amount below 10%, and the yield ratio decreases. Therefore, the Si content should be less than 0.5%, preferably 0.3% or less, more preferably 0.2% or less, and even more preferably 0.1% or less. From the viewpoint of obtaining the effects of the present invention, it is not necessary to include Si (the Si content may be 0%), but considering that excessively reducing the Si content may increase manufacturing costs, a Si content of 0.0001% or more is preferred. More preferably 0.0003% or more.

[0025] Mn: 2.0% to 3.0% Mn is an element that increases the tensile strength of steel sheets as a solid solution strengthening element. In addition, Mn has the effect of improving hardenability, and therefore suppresses the formation of ferrite during the cooling process in annealing. If the Mn content is less than 2.0%, it is difficult to suppress the amount of ferrite to 10% or less, the desired tensile strength is not reached, and the yield ratio decreases. Therefore, the Mn content should be 2.0% or more, preferably 2.2% or more, more preferably 2.3% or more, and even more preferably 2.4% or more. On the other hand, if the Mn content exceeds 3.0%, the improvement in hardenability becomes excessive, and the amount of martensite exceeds 90%, causing a decrease in bendability. Therefore, the Mn content should be 3.0% or less, preferably 2.8% or less, more preferably 2.7% or less, and even more preferably 2.6% or less.

[0026] P: 0.05% or less. P is inevitably included as an impurity element and has the effect of segregating at prior austenite grain boundaries, making the grain boundaries brittle and prone to fracture, thus reducing flexibility. Therefore, the P content should be as low as possible, preferably 0.05% or less, and more preferably 0.03% or less. From the viewpoint of obtaining the effects of the present invention, it is not necessary to include P (the P content may be 0%), but considering that excessively reducing the P content may increase manufacturing costs, a P content of 0.0001% or more is preferred. More preferably 0.0003% or more.

[0027] S: 0.005% or less. S is inevitably included as an impurity element, forming MnS inclusions in the steel, which act as crack initiation points during processing, thereby reducing bendability. Therefore, it is desirable to keep the S content as low as possible, with an S content of 0.005% or less, preferably 0.003% or less. From the viewpoint of obtaining the effects of the present invention, it is not necessary to include S (the S content may be 0%), but considering that excessively reducing the S content may increase manufacturing costs, an S content of 0.0001% or more is preferable. More preferably 0.0003% or more.

[0028] Al: 0.01% or more and 0.10% or less. Al is an element added as a deoxidizing agent. To obtain this effect, an Al content of 0.01% or more is required. Preferably, it is 0.02% or more, more preferably 0.03% or more, and even more preferably 0.04% or more. On the other hand, if the Al content exceeds 0.10%, it reduces the cleanliness of the steel, and Al also has the effect of raising the transformation point. Therefore, in order to suppress the formation of ferrite and obtain a high yield ratio, the Al content must be 0.10% or less, preferably 0.08% or less, more preferably 0.07% or less, and even more preferably 0.06% or less.

[0029] N: 0.010% or less. N is inevitably present as an impurity element and has the effect of reducing flexibility due to strain aging. Therefore, it is preferable to keep the N content as low as possible, and the N content should be 0.010% or less, preferably 0.007% or less. From the viewpoint of obtaining the effects of the present invention, it is not necessary to include N (the N content may be 0%), but considering that excessively reducing the N content may increase manufacturing costs, an N content of 0.0001% or more is preferable. More preferably 0.0003% or more.

[0030] In addition to the above ingredients, the following ingredients may also be included as optional ingredients.

[0031] B: 0.005% or less. B is an element that enhances hardenability and suppresses the formation of ferrite during annealing. To obtain this effect, the B content is preferably 0.0001% or more. More preferably 0.0005% or more, and even more preferably 0.0010% or more. However, the effect saturates when the content exceeds 0.005%. Therefore, to reduce costs, the B content is set to 0.005% or less. Preferably 0.004% or less, and even more preferably 0.003% or less.

[0032] Nb: 0.005% to 0.1%, Ti: 0.005% to 0.1%, V: 0.005% to 0.3%. One or more of these elements have the effect of improving tensile strength and yield strength without degrading ductility. If the content of any element is less than 0.005%, the above effect cannot be effectively exhibited. However, if the upper limit of each element is exceeded, coarse carbides are generated, becoming the starting point for void formation and degrading bendability. The above elements may be included individually or in combination of two or more. The preferred range for the lower limit is as follows: Nb is preferably 0.010% or more, more preferably 0.020% or more. Ti is preferably 0.010% or more, more preferably 0.020% or more. V is preferably 0.010% or more, more preferably 0.020% or more. The preferred range for the upper limit is as follows: Nb is preferably 0.09% or less. Ti is preferably 0.09% or less. V is preferably 0.2% or less, and more preferably 0.1% or less.

[0033] Mo: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, one or more of these elements are elements that have the effect of enhancing hardenability and suppressing the formation of ferrite during annealing. From the perspective of obtaining this effect, the Mo content is preferably 0.001% or more, more preferably 0.010% or more, and still more preferably 0.050% or more. The Cr content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more. The Cu content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more. The Ni content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more. However, for each element, even if it exceeds the upper limit value of each regulation, the effect saturates, so from the perspective of cost reduction, each element is set to be below the upper limit value of each regulation. Also, the preferable range for the upper limit value is as follows. Mo is preferably 0.15% or less. Cr is preferably 0.7% or less, more preferably 0.4% or less. Cu is preferably 0.9% or less, more preferably 0.7% or less. Ni is preferably 0.9% or less, more preferably 0.7% or less. Note that the above elements can be contained alone or in two or more kinds.

[0034] Ca: 0.0005% or more and 0.01% or less, REM: 0.0005% or more and 0.01% or less, one or two of these elements are elements that have the effect of refining inclusions, reducing the starting point of fracture, and increasing bendability. In order to obtain such an effect, it is effective to contain both Ca and REM at 0.0005% or more. More preferably 0.0010% or more, still more preferably 0.0015% or more. However, when the content of Ca and REM exceeds 0.01%, the inclusions coarsen, becoming the starting point of void generation and reducing bendability. Therefore, the content of Ca and REM is set to be 0.01% or less. Preferably 0.007% or less, more preferably 0.004% or less. Note that Ca and REM can be contained alone or in two or more kinds.

[0035] The balance other than the above components is Fe and inevitable impurities. When there is a lower limit value for the above optional components, even if the content is less than the lower limit value, it does not impair the effects of the present invention, so the above optional components less than the lower limit value are included as inevitable impurities.

[0036] Next, the steel structure of the high-strength cold-rolled steel sheet of the present invention will be described.

[0037] Total of martensite and tempered martensite: 20% or more and 90% or less In order to obtain a high tensile strength, it is necessary to have a steel structure containing martensite. When the total area ratio of martensite and tempered martensite to the entire steel structure is less than 20%, it becomes difficult to ensure high strength. Therefore, the total area ratio of martensite and tempered martensite is set to 20% or more. Preferably 25% or more, more preferably 30% or more, still more preferably 35% or more. On the other hand, when the total area ratio of martensite and tempered martensite to the entire steel structure exceeds 90%, it becomes difficult to ensure the desired bendability. Therefore, the total area ratio of martensite and tempered martensite is 90% or less, preferably 80% or less, more preferably 70% or less, still more preferably 60% or less. Also, tempered martensite has lower strength but better ductility than martensite, so the ratio of tempered martensite to martensite can be controlled according to the required characteristics. Therefore, although the respective contents are not particularly limited, the ratio of martensite to the total of martensite and tempered martensite is often 20% or less. The ratio of tempered martensite to the total of martensite and tempered martensite is often 80% or more.

[0038] Ferrite area ratio: 10% or less By suppressing the formation of soft ferrite, the yield ratio is increased. In order to effectively exert this effect, the area ratio of ferrite to the total steel structure must be 10% or less. Therefore, the area ratio of ferrite should be 10% or less, preferably 6% or less. More preferably less than 5%, and even more preferably 4% or less. Furthermore, from the viewpoint of obtaining the effects of the present invention, it is not necessary to include ferrite (the ferrite area ratio may be 0%), but if it is difficult to make the ferrite area ratio 0%, then 1% or more is preferable, and more preferably 2% or more.

[0039] Bainite Area Ratio: 10% to 80% Bainite is a hard structure with a uniform hardness distribution, resulting in a structure with an excellent balance of strength and ductility. If the bainite area ratio to the total steel structure is less than 10%, it becomes difficult to ensure the specified bendability. Therefore, the bainite area ratio is 10% or more, preferably 20% or more, more preferably 25% or more, and even more preferably 30% or more. On the other hand, since bainite is softer than martensite, if the bainite area ratio to the total steel structure exceeds 80%, it becomes difficult to ensure strength. Therefore, the bainite area ratio is 80% or less, preferably 70% or less. More preferably 60% or less, and even more preferably 55% or less. Note that bainite as used here includes both upper bainite and lower bainite.

[0040] Area ratio of MA in bainite: 1% or more and 10% or less. Since MA has the effect of improving ductility, in order to obtain this effect, the area ratio of MA in bainite must be 1% or more (with the entire steel structure being 100%). Preferably it is 2% or more, more preferably 3% or more, and even more preferably 4% or more. On the other hand, if the area ratio of MA exceeds 10%, voids are more likely to be formed due to the difference in hardness with adjacent structures, and the bendability decreases, so the area ratio of MA should be 10% or less. Preferably it is 9% or less, more preferably 8% or less, and even more preferably 7% or less. Note that the MA referred to here is island-shaped martensite precipitated in bainite, and is different from the above-mentioned martensite and tempered martensite.

[0041] Area ratio of cementite with an average particle size of 1 μm or less in bainite: 0.1% to 5.0% Cementite with an average particle size of 1 μm or less in bainite increases the strength of bainite and reduces the difference in hardness between bainite and martensite, thereby improving the yield ratio and bendability. To obtain this effect, the area ratio of cementite with an average particle size of 1 μm or less in bainite must be 0.1% or more (considering the entire steel structure as 100%). Preferably, it is 0.5% or more, more preferably 1.0% or more, and even more preferably 2.0% or more. Furthermore, if the above area ratio exceeds 5.0%, void formation is more likely to occur, causing a decrease in bendability. Therefore, the area ratio of cementite with an average particle size of 1 μm or less in bainite should be 5.0% or less. Preferably, it is 4.5% or less, more preferably 4.0% or less, and even more preferably 3.8% or less.

[0042] Furthermore, the high-strength cold-rolled steel sheet of the present invention may have a plating layer. The type of plating layer is not particularly limited. Examples include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.

[0043] Next, the method for manufacturing high-strength cold-rolled steel sheets according to the present invention will be described. The method for manufacturing high-strength cold-rolled steel sheets according to the present invention comprises a hot rolling step, a cold rolling step, and an annealing step. In addition, a plating step is included after the annealing step as needed. Each step will be described below. Note that temperature refers to surface temperature.

[0044] The hot rolling process involves rolling a slab having the above-mentioned component composition at a finish rolling temperature of 800°C to 1000°C, cooling it at an average cooling rate of 20°C / second or more, and winding it at a winding temperature of 550°C to 700°C.

[0045] The temperature of the slab to be subjected to hot rolling is preferably 1000 °C or higher. If the temperature of the slab is less than 1000 °C, carbides cannot be sufficiently dissolved, and furthermore, during rolling, the temperature of the steel may drop to the ferrite transformation temperature, and there is a risk of rolling in a state where ferrite is contained in the steel structure. Therefore, in order to complete rolling in the austenite single-phase temperature range and ensure bendability and strength after annealing, the temperature of the slab is preferably 1000 °C or higher. Regarding the upper limit of the slab heating temperature, 1350 °C or lower is preferable.

[0046] The finishing rolling end temperature is 800 °C or higher and 1000 °C or lower. If the finishing rolling end temperature is less than 800 °C, ferrite is generated during rolling, and pearlite occurs during subsequent cooling and coiling processes, and the cementite contained in the pearlite remains undissolved even during holding in the temperature range of Ac ３ point to 900 °C and coarsens in the temperature range of Ac ３ point to 900 °C, resulting in a decrease in bendability and yield ratio. Therefore, the finishing rolling end temperature is 800 °C or higher. Preferably 850 °C or higher, more preferably 870 °C or higher. On the other hand, since cooling to the subsequent coiling temperature becomes difficult, the finishing rolling end temperature is 1000 °C or lower. Preferably 960 °C or lower, more preferably 900 °C or lower. Here, the Ac ３ point is calculated by the following formula. In the following formula, (% element symbol) means the content (% by mass) of each element, and is 0 if not contained. Ac ３ = 910 - 203(%C) + 45(%Si) - 30(%Mn) - 20(%Cu) - 15(%Ni) + 11(%Cr) + 32(%Mo) + 104(%V) + 400(%Ti) + 460(%Al) After finishing rolling, cooling is performed to the coiling temperature at an average cooling rate of 20 °C / second or higher. If the average cooling rate after finishing rolling is less than 20 °C / second, the austenite grains are excessively coarsened, and pearlite occurs in the steel, and the cementite contained in the pearlite remains undissolved during subsequent annealing at Ac ３When held in the temperature range of 1°C to 900°C, it remains unmelted, reducing its flexibility and yield ratio. Therefore, an average cooling rate of 20°C / second or more is required. Preferably, it is 30°C / second or more, and more preferably 40°C / second or more. The preferred cooling method is water cooling. There is no particular upper limit to the average cooling rate, but 90°C / second or less is preferred. More preferably, it is 70°C / second or less.

[0047] After cooling as described above, the material is wound at a winding temperature of 550°C to 700°C. If the winding temperature is below 550°C, the steel structure will contain a hard layer, which will reduce the subsequent cold-rolling properties, and in the subsequent annealing process, recrystallization will be less likely to occur, resulting in reduced bendability. Therefore, a winding temperature of 550°C or higher is necessary. Preferably, it is 570°C or higher, and more preferably 590°C or higher. On the other hand, if the winding temperature exceeds 700°C, the ferrite grains will coarseen, and the resulting cementite will grow, causing the coarse cementite to affect Ac during the subsequent annealing process. ３ When held in the temperature range of 1 to 900°C, it remains unmelted, reducing its flexibility and yield ratio. Therefore, the winding temperature should be 700°C or lower. Preferably, it should be 660°C or lower, and more preferably 630°C or lower.

[0048] Cold rolling process: The cold rolling process is a process in which hot-rolled steel sheets obtained in the hot rolling process are cold-rolled under conditions of a reduction ratio of 20% or more.

[0049] In the cold rolling process, hot-rolled steel sheets are pickled and then cold-rolled to a reduction ratio of 20% or more to produce cold-rolled steel sheets. If the reduction ratio is less than 20%, recrystallization is less likely to occur in the subsequent annealing process, and the transformation to austenite is suppressed, resulting in a decrease in the yield ratio due to the remaining ferrite. Therefore, the reduction ratio should be 20% or more. Preferably it is 25% or more, and more preferably 30% or more. There is no particular upper limit to the reduction ratio, but it is preferably 90% or less, and more preferably 80% or less.

[0050] Annealing Process The annealing process is a process in which the cold-rolled steel sheet obtained in the cold-rolling process is subjected to primary annealing, secondary annealing, tertiary holding, and tertiary cooling, up to the upper limits specified below. (Primary Annealing) The cold-rolled steel sheet obtained in the cold-rolling process is subjected to Ac ３Primary heating involves heating to a temperature range of 15°C to 900°C, Ac ３ Primary annealing involves holding the material in a temperature range of 10°C to 900°C for 10 to 1200 seconds, followed by primary cooling at an average cooling rate of 3°C / second or more to a cooling stop temperature of 100°C or less. (Secondary annealing) After the primary annealing, Ac ３ Secondary heating, which involves heating to a temperature range above 1°C but below 900°C, Ac ３ Secondary annealing involves holding the material in a temperature range of 900°C or higher for 10 to 1200 seconds, followed by secondary cooling at an average cooling rate of 10°C / second or higher to a cooling stop temperature of 350°C to 600°C. (Tertiary holding) After the secondary annealing, the material is held in a temperature range of 350°C to 600°C for 1 to 1200 seconds. (Tertiary cooling) After the tertiary holding, the material is cooled at an average cooling rate of 5°C / second or higher.

[0051] In primary annealing, cold-rolled steel sheets are subjected to Ac in the austenite single-phase temperature range. ３ After heating to an annealing temperature of 1 to 900°C, the material is held for 10 to 1200 seconds and then cooled to a cooling stop temperature of 100°C or lower at an average cooling rate of 3°C / second or more. As described above, in this invention, it is necessary to suppress the formation of ferrite and create a composite structure of bainite and martensite. In order to suppress the formation of ferrite, it is necessary to create a structure with little difference in the concentration of components before starting secondary annealing, so in primary annealing, a treatment is performed to make the concentration of alloying elements in the structure uniform. At this time, if ferrite is formed, the alloying elements will be concentrated in the austenite, resulting in a structure with non-uniform alloying element concentrations. When this non-uniform structure is created, the transformation from ferrite to austenite is suppressed during subsequent secondary annealing, resulting in the formation of a large amount of ferrite in the final structure and a decrease in the yield ratio. Therefore, the annealing heating temperature for primary annealing is Ac ３ A minimum of 1 point is required. Preferably, Ac ３ Point + 10°C or higher, more preferably Ac ３ The temperature should be above +20°C. On the other hand, if the annealing heating temperature exceeds 900°C, the austenite becomes coarse and transforms into martensite or bainite while remaining coarse, leading to a decrease in the yield ratio. Therefore, the annealing heating temperature in the primary annealing process should be 900°C or lower. Preferably it is 880°C or lower, and more preferably 860°C or lower.

[0052] The holding time for primary annealing should be 10 to 1200 seconds. If the holding time for primary annealing is less than 10 seconds, the dissolution of carbides and the austenite transformation will not proceed sufficiently, resulting in a structure with residual ferrite and a difference in the concentration of alloying elements before secondary annealing. If secondary annealing is performed in this state, the transformation from ferrite to austenite will be suppressed, resulting in the formation of a large amount of ferrite in the final structure and a decrease in the yield ratio. Therefore, the holding time for primary annealing must be 10 seconds or more. Preferably, it should be 20 seconds or more, and more preferably 40 seconds or more. On the other hand, if the holding time for primary annealing exceeds 1200 seconds, the austenite grain size will coarseen, and it will transform into martensite or bainite while remaining coarse, leading to a decrease in the yield ratio. Therefore, it is preferable to keep the holding time for primary annealing at 1200 seconds or less. More preferably, it should be 1100 seconds or less, and even more preferably 1000 seconds or less.

[0053] The average cooling rate of the primary cooling performed after holding should be 3°C / second or higher. If the average cooling rate of the primary cooling is less than 3°C / second, a large amount of ferrite will be formed, and the remaining carbides will grow. Because the remaining ferrite results in a structure with differences in the concentration of alloying elements, if secondary annealing is performed as is, the transformation from ferrite to austenite will be suppressed, and a large amount of ferrite will be formed in the final structure, leading to a decrease in the yield ratio. Therefore, an average cooling rate of 3°C / second or higher is necessary for primary cooling. Preferably, it is 4°C / second or higher. Furthermore, there is no particular upper limit to the average cooling rate, but it is preferably 20°C / second or lower, and more preferably 10°C / second or lower.

[0054] Furthermore, the cooling stop temperature for primary cooling needs to be 100°C or lower in order to ensure an average cooling rate and suppress the coarsening of carbides. If the cooling stop temperature exceeds 100°C, the carbides coarseen, creating a carbon concentration distribution within the microstructure. If secondary annealing is performed in this state, the transformation from ferrite to austenite is suppressed in the regions with low carbon concentration, resulting in the formation of a large amount of ferrite in the final microstructure and a decrease in the yield ratio. Preferably, it is 80°C or lower. In addition, there is no particular lower limit to the above cooling stop temperature, but it is preferably 10°C or higher, and more preferably 20°C or higher.

[0055] In secondary annealing, the steel sheet after primary annealing is subjected to Ac in the austenite single-phase temperature range. ３ After heating to an annealing temperature of above 900°C or below, the material is held for 10 to 1200 seconds. In this invention, the formation of ferrite is suppressed, and a composite structure of bainite and martensite is formed. To achieve this, it is necessary to form a single-phase austenite structure during annealing. Therefore, the annealing temperature is Ac ３ It should exceed a certain point. Preferably, Ac ３ Point + 10°C or higher, more preferably Ac ３ The temperature should be above 20°C. On the other hand, if the annealing heating temperature exceeds 900°C, the austenite becomes coarse, and it transforms into martensite or bainite while remaining coarse, making it difficult to obtain a high yield ratio. Therefore, the annealing heating temperature should be 900°C or lower. Preferably it is 880°C or lower, and more preferably 860°C or lower.

[0056] The holding time for secondary annealing should be 10 to 1200 seconds. If the holding time for secondary annealing is less than 10 seconds, the dissolution of carbides and austenite transformation will not proceed sufficiently, resulting in ferrite remaining in the final structure and a decrease in the yield ratio. Therefore, the holding time for secondary annealing should be 10 seconds or more. Preferably, it should be 13 seconds or more, and more preferably 16 seconds or more. On the other hand, if the holding time for secondary annealing exceeds 1200 seconds, the austenite grain size will coarseen, and the coarse grain will transform into martensite or bainite, resulting in a decrease in the yield ratio. Therefore, the holding time for secondary annealing should be 1200 seconds or less. Preferably, it should be 1100 seconds or less, and more preferably 1000 seconds or less.

[0057] After the above holding stage, the material is cooled at an average cooling rate of 10°C / second or higher. If the average cooling rate of the secondary cooling is less than 10°C / second, ferrite will form, making it difficult to achieve a ferrite fraction of 10% or less in the final structure, and the yield ratio will decrease. Therefore, the average cooling rate of the secondary cooling should be 10°C / second or higher. Preferably, it should be 14°C / second or higher, and more preferably 17°C / second or higher. The upper limit of the above average cooling rate is not particularly limited, but it is preferably 100°C / second or lower, and more preferably 50°C / second or lower.

[0058] The cooling stop temperature for secondary cooling is 350-600°C. This cooling stop temperature is chosen to allow for the tertiary holding process described below to take place within this temperature range.

[0059] In tertiary holding, the steel sheet after secondary annealing is held at a temperature of 350 to 600°C for 1 to 1200 seconds. If the tertiary holding temperature is below 350°C, the diffusion of alloying elements becomes difficult, suppressing the bainite transformation and increasing the amount of martensite in the microstructure. As a result, the strength becomes excessively high, and the ductility and bendability decrease. Therefore, it is necessary to set the tertiary holding temperature to 350°C or higher. Preferably, it is 400°C or higher, more preferably 450°C or higher. On the other hand, if the tertiary holding temperature exceeds 600°C, cementite formation and growth occur during holding, resulting in the formation of a large amount of cementite larger than 1 μm in the bainite, leading to a decrease in the ductility of the bainite and a decrease in the bendability of the final microstructure. Therefore, the tertiary holding temperature should be 600°C or lower. Preferably, it is 570°C or lower, more preferably 550°C or lower.

[0060] The holding time for tertiary retention should be between 1 and 1200 seconds. If the holding time for tertiary retention is less than 1 second, bainite transformation will not occur sufficiently, making it difficult to secure a bainite fraction of 10% or more in the final structure, leading to a decrease in flexibility. Therefore, a holding time of tertiary retention of 1 second or more is necessary. Preferably, it is 20 seconds or more, more preferably 40 seconds or more. On the other hand, if the holding time for tertiary retention exceeds 1200 seconds, excessive bainite will be generated, the bainite fraction will exceed 80%, and it will be difficult to secure strength. Therefore, the holding time for tertiary retention should be 1200 seconds or less. Preferably, it is 1100 seconds or less, more preferably 1000 seconds or less.

[0061] In tertiary cooling, the material is cooled at an average cooling rate of 5°C / second or more from the holding temperature of tertiary holding. The cooling stop temperature is usually room temperature, which is defined as 0 to 50°C. If the average cooling rate is less than 5°C / second, the formation and growth of bainite and carbide will occur, making it difficult to obtain the desired microstructure fraction and cementite fraction in bainite, and causing a decrease in bendability. Therefore, the average cooling rate of tertiary cooling should be 5°C / second or more. Preferably, it should be 8°C / second or more. There is no particular upper limit, but it is preferably 40°C / second or less, and more preferably 30°C / second or less.

[0062] The annealing process described above may be followed by a plating process in which a plating treatment is applied to the surface. As described above, the type of plating layer is not particularly limited in the present invention, and therefore the type of plating treatment is also not particularly limited. For example, hot-dip galvanizing treatment or a plating treatment that involves alloying after the treatment can be mentioned.

[0063] The present invention will be described in detail with reference to examples.

[0064] 1. Manufacturing of the evaluation steel plate: Steel having the component composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a vacuum melting furnace and then bloc-rolled to obtain bloc-rolled material with a thickness of 27 mm. The obtained bloc-rolled material was hot-rolled to a plate thickness of 4.0 mm. The hot-rolling conditions were a slab heating temperature of 1250°C and hot-rolling was performed under the conditions shown in Table 2. Next, the hot-rolled steel plate was ground and cold-rolled under the conditions shown in Table 2 to produce cold-rolled steel plate. Next, the cold-rolled steel plate obtained above was heat-treated (annealed) under the conditions shown in Table 2 to produce annealed cold-rolled steel plate (high-strength cold-rolled steel plate). In this example, the secondary cooling stop temperature was set as the tertiary holding temperature.

[0065]

[0066]

[0067] 2. Evaluation Methods For annealed cold-rolled steel sheets obtained under various manufacturing conditions, the microstructure was analyzed to investigate the microstructure fraction, tensile properties such as elongation and yield ratio were evaluated by conducting tensile tests, and bendability was evaluated by bending tests. The methods for each evaluation are as follows.

[0068] (Area ratio of martensite (M) and tempered martensite (TM)) The thickness L section, which is the thickness cross section in the rolling direction of each annealed cold-rolled steel sheet, was mirror-polished, and the microstructure was revealed using Nital solution. After observation using a scanning electron microscope, a 16 × 15 grid with 4.8 μm spacing was placed on a region of actual length 82 μm × 57 μm on the SEM image at a magnification of 1500x, and the area ratio of martensite and tempered martensite was investigated by point counting, which counted the number of points on each phase. The area ratio was taken as the average of three area ratios obtained from separate SEM images at a magnification of 1500x. Martensite exhibits a white microstructure, and tempered martensite also exhibits a white microstructure, but because it is more susceptible to corrosion than martensite, it exhibits a microstructure in which blocks and packets appear within the prior austenite grain boundaries.

[0069] (Area ratio of ferrite (α)) The thickness L section of each annealed cold-rolled steel sheet, which is the thickness cross section in the rolling direction, was mirror-polished, and the microstructure was revealed with Nital solution. After observation using a scanning electron microscope, a 16 × 15 grid with 4.8 μm spacing was placed on a region of actual length 82 μm × 57 μm on the SEM image at a magnification of 1500x, and the area ratio of ferrite was investigated by point counting, which counted the number of points on each phase. The area ratio was taken as the average of three area ratios obtained from separate SEM images at a magnification of 1500x. Ferrite exhibits a dark gray microstructure that does not contain cementite.

[0070] (Area ratio of bainite (B)) The thickness L section of each annealed cold-rolled steel sheet, which is the thickness cross section in the rolling direction, was mirror-polished, and the microstructure was revealed with Nital solution. After observation using a scanning electron microscope, the area ratio of bainite was investigated by point counting, where a 16 × 15 grid with 4.8 μm spacing was placed on a region of actual length 82 μm × 57 μm on the SEM image at a magnification of 1500x, and the number of points on each phase was counted. The area ratio was taken as the average of three area ratios obtained from separate SEM images at a magnification of 1500x. Bainite exhibits a dark gray microstructure containing cementite, and MA can also be confirmed within the bainite.

[0071] (Area ratio of MA in bainite and area ratio of cementite (θ) with an average particle size of 1 μm or less in bainite) The thickness L section, which is the thickness cross section in the rolling direction of each annealed cold-rolled steel sheet, was mirror polished, the microstructure was revealed with Nital solution, and then immersed in a solution containing distilled water, sodium hydroxide aqueous solution, and picric acid. After immersion, the cementite dissolved from the MA and cementite in the bainite, and the area ratios of MA and cementite were investigated by distinguishing between MA and cementite. As an observation method, a scanning electron microscope was used, and the area ratio of MA was investigated from the number of MA and the average particle size in a region of actual length 82 μm × 57 μm on the SEM image at a magnification of 1500x, and the area ratio of cementite was investigated from the number of cementite and the average particle size. MA is a white microstructure and is characterized by being precipitated in an island-like manner in bainite. Cementite precipitated in bainite was measured, and the particle size was determined from the average value of the length in the rolling direction and the length perpendicular to the rolling direction of the cementite.

[0072] Figure 1 shows an example of a microstructure in which the area ratio was measured. In Figure 1, it is difficult to distinguish between cementite and MA in bainite, so first we calculate the sum of the area ratios of cementite and MA in bainite.

[0073] Figure 2 shows an example of the microstructure after two-stage etching. In Figure 2, the cementite has dissolved and become void, appearing black. Therefore, the area ratio of cementite was determined from Figure 2, and the area ratio of MA was calculated from the difference between the area ratio of cementite and the area ratio of cementite determined in Figure 1. Also, since the voided areas appear black, the average grain size can be calculated by assuming that these black areas are cementite.

[0074] (Tensile Test) From each annealed cold-rolled steel sheet, a JIS No. 5 test specimen with a gauge length of 50 mm, gauge width of 25 mm, and plate thickness of 1.2 mm was taken perpendicular to the rolling direction, and a tensile test was performed at a tensile speed of 10 mm / min to measure the tensile strength (TS), yield ratio (YR), and total elongation (El).

[0075] (Bending Test) From each annealed cold-rolled steel sheet, a bending test specimen measuring 25 mm in width and 100 mm in length was taken so that the rolling direction was the bending axis. Using the press bending method specified in JIS Z 2248, a pressing speed of 100 mm / second was used, and n=3 tests were conducted at each bending radius. The bending radius at which no cracks were observed in any of the three specimens was defined as the limit bending radius (R) and evaluated as a ratio to the plate thickness (t). Here, the presence or absence of cracks was observed on the outer side of the bent section using a 30x magnifying glass. A specimen with no cracks at all relative to its 25 mm width was marked "◎ (excellent)", a specimen with five or fewer microcracks of 0.2 μm or less relative to its 25 mm width was marked "○ (good)", and a specimen with one or more cracks longer than 0.2 μm or more relative to its 25 mm width was marked "× (NG)". "◎" and "○" indicated that no cracks were observed. The evaluation criterion for bendability was set as limit bending radius / plate thickness (R / t) ≤ 1.5. That is, R was defined as the limit bending radius for which a "good (○)" rating was obtained, and the evaluation was based on the presence or absence and condition of microcracks when R / t = 1.5.

[0076] 3. Evaluation Results The evaluation results are shown in Table 3.

[0077]

[0078] Materials were deemed acceptable if they had a tensile strength (TS) of 780 MPa or higher, a yield ratio (YR) of 0.65 or higher, and a bendability of "○" or higher. In Tables 1 to 3, items outside the scope of the present invention are underlined.

[0079] Table 3 shows examples of the invention, which are steel sheets of the inventive steel that satisfy all the conditions of the present invention, and obtained results of a tensile strength (TS) of 780 MPa or more, a yield ratio (YR) of 0.65 or more, and a bendability of "○" or higher. All of these steel sheets of the invention are high-strength cold-rolled steel sheets with a high yield ratio and excellent bendability.

[0080] On the other hand, in Table 3, the comparative steel sheet did not exhibit good tensile strength, yield ratio (YR), or bendability.

[0081] In test specimens No. 1, 4, 30, and 38, the secondary annealing temperature was below the range specified in this invention under the manufacturing conditions, resulting in a ferrite area ratio exceeding 10% of the total steel sheet and a poor yield ratio.

[0082] In test specimen No. 7, the finishing rolling temperature during hot rolling was below the range specified in this invention, resulting in a cementite area ratio exceeding 5.0% in bainite, and thus poor bendability.

[0083] In test material No. 8, the average cooling rate during hot rolling under the manufacturing conditions fell below the range specified in this invention. As a result, the area ratio of cementite with an average particle size of 1 μm or less in the bainite exceeded 5.0%, leading to poor bendability.

[0084] In test material No. 9, the winding temperature during hot rolling was below the range specified in this invention under the manufacturing conditions, resulting in a bainite area ratio of less than 10% and a martensite area ratio exceeding 90% of the total steel sheet, leading to poor bendability.

[0085] In test material No. 12, the winding temperature during hot rolling exceeded the range specified in this invention under the manufacturing conditions, resulting in a surface area ratio of cementite with an average particle size of 1 μm or less in bainite exceeding 5.0%, leading to poor bendability.

[0086] In test specimen No. 13, the reduction ratio during cold rolling under the manufacturing conditions fell below the range specified in this invention, resulting in a ferrite area ratio of 10% or more relative to the entire steel sheet, and thus a poor yield ratio.

[0087] Test specimen No. 14 was cooled immediately without a tertiary holding time of 0 seconds, resulting in a bainite area ratio of less than 10% and a martensite area ratio exceeding 90% of the entire steel plate, leading to poor bendability.

[0088] Test specimens No. 17, 20-22, and 26 had poor yield ratios because the primary annealing conditions in the continuous annealing process during manufacturing fell below the range specified in this invention.

[0089] In test samples No. 23 and 24, the secondary annealing conditions during continuous annealing fell below the range specified in this invention, resulting in a ferrite area ratio exceeding 10% of the total steel sheet area and a poor yield ratio.

[0090] In test specimen No. 25, the tertiary holding temperature during continuous annealing was below the range specified in this invention under the manufacturing conditions, resulting in a bainite area ratio of less than 10% and a martensite area ratio exceeding 90% of the entire steel sheet, leading to poor bendability.

[0091] In test material No. 29, the tertiary holding temperature during continuous annealing exceeded the range specified in this invention under the manufacturing conditions, resulting in a surface area ratio of cementite with an average particle size of 1 μm or less in the bainite exceeding 5.0%, thus causing poor bendability.

[0092] Test specimen No. 31 had a poor yield ratio because the secondary holding time was too long, resulting in a high amount of MA in the bainite.

[0093] Test specimens No. 35 and 36 had low strength because the final cooling rate and tertiary holding time during continuous annealing were outside the specified range under the manufacturing conditions, resulting in a decrease in the total amount of martensite and tempered martensite.

[0094] Test materials No. 41 and 42 had a high Si content, resulting in a ferrite area ratio of over 10% of the total steel plate area, and thus a poor yield ratio.

[0095] Test specimens No. 43 and 45 had tensile strengths of less than 780 MPa or poor yield ratios because their C content and Mn content were below the ranges specified in this invention.

[0096] Test specimens No. 44 and 46 exhibited poor bendability because their C and Mn content exceeded the ranges specified in this invention.

[0097] Test specimens No. 47, 48, and 50 had poor bendability because their P, S, and N content were outside the range specified by the present invention.

[0098] Test material No. 49 had a poor yield ratio because its Al content was outside the specified range.

[0099] Although the example described the case of annealed cold-rolled steel sheet manufacturing, similar effects were obtained with plated steel sheets.

Claims

DEPCT631. High-strength cold-rolled steel plate with the following compositional elements, based on percentage by mass: C: 0.12% or more and 0.25% or less; Si: less than 0.5%; Mn: 2.0% or more and 3.0% or less; P: 0.05% or less; S: 0.005% or less; Al: 0.01% or more and 0.10% or less;And N: 0.010% or less, the remainder being Fe and minor impurities, the percentage total area of ​​martensite and martensite after tempering corresponding to 20% or more and 90% or less, the percentage area of ​​ferrite corresponding to 10% or less, the percentage area of ​​bainite corresponding to 10% or more and 80% or less, the percentage area of ​​the martensite-austenite component in bainite with a value of 1% or more and 10% or less, the percentage area of ​​cementite with an average grain size...

1. High-strength cold-rolled steel sheet according to claim 1, where constituent elements are also included, on a percentage by mass basis, B: 0.005% or less; 3. High-strength cold-rolled steel sheet according to claim 1 or 2, where constituent elements are also included, on a percentage by mass basis, one or two or more elements selected from: Nb: 0.005% or more and 0.1% or less;Ti: 0.005% or more and 0.1% or less; and V: 0.005% or more and 0.3% or less.

4. High-strength cold-rolled steel sheet according to one of the claims 1 to 3, where the constituent elements are also included, on a percentage by mass basis, one or two or more elements are selected from: Mo: 0.2% or less; Cr: 1.0% or less; Cu: 1.0% or less; and Ni: 1.0% or less.

5. High-strength cold-rolled steel sheet according to one of the claims 1 to 4, where the constituent elements are also included, on a percentage by mass basis, one or two elements are selected from: Ca: 0.0005% or more and 0.01% or less;and REM: 0.0005% or more and 0.01% or less.

6. High-strength cold-rolled steel sheets according to one of the claims in 1 to 5, which also include a layer which is coated on its surface.

7. High-strength cold-rolled steel sheets according to claim 6, where the coated layer is a hot-dip galvanized layer, a hot-dip galvanized and annealed layer, or a Zn-Al coated layer.

8. Methods for the production of high-strength cold-rolled steel sheets, which include: the hot-rolling process of flat steel billets with a composition according to one of the claims in 1 to 5 at a final hot-rolling temperature of 800°C or higher and 1,000°C or lower, followed by cooling at an average cooling rate of 20°C / second or more, and rolling at a rolling temperature of 550°C or higher and 700°C or lower; the cold-rolling process of hot-rolled steel sheets which are produced in the hot-rolling process at a rolling attenuation of 20% or more;The annealing process of the primary annealing operation includes primary heating to heat the cold-rolled steel sheet produced in the cold rolling process at a temperature range of Ac3 to 900°C; primary resting to allow the cold-rolled steel sheet to rest at a temperature range of Ac3 to 900°C for 10 seconds or more and 1,200 seconds or less; and following the primary resting, primary cooling to cool the cold-rolled steel sheet to a stop-cooling temperature of 100°C or lower at an average cooling rate of 3°C / second or more.Following primary annealing, secondary annealing is performed, including secondary heating to heat the cold-rolled steel sheet to temperatures above Ac3 up to 900°C; secondary resting to allow the cold-rolled steel sheet to rest at temperatures above Ac3 up to 900°C for 10 seconds or more and 1,200 seconds or less; and following secondary resting, secondary cooling to cool the cold-rolled steel sheet to a freeze-stop temperature of 350°C or higher and 600°C or lower at an average cooling rate of 10°C / second or more; and following secondary annealing, tertiary resting to allow the cold-rolled steel sheet to rest at temperatures between 350°C and 600°C for 1 second or more and 1,200 seconds or less.And tertiary cooling to cool cold-rolled steel sheets at an average cooling rate of 5 degrees Celsius / second or more.

9. Methods for the production of high-strength cold-rolled steel sheets according to claim 8, which also includes, after the annealing step, the coating step of the coating operation on the surface of the cold-rolled steel sheet.