PROCESS FOR PRODUCTION OF COLD-ROLLED STEEL SHEET HAVING EXCELLENT PRESS MOLDABILITY, AND COLD-ROLLED STEEL SHEET

Description

Technical field

[0001] The present invention relates to a method of manufacturing a cold-rolled steel sheet having excellent press formability and a cold-rolled steel sheet manufactured by the manufacturing method.

Background art

[0002] It is often the case where liquid crystal frame parts used in a mobile phone or a notebook-type personal computer, gasket parts for an automobile engine and the like are formed by a cold press. Accordingly, a thin cold-rolled steel sheet which is used for forming these parts is required to have press formability.
Further, with respect to parts of an electronic device represented by a notebook-type personal computer, a mobile phone or a digital camera or parts such as a gasket for an automobile engine, these parts are required to satisfy a demand for the reduction of weight and the miniaturization. To realize the reduction of weight and for the miniaturization of these parts, it is necessary to make a steel sheet thin. On the other hand, when the steel sheet which is made thin has the same strength as the steel sheet which is not made thin, the steel sheet made thin cannot ensure a strength which a press product is required to have. Accordingly, it is necessary to provide a high strength steel sheet having a thin gauge and a high strength.
In view of such a technical background, as a steel sheet which is used in applications such as a reinforcing member for a bumper or a door of an automobile, there has been proposed an ultra high-strength steel sheet having the martensite or bainite structure and also having a tensile strength of 1180 MPa or more (see patent document 1).
Further, patent document 2 proposes a cold-rolled steel sheet for a gasket material having excellent spring property which is manufactured in such a manner where a steel sheet is heated at a recrystallization temperature or above in continuous annealing, is annealed after soaking, and is subjected to secondary cold rolling.

Prior Art Document

Patent Document

[0003] 

Patent document 1: Japanese Patent 3254106

Patent document 2: JP-A-9-194935

Summary of the Invention

Problems that the Invention is to Solve

[0004]  However, the high-strength steel sheets which are disclosed in patent document 1 and patent document 2 aim for the acquisition of high strength so that ductility is lowered and cracks occur at the time of press forming thus giving rise to a drawback that press formability is low.
Further, to realize a thin gauge at the time of cold-rolling a high-strength hot-rolled steel sheet, a load at the time of coldrolling is increased thus giving rise to a drawback that productivity is impeded due to lowering of operability and the increase of a yield loss.
Accordingly, the present invention has been made to overcome the above-mentioned drawbacks, and it is an object of the present invention to provide a cold-rolled steel sheet which undergoes a small load at the time of cold-rolling, has excellent press formability and has high strength, and a method of manufacturing the cold-rolled steel sheet.

Means for Solving the Problems

[0005] 

(1) A method of manufacturing a cold-rolled steel sheet according to the present invention is characterized in that a hot-rolled steel sheet which has the composition consisting of, by mass%, 0.10 to 0.30 C, 0.2 or more Mn, 0.01 or more Ni, 0.5 to 2.5 Mn+Ni, 1.2 to 9.0 Cr, and Fe and unavoidable impurities as a balance and has a tensile strength of 1000 MPa or less is subjected to pickling and, thereafter, is subjected to cold rolling at a total rolling reduction of 60 % or more thus forming a cold-rolled steel sheet, and final continuous annealing treatment is performed at a soaking temperature of 750°C or above and at a cooling rate of 3 °C/s to 100 °C/s thus manufacturing a cold-rolled steel sheet having a tensile strength of 1280 MPa or more, breaking elongation of 3 % or more and a thickness of 0.05 to 0.60 mm.

(2) The method of manufacturing a cold-rolled steel sheet according to the present invention is, in the above-mentioned constitution (1), characterized in that a thickness of the hot-rolled steel sheet is 1. 2 to 3.0 mm.

(3) A cold-rolled steel sheet according to the present invention is manufactured by a method where a hot-rolled steel sheet which has the composition consisting of, by mass%, 0.10 to 0.30 C, 0.2 or more Mn, 0.01 or more Ni, 0.5 to 2.5 Mn+Ni, 1.2 to 9.0 Cr, and Fe and unavoidable impurities as a balance and has a tensile strength of 1000 MPa or less is subjected to pickling and, thereafter, is subjected to cold rolling at a total rolling reduction of 60 % ormore thus forming a cold-rolled steel sheet, and final continuous annealing treatment is performed at a soaking temperature of 750°C or above and at a cooling rate of 3 °C/s to 100 °C/s, wherein
the cold-rolled steel sheet has a tensile strength of 1280 MPa or more, breaking elongation of 3 % or more and a thickness of 0.05 to 0.60 mm.

(4) The cold-rolled steel sheet according to the present invention is, in the above-mentioned constitution (3), a thickness of the hot-rolled steel sheet is 1. 2 to 3.0 mm.

Advantageous Effects of the Invention

[0006] According to the present invention, it is possible to provide a cold-rolled steel sheet which undergoes a small load at the time of cold-rolling, and has excellent press formability while ensuring high strength and ductility.
Further, according to the method of manufacturing a cold-rolled steel sheet of the present invention, it is also possible to provide a cold-rolled steel sheet having both strength and formability which has a thickness of 0.05 mm to 0.6 mm, a tensile strength of 1280 MPa or more, and breaking elongation of 3 % or more, wherein these are conditions which a raw material used for manufacturing a gasket of a gasoline engine for an automobile, a liquid crystal frame or a frame part of a notebook-type personal computer, a mobile phone and a digital camera or the like is required to have.

Mode for Carrying Out the Invention

<Composition of hot-rolled steel sheet>

[0007] The composition of hot-rolled steel sheet is set to the following range by mass%.
C: 0.10 to 0.30, Mn: 0.2 or more, Ni: 0.01 or more, Mn+Ni: 05 to 2.5, Cr: 1.2 to 9.0, balance: Fe and unavoidable elements.
C is an important element for forming the low-temperature transformed structure such as martensite in the steel sheet such that the steel sheet acquires high tensile strength.
When the content of C is extremely small, the required tensile strength cannot be acquired and hence, a lower limit of the content of C is set to 0.10. On the other hand, when the content of C is extremely large, a rolling load at the time of hot-rolling and a rolling load at the time of cold-rolling are increased so that the productivity is impeded due to the deterioration in shape or the like and hence, an upper limit of the content of C is set to 0.30. The content of C is more preferably set to a value which falls within a range from 0.15 to 0.25.

[0008] In the present invention, both Mn and Ni are elements which enhance hardenability and form the low-temperature transformed structure at the time of continuous annealing, and Mn and Ni impart high tensile strength to a cold-rolled steel sheet to which continuous annealing is applied. When a total amount of content of Mn and Ni is extremely small, the steel sheet cannot acquire strength after continuous annealing is applied to the steel sheet and hence, a lower limit of the total amount of content of Mn and Ni is set to 0.5. On the other hand, when the total amount of content of Mn and Ni is extremely large, the above-mentioned advantageous effect is saturated and, further, workability is lowered due to the segregation and the like and hence, an upper limit of the total amount of content of Mn and Ni is set to 2.5.
Here, with respect to respective ranges of the content of Mn and the content of Ni, it is necessary for the steel sheet to contain 0.20 or more of Mn since Mn prevents red heat caused by S which is an impurity, and it is necessary to add 0.01 or more of Ni to the steel sheet since Ni ensures toughness of the steel sheet after heat treatment. Since Ni is expensive, it is advantageous to adjust the total amount of content of Mn and Ni by adjusting the content of Mn in view of cost.

[0009] Cr is also an element which can enhance hardenability, and Cr forms the low-temperature transformed structure at the time of continuous annealing so that the steel sheet can acquire high tensile strength. When the content of Cr is extremely small, the steel sheet cannot acquire tensile strength after continuous annealing is applied to the steel sheet and hence, a lower limit of the content of Cr is set to 1.2. On the other hand, when the content of Cr is extremely large, the above-mentioned advantageous effect is saturated and, further, cost is increased wastefully and hence, an upper limit of the content of Cr is set to 9.0. The content of Cr is more preferably set to a value which falls within a range from 2.0 to 5.5.

[0010] Si is an element effective for greatly increasing strength of a cold-rolled steel sheet. The larger content of Si, the more easily the obj ect canbe achieved. However, when the content of Si exceeds 2.0 %, a load in cold-rolling is increased and a shape of the steel sheet is deteriorated and hence, an upper limit value of the content of Si is set to 2.0 %.

[0011] P is a component which makes crystal grains fine and hence, it is desirable to add a fixed amount of P to the steel sheet for enhancing strength of a cold-rolled steel sheet. However, P also causes segregation in a crystal grain boundary thus inducing brittleness of the steel sheet. Accordingly, the content of P is set to 0.06 % or less.

[0012] S is an impurity component which generates red-brittleness during hot rolling and hence, it is desirable that the steel sheet contains as little S as possible. However, it is impossible to completely prevent the mixing of S from a raw material or the like into a steel sheet, and the desulfurization in the manufacturing steps is also limited and hence, it is unavoidable that S remains in the steel sheet to some extent. The red-brittleness caused by a small amount of residual S can be alleviated by Mn and hence, an upper limit value of the content of S is set to 0.06 %.

[0013] A fixed amount of Cu may be added to the steel sheet for enhancing the strength of a steel sheet by solid solution strengthening or by precipitation strengthening. On the other hand, however, there exists a possibility that Cu will bring about brittleness in the steel sheet at the time of hot rolling. Accordingly, an upper limit of Cu is set to 0.5 %.

[0014] Al is added to a steel bath as a deoxidizing agent in making steel. Al reacts with solid solution N and segregates as AIN thus contributing to making the crystal grain fine. On the other hand, when the content of Al exceeds 0.10 %, the solidification of N becomes conspicuous thus lowering solid solution strengthening brought about by N and hence, the content of Al is set to 0.10 % or less.

[0015] The addition of Ti is effective for making the crystal grain fine, for suppressing a grain growth, for enhancing corrosion resistance and the like. However, even when the content of Ti added to the steel sheet is excessively large, the advantageous effect is saturated and hence, the content of Ti is set to 0.30 % or less.

[0016] N is , in the same manner as C and Mn, a component necessary for imparting high strength to a cold-rolled steel sheet and for increasing durability of the cold-rolled steel sheet. However, setting the content of N to less than 0.002 % is difficult in terms of making steel. On the other hand, the addition of the content of N which exceeds 0.015 % remarkably lowers a yield rate of ferro-nitride at the time of making steel thus making the steel sheet unstable, and also remarkably deteriorates anisotropy of the steel sheet at the time of press forming.
Further, cracks occur on a surface of a continuously-cast slab and the cracks become a defect in casting. Accordingly, the content of N is desirably set to 0.015 % or less in the present invention.

<Manufacturing steps>

[0017] Hereinafter, a method of manufacturing a cold-rolled steel sheet according to the present invention is explained.

<Making steel>

[0018] Raw materials are melted in a converter or an electric furnace to produce molten steel and molten steel is cast for producing a slab whose composition is adjusted to the above-mentioned range.

<Hot rolling>

[0019] A slab whose components are adjusted to the above-mentioned range is formed into a steel sheet having a sheet thickness of 1.2 to 3.0 mm by hot-rolling. When the sheet thickness is set small, a load at the time of hot-rolling is increased. Accordingly, a lower limit of the sheet thickness is set to 1.2. On the other hand, when the sheet thickness is set large, a load at the time of cold-rolling which is performed after hot-rolling is increased. Accordingly, an upper limit of the sheet thickness is set to 3.0.
In the hot-rolling step, a heat temperature of the slab having the above-mentioned composition is set to 1100 °C or above, and a coiling temperature (CT) is set to 600 °C or above. When the heating temperature of the slab is below 1100 °C, the positive decomposition solid solution of N becomes insufficient and a load at the time of hot-rolling is increased. Accordingly, such a heating temperature is not preferable.
Further, the winding temperature is set to a value which falls within a range from 600 °C to 800 °C. When the winding temperature is low, strength of the hot-rolled steel sheet is largely increased so that such a steel sheet is not preferable for cold-rolling. Accordingly, a lower limit of the winding temperature is set to 600°C.
On the other hand, when the winding temperature exceeds 800 °C, the generation of scales is accelerated at the time of hot rolling so that a load at the time of descaling by pickling is increased whereby an upper limit of the winding temperature is set to 800°C.
The hot-rolled steel sheet which is manufactured under the above-mentioned condition may have a tensile strength of 1000 MPa or less. This is because when tensile strength of the hot-rolled steel sheet exceeds 1000 MPa, a rolling load at the time of cold rolling is increased so that the tensile strength exceeding 1000 MPa is not preferable.

<Pickling>

[0020] Scales generated on a surface of the steel sheet at the time of hot-rolling are removed in an acid bath in accordance with a normal method and, thereafter, the steel sheet is subjected to cold-rolling and continuous annealing.

<Cold-rolling>

[0021] Cold-rolling is applied to the steel sheet once or several times at a rolling reduction of 60 % or more in total until the steel sheet, as a product, acquires a predetermined product thickness of 0.05 to 0.6 mm. This is because when the product thickness becomes 0.05 mm or less, rigidity of the steel sheet is lowered so that there may be a case where when the steel sheet is used for manufacturing products such as a gasket of a gasoline engine of an automobile, a liquid crystal frame or a frame part of a notebook-type personal computer, a mobile phone and a digital camera, the shape of the product is liable to be deformed and hence, the product cannot be manufactured.
On the other hand, when the product thickness becomes 0.6 mm or more, there may be a case where when the steel sheet is used for manufacturing a product such as a gasket of a gasoline engine of an automobile, a liquid crystal frame or a frame part of anotebook-typepersonal computer, a mobile phone and a digital camera, a weight of the product becomes larger than a designed value or the miniaturization of the product cannot be realized.

[0022] Although an upper limit of the total rolling reduction in cold-rolling is not particularly limited, the upper limit of the total rolling reduction is set to 98 %.
Further, by applying continuous annealing at a temperature of 500 °C or above or batch annealing at a temperature of 500 °C or above to the steel sheet after cold-rolling, the rolled sheet which is hardened by work hardening is softened so that cold-rolling can be applied to the steel sheet again.
A method of cold-rolling and the number of times that cold-rolling is performed are not particularly designated, and the method and the number of times that cold-rolling is performed can be suitably selected in accordance with a target sheet thickness.

<Continuous annealing>

[0023] Continuous annealing is applied to the steel sheet for removing strains in the sheet generated at the time of cold-rolling. When cold-rolling is performed plural times, continuous annealing can be performed in each cold-rolling.
According to the present invention, in the final continuous annealing, the steel sheet is soaked and held at a soaking temperature of 750°C or above for 1 second or more and 100 seconds or less. When the soaking temperature is below750°C, a temperature of the steel sheet does not exceed an Ae3 transformation temperature and hence, the steel sheet cannot acquire sufficient tensile strength.
On the other hand, even when the soaking temperature exceeds 1000°C, there is no particular advantageous effect, and such a temperature is wasteful from the industrial viewpoint and hence, an upper limit of the soaking temperature is set to 1000°C.
In the final continuous annealing, a part or the entire steel sheet is formed into austenite by heating the steel sheet and, thereafter, these parts are transformed into martensite or the like by cooling after such heating.
According to the present invention, corresponding to an amount of martensite and an amount of alloy element, the steel sheet can acquire a predetermined strength.

[0024]  After the steel sheet is soaked and held at a soaking temperature of 750°C or above for 1 second or more and 100 seconds or less, the steel sheet is cooled at a cooling rate of 3°C/s to 100°C/s. Due to such cooling, the austenite structure of the steel sheet is formed into structures such as martensite, tempered martensite and bainite. When the cooling rate is 3°C/s or less, the formation of the low-temperature transformed structure such as martensite becomes insufficient and hence, the steel sheet cannot acquire sufficient strength. On the other hand, when the cooling rate exceeds 100°C/s, although the steel sheet can acquire required strength, a shape of the steel sheet is deteriorated and hence, such a steel sheet is not preferable for press application.

[0025] In the present invention, after the continuous annealing, a steel sheet is cooled at a cooling rate which is remarkably slow compared with a cooling rate for general quenching using water or the like which is performed for ensuring a shape so that the low-temperature transformed structure is formed and thereby a high strength steel sheet is acquired.
With respect to Mn and Ni which are effective austenite stabilizing elements used as general quenching elements, it is necessary to add a considerable amount of Mn and Ni to the steel sheet to acquire a required strength at the above-mentioned cooling rate. However, the addition of a considerable amount of Mn and Ni to the steel sheet largely increases strength of the steel sheet after hot-rolling so that a load at the time of cold-rolling is increased whereby productivity is impeded thus inducing an increase in product cost.
According to the present invention, the content of Mn and the content of Ni are set relatively small, and Cr which is a ferrite stabilizing element is added to the steel sheet so as to form a composite and hence, it is possible to make the steel sheet relatively soft after hot-rolling and to manufacture a high-strength steel sheet by continuous annealing which is performed after cold-rolling whereby a load in the manufacture of the steel sheet can be significantly reduced.

[0026] Further, when necessary, tempering treatment below 600 °C may be applied to the steel sheet after the steel sheet is cooled after final continuous annealing. In the present invention, however, the steel sheet is formed into the quenched structure at a relatively low cooling rate and hence, an effect substantially equal to an effect obtained by tempering treatment can be acquired at the time of cooling whereby the tempering step can be omitted. The omission of the tempering step largely contributes to the reduction of a load in the manufacture of the steel sheet.

[0027] Due to the above-mentioned continuous annealing applied to the steel sheet, tensile strength of a cold-rolled steel sheet acquired by the manufacturing method of the present invention can be increased to 1280 MPa or more. Since the cold-rolled steel sheet has such strength, when the cold-rolled steel sheet is used for manufacturing a gasket of a gasoline engine of an automobile, there is no possibility that a gas leak will occur.
Further, when a sheet thickness of the cold-rolled steel sheet is made small for reduction of weight and the cold-rolled steel sheet is used for manufacturing a liquid crystal frame or frame parts of a notebook-type personal computer, a mobile phone, or a digital camera, and particularly when the cold-rolled steel sheet is used inmobile applications, theseparts can ensure the required rigidity as the parts.

[0028] Further, in the manufacturing method of the present invention, breaking elongation of a cold-rolled steel sheet can be set to 3 % or more. A value of breaking elongation indicative of ductility is important when a cold-rolled steel sheet is press-formed for manufacturing products such as gaskets for an automobile engine and electronic parts. When breaking elongation is less than 3 %, cracks are liable to occur inportions which are subjected to some degree of bulging forming, bending at 90° or the like.

[0029] A steel sheet manufactured by the above-mentioned manufacturing method has the structure formed of martensite, tempered martensite, bainite structure and the like by a volume% of 60 % or more, has high tensile strength of 1280 MPa or more, and ensures ductility.

<Temper rolling or the like>

[0030] The steel sheet obtained in this manner is, whennecessary, subjected to temper rolling for surface roughness adjustment or subjected to electro plating and chemical conversion coating using Zn, Ni or the like for rust prevention.

<Press forming>

[0031] The steel sheet obtained by the manufacturing method of the present invention can be used as a raw material for press forming. However, the steel sheet obtained by the manufacturing method of the present invention largely differs from a generally-used cold-rolled steel sheet in a strength range and hence, it is necessary to take into account spring-back or the like.

Examples

[0032] To explain a cold-rolled steel sheet of the present invention in further detail in conjunction with examples and comparison examples, slabs having compositions corresponding to specimens 1 to 18 shown in Table 1 are prepared. The specimens 1 to 11, 17, 18 are slabs having the compositions which fall within a composition range of the present invention, while the specimens 12 to 16 are slabs which do not fall within the composition range of the present invention.
In hot-rolling step, the slabs which are adjusted to the compositions corresponding to specimens 1 to 18 are heated at a temperature of 1230 °C, and hot-rolled steel sheets having a sheet thickness of 2.0 mm are manufactured at winding temperatures shown in Table 2. Properties of these hot-rolled steel sheets are shown in Table 2.
Next, after pickling these hot-rolled steel sheets, the hot-rolled steel sheets are cold-rolled to a thickness of 0.5 mm. Thereafter, inacontinuousannealingstep, thecold-rolled steel sheets are soaked and held at a soaking temperature of 900 °C for 20 seconds and, thereafter, the cold-rolled steel sheets are cooled at a cooling rate of 20 °C/s thus forming cold-rolled steel sheets. Properties of these cold-rolled steel sheets are shown in Table 2.
As can be understood from Table 2, in the examples 1 to 15, tensile strength of the hot-rolled steel sheets is 1000 MPa or less so that a thickness of the steel sheet can be reduced to targeted 0.5 mm in cold rolling which is performed after hot-rolling, and tensile strength of the cold-rolled steel sheet can be set to 1280 MPa or more and breaking elongation can be set to 3 % or more.
In the examples 3, 4, by changing a cooling rate after soaking to 3 °C/s, 100 °C/s respectively, cold-rolled steel sheets which have tensile strengths of 1320 MPa and 1405 MPa and have breaking elongation of 6.2 % and 6.0 % respectively can be obtained.

[0033] A hot-rolled steel sheet of a comparison example 1 is manufactured using the specimen 1 from which a hot-rolled steel sheet of the example 1 is also manufactured. However, the winding temperature is set to a low temperature of 480 °C and hence, tensile strength of a hot-rolled steel sheet is increased to 1108 MPa whereby the hot-rolled steel sheet becomes hard. Accordingly, a thickness of the steel sheet cannot be reduced to targeted 0.5 mm in cold rolling which is performed after hot-rolling, and when a rolling load and the number of times that rolling is performed are increased, cracks occur in the steel sheet and hence, cold-rolling is stopped.
A hot-rolled steel sheet of a comparison example 2 is manufactured using the specimen 2 from which hot-rolled steel sheets of the examples 2 to 4 are also manufactured. However, the winding temperature is set to a low temperature of 500 °C and hence, tensile strength of a hot-rolled steel sheet is increased to 1216 MPa whereby the hot-rolled steel sheet becomes hard. Accordingly, a thickness of the steel sheet cannot be reduced to targeted 0.5 mm in cold rolling which is performed after hot-rolling, and when a rolling load and the number of times that rolling is performed are increased, cracks occur in the steel sheet and hence, cold-rolling is stopped.
The content of C is set to 0.10 or less in comparison examples 3, 4, the content of Cr is set to 1.2 % or less in a comparison example 5, and the content of Mn and the content of Ni are small and the content of Mn+Ni is set to 0.5 % or less in a comparison example 7. Accordingly, in the respective comparison examples, tensile strength of a cold-rolled steel sheet becomes 1280 MPa or less so that when the cold-rolled steel sheet is used for manufacturing gaskets or frame parts, the cold-rolled steel sheet has insufficient strength whereby the cold-rolled steel sheet is not applicable to the manufacture of the gaskets and the frame parts.
In a comparison example 6, the total content of Mn+Ni is large, that is, 3.4 and hence, tensile strength of the hot-rolled steel sheet is increased whereby the steel sheet cannot be rolled to 0.5 mm which is a target thickness in cold-rolling which is performed after hot-rolling. To be more specific, due to high hardness, a thickness of the hot-rolled steel sheet cannot be reduced even when the hot-rolled steel sheet is rolled, or when a rolling load or the number of times that rolling is performed is increased, cracks occur in the steel sheet so that the cold-rolled steel sheet cannot be manufactured.

[0034] 

[Table 1]

	composition value [wt%]           slab
specimen	c	si	Mn	P	S	Cu	Ni	Cr	S.Al	Ti	N	Mn+Ni
1	0.15	0.18	0.8	0.004	0.005	0.19	0.1	3.1	0.019	0.00	0.009	0.9	example 1
													comparison example 1
2	0.14	0.17	1.6	0.006	0.003	0.20	0.1	2.5	0.026	0.10	0.010	1.7	example 2
													example 3
													example 4
													comparison example 2
3	0.15	0.18	1.61	0.006	0.005	0.19	0.10	3.1	0.024	0.00	0.012	1.7	example 5
4	0.15	0.20	0.89	0.004	0.003	0.19	1.02	2.6	0.026	0.10	0.010	1.9	example 6
5	0.18	1.12	0.78	0.003	0.003	0.05	0.07	2.5	0.010	0.03	0.010	0.9	example 7
6	0.15	0.21	0.82	0.005	0.006	0.19	0.11	5.1	0.017	0.00	0.013	0.9	example 8
7	0.14	0.19	0.42	0.015	0.005	0.31	0.11	3.0	0.031	0.00	0.008	0.5	example 9
8	0.16	0.28	1 60	0.012	0.005	0.19	0.66	3.0	0.012	0.00	0.011	2.3	example 10
9	0.10	0.31	1.62	0.005	0.002	0.18	0.13	2.9	0.025	0.00	0.009	1.8	example 11
10	0.30	0.55	1.19	0.003	0.004	0.15	0.12	2.0	0.031	0.00	0.008	1.3	example 12
11	0.11	0.25	0.44	0.011	0.010	0.11	0.05	8.9	0.025	0.00	0.011	0.5	example 13
12	0.08	0.21	0.82	0.006	0.005	0.19	0.11	7.2	0.008	0.00	0.016	0.9	comparison example 3
13	0.07	0.18	1.60	0.006	0.003	0.19	0.10	2.5	0.025	0.10	0.010	1.7	comparison example 4
14	0.18	1.17	0.72	0.004	0.005	0.12	0.09	1.0	0.017	0.11	0.008	0.8	comparison example 5
15	0.15	0.20	0.84	0.004	0.003	0.18	2.51	2.6	0.023	0.10	0.009	3.4	comparison example 6
16	0.19	1.17	0.36	0.001	0.005	0.05	0.06	2.1	0.020	0.01	0.010	0.4	comparison example 7
17	0.18	0.21	1.49	0.005	0.003	0.19	0.09	1.5	0.022	0.00	0.006	1.6	example 14
18	0.18	0.22	1.82	0.004	0.004	0.18	0.11	1.2	0.028	0.01	0.009	1.9	example 15

[0035] 

[Table 2]

	mechanical properties of hot-rolled steel sheet			properties after continuous annealing
	winding temperature [°C]	tensile strength [MPa]	breaking elongation [%]	temperature [°C]	cooling rate [°C/s]	tensile strength [MPa]	breaking elongation [%]
example 1	650	684	20.8	900	20	1304	6.1
comparison example 1	480	1108	12.2	-	20	-	-
example 2	650	646	19.6	900	20	1348	6.1
example 3				900	3	1320	6.2
example 4				900	100	1405	6.0
comparison example 2	500	1216	10.4	-	20	-	-
example5	650	585	22.8	900	20	1392	6.3
Example6	650	672	19.4	900	20	1439	7.1
example 7	620	635	18.8	900	20	1501	6.9
example 8	650	621	21.7	900	20	1359	6.0
example 9	650	584	22.2	900	20	1371	6.4
example 10	650	712	17.7	900	20	1381	6.2
example 11	650	532	22.8	900	20	1327	6.3
example 12	650	694	19.1	900	20	1588	6.0
example 13	650	579	18.9	900	20	1388	6.7
comparison example 3	650	547	26.7	900	20	1214	6.4
comparison example 4	650	625	19.8	900	20	1052	7.4
comparison example 5	620	532	21.3	900	20	853	18.5
comparison example 6	650	1282	7.7	900	20	-	-
comparison example 7	650	551	20.9	900	20	1227	8.3
example 14	650	588	21.1	900	20	1388	6.1
example 15	650	655	19.2	900	20	1421	6.5

[0036] According to the present invention, it is possible to provide a cold-rolled steel sheet which undergoes a small load at the time of cold-rolling, has excellent press formability while ensuring high strength and ductility. Further, it is also possible to provide a cold-rolled steel sheet having both strength and formability which has a thickness of 0.05 mm to 0.6 mm, a tensile strength of 1280 MPa or more, and breaking elongation of 3 % or more, wherein these are conditions which a raw material used for manufacturing a gasket of a gasoline engine for an automobile, a liquid crystal frame or a frame part of anotebook-typepersonal computer, a mobile phone and a digital camera or the like is required to have. Accordingly, the present invention has an extremely high industrial applicability.

Claims

1. A method of manufacturing a cold-rolled steel sheet in which a hot-rolled steel sheet which has the composition consisting of, by mass%, 0.10 to 0.30 C, 0.2 or more Mn, 0.01 or more Ni, 0.5 to 2.5 Mn+Ni, 1.2 to 9.0 Cr, and Fe and unavoidable impurities as a balance and has a tensile strength of 1000 MPa or less is subjected to pickling and, thereafter, is subjected to cold rolling at a total rolling reduction of 60 % or more thus forming a cold-rolled steel sheet, and final continuous annealing treatment is performed at a soaking temperature of 750 °C or above and at a cooling rate of 3 °C/s to 100 °C/s thus manufacturing a cold-rolled steel sheet having a tensile strength of 1280 MPa or more, breaking elongation of 3 % or more and a thickness of 0.05 to 0.60 mm.

2. The method of manufacturing a cold-rolled steel sheet according to claim 1, wherein a thickness of the hot-rolled steel sheet is 1. 2 to 3.0 mm.

3. A cold-rolled steel sheet which is manufactured by a method where a hot-rolled steel sheet which has the composition consisting of, by mass%, 0.10 to 0.30 C, 0.2 or more Mn, 0.01 or more Ni, 0.5 to 2.5 Mn+Ni, 1.2 to 9.0 Cr, and Fe and unavoidable impurities as a balance and has a tensile strength of 1000 MPa or less is subjected to pickling and, thereafter, is subjected to cold rolling at a total rolling reduction of 60 % or more thus forming a cold-rolled steel sheet, and final continuous annealing treatment is performed at a soaking temperature of 750 °C or above and at a cooling rate of 3 °C/s to 100 °C/s, wherein the cold-rolled steel sheet has a tensile strength of 1280 MPa or more, breaking elongation of 3 % or more and a thickness of 0.05 to 0.60 mm.

4. The cold-rolled steel sheet according to claim 3, wherein a thickness of the hot-rolled steel sheet is 1. 2 to 3.0 mm.