JP2012237042A - High-strength cold-rolled steel sheet excellent in workability and method for production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet with a tensile strength TS of 1,180 MPa or more, which is improved in workability such as elongation, stretch flanging property, and bendability by adjusting a metal structure without containing expensive rare metals from a viewpoint of weldability and formability.SOLUTION: The high-strength cold-rolled steel sheet has a component composition including, by mass%, 0.16-0.26% C, 1.2-2.2% Si, 2.6-3.6% Mn, 0.020% or less P, 0.0040% or less S, 0.005-0.08% Al, 0.008% or less N, 0.001-0.040% Ti, and 0.0001-0.0020% B, with the balance comprising Fe and unavoidable impurities and has a structure including, by volume fraction, 40-70% ferrite phase, 15-35% bainite phase, 5-25% tempered martensite phase, and 2-20% retained austenite phase, wherein the volume fraction of a martensite phase with a long axis length of ≥10 μm accounts for 30% or less of the total volume fraction of the tempered martensite phase.

Description

  The present invention relates to a high-strength cold-rolled steel sheet suitable for use in automobile parts and the like that are required to be press-formed into a complicated shape, and a method for producing the same, and in particular, Nb, V, Cu, Ni, Cr, Mo Elongation is achieved by utilizing residual austenite without actively adding expensive elements, making the metal structure a uniform structure mainly composed of a ferrite phase, and controlling the grain size of the tempered martensite phase. (El), stretch flangeability (usually evaluated by the hole expansion ratio (λ)) and bendability are improved at the same time, while at the same time achieving high strength with a tensile strength (TS) of 1180 MPa or more. To do.

In recent years, steel plates with a tensile strength (TS) of 590 MPa or more have been actively applied to automobile bodies for the purpose of improving fuel economy and collision safety by reducing the weight of automobile bodies. Application of steel sheets is being studied.
Conventionally, high-strength steel sheets of TS: 1180 MPa class or higher were often applied to light-worked parts, but recently, presses with complex shapes are required to achieve even greater collision safety and improved fuel economy by reducing vehicle weight. Applications to parts are being studied, and there is a great need for steel sheets with excellent workability.

  However, since steel sheets generally tend to have lower workability with higher strength, avoiding cracks during press forming is a major issue in expanding the application of high-strength steel sheets. In particular, when the strength is increased to TS: 1180 MPa class or higher, very expensive rare elements such as Nb, V, Cu, Ni, Cr and Mo are often positively added from the viewpoint of securing strength.

  For example, Patent Documents 1 to 4 mainly describe tempered martensite phase and retained austenite by limiting steel components and structures, optimizing hot-rolling conditions, and annealing conditions as conventional techniques related to high-strength cold-rolled steel sheets with excellent formability. The manufacturing technology of the high-strength cold-rolled steel sheet is disclosed.

JP 2004-308002 A JP 2005-179703 A JP 2006-283130 A JP 2004-359974 A

However, although the technique described in Patent Document 1 does not require an expensive element, in order to obtain fine massive martensite, it is necessary to repeat the treatment at a high temperature of A ₃ point or more and 1100 ° C. or less twice. Therefore, the cost becomes high. Moreover, although the knowledge which obtains high El in the component system with much C amount is disclosed, in addition to El at low C amount level, there is no knowledge about balancing stretch flangeability and bendability.
The technique described in Patent Document 2 has a disadvantage in that expensive Ni and Cu are essential as an austenite stabilizing element. Moreover, although the knowledge which achieves high El in TS: 780-980MPa level using residual austenite phase is disclosed, sufficient stretch flangeability is not obtained by TS: 1180MPa or more with much C amount, There is no knowledge about improvement of bendability.
The technique described in Patent Document 3 is sufficient because the Al content of the inventive steel sheet disclosed in the Examples is excessive, so that problems remain in weldability and the volume fraction of the tempered martensite phase is too large. TS x El balance may not be achieved. Furthermore, there is no knowledge regarding the improvement of stretch flangeability and bendability.
The technique described in Patent Document 4 not only requires expensive Mo and V, but also has no knowledge about workability, and in fact has a small volume fraction of retained austenite phase and a volume fraction of tempered martensite phase. Since there are too many, a problem remains in workability.

  The present invention advantageously solves the above-mentioned problems, and does not contain expensive rare metals from the viewpoint of weldability and formability, and can be stretched by adjusting the metal structure, workability such as stretch flangeability and bendability. An object of the present invention is to provide a high-strength cold-rolled steel sheet having an improved tensile strength TS of 1180 MPa or more together with its advantageous production method.

As a result of intensive research to solve the above problems, the inventors have obtained the following knowledge.
That is, by adjusting the metal structure such as ferrite phase, bainite phase, tempered martensite phase and residual austenite phase without containing expensive alloying elements Nb, V, Cu, Ni, Cr, Mo In addition, the volume fraction of the bainite phase that forms low temperature transformation from austenite, and the size and volume fraction of the tempered martensite phase that has been softened by annealing for softening, can be improved. It was found that by strictly controlling, workability such as elongation, stretch flangeability and bendability can be improved, and high tensile strength TS of 1180 MPa or more can be achieved.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) In mass%,
C: 0.16-0.26%
Si: 1.2-2.2%
Mn: 2.6-3.6%
P: 0.020% or less,
S: 0.0040% or less,
Al: 0.005-0.08%,
N: 0.008% or less,
Ti: 0.001 to 0.040% and B: 0.0001 to 0.0020%
The balance has a component composition consisting of Fe and inevitable impurities, and in volume fraction,
Ferrite phase: 40-70%,
Baynite phase: 15-35%
Tempered martensite phase: 5-25% and residual austenite phase: 2-20%
And the ratio of the tempered martensite phase with the major axis length ≧ 10 μm in the total volume fraction of the tempered martensite phase satisfies 30% or less (including 0%), and has excellent workability High strength cold rolled steel sheet.

(2) A steel slab having the composition described in 1 above is subjected to a first annealing in a temperature range of 400 to 800 ° C. after hot rolling, followed by cold rolling and then a temperature of 760 to 860 ° C. The second annealing is performed in the region, the cooling rate is 10 to 80 ° C./second, the cooling stop temperature is 300 to 500 ° C., the temperature is kept for 100 to 1000 seconds, the cooling is performed, and then 200 to 400 A method for producing a high-strength cold-rolled steel sheet having excellent workability, characterized by performing a third annealing in a temperature range of ° C.

  According to the present invention, it is possible to obtain a high-strength cold-rolled steel sheet having excellent elongation, stretch flangeability and bendability, and having a tensile strength of 1180 MPa or more, without containing an expensive alloy element. The high-strength cold-rolled steel sheet obtained by the present invention is particularly effective when used for automobile parts that are press-formed into a particularly severe shape.

Hereinafter, the present invention will be specifically described.
Now, as a result of intensive studies on improvement of workability of high-strength cold-rolled steel sheets, particularly workability such as elongation, stretch flangeability and bendability, the inventors have studied Nb, V, Cu, Ni, Cr, Even in a component system that does not contain expensive alloy elements such as Mo, 40 to 70% ferrite phase, 15 to 35% bainite phase, 5 to 25% tempered martensite phase, and 2 to 20 by volume fraction, respectively. % Of the retained austenite phase and the ratio of the tempered martensite phase with a major axis length ≧ 10 μm in the total volume fraction of the tempered martensite phase is 30% or less. It has been found that this is advantageously achieved.
Hereinafter, the reasons for limiting the component composition and structure of the present invention will be specifically described. The unit of the element content in the steel sheet is “mass%”, but hereinafter, it is simply indicated by “%” unless otherwise specified.

First, the appropriate range of the component composition of steel in the present invention and the reasons for limitation are as follows.
C: 0.16-0.26%
C is an element that contributes to strength, and effectively contributes to securing strength by solid solution strengthening and structure strengthening by a low-temperature transformation phase. However, if the amount of C is less than 0.16%, it is difficult to obtain a low temperature transformation phase having a required volume fraction. On the other hand, if it exceeds 0.26%, not only the spot weldability is remarkably deteriorated, but the low temperature transformation phase is excessively hardened. Decreasing moldability, especially stretch flangeability. Therefore, the C content is in the range of 0.16 to 0.26%.

Si: 1.2-2.2%
Si is an important element for promoting C concentration in austenite and stabilizing retained austenite. In order to obtain the above action, it is necessary to contain 1.2% or more, preferably 1.4% or more. On the other hand, if the Si content exceeds 2.2%, the steel sheet becomes brittle and cracks occur, resulting in a decrease in formability. Therefore, the Si content is 2.2% or less. Preferably it is 2.0% or less.

Mn: 2.6-3.6%
Mn is an element that improves hardenability and has an effect of easily ensuring a low-temperature transformation phase that contributes to strength. In order to obtain the above effect, it is necessary to contain 2.6% or more, but if it exceeds 3.6%, it becomes excessively hard, resulting in insufficient hot ductility and slab cracking. Therefore, the Mn content is in the range of 2.6 to 3.6%. Preferably it is 2.6 to 3.0% of range.

P: 0.020% or less Since P adversely affects spot weldability, it is preferable to reduce it as much as possible, but 0.020% is acceptable. However, excessively reducing the amount of P lowers the production efficiency in the steelmaking process and increases the cost, so the lower limit of the amount of P is preferably about 0.001%.

S: 0.0040% or less S not only segregates at the grain boundary to induce hot brittleness, but also forms sulfide inclusions such as MnS, and this MnS expands by cold rolling, In order to reduce the local deformability as a starting point of cracking, it is preferable to reduce as much as possible, but 0.0040% is acceptable. However, excessive reduction is industrially difficult and causes an increase in desulfurization cost in the steel making process, so the lower limit of the amount of S is preferably about 0.0001%. Preferably it is 0.0001 to 0.0030% of range.

Al: 0.005-0.08%
Al is mainly added for the purpose of deoxidation. Moreover, it is effective in suppressing the formation of carbides and generating a retained austenite phase, and is a useful element for improving the strength-elongation balance. In order to achieve the above object, it is necessary to contain 0.005% or more, but if it exceeds 0.08%, there arises a problem of deterioration of workability due to an increase in inclusions such as alumina. Therefore, the Al content is in the range of 0.005 to 0.08%. Preferably it is 0.02 to 0.06% of range.

N: 0.008% or less N is an element that deteriorates aging resistance. When the N content exceeds 0.008%, deterioration of aging resistance becomes remarkable. Moreover, it combines with the contained B to form BN to consume B, lower the hardenability of the solid solution B, and it becomes difficult to secure a martensite phase having a predetermined volume fraction. Moreover, since it exists as an impurity element in ferrite and lowers the ductility by strain aging, it is preferable that the N content is low, but it is acceptable up to 0.008%. However, excessive reduction of the amount of N causes an increase in denitrification costs in the steelmaking process, so the lower limit of the amount of N is preferably about 0.0001%. More preferably, it is 0.0010 to 0.0060% of range.

Ti: 0.001 to 0.040%
Ti forms carbonitrides and sulfides and contributes effectively to improving strength. Moreover, it is an element effective in suppressing the formation of BN by fixing N as TiN and exhibiting hardenability by B. In order to obtain these effects, it is necessary to contain 0.001% or more. However, if the Ti content exceeds 0.040%, excessive precipitates are generated in the ferrite phase, and excessive precipitation strengthening causes a decrease in elongation. Therefore, the Ti amount is in the range of 0.001 to 0.040%. Preferably it is 0.010 to 0.030% of range.

B: 0.0001-0.0020%
B is an element effective for increasing the hardenability and effectively contributing to securing low-temperature transformation phases such as a martensite phase and a retained austenite phase, and obtaining an excellent strength-elongation balance. In order to obtain this effect, it is necessary to contain B in an amount of 0.0001% or more. However, if the amount of B exceeds 0.0020%, the above effect is saturated. Therefore, the B content is in the range of 0.0001 to 0.0020%.

  In the steel sheet of the present invention, components other than those described above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

Next, the appropriate range of the steel structure, which is one of the important requirements for the present invention, and the reason for the limitation will be described.
Ferrite phase volume fraction: 40-70%
The ferrite phase is softer than the martensite phase, which is a low-temperature transformation phase from austenite, and the tempered martensite phase obtained by further heat-treating it, and contributes to the improvement of ductility. To obtain the desired elongation, a ferrite phase with a volume fraction of 40% or more is required. If the ferrite phase is less than 40%, the volume fraction of the hard bainite phase and martensite phase increases. The strength becomes excessively high, and the stretch and the stretch flange deteriorate. On the other hand, when the ferrite phase exceeds 70%, it is difficult not only to secure the strength: 1180 MPa, but also to secure a predetermined amount of retained austenite phase that contributes to ductility. Therefore, the volume fraction of the ferrite phase is in the range of 40 to 70%.

Volume fraction of bainite phase: 15-35%
The bainite phase is transformed at a higher temperature than the martensite phase, which is also a low-temperature transformation phase from austenite, and is softer than the martensite phase, but is harder than the ferrite phase and contributes to an improvement in strength. Further, by proceeding with the bainite transformation, C concentration in the austenite phase is promoted, and a predetermined amount of residual austenite phase that ultimately contributes to elongation can be secured. To achieve these objectives, the volume fraction of the bainite phase needs to be 15% or more. On the other hand, if the bainite phase exceeds 35%, the strength is excessively increased, so that it is difficult to ensure elongation.

Volume fraction of tempered martensite phase: 5-25%
The tempered martensite phase obtained by reheating the hard martensite phase contributes to the improvement of strength. TS: To ensure a tensile strength of 1180 MPa or more, a tempered martensite phase with a volume fraction of 5% or more. Need. However, when the volume fraction of the tempered martensite phase increases, the strength increases excessively and the elongation decreases, so the volume fraction of the tempered martensite phase is limited to 25% or less. And by adjusting the tempered martensite phase in the range of 5 to 25% in terms of volume fraction, a material balance with good strength, elongation, stretch flangeability and bendability can be obtained.

Volume fraction of retained austenite phase: 2-20%
Residual austenite phase has strain-induced transformation, that is, when the material is deformed, the strained portion transforms into the martensite phase, the deformed portion becomes hard, and has the effect of improving the ductility by preventing strain concentration. In order to increase ductility, it is necessary to contain 2% or more of retained austenite phase. However, since the residual austenite phase has a high C concentration and is hard, if it exceeds 20% excessively in the steel sheet, a local hard part will be present, so that the uniform deformation of the material during stretch flange molding Therefore, it is difficult to ensure excellent elongation and stretch flangeability. In particular, from the viewpoint of stretch flangeability, it is preferable that the retained austenite is small. Therefore, the volume fraction of the retained austenite phase is in the range of 2 to 20%.

Ratio of tempered martensite phase with major axis length ≧ 10μm in total volume fraction of tempered martensite phase: 30% or less (however, including 0%)
The tempered martensite phase is harder than the ferrite phase which is the base structure. When the total volume fraction of the tempered martensite phase is the same, the coarse tempered martensite phase with a major axis of 10 μm or more is localized compared to the tempered martensite phase of less than 10 μm, which inhibits uniform deformation. However, it is disadvantageous for stretch flangeability as compared with a fine and uniform structure that deforms more uniformly. When the ratio of such a coarse tempered martensite phase exceeds 30%, the tempered martensite phases are adjacent to each other, and non-uniform deformation becomes remarkable, which adversely affects elongation and stretch flangeability. Therefore, the smaller the ratio of the tempered martensite phase whose major axis length is 10 μm or more, the better. Therefore, the volume fraction of the tempered martensite phase with the long axis length ≧ 10 μm is set to a range of 30% or less. It may be 0%. The size of the tempered martensite phase is preferably small, and the volume fraction of the tempered martensite phase having a major axis length ≧ 5 μm is more preferably 30% or less.

Next, the manufacturing conditions of the high-strength cold-rolled steel sheet according to the present invention and the reasons for limitation will be described.
In the present invention, the process before hot finish rolling may be performed in accordance with a conventional method. For example, a steel slab obtained by melting and casting steel prepared in the above component composition range can be used. In the present invention, not only continuous casting slabs and ingot-splitting slabs, but also thin slabs with a thickness of about 50 to 100 mm can be used. Especially in the case of thin slabs, direct heating without reheating is possible. It can use for a hot rolling process.

The hot rolling is not particularly limited, and may be performed according to a conventionally known method. The preferred conditions are as follows.
The heating temperature during hot rolling is preferably 1100 ° C. or higher. The upper limit is preferably set to 1300 ° C. from the viewpoint of reducing scale generation and reducing fuel consumption. The finishing temperature in hot rolling is preferably 850 ° C. or higher so as to avoid a layered structure of a low-temperature transformation phase such as ferrite and pearlite. In addition, the upper limit is preferably set to 950 ° C. from the viewpoint of reduction of scale formation and fine homogenization of the structure by suppressing coarsening of the crystal grain size.
The coiling temperature after completion of hot rolling is preferably 450 to 600 ° C. from the viewpoint of cold rollability and surface properties, and the hot rolled sheet, which is a steel sheet after winding, is subjected to a pickling process and then the next process. To be served.

Then, after annealing, cold rolling is performed. In the present invention, the steps after the annealing step are important, and annealing (heat treatment) is performed three times conveniently with the cold rolling in between.
Annealing temperature (first time): 400-800 ° C
If the annealing temperature in the first annealing, that is, the annealing of the hot-rolled sheet after hot rolling is less than 400 ° C., the tempering after hot-rolling is insufficient, and in the finally obtained cold-rolled steel sheet, The influence of the structure cannot be removed, and a non-uniform structure resulting from the layered hot-rolled sheet structure composed of ferrite and pearlite is obtained, and sufficient stretch flangeability cannot be obtained. Further, the hot-rolled sheet is hardened and the cold rolling load increases, resulting in high costs. On the other hand, when annealing at a temperature exceeding 800 ° C., the structure after annealing becomes a low-temperature transformation phase such as martensite, which is harder than ferrite, and the structure becomes non-uniform and hardened. Flangeability is significantly reduced. Therefore, in order to obtain a very uniform structure before cold rolling, the annealing temperature after hot rolling is in the range of 400 to 800 ° C.

Cold rolling Cold rolling is performed after the first annealing, but this cold rolling process is not particularly limited and may be performed according to a conventional method.
Preferably, it is preferable to cold-roll at a rolling reduction of 30 to 60%. This is because if the rolling reduction is less than 30%, strain is introduced non-uniformly in the steel sheet, partial recovery, recrystallization and grain growth proceed, resulting in a non-uniform structure and adversely affecting stretch flangeability. On the other hand, even if it exceeds 60%, there is no problem in the material, but the cold rolling load increases.

Annealing temperature (second time): 760-860 ° C
If the annealing temperature in the second annealing, that is, the annealing after cold rolling is lower than 760 ° C, the volume fraction of the ferrite phase becomes high during annealing, and the volume fraction of the ferrite phase in the finally obtained structure is large. Therefore, it is difficult to secure TS: 1180 MPa. Moreover, C concentration to an austenite phase is accelerated | stimulated during annealing, the martensite phase before tempering hardens | cures excessively, it hardens also after a tempering process, and stretch flangeability falls. On the other hand, when heated to a high temperature range of austenite single phase exceeding 860 ° C., the austenite grain size becomes excessively coarse, the crystal grain size of ferrite phase and low temperature transformation phase becomes coarse, and stretch flangeability deteriorates. Therefore, the annealing temperature in the second annealing is in the range of 760 to 860 ° C.

Cooling rate: 10 to 80 ° C./second The cooling rate after the second annealing is important for obtaining the desired volume fraction of the low-temperature transformation phase. When the average cooling rate is less than 10 ° C./second, it is difficult to secure a bainite phase and a martensite phase, and it becomes difficult to secure strength because of softening. On the other hand, when it exceeds 80 ° C./second, a martensite phase is excessively generated and hardened excessively, so that workability such as elongation and stretch flangeability deteriorates. Accordingly, the cooling rate is in the range of 10 to 80 ° C./second.
The cooling in this case is preferably gas cooling, but other methods such as furnace cooling, mist cooling, roll cooling, and water cooling can be used as appropriate, or a combination thereof can also be used. is there.

Cooling stop temperature: 300 ~ 500 ℃
When the cooling stop temperature after the second annealing is less than 300 ° C., the generation of retained austenite is suppressed and the martensite phase is excessively generated, so that the strength becomes too high and it becomes difficult to ensure elongation. On the other hand, when the temperature exceeds 500 ° C., the bainite transformation during holding after the cooling stop is delayed, and accordingly, the formation of retained austenite is suppressed, and it becomes difficult to obtain excellent ductility. In order to maintain the strength of TS: 1180MPa class or higher, and at the same time obtain a well-balanced stretch and stretch flangeability, the cooling stop temperature is 300 ° C, mainly composed of ferrite phase, controlling the ratio of martensite phase and residual austenite phase. Must be in the range of ~ 500 ° C.

Holding time: 100 to 1000 seconds If the holding time in the above-described cooling stop temperature range (also the holding temperature range) is less than 100 seconds, the time for C to concentrate in the austenite phase becomes insufficient, and finally the desired amount It is difficult to obtain the retained austenite, and the martensite phase is excessively formed to increase the strength, and the elongation and stretch flangeability are deteriorated. On the other hand, even if retained for more than 1000 seconds, the amount of retained austenite does not increase, and no significant improvement in elongation is observed. Accordingly, the holding time is in the range of 100 to 1000 seconds.
In addition, as a means for maintaining the steel plate after the cooling stop in the residence temperature range, for example, a means for adjusting a temperature of the steel plate to the residence temperature by providing a heat retaining device or the like in the downstream process of the cooling equipment after annealing, etc. Can be mentioned. Moreover, the steel plate after residence is cooled to a desired temperature by any conventionally known method.

Annealing temperature (third time): 200-400 ° C
If the annealing temperature in the third annealing, that is, tempering annealing is lower than 200 ° C., the martensite phase becomes insufficiently softened, and the tempered martensite phase becomes excessively hardened, resulting in reduced stretch flangeability and bendability. . On the other hand, when the annealing temperature exceeds 400 ° C., the retained austenite phase obtained after the second annealing is decomposed, and the desired amount of retained austenite is not finally obtained, making it difficult to obtain a steel sheet having excellent elongation. It becomes. Further, since the bainite phase and the martensite phase are decomposed into a ferrite phase and cementite, it is difficult to ensure strength. Therefore, the 3rd annealing temperature shall be the range of 200-400 degreeC. It should be noted that the soaking time at the annealing temperature is less than 50 seconds, the softening of the martensite phase tends to be insufficient, while if it exceeds 1000 seconds, the retained austenite phase is easily decomposed, so it is set to 50 seconds to 1000 seconds. It is preferable.
In addition, there is no restriction | limiting in particular about the cooling rate after the above-mentioned 3rd annealing, What is necessary is just to cool in accordance with a conventional method.

  After the third annealing described above, the cold-rolled steel sheet finally obtained may be subjected to temper rolling (skin pass rolling) for the purpose of shape correction or surface roughness adjustment. Since strain is introduced into the steel sheet, the crystal grains are expanded to form a rolled structure, and the ductility may be reduced. Therefore, the rolling reduction of skin pass rolling is preferably about 0.05% to 0.5%.

A steel having the composition shown in Table 1 is melted to form a slab, heated to 1220 ° C., finished rolling exit temperature: 880 ° C., immediately after completion of rolling, first half cooling at a rate of 50 ° C./sec, winding temperature: Hot rolling at 580 ° C. was performed, followed by pickling with hydrochloric acid, followed by annealing and cold rolling under the conditions shown in Table 2 to produce a cold-rolled steel sheet having a thickness of 1.6 mm.
About the obtained cold-rolled steel sheet, the material characteristic was investigated by the material test shown below.
The obtained results are shown in Table 3.

(1) Structure of steel plate The cross section in the rolling direction was examined by observing a surface at 1/4 position of the plate thickness with a scanning electron microscope (SEM). The observation was carried out at N = 5 (5 observation fields). The volume fraction of the ferrite phase was determined by calculating the area occupied by the 50μm x 50μm square area that was arbitrarily set by image analysis using a cross-sectional structure photograph with a magnification of 2000 times. It was a fraction.
The amount of residual austenite phase was determined by X-ray diffraction using Mo Kα rays. That is, using a test piece having a surface near a thickness of 1/4 of the steel sheet as a measurement surface, the peaks of the (211) surface and (220) surface of the austenite phase and the (200) surface and (220) surface of the ferrite phase The volume ratio of the retained austenite phase was calculated from the strength.
Since the distinction between the bainite phase and the tempered martensite phase may be difficult after tempering, the structure after etching with nital on the plate thickness section parallel to the rolling direction of the steel plate before the third annealing, which is tempered annealing. SEM observation at 2000 to 3000 times, the martensite phase observed, that is, the structure observed as a lump shape with a relatively smooth surface is finally tempered to become a tempered martensite phase And judged.
In addition, the confirmation that the martensite phase is tempered is that the thickness cross-sectional structure before and after temper annealing is observed by SEM at a magnification of 5000, and by temper annealing, carbide is precipitated in the martensite phase. This was done by checking.
Long axis length: The ratio of the tempered martensite phase of 10 μm or more is the same reason as described above. The magnification of the steel plate before the third annealing: 100 μm arbitrarily set by image analysis using a cross-sectional structure photograph of 1000 times Obtain the occupied area of the martensite phase with a long axis length of 10 μm or more present in the square area of × 100 μm square, and consider this as the occupied area of the tempered martensite phase to determine the occupied area ratio of the tempered martensite phase. The volume fraction was obtained. The extraction of the martensite phase with a long axis length of 10 μm or more is possible when the long axis length of the martensite phase is 10 μm or more when the long axis of the martensite phase, that is, when the maximum diameter is the same as or larger than a circle with a diameter of 10 μm. It was supposed to be.
The volume fraction of each phase is determined by first determining the volume fraction of the ferrite phase by distinguishing between the ferrite phase and the low temperature transformation phase, and then determining the volume fraction of the retained austenite phase by X-rays, and the remaining volume fraction. Was the sum of the bainite phase and the martensite phase, and the distinction between the two phases was judged visually by SEM images as described above.

(2) Tensile properties Evaluation was performed by conducting a tensile test based on JIS Z 2241 using No. 5 test piece described in JIS Z 2201 with the rolling direction and 90 ° as the longitudinal direction (tensile direction). The evaluation criteria for tensile properties were TS × El ≧ 20,000 MPa ·% or more (TS: tensile strength (MPa), El: total elongation (%)).

(3) Hole expansion rate This was carried out based on the Japan Iron and Steel Federation standard JFST1001. When a hole with an initial diameter of d ₀ = 10 mm was punched and the conical punch with an apex angle of 60 ° was raised to widen the hole, when the crack penetrated the plate thickness, the rise of the punch was stopped. The punched hole diameter d was measured and calculated by the following formula.
Hole expansion rate (%) = ((d−d ₀ ) / d ₀ ) × 100
In addition, the test was implemented 3 times about the steel plate of the same number, and the average value ((lambda)) of the hole expansion rate was calculated | required. Here, TS × λ ≧ 36000 MPa ·% or more was regarded as good as the evaluation standard for the hole expansion rate.

(4) Bending characteristics Thickness: Using a steel plate with a thickness of 1.6 mm, a sample was taken so that the ridgeline of the bent part and the rolling direction were parallel, and the sample size was 40 mm x 100 mm (the sample length was the direction perpendicular to the rolling). . The obtained sample is bent at 90 ° V using a die with a tip bending radius of R = 3.0 mm, and the presence or absence of cracks is visually determined at the top of the bend. It was evaluated.

As is apparent from Table 3, all of the inventive examples Nos. 1 to 5 satisfy TS ≧ 1180 MPa, satisfy TS × El ≧ 20000 MPa ·%, TS × λ ≧ 36000 MPa ·%, and R / t = A high-strength cold-rolled steel sheet excellent in elongation, stretch flangeability and bendability satisfying 90 ° V bending without cracking at 3.0 / 1.6 = 1.875 has been obtained.
On the other hand, No. 6 in which the amount of C in steel is outside the proper range of the present invention is inferior in bendability, tensile properties, and stretch flangeability.
No. 7, which has a low annealing temperature for the first time, has an excessive proportion of tempered martensite phase with a long axis length ≧ 10 μm, and is also inferior in bendability, tensile properties and stretch flangeability.
In addition, No. 9, which has a low annealing temperature, No. 11, which has a slow cooling rate after the second annealing, and No. 17, which has a high third annealing temperature, both have a high volume fraction of ferrite phase. : Not satisfied with 1180MPa.
Furthermore, in No. 8 where the first annealing temperature is high, No. 12 where the cooling rate is fast after the second annealing, No. 13 where the cooling stop temperature is low, No. 14 where the cooling stop temperature is high, and the cooling stop temperature range No. 15, which has a short retention time, has a volume fraction of the bainite phase or tempered martensite phase which is a low-temperature transformation phase, is excessively high in strength, and is inferior in elongation and stretch flangeability.
No. 10 having a high annealing temperature has a low volume fraction of ferrite phase, an excessively high strength, and is inferior in elongation and stretch flangeability.
Furthermore, No. 16, which has a low third annealing temperature, has an excessively high strength and poor stretch flangeability because the martensite phase is not tempered.

According to the present invention, the volume fraction of each of the ferrite phase, the bainite phase, the tempered martensite phase, and the retained austenite phase without actively containing expensive elements such as Nb, V, Cu, Ni, Cr, and Mo in the steel sheet. By appropriately controlling, high-strength cold-rolled steel sheets that are inexpensive and have excellent elongation, stretch flangeability, and bendability and that have a tensile strength (TS) of 1180 MPa or more can be obtained.
The high-strength cold-rolled steel sheet of the present invention is not only suitable for automobile parts, but also suitable for applications that require strict dimensional accuracy and workability, such as in the field of architecture and home appliances.

Claims (2)

% By mass
C: 0.16-0.26%
Si: 1.2-2.2%
Mn: 2.6-3.6%
P: 0.020% or less,
S: 0.0040% or less,
Al: 0.005-0.08%,
N: 0.008% or less,
Ti: 0.001 to 0.040% and B: 0.0001 to 0.0020%
The balance has a component composition consisting of Fe and inevitable impurities, and in volume fraction,
Ferrite phase: 40-70%,
Baynite phase: 15-35%
Tempered martensite phase: 5-25% and residual austenite phase: 2-20%
And the ratio of the tempered martensite phase with the major axis length ≧ 10 μm in the total volume fraction of the tempered martensite phase satisfies 30% or less (including 0%), and has excellent workability High strength cold rolled steel sheet.   The steel slab having the component composition according to claim 1 is subjected to a first annealing in a temperature range of 400 to 800 ° C after hot rolling, followed by cold rolling, and then in a temperature range of 760 to 860 ° C. Second annealing, cooling rate: 10-80 ° C / second, cooling stop temperature: 300-500 ° C, hold in this temperature range for 100-1000 seconds, then cool, then 200-400 ° C A method for producing a high-strength cold-rolled steel sheet having excellent workability, characterized by performing a third annealing in a temperature range.