JP6098537B2 - High-strength cold-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet having a suitable tensile strength of 980 MPa or more and a method for producing the same for use in a skeletal structure part for an automobile body that is press-molded into a complicated shape.

  Conventionally, high-strength cold-rolled steel sheets with a tensile strength (hereinafter sometimes referred to as TS): 980 MPa class or higher were often applied to light-worked parts. However, recently, in order to achieve both higher collision safety and improved fuel efficiency by reducing the weight of the vehicle body, application to press parts with complex shapes has been studied, and there is a great need for steel sheets with excellent workability.

  The processing methods for stamped parts with complex shapes include stretch processing, stretch-flange processing, bending processing, deep drawing processing, but for TS: 980 MPa class or higher high-strength cold-rolled steel, Elongation-flanging and bending are the main components. As an example of the workability evaluation method, the stretch workability is evaluated by elongation in the tensile test, the stretch-flange workability is evaluated by the hole expansion rate in the hole expansion test, and the bending workability is evaluated by the limit bending radius by the bending test. Has been.

  However, in general, since the workability of steel sheets tends to decrease with increasing strength, avoiding cracks during press forming is a major issue in expanding the application of high-strength steel sheets. Also, when increasing the strength beyond TS: 980 MPa class, it is often necessary to actively add extremely expensive rare elements such as Cu, Ni, Cr, Mo, Nb and V from the viewpoint of securing strength. .

  Furthermore, with regard to bendability, when press-punching a member for an automobile, when taking the length of the part in the longitudinal direction of the steel sheet according to the shape of the part from the yield of the steel sheet (taken in the L direction) Also, when parts are collected in the direction perpendicular to the longitudinal direction (C direction), it will come out. In that case, in steel plates according to the prior art, C-bending (when bending a sample whose length is taken in the direction perpendicular to the rolling direction) is L-bending (when bending a sample whose length is taken in the rolling direction). There is a problem that it is extremely inferior.

  In response to the above, for example, Patent Documents 1 to 4 disclose techniques for obtaining a high-strength cold-rolled steel sheet having excellent workability.

JP 2004-332100 A JP 2005-298964 A JP 2005-179703 A JP 2011-157583 A

  In the technique described in Patent Document 1, expensive elements such as Cu, Ni, Cr, Mo, and Nb are essential. Therefore, the cost is high. Moreover, since it is a single phase structure which does not contain a retained austenite, a high hole expansion rate is obtained, but there is no knowledge about bendability improvement.

  Patent Document 2 discloses a steel sheet that has expensive Mo as an essential element and that has a ferrite phase as a main phase and has excellent stretch flangeability. However, there is no knowledge about improvement of bendability.

  In patent document 3, expensive Ni, Cu, etc. are essential as an austenite stabilization element. Therefore, the cost is high. Moreover, although the knowledge which achieves high El by TS: 780-980MPa class using a retained austenite phase is disclosed, there is no knowledge regarding improvement of bendability.

  Patent Document 4 discloses a steel sheet that is excellent in stretch flangeability having a main phase of a high-temperature region-generated bainite phase, a low-temperature region-generated bainite phase, and a tempered martensite phase. Moreover, the knowledge which achieves the outstanding balance of elongation, stretch flangeability, and bendability in TS: 980-1270MPa class is disclosed using the retained austenite phase. However, it is not intended to make the tissue uniform, but in order to obtain a plurality of tissues having different strength levels, in the holding process after heating and cooling, two-stage cooling that requires a step time or a slow temperature drop is required. Cooling is necessary, and a plurality of temperature maintaining facilities are necessary, resulting in high costs. In the evaluation of bendability according to Patent Document 4, a test piece is taken from the longitudinal direction and is evaluated by L-bending. As described above, depending on the plate-taking direction of the component, C bending (the width of the test piece is In some cases, cracking occurs during bending.

  The present invention was made in order to advantageously solve the above problems, and does not contain Cu, Ni, Cr, Mo, Nb, and V, which are expensive alloy elements, and has low anisotropy bendability, An object of the present invention is to provide a high-strength cold-rolled steel sheet having excellent elongation and stretch flangeability and a production method.

  The present inventors have intensively studied to solve the above problems. As a result, the volume fraction of the bainite phase that forms low-temperature transformations from austenite, particularly the retained austenite phase and martensite phase, even in the absence of expensive rare metals from the viewpoint of workability Further, it was found that by controlling the size strictly, a high-strength cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more, which is excellent in bendability with low anisotropy, elongation and stretch flangeability is obtained. The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) By mass%, C: 0.05 to 0.25%, Si: 0.5 to 2.0%, Mn: 1.0 to 3.0%, P: 0.020% or less, S: 0.0030% or less, Al: 0.005 to 0.08% and N: 0.008 %, With the balance being a component composition consisting of Fe and inevitable impurities, in volume fraction, bainite phase: 80-94%, martensite phase: 1-5% and residual austenite phase: 5-15 The volume ratio of the sum of the two phases of the martensite phase with a major axis length of 5 μm or less and the retained austenite phase with a major axis length of 5 μm or less in the total volume ratio of the martensite phase and the retained austenite phase is 80 to 100 % High-strength cold-rolled steel sheet, characterized by comprising a structure including
(2) The steel sheet further contains at least one of Ca: 0.0001 to 0.0050%, Sb: 0.0001 to 0.1%, Ti: 0.001 to 0.050%, and B: 0.0001 to 0.0050% in mass%. The high-strength cold-rolled steel sheet according to (1) above, which is characterized.
(3) The method for producing a high-strength cold-rolled steel sheet according to (1) or (2) above, wherein the steel slab is subjected to heat treatment at 400 to 800 ° C. after hot rolling, pickling and cold rolling. After annealing, annealing temperature: 860-960 ° C, soaking time: 5 seconds or more and less than 50 seconds, cooling rate: 5-80 ° C / second, cooling stop temperature: 350-450 ° C, then A method for producing a high-strength cold-rolled steel sheet, characterized by holding at a temperature range of 350 to 450 ° C. for 100 seconds or longer.

  In the present invention, the high strength cold rolled steel sheet is a cold rolled steel sheet having a tensile strength (TS) of 980 MPa or more.

  According to the present invention, it is possible to obtain a high-strength cold-rolled steel sheet having low anisotropy bendability, elongation and stretch flangeability without containing an expensive alloy element, and having a tensile strength of 980 MPa or more. . The high-strength cold-rolled steel sheet obtained by the present invention is suitable as an automobile part that is press-formed into a particularly severe shape and has a degree of freedom in picking a part from the steel sheet.

FIG. 3 is a view showing martensite and retained austenite having a major axis length exceeding 5 μm.

  Hereinafter, the present invention will be specifically described.

  The inventors of the present invention have made extensive studies on the workability of a high-strength cold-rolled steel sheet, particularly on the improvement of excellent bendability, elongation and stretch flangeability with small anisotropy. As a result, even in the component system not containing Cu, Ni, Cr, Mo, Nb, V, the volume fraction of the bainite phase is 80 to 94%, the volume fraction of the martensite phase is 1 to 5%, and the residual austenite phase The structure is adjusted so that the volume fraction of the material satisfies 5 to 15%, and further, the martensite phase with a major axis length of 5 μm or less and the residual with a major axis length of 5 μm or less in the total volume ratio of the martensite phase and residual austenite phase The inventors have found that the intended effect of the present invention can be advantageously obtained by controlling the volume ratio of the sum of the two phases of the austenite phase within the range of 80 to 100%, and the present invention has been completed.

  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%”, and hereinafter, 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: 0.05-0.25%
C contributes to TS, and if it is less than 0.05%, it is difficult to ensure a desired tensile strength of 980 MPa or more. On the other hand, if it exceeds 0.25%, not only the spot weldability is remarkably deteriorated, but also the martensite phase is hardened and the bendability, elongation and stretch flangeability intended in the present invention are lowered. Accordingly, the C content is in the range of 0.05 to 0.25%. A preferable amount of C for stably producing retained austenite, which is a feature of the present invention, is 0.15% or more. Further, the preferable amount of C for further enhancing the stretch flangeability intended in the present invention is 0.23% or less.

Si: 0.5-2.0%
Si is an important element for stabilizing retained austenite. In order to obtain the above action, it is necessary to contain 0.5% or more. On the other hand, if added over 2.0%, not only the effect is saturated, but also the steel sheet becomes brittle and cracks occur, and the bendability, elongation and stretch flangeability intended in the present invention are lowered. Further, when excessively contained, a scale with difficulty of peeling is generated during hot rolling, and the surface properties of the steel sheet deteriorate, and in addition, segregation and concentration on the steel sheet surface, crystal grain boundaries, and the like are intended in the present invention. It causes deterioration of bendability. Accordingly, the Si content is in the range of 0.5 to 2.0%. A preferable amount of Si is in the range of 1.2 to 1.8%.

Mn: 1.0-3.0%
Mn is an element that improves hardenability, and has the effect of suppressing the formation of ferrite and pearlite and ensuring the strength easily. In order to obtain the above action, it is necessary to contain 1.0% or more. On the other hand, if it exceeds 3.0%, it becomes excessively hard, resulting in insufficient hot ductility and slab cracking. Further, due to segregation of Mn and the like, the transformation point partially becomes a different structure, and as a result, the ferrite phase and the martensite phase become a non-uniform structure existing in a band shape, and the bendability, elongation and The stretch flangeability is reduced. Furthermore, the material is deteriorated by segregating and concentrating on the steel plate surface, crystal grain boundaries, and the like. Therefore, the Mn content is in the range of 1.0 to 3.0%. A preferable amount of Mn is in the range of 1.5 to 2.5%.

P: 0.020% or less P is an element contributing to strength. On the other hand, since it adversely affects spot weldability, it is preferable to reduce it as much as possible, but it is acceptable up to 0.020%. Therefore, the P content is 0.020% or less. The upper limit of the preferable amount of P from the viewpoint of spot weldability is 0.008% or less. To reduce the amount of P excessively decreases the production efficiency in the steel making process and increases the cost, so the lower limit of the amount of P is preferably 0.001%.

S: 0.0030% or less When the amount of S increases, sulfide inclusions such as MnS are formed, and this MnS expands by cold rolling and exists as a plate-like inclusion, which causes cracks during deformation. As a starting point, the bendability, elongation and stretch flangeability intended in the present invention are lowered. Therefore, it is desirable to reduce S as much as possible, but it is acceptable up to 0.0030%. For this reason, the amount of S is made 0.0030% or less. A preferable amount of S for further improving the stretch flangeability is 0.0010% or less. An excessive reduction of the amount of S is industrially difficult and causes an increase in desulfurization cost in the steel making process, so a preferable range of the amount of S is set to about 0.0001%.

Al: 0.005-0.08%
Al is effective as a deoxidizing agent in the steelmaking process, and is also useful in separating non-metallic inclusions that lower bendability and stretch flangeability into slag. Addition of 0.005% or more is necessary to achieve the above objective. On the other hand, if the content 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 From the viewpoint of improving elongation by cleaning the steel sheet, it is preferable that the amount of N is small. If the N content is 0.008% or less, the effect of the present invention is not impaired, so the upper limit of the N content is 0.008%. A preferable range of the N amount is 0.0060% or less. Since excessive reduction of the amount of N causes an increase in denitrification costs in the steel making process, the lower limit of the amount of N is preferably 0.0010%.

  Components other than the above are Fe and inevitable impurities. However, the following selective elements can be appropriately added as necessary within the range not impairing the effects of the present invention.

Ca: 0.0001 to 0.0050%, Sb: 0.0001 to 0.1%, Ti: 0.001 to 0.050%, B: 0.0001 to 0.0050% Any one or more of Ca: 0.0001 to 0.0050%
Ca is an element that can further improve bendability and stretch flangeability by controlling the form of sulfide in steel. In order to exhibit such an effect, it is preferable to contain 0.0001% or more. On the other hand, if the content is excessive, a large number of inclusions are generated on the surface layer of the steel sheet, which may lower the bendability and stretch flangeability intended in the present invention. Therefore, the Ca content is preferably in the range of 0.0001 to 0.0050%.

Sb: 0.0001 to 0.1%
Sb has an effect of suppressing decarburization from the inside of the steel plate during heat treatment such as temperature rising heating and soaking. When such decarburization occurs, the surface layer becomes coarse ferrite, which causes rough skin after molding and poor surface properties. In addition, excessive decarburization causes a decrease in strength (TS). In particular, Sb has a remarkable effect of suppressing decarburization when the C content is 0.05% or more as in the steel of the present invention. In order to obtain these effects, it is preferable to add 0.001% or more of Sb. On the other hand, when the amount of Sb exceeds 0.1%, the above effect is saturated. Therefore, the Sb content is preferably in the range of 0.0001 to 0.1%.

Ti: 0.001 to 0.050%
Ti forms carbonitrides and sulfides in steel, thereby effectively contributing to strength improvement by refinement and precipitation strengthening of the hot-rolled sheet structure and the steel sheet structure after annealing. Moreover, when adding B, it is an element effective also in suppressing the formation of BN and fixing the hardenability by B by fixing N as TiN. In order to obtain these effects, 0.001% or more is preferably contained. If the Ti content exceeds 0.050%, precipitates are excessively generated in α-iron (ferrite and bainite), and excessive precipitation strengthening may cause a decrease in elongation. Therefore, the Ti amount is preferably in the range of 0.001 to 0.050%. More preferably, it is 0.010 to 0.030% of range.

B: 0.0001-0.0050%
B enhances the hardenability and suppresses the formation of ferrite that occurs during the annealing and cooling process, effectively contributes to securing a desired amount of bainite phase, martensite phase and residual austenite phase, and has an excellent balance between strength and elongation. It is an element useful for obtaining. In order to acquire this effect, it is desirable to contain B 0.0001% or more. On the other hand, when the amount of B exceeds 0.0050%, the above effect is saturated. Accordingly, the B content is preferably in the range of 0.0001 to 0.0050%.

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.
In the present invention, the formation of ferrite phase and pearlite phase is suppressed as much as possible in the annealing, cooling, and holding processes, and the bainite transformation is advanced from the austenite phase in the holding process, and the C concentration to the austenite phase is promoted. Subsequently, it is cooled to room temperature, and finally a predetermined amount of retained austenite phase is secured. At the same time, by adjusting the amount of hard martensite generated in the cooling process after holding, a structure mainly composed of a bainite phase, a martensite phase, and a retained austenite phase not containing a soft ferrite phase is obtained.

Bainite phase volume fraction: 80-94%
The bainite phase is transformed at a higher temperature than the martensite phase transformed from austenite, and is softer than the martensite phase. Therefore, by having bainite, it is possible to ensure the bendability and stretch flangeability intended in the present invention while securing strength, and by promoting bainite transformation, C concentration in the austenite phase is promoted. A predetermined amount of retained austenite phase that ultimately contributes to elongation can be secured.

  In order to secure the desired TS980 MPa or more, the volume fraction of the bainite phase needs to be 80% or more. However, when the volume fraction of the bainite phase is excessively large, not only is the strength increased excessively, but it becomes difficult to secure a predetermined amount of retained austenite, and the elongation decreases. The rate should be below 94%. By setting the bainite phase to a structure containing a volume fraction of 80 to 94%, a material balance with good strength, bendability, elongation and stretch flangeability can be obtained.

Volume fraction of martensite phase: 1-5%
After the annealing, soaking, and holding steps after cooling, the martensite phase generated in the process of cooling to room temperature is not tempered and is extremely hardened, contributing to the improvement of TS. In order to obtain such an effect, a martensite phase of 1% or more is required. On the other hand, if the martensite exceeds 5%, the interface between the hard martensite phase and other phases becomes the starting point of void generation and cracking during molding, which adversely affects bendability and stretch flangeability. Therefore, the volume fraction of the martensite phase is in the range of 1 to 5%.

Volume fraction of retained austenite phase: 5-15%
Residual austenite phase has the effect of improving ductility by strain-induced transformation, that is, when the material is deformed, the strained part transforms into the martensite phase, the deformed part becomes hard, and strain concentration is prevented. In order to increase ductility, it is necessary to contain 5% or more. However, since the residual austenite phase has a high C concentration and is hard, if it is excessively present in the steel sheet in an amount exceeding 15%, a locally hard portion will be present, and the bending and stretch flanges intended by the present invention are present. Since it becomes a factor which inhibits the uniform deformation | transformation of the material at the time of shaping | molding, it becomes difficult to ensure the outstanding bending and stretch flangeability. Therefore, the volume fraction of the retained austenite phase is in the range of 5 to 15%.

The volume ratio of the sum of the two phases of the martensite phase with a major axis length of 5 μm or less and the retained austenite phase with a major axis length of 5 μm or less in the total volume ratio of the martensite phase and the retained austenite phase is 80 to 100%.
When the ratio of the volume of the sum of the two phases of the martensite phase with a major axis length exceeding 5 μm and the residual austenite phase with a major axis length exceeding 5 μm in the total volume ratio of the martensite phase and the retained austenite phase exceeds 20%, Since there are locally hard parts in the steel sheet, it becomes a non-uniform structure, and this is a factor that hinders uniform deformation of the material during bending and stretch flange forming. It becomes difficult to ensure flangeability. Therefore, the ratio of the volume of the sum of the two phases of the martensite phase with a major axis length of 5 μm or less and the retained austenite phase with a major axis length of 5 μm or less in the total volume ratio of the martensite phase and the retained austenite phase is in the range of 80 to 100%. And The martensite having a major axis length exceeding 5 μm and the retained austenite having a major axis length exceeding 5 μm are massive martensite or retained austenite exceeding a circle having a diameter of 5 μm when the microstructure is observed as shown in FIG. When martensite with a major axis length exceeding 5 μm and residual austenite with a major axis length exceeding 5 μm are present, the martensite phase exceeding the major axis length of 5 μm is counted by counting the volume fraction of those martensite or retained austenite. And the ratio of the volume of the sum of the two phases of the retained austenite phase with a major axis length exceeding 5 μm can be determined.

  The evaluation method of the ratio of a martensite phase, a bainite phase, and a retained austenite phase is based on, for example, the following microstructure determination.

  In the present invention, the following microstructural quantification was performed by measuring the area ratio of each metal phase in the cross section in the rolling direction, and using the obtained area ratio as the volume fraction.

  The observation surface at the 1/4 position of the plate thickness is observed with a scanning electron microscope (SEM) in the cross section in the rolling direction. The observation was carried out at N = 5 (5 observation fields). The volume fraction of the bainite phase and martensite phase and / or retained austenite phase is determined by image analysis using a cross-sectional structure photograph with a magnification of 2000 times, and each phase existing in a square area of 50 μm × 50 μm square set arbitrarily. Is obtained and averaged to obtain the volume fraction of each phase. The volume fraction of each phase is determined by SEM observation. If the ferrite phase and pearlite phase that are inevitably generated in advance are generated, the volume fraction of those metal phases is obtained, and the volume fraction other than the ferrite phase and pearlite phase is determined. The structure that has a relatively smooth surface and is observed in an island shape as a granular, acicular, and massive shape is determined as a retained austenite phase or a martensite phase, and the rest is defined as a bainite phase. Next, the amount of retained austenite phase is determined by X-ray diffraction using Mo Kα rays. That is, using a test piece having an observation surface near a thickness of 1/4 of the steel sheet as a measurement surface, 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 is calculated from the peak intensity, and is used as a volume ratio value. Next, the volume fraction of the retained austenite phase is determined as the volume fraction of the martensite phase from the volume ratio of the martensite phase including the retained austenite phase.

  In addition, the ratio of the sum of the two phases of the martensite phase with a major axis length of 5 μm or less and the retained austenite phase with a length of 5 μm or less in the total volume ratio of the martensite phase and the retained austenite phase is relatively smooth according to the above SEM photograph. The long axial length (major axis length) in the island-like form of the structure in which the structure observed in the form of islands as granular, needle-like and massive shapes is regarded as the retained austenite phase or martensite phase. ) Is more than 5 μm, and the total of those areas is calculated and the value is taken as the total volume, and divided by the volume of the sum of the two phases of the retained austenite phase and martensite phase observed in the island shape described above. Can be sought.

  As the balance other than the above-mentioned phase, an unavoidably generated metal phase, that is, a ferrite phase, a pearlite phase, or the like may be recognized, but the total of such unavoidably generated phases is a volume fraction. If it is less than 3%, the effect of the present invention is not affected.

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 the hot rolling is preferably 400 to 600 ° C. from the viewpoint of cold rollability and surface properties. Next, heat treatment is performed. In the present invention, the heat treatment step after the hot rolling is important.

Heat treatment temperature after hot rolling: 400-800 ° C
When the heat treatment temperature after hot rolling and winding is less than 400 ° C., tempering after hot rolling is insufficient, and the influence of the structure after hot rolling cannot be removed. It becomes a non-uniform structure resulting from a non-uniform bainite single-phase structure or a martensite single-phase structure in which coarse crystal grains and fine crystal grains are mixed, or a layered hot-rolled sheet structure composed of ferrite and pearlite. As a result, uniform crystal grains cannot be obtained in the structure finally obtained after cold rolling and annealing, and the martensite phase and the major axis length exceeding 5 μm in the major axis length in the total volume ratio of the martensite phase and the retained austenite phase. The ratio of the volume of the sum of the two phases of the retained austenite phase exceeding 5 μm exceeds 20%, and there are locally hard portions in the steel sheet, resulting in a non-uniform structure, and the small anisotropy intended in the present invention Flexibility, elongation and excellent elongation, stretch flangeability cannot be obtained. Also, the hot-rolled sheet becomes hard and the cold rolling load increases, resulting in high costs. On the other hand, when heat treatment is performed at a temperature exceeding 800 ° C., pearlite and hard martensite are generated, and a uniform structure cannot be obtained. In addition, P segregates at the grain boundaries, the steel sheet becomes brittle, and the elongation and stretch flangeability are significantly reduced. By heat-treating in the range of 400 to 800 ° C, the final structure obtained after cold rolling and annealing becomes uniform crystal grains, and high strength cold rolling with excellent bendability, elongation and stretch flangeability with small anisotropy. A steel plate is obtained. Therefore, in order to obtain a very uniform structure before cold rolling, the heat treatment temperature after hot rolling is in the range of 400 to 800 ° C.

  When the heat treatment holding time after hot rolling is less than 5 minutes, tempering after hot rolling is insufficient, and the influence of the structure after hot rolling may not be removed. The heat treatment holding time after hot rolling may be long, but it is preferably 5 minutes or more and 5 hours or less because it hinders productivity.

  The steel plate after the heat treatment is pickled according to a conventional method, and then subjected to the next step, that is, annealing through a cold rolling step. In the present invention, the steps after the annealing step are important.

Annealing temperature: 860 ~ 960 ℃
In order to suppress the formation of ferrite, it is important to coarsen the austenite grain size by annealing and reduce the number of ferrite formation sites. When the annealing temperature is 860 ° C. or higher, the formation of ferrite is suppressed during continuous cooling even at the cooling rate level obtained by gas jet cooling. The upper limit of the annealing temperature is not particularly limited, but is set to 960 ° C. from the viewpoint of high cost and heating furnace damage. On the other hand, when the annealing temperature is lower than 860 ° C., a ferrite phase is generated during cooling, and the volume fraction of the ferrite phase in the finally obtained structure increases, making it difficult to ensure TS: 980 MPa. Furthermore, C concentration to the austenite phase is promoted during cooling, the martensite phase is excessively hardened, and bendability and stretch flangeability are deteriorated. Accordingly, the annealing temperature is in the range of 860 to 960 ° C.

Soaking time: 5 seconds or more and less than 50 seconds When the soaking time is less than 5 seconds, the austenite grain size during annealing is insufficiently coarsened, and ferrite is generated during cooling. On the other hand, the upper limit of the soaking time is less than 50 seconds from the viewpoint of productivity. Therefore, the soaking time is in the range of 5 seconds to less than 50 seconds.

Cooling rate: The cooling rate after annealing at 5-80 ° C./sec is important to obtain the desired amount of bainite phase, martensite phase and residual austenite phase. When the cooling rate is less than 5 ° C./second on average, a ferrite phase is generated, and it becomes difficult to secure a predetermined amount of bainite phase and martensite phase, and softening makes it difficult to secure strength. On the other hand, even if the cooling rate exceeds 80 ° C./second on average, there is no problem in the material, but the upper limit is set to 80 ° C./second from the viewpoint of controllability such as supercooling in the cooling stop temperature range. Therefore, the cooling rate is in the range of 5 to 80 ° C./second. Considering the cooling stop temperature fluctuation during mass production, the upper limit of the cooling rate is preferably 50 ° 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, or a combination thereof can also be used. .

Cooling stop temperature: 350-450 ° C
When the cooling stop temperature is below 350 ° C, not only the volume fraction of the martensite phase becomes excessive, but also the volume fraction of the untransformed austenite phase decreases, so that the residual austenite accompanying the progress of the bainite transformation during holding It becomes difficult to secure a desired amount of the volume fraction of the phase, and it becomes difficult to secure a high-strength cold-rolled steel sheet having excellent bendability, elongation, and stretch flangeability with small anisotropy. On the other hand, when the cooling stop temperature is higher than 450 ° C., the start and end of bainite transformation is on the long side, the generation of retained austenite with the progress of bainite transformation is delayed, and it becomes difficult to secure the desired volume ratio, and excellent ductility is achieved. It becomes difficult to obtain. Further, since the untransformed austenite phase is transformed into the martensite phase in the cooling process after holding, the strength is excessively increased, and the bendability, elongation and stretch flangeability are deteriorated. Mainly composed of bainite phase, controlling the abundance ratio of martensite phase and residual austenite phase, ensuring strength of TS: 980MPa or higher, and obtaining a low anisotropy bendability, elongation and stretch flangeability in a balanced manner For this, the cooling stop temperature needs to be in the range of 350 to 450 ° C. When the cooling stop temperature is high, the bainite transformation start temperature becomes a long time side, and since it remains austenite immediately after the cooling stop, in particular, the bainite transformation is further promoted and the retained austenite is stably secured, which is favorable. In order to obtain elongation, the upper limit of the cooling stop temperature range (holding temperature range) is preferably 400 ° C. If the cooling stop temperature becomes too low, or if the temperature drops from the cooling stop temperature and the holding temperature decreases before or during holding, the bainite transformation starting temperature becomes the longer side, and the desired bainite phase or residual austenite phase Not only can be obtained, but the volume fraction of the hard martensite phase increases and high elongation cannot be obtained.

Holding temperature: When the holding temperature in the temperature range of 350 to 450 ° C is lower than 350 ° C, not only the volume fraction of the martensite phase becomes excessive, but also the volume fraction of the untransformed austenite phase decreases, As the bainite transformation progresses, it becomes difficult to secure the desired volume fraction of retained austenite phase, making it difficult to secure high-strength cold-rolled steel sheets with low anisotropy bendability, elongation, and stretch flangeability. . On the other hand, when the holding temperature is higher than 450 ° C., the start and end of bainite transformation is on the long side, the generation of retained austenite with the progress of bainite transformation is delayed, and it becomes difficult to secure a desired volume ratio, and excellent ductility is obtained. It becomes difficult. Further, since the untransformed austenite phase is transformed into the martensite phase in the cooling process after holding, the strength is excessively increased, and the bendability, elongation and stretch flangeability are deteriorated. Mainly composed of bainite phase, controlling the abundance ratio of martensite phase and residual austenite phase, ensuring strength of TS: 980MPa or higher, and obtaining a low anisotropy bendability, elongation and stretch flangeability in a balanced manner For this, the holding temperature needs to be in the temperature range of 350 to 450 ° C.

Holding time: 100 seconds or more After the above cooling, holding is performed in the above holding temperature range. If the holding time in this temperature range is less than 100 seconds, the C concentration in the austenite phase accompanying the progress of the bainite transformation during holding It is difficult to obtain the desired retained austenite volume fraction in the end, and the martensite phase is excessively formed from untransformed austenite during the cooling process after the end of holding, resulting in high strength. And bendability, elongation and stretch flangeability are reduced. Accordingly, the holding time is 100 seconds or more. Although the upper limit of the holding time is not particularly defined, the retained austenite amount does not increase even if the holding time exceeds 10,000 seconds, and no significant improvement in elongation is observed. In particular, in order to further promote the bainite transformation to stably secure retained austenite and obtain good elongation, the holding time is preferably longer, and is preferably 150 seconds to 1,000 seconds.

  Further, in order to promote the bainite transformation and stably secure the retained austenite and obtain better elongation, the isothermal holding (cooling stop temperature = holding temperature) is performed in the temperature range of 350 to 450 ° C. of the cooling stop temperature described above. It is preferable to do.

  As a means for holding the steel plate after cooling is stopped in the holding temperature range, for example, a means for adjusting the temperature of the steel plate to the holding temperature by providing a heat holding device or the like in the downstream process of the cooling equipment after annealing, etc. Can be mentioned. Moreover, the steel plate after holding is cooled to a desired temperature by any conventionally known method.

  The cold-rolled steel sheet obtained as described above may be subjected to temper rolling (skin pass rolling) for the purpose of shape correction and surface roughness adjustment. However, excessive skin pass rolling introduces strain into the steel sheet. For this reason, 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%.

  Steel with the composition shown in Table 1 is melted to form a slab, heated to 1180 ° C, hot rolled at the finishing mill exit temperature: 880 ° C, and after rolling, at a rate of 60 ° C / sec. After cooling and winding at 400 ° C., heat treatment was performed under the conditions shown in Table 2. Next, after pickling with hydrochloric acid, cold rolling was performed to finish a cold-rolled steel sheet having a thickness of 1.4 mm, and then an annealing treatment was performed under the conditions shown in Table 2. Measurement of the temperature in each step described above was performed by measuring the temperature of the steel sheet surface with a radiation thermometer.

  About the obtained cold-rolled steel sheet, the material characteristic was investigated by the material test shown below.

(1) In the cross section in the structure rolling direction of the steel plate, the surface at 1/4 position of the plate thickness was examined by observing with a scanning electron microscope (SEM). The observation was carried out at N = 5 (5 observation fields). The volume fraction of the bainite phase, martensite phase and residual austenite phase is the magnification: occupancy of each phase existing in a square area of 50 μm x 50 μm square set by image analysis using a cross-sectional structure photograph of 2000 times The area was obtained and averaged to obtain the volume fraction of each phase. The volume fraction of each phase is determined by SEM observation. If the ferrite phase and pearlite phase that are inevitably generated in advance are generated, the volume fraction of those metal phases is obtained, and the volume fraction other than the ferrite phase and pearlite phase is determined. The structure observed in an island shape as a granular, acicular and massive shape having a relatively smooth surface was judged as a retained austenite phase or a martensite phase, and the rest was regarded as a bainite phase. Next, the amount of retained 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 fraction of the retained austenite phase was calculated from the strength and used as the volume fraction value. Next, the difference between the volume fraction of the retained austenite phase and the volume fraction of the retained austenite phase was determined as the martensite volume fraction.
Further, the volume ratio of the sum of the two phases of the martensite phase and the retained austenite phase with a major axis length of 5 μm or less in the total area ratio of the martensite phase and the retained austenite phase has a relatively smooth surface according to the SEM photograph. Among the structures in which the structures observed in the shape of islands as granular, acicular and massive shapes are regarded as retained austenite phase or martensite phase, the long axial length (major axis length) in the island shape is For those exceeding 5 μm, the total volume was calculated and divided by the volume of the residual austenite phase or martensite phase observed in the island shape (see FIG. 1).

(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 ≧ 18000 MPa ·% or more (TS: tensile strength (MPa), El: total elongation (%)).

(3) Hole expansion rate The hole expansion rate was implemented based on JFST1001. When a hole with an initial diameter of d0 = 10 mm is punched and the hole is widened by raising a conical punch with an apex angle of 60 °, the punch stops rising when the crack penetrates the plate thickness, and punching is performed after crack penetration. The hole diameter d is measured and the following formula: Hole expansion rate (%) = ((d−d0) / d0) × 100
Calculated with Three tests were performed on the same number of steel plates, and the average value (λ) of the hole expansion rate was obtained. The evaluation standard for stretch flangeability (TS × λ) was TS × λ ≧ 46000 MPa ·% or more.

(4) Using a steel plate with a bending test thickness of 1.4 mm, samples were taken in two directions so that the ridge line of the bending part and the rolling direction were parallel and perpendicular, and the sample size was 40 mm x 100 mm (the sample length was the right angle of rolling) Direction (C bending) and parallel direction (L bending)). Perform 90 ° V bending with a pressing load of 88.2kN at the bottom dead center, visually determine the presence or absence of cracks at the top of the bend, find the minimum limit bend radius (R) that does not cause cracks and hair cracks, and limit bend radius R / When the thickness t ≦ 1.0 and −0.5 ≦ R / t (C bending) −R / t (L bending) ≦ 0.5 are satisfied, the bendability is considered to be excellent with small anisotropy. The minimum bending radius of the mold used in the bending test was 0.25 mm. When there was no crack, the minimum bending radius / plate thickness = 0.25 mm / 1.4 mm = 0.18 and the anisotropy were determined as 0.18 / 0.18.

  The obtained results are shown in Table 3.

  As shown in Table 3, the cold-rolled steel sheets of the examples of the present invention had good bendability, elongation, and stretch flangeability. On the other hand, the cold rolled steel sheet of the comparative example could not obtain sufficient characteristics in any of bendability, elongation and stretch flangeability.

  The high-strength cold-rolled steel sheet of the present invention is suitable as an automobile part, and besides that, it is also useful for applications that require strict dimensional accuracy and workability such as in the field of construction and home appliances.

Claims (3)

By mass%, C: 0.15 ~0.25%, Si: 0.5~2.0%, Mn: 1.0~ 2.30%, P: 0.020% or less, S: 0.0030% or less, Al: 0.005 to 0.08% and N: 0.008% or less And the balance has a component composition consisting of Fe and inevitable impurities, and in volume fraction, bainite phase: 80-94%, martensite phase: 1-5% and residual austenite phase: 5-15% In addition, the volume ratio of the sum of the two phases of the martensite phase with a major axis length of 5 μm or less and the retained austenite phase with a major axis length of 5 μm or less in the total volume ratio of the martensite phase and the retained austenite phase includes 80 to 100% A high-strength cold-rolled steel sheet characterized by comprising a structure.   The steel sheet further contains one or more of Ca: 0.0001 to 0.0050%, Sb: 0.0001 to 0.1%, Ti: 0.001 to 0.050%, and B: 0.0001 to 0.0050% in mass%. The high-strength cold-rolled steel sheet according to claim 1.   The method for producing a high-strength cold-rolled steel sheet according to claim 1 or 2, wherein the steel slab is subjected to heat treatment at 400 to 800 ° C after hot rolling, pickling and cold rolling, and then annealing. Temperature: 860-960 ° C, soaking time: After annealing at 5 seconds to less than 50 seconds, cooling rate: 5-80 ° C / second, cooling stop temperature: 350-450 ° C, then 350-450 ° C A method for producing a high-strength cold-rolled steel sheet, characterized by holding in a region for 100 seconds or more.

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