JP5807368B2 - High-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction and a method for producing the same - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet having extremely high ductility required for structural materials such as automobiles that are mainly used by pressing and a method for producing the same.

  High press workability and strength are required for steel plates used in automobile body structures. Elongation and local ductility can be cited as factors governing the press workability of high-strength steel sheets. If the elongation can be extremely increased only in one direction, even a member that is difficult to press-mold can be press-molded by matching the direction in which the elongation is high and the direction of the difficult-to-press part.

  It is known that a retained austenitic steel having retained austenite in a steel structure in a high-strength steel sheet has a very high elongation despite its high strength using the TRIP effect. The TRIP effect is a phenomenon in which high strength and high elongation can both be achieved by austenite remaining at room temperature causing martensitic transformation during deformation.

  In this retained austenitic steel, in order to further increase the elongation, for example, Patent Document 1 controls two types of ferrites (bainitic ferrite and polygonal ferrite) while ensuring a high fraction of retained austenite, A technique for ensuring uniform elongation is disclosed.

  On the other hand, Patent Document 2 discloses a technique for defining the shape of an austenite phase by an aspect ratio for the purpose of securing elongation and shape freezing property. Patent Document 3 discloses that the austenite phase distribution is optimized to ensure higher elongation.

  On the other hand, although the present inventors have disclosed the technique regarding the retained austenitic steel which improved the shape freezing property by controlling the crystal orientation of a ferrite phase until now, in patent documents 4 and patent documents 5. However, it is not intended to improve the uniform elongation by actively controlling the crystal orientation of the austenite phase. Patent document 5 is directed to a hot-rolled steel sheet.

JP 2006-274418 A JP 2007-154283 A JP 2008-56993 A JP 2002-97545 A JP 2004-250744 A

  The present invention has been made to solve the conventional problems, and is based on the knowledge found as a result of intensive studies to increase the ductility of retained austenitic steel as much as possible. It is an object to provide a rolled steel sheet and a manufacturing method thereof.

  Residual austenitic steel exhibits a TRIP effect by austenite undergoing martensitic transformation during deformation. If austenite is unstable and transforms with a small amount of strain, the high TRIP effect does not appear. On the other hand, if austenite becomes sufficiently stable, the TRIP effect cannot be obtained even if processing strain is introduced.

  As a result of intensive studies by the present inventors, it has been newly found that if the degree of accumulation of crystal orientation of the austenite phase is increased, extremely high ductility can be obtained in one direction. The direction in which the ductility is extremely high is a direction of 45 ° with respect to the rolling direction.

  The gist of the high-strength cold-rolled steel sheet of the present invention having extremely high ductility in the direction of 45 ° with respect to the rolling direction is as follows.

(1) In mass%,
C: 0.05% or more, 0.35% or less,
Si: 0.05% or more, 2.0% or less,
Mn: 0.8% or more, 3.0% or less,
P: 0.0010% or more, 0.1% or less,
S: 0.0005% or more, 0.05% or less,
N: 0.0010% or more, 0.010% or less,
Al: 0.01% or more, 2.0% or less,
Mo: 0.02% or more, 0.5% or less,
1 type or 2 types of Nb or Ti, respectively,
Nb: 0.005% or more, 0.05% or less,
Ti: 0.005% or more, 0.1% or less,
In the steel sheet containing the balance iron and inevitable impurities and having a metal structure mainly composed of ferrite or bainite and / or tempered martensite and containing 6% or more of retained austenite phase, the thickness of the steel sheet is 1 Direction of 45 ° with respect to the rolling direction, characterized in that the average value of the random intensity ratio of the {110} <111> to {110} <211> orientation groups of the austenite phase in the / 2 layer is 8.7 or more High strength cold-rolled steel sheet with extremely high uniform elongation.

(2) The steel sheet is mass%,
B: 0.0003% or more, 0.005% or less,
Cr: 0.1% or more, 5.0% or less,
W: 0.01% or more, 5.0% or less,
V: 0.001% or more, 0.1% or less,
Cu: 0.04% or more, 2.0% or less, and
Ni: 0.02% or more, 1.0% or less,
The high strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction as described in (1) above.

  (3) The steel sheet further contains 0.0005% or more and 0.05% or less of one or more of Ca, Mg, Zr, and REM, alone or in total, by mass%. A high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction as described in (1) or (2) above.

(4) In the method for producing a high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction according to any one of (1) to (3) , (1) to (3) The cast slab having the steel composition described in any one of the above , after casting, or after being cooled to 1100 ° C. or lower after being cast, is subjected to hot rolling and subjected to hot rolling, and a finishing temperature of 850 ° C. As described above, after the hot rolling is finished at 970 ° C. or less, and then cooled to a temperature range of 450 ° C. or less at an average cooling rate of 10 ° C./second or more and 200 ° C./second or less, a temperature range of less than 450 ° C. After rolling and pickling, cold rolling is performed at a cold rolling rate of 60% or more and 90% or less, and then a heating rate at 600 ° C. or more and 680 ° C. or less at annealing is 3 ° C./second or more, 20 Heat to ℃ / second or less, 750 ℃ or more, 900 ℃ Annealed below, and then cooled to a temperature range of 250 ° C. or higher and 480 ° C. or lower at an average cooling rate of 0.1 ° C./second or higher and 100 ° C./second or lower, and then continued for 1 second or longer in the same temperature range. A method for producing a high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction, characterized by holding for 1000 seconds or less.

(5) In the method for producing a high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction according to any one of (1) to (3) , (1) to (3) The cast slab having the steel composition described in any one of the above , after casting, or after being cooled to 1100 ° C. or lower after being cast, is subjected to hot rolling and subjected to hot rolling, and a finishing temperature of 850 ° C. As described above, after the hot rolling is finished at 970 ° C. or less, and then cooled to a temperature range of 450 ° C. or less at an average cooling rate of 10 ° C./second or more and 200 ° C./second or less, a temperature range of less than 450 ° C. After rolling and pickling, cold rolling is performed at a cold rolling rate of 60% or more and 90% or less, and then a heating rate at 600 ° C. or more and 680 ° C. or less at annealing is 3 ° C./second or more, 20 Heat to ℃ / second or less, 750 ℃ or more, 900 ℃ Annealed below, and then cooled to a temperature range of 250 ° C. or higher and 480 ° C. or lower at an average cooling rate of 0.1 ° C./second or higher and 100 ° C./second or lower, and then continued for 1 second or longer in the same temperature range. A method for producing a high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction, which is maintained for 1000 seconds or less and then immersed in a hot dip galvanizing bath.

  (6) In the method for producing a high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction described in (5) above, after being immersed in the hot-dip galvanizing tank, 500 ° C. or more and 580 A high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction, wherein the alloying treatment is performed in a temperature range of ℃ or less.

  The high-strength cold-rolled steel sheet of the present invention has an extremely high elongation despite the high strength due to the TRIP effect of retained austenite. The crystal orientation control of the austenite phase that is the basis of the high-strength cold-rolled steel sheet of the present invention is a very efficient and effective technique for improving the stability of retained austenite.

  This technique makes it possible to ensure extremely high ductility in one direction. That is, the above effect is obtained by accumulating <100> directions having high stability against deformation only in the vicinity of 45 ° with respect to the rolling direction.

  This effect can be continued if the crystal orientation of the residual austenite phase finally obtained can be optimized. That is, the same effect can be obtained not only in cold-rolled steel sheets and hot-dip galvanized steel sheets but also in hot-rolled steel sheets. Moreover, since the effect will continue unless the formed crystal orientation is broken, it can also be applied to an electroplated steel sheet.

  Moreover, the high-strength cold-rolled steel sheet of the present invention is not affected by casting conditions. For example, a special casting-hot rolling method such as a thin slab may be used because the influence of the casting method (continuous casting or ingot casting) and slab thickness is small.

It is a figure which shows the main direction of an austenite phase on ODF of (phi) ₂ = 45 degree cross section. It is a figure which shows the relationship between the intensity | strength and uniform elongation in invention steel and comparative steel.

  The high-strength cold-rolled steel sheet of the present invention focuses on increasing the stability of the retained austenite phase in the retained austenitic steel, and as a result of extensive studies, the stability of the retained austenite phase is controlled to the limit by controlling the crystal orientation of the retained austenite phase. It was made by finding that the strength and elongation can be compatible at a high level.

  The structure is mainly composed of a ferrite phase, a bainite phase and / or a tempered martensite phase, and needs to contain 6% or more of a retained austenite phase. If the residual austenite phase is less than 6%, the effect of controlling the crystal orientation cannot be obtained sufficiently, so 6% is made the lower limit. The lower limit of the retained austenite phase fraction is desirably 8%. More desirably, it is 10%.

  When higher strength is desired, fresh martensite may be contained, but when the ferrite phase, bainite phase and / or tempered martensite are not mainly used, the elongation is remarkably reduced. Further, if the pearlite is 5% or less, the material is not significantly deteriorated. Therefore, the pearlite is preferably 5% or less. Here, fresh martensite means martensite that has not been tempered.

  The total of the ferrite phase and the bainite phase and / or the tempered martensite phase as the parent phase needs to be 50% or more in terms of the volume fraction with respect to the entire structure. If it is less than 50%, the C concentration in the austenite phase cannot be increased, so that the elongation deteriorates. On the other hand, if it exceeds 95%, it will be difficult to ensure the necessary fraction of the retained austenite phase and cause deterioration of elongation, so 95% or less is desirable.

  The crystal orientation distribution of retained austenite is most important in the present invention. Since austenite is stable against deformation of the crystal orientation in the <100> direction, it is important in the present invention to accumulate crystal orientations including <100> in one direction within the plate surface.

  When the cold-rolled steel sheet is annealed and exceeds the α → γ transformation temperature, an austenite phase having a specific orientation relationship with the crystal orientation of the ferrite phase nucleates. Therefore, by controlling the crystal orientation of the ferrite before transformation, as a result, the crystal orientation of the austenite phase can also be controlled.

  As the crystal orientation, the orientation perpendicular to the plate surface is usually represented by [hkl] or {hkl}, and the orientation parallel to the rolling direction is represented by (uvw) or <uvw>. {Hkl} and <uvw> are generic terms for equivalent planes, and [hkl] and (uvw) refer to individual crystal planes. In the description of the present invention, the former {hkl} and <uvw> are used below.

  In the case of {100} <001>, the <001> direction is aligned in a direction parallel to the rolling direction and a direction parallel to the width direction. Therefore, uniform elongation cannot be increased in one direction. On the other hand, in the {110} orientation group, there is one <100> parallel to the plate surface.

  For example, in the case of {110} <111>, <100> is oriented in the 55 ° direction from the rolling direction. Therefore, when the retained austenite with such orientation increases, the uniform elongation in the direction of 55 ° from the rolling direction increases. In the case of {110} <112>, the uniform elongation in the 35 ° direction with respect to the rolling direction increases.

From the above viewpoint, among the {110} <111> to {110} <001> orientation groups, in particular, by increasing the random strength of the {110} <111> to {110} <112> orientation groups, rolling Uniform elongation can be increased in a direction of 45 ° with respect to the direction. In order to exert such an effect, the average value of the random intensity ratios of the {110} <111> to {110} <112> orientation groups needs to be 8 or more. In the claims, the lower limit of the average value of the random intensity ratio was set to 8.7 confirmed in the examples.

  The average values of the random intensity ratios of the {110} <111> to {110} <112> orientation groups of the austenite phase described above are {200}, {311}, and {200 of the austenite phase measured by X-ray diffraction. 220} may be obtained from a crystal orientation distribution function (Orientation Distribution Function, ODF) representing a three-dimensional texture calculated by the series expansion method based on the pole figure of 220}.

  The random intensity ratio refers to the X-ray intensity of a standard sample that does not accumulate in a specific orientation and the test material under the same conditions as measured by the X-ray diffraction method. It is a numerical value obtained by dividing the intensity by the X-ray intensity of the standard sample.

  FIG. 1 shows an ODF of a φ2 = 45 ° cross section in which the crystal orientation of the present invention is displayed. This figure is a Bunge display showing a three-dimensional texture by a crystal orientation distribution function, where Euler angle φ2 is 45 °, and a specific crystal orientation (hkl) [uvw] is Euler angle of the crystal orientation distribution function. These are indicated by φ1 and φ.

  For example, as indicated by the point on the axis of Φ = 90 ° in FIG. 1, the {110} <111> to {110} <211> orientation groups are φ1 = 35 to 55 °, Φ = 90 °, φ2 = 45 °. Therefore, by determining the average value of φ1 = 35 to 55 °, the average value of the random intensity ratios of the {110} <111> to {110} <211> orientation groups can be determined.

  The sample for X-ray diffraction is produced as follows. The steel plate is polished to a specified position in the plate thickness direction by mechanical polishing or chemical polishing, and finished to a mirror surface by buffing, and then the distortion is removed by electrolytic polishing or chemical polishing, and at the same time the 1/2 plate thickness part is measured. Adjust so that it becomes the surface.

  In the case of cold-rolled sheets, the texture change within the sheet thickness is considered to be not so large, but the closer the sheet thickness surface is, the more likely the structure changes due to shearing by the roll or some decarburization. Therefore, measurement is performed at the 1/2 thickness position.

  In addition, since it is difficult to accurately set the measurement surface to a 1/2 plate thickness portion, a sample is prepared so that the measurement surface is within a range of 3% of the plate thickness with the target position as the center. do it.

  If there is center segregation, the position may be shifted to a portion where the influence of segregation can be excluded. When measurement by X-ray diffraction is difficult, a statistically sufficient number of measurements may be performed by an EBSP (Electron Back Scattering Pattern) method or an ECP (Electron Channeling Pattern) method.

  Next, the reason for limiting the chemical components of the high-strength cold-rolled steel sheet of the present invention will be described. Hereinafter,% means mass%.

  C is an essential element from the viewpoint of securing strength and as a basic element for stabilizing austenite. If C is less than 0.05%, the strength is not satisfactory, and residual austenite is not formed. On the other hand, if it exceeds 0.35%, the strength is excessively increased, the ductility is insufficient, and it cannot be used as an industrial material. Moreover, spot weldability is remarkably deteriorated.

  When high elongation is required, 0.2% or more is desirable. On the other hand, when weldability is required, 0.25% or less is desirable.

  Although Si is added from the viewpoint of securing strength, it is an element that delays the formation of cementite and is an element effective for producing retained austenite. Therefore, it is usually added to ensure ductility. However, if added over 2.0%, the effect of addition is saturated and brittleness is likely to occur. When hot dip galvanizing properties and ease of chemical conversion treatment are required, 1.5% or less is desirable.

  On the other hand, addition of less than 0.05% provides a cementite suppressing effect, so 0.05% is made the lower limit. When the addition amount of Al that provides the same effect as Si is 0.1% or less, addition of 1% or more is desirable.

  Mn is added from the viewpoint of securing strength, but is an element that delays the formation of carbides and is an effective element for the production of retained austenite. If Mn is less than 0.8%, the strength is not improved, and the formation of residual austenite becomes insufficient, resulting in deterioration of ductility.

  On the other hand, if Mn exceeds 3.0%, the hardenability is increased, so that martensite is generated instead of retained austenite, which tends to cause an increase in strength, thereby increasing product variation and ductility. Insufficient to use as industrial material. Therefore, Mn is 0.8% or more and 3.0% or less. In terms of material, 1.0% or more and 2.4% or less are desirable.

  P is an element that increases the strength of the steel sheet, and is added according to the required strength level. However, if the addition amount is large, it segregates to the grain boundary and deteriorates the local ductility, and also deteriorates the weldability. Therefore, the upper limit of P is 0.1%. On the other hand, if it is less than 0.0010%, the effect of deteriorating P is lost, but the cost is increased, so the lower limit is made 0.0010%.

  S is an element that generates MnS to deteriorate local ductility and weldability, and is preferably an element that does not exist in steel. Therefore, the upper limit is made 0.05%. On the other hand, if it is less than 0.0005%, the cost increases, so the lower limit is made 0.0005%.

  Al, like Si, has an effect of promoting the formation of ferrite and is one of important elements that can also suppress cementite. That is, it has the effect of stabilizing the retained austenite. If it is less than 0.01%, this effect cannot be expected. On the other hand, even if Al is added excessively, the above effect is saturated and the steel is embrittled, so 2.0% is made the upper limit. Considering hot dip galvanizing properties, Al deteriorates this, so the upper limit is desirably 1.8%.

  N is an element that is inevitably included, but if it is contained in a large amount, not only the aging property is deteriorated, but also the AlN precipitation amount is increased to reduce the effect of Al addition, so 0.010% The following. Further, unnecessarily reducing N increases the cost in the steelmaking process, so 0.0010% or more is usually preferable.

  Mo is an element necessary for enhancing the variant selectivity during transformation from ferrite to austenite and for developing the austenite texture. If less than 0.02%, the austenite texture does not develop sufficiently, so 0.02% is made the lower limit. When Mo is added in an amount of 0.5% or more, carbides are generated and ductility deteriorates, so 0.5% is made the upper limit.

  Nb and Ti are elements required to remarkably suppress recrystallization of steel and promote the development of the texture. Both are added in an amount of 0.005% or more. However, if carbides such as NbC and TiC are precipitated, the ductility deteriorates, so the upper limits are Nb: 0.05% and Ti: 0.1%.

  B is an element that improves hardenability and is effective in securing the strength, so 0.0003% or more is added as necessary. On the other hand, if B is added excessively, the effect is not only saturated but also brittle, so 0.005% is made the upper limit.

  Cr, W, and V are elements that generate fine carbides, nitrides, or carbonitrides, and are effective in securing strength. Therefore, if necessary, Cr is 0.1% and W is 0.1%. Add 0.001% at 01%, V. On the other hand, in order to achieve this, excessive addition increases the strength too much and lowers the ductility, so the upper limit is Cr: 5.0%, W: 5.0%, V: 0.1% And

  Since Cu and Ni delay the transformation and increase the strength of the steel, they are added as necessary. If Cu: less than 0.04% and Ni: less than 0.02%, the hardenability is weak, and the required strength cannot be obtained to promote ferrite formation at high temperatures. On the other hand, if Cu: more than 2.0% and Ni: more than 1.0%, the hardenability becomes too strong, the ferrite and bainite transformations are delayed, and the C concentration to the residual austenite phase is delayed.

  The steel can further contain one or more of Ca, Mg, Zr, and REM (rare earth elements) alone or in total of 0.0005% to 0.05%. Ca, Mg, Zr, and REM control the shape of sulfides and oxides to improve local ductility and hole expansibility. For this reason, 1 type (s) or 2 or more types of these elements are added individually or in total 0.0005% or more. However, excessive addition degrades workability, so the upper limit is made 0.05%.

  In addition to the above elements, steel contains elements inevitably mixed in such as Sn and As, and the balance is iron.

  Below, the manufacturing method of the high intensity | strength cold-rolled steel plate of this invention is demonstrated.

  As a result of intensive studies, the present inventors have found that it is very important to control the crystal orientation of the austenite phase when manufacturing a high-strength cold-rolled steel sheet having high ductility. In order to control the texture of the austenite phase, it is extremely important to control the texture of the ferrite formed during annealing.

  The residual austenite phase remaining on the product plate is generated by reverse transformation from the interface of the ferrite phase formed during annealing, and thus is significantly affected by the crystal orientation of the ferrite phase. Therefore, in order to realize the effect of the present invention, the most important thing is to control the texture of the ferrite before transformation, and to allow the crystal orientation to be inherited by austenite during the subsequent reverse transformation.

  That is, it is necessary to remarkably develop an optimum texture in ferrite. That is, by controlling the coiling temperature in hot rolling, the hot-rolled sheet has a bainite single-phase structure and is homogenized to increase the accumulation of texture.

  By rolling this hot-rolled sheet at a high cold rolling rate, the desired crystal orientation is developed, and during annealing, recrystallization is delayed, and the temperature is increased to a two-phase region while remaining unrecrystallized, so that ferrite The phase texture is transferred to the austenite phase.

  The slab before hot rolling is set to 1100 ° C. or higher as it is or after re-heating after continuous casting. If it is less than 1100 degreeC, a homogeneous process will be inadequate and will raise | generate a strength fall.

  Next, the slab is hot-rolled at a finishing temperature of 850 ° C. or higher and 970 ° C. or lower. When the finishing temperature is less than 850 ° C., it becomes (α + γ) two-phase region rolling, resulting in a decrease in ductility. On the other hand, when it exceeds 970 ° C., the austenite grain size becomes coarse, the ferrite phase fraction decreases, and the ductility decreases.

  Then, it cools to a temperature range of 450 ° C. or less at an average cooling rate of 10 ° C./second or more and 200 ° C./second or less, and then winds up in a temperature range of 450 ° C. or less. When the cooling rate is less than 10 ° C./second and the coiling temperature exceeds 450 ° C., a ferrite phase or a pearlite phase is generated, and a bainite single phase structure cannot be obtained. When the cooling rate exceeds 200 ° C./second, the ferrite suppressing effect is saturated, and the variation in the cooling end point temperature increases, making it difficult to ensure a stable material. Therefore, the cooling rate is set to 200 ° C./second or less.

  After pickling the hot-rolled sheet, cold rolling is performed at a cold rolling rate of 60% or more and 90% or less. If the cold rolling rate is less than 60%, the texture is difficult to develop, so the cold rolling rate is 60% or more. On the other hand, if the cold rolling rate exceeds 90%, ear cracks are likely to occur during cold rolling, so 90% is made the upper limit.

  At the time of heating during annealing, the average heating rate in the range of 600 ° C. or more and 680 ° C. or less is controlled to be 3 ° C./second or more and 20 ° C./second or less. By increasing the heating rate in this temperature range and shortening the residence time, recrystallization is suppressed, and as a result, the accumulation degree of the retained austenite texture improves.

  However, if the heating rate is less than 3 ° C./second, recrystallization easily proceeds, so 3 ° C./second is set as the lower limit. If the heating rate exceeds 20 ° C./second, the effect of suppressing recrystallization will not change, and soaking in the plate width direction will decrease, so 20 ° C./second is the upper limit.

  The maximum temperature at the time of annealing shall be 750 degreeC or more and 900 degrees C or less. If it is less than 750 ° C., the austenite fraction generated during annealing is low and the retained austenite fraction is lowered, so 750 ° C. is the lower limit. On the other hand, when the maximum temperature exceeds 900 ° C., the martensite fraction increases, degradation of elongation occurs, and the austenite phase grain growth is promoted, and the formed texture is destroyed. Is the upper limit. The upper limit is preferably 860 ° C, and more preferably 840 ° C.

  In cooling after soaking in the annealing process, in order to freeze the structure and efficiently cause bainite transformation, the cooling rate should be fast. However, the transformation cannot be controlled at less than 0.1 ° C./second. On the other hand, if it exceeds 100 ° C./second, the effect is saturated, and the temperature controllability of the cooling end point temperature, which is most important for the formation of retained austenite, is remarkably deteriorated.

  For this reason, the cooling rate after annealing is set to 0.1 ° C./second or more and 100 ° C./second or less on average. In order to leave the retained austenite stably, it is preferably 1.0 ° C./second or more and 100 ° C./second or less.

  The longer the holding time, the better in terms of C concentration to retained austenite. If it is less than 1 second, the bainite transformation does not occur sufficiently and C concentration becomes insufficient. On the other hand, when the time exceeds 1000 seconds, cementite is generated in the austenite phase, and the concentration of C tends to decrease. Therefore, the holding time is 1 second or more and 1000 seconds or less.

  The present invention can also be applied to hot dip plated steel sheets. When applied to a hot dip galvanized steel sheet, it is immersed in a hot dip galvanizing tank after being held at 250 ° C to 480 ° C. Moreover, the present invention can also be subjected to an alloying treatment after being immersed in a plating tank for plating. At this time, the alloying process of plating is performed in the range of 500 ° C. or higher and 580 ° C. or lower. If it is less than 500 ° C, alloying is insufficient, and if it exceeds 580 ° C, it becomes an overalloy and the corrosion resistance is remarkably deteriorated.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example 1)
Steel pieces having the composition shown in Table 1 were melted to produce steel pieces. The steel slab was heated and hot rolled, followed by rough rolling and finish rolling. The slab reheating temperature was 1200 ° C., and the finishing temperature was 850 to 970 ° C. Cooling was performed at an average cooling rate of up to 450 ° C. at 30 ° C./second, and then winding was performed at the winding temperature shown in Table 2. After pickling, cold rolling with a cold rolling rate shown in Table 2 was performed, and then annealing and plating were performed under the conditions shown in Table 2.

  The volume fraction of retained austenite was determined by X-ray diffraction as described in JP-A-11-193435. That is, the volume fraction Vγ of retained austenite was calculated from the data obtained using the Mo—Kα ray by the following formula.

Vγ = (2/3) {100 / (0.7 × α (111) / γ (200) +1)}
+ (1/3) {100 / (0.78 × α (211) / γ (311) +1)}
However, α (211), γ (200), α (211), and γ (311) represent the surface strength of the ferrite phase α and the austenite phase γ.

  Further, the random strength ratio of {110} <111> to {110} <211> orientation groups of the retained austenite phase in the ½ plate thickness part of the steel plate was measured as follows.

  First, after mechanically polishing and buffing the steel sheet, X-ray diffraction was performed using a sample which was further subjected to electrolytic polishing to remove strain and adjusted so that the 1/2 plate thickness portion became the measurement surface. Note that X-ray diffraction of a standard sample having no accumulation in a specific orientation was performed under the same conditions.

  Next, ODF was obtained by the series expansion method based on the {200}, {311}, and {220} pole figures of the austenite phase obtained by X-ray diffraction. From this ODF, the random intensity ratio of {110} <111> to {110} <211> orientation groups was determined.

  Table 3 shows the measurement results of the mechanical properties. In addition, the tensile test was done by the JIS No. 5 tensile test. Moreover, the value of the test piece cut out in the direction (C direction) parallel to the plate width direction was used for the tensile strength. Elongation shows the value of uniform elongation. The uniform elongations in the rolling direction (L direction), 45 ° direction, and C direction are U-El (L), U-El (45), and U-El (C), respectively.

  As apparent from Tables 2 and 3, when the steel having the composition of the present invention is hot-rolled, cold-rolled and then annealed under appropriate conditions, it is ductile in the 45 ° direction with respect to the rolling direction. Steel plate with high height can be obtained. FIG. 2 shows the U-El relationship in the 45 ° direction in the strength and rolling direction. Also from this, it is clear that the inventive steel is excellent in uniform elongation.

  Comparative Example No. In e1 and f1, an element that delays recrystallization, such as Mo, Nb, and Ti, is contained in an amount below the lower limit defined in the present invention, so that the ferrite texture does not develop, As a result, it is considered that the austenite texture did not develop.

  On the other hand, Comparative Example No. In B4 and N2, the heating rate in the temperature range of 600 to 680 ° C. was too slow, so that the texture of recrystallized ferrite was developed and the orientation was considered to be inherited by austenite.

  Comparative Example O2 is an example in which the cold rolling rate is too low. Comparative Example B3 is an example in which the coiling temperature was high and the ferrite texture after cold rolling did not develop.

  In Comparative Example I2, the annealing temperature was too high, and the austenite concentration direction was randomized.

  Thus, when the austenite texture does not develop, the uniform elongation is inferior to that of the inventive examples. In Comparative Example R2, the annealing temperature is too low, and in Comparative Example b1, the amount of C is too low, so that no residual γ remains. Comparative Example F2 and Comparative Example Q2 are examples in which the retention temperature or the retention time is not appropriate, so that the residual γ is not sufficiently generated and high ductility is not obtained.

  The high-strength cold-rolled steel sheet of the present invention has extremely high uniform elongation, and is therefore used for automobiles, home appliances, buildings, and the like. In addition, the high-strength cold-rolled steel sheet of the present invention includes a cold-rolled steel sheet in a narrow sense that does not undergo surface treatment, and a cold-worked sheet in a broad sense that has been subjected to surface treatment such as hot-dip Zn plating, alloyed hot-dip Zn plating, and electroplating for rust prevention. Includes rolled steel sheets. The surface treatment includes aluminum-based plating, formation of an organic film on the surface of various plated steel sheets, formation of an inorganic film, painting, and a combination thereof.

  Since the high-strength cold-rolled steel sheet of the present invention has a high uniform elongation in any direction, it is more complicated to press than conventional steel sheets. It is possible to reduce the thickness of the parts that could not be achieved, that is, to reduce the weight and contribute to the preservation of the global environment. Moreover, since the member obtained by shaping and processing the high-strength cold-rolled steel sheet of the present invention is excellent in collision energy absorption characteristics, it contributes to the improvement of automobile safety.

Claims (6)

% By mass
C: 0.05% or more, 0.35% or less,
Si: 0.05% or more, 2.0% or less,
Mn: 0.8% or more, 3.0% or less,
P: 0.0010% or more, 0.1% or less,
S: 0.0005% or more, 0.05% or less,
N: 0.0010% or more, 0.010% or less,
Al: 0.01% or more, 2.0% or less,
Mo: 0.02% or more, 0.5% or less,
1 type or 2 types of Nb or Ti, respectively,
Nb: 0.005% or more, 0.05% or less,
Ti: 0.005% or more, 0.1% or less,
In the steel sheet containing the balance iron and inevitable impurities and having a metal structure mainly composed of ferrite or bainite and / or tempered martensite and containing 6% or more of retained austenite phase, the thickness of the steel sheet is 1 Direction of 45 ° with respect to the rolling direction, characterized in that the average value of the random intensity ratio of the {110} <111> to {110} <211> orientation groups of the austenite phase in the / 2 layer is 8.7 or more High strength cold-rolled steel sheet with extremely high uniform elongation. The steel sheet is in mass%,
B: 0.0003% or more, 0.005% or less,
Cr: 0.1% or more, 5.0% or less,
W: 0.01% or more, 5.0% or less,
V: 0.001% or more, 0.1% or less,
Cu: 0.04% or more, 2.0% or less, and
Ni: 0.02% or more, 1.0% or less,
The high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction according to claim 1, characterized by containing one or more of the following.   The steel sheet further contains, in mass%, one or more of Ca, Mg, Zr, and REM, individually or in total, 0.0005% or more and 0.05% or less. A high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction according to claim 1 or 2. In the manufacturing method of the high intensity | strength cold-rolled steel plate in which the uniform elongation of a 45 degree direction is very high with respect to the rolling direction of any one of Claims 1-3, It is any one of Claims 1-3. The cast slab having the following steel composition is cast as it is, or once cooled to 1100 ° C. or lower, and then reheated to 1100 ° C. or higher and subjected to hot rolling, the finishing temperature is 850 ° C. or higher and 970 ° C. or lower. After the hot rolling is completed, the steel sheet is cooled to a temperature range of 450 ° C. or less at an average cooling rate of 10 ° C./second or more and 200 ° C./second or less, and then wound in a temperature region of less than 450 ° C. After washing, cold rolling is performed at a cold rolling rate of 60% or more and 90% or less, and then the heating rate at 600 ° C or more and 680 ° C or less is 3 ° C / second or more and 20 ° C / second or less during annealing. And annealed at 750 ° C. or higher and 900 ° C. or lower. Then, it is cooled to a temperature range of 250 ° C. or higher and 480 ° C. or lower at an average cooling rate of 0.1 ° C./second or higher and 100 ° C./second or lower, and then maintained at the same temperature range for 1 second or longer and 1000 seconds or shorter. A method for producing a high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction.
In the manufacturing method of the high intensity | strength cold-rolled steel plate in which the uniform elongation of a 45 degree direction is very high with respect to the rolling direction of any one of Claims 1-3, It is any one of Claims 1-3. The cast slab having the following steel composition is cast as it is, or once cooled to 1100 ° C. or lower, and then reheated to 1100 ° C. or higher and subjected to hot rolling, the finishing temperature is 850 ° C. or higher and 970 ° C. or lower. After the hot rolling is completed, the steel sheet is cooled to a temperature range of 450 ° C. or less at an average cooling rate of 10 ° C./second or more and 200 ° C./second or less, and then wound in a temperature region of less than 450 ° C. After washing, cold rolling is performed at a cold rolling rate of 60% or more and 90% or less, and then the heating rate at 600 ° C or more and 680 ° C or less is 3 ° C / second or more and 20 ° C / second or less during annealing. And annealed at 750 ° C. or higher and 900 ° C. or lower. After that, it is cooled to a temperature range of 250 ° C. or higher and 480 ° C. or lower at an average cooling rate of 0.1 ° C./second or higher and 100 ° C./second or lower, and then kept at the same temperature range for 1 second or longer and 1000 seconds or shorter. And thereafter, dipping in a hot dip galvanizing bath, a method for producing a high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° relative to the rolling direction.   The method for producing a high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction according to claim 5, wherein the temperature is 500 ° C or higher and 580 ° C or lower after being immersed in the hot dip galvanizing bath. A method for producing a high-strength cold-rolled steel sheet having a very high uniform elongation in the direction of 45 ° with respect to the rolling direction, characterized by performing an alloying treatment in a region.

Images