JP6509187B2 - High strength cold rolled steel sheet excellent in bending workability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high strength cold rolled steel sheet excellent in bending workability used in automobiles and the like, and a method of manufacturing the same.

  In recent years, steel sheets for automobiles have increasingly adopted steel materials having very high levels of strength in order to secure fuel efficiency regulations for global environment protection and collision safety of passengers. In order to produce such a high strength steel, it is difficult to secure sufficient strength and ductility only with a steel material utilizing general solid solution strengthening or a steel material employing precipitation strengthening.

  As a result, the developed steel sheet is a transformation strengthened steel that utilizes a transformation structure. Such transformation strengthened steels include dual phase steels (DP steels), composite structure steels (CP steels), and transformation induced plasticity steels (TRIP steels). Patent Document 1 is a representative technique of the above-mentioned TRIP steel.

  However, in spite of utilizing such transformation structure, it is difficult to secure high strength and a sufficient draw ratio. In addition, although most of the actual processing is performed through bending and roll forming, there is a problem that both bending workability needs to be secured in order to suppress the cracks generated during such bending.

  In order to ensure bending workability, ferrite single phase steel or bainite single phase steel having a uniform material is suitable. However, high strength steel can not be manufactured with ferrite single phase steel, and in the case of bainite single phase steel, the content of carbon must be increased to ensure high strength, and in such a case, drawing It is difficult to use practically because the rate is low and the weldability is also low.

  Therefore, there is a real need for development of a steel having high bending workability, which is excellent in resistance to cracks at bending portions during bending while maintaining high strength.

Japanese Patent Application Laid-Open No. 6-145892

  An object of the present invention is to provide a cold-rolled steel sheet having high strength, excellent in bending workability, improved resistance to micro cracks generated in a bending portion at the time of molding, and a method of manufacturing the same.

  The problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

  One aspect of the present invention, by weight, C: 0.1 to 0.25%, Si: 0.01 to 0.6%, Mn: 2 to 3%, P: 0.001 to 0.1% S: 0.0001 to 0.01%, Cr: 0.3 to 1.0%, Al: 0.01 to 0.1%, Ti: 0.01 to 0.1%, Ca: 0.01 %, Nb: 0.02 to 0.05%, B: 0.001 to 0.003%, N: 0.001 to 0.01%, the balance contains Fe and unavoidable impurities, and the above Ti and N The content satisfies the relationship of Ti / N ≧ 3.4, and the content of Ti, Al, and Ca satisfies the relationship of Ti / (Al + 8Ca) ≦ 0.6 and exists within 1/4 of the plate thickness from the surface of the steel plate. The present invention provides a high strength cold rolled steel sheet excellent in bending workability in which the content of Ti of Al-Ti inclusions having a major axis length of 5 μm or more is 20% or less.

Still another aspect of the present invention relates to C: 0.1 to 0.25%, Si: 0.01 to 0.6%, Mn: 2 to 3%, P: 0.001 to 0 in weight%. %, S: 0.0001 to 0.01%, Cr: 0.3 to 1.0%, Al: 0.01 to 0.1%, Ti: 0.01 to 0.1%, Ca: 0.01% or less, Nb: 0.02 to 0.05%, B: 0.001 to 0.003%, N: 0.001 to 0.01%, the balance contains Fe and unavoidable impurities, and the above Ti And N contents satisfy the relation of Ti / N ≧ 3.4, and the contents of Ti, Al, and Ca satisfy the relation of Ti / (Al + 8Ca) ≦ 0.6, and cold rolling is performed. The step of annealing heat treating the cold rolled steel sheet in a temperature range of 750 to 850 ° C., and the cooling heat treated steel sheet at a cooling rate of 100 ° C./min or more Cooling in a temperature range between T1 and T2 defined by the equation and then cooling at a cooling rate of 30 ° C./min or less; producing a high strength cold rolled steel sheet having excellent bendability Provide a way.
T1 = 606-161 * C-53.6 * Si-30.8 * Mn-18.3 * Cr (° C.)
T2 = 535-386 * C-15.4 * Si-38.7 * Mn-15.4 * Cr (° C.)
(In the above T1 and T2, the unit of each content of C, Si, Mn and Cr is% by weight)

  According to the present invention, since a micro crack of a bending portion due to inclusions does not occur at the time of forming a steel plate, it is possible to provide a high strength cold rolled steel plate having improved crack resistance.

The test method for evaluating the bending characteristic of this invention is shown. It is the photograph which showed the typical shape of the micro crack in the bending part formed of the inclusion under the surface layer by the comparative example 1 among the Examples of this invention. It is the photograph which observed the torn surface after immersing the micro crack of FIG. 2 in liquid nitrogen and making it fracture along a crack.

  In the process of producing a high strength steel with a tensile strength of 1200 MPa (1.2 GPa) or more, the presence of inclusions in the steel can not be avoided in the ordinary steelmaking process, and particularly in the case of steel utilizing Ti, Ti inclusions It is impossible to avoid the nozzle clogging phenomenon accompanying formation and cluster inclusions existing in such inclusions and the like.

  The inventors of the present invention have found that they are influenced by the composition of inclusions present in the surface layer of the steel sheet as a result of conducting researches to prevent the occurrence of cracks in the bending section of high strength steel. The present invention has been achieved.

  First, the alloy composition of the cold rolled steel sheet of the present invention will be described in detail (hereinafter, referred to as weight%).

  The cold rolled steel sheet of the present invention is, by weight%, C: 0.1 to 0.25%, Si: 0.01 to 0.6%, Mn: 2 to 3%, P: 0.001 to 0.1 %, S: 0.0001 to 0.01%, Cr: 0.3 to 1.0%, Al: 0.01 to 0.1%, Ti: 0.01 to 0.1%, Ca: 0. 01% or less, Nb: 0.02 to 0.05%, B: 0.001 to 0.003%, N: 0.001 to 0.01%, the balance includes Fe and unavoidable impurities.

Carbon (C): 0.1 to 0.25%
C in the steel is an important element to secure the strength of the transformation structure steel. When the content of C is less than 0.1%, it is difficult to secure high strength (for example, 1.2 GPa), and when it exceeds 0.25%, the ductility and the bending workability and the weldability decrease and the steel plate for automobile Difficult to apply to. Therefore, the content of C in the present invention is preferably 0.1 to 0.25%.

Silicon (Si): 0.01 to 0.6%
Si is an element capable of improving the strength and the elongation at the time of addition, but if its content is less than 0.01%, not only such an effect can not be obtained, but also the degree of heterogeneity of the structure increases. And may cause problems such as material anisotropy. When the content of Si exceeds 0.6%, not only surface-scale defects are induced in relation to the surface quality, but also oxides are formed on the surface to induce non-plating during plating, resulting in non-plating and plating It may cause problems such as peeling. Therefore, the content of Si in the present invention is preferably 0.01 to 0.6%.

Manganese (Mn): 2 to 3%
When Mn is present in the steel material, it is not only an element that greatly contributes to solid solution strengthening, but is also required to increase hardenability. If the content of Mn is less than 2%, the hardenability is insufficient, and excessive ferrite transformation occurs during cooling after annealing, making it difficult to secure the target high strength. On the other hand, when the content of Mn exceeds 3%, not only the effect of improving hardenability, which is the purpose of adding Mn, is saturated, but also the bending characteristics are deteriorated due to the Mn segregation zone existing in the rolling direction in the steel sheet. The problem is that Therefore, the content of Mn in the present invention is preferably 2 to 3%.

Phosphorus (P): 0.001 to 0.1%
P is an element that plays a role of strengthening the steel plate, but is an element that may be mixed as an impurity at the time of steel production. If the content of P is less than 0.001%, not only the effect of the addition of P can not be derived, but also it may cause a problem of increased manufacturing cost in the smelting process for removing impurities. On the other hand, if the content exceeds 0.1%, brittleness of the steel may occur. Therefore, the content of P is preferably 0.001 to 0.1%.

Sulfur (S): 0.0001 to 0.01%
S is an impurity which is inevitably contained in steel, and is an element which inhibits not only bending characteristics at the time of press forming but also ductility and weldability. In the present invention, it is preferable to suppress the content of S as much as possible. However, if the content of S is less than 0.0001%, there is a problem that the manufacturing cost in the refining process is significantly increased, and if it exceeds 0.01%, the bending properties may be greatly reduced. Therefore, the content of S in the present invention is preferably 0.0001 to 0.01%.

Chromium (Cr): 0.3 to 1.0%
Cr is a component added to improve the hardenability of the steel and to ensure high strength. In the present invention, it is an element that induces the formation of bainite through the delay of ferrite transformation. When the content of Cr is less than 0.3%, it is difficult to secure the above effect. On the other hand, when it exceeds 1.0%, the effect is saturated, and not only cold rolling load is increased due to high strength after hot rolling, but also the manufacturing cost is greatly increased. Therefore, the content of Cr in the present invention is preferably 0.3 to 1.0%.

Aluminum (Al): 0.01 to 0.1%
Al is an element effective to combine with oxygen in steel to perform a deoxidizing action, and to distribute C in ferrite to austenite to improve the hardenability of martensite. Further, in the present invention, it is an important element for converting again Ti inclusions generated by the introduction of Ti alloy iron in the steel making process into Al inclusions. When the content of Al is less than 0.01%, it is difficult to sufficiently ensure the above-described effects. On the other hand, if the content of Al exceeds 0.1%, there is a problem that the surface quality of the slab is reduced along with the reduction of high temperature ductility due to the precipitation of excessive AlN, and there is a problem that the manufacturing cost increases. . Therefore, the content of Al in the present invention is preferably 0.01 to 0.1%.

Titanium (Ti): 0.01 to 0.1%
Ti is added for scavenging of N present in the steel so that B is present in a solid solution state without reacting with N when B is added to increase the strength and hardenability of the steel sheet. Element that is The scavenging is to add a small amount of a substance that is particularly reactive with a specific chemical species, and remove the chemical species from the system by reaction without significantly affecting others. . At this time, the substance to be added is called a scavenger. Ti is an element added to remove N as a scavenger. When the content of Ti is less than 0.01%, the unavoidable addition of N can not be sufficiently scavenged, and B in the steel precipitates as BN, so that the solid solution B is reduced and the annealing in the annealing process is performed. It is difficult to secure high tensile strength because ferrite is formed excessively due to the lack of penetration. On the other hand, when the content of Ti exceeds 0.1%, while the above-mentioned effect is slightly increased, Ti-based inclusions which cause nozzle clogging at the time of casting are excessively generated, and As the dropout occurs, fine cracks may frequently occur in the bending portion. Moreover, precipitates such as TiN and TiC are excessively formed, and there is a possibility that the surface quality of the slab becomes inferior as the high temperature ductility decreases, and the load increase at the time of hot rolling and the manufacturing cost increase. There's a problem. Therefore, the content of Ti in the present invention is preferably 0.01 to 0.1%.

Calcium (Ca): 0.01% or less Ca is a strong deoxidizing element, and is introduced to form low melting point inclusions in a steel making process to produce a cleaner steel plate. Further, in the present invention, by replacing Ti inclusions, which cause nozzle clogging at the time of casting like Al, when present in steel, by Ca inclusions, it is possible to form minute portions in a bend forming portion by the nozzle clogging material. It can contribute to the reduction of a crack. However, when Al is sufficiently present, it may not be added. If the content of Ca exceeds 0.01%, there is a problem of an increase in production cost due to the volatilization of Ca, so Ca in the present invention is preferably contained 0.01% or less.

Niobium (Nb): 0.02 to 0.05%
Nb is an element that is added to increase the strength of the steel sheet and to refine the crystal grains. If the content of Nb is less than 0.02%, it is difficult to expect the above effects, and if it exceeds 0.05%, bending cost and ductility may be reduced due to increased manufacturing cost and excessive precipitates. . Therefore, the content of Nb in the present invention is preferably 0.02 to 0.05%.

Boron (B): 0.001 to 0.003%
B is an element that plays an important role in increasing the hardenability that suppresses the ferrite transformation during cooling. When the content of B is less than 0.001%, the above-described effects can not be exhibited, and ferrite transformation in the annealing step becomes excessive, and high strength aimed at in the present invention can be secured. difficult. On the other hand, when the content exceeds 0.003%, not only the above effect is saturated due to grain boundary segregation of B, but also there is a problem that the brittleness at the time of hot rolling increases. Therefore, the content of B is preferably 0.001 to 0.003%.

Nitrogen (N): 0.001 to 0.01%
N is a solid solution strengthening element that can increase the strength of the steel sheet, and is an element generally mixed from the atmosphere. The content is controlled in the degassing process of the steelmaking process. When the content of N is less than 0.001%, excessive degassing treatment is required, which causes an increase in manufacturing cost. On the other hand, when it exceeds 0.01%, precipitates such as AlN and TiN are excessively formed, and there is a problem that the surface quality of the slab is lowered with the decrease of high temperature ductility. Therefore, the content of N in the present invention is preferably 0.001 to 0.01%.

  The balance other than the above composition is iron (Fe), and unintended impurities from the raw material or the surrounding environment may be inevitably mixed in the normal production process. On the other hand, the present invention does not exclude the addition of other alloys in addition to the above mentioned alloy composition.

  In the present invention, the Ti and N contents preferably satisfy the relationship of Ti / N ≧ 3.4. When the Ti / N value is less than 3.4, the Ti addition amount is insufficient compared to the dissolved N amount, and the formation of BN and the like due to residual N accompanying the lack of scavenging effect by Ti causes B The strength increase effect due to the addition may be reduced to cause strength reduction.

  On the other hand, in the present invention, the contents of Ti, Al and Ca preferably satisfy the relationship of Ti / (Al + 8Ca) ≦ 0.6. Suppresses the occurrence of micro cracks in the bends due to cluster inclusions under the surface layer of the steel plate (inclusions in the form of massively solidified located below the surface layer, inclusions below the surface layer) caused by the dropout of nozzle clogging substances during casting For this purpose, it is necessary to rapidly remove Ti-based inclusions at the time of addition of Ti in the steel making process. Ti-based inclusions are thermodynamically unstable when an oxidizable element is present rather than Ti such as Al or Ca, and it is difficult to secure a sufficient time until equilibrium is reached in an actual process. If system inclusions remain, it may cause nozzle clogging. In addition, when the value of Ti / (Al + 8Ca) exceeds 0.6, the removal rate of Ti-based inclusions is not sufficient, and bending workability due to cluster inclusions under the surface layer may be inferior. Therefore, it is preferable to satisfy the relation of Ti / (Al + 8Ca) ≦ 0.6 in the present invention.

  Hereinafter, the microstructure of the cold rolled steel sheet of the present invention will be described in more detail.

  In the cold-rolled steel sheet of the present invention, the average Ti content in Al-Ti inclusions having a major axis length of 5 μm or more within 1⁄4 of the surface of the steel sheet is 20% or less by weight preferable. In the case of steel products utilizing Ti, the presence of inclusions can not be avoided in a normal steelmaking process, and in the case of Ti based inclusions, the nozzle clogging phenomenon associated with the formation of Ti-based inclusions and cluster inclusions caused by substances causing such nozzle clogging. The existence of can not be avoided. However, the nozzle clogging of the Ti-based inclusions is influenced by the composition of the Al-Ti-based inclusions through the steel making process as well as the above-described component ratio of Ti, Al and Ca. When the average content of Ti in Al-Ti inclusions having a major axis length of 5 μm or more from the surface of the steel sheet exceeds 20%, the nozzles are severely clogged by Ti inclusions and clusters under the surface layer of the steel sheet due to nozzle clogging material There is a problem that micro cracks are generated in the bending portion due to the inclusions.

  The cold rolled steel sheet of the present invention preferably has a microstructure of 40 to 80% of bainite, 10 to 40% of martensite, and 20% or less (including 0) of ferrite in area fraction. Thereby, the strength and bendability targeted by the present invention can be secured at a certain level or more.

  If the bainite fraction is less than 40%, it is difficult to secure an excellent bendability because the inter-phase hardness difference is greatly increased. On the other hand, if it exceeds 80%, it is difficult to relatively reduce the martensite fraction and to secure the strength targeted in the present invention. On the other hand, if the martensite fraction is less than 10%, it may be difficult to secure the strength, and if it exceeds 40%, the generation of an excessive hard phase may deteriorate the bending properties. The above-mentioned ferrite is a phase which does not have to be provided in order to properly secure the strength and bendability of the present invention, but if its fraction exceeds 20%, the difference in hardness between the phases may increase and the bending properties may deteriorate. Absent.

  On the other hand, although not necessarily formed, retained austenite can be formed at 5% or less.

  Hereinafter, the method of manufacturing the cold rolled steel sheet of the present invention will be described in detail.

  The cold rolled steel sheet of the present invention prepares a cold rolled steel sheet manufactured using a steel slab satisfying the above-mentioned alloy composition.

  The present invention is not particularly limited with respect to the steps up to the cold rolling described above, and the steps are usually performed in the technical field to which the present invention belongs. For example, a steel slab satisfying the above composition is prepared, reheated, and subjected to hot rolling and cold rolling to prepare the cold rolled steel sheet.

  The cold rolled steel sheet is subjected to annealing heat treatment. The annealing heat treatment is performed by heating to a range of 750 to 850 ° C. and cooling to a temperature range of T1 to T2 at a cooling rate of 100 ° C./min or more. Thereafter, it is cooled at a cooling rate of 30 ° C./min or less.

T1 = 606-161 * C-53.6 * Si-30.8 * Mn-18.3 * Cr (° C.)
T2 = 535-386 * C-15.4 * Si-38.7 * Mn-15.4 * Cr (° C.)
(In the above T1 and T2, the unit of each content of C, Si, Mn and Cr is% by weight)

  It is preferable that the said annealing temperature is 750-850 degreeC. If the temperature is less than 750 ° C., it is difficult to secure strength with a ferrite fraction exceeding 20%, and bending workability may be reduced. On the other hand, when the temperature exceeds 850 ° C., the bending workability is improved, but the amount of surface concentrated substances such as Si, Mn, B and the like generated by the high temperature annealing is significantly increased to generate a large amount of surface defects. There is. Therefore, it is preferable that the said annealing temperature is 750-850 degreeC.

  On the other hand, after annealing, cooling is performed at a cooling rate of 100 ° C./min or more. The cooling rate of 100 ° C./min or more is required because ferrite and pearlite are formed when cooling at a rate lower than the above-mentioned cooling rate, and it becomes difficult to secure the strength targeted in the present invention.

Moreover, it is preferable that the cooling stop temperature at the time of cooling at the said cooling rate is a temperature range of said T1-T2. If the above cooling stop temperature exceeds the temperature T1, the bainite region falls under, but the bainite transformation rate is low, and it is difficult to secure a sufficient amount of bainite, so the bending workability becomes inferior. There is. On the other hand, when the cooling stop temperature is less than T2, there is a problem that martensite is formed without maintaining the bainite region during cooling and bending workability becomes inferior.

  After the above cooling, cooling is performed at a cooling rate of 30 ° C./min or less. The reason for gradually cooling in this manner is that sufficient bainite can not be secured in the case of rapid cooling at the above-mentioned speed or more, and bending workability may be lowered.

  On the other hand, in the present invention, the plated steel sheet can be manufactured by further performing a plating process. The plating is not particularly limited in the present invention with respect to the type and method thereof, such as zinc plating and aluminum plating, and a common plating method can be applied in the technical field to which the present invention belongs.

  Hereinafter, examples of the present invention will be described in detail. The following examples are only for the purpose of enhancing the understanding of the present invention, and are not intended to limit the present invention.

  After preparing a steel slab having the alloy composition of Table 1 below, the steel slab was reheated at 1200 ° C. and hot-rolled to produce a hot-rolled steel plate having a thickness of about 3 mm. The finish hot rolling at the time of the above-mentioned hot rolling was performed at a temperature of 930 ° C. Then, it wound up at 680 degreeC and cold-rolled by the rolling-reduction | draft ratio of 50%, and manufactured the cold-rolled steel plate of thickness about 1.5 mm. The steel sheet prepared in this manner was subjected to annealing heat treatment under the conditions of Table 2 below to produce a cold rolled steel sheet. After cooling between T1 and T2 in Table 2, cooling was performed at a cooling rate of about 7 to 8 ° C./min.

  On the other hand, in Table 3, with respect to the manufactured cold rolled steel sheet, the content, phase fraction and physical properties of Ti of Al-Ti inclusions having a major axis length of 5 μm or more existing within 1/4 of the surface of the steel sheet. Identified the results.

  The content of Ti in the Al-Ti inclusions was observed at 10 points at a magnification of 500 times using an SEM at a point within 1⁄4 of the plate thickness, and among them, the length of the major axis is 5 μm or more. Based on the content of Ti obtained by analyzing the components of Ti inclusions by EDS. Moreover, in the case of a tensile strength, a yield strength, and an elongation rate among physical characteristics, it confirmed through the tension test using a JIS5 test piece.

  The bending angle is indicated using the angle at the point of maximum load applied during bending deformation as shown in FIG. 1 according to the VDA 238 standard, using a test piece (thickness 1.5 mm) of plate size 30 mm × 60 mm. The At this time, the test punch 101 was 0.4 R and the deformation speed was 20 mpm.

  As shown in Table 3 above, in the case of the invention examples satisfying the conditions of the present invention, it is possible to ensure the characteristics excellent in bending workability having a tensile strength of 1.2 GPa or more and a bending angle of 70 ° or more. it can.

  On the other hand, in Comparative Examples 1 to 3, the value of the relational expression (Ti / (Al + 8Ca)) of Ti, Al, and Ca in the steel exceeds 0.6, or the content of Ti in the Al-Ti inclusions. Since it exceeds 20%, there are cluster inclusions due to nozzle clogging due to Ti-based inclusions at the time of casting, and the bending workability becomes inferior.

  In particular, the microcracks in the bends formed by the inclusions below the surface layer formed in Comparative Example 1 are as shown in FIG. Moreover, the micro crack of the said FIG. 2 was immersed in liquid nitrogen, and the photograph which observed the torn surface after making it fracture along a crack was shown in FIG.

Comparative Examples 4 and 5 correspond to the case where the cooling stop temperature after annealing exceeds T1 or is less than T2, but since sufficient bainite can not be secured, bending workability is increased according to the increase in the interphase strength difference. It became inferior. Comparative Examples 6 to 9 are cases in which the alloy composition range of the present invention can not be satisfied, and the strength targeted in the present invention could not be secured. Further, in Comparative Example 10, when the value of Ti / N is less than 3.4, the hardenability due to the insufficient scavenging effect of N by Ti is insufficient, and a sufficient strength can not be secured.

101 test punch 102 test piece 103 thickness of test piece 104 curvature radius of test punch (R)

Claims (4)

C: 0.1-0.25%, Si: 0.01-0.6%, Mn: 2-3%, P: 0.001-0.1%, S: 0.0001-% by weight 0.01%, Cr: 0.3 to 1.0%, Al: 0.01 to 0.1%, Ti: 0.01 to 0.1%, Ca: 0.01% or less, Nb: 0.. 02 to 0.05%, B: 0.001 to 0.003%, N: 0.001 to 0.01%, the balance being Fe and unavoidable impurities,
The contents of Ti and N satisfy the relation of Ti / N ≧ 3.4, the contents of Ti, Al and Ca satisfy the relation of Ti / (Al + 8Ca) ≦ 0.6, and 1/1 of the plate thickness from the steel sheet surface 4 exists within a content from 3.2 to 16.2% of Ti of Al-Ti inclusions is is 5μm or more length of the major axis, microstructure, in area%, of 59 to 75% High strength cold rolled steel sheet excellent in bending workability, comprising bainite, 13 to 28 % martensite and 5 to 15% ferrite.   The high strength cold rolled steel sheet having excellent bendability according to claim 1, wherein the microstructure contains 5% or less of retained austenite. C: 0.1-0.25%, Si: 0.01-0.6%, Mn: 2-3%, P: 0.001-0.1%, S: 0.0001-% by weight 0.01%, Cr: 0.3 to 1.0%, Al: 0.01 to 0.1%, Ti: 0.01 to 0.1%, Ca: 0.01% or less, Nb: 0.. 02 to 0.05%, B: 0.001 to 0.003%, N: 0.001 to 0.01%, the balance being Fe and unavoidable impurities, the content of Ti and N is Ti / NN3 Preparing a steel material satisfying the relation of .4 and satisfying the relation of Ti / (Al + 8Ca) ≦ 0.6, and cold rolling the steels;
Annealing-heat treating the cold-rolled steel plate in a temperature range of 750 to 850 ° C., and a cooling rate of 100 ° C./min or more between the annealing steel plate and T1 and T2 defined by the following relational expression Cooling in the following temperature range and then cooling at a cooling rate of 30.degree. C./min or less;
The Ti content of Al-Ti inclusions present within 1/4 of the thickness of the steel sheet and having a major axis length of 5 μm or more is 3.2 to 16.2% , and the microstructure is A method for producing a high strength cold rolled steel sheet having excellent bending workability, comprising 59 to 75 % of bainite, 13 to 28 % of martensite, and 5 to 15% of ferrite.
T1 = 606-161 * C-53.6 * Si-30.8 * Mn-18.3 * Cr (° C.)
T2 = 535-386 * C-15.4 * Si-38.7 * Mn-15.4 * Cr (° C.)
(In said T1 and T2, the unit of each content of C, Si, Mn, Cr is weight%)   The method for producing a high strength cold rolled steel sheet excellent in bending workability according to claim 3, further comprising the step of reheating the slab before cold rolling and performing hot rolling.