JP5205795B2 - High-strength steel sheet with excellent stretch flangeability and fatigue characteristics and method for producing the molten steel - Google Patents

Description

  The present invention relates to a high-strength steel sheet suitable for use in undercarriage parts of transportation equipment, and the like, and relates to a high-strength steel sheet excellent in stretch flangeability and fatigue characteristics and a method for melting the molten steel.

  There is an increasing demand for high strength and light weight hot-rolled steel sheets for automobiles from the viewpoint of improving the safety of automobiles and improving fuel efficiency leading to environmental conservation. Among the parts for automobiles, the mass of frames and arms, especially called undercarriage systems, occupies a large proportion of the mass of the entire vehicle body, so by reducing the thickness by increasing the strength of the materials used for these parts It becomes possible to realize the weight reduction. Moreover, the material used for this suspension system is required to have high fatigue characteristics from the viewpoint of durability against vibration during running, and high-strength steel sheets are widely used. Among these, hot rolled steel sheets are mainly used because of price advantages.

  Among them, high strength, good workability, and good formability can be achieved by combining a low yield ratio DP steel plate that combines a ferrite phase and a martensite phase, or a composite of a ferrite phase and a (residual) austenite phase. TRIP steel sheets are known. However, although these steel sheets are excellent in high strength, workability and ductility, they cannot be said to have excellent hole expandability, that is, stretch flangeability, and require stretch flange formability such as undercarriage parts. In structural parts, bainite-based steel sheets are generally used, although the ductility is somewhat inferior.

  One of the reasons why composite steel sheets such as DP steel sheets are inferior in stretch flangeability is a composite of a soft ferrite phase and a hard martensite phase, so stress is concentrated at the boundary between both phases during hole expansion processing. It is considered that this is because it cannot follow the deformation and tends to be a starting point of fracture.

  In order to overcome these problems, several steel sheets have been proposed with the aim of achieving both mechanical strength characteristics, fatigue characteristics and hole expandability (workability) based on DP steel sheets. The first is a technology aimed at stress relaxation by finely dispersed particles. For example, Patent Document 1 proposes a steel sheet in which fine Cu precipitates or solid solutions are dispersed in a ferrite structure-martensite phase composite structure steel sheet (DP steel sheet). In the disclosed technique shown in Patent Document 1, Cu precipitates having a particle size of 2 nm or less composed of solid solution of Cu or Cu alone are very effective for improving fatigue characteristics, and workability is not impaired. As a result, the composition ratio of various components is limited.

The second is a technique aimed at stress relaxation by reducing the strength difference of the composite phase. (For example, refer to Patent Document 2.) In the disclosed technique of Patent Document 2, the main phase is made a bainite structure by reducing C as much as possible, and a solid solution strengthened or precipitation strengthened ferrite structure is formed at an appropriate volume ratio. The gist is to reduce the hardness difference between these ferrites and bainite and to avoid the formation of coarse carbides.
Japanese Patent Laid-Open No. 11-199973 Japanese Patent Laid-Open No. 2001-200331

  By the way, although the steel plate which disperse | distributed the fine Cu precipitation or solid solution in DP steel plate as disclosed in the said patent document 1 certainly shows high fatigue strength, the remarkable improvement in stretch flangeability has been confirmed. Not done. Further, as disclosed in Patent Document 2, a high-strength hot-rolled steel sheet having a steel sheet structure mainly composed of a bainite phase and suppressing the formation of coarse carbides certainly exhibits excellent stretch flangeability, but Cu It cannot be said that the fatigue properties are necessarily superior to those of the contained DP steel sheet. In addition, the generation of cracks cannot be prevented when severe hole expansion processing is performed only by suppressing the formation of coarse carbides. According to the study by the present inventors, it has been found that these causes are the presence of elongated sulfide inclusions mainly composed of MnS in the steel sheet.

  That is, when subjected to repeated deformation, internal defects are generated around the extended coarse MnS inclusions on the surface layer or in the vicinity thereof, and the fatigue properties are deteriorated by propagating as cracks. Since it tends to be the starting point of the occurrence, it becomes a factor of reducing the stretch flangeability. At the same time, by increasing the acid-soluble Ti, the crystal grains can be refined by the effect of solid solution Ti or Ti carbonitride, and both high fatigue characteristics and excellent stretch flangeability can be achieved. I understood.

  For this reason, it is desirable to make the MnS inclusions in the steel finer and spheroidized as much as possible while increasing the acid-soluble Ti.

  However, Mn is an element that contributes effectively to increasing the strength of the material together with C and Si. In high-strength steel sheets, it is common to set the Mn concentration high to ensure strength, and further to normal steelmaking. In the processing of the process, the S concentration is about 50 ppm. For this reason, MnS is usually present in the slab. At the same time, when soluble Ti is increased, (Mn, Ti) S is precipitated by partially combining with coarse TiS and MnS. When the slab is hot-rolled and cold-rolled, these MnS inclusions are easily deformed, so that they become stretched MnS inclusions, which cause the fatigue characteristics and stretch flangeability (hole expansion workability) to deteriorate. It becomes. However, there have been no examples of proposing a high-strength steel sheet excellent in stretch flangeability and fatigue characteristics and a method for producing the molten steel from the viewpoint of controlling the precipitation and deformation of MnS inclusions.

  Therefore, the present invention has been devised in view of the above-mentioned problems, and the object is to precipitate as fine MnS, TiS, (Mn, Ti) S in a slab, and further rolling High-strength steel sheet with excellent stretch flangeability and fatigue characteristics, and its molten steel, which is improved in stretch flangeability and fatigue characteristics by being dispersed in the steel sheet as fine spherical inclusions that are sometimes not deformed and hardly start cracking It is to provide a melting method.

  In order to solve the above-described problems, the present inventor precipitated fine MnS, TiS, (Mn, Ti) S in the slab (in the present invention, MnS, TiS, (Mn, Ti) S). These three inclusions are referred to as MnS inclusions for convenience.) A method of improving the stretch flangeability by dispersing the inclusions in the steel sheet as fine spherical inclusions that are not deformed during rolling and are unlikely to start cracking. In addition, intensive research was conducted focusing on the elucidation of additive elements that do not degrade fatigue properties. As a result, MnS, TiS, (Mn, Ti) S precipitates on the fine and hard Ce oxide, La oxide, cerium oxysulfide, lanthanum oxysulfide produced by deoxidation by addition of Ce, La, Since deformation of the precipitated MnS, TiS, (Mn, Ti) S hardly occurs even during rolling, the stretched coarse MnS is remarkably reduced in the steel sheet, and these MnS are repeatedly deformed or subjected to hole expansion processing. It was clarified that the system inclusions hardly become the starting point of crack generation and the path of crack propagation, which leads to the improvement of fatigue resistance as described above. In order to obtain fine oxides and MnS inclusions, first, deoxidation is performed with Si, then deoxidation is performed with Ti, and finally deoxidation is performed by adding Ce and La. It was clarified that deoxidation leads to the refinement of oxides produced at each stage and is effective.

  It was observed that TiN was finely and hardly precipitated with Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide together with MnS inclusions. Since it was confirmed that there is almost no effect, TiN is not a target of MnS inclusions.

  The summary of the high-strength steel sheet excellent in stretch flangeability and fatigue characteristics according to the present invention is as follows.

(1) By mass%, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0030% or more , acid-soluble Ti: 0.008 to 0.20%, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, one or two of Ce or La Total of seeds: steel plate containing 0.0005-0.04%, the balance being iron and unavoidable impurities, inclusions present in the steel plate with an equivalent circle diameter of 1 μm or more, and long diameter / short The stretch flangeability and fatigue characteristics are characterized in that the ratio of the number of stretched inclusions having a diameter of 5 or more to a plurality of inclusions having a circle-equivalent diameter of 1 μm or more randomly selected using SEM is 20% or less. Excellent high strength steel plate.

(2) By mass%, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0030% or more , acid-soluble Ti: 0.008 to 0.20%, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, one or two of Ce or La Total of seeds: 0.0005-0.04% of the steel sheet, the balance being iron and inevitable impurities, and the steel sheet contains one or two of oxides of Ce and La, or this Using SEM, inclusions in which one or more of MnS, TiS, or (Mn, Ti) S are precipitated in combination with an oxide or oxysulfide containing one or two of Si and Ti are used. 10% or less of the number of inclusions with a circle equivalent diameter of 1 μm or more selected at random Fatigue high strength steel sheet and stretch flangeability, which comprises.

(3) By mass%, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0030% or more , acid-soluble Ti: 0.008 to 0.20%, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, one or two of Ce or La Total of seeds: steel plate containing 0.0005-0.04%, the balance being iron and unavoidable impurities, inclusions present in the steel plate with an equivalent circle diameter of 1 μm or more, and long diameter / short Stretch flangeability and fatigue characteristics high strength steel sheet, wherein the number density of stretched inclusions having a diameter of 5 or more is 1.0 × 10 ⁴ pieces / mm ³ or less.

(4) By mass%, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0030% or more , acid-soluble Ti: 0.008 to 0.20%, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, one or two of Ce or La Total of seeds: 0.0005-0.04% of the steel sheet, the balance being iron and inevitable impurities, and the steel sheet contains one or two of oxides of Ce and La, or this The volume number density of inclusions in which one or more of MnS, TiS, or (Mn, Ti) S is precipitated in combination with an oxide or oxysulfide containing one or two of Si and Ti is 1 High-strength steel with stretch flangeability and fatigue characteristics characterized by 0.0 × 10 ³ pieces / mm ³ or more Board.

(5) By mass%, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0030% or more , acid-soluble Ti: 0.008 to 0.20%, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, one or two of Ce or La Total of seeds: steel plate containing 0.0005-0.04%, the balance being iron and unavoidable impurities, inclusions present in the steel plate with an equivalent circle diameter of 1 μm or more, and long diameter / short Stretch flangeability and fatigue characteristics high-strength steel sheet, characterized in that the average equivalent circle diameter of drawn inclusions having a diameter of 5 or more is 10 μm or less.

(6) By mass%, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0030% or more , acid-soluble Ti: 0.008 to 0.20%, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, one or two of Ce or La Total of seeds: 0.0005-0.04% of the steel sheet, the balance being iron and inevitable impurities, and the steel sheet contains one or two of oxides of Ce and La, or this In addition, there is an inclusion in which one or more of MnS, TiS, or (Mn, Ti) S is precipitated in the oxide or oxysulfide containing one or two of Si and Ti. 0.5 to 95% by mass of the total of one or two of Ce or La in the average composition High-strength steel sheet with stretch flangeability and fatigue characteristics.

  (7) Stretch flangeability and fatigue property high-strength steel sheet according to any one of (1) to (6), wherein the mass ratio is (Ce + La) / S is 0.1 to 70.

(8) By mass%, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0030% or more , acid-soluble Ti: 0.008 to 0.20%, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, one or two of Ce or La Total of seeds: Elongation characterized in that 0.0005-0.04% is contained, the balance is steel plate made of iron and inevitable impurities, and the average grain size of crystals in the structure of the steel plate is 10 μm or less Flange and fatigue characteristics High strength steel plate.

  (9) By mass%, Nb: 0.01-0.10%, V: 0.01-0.05%, Cr: 0.01-0.6%, Mo: 0.01-0.4% B: 0.0003 to 0.003%, Ca: 0.0001 to 0.004%, or any one of (1) to (8) Stretch-flangeability and fatigue characteristics as described Steel sheet with low yield ratio and high strength.

(10) In a refining process in steelmaking, C is 0.03 to 0.20% and Si is 0.00% in molten steel that is processed by mass%, P is 0.05% or less, and S is 0.0030% or more . Add or adjust so that 08 to 1.5%, Mn is 1.0 to 3.0%, and N is 0.0005 to 0.01%, and then Ti is substantially added without adding Al. Then, one or two of Ce or La is added, the acid-soluble Ti is 0.008-0.20%, and the total of one or two of Ce or La is 0.0005-0. .04% Stretch flangeability and fatigue characteristics, characterized in that the molten steel for high strength steel sheet is melted.

  (11) In the refining step, before adding one or two kinds of Ce or La, Nb is further 0.01 to 0.10% and V is 0.01 to 0.05% by mass%. , Cr 0.01-0.6%, Mo 0.01-0.4%, B 0.0003-0.003%, Ca 0.0001-0.004% The method for melting molten steel for high-strength steel sheets according to (10), characterized in that it is added so as to be more than seeds.

  Incidentally, the high-strength steel plate in the present invention includes a case where it is used as it is with a normal hot-rolled / cold-rolled steel plate as it is, or subjected to surface treatment such as plating or painting.

  According to the method of the present invention, fine MnS inclusions are precipitated in the slab, and are not deformed during rolling, and can be dispersed in the steel sheet as fine spherical inclusions that are unlikely to become the starting point of cracking. Further, since the crystal grain size of the structure can be made fine, a high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics can be obtained.

  Hereinafter, as the best mode for carrying out the present invention, a high-strength steel sheet excellent in stretch flangeability and fatigue characteristics will be described in detail. Hereinafter, the mass% in the composition is simply described as%.

  First, the experiment that led to the completion of the present invention will be described.

  The inventor contains C: 0.06%, Si: 0.7%, Mn: 1.4%, P: 0.01% or less, S: 0.005%, N: 0.003%. The molten steel whose balance is Fe was deoxidized using various elements to produce a steel ingot. The obtained steel ingot was hot-rolled to obtain a 3 mm hot-rolled steel sheet. These manufactured hot-rolled steel sheets were subjected to a tensile test, a hole expansion test, and a fatigue test, and the inclusion number density, form, and average composition in the steel sheets were investigated.

  As a result, Si was added with almost no deoxidation with Al, and then Ti was added and stirred for about 2 minutes. Then, at least Ce and La were further added, and then sequential desorption in three stages was performed. It was found that the acidified steel sheet has the most excellent stretch flangeability and fatigue characteristics. The reason is that MnS based on MnS, TiS, (Mn, Ti) S on fine and hard Ce oxide, La oxide, cerium oxysulfide and lanthanum oxysulfide produced by deoxidation by addition of Ce and La. Inclusions are precipitated, and the MnS inclusions thus precipitated are not easily deformed during rolling, so that the stretched coarse MnS inclusions are remarkably reduced in the steel sheet. In addition, the crystal grain size of the steel sheet structure becomes fine. As a result, at the time of repeated deformation or hole expansion processing, these MnS inclusions are unlikely to become a starting point of crack generation or a path of crack propagation, which leads to improvement in fatigue resistance as described above.

The reason why Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide are refined is that the Ti added to the SiO ₂ -based inclusions initially generated by Si deoxidation is reductively decomposed and refined finely. Ti oxide is formed, and then Ce and La are further reduced and decomposed to form fine Ce oxide, La oxide, cerium oxysulfide and lanthanum oxysulfide, and further generated Ce oxide and La oxidation. This is because, since the interfacial energy between the molten steel, the cerium oxysulfide and the lanthanum oxysulfide itself and the molten steel is low, agglomeration and coalescence after generation is also suppressed.

  Based on the knowledge obtained from these experimental studies, the present inventor has studied the chemical composition conditions of the steel sheet and completed the present invention as will be described below.

  Hereinafter, the reason why the chemical component is limited in the present invention will be described.

C: 0.03-0.20%
C is the most basic element for controlling the hardenability and strength of steel, and contributes to the improvement of fatigue strength by increasing the hardness and depth of the hardened hardened layer. That is, this C is an essential element for securing the strength of the steel sheet, and at least 0.03% is required to obtain a high-strength steel sheet. However, if this C is excessively contained and exceeds 0.20%, workability and weldability deteriorate. In order to achieve the required strength and ensure workability and weldability, the C concentration is set to 0.20% or less in the present invention.

Si: 0.08 to 1.5%
Si is one of the main deoxidizing elements, and has the function of increasing the number of austenite nucleation sites during quenching heating, suppressing austenite grain growth, and reducing the grain size of the quenched hardened layer. This Si suppresses the formation of carbides, suppresses the decrease in grain boundary strength due to carbides, and is also effective for the generation of bainite structure, thus improving the strength without greatly impairing the elongation, and with a low yield ratio. It is an important element for improving hole expansibility. Reducing the dissolved oxygen concentration in the molten steel, once to generate the SiO ₂ inclusions (Ti added later the SiO ₂ inclusions are reduced decomposed to produce a fine Ti oxide, then further In order to make inclusions finer by reduction of Ce, La), it is necessary to add 0.08% or more of Si. Therefore, in the present invention, the lower limit of Si is set to 0.08%. On the other hand, if the Si concentration is too high, the SiO ₂ concentration in the inclusions becomes high and large inclusions are likely to be formed, and the toughness becomes extremely poor, resulting in increased surface decarburization and surface flaws. Therefore, the fatigue characteristics are worsened. In addition, excessive addition of Si adversely affects weldability and ductility. Therefore, in the present invention, the upper limit of Si is set to 1.5%.

Mn: 1.0-3.0%
Mn is an element useful for deoxidation in the steelmaking stage, and is an element effective for increasing the strength of the steel sheet together with C and Si. In order to obtain such an effect, it is necessary to contain 1.0% or more of this Mn. However, when Mn is contained in an amount exceeding 3.0%, ductility is lowered due to segregation of Mn and increase in solid solution strengthening. Further, since the weldability and the base metal toughness are also deteriorated, the upper limit of Mn is set to 3.0%.

P: 0.05% or less P is effective in that it acts as a substitutional solid solution strengthening element smaller than Fe atoms, but it segregates at the austenite grain boundaries and lowers the grain boundary strength, thereby torsional fatigue strength. And the processability is liable to be deteriorated, so 0.05% or less. Further, if solid solution strengthening is not necessary, it is not necessary to add P, and the lower limit value of P includes 0%.

S: 0.0005% or more Since S is segregated as an impurity, and S forms a coarse MnS-based stretch inclusion to deteriorate stretch flangeability, it is desirable that the concentration be as low as possible. Conventionally, in order to ensure stretch flangeability, it has been necessary to extremely low sulfurize the concentration of S. However, in order to improve the quality of the steel sheet, if it is less than 0.0005%, the desulfurization load in the secondary refining is too large, the desulfurization cost is increased, and a material commensurate with it cannot be obtained. Therefore, the lower limit of the S concentration when desulfurization in secondary refining is assumed is set to 0.0005%.

  In the present invention, the MnS inclusions can be deposited on the fine and hard Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide, and the morphology of the MnS inclusions can be controlled. Moreover, the merit that the desulfurization process of the secondary refining can be omitted can be enjoyed. . That is, Ce and La concentration balanced with S concentration in steel can be set, MnS inclusions can be controlled in form, deformation is difficult to occur during rolling, and extension of inclusions can be prevented. The upper limit of concentration is not specified.

Acid-soluble Ti: 0.008 to 0.20%
Ti is one of the main deoxidizing elements, and forms carbides, nitrides, carbonitrides, increases the number of austenite nucleation sites during hot rolling, and suppresses austenite grain growth. It contributes to miniaturization, effectively acts on dynamic recrystallization during hot rolling, and has a function of remarkably improving stretch flangeability. For this, it is necessary to add 0.008% or more of acid-soluble Ti. For this reason, in this invention, the minimum of acid-soluble Ti was made into 0.008%.

  On the other hand, if it contains more than 0.2%, not only the effect in deoxidation is saturated, but also coarse carbides, nitrides, carbonitrides are formed at the time of heating before hot rolling, leading to deterioration of the material, The effect commensurate with the content cannot be expected. For this reason, in the present invention, the upper limit of the concentration of acid-soluble Ti is set to 0.2%.

Moreover, in order to reduce the dissolved oxygen concentration in molten steel, it deoxidizes with Si, deoxidizes sequentially with Ti, and then further deoxidizes with Ce and La. By such sequential deoxidation, SiO ₂ inclusions are once generated, and the SiO ₂ inclusions are reduced and decomposed with Ti added later to produce fine Ti oxide, and further Ti oxide is reduced with Ce and La. By doing so, the inclusions can be made finer.

N: 0.0005 to 0.01%
N is an element that is inevitably mixed in steel because nitrogen in the air is taken in during the treatment of molten steel. N forms nitrides with Al, Ti, etc., and promotes refinement of the base material structure. However, if this N is added too much, coarse precipitates such as Al and Ti are generated, and the stretch flangeability is deteriorated. For this reason, in the present invention, the upper limit of the concentration of N is set to 0.01%. On the other hand, if the N concentration is less than 0.0005%, the cost increases, so 0.0005% is set as the lower limit.

Acid-soluble Al: 0.01% or less Acid-soluble Al is desirable to be suppressed as much as possible because its oxides are likely to be clustered and become coarse and deteriorate stretch flangeability and fatigue characteristics. However, up to 0.01% is permitted as a preliminary deoxidizer. This is because when the acid-soluble Al concentration exceeds 0.01%, the Al ₂ O ₃ content in the inclusions exceeds 50%, and inclusions cluster. From the viewpoint of preventing clustering, the acid-soluble Al concentration should be low, and the lower limit includes 0%. The acid-soluble Al concentration is an analytical method using the measurement of the concentration of Al dissolved in an acid. The dissolved Al dissolves in an acid and Al ₂ O ₃ does not dissolve in an acid. Here, examples of the acid include a mixed acid mixed at a ratio (mass ratio) of hydrochloric acid 1, nitric acid 1, and water 2. By using such an acid, it can be separated into Al soluble in acid and Al ₂ O ₃ not soluble in acid, and the acid soluble Al concentration can be measured.

Total of one or two of Ce or La: 0.0005 to 0.04%
Ce and La reduce SiO ₂ produced by Si deoxidation, and successively TiO ₂ and Ti ₂ O ₃ produced by Ti deoxidation, tend to become precipitation sites for MnS-based inclusions, and are hard and fine during rolling. Non-deformable Ce oxides (eg, Ce ₂ O ₃ , CeO ₂ ), cerium oxysulfide (eg, Ce ₂ O ₂ S), La oxides (eg, La ₂ O ₃ , LaO ₂ ), lanthanum oxysulfide (For example, La ₂ O ₂ S), Ce oxide-La oxide, or cerium oxysulfide-lanthanum oxysulfide has an effect of forming inclusions having a main phase (50% or more as a guide). doing.

Here, the inclusions may contain a part of MnO, SiO ₂ , TiO ₂ , Ti ₂ O ₃ , or Al ₂ O ₃ depending on deoxidation conditions. For example, it functions sufficiently as a precipitation site for MnS inclusions, and the effect of making the inclusions fine and hard is not impaired. In order to obtain such inclusions, the total concentration of one or two of Ce or La needs to be 0.0005% or more and 0.04% or less. If the total concentration of one or two of Ce or La is less than 0.0005%, the inclusions of SiO ₂ , TiO ₂ , and Ti ₂ O ₃ cannot be reduced, and if it exceeds 0.04%, cerium oxysulfide and lanthanum oxysulfide Are produced in large quantities, resulting in coarse inclusions, which deteriorate stretch flangeability and fatigue characteristics.

  Hereinafter, the reason why the chemical component is limited for the selected element in the present invention will be described. Since these elements are selective elements, the presence or absence of addition is arbitrary.

Nb: 0.01 to 0.10%
Nb forms carbides, nitrides, and carbonitrides with C or N and promotes refinement of the base material structure. In order to obtain this effect, at least 0.01% is preferable. However, even if it is contained in a large amount exceeding 0.10%, the effect is saturated and the cost becomes high, so 0.10% is made the upper limit.

V: 0.01-0.05%
V forms carbides, nitrides, and carbonitrides with C or N to promote the refinement of the base material structure. In order to obtain this effect, at least 0.01% is preferable. However, even if it is contained in a large amount exceeding 0.05%, the effect is saturated and the cost becomes high, so 0.05% is made the upper limit.

Cr: 0.01 to 0.6%
Cr can be contained as necessary in order to improve the hardenability of the steel and ensure the strength of the steel sheet, and at least 0.01% is preferable for obtaining this effect. However, the inclusion of a large amount deteriorates the balance between strength and ductility. Therefore, the upper limit is 0.6%.

Mo: 0.01 to 0.4%
Mo can be contained as necessary in order to improve the hardenability of the steel and ensure the strength of the steel sheet. To obtain this effect, Mo is preferably at least 0.01%. However, the inclusion of a large amount deteriorates the balance between strength and ductility. Therefore, 0.4% is made the upper limit.

B: 0.0003 to 0.003%
B can be contained as necessary in order to improve the hardenability of the steel, strengthen the grain boundaries, and improve the workability. To obtain this effect, B is preferably at least 0.0003%. . However, a large amount on the contrary deteriorates the cleanliness of the steel and deteriorates the ductility. Therefore, the upper limit is set to 0.003%.

Ca: 0.0001 to 0.004%
Ca can be contained as necessary in order to improve the workability of steel by controlling the form of sulfide, and at least 0.0001% is preferable for obtaining this effect. However, a large amount on the contrary deteriorates the cleanliness of the steel and deteriorates the ductility. Therefore, the upper limit is made 0.004%.

Next, the presence conditions of inclusions in the steel sheet of the present invention will be described. The steel sheet means a rolled sheet obtained through hot rolling or further cold rolling.

  In order to obtain a steel sheet excellent in stretch flangeability and fatigue properties, a steel sheet is formed by using an extended coarse MnS-based inclusion (MnS, TiS, (Mn, Ti) S inclusion) that tends to be a starting point of crack generation or a path of crack propagation. It is important to reduce as much as possible. The present inventor has known through experiments that MnS inclusions having an equivalent circle diameter of less than 1 μm are harmless as crack initiation points and do not deteriorate stretch flangeability and fatigue characteristics. Since inclusions with a diameter of 1 μm or more can be easily observed with a scanning electron microscope (SEM) or the like, the shape and composition of the inclusions with a circle equivalent diameter of 1 μm or more in a steel sheet are investigated, and MnS inclusions are investigated. The distribution state of was evaluated. Here, the equivalent circle diameter is defined as (major axis × minor axis) · 0.5 based on the length and minor axis of the inclusion observed in the cross section. The upper limit of the equivalent circle diameter of the MnS-based inclusion is not particularly specified, but in reality, an MnS-based inclusion of about 1 mm may be observed.

  The ratio of the number of stretched inclusions was determined by analyzing the composition of a plurality of inclusions (for example, about 50) having a circle-equivalent diameter of 1 μm or more selected at random using SEM, and determining the major axis and minor axis of the inclusions from the SEM image. taking measurement. Here, when the elongated inclusion is an inclusion having a major axis / minor axis (ratio of stretching) of 5 or more, the number of the detected elongated inclusions is the total number of inclusions examined (50 in the above example). The ratio of the number of the stretched inclusions can be obtained by dividing by the degree.

  The reason why the inclusion stretching ratio was set to 5 or more is that inclusions with a stretching ratio of 5 or more in the comparative steel sheet to which Ce and La were not added were mostly MnS inclusions. In addition, although the upper limit of the extending | stretching ratio of MnS type inclusions is not prescribed | regulated in particular, MnS type inclusions of about 50 extending | stretching ratio may be observed actually.

  As a result, it has been found that stretch flangeability and fatigue characteristics are improved in a steel sheet whose form is controlled to have a number ratio of stretched inclusions having a stretching ratio of 5 or more to 20% or less. That is, when the number ratio of stretch inclusions with a stretch ratio of 5 or more exceeds 20%, the number ratio of MnS-based stretch inclusions that are likely to be the starting point of cracking increases, and the stretch flangeability and fatigue characteristics decrease. In the present invention, the ratio of the number of stretched inclusions having a stretching ratio of 5 or more is 20% or less. Moreover, since the stretch flangeability and fatigue characteristics are so good that there are few stretched MnS inclusions, the lower limit of the number ratio of stretch inclusions having a stretch ratio of 5 or more includes 0%.

  Here, an inclusion having a circle equivalent diameter of 1 μm or more and a lower limit value of the number ratio of the drawing inclusions having a drawing ratio of 5 or more means 0% is an inclusion having a circle equivalent diameter of 1 μm or more. This is the case where there is no stretch ratio of 5 or more, or even if the stretch inclusions have a stretch ratio of 5 or more, the equivalent circle diameter is less than 1 μm.

  In addition, in the steel sheet in which the shape ratio of the stretched inclusions having a stretching ratio of 5 or more is controlled to 20% or less, correspondingly, an oxide composed of one or two of Ce and La, or Si, MnS inclusions are deposited on oxides or oxysulfides containing one or two of Ti. As the form of this inclusion, MnS inclusions are deposited on an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti. Although it is not particularly specified, an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti in the core is used as a nucleus. In many cases, MnS inclusions are deposited around the periphery.

  In some cases, TiN may be precipitated together with MnS inclusions on fine and hard Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide. However, as described above, it has been confirmed that TiN has little influence on stretch flangeability and fatigue characteristics, and therefore TiN is not a target of the MnS inclusions of the present invention.

  In addition, an oxide composed of one or two kinds of Ce and La, an oxide containing one or two kinds of Si and Ti, or an inclusion in which MnS-based inclusions are precipitated on oxysulfide, Since deformation is difficult to occur, the steel sheet has a shape that is not stretched even in the steel sheet, that is, a substantially spherical inclusion.

  Here, the spherical inclusions judged not to be stretched are not particularly defined, but are inclusions having a stretching ratio of 3 or less in the steel sheet, preferably inclusions of 2 or less. This is because MnS-based inclusions are contained in an oxide comprising one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti in the slab stage before rolling. This is because the stretch ratio of the inclusion in the precipitated form was 3 or less. In addition, since the spherical inclusion that is determined not to be stretched is completely spherical, the stretch ratio is 1, so the lower limit of the stretch ratio is 1.

  Such investigation of the number ratio of inclusions was performed in the same manner as the number ratio investigation of stretched inclusions. As a result, an oxide composed of one or two kinds of Ce and La, or an oxide containing one or two kinds of Si and Ti, or inclusions in the form in which MnS inclusions are deposited on oxysulfide. It was found that the steel sheet whose precipitation rate was controlled to 10% or more improved the stretch flangeability and the fatigue characteristics. The number ratio of inclusions in the form of MnS-based inclusions deposited on oxides of one or two types of Ce and La, oxides containing one or two types of Si and Ti, or oxysulfides. If it is less than 10%, the proportion of the number of MnS-based stretch inclusions correspondingly increases, and the stretch flangeability and fatigue characteristics deteriorate. For this reason, the oxide of Ce or La, or an oxide containing MnS-based inclusions in the oxide or oxysulfide containing one or two of Si and Ti or oxides thereof. The number ratio is 10% or more. In addition, stretch flangeability and fatigue properties are determined by using MnS inclusions in oxides or oxysulfides containing one or two kinds of oxides of Ce and La, or one or two kinds of Si and Ti. Since it is better to deposit many, the upper limit of the number ratio includes 100%.

  In addition, the inclusion in the form in which MnS-based inclusions are deposited on an oxide composed of one or two kinds of Ce and La, an oxide containing one or two kinds of Si and Ti, or oxysulfide, Since deformation hardly occurs during rolling, the equivalent circle diameter is not particularly specified, and may be 1 μm or more. However, since it is feared that if it is too large, it becomes a starting point of cracking, the upper limit is preferably about 50 μm.

  On the other hand, this inclusion is not easily deformed even during rolling, and when the equivalent circle diameter is less than 1 μm, it does not become a starting point of cracking, so the lower limit of the equivalent circle diameter is not particularly specified.

  Next, the existence condition of inclusions in the steel sheet of the present invention described above is defined by the number density of inclusions per unit volume.

The particle size distribution of the inclusion was carried out by SEM evaluation of the electrolytic surface by the speed method. The SEM evaluation of the electrolytic surface by the speed method is to evaluate the size and number density of inclusions by polishing the surface of the sample piece, performing electrolysis by the speed method, and directly observing the sample surface by SEM. The speed method is a method of electrolyzing the sample surface using 10% acetylacetone-1% tetramethylammonium chloride-methanol to extract inclusions, and the amount of electrolysis is 1 C per 1 cm ² area of the sample surface. Was electrolyzed. The SEM image of the surface electrolyzed in this manner was subjected to image processing, and the frequency (number) distribution with respect to the equivalent circle diameter was obtained. The average equivalent circle diameter was calculated from the frequency distribution of the particle diameters, and the number density of inclusions per volume was also calculated by dividing the frequency by the area of the observed visual field and the depth determined from the amount of electrolysis.

As a result of evaluating the volume number density of inclusions having an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more, which becomes the starting point of crack generation and deteriorates stretch flangeability and fatigue characteristics, it is 1.0 × 10 ⁴ pieces / mm ³ or less. It has been found that stretch flangeability and fatigue properties are improved. When the volume number density of stretched inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 5 or more exceeds 1.0 × 10 ⁴ pieces / mm ³ , the number density of MnS-based stretched inclusions that are likely to be the starting point of cracking is increased. Too much and the stretch flangeability and fatigue characteristics deteriorate. Therefore, the volume number density of stretched inclusions having an equivalent circle diameter of 1 μm or more and a stretch ratio of 5 or more is set to 1.0 × 10 ⁴ pieces / mm ³ or less. Further, since the stretched flangeability and fatigue characteristics are better as the number of MnS-based inclusions stretched is smaller, the lower limit value of the volume number density of stretched inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 5 or more is 0%. Including.

  Here, the lower limit value of the volume number density of stretched inclusions having an equivalent circle diameter of 1 μm or more and a stretch ratio of 5 or more means 0%, which is the same as described above.

Further, in a steel sheet whose shape is controlled to have a volume number density of a stretched inclusion having a diameter of 1 μm or more and a draw ratio of 5 or more of 1.0 × 10 ⁴ pieces / mm ³ or less, it is not stretched correspondingly. MnS-based inclusions are in the form of MnS-based inclusions deposited on an oxide consisting of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti. Its shape was almost a spherical inclusion.

  As the form of the inclusion, as described above, an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti, and MnS type There is no particular limitation as long as the inclusions are precipitated, and it is not particularly limited, but an oxide comprising one or two of Ce and La, or an oxide containing one or two of Si and Ti, or In many cases, MnS inclusions are deposited around oxysulfide as a nucleus.

  The spherical inclusion is not particularly specified, but is an inclusion having a drawing ratio of 3 or less in the steel plate, preferably an inclusion having 2 or less. Here, since the stretching ratio is 1 if it is completely spherical, the lower limit of the stretching ratio is 1.

As a result of investigating the volume number density of such inclusions, as a result, oxides of one or two kinds of Ce and La, or oxides or oxysulfides containing one or two kinds of Si and Ti are cored. In the case of a steel sheet in which the volume number density of inclusions in the form in which MnS inclusions are precipitated is controlled to be 1.0 × 10 ³ pieces / mm ³ or more, the stretch flangeability and fatigue characteristics may be improved. found. Volume number density of inclusions in which MnS inclusions are deposited on oxides containing one or two types of Ce and La, oxides containing one or two types of Si and Ti, or oxysulfides When the ratio is less than 1.0 × 10 ³ pieces / mm ³ , the number ratio of MnS-based stretched inclusions is excessively increased, and the stretch flangeability and fatigue characteristics are lowered. The volume number density of one or two kinds of oxides, or oxides containing one or two kinds of Si and Ti, or inclusions in the form of MnS inclusions precipitated in oxysulfide is 1.0. × 10 ³ / mm ³ or more. Stretch flangeability and fatigue strength are based on oxides of one or two kinds of Ce and La, or oxides or oxysulfides containing one or two kinds of Si and Ti. Since it is better to deposit a large number of objects, the upper limit of the volume number density is not particularly specified.

  In addition, a circle of inclusions in a form in which MnS inclusions are deposited on an oxide composed of one or two types of Ce and La, an oxide containing one or two types of Si and Ti, or oxysulfide. The equivalent diameter is not particularly defined as described above, and may be 1 μm or more. However, if this equivalent circle diameter is too large, there is a concern that cracking will start, so the upper limit is preferably about 50 μm.

  On the other hand, when the circle equivalent diameter of the inclusion is less than 1 μm, there is no problem at all, and therefore the lower limit is not particularly specified.

  Next, the existence condition of the drawn inclusions in the steel sheet of the present invention described above was defined by the upper limit of the equivalent circle diameter. Specifically, as a result of evaluating the average equivalent circle diameter of inclusions having a circle equivalent diameter of 1 μm or more and an elongation ratio of 5 or more, which becomes the starting point of crack generation and deteriorates stretch flangeability and fatigue characteristics, the average of the extension inclusions was evaluated. It has been found that stretch flangeability and fatigue characteristics are improved when the equivalent circle diameter is 10 μm or less. This is because the average circle equivalent diameter of the stretched inclusions increases as the number ratio of stretched inclusions with a circle equivalent diameter of 1 μm or more and a stretch ratio of 5 or more increases. The equivalent diameter is defined as an index. This is presumed that as the amount of Mn or S in the molten steel increases, the number of MnS-based inclusions generated increases and the size of the generated MnS-based inclusions also increases.

  Therefore, if the number of stretched inclusions with an equivalent circle diameter of 1 μm or more and a stretch ratio of 5 or more exceeds 10 μm, the number ratio of the stretch inclusions exceeds 20% accordingly, which is the starting point for cracking. Since the number ratio of easy and coarse MnS-based stretch inclusions increases too much, and the stretch flangeability and fatigue characteristics deteriorate, the average equivalent circle diameter of stretch inclusions with a circle equivalent diameter of 1 μm or more and a stretch ratio of 5 or more is 10 μm. The following.

  The definition that the average equivalent circle diameter of stretched inclusions having a circle equivalent diameter of 1 μm or more and a stretching ratio of 5 or more is 10 μm or less is that the inclusion having a circle equivalent diameter of 1 μm or more exists in the steel sheet. This means that the lower limit of the equivalent circle diameter is 1 μm.

  On the other hand, in the steel sheet of the present invention described above, MnS is added to an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti. The presence condition of inclusions in the form of precipitation of system inclusions was defined by the content of the average composition of Ce or La in the inclusions of precipitation of MnS inclusions.

  Specifically, as described above, in order to improve stretch flangeability and fatigue characteristics, one or two kinds of oxides of Ce and La, or one or two kinds of Si and Ti are contained therein. It is important to deposit MnS inclusions on the oxide or oxysulfide to prevent the MnS inclusions from stretching.

  As the form of the inclusion, as described above, an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti, and MnS type There is no particular limitation as long as the inclusions are precipitated, and it is not particularly limited, but an oxide comprising one or two of Ce and La, or an oxide containing one or two of Si and Ti, or In many cases, MnS inclusions are deposited around oxysulfide as a nucleus.

  The spherical inclusion is not particularly specified, but is an inclusion having a drawing ratio of 3 or less in the steel plate, preferably an inclusion having 2 or less. Here, if it is perfectly spherical, the stretching ratio is 1, so the lower limit of the stretching ratio is 1.

  Therefore, in order to clarify an effective composition for suppressing the stretching of MnS inclusions, an oxide composed of one or two of Ce and La, or an oxide containing one or two of Si and Ti. Or the composition analysis of the inclusion of the form which MnS type inclusion precipitated in oxysulfide was implemented.

  However, if the inclusion has a circle equivalent diameter of 1 μm or more, the observation is easy, and therefore, for convenience, a circle equivalent diameter of 1 μm or more was targeted. However, if the observation is possible, inclusions having an equivalent circle diameter of less than 1 μm may be included.

  In addition, an oxide composed of one or two kinds of Ce and La, an oxide containing one or two kinds of Si and Ti, or an inclusion in a form in which an MMnS inclusion is precipitated on oxysulfide, Since it was not stretched, it was confirmed that all the stretching ratios were inclusions of 3 or less. Therefore, a composition analysis was performed on inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 3 or less.

  As a result, stretch flangeability and fatigue when inclusion of 0.5 to 50% of the average composition of one or two of Ce or La in an inclusion having an equivalent circle diameter of 1 μm or more and an elongation ratio of 3 or less is included. It was found that the characteristics were improved. When the average content of one or two of Ce or La in the inclusion having an equivalent circle diameter of 1 μm or more and a stretching ratio of 3 or less is less than 0.5 mass%, one or two of Ce and La Corresponding to this, the proportion of inclusions in the form of MnS inclusions in oxides consisting of seeds, oxides containing one or two of Si and Ti, or oxysulfide in this form is greatly reduced. As a result, the number ratio of MnS-based stretch inclusions, which are likely to be the starting point of cracking, increases too much, and the stretch flangeability and fatigue characteristics deteriorate.

  On the other hand, when the average content of one or two of Ce or La in inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 3 or less exceeds 50%, cerium oxysulfide and lanthanum oxysulfide are obtained. Since it is produced in a large amount and becomes a coarse inclusion having an equivalent circle diameter of about 50 μm or more, stretch flangeability and fatigue characteristics are deteriorated.

  Further, in the steel sheet of the present invention described above, an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti, and MnS-based oxide. As an existence condition of inclusions in a form in which inclusions are deposited, an oxide in which MnS inclusions are one or two of Ce and La, or an oxide containing one or two of Si and Ti therein Alternatively, focusing on S to capture how it is modified with oxysulfide, the idea was to define and organize the chemical composition (Ce + La) / S mass ratio of the steel sheet. Specifically, when this mass ratio is small, MnS intervenes in an oxide or oxysulfide containing one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti. A large number of objects are precipitated alone. When this mass ratio is increased, as compared with MnS inclusions, an oxide comprising one or two of Ce and La, or an oxide containing one or two of Si and Ti, or Oxysulfide is increasing, and MnS inclusions are deposited on oxides of one or two of these Ce and La, or oxides or oxysulfides containing one or two of Si and Ti. Inclusions in the form will increase. That is, the MnS-based inclusion is modified with an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti. Thus, in order to improve stretch flangeability and fatigue characteristics, MnS is added to an oxide or oxysulfide containing one or two kinds of Ce and La, or one or two kinds of Si and Ti. It is a chemical component ratio for precipitating system inclusions and preventing the extension of MnS type inclusions, and arranging this effect.

  Therefore, in order to clarify the chemical component ratio effective for suppressing the stretching of the MnS inclusions, the (Ce + La) / S ratio of the steel sheet was changed to evaluate the inclusion morphology, stretch flangeability and fatigue characteristics. As a result, it was found that stretch flangeability and fatigue characteristics were improved when the (Ce + La) / S ratio was 0.1 to 70. When the (Ce + La) / S ratio is less than 0.1, MnS is added to an oxide comprising one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti. Since the number ratio of inclusions in the form of precipitation of system inclusions is greatly reduced, the number ratio of MnS-based extension inclusions, which are likely to be the starting point of cracking, increases correspondingly, and the stretch flangeability and fatigue characteristics are increased. descend.

  On the other hand, when the (Ce + La) / S ratio exceeds 70, a large amount of cerium oxysulfide and lanthanum oxysulfide are formed, resulting in coarse inclusions having a circle-equivalent diameter of about 50 μm or more. Deteriorate.

  Next, the structure of the steel plate will be described.

  In the present invention, fine MnS inclusions are precipitated in the slab and further dispersed in the steel sheet as fine spherical inclusions that are not deformed during rolling and are unlikely to start cracking. The characteristic is improved, and the microstructure of the steel sheet is not particularly limited.

  In the present invention, the crystal grain size in the steel sheet structure is preferably 10 μm or less. The microstructure of the steel sheet is not particularly limited, but a steel sheet having a structure with bainitic ferrite as the main phase, a steel sheet with a composite structure having the ferrite phase as the main phase, the martensite phase, and the bainite phase as the second phase. And any structure of a steel sheet having a composite structure composed of ferrite, retained austenite, and low-temperature transformation phase (martensite or bainite).

  Further, in any structure, it is preferable to refine the crystal grain size to 10 μm or less because hole expandability and fatigue characteristics can be improved. When the average particle size exceeds 10 μm, the improvement in ductility and fatigue characteristics becomes small. In order to improve hole expansibility and fatigue characteristics, the thickness is more preferably 8 μm or less. However, in general, in order to obtain excellent stretch flangeability such as undercarriage parts, it is desirable that the ferrite or bainite phase is the largest phase by area ratio although it is slightly inferior in ductility.

  Next, manufacturing conditions will be described.

In the present invention, deoxidization and component adjustment are performed by adding and stirring an alloy such as C, Si, Mn, etc. in molten steel blown in a converter and decarburized, or further decarburized using a vacuum degasser. I do.

At this time, Al is not substantially added, but it may be deoxidized as long as a small amount of Al is added so that acid-soluble Al remains slightly. Including that it is not substantially added. At that time, it is preferable to take a flying time of about 3 minutes according to Al ₂ O ₃ produced by a small amount of Al.

  Moreover, about S, since it is not necessary to desulfurize by a refining process as above-mentioned, a desulfurization process can be abbreviate | omitted. However, when molten steel desulfurization is necessary in secondary refining in order to melt extremely low-sulfur steel with S ≦ 20 ppm, component adjustment may be performed by desulfurization.

  Then, after adding Si, about 3 minutes, after adding Ti and stirring for about 2 minutes, one or two of Ce or La are sequentially added to adjust the components. Here, when adding the selective element, it is carried out before adding one or two of Ce or La, sufficiently stirred, and after adjusting the components of the selective element as necessary, Ce or La is added. Add one or two of the following. The molten steel thus produced is continuously cast to produce a slab.

  For continuous casting, not only is it applied to normal slab continuous casting of about 250 mm thickness, but it is also used for thin slab continuous casting where the mold thickness of blooms and billets and slab continuous casting machines is thinner than usual, for example 150 mm or less. And is fully applicable.

  The hot rolling conditions for producing a high strength hot rolled steel sheet will be described.

  The heating temperature of the slab before hot rolling is preferably 1150 ° C. or higher in order to dissolve carbonitrides in the steel. By dissolving these carbonitrides in solid solution, a preferable ferrite content can be obtained for improving ductility in the cooling process after rolling. On the other hand, when the heating temperature of the slab before hot rolling exceeds 1250 ° C., oxidation of the slab surface becomes remarkable, and in particular, wedge-shaped surface defects due to selective oxidation of grain boundaries remain after descaling, Since the surface quality after rolling is impaired, the upper limit is preferably set to 1250 ° C.

After being heated to the above temperature range, normal hot rolling is performed. In the process, the finish rolling completion temperature is important when the structure of the steel sheet is controlled. If the finish rolling completion temperature is less than Ar ₃ point + 30 ° C., the crystal grain size of the surface layer portion tends to be coarse, which is not preferable in terms of fatigue characteristics. On the other hand, if the Ar ₃ point exceeds + 200 ° C., the austenite grain size after the rolling becomes coarse, and it becomes difficult to control the composition and fraction of the phase generated during cooling, so the upper limit may be set to Ar ₃ point + 200 ° C. preferable.

  In addition, when the average cooling rate of the steel sheet after finish rolling is 10 to 100 ° C./second and the coiling temperature is in the range of 450 to 650 ° C., the air cooling is maintained at about 5 ° C./second until 680 ° C. after finish rolling. Then, cooling is performed at a cooling rate of 30 ° C./second or more, and the coiling temperature is set to 400 ° C. or less, and the selection is made according to the target tissue configuration. By controlling the cooling rate and coiling temperature after rolling, the former rolling conditions had one or more microstructures and fractions from polygonal ferrite, bainitic ferrite, and bainite phase. With the latter rolling conditions, a DP steel sheet having a large amount of a polygonal ferrite phase and a martensite phase composite structure excellent in ductility can be obtained.

  If the above average cooling rate is less than 10 ° C./second, pearlite which is unfavorable for stretch flangeability tends to be generated, which is not preferable. On the other hand, there is no need to set an upper limit on the cooling rate in terms of structure control, but too high a cooling rate may cause uneven cooling of the steel sheet, and it is expensive to manufacture equipment that enables such cooling. It is thought that this will lead to an increase in the price of the steel sheet. From such a viewpoint, the upper limit of the cooling rate is preferably set to 100 ° C./second.

  The high-strength cold-rolled steel sheet according to the present invention is manufactured by cold rolling and annealing a steel sheet that has undergone processes such as pickling and skin pass after hot rolling and winding. The final cold-rolled steel sheet is obtained by annealing in an annealing process such as batch annealing or continuous annealing.

  Needless to say, the high-strength steel sheet according to the present invention may be applied as a steel sheet for electroplating. Even if electroplating is applied, there is no change in the mechanical properties of the high-strength steel sheet of the present invention.

  Examples of the present invention will be described below together with comparative examples.

  The slab which shows a chemical component in Table 1 was hot-rolled on the conditions shown in Table 2, and the hot rolled sheet of thickness 3.2mm was obtained.

In Table 1, steel numbers (hereinafter referred to as steel numbers) 1, 5, 7, 9 , 11, and 13 are composed of compositions within the range of the high-strength steel plate according to the present invention, and steel numbers 2, 4, 6, 8, 10, 12, and 14 are configured as comparative steels that deviate from the range of the high-strength steel sheet according to the present invention. Steel Nos. 2, 4, 6, 8, 10, and 14 are slabs containing acid-soluble Al in excess of 0.01%. Steel Nos. 8, 10, 12, and 14 are Ce or La. It is configured as a slab in which the total of one or two types is reduced to less than 0.0005.

By the way, in Table 1, steel numbers 1 and 2 , steel numbers 5 and 6, steel numbers 7 and 8, so that they can be compared with each other, have almost the same composition. In addition, acid-soluble Al and the like are different from each other. Moreover, after having comprised with the substantially same composition mutually so that it can each compare between the steel number 9 and the steel number 10, the steel plate 11 and the steel number 12, and the steel number 13 and the steel number 14. , Ce + La and the like are different from each other.

  In Table 2, as Condition A, the heating temperature is 1250 ° C., the finish rolling completion temperature is 845 ° C., the cooling rate after finish rolling is 75 ° C./second, and the winding temperature is 450 ° C. As condition B, the heating temperature is 1200 ° C., the finish rolling completion temperature is 860 ° C., and after the finish rolling is maintained at air cooling at about 5 ° C./second until 680 ° C., and then the cooling rate of 30 ° C./second or more and the winding temperature is 400 ° C. It is said. As Condition C, the heating temperature is 1200 ° C., the finish rolling completion temperature is 825 ° C., the cooling rate after finish rolling is 45 ° C./second, and the winding temperature is 450 ° C.

For Steel No. 1 and Steel No. 2, Condition A, and for Steel No. 4 , Condition B,
For steel numbers 5 and 6, condition A, for steel numbers 7 and 8, condition C, for steel numbers 9 and 10, condition A, steel By applying Condition C to No. 11 and Steel No. 12 and to Steel No. 13 and Steel No. 14, the effects of chemical composition can be compared under the same manufacturing conditions. .

  As basic properties of the steel sheet thus obtained, strength, ductility, stretch flangeability, and fatigue limit ratio were examined.

  In addition, as the state of the presence of the stretched inclusions in the steel sheet, the number ratio, volume number density, and average equivalent circle diameter of the inclusions with a stretching ratio of 5 or more were examined for all inclusions of 1 μm or more.

  Furthermore, as the presence state of the non-stretched inclusions in the steel plate, all of inclusions of 1 μm or more, Ce, La, one or two oxides, or Si, Ti, or The number ratio and volume number density of inclusions in which MnS-based inclusions are precipitated in oxides or oxysulfides containing two kinds, and the total of one or two kinds of Ce or La in inclusions having a drawing ratio of 3 or less The average content was examined.

  The reason why inclusions of 1 μm or more are used is that, in addition to easy observation, inclusions of less than 1 μm do not affect stretch flangeability and deterioration of fatigue characteristics.

  The results are shown in Table 3 for each combination of steel and rolling conditions.

The strength and ductility were obtained by a tensile test of a JIS No. 5 specimen taken in parallel with the rolling direction. Stretch flangeability is measured by measuring the hole diameter D (mm) at the time when a through-thickness crack is generated by expanding a punched hole with a diameter of 10 mm with a 60 ° conical punch in the center of a 150 mm × 150 mm steel plate. The hole expansion value λ = (D−10) / 10. Further, the fatigue limit ratio used as an index representing the fatigue characteristics is a value obtained by dividing the 2 × 10 ⁶ time strength (σW) obtained by a method according to JIS Z 2275 by the strength (σB) of the steel sheet (σW / σB) was evaluated.

  Note that the test piece is a No. 1 test piece defined in the same standard, with a parallel part of 25 mm, a radius of curvature R of 100 mm, and a thickness of 3.0 mm obtained by equally grinding both surfaces of the original plate (hot rolled plate). .

  Further, the inclusions were observed by SEM, and the major axis and the minor axis were measured for 50 inclusions having a circle-equivalent diameter of 1 μm or more selected at random. Further, using the quantitative analysis function of SEM, composition analysis was performed on 50 inclusions with a diameter of 1 μm or more selected at random. Using these results, the number ratio of inclusions with a stretching ratio of 5 or more, the average equivalent circle diameter of inclusions with a stretching ratio of 5 or more, an oxide composed of one or two of Ce and La, or Si, The ratio of the number of inclusions in which MnS inclusions are deposited on oxides or oxysulfides containing one or two kinds of Ti, and the total of one or two kinds of Ce or La in inclusions with a stretching ratio of 3 or less The average value of was obtained. Further, the volume number density of inclusions by shape was calculated by SEM evaluation of the electrolytic surface by the speed method.

As is apparent from Table 3, in steel Nos. 1 , 5 , 7 , 9 , 11, and 13 to which the method of the present invention was applied, an oxide composed of one or two of Ce and La, or Si, Ti By depositing MnS-based inclusions on oxides or oxysulfides containing one or two of the above, it was possible to reduce the stretched MnS-based inclusions in the steel sheet. That is, inclusions in which MnS inclusions are deposited on an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti in steel plate The number ratio of the inclusions is 10% or more, the volume number density of the inclusions is 1.0 × 10 ³ pieces / mm ³ or more, and one kind of Ce or La in the inclusions having a stretching ratio of 3 or less present in the steel sheet or By setting the average content of the two types to 0.5% to 50%, the number ratio of stretched inclusions having an equivalent circle diameter of 1 μm or more and a stretch ratio of 5 or more is 20% or less, and the volume number density of the inclusions 1.0 × 10 ⁴ pieces / mm ³ or less, and the average equivalent circle diameter of the inclusions was 10 μm or less. In any steel sheet structure, the average crystal grain size was 1 to 8 μm.

As a result, compared with the comparative steel, steel Nos. 1 , 5 and 7 as the steels of the present invention were able to obtain a steel plate excellent in stretch flangeability and fatigue characteristics. However, in the comparative steels (steel numbers 2, 4, 6, 8, 10, 12, and 14), although the average crystal grain size was 1 to 8 μm, the stretched MnS inclusions and Ce The distribution state of inclusions in which MnS inclusions are precipitated in oxides composed of one or two of La, oxides containing one or two of Si and Ti, or oxysulfides in the present invention Therefore, the MnS inclusions stretched during the processing of the steel sheet became the starting point of cracking, and the stretch flangeability and fatigue characteristics were reduced.

Claims (11)

C: 0.03 to 0.20%
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.0030% or more ,
Acid-soluble Ti: 0.008 to 0.20%
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
A plurality of inclusions with an equivalent circle diameter of 1 μm or more, which are randomly selected using an SEM of inclusions with an equivalent circle diameter of 1 μm or more existing in the steel sheet and whose major axis / minor axis is 5 or more. A high-strength steel sheet excellent in stretch flangeability and fatigue characteristics, characterized in that the ratio of the number relative to is 20% or less. % By mass
C: 0.03 to 0.20%
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.0030% or more ,
Acid-soluble Ti: 0.008 to 0.20%
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
In the steel sheet, an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti.
Inclusions in which one or more of MnS, TiS, or (Mn, Ti) S are combined and precipitated are randomly selected using SEM at a ratio of 10 to a plurality of inclusions having a circle equivalent diameter of 1 μm or more. % High-strength steel sheet with excellent stretch flangeability and fatigue characteristics. % By mass
C: 0.03 to 0.20%
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.0030% or more ,
Acid-soluble Ti: 0.008 to 0.20%
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
The volume number density of the inclusions having an equivalent circle diameter of 1 μm or more present in the steel sheet and having a major axis / minor axis of 5 or more is 1.0 × 10 ⁴ pieces / mm ³ or less. High-strength steel sheet with excellent stretch flangeability and fatigue characteristics. % By mass
C: 0.03 to 0.20%
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.0030% or more ,
Acid-soluble Ti: 0.008 to 0.20%
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
In the steel sheet, an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti.
Stretch flangeability, characterized in that the volume number density of inclusions in which one or more of MnS, TiS, or (Mn, Ti) S is combined and precipitated is 1.0 × 10 ³ pieces / mm ³ or more. High strength steel plate with excellent fatigue properties. % By mass
C: 0.03 to 0.20%
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.0030% or more ,
Acid-soluble Ti: 0.008 to 0.20%
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
Stretch flangeability and fatigue characteristics characterized by inclusions having an equivalent circle diameter of 1 μm or more present in the steel sheet, and an average equivalent circle diameter of elongated inclusions having a major axis / minor axis of 5 or more being 10 μm or less. Excellent high strength steel plate. % By mass
C: 0.03 to 0.20%
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.0030% or more ,
Acid-soluble Ti: 0.008 to 0.20%
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
In the steel sheet, an oxide composed of one or two of Ce and La, or an oxide or oxysulfide containing one or two of Si and Ti.
There are inclusions in which one or more of MnS, TiS, or (Mn, Ti) S are complex-precipitated, and the total of one or two of Ce or La in the average composition is 0. A high-strength steel sheet excellent in stretch flangeability and fatigue characteristics, characterized by containing 5 to 95% by mass. % By mass
(Ce + La) / S ratio is 0.1-70, The high-strength steel plate excellent in stretch flangeability and fatigue characteristics in any one of Claims 1-6 characterized by the above-mentioned. % By mass
C: 0.03 to 0.20%
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.0030% or more ,
Acid-soluble Ti: 0.008 to 0.20%
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
A high-strength steel sheet excellent in stretch flangeability and fatigue characteristics, wherein the average grain size of crystals in the structure of the steel sheet is 10 μm or less. % By mass
Nb: 0.01 to 0.10%
V: 0.01-0.05%
Cr: 0.01 to 0.6%
Mo: 0.01 to 0.4%
B: 0.0003 to 0.003%
Ca: 0.0001 to 0.004%
The high-strength steel sheet excellent in stretch flangeability and fatigue characteristics according to any one of claims 1 to 8, wherein the high-strength steel sheet is excellent in stretch flangeability and fatigue characteristics. In the refining process in steelmaking, C is 0.03 to 0.20% and Si is 0.08 to 1% by mass% with P being 0.05% or less and S being 0.0030% or more. 0.5%, Mn 1.0-3.0%, N is added or adjusted to be 0.0005-0.01%, and then Ti is added without substantially adding Al. Then, one or two of Ce or La are added, and the acid-soluble Ti is 0.008 to 0.20%, and the total of one or two of Ce or La is 0.0005 to 0.04% A method for producing molten steel for high-strength steel sheets excellent in stretch flangeability and fatigue characteristics, characterized by:   In the refining step, before adding one or two of Ce or La, in addition, by mass%, Nb is 0.01 to 0.10%, V is 0.01 to 0.05%, and Cr is 0.01-0.6%, Mo 0.01-0.4%, B 0.0003-0.003%, Ca 0.0001-0.004%, one or more The method for producing molten steel for high-strength steel sheets excellent in stretch flangeability and fatigue characteristics according to claim 10, wherein the molten steel is added as described above.