WO2008007477A1 - High-strength steel sheet excellent in stretch flangeability and fatigue property - Google Patents

Abstract

A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue properties. The steel sheet contains 0.03-0.20% C, 0.08-1.5% Si, 1.0-3.0% Mn, up to 0.05% P, at least 0.0005% S, 0.0005-0.01% N, up to 0.01% acid-soluble Al, less than 0.008% acid-soluble Ti, and 0.0005-0.04% at least either of Ce and La, the remainder being iron and unavoidable impurities. The steel sheet contains stretched inclusions which have a diameter, in terms of equivalent-circle diameter, of 1 µm or larger and in which the ratio of the major-axis length to the minor-axis length is 5 or higher, in a proportion of 20% or higher by number.

Description

 Description High-strength steel sheet with excellent stretch flangeability and fatigue characteristics

 The present invention relates to a high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics, which is suitable as a material for an automobile underbody member. Background art

 There is an increasing demand for high strength and light weight hot-rolled steel sheets for automobiles from the viewpoint of improving automobile safety and improving fuel efficiency leading to environmental conservation. Among the parts for automobiles, the weight of frames and arms, especially the underbody system, is high in the weight of the entire vehicle body, so the material used for these parts is made thinner by increasing the strength. This makes it possible to reduce the weight. In addition, the materials used for the undercarriage system are required to have high fatigue characteristics from the viewpoint of durability against vibration during running.

 However, with increasing strength and fatigue resistance, hole expansibility tends to decrease in the same way as ductility, and when applying high-strength steel sheets to undercarriage systems of automobiles with complex shapes, The hole expansibility is an important consideration. .

For this reason, several steel sheets have been proposed with the aim of achieving both mechanical strength characteristics, fatigue characteristics, and hole expandability (workability). For example, JP-A-11-199973 proposes a steel sheet (generally called DP steel sheet) in which fine Cu precipitates or solid solution is dispersed in a ferrite steel sheet and martensite phase composite steel sheet. . In the disclosed technique disclosed in Japanese Patent Laid-Open No. 11-199973, solid solution of Cu or simple substance is used. It has been found that Cu precipitates with a particle size of 2 nm or less, which are composed by themselves, are extremely effective in improving fatigue properties and do not impair workability, thus limiting the composition ratio of various components.

 The DP steel sheet is known to be excellent in the balance between strength and ductility and fatigue properties, but still has poor stretch flangeability as evaluated in the hole expansion test. One reason for this is that DP steel is a composite of a soft ferrite phase and a hard martensite phase, so the boundary between the two phases cannot follow the deformation during hole expansion, making it easy to start fracture. It is thought that.

 On the other hand, high strength hot-rolled steel sheets have been proposed that satisfy not only fatigue characteristics but also severe stretch flangeability requirements of recent materials for wheels and suspension members (for example, JP 2001 — See 20033 No. 1). In the disclosed technology disclosed in Japanese Patent Laid-Open No. 2001-200331, the main phase is made to be a bainitic structure by reducing C as much as possible, and a solid structure strengthened or precipitation strengthened ferrite structure at an appropriate volume ratio. The main point is to reduce the hardness difference between ferrite and bainite and to avoid the formation of coarse carbides. Disclosure of the invention

The high-strength hot-rolled steel sheet, which is mainly composed of a steel sheet structure and suppresses the formation of coarse carbides, as disclosed in the above-mentioned JP-A-2001-200331, certainly exhibits excellent stretch flangeability. Fatigue properties are not necessarily superior to DP steel containing Cu. In addition, cracking cannot be prevented when severe hole enlargement processing is performed simply 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. Repeated deformation As a result, internal defects are generated around the extended coarse Mn S inclusions on or near the surface layer and propagate as cracks, thereby deteriorating fatigue characteristics and also extending coarse Mn S inclusions. This is because the object tends to be the starting point of cracking during hole expansion. For this reason, it is desirable to make the Mn S inclusions in steel as fine as possible without stretching them.

 However, Mn is an element that contributes to increasing the strength of the material together with C and Si. However, in high-strength steel sheets, the Mn concentration is generally set high to ensure strength. If heavy desulfurization is not performed in the next purification process, the S concentration will be more than 50 PPDI. For this reason, MnS is usually present in the piece. When hot-rolled and cold-rolled, the Mn S deforms easily and becomes a stretched Mn S-based inclusion, which causes deterioration in fatigue properties and stretch flangeability (hole expansion workability). . However, there is no example of proposing a hot-rolled steel sheet with excellent stretch flangeability and fatigue properties from the viewpoint of Mn S precipitation / deformation control.

 Therefore, the present invention has been devised in view of the above-described problems, and the object of the present invention is to cause precipitation as fine MnS in the flakes, and further, without being deformed during rolling and cracking. It is an object of the present invention to provide a high-strength steel sheet excellent in stretch flangeability and fatigue properties by improving the stretch flangeability and fatigue properties by dispersing in the steel plate as fine spherical inclusions that are unlikely to be the starting point of occurrence.

In order to solve the above-mentioned problems, the present inventor has made steel sheets as fine spherical inclusions which are precipitated as fine Mn S in the flakes and are not deformed during rolling, and are unlikely to start cracking. Research was conducted with a focus on elucidating additive elements that do not degrade fatigue properties and the method of dispersing them. As a result, the fine and hard produced by the deoxidation by the addition of Ce and La. MnS precipitates on high-quality Ce oxides, La oxides, cerium oxide sulfides, and lanthanum sulfides, and deformation of the precipitated MnS hardly occurs during rolling. The stretched coarse MnS is remarkably reduced, and it becomes difficult for these MnS inclusions to become the starting point of cracking and the path of crack propagation during repeated deformation and hole expansion processing. It has been elucidated that this will lead to improvements.

 The outline 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: 0.03-0.20%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005-0.01%, acid-soluble A1: 0.01% or less, acid-soluble Ti: less than 0.008%, Ce or La 1 or 2 types in total: 0.0005-0.04% steel, the balance being steel and inevitable impurities, The number ratio of the inclusions having an equivalent circle diameter of 1 m or more present in the steel sheet and having a major axis Z minor axis of 5 or more is 20% or less.

 (2) By mass%, C: 0.03-0.20%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005-0.01%, acid-soluble A1: 0.01% or less, acid-soluble Ti: less than 0.008%, a total of one or two of Ce or La: 0.0005 to 0.04%, the balance being steel and unavoidable impurities, The steel sheet is characterized by containing at least 10% of inclusions in which MnS is deposited on one or two kinds of oxides or oxysulfides of Ce or La.

(3) By mass%, C: 0.03-0.20%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005-0.01%, acid-soluble A1: 0.01% or less, acid-soluble Ti: less than 0.008%, one or two of Ce or La: Total: 0.0005-0.04%, the balance being iron A steel plate composed of unavoidable impurities, and an inclusion with an equivalent diameter of 1 ^ m or more and an elongated inclusion with a major axis Z minor axis of 5 or more has a volume density of 1.0X10 ⁴ inclusions. It is characterized by Z 匪³ or less.

(4) By mass%, C: 0.03-0.20%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005-0.01%, acid-soluble A1: 0.01% or less, acid-soluble Ti: less than 0.008%, Ce or La total of one or two: 0.0005 to 0.04%, the balance being steel and unavoidable impurities, The volume number density of inclusions in which MnS is deposited on one or two oxides or oxysulfide of Ce or La in the steel sheet is 1. OX 10 ³ pieces Zmm ³ or more Features. .

 (5) By mass%, C: 0.03-0.20%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005-0.01%, acid-soluble A1: 0.01% or less, acid-soluble Ti: less than 0.008%, Ce or a total of one or two: 0.0005 to 0.04%, the balance being steel and unavoidable impurities. Elongation flange property, characterized by the inclusion of an inclusion with an equivalent diameter of 1 m or more present in a steel plate and an average equivalent circle diameter of an extension inclusion with a major axis / minor axis of 5 or more of 10 m or less. High strength steel plate with excellent fatigue characteristics.

(6) By mass%, C: 0.03-0.20%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005-0.01%, acid-soluble A1: 0.01% or less, acid-soluble Ti: less than 0.008%, Ce or La 1 or 2 types in total: 0.0005-0.04% steel, the balance being steel and inevitable impurities, In the steel sheet, there is an inclusion in which MnS is precipitated on an oxide or oxysulfide containing one or two kinds of Ce or La, and Ce has an average composition in the inclusion. A high-strength steel sheet with excellent stretch flangeability and fatigue characteristics, characterized by containing 0.5 to 50% by mass of one or two of La.

 (7) By mass%, C: 0.03-0.20%, Si: 0.08-1.5%, Mil: 1.0-3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005-0.01%, acid-soluble A1: 0.01% or less, Acid-soluble Ti: Less than 0.008%, Ce or La Total of one or two: 0.0005 to 0.04%, the balance being steel and unavoidable impurities, (Ce + La) A high-strength steel sheet with excellent stretch flangeability and fatigue characteristics characterized by a ZS ratio of 0.1 to 70.

 (8) Any one of the following: Nb: 0.01 to 0.10%, V: 0.01 to 0.05%, Cr: 0.01 to 0.6%, Mo: 0.01 to 0.4%, B: 0.0003 to 0.03% One or two or more, the balance being a steel plate made of iron and inevitable impurities, characterized by excellent stretch flangeability and fatigue properties according to any one of (1) to (7) High strength steel plate. Brief Description of Drawings

 Figure 1 shows the relationship between Ce + La (%) and S (). BEST MODE FOR CARRYING OUT THE INVENTION

 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 present inventor made various changes to molten steel containing C: 0.07%, Si: 0.2%, Mn: 1.2%, P: 0.01% or less, S: 0.005%, N: 0.003%, and the balance being Fe. Deoxidation was performed using various elements to produce a steel ingot. The obtained steel ingot was hot rolled into a 3 mm hot rolled steel sheet. These manufactured hot rolled steel sheets In addition to the hole expansion test and fatigue test, the density, morphology and average composition of inclusions in the steel sheet were investigated.

 As a result, it was found that the steel sheet that had been deoxidized with addition of at least Ce and La after adding Si with almost no deoxidation with A 1 had the best stretch flangeability and fatigue characteristics. The reason is that MnS was deposited on fine and hard Ce oxide, La oxide, cerium oxide sulfide, and lanthanum sulfide produced by deoxidation by addition of Ce and La, and this precipitation was also observed during rolling. Since deformation of MnS hardly occurs, the stretched coarse MnS is remarkably reduced in the steel sheet. As a result, during repeated deformation or hole expansion, these MnS inclusions are less likely to become crack initiation points and crack propagation paths, leading to improvements in fatigue resistance as described above.

Incidentally, Ce oxides, La oxides, why cell Liu Muo carboxymethyl monkey phi Dooyo beauty lanthanum O carboxymethyl sulfates eye de is miniaturized, the first S i 0 ₂ type inclusions generated by S i deoxidation The Ce and La added later are reduced and decomposed to form fine Ce oxide, La oxide, cerium oxysulfide and lanthanum sulfite, and the generated Ce oxide, This is because, since the interface energy between La oxide, cerium ogi sulfide and lanthanum sulfide itself and molten steel is low, agglomeration after formation is also suppressed.

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

 The reason why the chemical components are limited in the present invention will be described below. '

 C: 0.03 to 0.20%

C is the most basic element that controls the hardenability and strength of steel. Enhances the fatigue strength by increasing the hardness and depth of the hardened hardened layer. In other words, this C is an essential element for securing the strength of the steel sheet, and at least 0.03% is necessary to obtain a high-strength steel sheet. However, if this C is excessively contained, a cementite phase will be generated even if C is fixed by Ti carbide generation as in the past, or even if cooling conditions are used. This cementite phase induces work hardening of the steel sheet and is not preferable for improving stretch flange characteristics. Therefore, in the present invention, from the viewpoint of improving workability, the C concentration is set to 0.20% or less.

 Si: 0.08 to 1.5%

Since Si is a major deoxidizing element in molten steel to which A1 and Ti are not added as much as in the present invention, it is extremely important in the present invention. Si also has the function of increasing the number of nucleation sites of austenite during quenching heating, suppressing austenite grain growth, and reducing the grain size of the quenched hardened layer. This Si suppresses the formation of carbides and suppresses the decrease in grain boundary strength due to carbides. In addition, this Si is effective for the generation of the Painai microstructure, and plays an important role in terms of ensuring the strength of the entire material. Reducing the dissolved oxygen concentration in the molten steel, once Si0 in order to produce a _2-based inclusions (Ce to added pressure after the Si0 ₂ -based inclusions, in order to fine inclusions by reduction with 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%. In contrast, when the concentration of Si is too high, liable to generate a large inclusions becomes high Si0 ₂ concentration in inclusions, also toughness and ductility become extremely poor, the surface decarburization and surface flaws increase 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 to 3.0%

 Mn is an element useful for deoxidation in the steelmaking stage, and is an element effective for increasing the strength of steel sheets 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, if Mn exceeds 3.0%, the segregation of Mn decreases the ductility due to the increase in solid solution strengthening. Also, since the weldability and base metal toughness deteriorate, 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 reduces the torsional fatigue strength by praying to the austenite grain boundaries and lowering the grain boundary strength. Since there is a concern about deterioration of workability, the content should be 0.05% or less. If solid solution strengthening is not required, it is not necessary to add P, and the lower limit of P includes 0%.

 S: 0.0005% or more

 S is prayed as an impurity, and S forms coarse inclusions of MnS and deteriorates stretch flangeability. Therefore, it is desirable that the concentration be as low as possible. Conventionally, in order to ensure stretch flangeability, it was necessary to extremely low sulfurize the S concentration to less than 0.005%. However, in the present invention, MnS is precipitated on fine and hard Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide, and deformation hardly occurs during rolling. Since stretching is prevented, the upper limit of the concentration of S is not specified.

 In addition, in order to reduce the S concentration to less than 0.0005%, it is necessary to considerably strengthen the desulfurization treatment by secondary scouring, and the desulfurization treatment cost for realizing the concentration becomes too high. In addition, since the effect of controlling the morphology of MnS is difficult to develop, the lower limit of the S concentration is set to 0.0005%.

N: 0.0005 to 0.0 1% N is an element that is inevitably mixed into steel because nitrogen in the air is taken in during the treatment of molten steel. N forms nitrides with ΑΓ, Ti, etc., and promotes the refinement of the matrix structure. However, if this N is added too much, coarse precipitates are generated even with a trace amount of A1 or a trace amount of Ti, and the stretch flangeability is deteriorated. Therefore, in the present invention, the upper limit of the N concentration 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 the lower limit.

 Acid soluble A1: 0.01% or less

Acid-soluble A1 is desirable to be suppressed as much as possible because its oxides are likely to be clustered and become coarse, and the stretch flangeability deteriorates fatigue properties. However, up to 0.01% is allowed as a preliminary deoxidizer. This is because if the acid soluble A1 concentration of 0.01 percent, A 1 ₂ 0 ₃ content in the inclusions exceeds 50%, it is because the clustered was place of inclusions. From the viewpoint of preventing clustering, it is better that the acid-soluble A1 concentration is lower, and the lower limit is 0%. Further, the acid soluble A1 concentration, obtained by measuring the concentration of A1 dissolved in acid, dissolved A1 is dissolved in acid, A 1 ₂ 0 ₃ is the analysis method using not to dissolve the acid . Here, examples of the acid include a mixed acid mixed at a ratio of hydrochloric acid 1, nitric acid 1 and water 2 ('mass ratio). Using such a acid, a soluble A1 acid, can classified into the A1 ₂ 0 ₃ which does not dissolve in acid, it can be measured acid soluble M concentration.

 Acid soluble Ti: less than 0.008%

The acid-soluble Ti is also likely to become coarse due to its oxides clustering, and it is easy to form coarse TiN inclusions combined with N in the steel, so the acid-soluble Ti is less than 0.008%. Contains 0%. The acid-soluble Ti concentration is a measure of the concentration of Ti dissolved in acid, and is an analytical method that utilizes dissolved Ti dissolved in acid and Ti oxide not dissolved in acid. . Here, the acid is, for example, the ratio of hydrochloric acid 1, nitric acid 1, water 2 (mass ratio Examples of the mixed acid mixed in). Using such an acid, it is soluble in acid

It can be separated into Ti and Ti oxide that does not dissolve in acid ', and acid-soluble Ti concentration can be measured.

 Total of one or two of Ce or La: 0.0005-0.04%

Ce, La is reduced ₂ Si0 produced by Si deoxidation, easy precipitation site next to the MnS, and the hard, difficult to deform Ce oxide during rolling fine _{(eg, Ce 2 0 3, Ce0 2} ), Cerium oxide sulfide (eg Ce ₂ 0 ₂ S), La oxide (eg La ₂ 0 ₃ , La0 ₂ ), lanthanum oxide sulfide (eg La ₂ 0 ₂ S), Ce oxidation Materials—Has the effect of forming inclusions whose main phase is 50% or more of La oxide or cerium oxysulfide or lanthanum oxysulfide.

Here, in the above inclusions, MnO, Si0 ₂ by deoxidation conditions, or in some cases to partially containing A 1 ₂ 0 _3, main phase as a precipitation site of MnS within the above oxide It functions well, and the effect of the micro-hardening of inclusions is not impaired. In order to obtain such inclusions, the total concentration of one or two of Ce or La must be 0.0005% or more and 0.04% or less. One or two total concentration of Ce or La can not reducing 'the Si0 ₂ inclusions is less than 0.00 0.05%, Seriyuumuo Kishisarufuai de is 0.04 percent, lanthanum O carboxymethyl monkey sulfide is produced in large quantities, coarse It becomes an inclusion and stretch flangeability deteriorates fatigue characteristics.

 Nb: 0.01-0.10%

 Nb forms carbides, nitrides, and carbonitrides with C or N and promotes refinement of the base material structure. To obtain this effect, at least 0.01% is required. 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 to 0.05%

V is a base material that forms carbide, nitride, carbonitride with C or N Promotes tissue refinement. To obtain this effect, at least 0.01% is required. However, even if it is contained in a large amount exceeding 0. Q5%, the effect is saturated and the cost becomes high, so 0.05% is made the upper limit.

 C r: 0.01 to 0.6%

 Cr can be included as necessary to improve the hardenability of the steel and ensure the strength of the steel sheet, and at least 0.01% is necessary to obtain this effect. . However, a large amount of content deteriorates the balance between strength and ductility. Therefore, the upper limit is 0.6%.

 Mo: 0.01-0.4%

 Mo can be added as needed to improve the hardenability of the steel and ensure the strength of the steel sheet, and at least 0.01% is necessary to obtain this effect. However, a large amount of content deteriorates the balance of strength ductility. Therefore, the upper limit is 0.4%.

 B: 0.0003 to 0.003%

 B can be included as needed to improve the hardenability of the steel, strengthen the grain boundaries, and improve the workability. To obtain this effect, B is at least 0.0003%. is required. However, if a large amount is contained, the cleanliness of the steel is impaired and the ductility is deteriorated. Therefore, the upper limit is 0.003%.

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

In order to obtain a steel sheet with excellent stretch flangeability and fatigue properties, it is important to reduce as much as possible the stretched and coarse MnS inclusions in the steel sheet, which are likely to be the origin of crack initiation and the path of crack propagation. The inventor has experimentally found that MnS inclusions with an equivalent circle diameter of less than 1 m are harmless as a starting point of cracking and do not deteriorate fatigue characteristics if stretch flangeability occurs. In addition, inclusions with a circle equivalent diameter of 1 or more can be easily observed with a scanning electron microscope (SEM), etc. The morphology and composition were investigated, and the distribution of MnS inclusions was evaluated. Here, the equivalent circle diameter is defined as (major axis X minor axis) obtained from the major axis and minor axis of the inclusion observed in the cross section.

 The upper limit of the circle equivalent diameter of MnS inclusions is not particularly specified, but in reality, about 1 MnS inclusions may be observed.

 The number of stretched inclusions was determined by analyzing the composition of multiple inclusions (for example, about 50) with a diameter equivalent to a circle l ^ m or more randomly selected using SEM, and determining the major axis and minor axis of the inclusions. Measure from SEM image. Here, when the elongated inclusion is an inclusion having a major axis Z minor axis (stretching ratio) of 5 or more, the number of the detected elongated inclusions is the total number of investigated inclusions (50 in the above example). The ratio of the number of the stretched inclusions can be obtained.

 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. The upper limit of the stretching ratio of MnS inclusions is not particularly specified, but in reality, MnS inclusions with a stretching ratio of about 50 may be observed.

As a result, it was found that the stretch flangeability and fatigue characteristics of the steel sheet whose form ratio was controlled to 20% or less in the number ratio of stretch inclusions with a stretch ratio of 5 or more were improved. That is, if the number ratio of stretch inclusions with a stretch ratio of 5 or more exceeds 20%, the number ratio of MnS-based stretch inclusions, which are likely to start cracking, increases too much, and the stretch flangeability and fatigue characteristics deteriorate. In the present invention, the number ratio of stretch inclusions having a stretch ratio of 5 or more is 20%. The following. In addition, since the stretch flangeability and fatigue properties are better as the number of stretched MnS inclusions is smaller, the lower limit of the number ratio of stretch inclusions with a stretch ratio of .5 or more includes 0%.

 Here, an inclusion with an equivalent circle diameter of 1 z ^ rn or more and a lower limit of the number ratio of the extension inclusions with a drawing ratio of 5 or more means 0%. This is the case when there is no stretch ratio of 5 or more, or when the circular equivalent diameter is less than 1 m even for stretch inclusions with a stretch ratio of 5 or more.

 In addition, in steel sheets whose shape ratio is 20% or less, the number of stretched inclusions with a stretching ratio of 5 or more is correspondingly reduced by adding MnS to oxides or oxysulfide containing one or two of Ce or La. It is in the form of precipitated. As for the form of this inclusion, it is sufficient that MnS is precipitated in one or two kinds of oxides or oxysulfides of Ce or La, and it is not particularly specified, but one kind of Ce or La is used. In many cases, MnS is deposited around two oxides or oxysulfide as a nucleus.

 In addition, inclusions in which MnS is precipitated in one or two oxides or oxysulfide of Ce or La are not easily deformed during rolling. It is almost 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, preferably 2 or less, in the steel sheet. This is because, in the flake stage before rolling, the stretching ratio of inclusions in the form of 'MnS precipitated on oxide or oxysulfide consisting of one or two of Ce or La was 3 or less. is there. In addition, spherical inclusions that are judged not to be stretched have a stretching ratio of 1 if they are completely spherical, so the lower limit of the stretching ratio is 1.

 The survey of the number ratio of inclusions was conducted in the same manner as the number ratio survey of stretched inclusions. As a result, in steel sheets in which the number ratio of inclusions in the form of MnS precipitated in one or two types of oxides or oxysulfides of Ce or La was controlled to be 10% or more, elongation flangeability and It was found that the fatigue characteristics were improved. When the number of inclusions in the form of MnS deposited on one or two types of oxides or oxysulfides of Ce or La becomes less than 10%, MnS-based stretched inclusions correspond to this. The ratio of the number of steels increases so much that the stretch flangeability and fatigue characteristics deteriorate. For this reason, the number ratio of inclusions in the form of MnS deposited on one or two kinds of oxides or oxysulfides of Ce or La should be 10% or more. In addition, the stretch flangeability and fatigue properties are better when a large amount of MnS is precipitated in one or two oxides or oxysulfides of Ce or La. Contains 1 00%.

 It should be noted that inclusions in the form of one or two oxides of Ce or La, or inclusions in which MnS is precipitated on oxysulfide are unlikely to deform during rolling, so the circle equivalent It diameter is specified in particular. It is not a thing, l ^ m or more may be sufficient. However, if it is too large, there is a concern that cracking will start, so the upper limit is preferably about 50 m.

 On the other hand, this inclusion is not easily deformed during rolling, and when the equivalent circle diameter is less than 1 m, it does not become a starting point for 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 inclusion particle size distribution was determined by SEM evaluation of the electrolytic surface by the speed method. The SEM evaluation of the electrolytic surface by the speed method means that the surface of the sample piece is polished. After polishing, electrolysis is performed by the speed method, and the size and number density of inclusions are evaluated by direct SEM observation of the sample surface. The speed method is a method of electrolyzing the sample surface using 10% acetylacetone and 1% tetramethylammonium diumurium chloride methanol to extract inclusions. The amount of electrolysis is 1 1 C was electrolyzed per cm ² . The SEM image of the surface electrolyzed in this way was image-processed, and the frequency (number) distribution for the equivalent circle diameter was obtained. The average equivalent circle diameter was calculated from the frequency distribution of the particle size, and the number density per volume of inclusions was calculated by dividing the frequency by the area of the observed field and the depth determined from the amount of electrolysis.

As a result of evaluating the volume number density of inclusions with an equivalent diameter of 1 m or more and an elongation ratio of 5 or more as a starting point of crack generation and stretch flangeability and fatigue characteristics are reduced, 1. OX 10 ⁴ pieces Zmin ³ or less It has been found that stretch flangeability and fatigue properties are improved. If the volume number density of stretched inclusions with an equivalent circle diameter of 1 m or more and an elongation ratio of 5 or more exceeds 1. OX 10 ⁴ pieces Zmm ³ , the number density of MnS-type stretch inclusions that are likely to start cracking. becomes too large, to lower the fatigue characteristics stretch flange formability, an equivalent circle diameter of 1 / xm or more, and a volume number density of the stretch ratio of 5 or more stretched inclusions 1.0X 10 ⁴ cells / mm ³ or less To do. In addition, since the stretch flangeability and fatigue properties are better as the number of stretched MnS inclusions is smaller, the lower limit of the volume number density of stretched inclusions with a circle equivalent diameter of l ^ m or more and a stretch ratio of 5 or more Contains 0%.

 Here, the lower limit value of the volume number density of stretched inclusions having an equivalent circle diameter of l ^ m or more and a stretching ratio of 5 or more means 0%, as described above.

In addition, in the case of a steel sheet whose shape is controlled to be 1.0 × 10 ⁴ pieces / mm ³ or less in the volume number density of drawn inclusions having a diameter of l ^ m or more and a draw ratio of 5 or more Corresponding to the above, unstretched MnS inclusions are in the form of MnS deposited on one or two oxides or oxysulfides of Ce or La, and the shape is almost spherical inclusions. It was.

 The form of the inclusion is not particularly limited as long as MnS is precipitated in the oxide or oxysulfide containing one or two kinds of Ce or La as described above. In many cases, MnS is deposited around one or two oxides or oxysulfide as a nucleus.

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

As a result of investigating the volume number density of such inclusions, the volume number density of inclusions in the form of MnS precipitated around one or two oxides or oxysulfide of Ce or La as the core. However, it was found that the stretch flangeability and fatigue properties of steel sheets with a controlled precipitation of 1.0 x 10 ³ Z mm ³ or more improved. When the volume number density of inclusions in the form of MnS deposited on one or two oxides or oxysulfide of Ce or La is less than 1.0 X 10 ³ / mm ³ As a result, the number ratio of MnS-based stretch inclusions increases so much that the stretch flangeability and fatigue properties decrease, so that MnS precipitates in oxides or oxysulfides of one or two of Ce or La. The volume number density of inclusions in this form is specified to be 1.0 X 1 O ³ pieces / mm ³ or more. In addition, the fatigue strength of stretch flangeability is better when a large amount of MnS is precipitated with one or two oxides of Ce or La or oxysulfide as the core. The value is not particularly specified. In addition, the equivalent circle diameter of inclusions in which MnS is deposited on one or two types of oxides or oxysulfide of Ce or La is not particularly specified, as described above. That's all. 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 m.

 On the other hand, if the equivalent circle diameter of this inclusion is less than 1 m, there is no problem, so the lower limit is not 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 with a circle equivalent diameter of 1 m or more and an elongation ratio of 5 or more, which is the starting point of crack generation and deteriorates the stretch flangeability and fatigue characteristics, It was found that when the average equivalent circle diameter is 10 m or less, stretch flangeability and fatigue characteristics are improved. This is because the average equivalent circle diameter of the stretched inclusions increases as the number ratio of stretched inclusions with a circle equivalent diameter of 1 / im or more and a stretching ratio of 5 or more increases. The average equivalent circle diameter is defined as an index. This is presumed that as the amount of Mn and S in the molten steel increases, the number of MnS produced increases and the size of MnS produced increases.

 Therefore, if the elongation inclusion with a circle equivalent diameter of 1 m or more and a stretching ratio of 5 or more exceeds 10 m, the number ratio of the stretching inclusions exceeds 20%. The ratio of the number of coarse MnS-based stretch inclusions that are likely to be the starting point of the steel increases too much, and the stretch flangeability and fatigue characteristics decrease. The equivalent diameter shall be 10 m or less.

In addition, the provision that the average equivalent circle diameter of stretched inclusions with a circle equivalent diameter of 1 / im or more and a stretch ratio of 5 or more is lOm or less is equivalent to 1m This means that the inclusions exist in the steel sheet, so the lower limit of the equivalent circle diameter is 1 m.

 On the other hand, in the steel sheet of the present invention described above, as an existence condition of inclusions in the form of MnS precipitated on one or two kinds of oxides or oxysulfide of Ce or La, inclusions with MnS precipitated It was defined by the content of the average composition of Ce or La.

 Specifically, as described above, in order to improve stretch flangeability and fatigue characteristics, MnS is deposited on the oxide or oxysulfide of Ce or Mo, which is one or two kinds of La, and MnS is stretched. It is important to prevent this.

 The form of the inclusion is not particularly limited as long as MnS is precipitated in the oxide or oxysulfide of one or two kinds of Ce or La, as described above. In many cases, MnS is deposited around an oxide or oxysulfide consisting of one or two kinds of La as a nucleus.

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

 Therefore, in order to clarify the effective composition for suppressing the stretching of Mn S inclusions, the composition of inclusions in the form of MnS deposited on oxides or oxysulfide consisting of one or two of Ce or La Analysis was performed. However, if this inclusion has an equivalent circle diameter of 1 // m or more, it is easy to observe. Therefore, for the sake of convenience, the equivalent circle diameter of 1 m or more was used. However, if observation is possible, inclusions with a circle equivalent diameter of less than 1 m may be included.

Also, oxides or oxides of one or two of Ce or La The inclusions in the form of MnS deposited on the sisalide were not stretched, so it was confirmed that all the stretch ratios were inclusions of 3 or less. Therefore, a composition analysis was performed for inclusions with an equivalent circle diameter of l ^ m or more and a stretching ratio of 3 or less.

 As a result, when inclusions of one or two of Ce or La with an average composition in an inclusion with an equivalent circle diameter of l ^ m or more and an elongation ratio of 3 or less are contained in an amount of 0.5 to 50%, stretch flangeability It was found that the fatigue characteristics improved. When the average content of one or two of Ce or La in inclusions with an equivalent circle diameter of l ^ m or more and a stretch ratio of 3 or less is less than 0.5 mass%, Corresponding to this, the ratio of inclusions in the form of MnS deposited on one or two oxides or oxysulfide is greatly reduced. The number ratio increases too much, and the stretch flangeability and fatigue characteristics decrease.

 On the other hand, when the average content of one or two of Ce or La in inclusions with an equivalent circle diameter of lm or more and an extension ratio of 3 or less exceeds 50%, cerium oxysulfide, lanthanum A large amount of oxysulfide is generated and becomes a coarse inclusion with an equivalent circle diameter of about 50 xm or more. Therefore, stretch flangeability deteriorates fatigue characteristics.

 Further, in the steel sheet of the present invention, the chemical composition of the steel sheet (Ce + La) ZS ratio is the presence condition of inclusions in the form of MnS precipitated in one or two kinds of oxides or oxysulfide of Ce or La. Stipulated in

Specifically, as described above, in order to improve stretch flangeability and fatigue characteristics, MnS is deposited on one or two kinds of oxides or oxysulfide of Ce or La to prevent the extension of MnS. The chemical component ratio for Therefore, in order to clarify the effective chemical composition ratio for suppressing the elongation of MnS inclusions, the (Ce + La) ZS ratio of the steel sheet was changed to evaluate the inclusion morphology, stretch flangeability and fatigue properties ( Figure 1 ) . As a result, it was found that stretch flangeability and fatigue characteristics were improved when the (Ce + La) ratio was 0.1 to 70. When the (Ce + La) -no-S ratio is less than 0.1, the ratio of the number of inclusions in the form of MnS deposited on one or two oxides or oxysulfide of Ce or La greatly decreases. Correspondingly, the number ratio of MnS-based stretch inclusions, which are likely to be the starting point of cracking, increases too much and stretch flangeability and fatigue characteristics deteriorate. On the other hand, when the (Ce + La) / S ratio exceeds 70, a large amount of cerium oxysulfide and lanthanum oxysulfide are generated, and the coarse equivalent diameter is about 50 m or more. As a result, the stretch flangeability and fatigue characteristics are degraded.

 Next, the steel sheet weaving will be described.

In the present invention, stretch flangeability and fatigue properties are improved by controlling MnS inclusions, and the microstructure of the steel sheet is not particularly limited. Steel sheet with a structure composed mainly of patient ferrite, steel sheet with a complex structure composed mainly of the ferri phase, martensite phase and bainite phase, and ferritic phase, residual austenite The effect of the present invention can be obtained with any steel sheet of a composite structure comprising a low temperature transformation phase (martensite or bainite), but in order to obtain excellent stretch flangeability, It is preferable that the structure has a main phase as a main phase. Desirably, it is necessary that the pay- ferrite or the vinyl phase is the largest phase ratio. The area ratio of the vane-ferrite phase in the steel sheet is preferably 50% or more, more preferably 80% or more, and even more preferably 100%. The balance is the bain phase or polygo It can contain 20% or more of the null-ferrite phase.

 Next, manufacturing conditions will be described. In the present invention, decarburization is performed by blowing in a converter, or decarburization is further performed using a vacuum degassing apparatus, and Si, Mn are contained in a molten steel having a C concentration of 0.03 to 0.1%. , P and other alloys are added for deoxidation and component adjustment, and if A 1 or T i is not added or oxygen adjustment is required, acid soluble A 1 or acid soluble T i Add a small amount of A 1 or Ti so that a slight amount remains, and then add one or two of Ce or La to adjust the ingredients. A piece is produced by continuously forging the molten steel thus produced.

 Continuous forging is not only applied to normal continuous slabs with a thickness of about 250nim, but is also used for bloom pallets and slab continuous forgings. It is fully applicable to subsequent fabrication.

 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 1 150 ° C or higher so as to dissolve carbonitrides in the steel. By dissolving these in solid solution, the formation of polygonal ferrite is suppressed in the cooling process after rolling, and a structure mainly composed of a plastic ferrite phase that is favorable for stretch flangeability can be obtained. On the other hand, when the heating temperature of the slab before hot rolling exceeds 1250 ° C, the slab surface is significantly oxidized. In particular, wedge-shaped surface defects caused by selective oxidation of the grain boundaries are observed after descaling. The upper limit is preferably set to 1 250 ° C because the remaining quality deteriorates the surface quality after rolling.

After being heated to the above temperature range, normal hot rolling is performed. In this process, the finish rolling completion temperature is important for controlling the structure of the steel sheet. If the finish rolling completion temperature is less than Ar ₃ points + 30 ° C, the crystal grain size of the surface layer portion tends to be coarse, which is not preferable in terms of fatigue characteristics. Meanwhile, Ar ₃ Detrimental to stretch flangeability than a point + 200 ^P C than polygonal • ferrite phase is generated easily Runode, it is preferable that the upper limit 'and Ar _3. Point + 200 ° C.

 In addition, the average cooling rate of the steel sheet after finish rolling should be 40 ° C nosec or more, and cooling to the range of 300-500 ° C will suppress the formation of polygonal 'ferrite phase, and the · It is effective for obtaining an organization mainly composed of ferrite phase.

 If the average cooling rate is less than 40 ° C / second, a polygonal ferri phase is likely to be formed, which is not preferable. On the other hand, there is no need to set an upper limit on the cooling rate in terms of microstructure control, but too high a cooling rate may cause uneven cooling of the steel sheet, and for the production of equipment that enables such cooling. A large amount of money is required, which is thought to increase the price of steel sheets. From this point of view, the upper limit of the cooling rate is preferably 100 ° C / sec.

 In addition, when the cooling stop temperature is lower than 300 ° C, a martensite phase that is unfavorable for stretch flangeability is generated, so the lower limit is set to 300 ° C. Accordingly, the coiling temperature of the hot-rolled coil is preferably set to 300 ° C. or higher in order to suppress the formation of a martensite phase that extremely deteriorates the stretch flangeability.

On the other hand, when the temperature exceeds 500 ° C, the formation of polygonal ferrite phase cannot be suppressed, and in steels containing Cu, Cu may be deposited locally in the ferrite phase and reduce the fatigue property improvement effect. Therefore, the winding temperature is preferably 500 ° C or lower. Therefore, by winding at 500 ° C or less, carbonitride precipitates during the subsequent cooling process, reducing the amount of dissolved C and N in the ferritic phase and improving stretch flangeability. Example

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

Table 1 shows the chemical composition of the slab, which was hot-rolled under the conditions shown in Table 2 to obtain a hot rolled sheet having a thickness of 3.2 mni.

table 1

Table 2

In Table 1, steel numbers (hereinafter referred to as steel numbers) 1, 3, 5, 7, 9, 11, 13 are composed of compositions within the range of the high-strength steel sheet according to the present invention. 2, 4, 6, 8, 8, 10, 12, and 14 are configured as comparative steels that deviate from the range of high-strength steel sheets according to the present invention. Steel numbers 2, 4, and 6 are slabs containing more than 0.01% acid-soluble A1, and steel numbers 8, 10, 12, and 14 are the sum of one or two of Ce or La. It is constructed as a slab with a reduced to less than 0.0005.

 By the way, in this table 1, steel number 1 and steel number 2, steel number 3 and steel number 4, steel number 5 and steel number 6, steel number 7 and steel number 8 can be compared respectively. In this way, the acid-soluble A1 and the like are made different from each other after having the same composition. In addition, in order to be able to compare steel number 9 and steel number 10, steel number 11 and steel number 12, steel number 13 and steel number 14, respectively, Ce + La etc. 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 / sec, the winding temperature is 450 ° C, As condition B, the heating temperature is 1200 ° C, the finish rolling completion temperature is 825, the cooling rate after finish rolling is 45 Zsec, and the winding temperature is 450. For Steel No. 1 and Steel No. 2, Condition A, for Steel No. 3 and Steel No. 4, Condition B, for Steel No. 5 and Steel No. 6, Condition A Furthermore, by applying condition B to steel number 7 and steel number 8, steel number 9 and steel number 10, steel number 1 1 and steel number 12, steel number 13 and steel number 14, The effect of chemical composition can be compared under the same manufacturing conditions.

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

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

 Furthermore, as for the presence of unstretched inclusions in the steel sheet, all inclusions with a length of 1 m or more were used, and inclusions in which MnS precipitated on oxides or oxysulfides of one or two types of Ce or La The average value of the total content of one kind or two kinds of Ce or La in inclusions having an object number ratio and volume number density and an elongation ratio of 3 or less was examined.

 The inclusions of 1 / xm or more are targeted because they are easy to observe, and inclusions of less than 1βm do not affect the fatigue characteristics of stretch flangeability.

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

Table 3

Strength and rolling were obtained by a tensile test of JIS No. 5 specimens taken in parallel with the rolling direction. Stretch flangeability is measured by measuring the hole diameter D (mm) when a through-thickness crack is generated by pushing a lOmm diameter punched hole in the center of a 150mm x 150MI steel plate with a 60 ° conical punch. Expansion value λ = (D− 10) Evaluation was performed using λ obtained from Z10. The fatigue limit ratio used as an indicator of fatigue characteristics is the value obtained by dividing the 2 × 10 ⁶ time strength (o W) obtained by the method based on JIS Ζ 2275 by the strength of the steel sheet (σ Β) (o W / σ B).

 The test piece is a No. 1 test piece specified in the same standard, with a parallel part of 25 mm, a radius of curvature R of 100 mm, and a thickness of 3. Omm with both sides of the original plate (hot rolled plate) ground equally. .

 Furthermore, the inclusions were subjected to SEM observation, and the major axis and minor axis were measured for 50 inclusions with a circle-equivalent diameter of 1 m or more selected at random. Furthermore, using the quantitative analysis function of SEM, composition analysis was performed on 50 inclusions with a circle-equivalent diameter of 1 m 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, MnS in oxides or oxysulfide consisting of one or two of Ce or La The ratio of the number of inclusions deposited, and the average value of the total of one or two of Ce or La in inclusions with a stretching ratio of 3 or less were determined. The volume 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, 3, 5, 7, 9, 11, and 13 to which the method of the present invention was applied, MnS was added to the oxide or oxysulfide containing one or two of Ce or La. It was possible to reduce the stretched MnS inclusions in the steel sheet. That is, the ratio of the number of inclusions in which MnS is precipitated in one or two oxides or oxysulfide of Ce or La in the steel plate is 10% or more, The volume number density of inclusions is 1.0X 10 ³ mm ³ or more, and the average content of 0.5% or more of Ce or La in inclusions with an elongation ratio of 3 or less is 3 or less. -50%, the number ratio of stretch inclusions with a circle equivalent diameter of 1 m or more and a stretch ratio of 5 or more is 20% or less, and the volume number density of the inclusions is 1.0 × 10 ⁴ pieces ³ or less, The average equivalent circle diameter of the inclusions was less than lO trn. As a result, compared with the comparative steel, steel plates 1, 3, 5, 7, 9, 11 and 13 as the steels of the present invention were able to obtain steel sheets with excellent stretch flangeability and fatigue characteristics. However, in the comparative steels (steel numbers 2, 4, 6, 8, 10, 12, 14), MnS is added to the stretched MnS inclusions and one or two oxides or oxysulfide of Ce or La. Since the distribution of the inclusions precipitated was different from the distribution specified in the present invention, the MnS inclusions that were stretched during the processing of the steel sheet were the starting points for cracking, and the stretch flangeability and fatigue characteristics were reduced. Industrial applicability

 According to the method of the present invention, the fine MnS precipitates in the flakes, and is not deformed during rolling and is dispersed in the steel sheet as fine spherical inclusions that are unlikely to start cracking. A high-strength hot-rolled steel sheet with excellent flangeability and fatigue characteristics can be obtained.

Claims

1. By mass%

 C: 0.03-0.20%,

 Si: 0.08 ~ 5%

 Mn: 1.0-3.0%,

 P: 0.05% or less

 S: 0.0005% or more,

 N: 0.0005-0.01%,

 Acid soluble A1: 0.01% or less,

 Acid soluble Ti: less than 0.008%,

 Surrounding

 Ce or La 1 type or 2 types in total: 0.0005 to 0.04% contained, the balance is a steel plate made of iron and inevitable impurities, with inclusions with an equivalent circle diameter of 1 m or more present in the steel plate, A high-strength steel sheet with excellent stretch flangeability and fatigue characteristics, wherein the number ratio of elongated inclusions having a major axis / minor axis of 5 or more is 20% or less.

 2. In quantity%

 C 0.03 ~ 0.20%,

 Si 0.08-1.5%,

 Mn 1.0-3.0%,

 P 0.05% or less,

 S 0. 0005% or more,

 N 0. 0005-0.01%,

 Acid soluble Al: 0.01% or less,.

 Acid 'soluble Ti: less than 0.008%,

Ce or La 1 type or 2 types in total: 0.0005 to 0.04% containing, the balance is a steel plate made of iron and inevitable impurities, in the steel plate Is excellent in stretch flangeability and fatigue characteristics, characterized by containing 10% or more of inclusions with MnS precipitated on one or two kinds of oxides of Ce or La or oxysulfide High strength steel plate.

 3. By mass%

 C: 0.03-0.20%,

 Si: 0.08 to 1.5%,

 Mn: 1.0-3.0%,

 P: 0.05% or less,

 S: 0.0005% or more,

 N: 0.0005-0.01%,

 Acid soluble A1: 0.01% or less,

 Acid soluble Ti: less than 0.008%,

Ce or La 1 type or 2 types in total: 0.0005-0.04% containing, the balance is a steel plate made of iron and inevitable impurities, with inclusions with an equivalent circle diameter of 1 m or more present in the steel plate, A high-strength steel sheet with excellent stretch flangeability and fatigue properties, characterized in that the volume number density of stretched inclusions having a major axis Z minor axis of 5 or more is 1.0 × 10 ⁴ kn mm ³ or less.

 4. Mass%

 C: 0.03-0.20%,

 Si: 0.08 to 1.5%

 Mn: 1.0-3.0%,

 P: 0.05% or less,

 5: 0.0005% or more,

 N: 0.0005 to 0.01%,.

 Acid 'soluble A1: 0.01% or less,

 Acid soluble Ti: less than 0.008%,

Total of one or two of Ce or La: Contains 0.0005-0.04% The balance is a steel plate made of iron and unavoidable impurities, and the volume number density of inclusions in which MnS is deposited on the oxide or oxysulfide consisting of one or two of Ce or La is contained in the steel plate. 1. OX 10 ³ cells / Ran ³ or more in high-strength steel sheets were excellent in stretch flange formability and fatigue properties, characterized in that.

 5. By mass%,

 C: 0.03-0.20%,

 Si: 0.08 to 1.5%>

 Mn: 1.0-3.0%,

 P: 0.05% or less,

 5: 0.0005% or more,

 N: 0.0005-0.01%,

 Acid-soluble A1: 0.01% or less,

 Acid soluble Ti: less than 0.008%,

 Ce or La 1 type or 2 types in total: 0.0005 to 0.04% contained, the balance is a steel plate made of iron and inevitable impurities. A high-strength steel sheet excellent in stretch flangeability and fatigue characteristics, characterized in that the average equivalent circle diameter of elongated inclusions having a major axis / minor axis of 5 or more is 10 'm or less.

 6. By mass%

 C: 0.03-0.20%,

 Si: 0.08 to 1.5%,

 Mn: 1.0-3.0%,

 P: 0.05% or less,

 S ': 0.0005% or more,

 N: 0.0005-0.01%,

Acid-soluble A1: 0.01% or less, Acid soluble Ti: less than 0.008%,

 Ce or La 1 type or 2. Total of species: 0.0005-0.04% containing, the balance is a steel plate made of iron and inevitable impurities, and the steel plate consists of 1 or 2 types of Ce or La Inclusions in which MnS is precipitated are present on oxide or oxysulfide, and the inclusions contain an average composition of one or two of Ce or La in an amount of 0.5 to 50% by mass. High-strength steel plate with excellent stretch flangeability and fatigue characteristics.

 7. By mass%

 C: 0.03-0.20%,

 Si: 0.08 to 1.5%,

 Μη: 1.0-3.0%,

 ：: 0.05% or less,

 S: 0.0005% or more,

 Ν: 0.0005-0.01%,

 Acid-soluble A1: 0.01% or less,

 Acid soluble Ti: less than 0.008%,

 Ce or La 1 type or 2 types in total: 0.0005-0.04% containing, the balance is iron and inevitable impurities steel plate, (Ce + La) / S ratio is 0.0 to 70 A high-strength steel sheet with excellent stretch flangeability and fatigue characteristics.

 8. By mass%

 Nb: 0.01 ~ 0 · 10%,

 V: 0.0 to 0.05%,.

 Cr: 0.01-0.6%

 Mo: 0.01-0.4%

B: 0.0003-0.03% The stretch flangeability and fatigue characteristics according to any one of claims 1 to 7, characterized in that it is a steel plate containing any one or more of the following, with the balance being iron and inevitable impurities. Excellent high strength steel plate.