CN101490295A - High-strength steel sheet with excellent tensile flangeability and fatigue properties - Google Patents

Abstract

The present invention provides a high-strength hot-rolled steel sheet excellent in tensile flangeability and fatigue properties, the steel sheet containing 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%, one or both of Ce and La: 0.0005-0.04%, the remaining Partly composed of iron and unavoidable impurities; the number ratio of elongated inclusions is 20% or more, and the elongated inclusions are inclusions with an equivalent circle diameter of 1 μm or more existing in the steel plate, and the major diameter/short diameter 5 or more.

Description

High-strength steel sheet with excellent tensile flangeability and fatigue properties

technical field

The present invention relates to a high-strength hot-rolled steel sheet which is excellent in tensile flangeability and fatigue properties and is suitable as a raw material for automobile running parts.

Background technique

From the standpoint of improving the safety of automobiles and increasing fuel costs related to environmental protection, there is an increasing demand for high strength and light weight of hot-rolled steel sheets for automobiles. Among automotive parts, the weight of the frame and arms, which are called the running system, accounts for a high proportion of the total weight of the vehicle body. Therefore, it can be realized by increasing the strength of the raw materials used in these parts and reducing the thickness. lightweight. In addition, from the viewpoint of durability against vibration during running, materials used for the running system are required to have high fatigue properties.

However, with the increase in strength and fatigue resistance, both hole expandability and ductility tend to decrease. When high-strength steel sheets are used in automobile running systems with complex shapes, their hole expandability becomes an important research. topic.

Therefore, various steel sheets aiming at achieving both mechanical strength properties, fatigue properties, and hole expandability (workability) have been proposed. For example, Japanese Patent Laid-Open No. 11-199973 proposes a steel plate in which fine Cu or dispersed solid solution is precipitated in a steel plate with a composite structure of ferrite phase and martensite phase (generally referred to as DP steel plate). In the disclosed technology disclosed in Japanese Patent Laid-Open No. 11-199973, it was found that solid solution Cu or Cu precipitates composed of Cu alone with a particle size of 2 nm or less are very effective in improving fatigue characteristics without affecting workability , so the composition ratio of each component is limited.

It is known that such a DP steel sheet is excellent in balance between strength and ductility and fatigue properties, but is still poor in tensile flangeability as evaluated by a hole expansion test. One of the reasons is considered to be that the DP steel sheet is a complex of soft ferrite phase and hard martensite phase, so the boundary between the two phases cannot adapt to deformation during hole expansion and is likely to become the starting point of fracture.

In this regard, a high-strength hot-rolled steel sheet that satisfies not only the fatigue characteristics but also the severe tensile flangeability requirements required for the materials of recent wheels and running parts has been proposed (for example, refer to Japanese Patent Laid-Open No. 2001-200331). . The main technical content of the technology disclosed in Japanese Patent Application Laid-Open No. 2001-200331 is to make the main phase a bainite structure by reducing C as much as possible, and at the same time contain iron after solid solution strengthening or precipitation strengthening at an appropriate volume ratio. The ferrite structure reduces the hardness difference between ferrite and bainite, and avoids the generation of coarse carbides.

Contents of the invention

The high-strength hot-rolled steel sheet disclosed in the above-mentioned Japanese Patent Laid-Open No. 2001-200331, in which the structure of the steel sheet is mainly composed of bainite phase and the formation of coarse carbides is suppressed, does have excellent tensile flangeability, but its fatigue properties are not good. It must be better than DP steel plate containing Cu. Furthermore, if only the formation of coarse carbides is suppressed, the occurrence of cracks cannot be prevented during severe hole reaming. According to the studies of the present inventors, it is found that the cause is the presence of extended sulfide-based inclusions mainly composed of MnS in the steel sheet. If subjected to repeated deformation, internal defects will be generated around the extended coarse MnS inclusions located on or near the surface, and will propagate in the form of cracks to degrade the fatigue properties, and the extended coarse MnS inclusions will easily become hole expansion. The starting point of cracking during processing. For this reason, it is desired to prevent the MnS-type inclusions in the steel from extending and forming fine spherical inclusions as much as possible.

However, Mn, like C and Si, is an element that effectively increases the strength of the material. Therefore, in high-strength steel sheets, the concentration of Mn is usually set high in order to ensure the strength. In addition, if the secondary refining process is not carried out For the heavy treatment of desulphurization, the S concentration will also be above 50ppm. Therefore, MnS is usually present in cast flakes. When the cast slab is hot-rolled or cold-rolled, MnS is easily deformed to become elongated MnS-type inclusions, which is a cause of deterioration of fatigue characteristics and stretch flangeability (hole expansion workability). However, no example has been proposed of a hot-rolled steel sheet excellent in tensile flangeability and fatigue properties from the viewpoint of controlling the precipitation and deformation of MnS.

Therefore, the present invention was made in view of the above-mentioned problems, and an object of the present invention is to provide a high-strength steel sheet excellent in stretch flangeability and fatigue properties by precipitating fine MnS in the cast sheet and allowing the MnS to form The form of fine spherical inclusions that are not easily deformed during rolling and are not likely to be the origin of cracking is dispersed in the steel sheet, thereby improving the tensile flangeability and fatigue properties.

In order to solve the above-mentioned problems, the present inventors precipitated fine MnS in the cast slab and dispersed it in the steel plate in the form of fine spherical inclusions that are not easily deformed during rolling and are not easy to become the origin of cracking, and the method of not using Focusing on the elucidation of additive elements that reduce fatigue properties, intensive research has been conducted. As a result, it was found that MnS precipitated on the fine and hard Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide produced by adding Ce and La for deoxidation, and the precipitated MnS was not easily deformed during rolling, so Coarse MnS extended in the steel plate is significantly reduced, and these MnS-type inclusions are less likely to become the starting point of cracking and the path of crack propagation during repeated deformation and hole expansion, which explains the reason for the above-mentioned improvement in fatigue resistance.

The main contents of the high-strength steel sheet excellent in tensile flangeability and fatigue properties according to the present invention are as follows.

(1) The steel sheet of the present invention is characterized by containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N : 0.0005-0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, total of one or both of Ce and La: 0.0005-0.04%, the rest is iron and unavoidable The composition of impurities; the proportion of the number of elongated inclusions, the elongated inclusions are inclusions with an equivalent circle diameter of 1 μm or more existing in the steel plate, and the major axis/short axis ratio is 5 or more.

(2) The steel sheet of the present invention is characterized by containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N : 0.0005-0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, total of one or both of Ce and La: 0.0005-0.04%, the rest is iron and unavoidable The composition of impurities; the steel sheet contains 10% or more of inclusions in which MnS is precipitated on oxides or sulfur oxides formed of one or two of Ce or La in a number ratio.

(3) The steel sheet of the present invention is characterized by containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N : 0.0005-0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, total of one or both of Ce and La: 0.0005-0.04%, the rest is iron and unavoidable The composition of impurities; the volume number density of extension inclusions is 1.0×10 ⁴ pieces/mm ³ or less, and the extension inclusions are inclusions with an equivalent circle diameter of 1 μm or more existing in the steel plate, and the long diameter/short The diameter is more than 5.

(4) The steel sheet of the present invention is characterized by containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N : 0.0005-0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, total of one or both of Ce and La: 0.0005-0.04%, the rest is iron and unavoidable Composition of impurities; in this steel sheet, the volume number density of MnS inclusions precipitated on the oxide or sulfur oxide formed by one or two of Ce or La is 1.0×10 ³ /mm ³ or more .

(5) The steel sheet excellent in tensile flangeability and fatigue properties of the present invention is characterized by containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, and P: 0.05 in mass %. % or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, one or both of Ce and La: 0.0005 to 0.04%, the rest is composed of iron and unavoidable impurities; the average equivalent circle diameter of the extended inclusions is 10 μm or less, and the extended inclusions are inclusions with an equivalent circle diameter of 1 μm or more existing in the steel plate, and the length The diameter/short diameter is 5 or more.

(6) The steel sheet excellent in tensile flangeability and fatigue properties of the present invention is characterized by containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, and P: 0.05% by mass % % or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, one or both of Ce and La: 0.0005 to 0.04%, and the rest is composed of iron and unavoidable impurities; in this steel plate, there are inclusions with MnS precipitated on oxides or sulfur oxides formed by one or two of Ce or La, and in the inclusions The product contains 0.5 to 50% by mass of Ce or La in an average composition or a total of both of them.

(7) The steel sheet excellent in stretch flangeability and fatigue properties of the present invention is characterized by containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, and P: 0.05% by mass. % or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, one or both of Ce and La: 0.0005 to 0.04%, the remainder is composed of iron and unavoidable impurities; (Ce+La)/S ratio is 0.1-70.

(8) The high-strength steel sheet excellent in tensile flangeability and fatigue properties according to any one of (1) to (7), characterized by containing Nb: 0.01 to 0.10%, V: Any one or two or more of 0.01 to 0.05%, Cr: 0.01 to 0.6%, Mo: 0.01 to 0.4%, and B: 0.0003 to 0.03%, and the remainder is composed of iron and unavoidable impurities.

Description of drawings

FIG. 1 is a graph showing the relationship between Ce+La (%) and S (%).

Detailed ways

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

First, experiments for carrying out the present invention will be described.

The present inventors deoxidized 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 remainder was Fe, using various elements, Create steel blocks. The obtained steel block was hot-rolled into a 3 mm hot-rolled steel plate. These manufactured hot-rolled steel sheets were subjected to hole expansion tests and fatigue tests, and the number density, shape, and average composition of inclusions in the steel sheets were analyzed.

From the results, it can be seen that the steel sheet deoxidized by adding Si and at least Ce and La after deoxidation with almost no Al is the best in terms of tensile flangeability and fatigue properties. The reason for this is that MnS precipitates on the fine and hard Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide generated by adding Ce and La for deoxidation, and the precipitated MnS is not easily deformed during rolling. , so the extended coarse MnS in the steel plate is significantly reduced. As a result, these MnS-based inclusions are less likely to become the origin of cracking and the path of crack propagation during repeated deformation and hole expansion, which improves the above-mentioned fatigue resistance and the like.

In addition, the reason for the miniaturization of Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide is that SiO ₂ -type inclusions generated by Si deoxidation initially are reductively decomposed by Ce and La added later to form fine Ce oxides. compound, La oxide, cerium oxysulfide and lanthanum oxysulfide, and the interfacial energy between the generated Ce oxide, La oxide, cerium oxysulfide and lanthanum oxysulfide itself and the molten steel is low, so it can also suppress the generated agglomerates.

Based on the findings obtained from these experimental studies, the present inventors studied the chemical composition conditions of steel sheets as follows, and completed the present invention.

Hereinafter, the reasons for limiting the chemical components in the present invention will be described.

C: 0.03 to 0.20%

C is the most basic element to control the hardenability and strength of steel, and it helps to effectively increase the hardness and depth of the quench hardened layer and improve the fatigue strength. That is, this C is an essential element for securing the strength of the steel sheet, and must be at least 0.03% in order to obtain a high-strength steel sheet. However, if this C is contained too much, C will be fixed by forming Ti carbides as in the past, and even if cooling conditions are applied, a cementite phase will still be formed. This cementite phase induces work hardening of the steel sheet, which is not conducive to the improvement of the tensile flangeability. Therefore, in the present invention, the concentration of C is made 0.20% or less from the viewpoint of improving workability.

Si: 0.08～1.5%

Si is the main deoxidizing element in the molten steel in which Al and Ti are not added as much as possible in the present invention, so it is very important in the present invention. Furthermore, Si increases the number of nucleation sites of austenite during quenching heating, suppresses the grain growth of austenite, and plays a role of making the grain size of the quench-hardened layer finer. This Si suppresses the formation of carbides and suppresses the reduction of grain boundary strength caused by carbides. In addition, this Si is also effective in forming a bainite structure, and plays an important role from the viewpoint of securing the strength of the entire material. In order to reduce the dissolved oxygen concentration in molten steel and temporarily generate SiO ₂ -type inclusions (the SiO ₂ -type inclusions are reduced by Ce and La added later to make the inclusions finer), it is necessary to add 0.08% or more of Si. Therefore, in the present invention, the lower limit of Si is 0.08%. In contrast, if the concentration of Si is too high, the concentration of SiO ₂ in the inclusions will increase and large inclusions will be easily formed, and the toughness and ductility will be extremely poor, and the fatigue properties will decrease due to the increase of surface decarburization and surface defects. In addition, if Si is added too much, it will also have a bad influence on weldability and ductility. Therefore, in the present invention, the upper limit of Si is limited to 1.5%.

Mn: 1.0-3.0%

Mn is an element useful for deoxidation at 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, if more than 3.0% of Mn is contained, the ductility will decrease due to the segregation of Mn and the increase of solid solution strengthening. In addition, since weldability and substrate toughness also decrease, the upper limit of Mn is limited to 3.0%.

P: less than 0.05%

P can effectively function as a substitutional solid-solution strengthening element smaller than Fe atoms, but due to segregation at the grain boundary of austenite, the strength of the grain boundary decreases, resulting in a decrease in torsional fatigue strength, which may decrease the workability. Therefore, it is made 0.05% or less. In addition, since solid solution strengthening is not required, it is not necessary to add P, so the lower limit of P includes 0%.

S: 0.0005% or more

S segregates as an impurity and forms coarse and elongated inclusions of MnS to degrade the stretch-flangeability, so the concentration is desirably as low as possible. Conventionally, in order to ensure stretch-flangeability, it was necessary to perform extremely low vulcanization so that the concentration of S was less than 0.0005%. However, in the present invention, since MnS is precipitated on Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide, it is not easily deformed during rolling, and the extension of inclusions is prevented. Therefore, the upper limit of the concentration of S No special provisions are made.

In addition, in order to reduce the S concentration to the same level of less than 0.0005% as in the past, it is necessary to strengthen the desulfurization treatment to a considerable extent in the secondary refining. The effect of shape control is performed, so the lower limit of the S concentration is set to 0.0005%.

N: 0.0005～0.01%

N is an element that is inevitably mixed into steel due to intake of nitrogen in the air during molten steel treatment. N forms nitrides with Al, Ti, etc., and promotes fine-graining of the substrate structure. However, if this N is added too much, coarse precipitates will be generated even with a small amount of Al and a small amount of Ti, resulting in a decrease in stretch flangeability. Therefore, in the present invention, the upper limit of the concentration of N is limited to 0.01%. On the other hand, if the concentration of N is less than 0.0005%, the cost will increase, so 0.0005% is made the lower limit.

Acid-soluble Al: less than 0.01%

Acid-soluble Al tends to be coarsened due to clustering of oxides, resulting in a decrease in tensile flangeability and fatigue properties, so it is best to control it as much as possible. However, it is allowed to use 0.01% or less as a preliminary deoxidizing material. This is because if the acid-soluble Al concentration exceeds 0.01%, the Al ₂ O ₃ content in the inclusions exceeds 50%, causing clustering of the inclusions. From the viewpoint of preventing clustering, the lower the acid-soluble Al concentration, the better, and the lower limit includes 0%. In addition, the acid-soluble Al concentration refers to the concentration obtained by measuring the concentration of Al dissolved in an acid, and is an analysis method that utilizes the phenomenon that dissolved Al dissolves in an acid and Al ₂ O ₃ does not dissolve in an acid. Here, as the acid, for example, a mixed acid obtained by mixing 1 hydrochloric acid, 1 nitric acid, and 2 water (mass ratio) can be exemplified. Using this acid, Al soluble in acid can be distinguished from Al ₂ O ₃ insoluble in acid, and the acid-soluble Al concentration can be measured.

Acid soluble Ti: less than 0.008%

Acid-soluble Ti also becomes coarse because its oxides are easy to cluster, and it is easy to combine with N in the steel to form coarse TiN inclusions, so the acid-soluble Ti is lower than 0.008%, and the lower limit includes 0%. In addition, the acid-soluble Ti concentration refers to the concentration obtained by measuring the concentration of Ti dissolved in acid, and is an analysis method utilizing the phenomenon that dissolved Ti dissolves in acid and Ti oxides do not dissolve in acid. Here, as the acid, for example, a mixed acid obtained by mixing 1 hydrochloric acid, 1 nitric acid, and 2 water (mass ratio) can be exemplified. Using this acid can distinguish between acid-soluble Ti and acid-insoluble Ti oxides, and can determine the concentration of acid-soluble Ti.

One or two of Ce or La: 0.0005 to 0.04%

Ce and La reduce SiO ₂ generated by deoxidation of Si, easily become precipitation sites of MnS, and have Ce oxides (such as Ce ₂ O ₃ , CeO ₂ ), which are hard, fine, and difficult to deform during rolling, Cerium oxysulfide (e.g. Ce ₂ O ₂ S), La oxide (e.g. La ₂ O ₃ , LaO ₂ ), Lanthanum oxysulfide (e.g. La ₂ O ₂ S), Ce oxide-La oxide or cerium oxysulfide- The effect of inclusions of lanthanum oxysulfide as the main phase (more than 50% as the standard).

Here, the above-mentioned inclusions may contain a part of MnO, SiO ₂ or Al ₂ O ₃ depending on the deoxidation conditions, but as long as the main phase is the above-mentioned oxides, they can fully function as the precipitation sites of MnS, and Does not impair the fineness and hardening effect of inclusions. In order to obtain the aforementioned inclusions, the total concentration of one or both of Ce and La must be 0.0005% or more and 0.04% or less. If the total concentration of one or both of Ce or La is less than 0.0005%, the _SiO2 inclusions cannot be reduced, and if it exceeds 0.04%, a large amount of cerium oxysulfide and lanthanum oxysulfide will be formed to form coarse inclusions , leading to a decrease in tensile flangeability and fatigue properties.

Nb: 0.01 to 0.10%

Nb forms carbides, nitrides, and carbonitrides with C or N to promote the fine-graining of the matrix structure. In order to obtain this effect, it must be at least 0.01%. However, even if it is contained in a large amount exceeding 0.10%, the effect is saturated and the cost increases, so 0.10% is made the upper limit.

V: 0.01～0.05%

V and C or N form carbides, nitrides, and carbonitrides to promote the fine-graining of the matrix structure. In order to obtain this effect, it must be at least 0.01%. However, even if it is contained in a large amount exceeding 0.05%, the effect is saturated and the cost increases, so 0.05% is made the upper limit.

Cr: 0.01-0.6%

Cr may be contained as needed to improve the hardenability of the steel and ensure the strength of the steel sheet, and in order to obtain this effect, it must be at least 0.01%. However, if it is contained in a large amount, the strength-ductility balance will be lowered instead. Therefore, 0.6% is taken as an upper limit.

Mo: 0.01 to 0.4%

Mo may be contained as needed to improve the hardenability of the steel and ensure the strength of the steel sheet, and to obtain this effect, it must be at least 0.01%. However, if it is contained in a large amount, the strength-ductility balance will be lowered instead. Therefore, 0.4% is taken as the upper limit.

B: 0.0003～0.003%

B may be contained as needed to improve the hardenability of steel, strengthen grain boundaries, and improve workability, but in order to obtain this effect, it must be at least 0.0003%. However, if it is contained in a large amount, it will impair the washing performance of the steel and reduce the ductility. Therefore, 0.003% is made the upper limit.

Next, conditions for the presence of inclusions in the steel sheet of the present invention will be described. In addition, the steel plate refers to a rolled plate obtained by hot rolling or further cold rolling.

In order to obtain a steel sheet excellent in tensile flangeability and fatigue properties, it is important to minimize the coarse MnS-type inclusions in the steel sheet that tend to be the starting point of crack generation and the extension of the crack propagation path. The present inventors have found through experiments that MnS-type inclusions with an equivalent circle diameter of less than 1 μm are not harmful as the starting point of cracking, and will not reduce the tensile flangeability and fatigue properties, and inclusions with an equivalent circle diameter of more than 1 μm can be used Since it is easy to observe with a scanning electron microscope (SEM), etc., the distribution state of MnS-type inclusions is evaluated by analyzing the morphology and composition of inclusions with an equivalent circle diameter of 1 μm or more in steel sheets. Here, the circle-equivalent diameter is defined as a value obtained by (major diameter×short diameter) ^0.5 from the major axis and minor axis of the inclusions obtained from cross-sectional observation.

In addition, the upper limit of the circle-equivalent diameter of the MnS-type inclusions is not particularly specified, but in fact, MnS-type inclusions of about 1 mm may be observed in some cases.

The ratio of the number of elongated inclusions is determined by analyzing the composition of a plurality (for example, about 50) of randomly selected inclusions with an equivalent circle diameter of 1 μm or more using SEM and measuring the major axis and minor axis of the inclusions from the SEM image. Here, when the elongated inclusions are inclusions with a major axis/short axis (extension ratio) of 5 or more, the number of the above-mentioned elongated inclusions detected is divided by the total number of inclusions analyzed (in the above example, For example, about 50), the number ratio of the above-mentioned elongated inclusions can be obtained.

In addition, the reason why the elongation ratio of the inclusions is set to be 5 or more is because all the inclusions having an elongation ratio of 5 or more in the comparative steel sheet to which Ce and La are not added are MnS-type inclusions. In addition, the upper limit of the elongation ratio of the MnS-type inclusions is not particularly specified, but MnS-type inclusions with an elongation ratio of about 50 are sometimes observed in practice.

The results show that the tensile flangeability and fatigue properties of the steel plate are improved in the steel plate having a stretched inclusion ratio of 5 or more and a ratio of the number of stretched inclusions of 20% or less through shape control. That is, if the number ratio of stretched inclusions with a stretch ratio of 5 or more exceeds 20%, the number ratio of MnS-type stretched inclusions that are likely to be the origin of cracking is too large, and the tensile flangeability and fatigue properties are reduced. In the present invention, the number ratio of elongated inclusions having an elongated ratio of 5 or more is set to 20% or less. The lower limit of the number ratio of elongated inclusions having an elongated ratio of 5 or more includes 0% because the less elongated MnS-based inclusions are, the better the tensile flangeability and fatigue properties are.

Here, the lower limit of the number ratio of elongated inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more is 0%, which means that there are no inclusions with an equivalent circle diameter of 1 μm or more but an elongation ratio of 5 In the case of the above-mentioned inclusions, or in the case of elongated inclusions having an elongation ratio of 5 or more, the circle-equivalent diameters are all less than 1 μm.

In addition, the steel sheet in which the number ratio of elongation inclusions with an elongation ratio of 5 or more is 20% or less is formed on oxides or sulfur oxides formed of one or both of Ce and La according to morphology control. There is a form of MnS. The form of the inclusions is not particularly specified as long as MnS is deposited on the oxide or oxysulfide formed of one or both of Ce or La, and in many cases, it is formed of one or more of Ce or La. One or two kinds of oxides or sulfur oxides are formed as the core, and MnS is precipitated around it.

In addition, since the inclusion of MnS precipitated on the oxide or sulfur oxide formed of one or both of Ce and La is not easily deformed during rolling, it has an unstretched shape in the steel sheet, that is, a nearly spherical shape. inclusions.

Here, the spherical inclusions judged to be unstretched are not specifically defined, and refer to inclusions in the steel plate with a stretch ratio of 3 or less, preferably inclusions with a stretch ratio of 2 or less. This is because the elongation ratio of inclusions in the form of MnS precipitated on oxides or oxysulfides formed of one or both of Ce and La in the slab stage before rolling is 3 or less. In addition, if the spherical inclusions judged not to be elongated are completely spherical, the elongation ratio is 1, so the lower limit of the elongation ratio is 1.

The number ratio of the above-mentioned inclusions was analyzed by the same method as the number ratio analysis of the extension inclusions. The results showed that the tensile strength of the steel sheet was 10% or more in the number ratio of inclusions in the form of MnS precipitated on the oxide or sulfur oxide formed of one or both of Ce or La through precipitation control. Stretch flangeability and fatigue properties are improved. If the ratio of the number of inclusions in the form of MnS precipitated on the oxide or oxysulfide formed by one or both of Ce or La is less than 10%, the corresponding MnS-type extended inclusions If the number ratio is too large, the stretch flangeability and fatigue properties will be reduced. Therefore, the number ratio of inclusions in the form of MnS precipitated on the oxide or oxysulfide formed of one or both of Ce and La is set to be 10% or more. In addition, the more MnS is precipitated in the oxide or oxysulfide formed of one or both of Ce and La, the better the tensile flangeability and fatigue properties are, so the upper limit of the number ratio includes 100%. .

In addition, inclusions in the form of MnS precipitated on oxides or oxysulfides formed of one or both of Ce and La are not easily deformed during rolling, so the equivalent circle diameter is not particularly specified, and can be 1 μm or more. However, if it is too large, it may become a starting point of cracking, so the upper limit is preferably about 50 μm.

On the other hand, the inclusions are not only difficult to deform during rolling, but also do not become the starting point of cracking when the equivalent circle diameter is less than 1 μm, so there is no special regulation on the lower limit of the equivalent circle diameter.

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

The particle size distribution of the inclusions was evaluated by SEM on the electrolytic surface using the velocity method. The SEM evaluation of the electrolytic surface by the speed method means that after the surface of the sample is ground, it is electrolyzed by the speed method, and the SEM observation of the sample surface is directly performed to evaluate the size and number density of the inclusions. In addition, the speed method refers to a method in which inclusions are extracted by electrolyzing the surface of the sample using 10% acetylacetone-1% tetramethylammonium chloride-methanol. As the amount of electrolysis, the electrolysis is carried out at 1 C relative to the area of the sample surface 1 cm ² . The SEM image of the surface obtained by electrolysis in this way was image-processed, and the frequency (number) distribution with respect to the equivalent circle diameter was calculated|required. The average equivalent circle diameter was calculated from the frequency distribution of particle diameters, and the number density per unit volume of inclusions was calculated by dividing the frequency by the area of the observation field and the depth obtained from the amount of electrolysis.

The evaluation results of the volume number density of inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more, which become the starting point of cracking and degrade the tensile flangeability and fatigue properties, show that if it is 1.0×10 ⁴ /mm ³ or less, the stretch flangeability and fatigue properties are improved. If the volume number density of elongated inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more exceeds 1.0×10 ⁴ pieces/mm ³ , the number density of MnS-type elongated inclusions that are likely to be the starting point of fracture is too large , the tensile flangeability and fatigue properties deteriorate, so the volume number density of elongated inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more is set to be 1.0×10 ⁴ pieces/mm ³ or less. In addition, since the tensile flangeability and fatigue properties are better with fewer elongated MnS inclusions, the lower limit value of the volume number density of elongated inclusions having an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more includes 0%.

Here, the lower limit of the volume number density of elongated inclusions having a circle-equivalent diameter of 1 μm or more and an elongation ratio of 5 or more means 0% in the same sense as above.

In addition, in a steel plate whose shape is controlled so that the volume density of elongated inclusions with a diameter of 1 μm or more and an elongation of 5 or more is 1.0×10 ⁴ pieces/mm ³ or less, correspondingly, unextended MnS inclusions The object is in the form of MnS precipitated on the oxide or oxysulfide formed of one or both of Ce and La, and its shape is a nearly spherical inclusion.

The form of the inclusions is not particularly limited as long as MnS is deposited on oxides or oxysulfides formed of one or both of Ce and La in the same manner as above. One or two kinds of oxides or oxysulfides formed in La serve as the core, and MnS is deposited around it.

The spherical inclusions are not specifically defined, and refer to inclusions with an elongation ratio of 3 or less in the steel sheet, preferably inclusions with an elongation ratio of 2 or less. Here, since the stretch ratio is 1 if it is completely spherical, the lower limit of the stretch ratio is 1.

The analysis results of the volume number density of the above-mentioned inclusions show that the inclusions in the form of MnS are precipitated around the oxide or sulfur oxide formed by one or both of Ce or La through the precipitation control. A steel plate having a volume number density of 1.0×10 ³ pieces/mm ³ or more has improved tensile flangeability and fatigue properties. If the volume number density of inclusions in the form of MnS precipitated on the oxide or sulfur oxide formed by one or both of Ce or La is lower than 1.0×10 ³ /mm ³ , correspondingly MnS If the ratio of the number of elongated inclusions is too large, the tensile flangeability and fatigue properties will decrease, so the form of MnS will be precipitated on the oxide or sulfur oxide formed of one or both of Ce or La. The volume number density of inclusions is specified to be 1.0×10 ³ /mm ³ or more. Furthermore, the more MnS precipitated with oxides or oxysulfides formed of one or both of Ce and La as the core, the better the tensile flangeability and fatigue strength. Limits are not specified.

In addition, the equivalent circle diameter of inclusions in the form of MnS precipitated on the oxide or oxysulfide formed of one or both of Ce and La is not particularly limited as above, and may be 1 μm or more. However, if the circle-equivalent diameter is too large, there is a possibility that cracks may be generated, so the upper limit is preferably about 50 μm.

On the other hand, when the equivalent circle diameter of the inclusion is less than 1 μm, there is no problem, so there is no particular regulation on the lower limit.

Next, the conditions for the presence of elongated inclusions in the steel sheet of the present invention are defined by the upper limit of the equivalent circle diameter. Specifically, the evaluation of the average equivalent circle diameter of inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more, which become the starting point of cracking and degrade the tensile flangeability and fatigue properties, shows that when the elongation When the average equivalent circle diameter of the inclusions is 10 μm or less, the tensile flangeability and fatigue properties are improved. This is to focus on the fact that as the number ratio of extension inclusions with an equivalent circle diameter of 1 μm or more and an extension ratio of 5 or more increases, the average equivalent circle diameter of the extension inclusions increases, while the average equivalent circle diameter of the extension inclusions increases. The diameter is specified as an index. This is presumably because as the amounts of Mn and S in the molten steel increase, the number of formed MnS increases and the size of the formed MnS becomes coarser.

Therefore, if the extended inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more are larger than 10 μm, the proportion of the number of such elongated inclusions will exceed 20%, so they are likely to be coarse MnS-type elongated inclusions that are the starting point of cracking. If the ratio of the number of particles is too large, the tensile flangeability and fatigue properties will be reduced. Therefore, the average equivalent circle diameter of elongated inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more is set to 10 μm or less.

In addition, the regulation that the average equivalent circle diameter of elongated inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more is set to 10 μm or less means that there are inclusions with an equivalent circle diameter of 1 μm or more in the steel plate, so the equivalent The lower limit of the circle diameter is 1 μm.

On the other hand, as the existence conditions of the inclusions in the form of MnS precipitated on the oxide or sulfur oxide formed of one or both of Ce or La in the steel sheet of the present invention, the steel sheet with MnS precipitated is used. The content of the average composition of Ce or La in the inclusions is specified.

Specifically, as described above, in terms of improving tensile flangeability and fatigue properties, MnS is precipitated on oxides or oxysulfides formed of one or both of Ce or La, and the extension of MnS is prevented. , which is important.

The form of the inclusions is not particularly specified as long as MnS is deposited on the oxide or oxysulfide formed of one or both of Ce and La as described above. In many cases, it is made of Ce or La One or two kinds of oxides or oxysulfides formed in La serve as the core, and MnS is deposited around it.

In addition, the spherical inclusions are not specifically defined, but refer to inclusions with a stretch ratio of 3 or less in the steel sheet, preferably inclusions with a stretch ratio of 2 or less. Here, since the stretch ratio is 1 if it is completely spherical, the lower limit of the stretch ratio is 1.

Therefore, in order to clarify the composition that can effectively suppress the extension of MnS inclusions, composition analysis was performed on inclusions in the form of MnS precipitated on oxides or sulfur oxides formed of one or both of Ce and La.

However, since the inclusions are easy to observe when they have a circle-equivalent diameter of 1 μm or more, for convenience, a circle-equivalent diameter of 1 μm or more is targeted. However, inclusions having an equivalent circle diameter of less than 1 μm are preferably included if they can be observed.

In addition, inclusions in the form of MnS precipitated on oxides or oxysulfides formed of one or both of Ce and La were not elongated, so it was confirmed that they were all inclusions with an elongation ratio of 3 or less. Therefore, the composition analysis was performed on inclusions having a circle-equivalent diameter of 1 μm or more and an elongation ratio of 3 or less.

The results showed that when the inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 3 or less contained 0.5 to 50% of one or both of Ce or La in total on an average composition basis, the tensile flangeability and Fatigue properties improved. When the average content of one or both of Ce or La in inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 3 or less is less than 0.5% by mass, one of Ce or La Or the proportion of the number of inclusions in the form of MnS precipitated on the two formed oxides or sulfur oxides is greatly reduced, so the proportion of the number of MnS-type extended inclusions that are easy to become the starting point of cracking is too large, and the The stretch flangeability and fatigue properties are reduced.

On the other hand, if the average content of one or both of Ce and La in inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 3 or less exceeds 50%, a large amount of cerium oxysulfide will be generated. , Lanthanum oxysulfide, forming coarse inclusions with an equivalent circle diameter of about 50 μm or more, resulting in a decrease in tensile flangeability and fatigue properties.

As the existence condition of inclusions in the form of MnS precipitated on oxides or sulfur oxides formed of one or both of Ce and La in the steel sheet of the present invention, the chemical composition (Ce+La) of the steel sheet is used /S ratio to specify.

Specifically, as described above, in order to improve the tensile flange characteristics and fatigue characteristics, MnS is precipitated on oxides or oxysulfides formed of one or both of Ce or La to prevent MnS from elongating. chemical composition ratio.

Therefore, in order to clarify the chemical composition ratio that can effectively suppress the extension of MnS-type inclusions, the morphology, tensile flangeability, and fatigue properties of the inclusions were evaluated by changing the (Ce+La)/S ratio of the steel plate (Fig. 1 ). The results showed that when the (Ce+La)/S ratio was 0.1 to 70, the tensile flangeability and fatigue properties were improved. If the (Ce+La)/S ratio is lower than 0.1, the ratio of the number of inclusions in the form of MnS precipitated on the oxide or sulfur oxide formed by one or two of Ce or La is greatly reduced, so Correspondingly, the ratio of the number of MnS-type elongated inclusions, which tend to be the starting point of cracking, is too large, and the tensile flangeability and fatigue properties decrease.

On the other hand, if the (Ce+La)/S ratio exceeds 70, a large amount of cerium oxysulfide and lanthanum oxysulfide will be formed, and coarse inclusions with an equivalent circle diameter of about 50 μm or more will be formed, resulting in tensile flangeability and fatigue properties. decline.

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

In the present invention, the tensile flangeability and fatigue properties are improved by controlling the MnS inclusions, and there is no special limitation on the microstructure of the steel plate. In the steel plate with the structure of bainitic ferrite as the main phase, the composite structure steel plate with the ferrite phase as the main phase and the martensite phase and the bainite phase as the second phase, ferrite, The effect of the present invention can be obtained in any steel plate with a composite structure composed of retained austenite and low-temperature transformation phase (martensite or bainite), but in order to obtain excellent stretch flangeability, it is preferable to form the following Bainitic ferrite is the main phase structure. Preferably, the bainitic ferrite or bainite phase must be the largest phase in terms of area ratio. The area ratio of the bainitic ferrite phase in the steel sheet is preferably 50% or more, more preferably 80% or more, and still more preferably 100%. In addition, the remainder may contain 20% or more of bainite phase or polygonal ferrite phase.

Next, the manufacturing conditions will be described. In the present invention, carry out blowing and decarburization in the converter, or further use a vacuum degasser to carry out decarburization, add Si, Mn, P and other alloys to molten steel with a C concentration of 0.03 to 0.1%, and carry out deoxidation and composition Adjustment, and do not add Al and Ti or add a small amount of Al and Ti to the extent that a small amount of acid-soluble Al and acid-soluble Ti remain when oxygen adjustment is required, and then add one or both of Ce or La to adjust the composition . The molten steel thus smelted is continuously cast to produce cast slabs.

Continuous casting is not only suitable for continuous casting of slabs with a thickness of about 250 mm, but also for continuous casting of thin slabs such as steel ingots or slabs and slab continuous casting machines whose mold thickness is thinner than usual, for example, 150 mm or less.

Hot rolling conditions for producing high-strength hot-rolled steel sheets will be described. The heating temperature of the slab before hot rolling is preferably 1150° C. or higher in order to solid-solve carbonitrides and the like in the steel. By making them into a solid solution, the generation of polygonal ferrite in the cooling process after rolling is suppressed, and a structure mainly composed of a bainitic ferrite phase with good stretch-flangeability is obtained. On the other hand, if the heating temperature of the slab before hot rolling exceeds 1250°C, the oxidation of the slab surface will be significant, especially the wedge-shaped surface defects caused by the selective oxidation of the grain boundary will remain after descaling, which will affect the rolling process. Therefore, the upper limit is preferably set to 1250° C. because of the surface quality after processing.

After heating to the above temperature range, normal hot rolling is performed, but the temperature at which finish rolling is completed in this step is important for controlling the structure of the steel sheet. If the finishing temperature of the finish rolling is lower than the Ar ₃ point + 30°C, the grain size of the surface layer tends to become coarser, which is detrimental to the fatigue properties. On the other hand, when the temperature exceeds Ar ₃ point + 200°C, a polygonal ferrite phase that is detrimental to stretch flangeability is likely to be formed, so it is preferable to limit the upper limit to Ar ₃ point + 200°C.

In addition, the process of setting the average cooling rate of the steel plate after finish rolling to 40°C/s or more and cooling to the range of 300-500°C can effectively suppress the formation of polygonal ferrite phase, and obtain a bainitic ferrite phase. the main organization.

If the above-mentioned average cooling rate is lower than 40°C/sec, polygonal ferrite is likely to be formed, which is not preferable. On the other hand, although there is no need to set an upper limit on the cooling rate in terms of microstructure control, an excessively fast cooling rate may cause uneven cooling of the steel plate, and the cost of manufacturing equipment capable of such cooling will be high, resulting in an increase in the price of the steel plate . From this point of view, the upper limit of the cooling rate is preferably 100°C/sec.

In addition, if the stop cooling temperature is lower than 300°C, a martensite phase which is detrimental to stretch flangeability will be formed, so the lower limit is made 300°C. Therefore, the coiling temperature of the hot-rolled coil is preferably 300° C. or higher in order to suppress the formation of the martensitic phase that extremely deteriorates the stretch-flangeability.

On the other hand, if it exceeds 500°C, the formation of polygonal ferrite phase cannot be suppressed, and in steel containing Cu, Cu may be locally precipitated in the ferrite phase to reduce the effect of improving fatigue properties. The coiling temperature is set to be below 500°C. Therefore, by coiling at 500° C. or lower, carbonitrides are precipitated in the subsequent cooling process, the amount of solid solution C and N in the ferrite phase is reduced, and the stretch flangeability is improved.

Example

Next, examples of the present invention will be described together with comparative examples.

The slabs with the chemical compositions shown in Table 1 were hot-rolled under the conditions shown in Table 2 to obtain hot-rolled sheets with a thickness of 3.2 mm.

Table 2

condition Heating temperature (℃) Finish rolling finish temperature (°C) Cooling rate after finish rolling (°C/s) Coiling temperature (℃) A 1250 845 75 450 B 1200 825 45 450

In this Table 1, steel numbers (hereinafter referred to as steel numbers) 1, 3, 5, 7, 9, 11, and 13 have compositions within the range of the high-strength steel sheet of the present invention, and steel numbers 2, 4, and 6 , 8, 10, 12, and 14 were configured as comparative steels outside the range of the high-strength steel sheet of the present invention. Steel Nos. 2, 4, and 6 are composed of slabs containing more than 0.01% of acid-soluble Al. In addition, Steel Nos. 8, 10, 12, and 14 reduce the total amount of one or both of Ce and La To less than 0.0005% until the slab to constitute.

That is, in this Table 1, in order to make comparisons between 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, to mutually Almost the same composition to constitute, and make the acid-soluble Al and so on different from each other. In addition, in order to allow comparison between Steel No. 9 and Steel No. 10, Steel No. 11 and Steel No. 12, and Steel No. 13 and Steel No. 14, they are composed of almost the same composition, and Ce+La etc. Are not the same.

In Table 2, as condition A, the heating temperature is 1250°C, the finish rolling temperature is 845°C, the cooling rate after finish rolling is 75°C/sec, and the coiling temperature is 450°C, and as condition B, the heating temperature is 1200°C, finish rolling temperature of 825°C, cooling rate after finish rolling of 45°C/sec, and coiling temperature of 450°C.

Steel No. 1 and No. 2 adopt condition A, steel No. 3 and steel No. 4 adopt condition B, steel No. 5 and steel No. 6 adopt condition A, steel No. 7 and steel No. 8, steel No. 9 and steel No. 10, steel No. Steel No. 11 and Steel No. 12, and Steel No. 13 and Steel No. 14 used Condition B, thereby enabling comparison of the effects of chemical compositions under the same manufacturing conditions.

The basic properties of the steel sheets thus obtained, ie, strength, ductility, tensile flangeability, and fatigue limit ratio, were analyzed.

In addition, as the existence state of elongated inclusions in the steel plate, the number ratio, volume number density, and average equivalent circle diameter of inclusions with an elongation ratio of 5 or more were analyzed for inclusions with an elongation ratio of 5 or more. .

In addition, as the existence state of the unstretched inclusions in the steel sheet, the inclusions are all 1 μm or more, and MnS is precipitated on the oxide or sulfur oxide formed of one or both of Ce or La. The number ratio and volume number density of the inclusions and the average value of the total content of one or both of Ce or La in the inclusions with an extension ratio of 3 or less were analyzed.

In addition, inclusions of 1 μm or more are targeted not only because they are easy to observe, but also because inclusions of less than 1 μm do not affect the decrease in tensile flangeability and fatigue properties.

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

The strength and rolling were determined by a tensile test on a JIS No. 5 test piece taken parallel to the rolling direction. Tensile flangeability is evaluated by λ, which is measured by expanding a punch hole with a diameter of 10 mm in the center opening of a 150 mm x 150 mm steel plate using a 60° conical punch, and measuring the hole diameter D when cracks through the plate thickness occur. (mm), calculated according to the hole expansion value λ=(D-10)/10. In addition, the fatigue limit ratio used as an indicator of fatigue characteristics is the value (σW/σB) obtained by dividing the 2×10 ⁶ time strength (σW) obtained by the method of JIS Z 2275 by the strength of the steel plate (σB) to evaluate.

In addition, the test piece is the No. 1 test piece specified in the same specification, and a test piece with a thickness of 3.0 mm after the parallel portion is 25 mm, the radius of curvature R is 100 mm, and both sides of the original plate (hot-rolled plate) are equally ground.

In addition, the inclusions were observed by SEM, and the major and minor axes of 50 randomly selected inclusions having an equivalent circle diameter of 1 μm or more were measured. In addition, using the quantitative analysis function of SEM, composition analysis was performed on 50 randomly selected inclusions with an equivalent circle diameter of 1 μm or more. Using these results, the ratio of the number of inclusions with an elongation ratio of 5 or more, the average equivalent circle diameter of inclusions with an elongation ratio of 5 or more, and the ratio of the number of inclusions formed of one or both of Ce or La were determined. The ratio of the number of inclusions with MnS precipitated on sulfur oxides and the average value of the total of one or both of Ce and La in inclusions with an extension ratio of 3 or less. In addition, the SEM evaluation of the electrolytic surface was performed by the velocity method, and the volume number density of each form of inclusions was calculated.

As shown in Table 3, steel grades 1, 3, 5, 7, 9, 11, and 13 using the method of the present invention are precipitated on oxides or sulfur oxides formed by one or two of Ce or La. MnS can reduce the extended MnS inclusions in the steel plate. That is, by making the ratio of the number of inclusions in which MnS precipitates on oxides or sulfur oxides formed of one or both of Ce or La in the steel sheet to be 10% or more, the volume number of the inclusions The density is 1.0×10 ³ /mm ³ or more, and the elongation ratio in the steel plate is 3 or less. The average content of one or both of Ce and La is 0.5% to 50%. The number ratio of elongated inclusions with an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more is 20% or less, the volume number density of the inclusions is 1.0×10 ⁴ pieces/mm ³ or less, and the average equivalent weight of the inclusions The circle diameter is 10 μm or less. As a result, it was shown that the steel numbers 1, 3, 5, 7, 9, 11, and 13, which are the steels of the present invention, can provide steel sheets having excellent tensile flangeability and fatigue properties compared with the comparative steels. However, due to the extended MnS inclusions and the oxide or oxysulfide formed by one or both of Ce or La The distribution state of the inclusions containing precipitated MnS is different from the distribution state specified in the present invention, so the MnS-type inclusions elongated during steel plate processing become the origin of cracking, and the tensile flangeability and fatigue properties are reduced.

According to the present invention, the tensile flangeability and fatigue properties can be obtained by precipitating fine MnS in the slab and dispersing it in the steel sheet as fine spherical inclusions that are not deformed during rolling and are less likely to be cracking origins. Excellent high strength hot rolled steel plate.

Claims (8)

1, the high tensile steel plate of a kind of stretch flange and excellent in fatigue characteristics, it is characterized in that, this steel plate in quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made of iron and unavoidable impurities; The number ratio of extending inclusion is below 20%, and described extension inclusion is that the diameter of equivalent circle that exists in this steel plate is the inclusion more than the 1 μ m, and major diameter/minor axis is more than 5.

2, the high tensile steel plate of a kind of stretch flange and excellent in fatigue characteristics, it is characterized in that, this steel plate in quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made of iron and unavoidable impurities; In this steel plate, contain and separating out the inclusion that MnS is arranged by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide more than 10% with the number proportional meter.

3, the high tensile steel plate of a kind of stretch flange and excellent in fatigue characteristics, it is characterized in that, this steel plate in quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made of iron and unavoidable impurities; A volume number density of extending inclusion is 1.0 * 10 ⁴Individual/mm ³Below, described extension inclusion is that the diameter of equivalent circle that exists in this steel plate is the inclusion more than the 1 μ m, and major diameter/minor axis is more than 5.

4, the high tensile steel plate of a kind of stretch flange and excellent in fatigue characteristics, it is characterized in that, this steel plate in quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made of iron and unavoidable impurities; In this steel plate, be 1.0 * 10 by a volume number density of separating out the inclusion of MnS on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide ³Individual/mm ³More than.

5, the high tensile steel plate of a kind of stretch flange and excellent in fatigue characteristics, it is characterized in that, this steel plate in quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made of iron and unavoidable impurities; The average equivalent circular diameter that extends inclusion is below the 10 μ m, and described extension inclusion is that the diameter of equivalent circle that exists in this steel plate is the inclusion more than the 1 μ m, and major diameter/minor axis is more than 5.

6, the high tensile steel plate of a kind of stretch flange and excellent in fatigue characteristics, it is characterized in that, this steel plate in quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made of iron and unavoidable impurities; In this steel plate, exist in by on a kind among Ce or the La or 2 kinds of oxide compounds that form or the oxysulfide and separate out the inclusion that MnS is arranged, in this inclusion, contain the Ce of 0.5～50 quality % or a kind or 2 kinds total among the La in average composition.

7, the high tensile steel plate of a kind of stretch flange and excellent in fatigue characteristics, it is characterized in that, this steel plate in quality % contain C:0.03～0.20%, Si:0.08～1.5%, Mn:1.0～3.0%, below the P:0.05%, more than the S:0.0005%, N:0.0005～0.01%, below the sour solvable Al:0.01%, sour solvable Ti: be lower than 0.008%, a kind or 2 kinds total among Ce or the La: 0.0005～0.04%, remainder is made of iron and unavoidable impurities; (Ce+La)/the S ratio is 0.1～70.

8, according to the high tensile steel plate of each described stretch flange and excellent in fatigue characteristics in the claim 1～7, it is characterized in that, in quality % contain in Nb:0.01～0.10%, V:0.01～0.05%, Cr:0.01～0.6%, Mo:0.01～0.4%, B:0.0003～0.03% any or more than 2 kinds, remainder is made of iron and unavoidable impurities.

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