TWI424069B - A high-strength hot rolled steel sheet having stretch-flangeability and fatigue resistance - Google Patents

Description

High-strength steel sheet excellent in stretch flangeability and fatigue resistance Technical field

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

Background technique

From the viewpoint of improving the safety of automobiles and raising the fuel cost related to environmental protection, the demand for high strength and light weight of hot-rolled steel sheets for automobiles is increasing. In the automotive parts, in particular, the weight of the frame or the crossbar, which is called the chassis, is relatively high in the weight of the entire vehicle body. Therefore, the material used in the above-mentioned portion can be made thinner and thinner. The aforementioned parts are made lighter. Further, from the viewpoint of durability against running vibration, the material used in the above-described undercarriage system must have high fatigue resistance.

However, the ring forgeability and the ductility deteriorate with the increase in strength and fatigue resistance. For high-strength steel sheets suitable for a complex automobile chassis system, it is an important research topic to improve the ring forgeability. .

Therefore, several steel sheets have been proposed for the purpose of achieving both mechanical strength properties, fatigue resistance, and ring forgeability (processability). For example, Japanese Laid-Open Patent Publication No. Hei 11-199973 proposes a steel sheet which is a steel sheet in which a fine Cu precipitate or a solid solution is dispersed in a composite structure steel sheet of a ferrite iron phase and a granulated iron phase (generally referred to as DP). Steel plate). In the conventional technique disclosed in Japanese Laid-Open Patent Publication No. H11-199973, it is found that Cu precipitates having a particle size of 2 nm or less which are solid-solidified by Cu or Cu alone are very effective for improving fatigue resistance and are not destructive to workability, and The composition ratio of the various components is limited.

Although the DP steel sheet is excellent in strength and ductility balance or fatigue resistance, the stretch flangeability evaluated by the ring forging test is still poor. One of the reasons is that the DP steel sheet is a composite of a soft ferrite phase and a hard hemp iron phase. Therefore, in the ring forging process, the boundary portions of the two phases cannot be deformed and easily become the starting point of the fracture.

Compared with the above steel plate, a high-strength hot-rolled steel plate is proposed, which not only satisfies the fatigue resistance, but also satisfies the severe tensile flangeability requirements required for the material of the recent wheel or chassis member (for example, reference Opened the Gazette 2001-200331). The main idea of the technique disclosed in Japanese Laid-Open Patent Publication No. 2001-200331 is to make the main phase a tough steel structure by minimizing carbonization, and to contain a solid volume iron structure of a solid volume strengthening or precipitation strengthening at an appropriate volume ratio, and to reduce the aforementioned The hardness of the ferrite and tough steel is poor to avoid the formation of coarse carbides.

Invention

The high-strength hot-rolled steel sheet which can suppress the generation of coarse carbides by the steel sheet structure main body as a tough steel phase as disclosed in the above-mentioned JP-A-2001-200331, although it can exhibit excellent stretch flangeability, its resistance Fatigue is not necessarily superior to DP steel containing Cu. Further, it is only possible to suppress the generation of coarse carbides and prevent cracking from occurring during the ring forging process. According to the study by the present inventors, it is known that the above-mentioned reason is due to the presence of sulfide-based inclusions extending mainly from MnS in the steel sheet, and when subjected to deformation, the coarse MnS inclusions which are present in the vicinity of the surface layer or the surface layer are present. Internal defects are generated around the object. In addition to the deterioration of fatigue resistance due to crack propagation, the extended coarse MnS inclusions are also likely to be the starting point for cracking during ring forging. Therefore, it is preferable to prevent the MnS inclusions in the steel from being elongated as much as possible, and it is preferable to use fine spheroidization.

However, Mn is an element which contributes to the strength of the material like C or Si. Generally, in order to secure the strength of the high-strength steel sheet, the Mn concentration is set to be high, and if it is not removed in the secondary refining step, S For heavy treatment, the S concentration will also be as high as 50 ppm or more. Therefore, MnS is usually present in the cast piece. When the cast piece is subjected to hot rolling and cold rolling, MnS is easily deformed to become an extended MnS-based inclusion, which is a cause of deterioration of fatigue resistance and stretch flangeability (ring forging workability). However, an example of a hot-rolled steel sheet excellent in stretch flangeability and fatigue resistance has not been seen from the viewpoint of precipitation of MnS and deformation control.

Accordingly, the present invention has been made in view of the above problems, and an object thereof is to provide a high-strength steel sheet excellent in stretch flangeability and fatigue resistance, which is obtained by depositing fine MnS in a cast piece. The fine spherical inclusions which are not deformed by the pressing time and are not likely to be the starting point of the cracks are dispersed in the steel sheet, and the stretch flangeability and the fatigue resistance can be improved.

In order to solve the above problems, the present inventors mainly disperse fine MnS in a cast piece, and a method in which fine spherical inclusions which are not easily deformed by pressure and which are not likely to be a starting point of cracks are dispersed in the steel sheet, and The added elements that deteriorate fatigue resistance are being researched unremittingly. As a result, it was found that MnS precipitated on the fine and hard Ce oxide, La oxide, yttrium oxysulfide, and yttrium oxysulfide which are generated by the deoxidation of Ce and La, and the MnS precipitated in the above-mentioned pressure is not easily deformed. The coarse MnS extending in the steel sheet can be remarkably reduced, and the MnS-based inclusions are less likely to be the starting point of the crack or the path of the crack propagation during the reverse deformation or the ring forging process, so that the fatigue resistance can be improved as described above.

The gist of the high-strength steel sheet excellent in stretch flangeability and fatigue resistance of the present invention is as follows.

(1) A high-strength steel sheet excellent in stretch flangeability and fatigue resistance, in terms of mass%, 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 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and Ce or La in one or two totals : 0.0005 to 0.04%, and the remaining portion is a steel sheet composed of iron and unavoidable impurities, and an extension having a long diameter/minor diameter of 5 or more in inclusions having an equivalent area of 1 μm or more in a projected area of the steel sheet. The ratio of the number of inclusions is 20% or less.

(2) A high-strength steel sheet excellent in stretch flangeability and fatigue resistance, in terms of % by mass, C: 0.03 to 0.20%; Si: 0.08 to 1.5%; Mn: 1.0 to 3.0%; P: 0.05% or less; S: 0.0005% or more; N: 0.0005 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and Ce or La in one or two totals : 0.0005 to 0.04%, and the remaining portion is a steel sheet composed of iron and unavoidable impurities, and the steel sheet contains an oxide having one or two kinds of Ce or La in a ratio of 10% or more. The inclusions of MnS are precipitated on the oxysulfide.

(3) A high-strength steel sheet excellent in stretch flangeability and fatigue resistance, in terms of mass%, 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 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and Ce or La in one or two totals : 0.0005 to 0.04%, and the remaining portion is a steel sheet composed of iron and unavoidable impurities, and an extension having a long diameter/minor diameter of 5 or more in inclusions having an equivalent area of 1 μm or more in a projected area of the steel sheet. The number density of the inclusions is 1.0 × 104 / mm 3 or less.

(4) A high-strength steel sheet excellent in stretch flangeability and fatigue resistance, in terms of % by mass, C: 0.03 to 0.20%; Si: 0.08 to 1.5%; Mn: 1.0 to 3.0%; P: 0.05% or less; S: 0.0005% or more; N: 0.0005 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and Ce or La in one or two totals : 0.0005 to 0.04%, and the remaining portion is a steel sheet composed of iron and unavoidable impurities, and in the steel sheet, MnS is precipitated on an oxide or oxysulfide composed of one or two of Ce or La. The number density of the inclusions is 1.0 × 103 / mm 3 or more.

(5) A high-strength steel sheet excellent in stretch flangeability and fatigue resistance, in terms of % by mass, C: 0.03 to 0.20%; Si: 0.08 to 1.5%; Mn: 1.0 to 3.0%; P: 0.05% or less; S: 0.0005% or more; N: 0.0005 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and Ce or La in one or two totals : 0.0005 to 0.04%, and the remaining portion is a steel sheet composed of iron and unavoidable impurities, and an extension having a long diameter/short diameter of 5 or more in an inclusion having an equivalent area of 1 μm or more in a projected area of the steel sheet. The average projected area equivalent diameter of the inclusions is 10 μm or less.

(6) A high-strength steel sheet excellent in stretch flangeability and fatigue resistance, in terms of % by mass, 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 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and Ce or La in one or two totals : 0.0005 to 0.04%, and the remaining portion is a steel sheet composed of iron and unavoidable impurities, and the steel sheet is precipitated on an oxide or oxysulfide composed of one or two types of Ce or La. Inclusions of MnS, and the inclusions contain one or two types of Ce or La having an average composition of 0.5 to 50% by mass in total.

(7) A high-strength steel sheet excellent in stretch flangeability and fatigue resistance, in terms of % by mass, 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 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and Ce or La in one or two totals : 0.0005 to 0.04%, and the remaining portion is made of iron and inevitable impurities, and the (Ce+La)/S ratio is 0.1 to 70.

(8) A high-strength steel sheet excellent in stretch flangeability and fatigue resistance according to any one of claims 1 to 7, which is, by mass%, contains: Nb: 0.01 to 0.10%; V: 0.01 ~0.05%; Cr: 0.01~0.6%; Mo: 0.01~0.4%; and B: 0.0003~0.03% of either or both, and the remaining part is composed of iron and inevitable impurities.

Simple illustration

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

The best form of implementing the invention

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

First, an experiment up to the completion of the present invention will be described.

The present inventors used various elements for the 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 being Fe. Oxygen, while making steel blocks. The obtained steel block was hot rolled to prepare a hot rolled steel sheet of 3 mm. The hot-rolled steel sheets produced according to the above steps were subjected to a ring forging test and a fatigue test, and the number density, morphology, and average composition of the inclusions in the steel sheets were investigated.

As a result, it was found that it is almost impossible to deoxidize with Al, and after adding Si, at least Ce and La are added to deoxidize the steel sheet to have the most excellent stretch flangeability and fatigue resistance. The reason for this is that MnS precipitates on the fine and hard Ce oxide, La oxide, yttrium oxysulfide, or yttrium oxysulfide produced by deoxidation by adding Ce and La, even if the MnS precipitated as described above is delayed. It is also less likely to cause deformation, so the coarse MnS extending in the steel sheet can be significantly reduced. As a result, in the case of the reverse deformation or the ring forging process, the MnS-based inclusions are less likely to be the starting point of the fracture or the path of the crack propagation, and contribute to the above-mentioned effects of improving the fatigue resistance.

Further, Ce oxides, La oxides, cerium oxysulfide and lanthanum oxysulfide can be miniaturized reason is due Ce added later, SiO La will initially arising from _{2-deoxy-Si-based} inclusions formed by reductive decomposition Fine Ce oxide, La oxide, antimony oxysulfide and antimony oxysulfide. In addition, the Ce oxide, La oxide, antimony oxysulfide and antimony oxysulfide itself have lower interfacial energy with molten steel, so The aggregate after generation can be suppressed.

Based on the knowledge obtained from the above experimental studies, the inventors of the present invention conducted studies on the chemical composition conditions of steel sheets as described below, and completed the present invention.

Hereinafter, the reason for limiting the chemical components of the present invention will be described.

C: 0.03~0.20%

The C system controls the quenching and strength of steel as the most basic element, which can improve the hardness and depth of the quench hardened layer, and help to improve the fatigue strength. That is, the C system is an essential element for securing the strength of the steel sheet, and at least 0.03% is required in order to obtain a high-strength steel sheet. However, if too much C, even if C is fixed by the production of Ti carbide as in the prior art, and the cooling condition is driven, the ferritic carbon-iron phase is still generated. The aforementioned ferritic carbon-iron phase causes work hardening of the steel sheet and is not suitable for improving the tensile flange characteristics. Therefore, in the present invention, the C concentration is set to 0.20% or less from the viewpoint of improving workability.

Si: 0.08~1.5%

Since Si is a main deoxidizing element in a molten steel in which Al or Ti is not added as much as possible in the present invention, it is extremely important in the present invention. Further, Si can increase the number of nucleation sites of the Worthite iron during quenching heating, suppress the grain growth of the Worthite iron, and have a function of making the particle size of the quench hardened layer fine. The above Si suppresses the formation of carbides and suppresses the decrease in grain boundary strength due to carbides. Further, the aforementioned Si also contributes to the generation of the tough steel structure, and serves as an important role from the viewpoint of ensuring the strength of the entire material. In order to lower the dissolved oxygen concentration in the molten steel, SiO ₂ -based inclusions are temporarily generated (the SiO ₂ -based inclusions are reduced by the later addition of Ce and La to make the inclusions fine), and it is necessary to add 0.08% or more. Si. Therefore, in the present invention, the lower limit of Si is set to 0.08%. On the other hand, if the concentration of Si is too high, the concentration of SiO ₂ in the inclusions becomes high, and large inclusions are likely to be generated, and toughness and ductility are extremely poor, so that surface decarburization or surface flaws are increased, so The fatigue resistance is deteriorated. In addition, excessive addition of Si also adversely affects weldability or ductility. Therefore, in the present invention, the upper limit of Si is set to 1.5%.

Mn: 1.0~3.0%

Mn is a useful element for deoxidation at the steelmaking stage, and is an effective element that contributes to the strength of the steel sheet, like C and Si. In order to obtain the above effects, it is necessary to contain 1.0% or more of Mn. However, when Mn is contained in excess of 3.0%, the ductility is lowered due to segregation of Mn or increase in solid solution strengthening. Moreover, since the meltability and the base material toughness also deteriorate, the upper limit of the Mn is set to 3.0%.

P: 0.05% or less

P is an effective element when it is a substitutional solid solution strengthening element which is smaller than the Fe atom. However, it is segregated in the grain boundary of the Worthite iron, and the grain boundary strength is deteriorated, and the torsional fatigue strength is changed. If it is low, the workability may be impaired, so the content is set to 0.05% or less. Moreover, if there is no need for solid solution strengthening, it is not necessary to add P, so that the lower limit of P includes 0%.

S: 0.0005% or more

Since S segregates into impurities, and S forms coarse extended inclusions of MnS and deteriorates the stretch flangeability, it is preferable to keep the concentration as low as possible. Heretofore, in order to secure the stretch flangeability, it is necessary to carry out extremely low vulcanization so that the concentration of S is less than 0.0005%. However, since the present invention precipitates MnS on fine and hard Ce oxide, La oxide, yttrium oxysulfide, and yttrium oxysulfide, it is not easily deformed at the time of pressing, and the extension of inclusions can be prevented, so that S is not particularly specified. The upper limit of the concentration.

Further, in order to reduce the S concentration to less than about 0.0005%, it is necessary to sufficiently enhance the desulfurization treatment in the secondary refining, and the desulfurization treatment for achieving the above concentration is too expensive, and it is difficult to exhibit the effect of controlling the morphology of MnS. Therefore, 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 nitrogen in the air during molten steel processing. N forms a nitride with Al, Ti, or the like to promote fine graining of the base material structure. However, when N is excessively added, even a small amount of Al or a trace amount of Ti causes coarse precipitates, 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, since the N concentration is less than 0.0005%, the cost is increased, so 0.0005% is the lower limit.

Soluble in acid Al: 0.01% or less

Since the oxide of Al which is soluble in acid is easily crystallized and coarsened, the stretch flangeability or the fatigue resistance is deteriorated, so it should be suppressed as much as possible. However, as a preliminary oxygen-removing material, Al can be used up to 0.01%, because when the concentration of Al which is soluble in acid is more than 0.01%, the Al ₂ O ₃ content in the inclusions is more than 50%, resulting in The reason for the crystallization of inclusions. From the viewpoint of preventing crystallization, the concentration of Al which is soluble in acid is preferably the lower, and the lower limit is 0%. Further, the Al-soluble acid concentration is an analytical method in which the concentration of Al dissolved in an acid is measured, and the solubility of Al is soluble in an acid and the solubility of Al ₂ O ₃ in an acid is utilized. Here, the acid means a mixed acid in which, for example, a ratio (mass ratio) of hydrochloric acid 1, nitric acid 1, and water 2 is mixed. By using the above-mentioned acid, Al which is soluble in acid and Al ₂ O ₃ which is insoluble in acid can be separately obtained, and the concentration of Al which is soluble in acid can be measured.

Soluble in acid Ti: less than 0.008%

The oxide of Ti which is soluble in acid is also easily crystallized and coarse, and since it is easy to combine with N in the steel to produce coarse TiN inclusions, the Ti which is soluble in acid is set to be less than 0.008%, and the lower limit value is included. 0%. Further, the concentration of Ti which is soluble in acid is a method for measuring the concentration of Ti dissolved in an acid, and is an analytical method in which dissolved Ti is dissolved in an acid and the Ti oxide is not dissolved in an acid. Here, the acid means a mixed acid which is mixed, for example, in a ratio (mass ratio) of hydrochloric acid 1, nitric acid 1, and water 2. By using the above-mentioned acid, Ti which is soluble in acid and Ti oxide which is insoluble in acid can be separately obtained, and the Ti concentration which is soluble in acid can be measured.

1 or 2 of Ce or La total: 0.0005~0.04%

Ce and La can reduce SiO ₂ produced by deoxidation of Si, and easily become a precipitation position of MnS, and these effects can form a Ce oxide (for example, Ce ₂ O _{3 )} which is hard, fine, and hard to be deformed by a pressure delay. , CeO ₂ ), bismuth oxysulfide (eg Ce ₂ O ₂ S), La oxide (eg La ₂ O ₃ , LaO ₂ ), bismuth oxysulfide (eg La ₂ O ₂ S), Ce oxide-La oxide Or inclusions of yttrium oxysulfide-sulfur sulphide as the main phase (targeting 50% or more).

Here, in the above inclusions, although due to part-deoxy conditions comprising MnO, SiO ₂ or Al ₂ O _3, but similar to the above-described master oxide, can be sufficiently precipitated as MnS position, and without prejudice to inclusions The subtle and hard effect of the object. In order to obtain the inclusions, the total concentration of one or two of Ce or La must be 0.0005% or more and 0.04% or less. When the total concentration of one or two of Ce or La is less than 0.0005%, the SiO ₂ inclusions cannot be reduced, and if it is more than 0.04%, yttrium oxysulfide or yttrium oxysulfide is generated in a large amount to form coarse inclusions and stretch. Flangeability or fatigue resistance is deteriorated.

Nb: 0.01~0.10%

Nb promotes the formation of carbides, nitrides, and carbonitrides by C or N, and promotes the fine granulation of the base material. In order to achieve this effect, at least 0.01% must be present. However, when the content is more than 0.10%, the effect is saturated and the cost is also increased, so the upper limit is 0.10%.

V: 0.01~0.05%

V promotes the formation of carbides, nitrides, and carbonitrides by C or N, and promotes the fine granulation of the base material structure. In order to achieve this effect, at least 0.01% must be present. However, when the content is more than 0.05%, the effect is saturated and the cost is also increased, so the upper limit is 0.05%.

Cr: 0.01~0.6%

In order to improve the hardenability of steel and ensure the strength of the steel sheet, Cr may be contained as needed, and in order to obtain this effect at least Need 0.01%. However, if the content is too large, the balance of strength-ductility is deteriorated, so the upper limit is 0.6%.

Mo: 0.01~0.4%

In order to improve the hardenability of the steel and ensure the strength of the steel sheet, Mo may be contained as needed, and at least 0.01% is required in order to obtain this effect. However, the inclusion of a large amount adversely affects the balance of strength-ductility, so the upper limit is 0.4%.

B: 0.0003~0.003%

In order to improve the hardenability of steel, strengthen the grain boundary and improve the workability, B may be contained as needed, and at least 0.0003% is required in order to obtain this effect. However, excessive inclusion may adversely affect the cleanliness of the steel and deteriorate the ductility. Therefore, the upper limit is 0.003%.

Next, the conditions for the presence of inclusions in the steel sheet of the present invention will be described. Further, the steel sheet refers to a flat plate after rolling which is obtained by hot rolling or cold rolling.

In order to obtain a steel sheet excellent in stretch flangeability and fatigue resistance, it is very important to reduce coarse MnS-based inclusions in the steel sheet which are likely to be a fracture origin or a fracture propagation path as much as possible. The inventors have found through experiments that MnS-based inclusions having a projected area equivalent diameter of less than 1 μm have no adverse effect as a starting point of fracture, and do not deteriorate the stretch flangeability or fatigue resistance, and, due to the projected area equivalent diameter The inclusions of 1 μm or more are also easily observed by a scanning electron microscope (SEM). Therefore, the inclusions having an equivalent area of 1 μm or more on the steel sheet are investigated, and the morphology and composition of the particles are investigated, and the MnS inclusions are evaluated. The distribution state of the object. Here, the projected area equivalent diameter means the long diameter and the short diameter of the inclusion observed from the cross section, and is obtained by (long diameter × short diameter) ^0.5 .

Further, the upper limit of the equivalent area diameter of the projected area of the MnS-based inclusions is not particularly limited, but actually, MnS-based inclusions of about 1 mm are observed.

The ratio of the number of the extended inclusions was analyzed by SEM to analyze the composition of a plurality of (for example, about 50) inclusions having a projected area equivalent diameter of 1 μm or more, and the long diameter and the short diameter of the inclusions were measured from the SEM image. Here, when the definition of the extended inclusion is set to an inclusion having a long diameter/minor diameter (elongation ratio) of 5 or more, the number of the detected inclusions is divided by the total number of inclusions investigated. The number (in the above example, about 50) can be used to determine the ratio of the number of the extended inclusions.

In addition, the reason why the ratio of the inclusions is 5 or more is that the inclusions having a ratio of elongation of 5 or more in the comparative steel sheet to which Ce and La are not added are almost all MnS-based inclusions. Further, although the upper limit of the elongation ratio of the MnS-based inclusions is not particularly specified, an MnS-based inclusion having an elongation ratio of about 50 is actually observed.

As a result, it has been found that a steel sheet having a shape ratio controlled to have an elongation ratio of 5 or more and an elongation ratio of 20% or less is preferable in terms of stretch flangeability and fatigue resistance. In other words, when the ratio of the number of the extended inclusions of the elongation ratio of 5 or more exceeds 20%, the number of the MnS-based extended inclusions which are likely to be the starting point of the fracture is excessively large, and the stretch flangeability and the fatigue resistance are lowered. Therefore, in the present invention, the ratio of the number of the extended inclusions having an elongation ratio of 5 or more is set to 20% or less. In addition, the smaller the stretched MnS-based inclusions, the better the stretch flangeability or the fatigue resistance. Therefore, the lower limit of the ratio of the number of extended inclusions having the elongation ratio of 5 or more includes 0%.

Here, the lower limit value of the number of the extended inclusions having the projection area equivalent diameter of 1 μm or more and the elongation ratio of 5 or more is 0%, which means that there may be inclusions having a projected area equivalent diameter of 1 μm or more, but not There are cases in which the elongation ratio is 5 or more, or even if there are extended inclusions having an elongation ratio of 5 or more, the projected area equivalent diameter is all less than 1 μm.

In addition, in the steel sheet in which the ratio of the number of the extended inclusions of the elongation ratio of 5 or more is 20% or less, an oxide or oxysulfide composed of one or two types of Ce or La is formed. The morphology of MnS precipitated on the object. The form of the inclusions is not limited to the precipitation of MnS by an oxide or oxysulfide composed of one or two types of Ce or La, but most of them are one type of Ce or La or The oxide or oxysulfide composed of the two types is a nucleus, and MnS is precipitated around the oxide.

Further, inclusions in which MnS is precipitated in one or two types of oxides or oxysulfides composed of Ce or La are not easily deformed during rolling, and therefore have a non-extending shape in the steel sheet, that is, almost Spheroidal inclusions.

Here, the spherical inclusions that are determined to be non-extended are not particularly limited, but generally refer to inclusions having a ratio of 3 or less in the steel sheet, and more preferably 2 or less. In the case of the slab stage before rolling, the inclusions in the form of MnS precipitated from an oxide or oxysulfide composed of one or two types of Ce or La have an elongation ratio of 3 or less. Further, when it is judged that the spherical inclusions which are not extended are completely spherical, the elongation ratio is 1, so the lower limit of the elongation ratio is 1.

The investigation of the ratio of the number of inclusions was carried out in the same manner as the ratio of the number of extended inclusions. As a result, precipitation control is performed to obtain a steel sheet in which the ratio of the number of inclusions in the form of MnS precipitated from the oxide or oxysulfide of one or two types of Ce or La is 10% or more. Flangeability and fatigue resistance are preferred. When the ratio of the number of inclusions in the form of MnS precipitated on an oxide or oxysulfide composed of one or two types of Ce or La is less than 10%, the number of MnS-extended inclusions corresponds to this. If the ratio is too large, the stretch flangeability and fatigue resistance are deteriorated. Therefore, the ratio of the number of inclusions in the form of MnS precipitated on the oxide or oxysulfide composed of one or two types of Ce or La is 10% or more. Further, the stretch flangeability or the fatigue resistance is preferably good in the case where a large amount of MnS is precipitated on an oxide or oxysulfide composed of one or two types of Ce or La. Therefore, the upper limit of the number ratio includes 100%.

In addition, inclusions in the form of MnS precipitated on an oxide or oxysulfide composed of one or two types of Ce or La are not easily deformed by the pressurization, and thus the projected area equivalent diameter is not particularly specified. 1 μm or more is also acceptable. However, since it is considered to be a starting point of fracture when it is too large, the upper limit is preferably about 50 μm.

On the other hand, the inclusions are not easily deformed even after the pressing, and when the projected area equivalent diameter is less than 1 μm, the origin of the fracture does not become a starting point. Therefore, the lower limit of the equivalent diameter of the projected area is not particularly specified.

Next, the number density of the unit volume of the inclusions is defined as the existence condition of the inclusions in the steel sheet of the present invention.

The particle size distribution of the inclusions was subjected to SEM evaluation of the electrolytic surface by the SPEED method (non-aqueous electrolytic etching method). The SEM evaluation of the electrolytic surface by the SPEED method means that after the surface of the sample piece is polished, electrolysis by the SPEED method is performed, and the size or the number density of the inclusions is evaluated by directly observing the sample surface by SEM. In addition, the SPEED method refers to a method of extracting inclusions by electrolysis of the surface of a sample using 10% acetamidine-1% tetramethylammonium chloride-methanol, and the surface area of the electrolyzed sample is electrolyzed by 1 C per 1 cm ² . The SEM image of the surface after electrolysis as described above was subjected to image processing, and the frequency (number) distribution with respect to the projected area equivalent diameter was obtained. The average projected area equivalent diameter is calculated from the frequency distribution of the particle diameter, and the number density of each volume of the inclusions can be calculated by dividing the frequency by the observed field of view area and the depth obtained from the amount of electrolysis. .

The result of evaluating the number density of inclusions having a projected area equivalent diameter of 1 μm or more and an elongation ratio of 5 or more which is a starting point of the fracture and which deteriorates the stretch flangeability or the fatigue resistance is found to be 1.0 × 10 ⁴ /mm ^{3 .} In the following, the stretch flangeability and fatigue resistance can be improved. When the number of extended inclusions having a projected area equivalent diameter of 1 μm or more and an elongation ratio of 5 or more is more than 1.0×10 ⁴ /mm ³ , the number density of the MnS-based extended inclusions which are likely to be the starting point of the fracture is excessive. Since the stretch flangeability and the fatigue resistance are deteriorated, the volume density of the extended inclusions having a projected area equivalent diameter of 1 μm or more and an elongation ratio of 5 or more is 1.0 × 10 ⁴ /mm ³ or less. Further, since the amount of the MnS-based inclusions is less, and the stretch flangeability or the fatigue resistance is better, the lower limit value of the number-of-volume density of the extended inclusions having a projected area equivalent diameter of 1 μm or more and an elongation ratio of 5 or more includes 0%. .

Here, the meaning of the lower limit value of the volume number density of the extended inclusions having a projected area equivalent diameter of 1 μm or more and an elongation ratio of 5 or more is 0%, which is the same as described above.

Further, the steel sheet having a diameter of 1 μm or more and an elongation of 5 or more and having a volume density of 1.0 × 10 ⁴ /mm ³ or less is an unextended MnS-based inclusion by Ce. Or a form in which MnS is precipitated on an oxide or oxysulfide composed of one or two of La, and its shape is a substantially spherical inclusion.

The form of the inclusions may be the same as described above, and MnS may be deposited on an oxide or oxysulfide composed of one or two types of Ce or La, and there is no particular limitation, but one or two of Ce or La may be used. The oxide or oxysulfide is a core, and MnS is precipitated around it.

Further, although the definition of the spherical inclusions is not particularly defined, it is preferably an inclusion having a stretching ratio of 3 or less in the steel sheet, and more preferably an inclusion of 2 or less. Here, in the case of a completely spherical shape, the elongation ratio is 1, so the lower limit of the elongation ratio is 1.

As a result of investigating the number density of the inclusions, the precipitation control is performed by using an oxide or oxysulfide composed of one or two types of Ce or La as a core and an inclusion of MnS in the periphery thereof. A steel sheet having a volume density of 1.0 × 10 ³ /mm ³ or more was found to have better stretch flangeability and fatigue resistance. When the number density of inclusions in the form of MnS precipitated from an oxide or oxysulfide composed of one or two types of Ce or La is less than 1.0 × 10 ³ /mm ³ , the MnS system corresponds to this. The ratio of the number of the extended inclusions is too large, and the stretch flangeability and the fatigue resistance are lowered. Therefore, the form of MnS precipitated on the oxide or oxysulfide composed of one or two types of Ce or La is specified. The number density of the inclusions must be 1.0 × 10 ³ /mm ³ or more. In addition, since the tensile flangeability or the fatigue strength of a large amount of MnS precipitated by using an oxide or an oxysulfide composed of one or two types of Ce or La as a core is good, the number of the volume is not particularly limited. The upper limit of density.

In addition, the projected area equivalent diameter of the inclusions in which MnS is precipitated in an oxide or oxysulfide composed of one or two types of Ce or La is also the same as described above, and may be 1 μm or more. However, if the projected area equivalent diameter is too large, the starting point of the fracture may occur, so the upper limit is preferably about 50 μm.

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

Next, the existence conditions of the extended inclusions in the steel sheet of the present invention described above are defined by the upper limit of the projected area equivalent diameter. Specifically, the average projected area equivalent diameter of the inclusions having a projected area equivalent diameter of 1 μm or more and an elongation ratio of 5 or more which is the origin of the fracture and which deteriorates the stretch flangeability or the fatigue resistance is evaluated, and it is found that the above-mentioned extended inclusions When the average projected area equivalent diameter of the object is 10 μm or less, the stretch flangeability and the fatigue resistance can be improved. This is to increase the ratio of the number of extended inclusions with an equivalent area diameter of 1 μm or more and an extension ratio of 5 or more, and the average projected area equivalent diameter of the extended inclusions will become larger, thereby specifying the average projection of the extended inclusions. The area equivalent diameter is used as an indicator. We speculate that as the amount of Mn or S in the molten steel increases, the number of MnS produced increases, and the size of the produced MnS also coarsens.

Therefore, when the projected inclusions having an equivalent area diameter of 1 μm or more and an elongation ratio of 5 or more are larger than 10 μm, since the ratio of the number of the extended inclusions is more than 20%, the coarse MnS-based extension which is a starting point of the fracture is likely to occur. When the ratio of the number of inclusions is too large, the stretch flangeability and the fatigue resistance are lowered. Therefore, the average projected area equivalent diameter of the extended inclusions having a projected area equivalent diameter of 1 μm or more and an elongation ratio of 5 or more is set to 10 μm or less.

In addition, the average projected area equivalent diameter of the extended inclusions having a projected area equivalent diameter of 1 μm or more and an elongation ratio of 5 or more is 10 μm or less, which means that the inclusions having a projected area equivalent diameter of 1 μm or more are present in the steel sheet, so the projected area The lower limit of the equivalent diameter is 1 μm.

On the other hand, in the steel sheet of the present invention, inclusions in the form of MnS precipitated on an oxide or oxysulfide composed of one or two types of Ce or La are present, and inclusions of MnS are precipitated. The content of the average composition of Ce or La is specified.

Specifically, as described above, in addition to the improvement of the stretch flangeability and the fatigue resistance, MnS is precipitated by an oxide or oxysulfide composed of one or two types of Ce or La to prevent MnS from extending. important.

In the form of the inclusions, MnS may be deposited on an oxide or oxysulfide composed of one or two types of Ce or La, as described above, and is not particularly specified, but most of them are Ce or La. Or two kinds of oxides or oxysulfides are cores, and MnS is precipitated around the oxides.

In addition, although the spherical inclusions are not particularly limited, it is preferably an inclusion having a ratio of 3 or less in the steel sheet, and more preferably 2 or less. Here, in the case of a completely spherical shape, the elongation ratio is 1, and therefore the lower limit of the elongation ratio is 1.

Therefore, in order to understand the composition which can suppress the extension of the MnS-based inclusions, the composition of the inclusions in the form of MnS precipitated on the oxide or oxysulfide composed of one or two of Ce or La is analyzed.

However, since the projected area equivalent diameter of the inclusions is 1 μm or more, it is easier to observe, and therefore, for the sake of convenience, the projection area equivalent diameter of 1 μm or more is targeted. However, inclusions having an equivalent area diameter of less than 1 μm may be included if observation is possible.

In addition, since the inclusions in the form of MnS precipitated on the oxide or oxysulfide composed of one or two types of Ce or La are not extended, it is confirmed that the inclusions have an elongation ratio of 3 or less. Therefore, the composition analysis was performed on the inclusions having a projected area equivalent diameter of 1 μm or more and an elongation ratio of 3 or less.

As a result, it was found that when the average composition of the inclusions having an equivalent area of 1 μm or more and the elongation ratio of 3 or less and the average composition containing Ce or La is 0.5 to 50%, the stretch flangeability and the fatigue resistance are compared. good. When the average content of one or two of Ce or La in the inclusions having an equivalent area diameter of 1 μm or more and an elongation ratio of 3 or less is less than 0.5% by mass, one or two types of Ce or La are used. The ratio of the number of inclusions in the form of MnS precipitated on the oxide or oxysulfide is greatly reduced. Therefore, the ratio of the number of MnS-based extended inclusions which are likely to be the starting point of the fracture is excessive, and the stretch flangeability is lowered. With fatigue resistance.

On the other hand, when the average content of one or two of Ce or La in the inclusions having an equivalent area diameter of 1 μm or more and an elongation ratio of 3 or less is more than 50%, a large amount of strontium oxysulfide or strontium oxysulfide is generated. On the other hand, since coarse inclusions having a projected area equivalent diameter of about 50 μm or more are formed, the stretch flangeability or the fatigue resistance is deteriorated.

In addition, the existence conditions of inclusions in the form of inclusions of MnS on oxides or oxysulfides composed of one or two types of Ce or La in the steel sheet of the present invention are defined by the chemical composition (Ce+La)/S ratio of the steel sheet. .

Specifically, as described above, in addition to the improvement of the stretch flangeability and the fatigue resistance, the predetermined condition is that MnS is precipitated from an oxide or oxysulfide composed of one or two types of Ce or La, and The chemical composition ratio used to prevent MnS extension.

Therefore, in order to understand the chemical composition ratio which can effectively suppress the extension of MnS-based inclusions, the (Ce+La)/S ratio of the steel sheet was changed, and the morphology, stretch flangeability, and fatigue resistance of the inclusions were evaluated (Fig. 1). As a result, it was found that when the (Ce + La) / S ratio is from 0.1 to 70, the stretch flangeability and the fatigue resistance are better. On the other hand, when the (Ce+La)/S ratio is less than 0.1, the ratio of the number of inclusions in the form of MnS precipitated on the oxide or oxysulfide composed of one or two of Ce or La is greatly reduced. As a result, the number of MnS-based extended inclusions which are likely to be the starting point of the fracture is excessive, and the stretch flangeability and the fatigue resistance are deteriorated.

On the other hand, when the (Ce+La)/S ratio is more than 70, a large amount of yttrium oxysulfide or yttrium oxysulfide is generated, and coarse inclusions having a projected area equivalent diameter of about 50 μm or more are formed, so that stretch flangeability or resistance is caused. Fatigue deteriorates.

Next, the steel sheet structure will be described.

In the present invention, the stretch flangeability and the fatigue resistance are improved by performing MnS-based inclusion control, and the microstructure of the steel sheet is not particularly limited. a steel plate with a structure of a toughened ferrite grain iron phase; a composite microstructure steel plate with a ferrite grain iron phase as the main phase, a Matian iron phase and a tough steel phase as a second phase; and a ferrite iron and a residual Worth A composite structural steel plate composed of Tiantie and a low-temperature metamorphic phase (Mitiya loose iron or tough steel) can obtain the effect of the present invention from any of the above steel plates, but in order to obtain excellent stretch flangeability, the toughened ferrite iron is used. The organization of the main phase is better, and it is more necessary to be the toughest ferrite iron or tough steel phase as the largest phase from the area ratio. The area ratio of the ductile ferrite iron phase in the steel sheet is preferably 50% or more, more preferably 80% or more, and most preferably 100%. Further, the remaining portion may contain a tough steel phase or a polyhedral fat iron phase of 20% or more.

Next, the manufacturing conditions will be described. The invention removes carbon by blowing in a converter, or decarburizes by using a vacuum degassing device, and adds an alloy such as Si, Mn, P, etc. to a molten steel having a C concentration of 0.03 to 0.1% for deoxidation and composition. Adjust, and without adding Al or Ti, or if oxygen must be adjusted, add only a small amount of Al or Ti which is allowed to dissolve in acid Al or soluble in acid Ti, and then add Ce or La 1 The composition was adjusted in two or two types. A cast piece is produced by continuously casting a molten steel obtained by the above.

Regarding continuous casting, it can be applied not only to the continuous casting of a plate with a thickness of 250 mm. For a block or a steel or even a continuous casting machine, the thickness of the casting mold is thinner than usual, for example, a continuous casting of a thin plate of 150 mm or less is also suitable. .

The heat extension conditions for producing a high-strength hot-rolled steel sheet will be described below. In order to solid-solve carbonitrides or the like in the steel, the heating temperature of the flat plate before the heat extension is preferably 1150 ° C or higher. By solidifying such a solution in advance, it is possible to suppress the formation of polyhedral ferrite iron during the cooling process after rolling, and to obtain a structure mainly composed of a toughened ferrite iron phase which contributes to stretch flangeability. On the other hand, when the heating temperature of the flat plate before the heat extension is greater than 1250 ° C, the oxidation of the surface of the flat plate is remarkable, especially because the wedge-shaped surface defects caused by the selective oxidation of the grain boundaries remain after the rust removal. However, it is detrimental to the surface quality after rolling, so the upper limit is preferably 1250 ° C.

After heating in the above temperature range, general hot rolling is performed, but in this step, the precise rolling end temperature is extremely important in performing the structure control of the steel sheet. When the precise rolling end temperature is less than Ar ₃ point + 30 ° C, the crystal grain size of the surface layer portion tends to be coarse, which is not preferable in terms of fatigue resistance. On the other hand, if greater than Ar ₃ point + 200 ℃, it is not good enough in terms of stretch-flangeability, ferrite prone polyhedral phase, the upper limit is suitably Ar ₃ point + 200 ℃.

Moreover, the average cooling rate of the steel plate after precise rolling is 40 ° C / sec or more, and the cooling to 300 ~ 500 ° C range can suppress the generation of the polyhedral ferrite iron phase, and contribute to the iron phase of the toughened fat grain. Organization.

When the above average cooling rate is less than 40 ° C / sec, the polyhedral fat iron phase tends to occur, which is not preferable. On the other hand, although it is not necessary to set an upper limit on the cooling rate in the control of the structure, an excessively fast cooling rate may cause the steel sheet to be unevenly cooled, and an extra cost is required for manufacturing a device capable of rapid cooling, which may result in the price of the steel sheet. The rise. From the above point of view, the upper limit of the cooling rate is preferably 100 ° C / sec.

Further, when the cooling stop temperature is lower than 300 ° C, the tensile flange property is not good and the 麻田散铁 phase is generated, so the lower limit is made 300 ° C. Therefore, in order to suppress the formation of the granulated iron phase which deteriorates the stretch flangeability extremely much, the winding temperature of the hot-rolled steel ring is preferably 300 ° C or more.

On the other hand, if it is more than 500 °C, the iron phase of the polyhedral fat can not be suppressed. Moreover, the steel containing Cu will locally precipitate Cu in the ferrite phase, which may deteriorate the fatigue resistance, so the winding temperature It should be below 500 °C. Therefore, by winding at 500 ° C or lower, carbonitride is precipitated in the subsequent cooling process, and the amount of solid solution C and N in the ferrite-grained iron phase is reduced, and the stretch flangeability can be improved.

Example

Hereinafter, examples and comparative examples of the present invention will be described.

The flat plate of the chemical composition shown in Table 1 was hot rolled under the conditions shown in Table 2 to obtain a hot rolled sheet having a thickness of 3.2 mm.

In the above Table 1, the steel numbers (hereinafter referred to as steel numbers) 1, 3, 5, 7, 9, 11, and 13 are composed of the composition in the range of the high-strength steel sheets of the present invention, and steel numbers 2, 4, and 6 8, 10, 12, and 14 are comparative steels outside the range of the high-strength steel sheets of the present invention. Steel No. 2, 4, and 6 contain flat plates of acid-soluble Al greater than 0.01%, and one or two types of steel No. 8, 10, 12, and 14 series Ce or La are reduced to less than 0.0005%. .

Incidentally, in the foregoing Table 1, steel No. 1 and steel No. 2, steel No. 3 and steel No. 4, steel No. 5 and steel No. 6, steel No. 7 and steel No. 8 are mutually comparable, and are almost identical to each other. It is composed of the same composition, and is soluble in acid and the like. Further, the steel No. 9 and the steel No. 10, the steel No. 11 and the steel No. 12, the steel No. 13 and the steel No. 14 are mutually comparable, and each of them has almost the same composition, and that Ce+La and the like are different from each other.

Further, in the above Table 2, Condition A is a heating temperature of 1,250 ° C, a precise rolling end temperature of 845 ° C, a cooling rate of 75 ° C / sec after precise rolling, a winding temperature of 450 ° C, a condition B heating temperature of 1200 ° C, and precise calendering. The temperature was 825 ° C, the cooling rate after precision rolling was 45 ° C / sec, and the coil heat temperature was 450 ° C.

By using condition A for steel number 1 and steel number 2, and condition B for steel number 3 and steel number 4, condition A for steel number 5 and steel number 6, and steel number 7 and steel number 8, steel No. 9 and steel No. 10, steel No. 11 and steel No. 12, steel No. 13 and steel No. 14 use condition B, and the influence of chemical composition under the same manufacturing conditions can be compared.

Next, the strength, ductility, stretch flangeability, and fatigue limit ratio were investigated, and the basic characteristics of the steel sheet obtained as described above were obtained.

In addition, the inclusion ratio of the inclusions of 1 μm or more was investigated, and the number of the inclusions, the number density of the volume, and the equivalent projected area equivalent diameter of the inclusions having an elongation ratio of 5 or more were investigated as the existence state of the extended inclusions in the steel sheet.

In addition, the number of inclusions and the number density of the inclusions in which MnS is precipitated on the oxide or oxysulfide composed of one or two types of Ce or La, and the inclusions of 1 μm or more are investigated. The average value of the total content of one or two types of Ce or La in the inclusions of the elongation ratio of 3 or less is the existence state of the inclusions which are not extended in the steel sheet.

Further, in the case of inclusions of 1 μm or more, in addition to being easily observed, inclusions of less than 1 μm do not affect stretch flangeability or fatigue resistance.

The results are shown in Table 3 in accordance with the combination of each steel and rolling conditions.

The strength and the rolling were determined by a tensile test of JIS No. 5 test piece taken in parallel with the rolling direction. The stretch flangeability is expanded by piercing a diameter of 10 mm at the center of the 150 mm × 150 mm steel plate with a 60° conical punch, and the hole diameter D (mm) when the thickness of the plate is generated through the crack is measured, and the ring forging value λ = The evaluation of λ obtained by (D-10)/10 was carried out. In addition, the fatigue limit ratio used as an index indicating the fatigue resistance is a value obtained by dividing the 2×10 ⁶ time intensity (σW) obtained by the method according to JIS Z 2275 by the steel plate strength (σB) (σW). /σB) was evaluated.

Further, in the test piece, the test piece No. 1 of the same specification was used, and the parallel portion was 25 mm, the radius of curvature R was 100 mm, and the thickness of the original plate (hot rolled plate) was also ground to a thickness of 3.0 mm.

Further, SEM observation of the inclusions was carried out, and 50 randomly selected inclusions having an equivalent area diameter of 1 μm or more were measured, and the long diameter and the short diameter were measured. Further, using the quantitative analysis function of the SEM, composition analysis was performed on 50 randomly selected inclusions having a projected area equivalent diameter of 1 μm or more. Using the above results, the ratio of the number of inclusions having an elongation ratio of 5 or more, the average projected area equivalent diameter of the inclusions having an elongation ratio of 5 or more, and the oxide or oxygen composed of one or two types of Ce or La are obtained. The ratio of the number of inclusions of MnS precipitated on the sulfide and the average of one or two of Ce or La in the inclusions of the elongation ratio of 3 or less. Further, the SEM evaluation of the electrolytic surface by the SPEED method was used to calculate the volume number density of the different forms of the inclusions.

From Table 3, it can be found that the steel No. 1, 3, 5, 7, 9, 11, 13 using the method of the present invention is composed of an oxide or oxysulfide composed of one or two kinds of Ce or La. Precipitation of MnS reduces the presence of extended MnS inclusions in the steel sheet. In other words, the number of inclusions in which MnS is precipitated on the oxide or oxysulfide composed of one or two types of Ce or La in the steel sheet is 10% or more, and the number density of the inclusions is When the ratio is 1.0×10 ³ /mm ³ or more, the average content of one or two of Ce or La in the inclusions having a stretching ratio of 3 or less in the steel sheet is 0.5% to 50%. The ratio of the number of extended inclusions having a projected area equivalent diameter of 1 μm or more and an elongation ratio of 5 or more is 20% or less, and the number density of the inclusions is 1.0×10 ⁴ /mm ³ or less, and the inclusions are The average projected area equivalent diameter is 10 μm or less. As a result, compared with the comparative steel, the steel grades 1, 3, 5, 7, 9, 11, and 13 of the steel of the present invention can obtain a steel sheet excellent in stretch flangeability and fatigue resistance. However, the comparative steel (steel Nos. 2, 4, 6, 8, 10, 12, 14) is precipitated by the extension of the MnS-based inclusions and the oxide or oxysulfide composed of one or two kinds of Ce or La. Since the distribution state of the inclusions of MnS is different from the distribution state defined by the present invention, the extension of the MnS-based inclusions during the processing of the steel sheet becomes the starting point of the fracture, and the stretch flangeability and the fatigue resistance are lowered.

Industrial use possibility

According to the method of the present invention, by depositing fine MnS in the cast piece, it is more difficult to be a fine spherical inclusion which is difficult to be the starting point of the fracture, and the stretch flangeability and the fatigue resistance can be obtained. High-strength hot-rolled steel sheet with excellent properties.

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

Claims (8)

A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue resistance, 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 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and one or two of Ce and La The total amount is 0.0005 to 0.04%; the remainder is composed of iron and unavoidable impurities; and the inclusions having an equivalent diameter of 1 μm or more in the projected area of the steel sheet have a long diameter/short diameter of 5 or more. The number ratio of the objects is 20% or less. A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue resistance, 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 to 0.01%; Acid soluble Al: 0.01% or less; Acid soluble Ti: less than 0.008%; and Ce or La one or two total: 0.0005 to 0.04%; the remainder is made of iron and inevitable impurities In addition, the steel sheet contains inclusions in which MnS is precipitated in an oxide or oxysulfide composed of one or two of Ce and La, and is contained in an amount of 10% or more. A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue resistance, 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 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and one or two of Ce and La The total length is 0.0005 to 0.04%; the remainder is composed of iron and unavoidable impurities; and the inclusions having an equivalent area diameter of 1 μm or more in the steel sheet have an extension of a long diameter/minor diameter of 5 or more. The number density of the inclusions is 1.0 × 10 ⁴ /mm ³ or less. A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue resistance, 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 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and one or two of Ce and La The total amount is 0.0005 to 0.04%; the remainder is composed of iron and unavoidable impurities; and in the steel sheet, oxides or oxysulfides of one or two of Ce and La are precipitated. The number density of the inclusions of MnS is 1.0 × 10 ³ /mm ³ or more. A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue resistance, 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 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and one or two of Ce and La Total: 0.0005~0.04%; The remaining portion is composed of iron and unavoidable impurities; and the average projected area equivalent of the extended inclusion having a long diameter/minor diameter of 5 or more in inclusions having a projected area equivalent diameter of 1 μm or more in the steel sheet The diameter is 10 μm or less. A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue resistance, 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 to 0.01%; soluble in acid Al: 0.01% or less; soluble in acid Ti: less than 0.008%; and one or two of Ce and La The total amount is 0.0005 to 0.04%; the remainder is composed of iron and unavoidable impurities; and the steel sheet contains precipitates of oxides or oxysulfides composed of one or two of Ce and La. The inclusion of MnS is one or two or more of Ce and La in an amount of from 0.5 to 50% by mass based on the average composition. A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue resistance, 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 to 0.01%; Acid-soluble Al: 0.01% or less; Acid-soluble Ti: less than 0.008%; and Ce or La of 1 or 2 The total amount is 0.0005~0.04%; the remaining part is composed of iron and unavoidable impurities; and the (Ce+La)/S ratio is 0.1~70. The high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue resistance according to any one of claims 1 to 7, which contains, by mass%, V: 0.01 to 0.05%; Cr: 0.01 to 0.6 %; Mo: 0.01 to 0.4%; and B: 0.0003 to 0.03% of either or both, and the remainder consists of iron and unavoidable impurities.