TWI412609B - High strength steel sheet and method for manufacturing the same - Google Patents

Abstract

A high strength steel sheet having excellent workability and a tensile strength (TS) of 980 MPa or more is provided. Regarding composition, on a percent by mass basis, C: 0.17% or more, and 0.73% or less, Si: 3.0% or less, Mn: 0.5% or more, and 3.0% or less, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, and N: 0.010% or less are included while it is satisfied that Si + Al is 0.7% or more, and the remainder includes Fe and incidental impurities, wherein regarding the steel sheet microstructure, it is specified that the area percentage of a total amount of lower bainite and whole martensite is 10% or more, and 90% or less relative to the whole steel sheet microstructure, the amount of retained austenite is 5% or more, and 50% or less, the area percentage of bainitic ferrite in upper bainite is 5% or more relative to the whole steel sheet microstructure, as-quenched martensite is 75% or less of the total amount of lower bainite and whole martensite, the area percentage of polygonal ferrite is 10% or less (including 0%), and the average amount of C in the above-described retained austenite is 0.70% or more.

Description

High-strength steel plate and manufacturing method thereof

The present invention relates to a high-strength steel sheet which is excellent in workability (particularly ductility and stretch flangeability) and has a tensile strength (TS) of 980 MPa or more and is used in an industrial field such as an automobile or an electric machine, and a method for producing the same.

In recent years, from the perspective of global environmental protection, the improvement of fuel efficiency of automobiles has become an important issue. Therefore, the use of the high strength of the vehicle body material to achieve a thin plate, so that the car body itself is moving in a lighter direction.

In general, in order to increase the strength of the steel sheet, it is necessary to increase the ratio of the hard phase such as granulated iron or toughened iron to the entire structure of the steel sheet. However, since the increase in strength of the steel sheet due to the increase in the hardness ratio is caused, the workability is lowered, and development of a steel sheet having high strength and excellent workability is expected. Up to now, various composite structural steel sheets such as ferrite-magazine-dissolved iron two-phase steel (DP steel) and TRIP steel using metamorphic plasticity of residual Worth iron have been developed.

When a hard phase is added to a composite structure steel sheet, the workability of the steel sheet is strongly affected by the hard phase processability. The reason is that when there are many soft polygons with less hard particles, the deformability of the polygonal ferrite iron will dominate the workability of the steel sheet, and even if the hard phase processing is insufficient, the workability such as ductility can be ensured. When there are many harder cases, it is not dominated by the deformation of the polygonal ferrite iron, but the deformability of the hard phase directly affects the formability of the steel sheet. If the workability of the hard phase itself is insufficient, the workability deterioration of the steel sheet tends to obvious.

Therefore, in the case of cold-rolled steel sheet, after heat treatment for adjusting the amount of polygonal ferrite particles generated during annealing and subsequent cooling, the steel sheet is subjected to water quenching to form a granulated iron, and then the steel sheet is heated again. Keeping the high temperature, thereby tempering the granulated iron in the granules, and forming carbides in the granulated iron in the hard phase, can improve the processability of the granulated iron. However, in the quenching and tempering of such a granulated iron, special manufacturing equipment such as a continuous annealing apparatus having a water quenching function is required. Therefore, when ordinary steelmaking equipment which cannot heat up again and maintains high temperature after water quenching of a steel plate is used, although the high strength of the steel plate can be performed, the Matian iron processing property which is a hard phase cannot be improved.

In addition, as a steel sheet which is a hard phase other than the granulated iron, the main phase is defined as a polygonal ferrite iron, and the hard phase is defined as toughened iron or bead iron, and changes in the hard phase are made. A steel plate that forms carbides in ductile iron or bead iron. The steel sheet does not rely on the polygonal ferrite iron to improve the workability, but also forms a carbide in the hard phase, and also improves the workability of the hard phase itself, in particular, a steel sheet having an extended flange property. However, under the premise that the main phase is a polygonal ferrite iron, it is difficult to achieve both high strength and workability in which the tensile strength (TS) is 980 MPa or more. Further, since the workability of the hard phase itself is improved by the formation of carbides in the hard phase, the workability of the polygonal ferrite iron is still poor, so that the tensile strength (TS) is 980 MPa or more. When the strength is increased and the amount of polygonal ferrite is reduced, sufficient workability cannot be obtained.

Patent Document 1 proposes a method of forming a high-tensile steel sheet having excellent bending workability and impact properties by forming a steel composition into fine and uniform toughened iron having residual Worth iron.

Patent Document 2 proposes that by setting a predetermined alloy composition, the steel structure is a toughened iron having residual Worth iron, and the amount of Worstian iron in the toughened iron is defined, thereby obtaining excellent calcination hardenability. Composite tissue steel plate.

Patent Document 3 proposes that by setting a predetermined alloy composition and setting the steel structure to a ductile iron having a residual Worthite iron, the area ratio of the tough iron is 90% or more, and the amount of the remaining Worth iron in the toughened iron is The composite structure steel sheet excellent in impact resistance is obtained by setting the hardness (HV) of the toughened iron to 1% or more and 15% or less.

[Advanced technical literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. Hei 4-235253

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2004-76114

[Patent Document 3] Japanese Patent Laid-Open No. Hei 11-256273

However, the above steel sheet potentially has the following problems.

In the component composition described in Patent Document 1, when strain is applied to the steel sheet, it is difficult to secure a stable residual Worthite iron amount which exhibits a TRIP effect in a high strain region, and although flexibility can be obtained, the plasticity is unstable until the plasticity is unstable. Low ductility and poor stretchability.

In the steel sheet described in Patent Document 2, although the calcination hardenability is obtained, even if the tensile strength (TS) is to be increased to 980 MPa or more or even 1050 MPa or more, it is mainly composed of tough iron or even ferrite iron. In addition, it is difficult to ensure workability such as ductility and stretch flangeability when the strength is ensured or the strength is increased.

The steel sheet described in Patent Document 3 is mainly for the purpose of improving the impact resistance, and is a structure having a toughened iron having a hardness of HV 250 or less as a main phase (specifically, containing more than 90%), and thus tensile strength. (TS) is difficult to reach 980 MPa or more.

The present invention is advantageous in solving the above problems, and an object thereof is to provide a high-strength steel sheet excellent in workability (particularly, ductility and stretch flangeability) and having a tensile strength (TS) of 980 MPa or more, and an advantageous production method thereof.

The high-strength steel sheet of the present invention covers a steel sheet which is subjected to hot-dip galvanizing or alloying hot-dip galvanizing on the surface of the steel sheet.

In the present invention, "excellent workability" means that the TS × T.EL value is 20,000 MPa ‧ % or more, and the TS × λ value is 25,000 MPa ‧ % or more. Here, "TS" means tensile strength (MPa), "T.EL" means total elongation (%), and "λ" means ultimate expansion ratio (%).

In order to solve the above problems, the inventors have conducted intensive studies on the composition and microstructure of the steel sheet. As a result, it has been found that the use of the lower toughened iron structure and/or the granulated iron structure is increased, and the amount of C in the steel sheet is set to be more than 0.17%. The ductile iron metamorphosis can be lifted before the TRIP effect is obtained, ensuring a favorable stability of the residual Worthite iron, and by forming part of the granulated iron into the tempering granulated iron, the processability is excellent, especially A high-strength steel sheet having excellent balance between strength and ductility and a balance between strength and stretch flangeability, and a tensile strength of 980 MPa or more.

The present invention has been completed based on the above findings, and the subject matter is as follows:

A high-strength steel sheet comprising: C: 0.17% or more, 0.73% or less, Si: 3.0% or less, Mn: 0.5% or more, 3.0% or less, P: 0.1% or less, and S: 0.07% or less, Al: 3.0% or less, and N: 0.010% or less, and Si+Al satisfies 0.7% or more, and the rest is composed of Fe and unavoidable impurities, and the steel sheet structure satisfies: lower tough iron and whole anesthesia The area ratio of the total amount of the molten iron to the entire steel sheet structure is 10% or more and 90% or less, and the amount of the remaining Worthite iron is 5% or more and 50% or less, and the tough ferrite iron in the upper toughened iron is relative to the entire steel sheet structure. The area ratio is 5% or more, and the above-mentioned lower toughness iron and the total amount of the loose iron in the whole field are 75% or less in the quenched state, and the area ratio of the polygonal ferrite iron to the entire steel sheet structure is 10% or less. 0%) is contained, and the average C content in the above-mentioned residual Worthite iron is 0.70% or more, and the tensile strength is 980 MPa or more.

2. The high-strength steel sheet according to the above-mentioned item 1, wherein the steel sheet further contains, by mass%, Cr: 0.05% or more and 5.0% or less, V: 0.005% or more and 1.0% or less, and Mo: 0.005% or more. One or two or more elements selected from 0.5% or less.

3. The high-strength steel sheet according to the above-mentioned item 1 or 2, wherein the steel sheet further contains, in terms of % by mass, from the group consisting of Ti: 0.01% or more and 0.1% or less, and Nb: 0.01% or more and 0.1% or less. Or 2 kinds of elements.

The high-strength steel sheet according to any one of the above-mentioned items, wherein the steel sheet further contains, by mass%, B: 0.0003% or more and 0.0050% or less.

The high-strength steel sheet according to any one of the above-mentioned items, wherein the steel sheet further contains, by mass%, Ni: 0.05% or more and 2.0% or less, and Cu: 0.05% or more and 2.0% or less. One or two elements selected in the middle.

The high-strength steel sheet according to any one of the above-mentioned items, wherein the steel sheet further contains, by mass%, Ca: 0.001% or more and 0.005% or less, and REM: 0.001% or more and 0.005% or less. One or two elements selected in the middle.

A high-strength steel sheet provided with a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the steel sheet according to any one of the above 1 to 6.

A method for producing a high-strength steel sheet, which is obtained by subjecting a steel sheet having the composition described in any one of the above 1 to 6 to hot rolling, and then cold rolling to form a cold-rolled steel sheet, followed by cold rolling After the steel sheet is annealed in a single phase of the Vostian iron for 15 seconds or more and 600 seconds or less, after cooling to a cooling stop temperature determined by a first temperature region of 350 ° C or higher and 490 ° C or lower: T ° C, at least The average cooling rate is controlled to be 5° C./s or more at 550° C., and then held in the first temperature region for 15 seconds or longer and 1000 seconds or shorter, and then at a second temperature region of 200° C. or higher and 350° C. or lower. It is kept for 15 seconds or more and 1000 seconds or less.

9. The method for producing a high-strength steel sheet according to the above-mentioned item 8, wherein the hot-dip galvanizing treatment or the alloying hot-dip plating is performed during cooling up to the cooling stop temperature: T°C or in the first temperature region. Zinc treatment.

According to the present invention, it is possible to provide a high-strength steel sheet having excellent workability (particularly ductility and stretch flangeability) and a tensile strength (TS) of 980 MPa or more, and an advantageous production method thereof, in industries such as automobiles and electric machines. The value of use in the field is very large, especially for the weight reduction of automobile bodies.

Hereinafter, the present invention will be specifically described.

First, the present invention will be described with respect to the reason why the steel sheet structure is as defined above. Hereinafter, the "area ratio" is set to an area ratio with respect to the entire steel sheet structure.

The combined area ratio of the lower toughened iron and the whole hemp field loose iron: 10% or more and 90% or less

The lower toughened iron and the 麻田散铁 are the necessary tissues for increasing the strength of the steel sheet. If the combined area ratio of the lower toughened iron and the whole hemp field is less than 10%, the tensile strength (TS) of the steel sheet is less than 980 MPa. On the other hand, if the combined area ratio of the lower toughened iron and the whole ramie loose iron exceeds 90%, the upper toughened iron will be less, and as a result, the stable residual Worthite iron cannot be ensured by the concentration of C, resulting in ductility and the like. The problem of reduced sex occurs. Therefore, the total area ratio of the lower toughened iron and the whole field of the loose iron is set to be 10% or more and 90% or less. It is preferably in the range of 20% or more and 80% or less. More preferably, it is in the range of 30% or more and 70% or less.

The proportion of the granulated iron in the quenched state is less than 75% in the lower toughness iron and the total ramification iron

In the granulated iron in the field, the proportion of the granulated iron in the quenched state is more than 75% when the total amount of the lower toughened iron and the whole ramification iron present in the steel sheet is more than 7%, but the tensile strength is 980 MPa or more. It is bad. Since the quenched state of the granulated iron is extremely hard, the deformability of the granulated iron in the quenched state is extremely low, and the workability of the steel sheet (especially the stretch flangeability) is remarkably deteriorated. In addition, since there is a significant difference in the hardness difference between the granulated iron in the quenched state and the upper toughened iron, if there is much iron in the quenched state, the interface between the granulated iron and the upper toughened iron will increase. In the case of punching, etc., the interface between the quenched state of the granulated iron and the upper toughened iron may cause minute holes, which may cause the holes to be joined when the extended flange is formed after the punching process. The cracking condition is easy to progress, and thus the stretch flangeability is further deteriorated. Therefore, in the Ma Tian loose iron, the proportion of the granulated iron in the quenched state is determined to be 75% or less with respect to the total amount of the lower toughness iron and the total ramification iron in the steel sheet. It is preferably 50% or less. In addition, in the quenched state, the structure of the carbide was not found in the granulated iron of the methadrite, and it was observed by SEM.

Residual Worthite iron: 5% or more, 50% or less

In the processing of the residual Worth Iron, the use of the TRIP effect is used to carry out the transformation of the granulated iron, and the ductility is improved by increasing the strain dispersion ability.

In the steel sheet according to the present invention, the upper portion of the tough iron is metamorphosed, and in particular, the residual Worth iron having an increased C concentration is formed in the upper toughened iron. As a result, residual Worthite iron which exhibits a TRIP effect even at a high strain region during processing can be obtained. By coexisting the residual Worthfield iron and the granulated iron and using it, it is possible to obtain a good workability even in a high-strength region having a tensile strength (TS) of 980 MPa or more, specifically TS×T.El The value is set to be 20,000 MPa ‧ % or more, and a steel sheet excellent in balance between strength and ductility is obtained.

Here, the residual Worth iron in the upper toughened iron is formed between the tough ferrite iron slats in the upper toughened iron, and since it is finely distributed, the amount is determined by the observation of the structure (area ratio) When it is necessary, a large amount of measurement must be performed at a high magnification, and it is difficult to accurately quantify it. However, the amount of residual Worth iron formed between the toughened ferrite slabs is a certain degree of coordination with the amount of iron formed into a toughened fertilizer. Therefore, as a result of investigation by the inventors, it has been found that the area ratio of the toughened ferrite in the upper toughened iron is 5% or more, and the residual Vostian iron quantity measurement method performed by the self-learning method uses X-ray diffraction (XRD). The intensity of the applied force (specifically, the ratio of the X-ray diffraction intensity of the ferrite iron to the Vostian iron) is more than 5%, and a sufficient TRIP effect can be obtained. The tensile strength (TS) is 980 MPa or more and TS × T.EL is 20,000 MPa‧% or more. In addition, it is confirmed that the amount of residual Worth iron obtained by the residual Worthite iron amount measuring method performed by the self-study is equivalent to the area ratio of the remaining Worth iron to the entire steel plate structure.

If the amount of iron in the remaining Vostian is less than 5%, a sufficient TRIP effect cannot be obtained. On the other hand, when it exceeds 50%, the hard ramification iron which is generated after the TRIP effect is exhibited may be excessively large, causing problems such as deterioration of toughness. Therefore, the amount of residual Worthite iron is set to be in the range of 5% or more and 50% or less. It is preferably in the range of more than 5%, more preferably 10% or more and 45% or less. It is particularly preferably in the range of 15% or more and 40% or less.

Average C amount in residual Worthite iron: 0.70% or more

In order to obtain excellent workability by utilizing the TRIP effect, in the high-strength steel sheet having a tensile strength (TS) of 980 MPa to 2.5 GPa, the amount of C remaining in the Worthite iron is important. The steel sheet of the present invention concentrates C in the residual Worth iron formed between the toughened ferrite slabs in the upper toughened iron. Though it is difficult to accurately evaluate the amount of C concentrated in the residual Worthite iron between the slats, it has been found by the inventors that the steel sheet of the present invention is retained in the Worstian iron by the conventionally determined measurement. If the average C amount (the average amount of C in the residual Worthite iron) is obtained by the diffraction peak displacement of the X-ray diffraction (XRD), the average C amount in the residual Worthite iron is 0.70% or more. Excellent processability is obtained.

When the average C amount in the residual Worth iron is less than 0.70%, the numb iron metamorphosis is generated in the low strain region during processing, resulting in the TRIP effect in the high strain region where the workability is improved. Therefore, the average C amount in the residual Worthite iron is set to be 0.70% or more. It is preferably 0.90% or more. On the other hand, if the average C amount in the remaining Worthite iron exceeds 2.00%, the remaining Worthite iron is excessively stabilized, and the kinetic iron metamorphosis does not occur during the processing, and the TRIP effect is not exhibited, so that the ductility is lowered. Therefore, the average C amount in the residual Worthite iron is preferably 2.00% or less. More preferably, it is 1.50% or less.

Toughened ferrite grain area ratio in upper toughened iron: 5% or more

According to the metamorphic iron metamorphism of the upper tough iron, the iron is formed in order to obtain the concentration of C in the untransformed Worthite iron, and the TRIP effect is exhibited in the high strain region during processing, and the strain decomposition ability is improved. Residual Worth Iron is necessary. The metamorphosis from the Vostian iron to the toughened iron occurs in a wide temperature range spanning 150 to 550 ° C, and the toughened iron formed in this temperature range exists in various forms. In the conventional art, in many cases, such various toughened irons are simply defined as toughened iron. However, in the present invention, in order to obtain the target processability, it is necessary to clearly define the toughened iron structure, and thus the upper portion is toughened. "Iron" and "lower toughened iron" are defined as follows.

The upper toughened iron system consists of slat-like toughened ferrite iron and residual Worth iron and/or carbide present between the toughened ferrite and iron, characterized by: in the slat-like toughened ferrite iron There are no fine carbides arranged neatly. On the other hand, the lower toughened iron system is composed of slat-like toughened ferrite iron and residual Worthite iron and/or carbide existing between the toughened ferrite and iron, which is common to the upper toughened iron. However, the lower toughened iron is characterized by the presence of finely arranged fine carbides in the slab-shaped toughened ferrite.

That is, the upper toughened iron and the lower toughened iron are distinguished according to whether there is a finely arranged fine carbide in the toughened ferrite iron. The poor formation of carbides in such toughened ferrite irons has a considerable effect on the concentration of C in the residual Worthite iron. That is, when the area ratio of the toughened ferrite of the upper toughened iron is less than 5%, the amount of carbide formed in the tough ferrite iron is increased even in the case of the toughening iron metamorphosis. The amount of C concentration in the residual Worth iron present between the slats is reduced, resulting in a problem of a decrease in the amount of residual Worthite iron which exhibits a TRIP effect in a high strain region during processing. Therefore, the area ratio of the toughened ferrite in the upper toughened iron must be 5% or more with respect to the area ratio of the entire steel sheet structure. On the other hand, if the area ratio of the toughened ferrite iron of the upper toughened iron to the entire steel sheet structure exceeds 85%, it may be difficult to ensure strength, and therefore it is preferably 85% or less.

Area ratio of polygonal ferrite iron: 10% or less (including 0%)

If the area ratio of the polygonal ferrite is more than 10%, it is difficult to satisfy the tensile strength (TS): 980 MPa or more, and at the same time, during the processing, the soft polygonal ferrite iron mixed in the hard tissue is strain-concentrated. It is prone to cracking when processing is performed, and as a result, the desired workability cannot be obtained. Here, if the polygonal ferrite grain area ratio is 10% or less, even if there is polygonal ferrite iron, a small amount of polygonal ferrite iron in the hard phase is isolated and dispersed, and strain concentration can be suppressed, and workability can be prevented from deteriorating. Therefore, the area ratio of the polygonal ferrite iron is set to be less than 10%. It is preferably 5% or less, more preferably 3% or less, and may be 0%.

Further, in the case of the steel sheet of the present invention, the hardness of the hardest structure in the steel sheet structure is HV≦800. That is, in the steel sheet of the present invention, when there is no quarry iron in the quenched state, any of the tempered granulated iron or the lower toughened iron or the upper toughened iron becomes the hardest phase, and the structures are all Become the phase of HV≦800. In addition, when there is a quenched granulated iron in the quenched state, the granulated iron in the quenched state becomes the hardest structure. In the steel sheet of the present invention, even in the quenched state, the hardness of the granulated iron is HV ≦ 800, and there is no HV. >800's apparently harder the existence of loose iron in the field, ensuring good stretch flangeability.

In the steel sheet of the present invention, as the remaining structure, bead iron, Federmanite iron (Widmanstaettenn), and lower toughened iron may be contained. In this case, the allowable content of the remaining tissues is preferably 20% or less in terms of the area ratio. More preferably, it is 10% or less.

The above is a basic structure of the steel sheet structure of the high-strength steel sheet of the present invention, and the following configuration may be added as needed.

Next, the present invention will be described with respect to the reason why the chemical composition of the steel sheet is limited as described above. In addition, "%" indicated by the following component composition means "% by mass".

C: 0.17% or more and 0.73% or less

In order to ensure the high strength of the steel sheet and the indispensable element for ensuring the stability of the residual Worthite iron, the C system is an essential element for ensuring the amount of iron in the field and ensuring the retention of the Worthite iron at room temperature. If the amount of C is less than 0.17%, it is difficult to ensure the strength and workability of the steel sheet. On the other hand, when the amount of C exceeds 0.73%, the welded portion and the heat-affected portion are hardened, and the weldability is deteriorated. Therefore, the amount of C is set to be in the range of 0.17% or more and 0.73% or less. It is preferably in the range of more than 0.20% and 0.48% or less, more preferably 0.25% or more.

Si: 3.0% or less (including 0%)

The Si system is a useful element that contributes to the strength improvement of steel by solid solution strengthening. However, when the amount of Si exceeds 3.0%, the amount of solid solution in the polygonal ferrite and the tough ferrite is increased, and the workability and toughness are deteriorated, and the surface properties are deteriorated due to occurrence of red rust or the like. When the hot plating is performed, the plating adhesion and the adhesion are deteriorated. Therefore, the amount of Si is set to be 3.0% or less. It is preferably 2.6% or less. More preferably, it is 2.2% or less.

In addition, Si suppresses the formation of carbides and promotes the formation of useful elements of the residual Worthite iron. Therefore, the amount of Si is preferably 0.5% or more. When only the formation of carbides is inhibited by Al, Si is not added. The amount can also be 0%.

Mn: 0.5% or more and 3.0% or less

An effective element for strengthening Mn steel. When the amount of Mn is less than 0.5%, since carbides are precipitated in a temperature region higher than the temperature at which the toughened iron or the granulated iron is formed during cooling after annealing, it is not possible to secure a hard steel contribution. The amount of phase. On the other hand, when the amount of Mn exceeds 3.0%, deterioration of castability or the like is caused. Therefore, the amount of Mn is set to be in the range of 0.5% or more and 3.0% or less. It is preferably in the range of 1.5% or more and 2.5% or less.

P: 0.1% or less

P is an element which is useful for strengthening steel. When the amount of P exceeds 0.1%, it is embrittled by grain boundary segregation, and the impact resistance is deteriorated. When the alloy is subjected to alloying hot-dip galvanizing, the alloying speed is greatly changed. slow. Therefore, the amount of P is set to be less than 0.1%. It is preferably 0.05% or less. Further, the amount of P is preferably reduced, but if it is less than 0.005%, the cost is greatly increased, so the lower limit is preferably set to about 0.005%.

S: 0.07% or less

In the S system, MnS is formed to form inclusions, which causes deterioration of impact resistance and fracture of the welded portion along the metal flow. Therefore, it is preferable to reduce the amount of S as much as possible. However, if the amount of S is excessively reduced, the manufacturing cost is increased, and thus the amount of S is set to be 0.07% or less. It is preferably 0.05% or less, more preferably 0.01% or less. In addition, when S is less than 0.0005%, a large increase in manufacturing cost is derived. Therefore, from the viewpoint of production cost, the lower limit is about 0.0005%.

Al: 3.0% or less

Al is a useful element for steel strengthening and is a useful element added as a deoxidizer in the steel making step. When the amount of Al exceeds 3.0%, excessive inclusions in the steel sheet cause deterioration in ductility. Therefore, the amount of Al is set to be 3.0% or less. Preferably it is 2.0% or less.

In addition, Al is a useful element for suppressing the formation of carbides, and promotes the formation of residual Worthite iron. Further, in order to obtain a deoxidizing effect, the amount of Al is preferably 0.001% or more, more preferably 0.005% or more. Further, the amount of Al in the present invention is set to the amount of Al contained in the steel sheet after deoxidation.

N: 0.010% or less

The N-based element which causes the greatest deterioration in the aging resistance of steel is preferably minimized. If the amount of N exceeds 0.010%, the deterioration of the aging resistance tends to be conspicuous, so the amount of N is set to be 0.010% or less. In addition, since the manufacturing cost is greatly increased when N is set to less than 0.001%, the lower limit is about 0.001% from the viewpoint of production cost.

Although the basic components have been described above, the present invention is only insufficient in satisfying the above-described range of components, and it is necessary to satisfy the following formula.

Si+Al≧0.7%

Both Si and Al are as described above, and are useful elements for suppressing the formation of carbides and promoting the formation of residual Worth iron. The inhibition of the formation of carbides has an effect even if Si or Al alone is contained, but the total amount of Si and the amount of Al must satisfy 0.7% or more. Further, the amount of Al in the above formula is set to the amount of Al contained in the steel sheet after deoxidation.

Further, in the present invention, in addition to the above basic components, the following components may be appropriately contained.

One or two or more selected from the group consisting of Cr: 0.05% or more and 5.0% or less, V: 0.005% or more and 1.0% or less, and Mo: 0.005% or more and 0.5% or less.

Cr, V, and Mo have elements which suppress the formation of bead iron when cooling is performed from the annealing temperature. This effect is obtained by Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more. On the other hand, when it exceeds Cr: 5.0%, V: 1.0%, and Mo: 0.5%, the amount of the loose iron in the hard 麻田 becomes too large, and it is necessary to have a high strength. Therefore, when Cr, V, and Mo are contained, Cr: 0.05% or more and 5.0% or less, V: 0.005% or more and 1.0% or less, and Mo: 0.005% or more and 0.5% or less are set.

One or two kinds are selected from Ti: 0.01% or more and 0.1% or less, and Nb: 0.01% or more and 0.1% or less.

Ti and Nb are useful for precipitation strengthening of steel, and the effect is obtained when the content is 0.01% or more. On the other hand, when the content per content exceeds 0.1%, workability and shape freezeability are lowered. Therefore, when Ti and Nb are contained, it is set to be Ti: 0.01% or more and 0.1% or less, and Nb: 0.01% or more and 0.1% or less.

B: 0.0003% or more and 0.0050% or less

The B system suppresses the formation of useful elements from the Worthfield iron grain boundary. This effect is obtained when it contains more than 0.0003%. On the other hand, when the content exceeds 0.0050%, workability is lowered. Therefore, when B is contained, it is set to B: 0.0003% or more and 0.0050% or less.

One or two selected from the group consisting of Ni: 0.05% or more and 2.0% or less, and Cu: 0.05% or more and 2.0% or less

The strengthening of Ni and Cu-based steel is an effective element. Further, when the steel sheet is subjected to hot-dip galvanizing or alloying hot-dip galvanizing, the internal oxidation of the surface layer portion of the steel sheet is promoted and the plating adhesion is improved. These effects are each obtained in an amount of 0.05% or more. On the other hand, when the respective content exceeds 2.0%, the workability of the steel sheet is lowered. Therefore, when Ni and Cu are contained, it is set to be in the range of Ni: 0.05% or more and 2.0% or less, and Cu: 0.05% or more and 2.0% or less.

One or two kinds of Ca: 0.001% or more and 0.005% or less, and REM: 0.001% or more and 0.005% or less

Ca and REM form a useful element for spheroidizing the shape of the sulfide and improving the adverse effect of the sulfide on the stretch flangeability. This effect is obtained in accordance with the respective contents of 0.001% or more. On the other hand, when the content is more than 0.005%, the inclusions and the like are increased, and surface defects and internal defects are caused. Therefore, when Ca and REM are contained, it is set to Ca: 0.001% or more and 0.005% or less, and REM: 0.001% or more and 0.005% or less.

In the steel sheet of the present invention, the components other than the above are Fe and unavoidable impurities. However, it is not excluded to include components other than the above insofar as the effects of the present invention are not impaired.

Next, a method of producing the high-strength steel sheet of the present invention will be described.

After the steel sheet adjusted to have the composition of the above preferred composition is produced, hot rolling is performed, followed by cold rolling to form a cold rolled steel sheet. In the present invention, the treatment is not particularly limited and may be carried out in accordance with a conventional method.

Preferred manufacturing conditions are as follows. After the steel sheet is heated in a temperature range of 1000 ° C or more and 1300 ° C or less, hot rolling is performed in a temperature range of 870 ° C or more and 950 ° C or less, and the obtained hot rolled steel sheet is 350 ° C or more and 720 ° C or less. The temperature zone is subjected to coiling. Next, after the hot-rolled steel sheet is pickled, cold rolling is performed at a rolling reduction ratio in a range of 40% or more and 90% or less to form a cold-rolled steel sheet.

Further, in the present invention, the steel sheet is manufactured by various steps such as ordinary steel making, casting, hot rolling, pickling, and cold rolling. However, for example, in the case of thin steel billet casting or sheet continuous casting, Some or all of the hot rolling steps are omitted for manufacturing.

The heat treatment shown in Fig. 1 was applied to the obtained cold rolled steel sheet. Hereinafter, description will be made with reference to Fig. 1 .

Annealing is performed for 15 seconds or more and 600 seconds or less in the single phase of the Vostian iron. The steel sheet according to the present invention is a main phase in which an upper toughened iron, a lower toughened iron, and a granulated iron are metamorphosed in a lower temperature region in which the untransformed Worth iron is in a range of 350 ° C or more and 490 ° C or less. Therefore, it is best to reduce the polygonal ferrite iron as much as possible, and it is necessary to perform annealing in the single phase of the Vostian iron. The relevant annealing temperature, if it is in the single phase of the Vostian iron, is not particularly limited. If the annealing temperature exceeds 1000 ° C, the growth of the Worthfield iron particles tends to be obvious, which may result in subsequent cooling. The formation phase is coarsened, resulting in deterioration of toughness and the like. On the other hand, when the annealing temperature is less than A ₃ point (Worthfield iron metamorphic point), polygonal ferrite iron is formed in the annealing stage, in order to suppress the growth of the polygonal ferrite iron in cooling, resulting in the necessity of 500 ° C The above temperature range is extremely rapidly cooled. Therefore, the annealing temperature must be set to be equal to or higher than A ₃ point (Worstian iron metamorphic point), and preferably set to 1000 ° C or lower.

In addition, when the annealing time is less than 15 seconds, the inversion state of the Worthite iron may not be sufficiently performed, or the carbide in the steel sheet may not be sufficiently dissolved. On the other hand, if the annealing time exceeds 600 seconds, the cost will be increased with a large amount of energy consumption. Therefore, the annealing time is set to be in the range of 15 seconds or more and 600 seconds or less. It is preferably in the range of 60 seconds or more and 500 seconds or less. Here, the A ₃ point system can use the following formula:

A ₃ points (°C)=910-203×[C%]1/2+44.7×[Si%]-30×[Mn%]

+700×[P%]+130×[Al%]-15.2×[Ni%]

-11×[Cr%]-20×[Cu%]+31.5×[Mo%]

+104×[V%]+400×[Ti%]

Perform an approximation calculation. Among them, [X%] is the mass % of the steel component X.

The annealed cold-rolled steel sheet is cooled to a cooling stop temperature determined by a first temperature range of 350 ° C or higher and 490 ° C or lower: T ° C, but at least until 550 ° C, the average cooling rate is controlled at 5 ° C / Cool down above s. When the average cooling rate is less than 5 ° C / s, the polygonal ferrite iron may be excessively formed and grown, or precipitation of bead iron or the like may occur, or the desired steel sheet structure may not be obtained. Therefore, the average cooling rate from the annealing temperature to the first temperature region is set to 5 ° C / s or more. It is preferably 10 ° C / s or more. The upper limit of the average cooling rate is not changed at the cooling stop temperature, and the rest is not particularly limited. If the average cooling rate of the general equipment exceeds 100 ° C / s, the longitudinal direction of the steel sheet and the width direction of the steel sheet are The change in the structure is markedly large, so it is preferably set to be 100 ° C / s or less.

The steel sheet cooled to 550 ° C was continuously cooled to a cooling stop temperature: T ° C. In the temperature range of T ° C or more and 550 ° C or less, the cooling rate of the steel sheet is not particularly limited, except that the holding time in the first holding temperature region is 15 seconds or more and 1000 seconds or less. If the steel sheet is cooled at an excessively slow rate, there is a possibility that the desired structure cannot be obtained due to the formation of carbides from the never-deformed Worth iron. Therefore, in a temperature range of T ° C or more and 550 ° C or less, the steel sheet is preferably cooled at an average rate of 1 ° C/s or more.

The steel sheet cooled to the cooling stop temperature: T ° C is held in the first temperature region of 350 ° C or more and 490 ° C or less for 15 seconds or more and 1000 seconds or less. When the upper limit of the first temperature region exceeds 490 ° C, carbides are precipitated from the untransformed Worth iron, and the desired structure cannot be obtained. On the other hand, when the lower limit of the first temperature region is less than 350 ° C, the upper toughened iron is not formed, but the lower toughened iron is formed, which causes a problem that the concentration of C in the Worthite iron is small. Therefore, the range of the first temperature range is set to be in a range of 350 ° C or more and 490 ° C or less. It is preferably in the range of 370 ° C or more and 460 ° C or less.

Further, if the holding time in the first temperature region is less than 15 seconds, the amount of upper tough iron deformation is reduced, resulting in a problem that the amount of C concentration in the untransformed Worth iron is small. On the other hand, when the holding time in the first temperature region exceeds 1000 seconds, the final structure of the steel sheet is precipitated from the untransformed Worth iron which is the residual Worthite iron, and the stable residue which is concentrated by C cannot be obtained. Worth Iron, the result is not able to obtain the required processability. Therefore, the hold time is set to be 15 seconds or more and 1000 seconds or less. It is preferably in the range of 30 seconds or more and 600 seconds or less.

The steel sheet that has been held in the first temperature region is cooled to a second temperature region of 200° C. or higher and 350° C. or lower at an arbitrary speed, and held in the second temperature region for a period of 15 seconds or longer and 1000 seconds or shorter. When the upper limit of the second temperature region exceeds 350 ° C, the lower toughened iron metamorphosis is not performed, and as a result, there is a problem that the amount of the granulated iron in the quenched state increases. On the other hand, when the lower limit of the second temperature region is less than 200 ° C, the lower toughened iron metamorphosis is not performed in the same manner, and the amount of the granulated iron in the quenched state is increased. Therefore, the range of the second temperature range is set to be in the range of 200 ° C or more and 350 ° C or less. It is preferably in the range of 250 ° C or more and 340 ° C or less.

Further, if the holding time is less than 15 seconds, a sufficient amount of the lower toughened iron cannot be obtained, resulting in failure to obtain desired workability. On the other hand, if the holding time exceeds 1000 seconds, carbides are precipitated from the stable residual Worstian iron in the upper toughened iron generated in the first temperature region, and as a result, the desired workability cannot be obtained. Therefore, the retention time is set to a range of 15 seconds or more and 1000 seconds or less. It is preferably in the range of 30 seconds or more and 600 seconds or less.

Further, in the series of heat treatments of the present invention, the temperature is not necessarily constant if it falls within the predetermined temperature range, and the present invention is not impaired even if it varies within a predetermined temperature range. The relevant cooling rate is also the same. In addition, if the thermal experience can be satisfied, the steel plate can be heat treated according to any equipment. Further, after the heat treatment, surface treatment such as temper rolling or plating for the surface of the steel sheet for the purpose of shape correction is also included in the scope of the present invention.

In the method for producing a high-strength steel sheet according to the present invention, alloyed hot-dip galvanizing may be further performed by alloying treatment in hot-dip galvanizing or after hot-dip galvanizing. The hot-dip galvanizing or alloying hot-dip galvanizing may be carried out in the cooling up to the first temperature region or in the first temperature region. In this case, the holding time in the first temperature region is set to 15 seconds or more and 1000 seconds or less, including the treatment time in the first temperature region in which the hot-dip galvanizing treatment or the alloying galvanizing treatment is performed. Further, the hot-dip galvanizing treatment or the alloying hot-dip galvanizing treatment is preferably carried out using a continuous hot-dip galvanizing line.

Further, in the method for producing a high-strength steel sheet according to the present invention, after the high-strength steel sheet which has been subjected to the heat treatment is produced in accordance with the above-described production method of the present invention, the hot-dip galvanizing treatment is further performed, or the alloying is further performed. Melt galvanizing treatment.

Further, according to the production method of the present invention, after the holding in the second temperature region, the hot-dip galvanizing treatment or the alloying hot-dip galvanizing treatment may be performed.

The method of subjecting the steel sheet to hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment is as follows.

The steel sheet is immersed in a plating bath, and the amount of adhesion is adjusted by a gas whipping method or the like. The amount of dissolved Al in the plating bath is preferably in the range of 0.12% or more and 0.22% or less in the case of the hot-dip galvanizing treatment, and is preferably 0.08% or more in the case of the alloying hot-dip galvanizing treatment. And within the range of 0.18% or less.

The treatment temperature is in the case of hot-dip galvanizing treatment, and the temperature of the plating bath may be in the range of usually 450 ° C or more and 500 ° C or less, and when alloying treatment is performed, the temperature at the time of alloying is preferably set to Below 550 °C. When the alloying temperature exceeds 550 ° C, since carbides are precipitated from the never-deformed Worthite iron, and bead iron is formed depending on the case, strength or workability cannot be obtained, or neither of them can be obtained, and the plating layer is not obtained. The powdering properties will also deteriorate. On the other hand, if the temperature at the time of alloying is less than 450 ° C, alloying may not be performed, and therefore it is preferably set to 450 ° C or higher.

The plating adhesion amount is preferably set to be in the range of 20 g/m ² or more and 150 g/m ² or less per one side. If the plating adhesion amount is less than 20 g/m ² , the corrosion resistance is insufficient, and conversely, even if it exceeds 150 g/m ² g, the corrosion resistance effect is saturated, and only the cost is increased.

The alloying degree [Fe mass % (Fe content)) of the plating layer is preferably in the range of 7 mass% or more and 15 mass% or less. When the degree of alloying of the plating layer is less than 7% by mass, unevenness in alloying may result in deterioration of appearance quality, or a so-called "ζ phase" may be formed in the plating layer, resulting in deterioration of slidability of the steel sheet. On the other hand, if the degree of alloying of the plating layer exceeds 15% by mass, a hard and brittle Γ phase is formed in a large amount, resulting in deterioration of plating adhesion.

[Examples]

Hereinafter, the present invention will be described in more detail by way of examples, but the following examples are not intended to limit the invention. Further, the configuration changes that are within the scope of the gist of the present invention are also included in the scope of the present invention.

The cast piece obtained by melting the steel of the composition shown in Table 1 was heated to 1200 ° C, and hot-rolled steel sheet was obtained by hot-rolling finishing at 870 ° C, and then coiled at 650 ° C, followed by hot rolling. After the steel sheet was pickled, it was cold rolled at a 65% rolling ratio to form a cold rolled steel sheet having a thickness of 1.2 mm. The obtained cold-rolled steel sheet was subjected to heat treatment in accordance with the conditions shown in Table 2. In addition, the "cooling stop temperature: T" in Table 2 means the temperature at which the steel sheet is cooled when the steel sheet is cooled from the annealing temperature.

Further, for some of the cold-rolled steel sheets, a hot-dip galvanizing treatment or an alloying hot-dip galvanizing treatment is performed. Among them, the hot-dip galvanizing treatment was performed by double-sided plating in such a manner that the plating bath temperature was 463 ° C and the apparent amount (per single side): 50 g/m ² . In addition, the alloying hot-dip galvanizing treatment adjusts the alloying conditions in such a manner that the apparent amount (per single side): 50 g/m ² and the degree of alloying [Fe mass % (Fe content)] is 9% by mass. And perform double-sided plating. Further, the hot-dip galvanizing treatment and the alloying hot-dip galvanizing treatment were carried out after cooling to T °C shown in Table 2.

When the obtained steel sheet is not subjected to the plating treatment, after the heat treatment, the rolling reduction ratio (stretching ratio) is performed: 0.3% of the temper rolling, and when the hot-dip galvanizing treatment or the alloying hot-dip galvanizing is performed In the case of the treatment, the rolling reduction ratio (stretching ratio) was carried out after the treatment: 0.3% of the temper rolling.

The characteristics of the obtained steel sheets were evaluated by the following methods.

The sample was cut out from each of the steel sheets and polished, and the parallel surface of the rolling direction was observed by a scanning electron microscope (SEM) at a magnification of 3,000 times to observe the area ratio of each phase, and the phase structure of each crystal grain was identified. .

The residual Worthite iron amount is obtained by grinding the steel sheet in the direction of the plate thickness, grinding it to 1/4 of the plate thickness, and measuring it by X-ray diffraction intensity measurement. The incident X-ray system uses Co-Kα, and the diffraction intensity of each of the (200), (211), and (220) faces of the ferrite iron, (200), (220), and (311) of the Worthite Iron The average strength ratio of each surface is calculated as the average value of the residual Worthite iron.

The average C amount in the residual Worthite iron is determined from the intensity peaks of the Worstian iron (200), (220), and (311) planes measured by the X-ray diffraction intensity, and the lattice constant is obtained, and then calculated from the following formula. The average C amount (% by mass) in the residual Worthite iron was obtained by the formula.

a ₀ =0.3580+0.0033×[C%]+0.00095×[Mn%]+0.0056×[Al%]+0.022×[N%]

Wherein a ₀ : lattice constant (nm); [X%]: mass % of the element X. In addition, the elemental mass % other than C is the mass % with respect to the whole steel plate.

The tensile test was carried out in accordance with JIS No. 2241 using a JIS No. 5 test piece taken from the vertical direction of the rolling direction of the steel sheet. The TS (tensile strength) and T.El (total elongation) were measured, and the product of the strength and the total elongation (TS × T. El) was calculated, and the balance between the strength and the workability (ductility) was evaluated. Further, in the present invention, the case of TS × T. El ≧ 20000 (MPa ‧ %) is referred to as "good".

The extended flangeability is evaluated according to the Japan Iron and Steel Federation specification JFST1001. After the obtained steel sheets were cut into 100 mm × 100 mm, the gap was set to 12% of the sheet thickness, and a hole having a diameter of 10 mm was punched out, and then a mold having an inner diameter of 75 mm was used, and the force was pressed by the wrinkles: When 88.2 kN was pressed, a 60° conical punch was pressed into the hole, and the diameter of the hole at which the crack limit occurred was measured, and the ultimate hole expansion ratio λ (%) was obtained from the formula (1).

Ultimate hole expansion ratio λ(%)={(D _f -D _o )/D _o }×100 ...(1)

Among them, D _f is the pore diameter (mm) at which cracking occurs, and D _o is the initial pore diameter (mm).

Using the measured λ, the product of the intensity and the ultimate hole expansion ratio (TS × λ) was calculated, and the balance between the strength and the stretch flangeability was evaluated.

Further, in the present invention, when TS × λ ≧ 25000 MPa ‧ %, the stretch flangeability is regarded as "good".

Further, the hardness of the hardest structure in the steel sheet structure was judged by the following method. That is, when the observation result is observed in the quenched state of the granulated iron, the quenched state of the granulated iron is measured by an ultra-micro Vickers hardness meter at a load of 0.02 N, and the average value is It is regarded as the hardness of the hardest structure in the steel plate structure. In addition, when there is no quenching state of the granulated iron, as in the above, any of the tempered granulated iron, the upper toughened iron or the lower toughened iron will become the hardest phase in the steel sheet of the present invention. When the hardest phase is the steel sheet of the present invention, it becomes the phase of HV≦800.

The above evaluation results are shown in Table 3.

It is known from the same table that the steel sheet of the present invention can satisfy a tensile strength of 980 MPa or more, a TS × T. El value of 20,000 MPa ‧ % or more, and a TS × λ ≧ 25000 MPa ‧ % or more Strength and excellent processability (especially excellent stretch flangeability).

On the other hand, in sample No. 1, since the average cooling rate up to 550 ° C was out of the appropriate range, the desired steel sheet structure could not be obtained, and although TS × λ ≧ 25000 MPa ‧ % was satisfied, the tensile strength was not satisfied ( TS) ≧ 980 MPa and TS × T. EL ≧ 20000 MPa ‧ %. In sample No. 2, the holding time in the first temperature range was outside the appropriate range, and sample No. 5 was less than A ₃ point °C in the annealing temperature, and sample No. 6 was in the cooling stop temperature: T was outside the first temperature range. In the sample No. 8, since the holding temperature in the second temperature region exceeded the proper range, the sample No. 11 was not able to obtain the desired steel sheet structure because the holding time in the second temperature region exceeded the appropriate range. The tensile strength (TS) ≧ 980 MPa, but it was not satisfied with TS × T.EL ≧ 20000 MPa ‧ % and TS × λ ≧ 25000 MPa ‧ % Sample Nos. 30 to 34 were unable to obtain the desired steel sheet structure because the composition of the composition exceeded the appropriate range, and the tensile strength (TS) ≧ 980 MPa, TS × T.EL ≧ 20000 MPa ‧ %, and TS × λ ≧ 25000 MPa ‧ % There will be more than one item that cannot be met.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing the temperature at the time of heat treatment in accordance with the manufacturing method of the present invention.

Claims (5)

A high-strength steel sheet containing C: 0.17% or more, 0.73% or less, Si: 3.0% or less, Mn: 0.5% or more, 3.0% or less, P: 0.1% or less, and S: 0.07 in terms of % by mass. % or less, Al: 3.0% or less, and N: 0.010% or less, and Si+Al satisfies 0.7% or more, and the rest is composed of Fe and unavoidable impurities; the steel sheet structure is satisfied: the lower toughened iron and the whole Ma Tiansan The total amount of iron is 10% or more and 90% or less with respect to the area ratio of the entire steel sheet structure, and the amount of the remaining Worthite iron is 5% or more and 50% or less, and the tough ferrite iron in the upper toughened iron is relative to the entire steel sheet structure. The area ratio is 5% or more, and the total amount of the lower toughened iron and the whole ramie iron is 75% or less in the quenched state, and the area ratio of the polygonal ferrite iron to the entire steel sheet structure is 10% or less. (including 0%); and the average C content in the above-mentioned residual Worth iron is 0.70% or more, and the tensile strength is 980 MPa or more. The high-strength steel sheet according to claim 1, wherein the steel sheet further contains one or more of the following groups (A) to (E) by mass%: (A) one or more selected from the group consisting of Cr: 0.05% or more and 5.0% or less, V: 0.005% or more and 1.0% or less, and Mo: 0.005% or more and 0.5% or less; (B) Ti: 0.01% or more and 0.1% or less, and Nb: 0.01% or more and 0.1% or less, one or two selected; (C) B: 0.0003% or more and 0.0050% or less; (D) Ni: 0.05% or more 2.0% or less, and Cu: 0.05% or more and 2.0% or less, one or two selected; (E) Ca: 0.001% or more and 0.005% or less, and REM: 0.001% or more and 0.005% or less. 1 or 2 types. A high-strength steel sheet provided with a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of a high-strength steel sheet according to claim 1 or 2. A method for manufacturing a high-strength steel plate, which is to be made into a patent application scope A steel sheet composed of one or two components is subjected to hot rolling, and then cold-rolled to form a cold-rolled steel sheet, and then the cold-rolled steel sheet is subjected to a single-phase region of Vostian iron for 15 seconds or more and 600 seconds or less. After the annealing, when cooling to a cooling stop temperature determined by the first temperature region of 350 ° C or higher and 490 ° C or lower: T ° C, the average cooling rate is controlled to 5 ° C / s or more and cooled at least at 550 ° C. Then, it is held in the first temperature region for 15 seconds or more and 1000 seconds or less, and then held in the second temperature region of 200 ° C or more and 350 ° C or less for 15 seconds or more and 1000 seconds or less. The method for producing a high-strength steel sheet according to claim 4, wherein the hot-dip galvanizing treatment or the alloying hot-dip plating is performed at the cooling up to the cooling stop temperature: T°C or in the first temperature region. Zinc treatment.