TWI433961B - High strength galvanized steel sheet having excellent formability and stability of mechanical properties and method for manufacturing the same - Google Patents

Description

High-strength hot-dip galvanized steel sheet excellent in material stability and workability and method for producing same

The present invention relates to a high-strength hot-dip galvanized steel sheet which is excellent in material stability and workability, which is suitable for use in industrial fields such as automobiles and electric appliances, 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. As a result, there is a tendency to increase the strength of the vehicle body material and to make the body thinner, and the movement of the vehicle body itself is becoming lighter.

However, the high strength of the steel sheet leads to a decrease in the rolling ductility, that is, the formability is lowered. Therefore, the development of materials with high strength and high processability is expected.

Further, when the high-strength steel sheet is subjected to a forming process of a complicated shape such as an automobile part, there is a large problem that the protruding portion and the stretched flange portion are broken or necked. Therefore, there is also a need for a high-strength steel sheet which can overcome the problems of fracture and necking and which combines high rolling ductility and high hole expansion.

Furthermore, as the steel sheet is strengthened and thinned, the shape freezeability is remarkably lowered. In order to cope with this phenomenon, it is widely used to predict the shape change after demolding in press forming, and the mold is designed by predicting the amount of shape change. However, if the tensile strength (TS) of the steel sheet changes, the bias The situation of shifting a certain amount of prediction will become larger, resulting in a shape defect. After stamping and forming, corrections such as sheet metal processing for one shape are indispensable, resulting in a significant reduction in mass production efficiency. Therefore, the TS variation of the steel sheet is required to be as small as possible.

In view of the improvement in the formability of high-strength steel sheets, up to now, TRIP steels have been developed which have developed ferrite-magnesium-distributed iron-duplex steel (Dual-Phase steel) or residual transformation-induced plasticity (Transformation Induced Plasticity). Various types of composite high-strength hot-dip galvanized steel sheets.

For example, Patent Document 1 proposes to obtain a steel sheet excellent in rolling ductility by specifying a chemical composition and specifying a volume fraction of the remaining Worthite iron and the granulated iron, and a method for producing the same. Moreover, in the patent document 2, it is proposed to obtain a steel plate excellent in rolling property by specifying a chemical component and specifying a special manufacturing method. Patent Document 3 proposes to obtain a steel sheet excellent in rolling property by specifying a chemical composition and specifying a volume ratio of ferrite iron to tough ferrite iron and residual Worth iron. Further, Patent Document 4 proposes a method of producing a high-strength cold-rolled steel sheet having improved tensile variation in the sheet width direction.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2001-140022

[Patent Document 2] Japanese Patent Laid-Open No. Hei 04-026744

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2007-182625

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2000-212684

However, since the main purpose of Patent Documents 1 to 3 is to improve the rolling property of the high-strength steel sheet, the relevant hole expandability is not taken into consideration. Patent Document 4 describes the variation in the total elongation EL in the sheet width direction, but the material variation due to the composition of the components and the manufacturing conditions is not taken into consideration. Therefore, the present invention has been developed as a high-strength hot-dip galvanized steel sheet having both high rolling property and high hole expandability and excellent material stability.

The present invention has been made in view of the above circumstances, and an object of the invention is to provide a high-strength hot-dip galvanized steel sheet having a tensile strength TS of 540 MPa or more and excellent material stability and workability (high rolling property and high hole expansion). And its manufacturing method.

The inventors of the present invention have been able to obtain a high-strength hot-dip galvanized steel sheet having a tensile strength TS of 540 MPa or more and excellent in material stability and workability (high rolling ductility and high hole expandability), and have been intensively studied and found Said matter.

By the positive addition of Si, the work hardening ability of the ferrite iron can be improved to improve the rolling property, and the solid solution strengthening of the ferrite iron can be used to ensure the strength, and the hardness difference between the second phase and the second phase can be used to enhance the reaming. Sex. In addition, the use of the toughened ferrite iron and the bead iron can alleviate the hardness difference between the soft ferrite iron and the hard hemp field, and the porosity can be improved. Moreover, if there are more hard ramification loose irons in the final structure, a large hardness difference will occur at the heterojunction interface of the soft ferrite iron phase, resulting in a decrease in hole expandability, and thus will eventually become metamorphosed into 麻田. The untransformed Worth iron of the loose iron is fertilized by beads, and the structure of ferrite iron, tough ferrite iron, bead iron, and a small amount of granulated iron can be produced while maintaining high rolling ductility. The hole expandability is improved, and the material stability can be ensured by appropriately controlling the area ratio of each of the above phases.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

(1) A high-strength hot-dip galvanized steel sheet having excellent material stability and workability, and has a composition of C: 0.04% or more and 0.13% or less and Si: 0.7% or more and 2.3% or less. Mn: 0.8% or more and 2.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, and the balance of Fe and unavoidable impurities; According to the area ratio, there are 75% or more of the ferrite phase iron phase, 1.0% or more of the toughened ferrite iron phase, and 1.0% or more and 10.0% or less of the bead iron phase, and the area ratio of the Matian iron phase It is 1.0% or more and less than 5.0%, and satisfies the area ratio of the granulated iron / (the toughened ferrite iron area ratio + the bead iron area ratio) ≦ 0.6.

(2) The high-strength hot-dip galvanized steel sheet having excellent material stability and workability as described in (1), wherein the component composition further contains, by mass%, Cr: 0.05% or more and 1.0% or less. V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less of at least one element selected.

(3) The high-strength hot-dip galvanized steel sheet having excellent material stability and workability as described in (1) or (2), wherein the component composition further contains, by mass%, from Ti: 0.01% or more. 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less are at least one element selected.

(4) The high-strength hot-dip galvanized steel sheet excellent in stability and workability of the material according to any one of the items (1) to (3), wherein the component composition is contained in terms of % by mass. : 0.001% or more and 0.005% or less, and at least one element selected from REM: 0.001% or more and 0.005% or less.

(5) The high-strength hot-dip galvanized steel sheet excellent in stability and workability of the material according to any one of the items (1) to (4), wherein the component composition is further contained in terms of % by mass. : 0.001% or more and 0.010% or less, and at least one element selected from the group consisting of Sn: 0.002% or more and 0.2% or less.

(6) The high-strength hot-dip galvanized steel sheet having excellent material stability and workability as described in any one of (1) to (5), wherein the component composition contains Sb in terms of % by mass: 0.002% or more and 0.2% or less.

(7) A method for producing a high-strength hot-dip galvanized steel sheet having excellent material stability and workability, wherein the steel preform having the composition described in any one of (1) to (6) is subjected to hot rolling and pickling Or further cold rolling, and then heated to a temperature range of 650 ° C or higher according to an average heating rate of 5 ° C / s or more, and then maintained at a temperature of 750 to 900 ° C for 15 to 600 s, and cooled to a temperature of 450 to 550 ° C. After the domain, it is maintained at a temperature of 450 to 550 ° C for 10 to 200 s, followed by hot galvanizing.

(8) A method for producing a high-strength hot-dip galvanized steel sheet having excellent material stability and workability as described in (7), wherein after performing hot-dip galvanizing, it is further satisfied in a temperature range of 500 to 600 ° C The conditions of the formula are subjected to galvanizing alloying treatment.

0.45≦exp[200/(400-T)]×ln(t)≦1.0

among them,

T: average holding temperature in the temperature range of 500~600°C (°C)

t: 500~600 °C temperature domain holding time (s)

Exp(X) and ln(X) refer to the exponential function and the natural logarithm of X, respectively.

In addition, in this specification, all the "%" of the steel component are "mass %." In the present invention, the "high-strength hot-dip galvanized steel sheet" means a hot-dip galvanized steel sheet having a tensile strength TS of 540 MPa or more.

Further, in the present invention, a steel sheet which is subjected to zinc plating on a steel sheet by hot-dip galvanizing is collectively referred to as a "melted galvanized steel sheet" regardless of whether or not an alloying treatment is performed. In other words, the "hot-dip galvanized steel sheet" of the present invention covers both a hot-dip galvanized steel sheet which is not subjected to alloying treatment, and an alloyed hot-dip galvanized steel sheet which is subjected to alloying treatment.

According to the present invention, a high-strength hot-dip galvanized steel sheet having a tensile strength TS of 540 MPa or more and excellent workability and high material stability is obtained because of high rolling property and high hole expandability. By using the high-strength hot-dip galvanized steel sheet of the present invention as, for example, an automobile structural member, it is possible to achieve fuel efficiency improvement by weight reduction of the vehicle body, and the industrial use value is extremely large.

Hereinafter, the details of the present invention will be described.

It is generally known that the two-phase structure of soft ferrite iron and hard hemp iron is sufficient to ensure rolling ductility, but since the hardness difference between the ferrite iron and the granulated iron is large, sufficient hole expandability cannot be obtained. Here, the inventors further explored the use of the toughened ferrite iron and the bead iron, focusing on the ferrite iron and the toughened ferrite iron and the bead iron and the granulated iron (including the partially residual Worthfield). Iron Man), which is a detailed discussion of the possibility of improving the characteristics of composite organizations.

As a result, under the purpose of solid solution strengthening of ferrite iron and improvement of work hardening ability of ferrite iron, fermented iron and toughened ferrite iron and bead iron and a small amount of granulated iron were prepared by actively adding Si. The composite structure, which reduces the hardness difference between the heterophases, and the appropriate area fraction of the composite structure, can achieve both high rolling ductility and high hole expandability, as well as ensuring material stability.

The above is the technical feature of the present invention. Further, the present invention is characterized in that the component composition contains, by mass%, C: 0.04% or more and 0.13% or less, Si: 0.7% or more and 2.3% or less, Mn: 0.8% or more and 2.0% or less, and P: 0.1. % or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, and the balance is Fe and unavoidable impurities; wherein the steel structure has a ratio of area ratio of 75% or more of ferrite Phase, 1.0% or more of the toughened ferrite iron phase, and 1.0% or more and 10.0% or less of the bead iron phase, and the area ratio of the Matian iron phase is 1.0% or more and less than 5.0%, and satisfies Ma Tiansan Iron area ratio / (toughened ferrite grain area ratio + bead iron area ratio) ≦ 0.6.

(1) First, the composition of the components will be described.

C: 0.04% or more and 0.13% or less

The C-based Worthite iron-forming element is an indispensable element of steel reinforcement. If the amount of C is less than 0.04%, it is difficult to ensure the required strength. On the other hand, when the amount of C exceeds 0.13%, the weld portion and the heat-affected portion are hardly cured, and the mechanical properties of the welded portion are deteriorated, so that the spot weldability, the arc weldability, and the like are lowered. Therefore, C is set to be 0.04% or more and 0.13% or less.

Si: 0.7% or more and 2.3% or less

The Si system is a ferrite-forming element and is also an element effective for solid solution strengthening. However, in order to be able to utilize the work hardening ability of the ferrite grain iron phase to ensure good rolling ductility, it is necessary to add up to 0.7% or more. In addition, it is necessary to add up to 0.7% or more in order to secure the area ratio of the toughened ferrite grain iron phase and ensure good hole expandability. However, if Si is excessively added, surface properties such as red rust may be deteriorated, plating adhesion may be caused, and adhesion may be deteriorated. Therefore, the Si system is set to be 0.7% or more and 2.3% or less. It is preferably 1.2% or more and 1.8% or less.

Mn: 0.8% or more and 2.0% or less

An effective element for strengthening Mn steel. It is an element that stabilizes the Vostian Iron, and the adjustment of the fraction of the second phase is an essential element. Therefore, Mn must be added up to 0.8% or more. On the other hand, if more than 2.0% is added excessively, the area ratio of the granulated iron in the second phase will increase, making it difficult to ensure material stability. Moreover, in recent years, because of the high cost of Mn alloys, it is also related to the cost increase. Therefore, the Mn system is set to be 0.8% or more and 2.0% or less. It is preferably 1.0% or more and 1.8% or less.

P: 0.1% or less

When the effective element of the P-based steel reinforcement is excessively added in excess of 0.1%, embrittlement is caused by segregation at the grain boundary, and the impact resistance is deteriorated. Moreover, when it exceeds 0.1%, the alloying speed will be largely delayed. Therefore, the P system is set to be 0.1% or less.

S: 0.01% or less

S becomes an inclusion such as MnS, and causes deterioration of impact resistance and breakage of the metal flow along the welded portion. Therefore, it is preferable to reduce it as much as possible, and from the viewpoint of production cost, S is made 0.01% or less.

Al: 0.1% or less

When the Al system exceeds 0.1%, coarse Al ₂ O _{3 is formed} , which deteriorates the material. In addition, when Al is added to deoxidation of steel, if it is less than 0.01%, coarse oxides such as Mn and Si are mostly dispersed in steel, and the material is deteriorated. Therefore, it is preferable to set the addition amount to 0.01% or more. Therefore, the amount of Al is set to be 0.1% or less, preferably 0.01 to 0.1%.

N: 0.008% or less

The N system causes the most severe deterioration of the steel's aging resistance, and the less the better, if it exceeds 0.008%, the deterioration of the aging resistance tends to be conspicuous. Therefore, N is set to 0.008% or less.

The rest are Fe and unavoidable impurities. Among them, in addition to the elements, it is also possible to add at least one of the following elements as needed.

From Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less At least one of them is selected because Cr, V, and Mo have an effect of improving the balance between strength and rolling property, and thus may be added as needed. This effect is obtained by Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more. However, if excessive addition of Cr: 1.0%, V: 0.5%, and Mo: 0.5% is exceeded, the fraction of the second phase may become excessively large, and there is a concern that the strength is remarkably increased. Also, it will become a cause of cost increase. Therefore, when these elements are added, the amounts thereof are set to be Cr: 1.0% or less, V: 0.5% or less, and Mo: 0.5% or less.

The effective elements strengthened by Ni and Cu-based steels may be used for reinforcement of steel in the range specified by the present invention. Further, it has an effect of improving the adhesion of plating by promoting internal oxidation. In order to obtain these effects, it is necessary to achieve 0.05% or more. On the other hand, when both Ni and Cu are added in excess of 1.0%, the workability of the steel sheet is lowered. Also, it will become a cause of cost increase. Therefore, when Ni and Cu are added, the addition amount thereof is set to 0.05% or more and 1.0% or less, respectively.

It is effective to select at least one Ti and Nb-based steel for precipitation strengthening of Ti from 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less. It can be obtained by 0.01% or more, and it can be used for strengthening of steel if it is within the scope of the present invention. However, if it exceeds 0.1%, respectively, workability and shape freezeability will be lowered. Also, it will become a cause of cost increase. Therefore, when Ti and Nb are added, the amount of Ti is set to be 0.01% or more and 0.1% or less, and Nb is set to be 0.01% or more and 0.1% or less.

The B system has an effect of suppressing the formation/growth of the ferrite iron from the Worthite iron grain boundary, and thus can be added as needed. This effect is more than 0.0003% to be obtained. However, if it exceeds 0.0050%, workability will fall. It will also become a cause of cost increases. Therefore, when B is added, it is set to 0.0003% or more and 0.0050% or less.

At least one selected from the group consisting 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 are effective elements for spheroidizing the shape of sulfides and improving the adverse effects of sulfides on hole expandability. In order to obtain this effect, it must be 0.001% or more. However, excessive addition causes an increase in inclusions and the like, and causes surface and internal defects and the like. Therefore, when Ca and REM are added, the amount of addition is 0.001% or more and 0.005% or less, respectively.

When at least one type of Ta is selected from Ta: 0.001 to 0.010% and Sn: 0.002 to 0.2%, alloy carbides and alloyed nitrogen carbides are formed, which contribute not only to high strength but also to Some of them are dissolved in Nb carbides and Nb nitrogen carbides to form composite precipitates such as (Nb, Ta) (C, N), which significantly inhibits the coarsening of precipitates, and is judged to be capable of utilizing precipitation strengthening. The contribution to strength is stabilized. Therefore, when Ta is added, the content thereof is preferably set to 0.001% or more. However, when excessively added, not only the above-mentioned precipitate stabilization effect is saturated, but also the alloy cost is increased. Therefore, when Ta is added, the content thereof is preferably set to 0.010% or less.

The Sn system can be added from the viewpoint of suppressing nitriding, oxidation, or decarburization in the region of 10 μm from the surface layer of the steel sheet due to oxidation. By suppressing such nitriding and oxidation, it is possible to prevent the amount of granulated iron generated on the surface of the steel sheet from being reduced, and to improve fatigue characteristics and aging resistance. From the viewpoint of suppressing nitriding and oxidation, when Sn is added, the content thereof is preferably set to 0.002% or more, and if it exceeds 0.2%, the toughness is lowered. Therefore, it is preferable to set the content to 0.2% or less.

Sb: 0.002~0.2%

Sb is also the same as Sn, and can be added from the viewpoint of suppressing nitriding, oxidation, or decarburization in the region of 10 μm from the surface layer of the steel sheet due to oxidation. By suppressing such nitriding and oxidation, it is possible to prevent the amount of granulated iron generated on the surface of the steel sheet from being reduced, and to improve fatigue characteristics and aging resistance. From the viewpoint of suppressing nitriding and oxidation, when Sb is added, the content thereof is preferably set to 0.002% or more, and if it exceeds 0.2%, the toughness is lowered. Therefore, it is preferable to set the content to 0.2% or less.

(2) Next, the steel structure will be described.

Area ratio of ferrite iron phase: 75% or more

In order to ensure good rolling ductility of more than 75%, the ferrite-iron-dependent area ratio must be more than 75%.

Area ratio of ferro-tough ferrite iron phase: 1.0% or more

In order to ensure good hole expansion, it is possible to alleviate the hardness difference between the soft fat iron and the hard ram iron, and the area ratio of the tough ferrite iron phase must be 1.0% or more.

Area ratio of bead iron phase: 1.0% or more and 10.0% or less

In order to ensure good hole expandability, the area ratio of the bead iron phase is set to 1.0% or more. In order to ensure the required strength-rolling balance, the area ratio of the bead iron phase is set to be 10.0% or less.

Area ratio of granulated iron phase: 1.0% or more and less than 5.0%

In order to ensure the required strength-rolling balance, the area ratio of the granulated iron phase is set to 1.0% or more. In order to ensure good material stability, the area ratio of the granules of the granules, which has a large influence on the tensile properties (TS, EL), must be less than 5.0%.

Area ratio of granulated iron in the field / (toughened ferrite grain area ratio + bead iron area ratio) ≦ 0.6

In order to ensure good material stability, the phase structure of the second phase must reduce the amount of granulated iron in the material change factor, and use the granulated iron to increase the amount of soft tough ferrite iron and bead iron, which must be satisfied. The area ratio of the loose iron in Ma Tian / (the area ratio of the toughened fertiliser iron area + the area ratio of the bead iron) is 0.6.

In addition, in addition to the ferrite iron, the tough ferrite iron, the bead iron, and the Ma Tian loose iron, there are cases in which carbides such as residual Worthite iron, tempered Matian loose iron, and carbonized iron are formed, but The object of the present invention can be attained by satisfying the above area ratio of ferrite iron, tough ferrite iron, bead iron, and granulated iron.

In addition, in the present invention, the "area ratio of ferrite iron, tough ferrite iron, bead iron, and granulated iron" refers to the ratio of the area of each phase in the observed area.

The high-strength hot-dip galvanized steel sheet according to the present invention is characterized in that the steel sheet having the above-described composition and the steel structure is used as a base steel sheet, and a plating film formed by hot-dip galvanizing or an alloy after performing hot-dip galvanizing is provided thereon. The treated plating film.

(3) Next, the manufacturing conditions will be described.

The high-strength hot-dip galvanized steel sheet according to the present invention is a steel preform having a composition suitable for the composition range of the above composition, subjected to hot rolling, pickling, or cold rolling, and then heated at an average heating rate of 5 ° C / s or more. To the temperature range above 650 °C, and maintained in the temperature range of 750~900 °C for 15~600s, then cooled to the temperature range of 450~550 °C, and maintained in the temperature range of 450~550 °C for 10~200s, and then melted. It can be made by galvanizing.

When a high-strength hot-dip galvanized steel sheet subjected to alloying treatment is produced, after hot-dip galvanizing, an alloying treatment of galvanizing is performed in a temperature range of 500 to 600 ° C in accordance with the following formula.

0.45≦exp[200/(400-T)]×ln(t)≦1.0

among them,

T: average holding temperature in the temperature range of 500~600°C (°C)

t: 500~600 °C temperature domain holding time (s)

Exp(X) and ln(X) refer to the exponential function and the natural logarithm of X, respectively.

The details will be described below.

The steel having the above-described composition is melted by a known method, and then formed into a flat blank by block or continuous casting, and hot rolled to form a hot rolled sheet. When hot rolling is performed, it is preferred to heat the slab to 1100 to 1300 ° C, and to set the final finishing temperature to 850 ° C or higher and perform hot rolling, and to take up the steel strip at 400 to 650 ° C. When the coiling temperature exceeds 650 ° C, the carbides in the hot-rolled sheet are coarsened because the coarsened carbides are not completely melted during the soaking during annealing, and thus the necessary strength may not be obtained. . Then, pickling treatment is carried out according to a known method. Or, after pickling, further cold rolling is performed. When the cold rolling is performed, the conditions are not particularly limited, but it is preferable to carry out cold rolling at a cold rolling reduction ratio of 30% or more. If the cold rolling reduction rate is low, the recrystallization of the ferrite iron is not promoted, and there is a possibility that the unrecrystallized ferrite remains, resulting in a decrease in rolling ductility and hole expandability.

The acid-washed hot-rolled sheet or the cold-rolled steel sheet is subjected to the following annealing, and then cooled and then subjected to hot-dip galvanizing.

Heating to a temperature range above 650 ° C at an average heating rate of 5 ° C / s or more

When the average heating rate in the temperature range above 650 ° C is less than 5 ° C / s, the fine and uniformly dispersed Wolster iron phase is not formed during annealing, resulting in an increase in the area ratio of the final structure of the field. It is difficult to ensure good hole expandability. Further, it is necessary to have a furnace which is longer than usual, resulting in an increase in cost and deterioration in production efficiency due to a large energy consumption. The heating furnace is preferably a DFF (Direct Fired Furnace). The reason is that the formation of the internal oxide layer by rapid heating by DFF prevents the oxides such as Si and Mn from being concentrated toward the outermost layer of the steel sheet, and ensures good plating properties.

15~600s in the temperature range of 750~900°C

In the temperature range of 750~900 °C, the specific phase is in the single phase of the Worthite iron, or in the two-phase domain of the Worthite iron and the ferrite iron, and the annealing is maintained for 15~600s. If the annealing temperature is less than 750 ° C and the holding time is less than 15 s, the hard carbon carbide in the steel sheet will not be sufficiently melted, resulting in a decrease in hole expandability, and the desired area ratio of the loose iron in the field cannot be obtained, thereby causing a decrease in the rolling ductility. . On the other hand, if the annealing temperature exceeds 900 ° C, the growth of the Worthite iron particles is obvious, and the tough ferrite iron formed by the toughening iron metamorphism generated during the maintenance after cooling is ensured to be difficult, resulting in a decrease in hole expandability. And because the area ratio of the granulated iron area / (the area ratio of the toughened ferrite iron area + the area ratio of the bead iron) will exceed 0.6, good material stability cannot be obtained. Moreover, if the holding time exceeds 600 s, the Worthite iron will be coarsened, and the required strength will be difficult to ensure, and the cost will increase due to the large energy consumption.

10~200s in the temperature range of 450~550°C

After the above annealing, it is cooled to a temperature range of 450 to 550 ° C, and then maintained at a temperature of 450 to 550 ° C for 10 to 200 s. If the temperature is maintained above 550 ° C or the holding time is less than 10 s, the toughened iron metamorphism is not promoted, and the area ratio of the toughened ferrite iron is less than 1.0%, resulting in failure to obtain the desired hole expandability. In addition, if the temperature is less than 450 ° C or the holding time exceeds 200 s, the majority of the second phase will become a Worstian iron and a toughening fertilizer having a large amount of solid solution carbon which is generated by the promotion of the toughening iron metamorphism. In the case of the granular iron, the area ratio of the bead iron of 1.0% or more is not obtained, and the area ratio of the loose iron phase of the hard hemp field becomes 5.0% or more, resulting in failure to obtain good hole expandability and material stability.

Then, the steel sheet is immersed in a plating bath of a normal bath temperature to perform hot-dip galvanization, and the amount of plating adhesion is adjusted by an air brush method or the like, and by cooling, a hot-dip galvanized steel sheet which is not alloyed with the plating layer is obtained. .

When the hot-dip galvanized steel sheet subjected to the alloying treatment is produced, after the hot-dip galvanizing is performed, the alloying treatment of galvanizing is performed in a temperature range of 500 to 600 ° C in accordance with the following formula.

0.45≦exp[200/(400-T)]×ln(t)≦1.0

among them,

T: average holding temperature in the temperature range of 500~600°C (°C)

t: 500~600 °C temperature domain holding time (s)

Exp(X) and ln(X) refer to the exponential function and the natural logarithm of X, respectively.

If exp[200/(400-T)]×ln(t) is less than 0.45, there will be more 麻田散铁 in the alloyed steel structure, resulting in the above-mentioned hard 麻田 loose iron adjacent to the soft fertilizer Iron causes a large difference in hardness between the opposite phases, resulting in a decrease in hole expandability. Moreover, since the area ratio of the granulated iron area/(the area ratio of the toughened ferrite iron to the area of the iron of the bead iron) exceeds 0.6, the material stability is impaired. Moreover, the adhesion of the hot-dip galvanized layer is deteriorated.

If exp[200/(400-T)]×ln(t) exceeds 1.0, the untransformed Worthite iron will almost become metamorphosed as carbon carbide or bead iron, resulting in failure to obtain the required strength and rolling ductility. balanced.

In the temperature range of less than 500 ° C, the alloying of the plating layer is not promoted, resulting in difficulty in obtaining an alloyed hot-dip galvanized steel sheet. Further, in the temperature range of more than 600 ° C, the second phase becomes almost all of the bead iron, resulting in failure to obtain the required area ratio of the granulated iron, resulting in a balanced decrease in strength and rolling property. The alloying of the relevant plating layer can be carried out without any problem in the temperature range of 500 to 600 ° C if it falls within the scope of the present invention satisfying the above exp [200/(400-T)] × ln(t) conditions. .

Further, in the series of heat treatments of the production method of the present invention, if the temperature is within the above temperature range, the temperature is not necessarily constant, and even if the cooling rate changes during cooling, it does not occur within the predetermined range. The subject matter of the present invention is impaired. Moreover, if the heat history can be satisfied, the steel plate may be subjected to heat treatment according to any equipment. In addition, it is also within the scope of the present invention to perform the shape correction after the heat treatment to perform the temper rolling of the steel sheet of the present invention. Further, in the present invention, it is assumed that the steel material is produced by various steps such as ordinary steel making, casting, and hot rolling, but it may be manufactured by, for example, thin-plate casting or the like, and some or all of the hot rolling steps are omitted.

Figs. 1 and 2 show Nos. 15, 16, 17 (Table 2, Table 5) of the steel A of the present invention, and No. 18, 19, and 20 belonging to the steel of Comparative Example H (Tables). 2. Table 5), the relationship between TS, EL, and annealing temperature (T ₁ ). As is apparent from Fig. 1 and Fig. 2, in the steel of the present invention, the TS and EL derived from the change in the annealing temperature were small, and the steels of the comparative example H and the EL were greatly changed.

Further, the examples described later in Figs. 3 and 4 belong to No. 21, 22, and 23 (Table 2, Table 5) of the steel A of the present invention, and No. 24, 25, and 26 belonging to the steel of Comparative Example H. (Table 2, Table 5), a relationship diagram between the finishing of TS, EL, and the cooling average holding temperature (T ₂ ) after annealing. 3 and 4, in the example steel of the present invention, the TS and EL which are derived from the change in the average holding temperature are small, and in contrast, the steels of the comparative example H are TS and EL vary greatly.

[Examples]

The steel composed of the components having the composition shown in Table 1, and the balance of Fe and unavoidable impurities was melted in a converter, and a flat embryo was formed by a continuous casting method. The obtained spheroids were heated to 1200 ° C, and then hot rolled at a finishing temperature of 870 to 920 ° C until the thickness of the plate was 3.2 mm, and then coiled at 520 ° C. Next, the obtained hot-rolled sheet was subjected to pickling, a part of which was formed into a hot-rolled steel sheet which was maintained in a pickled state, and a part of which was further subjected to cold rolling to produce a cold-rolled steel sheet. Next, the hot-rolled steel sheet (after pickling) and the cold-rolled steel sheet obtained by the above are subjected to annealing treatment by a continuous hot-dip galvanizing production line according to the manufacturing conditions shown in Tables 2 to 4, and subjected to hot-dip galvanizing treatment, and further performed. A molten galvanized steel sheet is obtained by alloying the plating layer. The amount of plating adhesion is 30 to 50 g/m ² per one side. A part of the hot-dip galvanized steel sheet which was not subjected to alloying treatment after being subjected to hot-dip galvanizing treatment.

For the obtained galvanized steel sheet, the area ratio of the ferrite iron, the tough ferrite iron, the bead iron, and the granitic iron phase is ground, and the parallel plate thickness profile is ground toward the rolling direction of the steel sheet, and then Corrosion was performed on 3% Nital, and then 10 fields of view were observed using a SEM (Scanning Electron Microscope) at a magnification of 2000 times, and were obtained using Image-Pro of Media Cybernetics. At this time, since the distinction between the Ma Tian loose iron and the residual Worth iron is difficult, the obtained hot-dip galvanized steel sheet is subjected to tempering treatment at 200 ° C for 2 hours, and then, in the rolling direction of the steel sheet, it is observed in parallel according to the above method. The structure of the plate thickness profile, and the area ratio of the tempered iron phase of the tempered granules obtained by the above method is regarded as the "area ratio of the granulated iron phase of the granules". Further, the volume fraction of the iron phase remaining in the Vostian was obtained by polishing the steel sheet to a quarter surface in the thickness direction and then obtaining the diffraction X-ray intensity of the 1/4 surface of the sheet. The incident X-ray system uses CoKα rays, and {111}, {200}, {220}, {311} faces of the residual Worthfield iron phase, {110}, {200}, {211 with the ferrite grain iron phase. } The peak intensity of the face is all combined, and the intensity ratio is obtained, and the average value is regarded as the "volume rate of the residual Worth iron phase".

Further, in the tensile test, a JIS No. 5 test piece in which the sample was taken at a right angle to the rolling direction of the steel sheet was used, and the TS (tensile strength) and EL (total elongation) were measured in accordance with JIS Z 2241. ). Further, in the present invention, the case where TS × EL ≧ 19000 MPa ‧ % was judged to be good in the rolling property.

The material stability is determined by (1) the conditions other than the annealing temperature T ₁ are the same, and only the steel sheets having different annealing temperatures T ₁ are investigated, and the fluctuation amounts of TS and EL are investigated, and the fluctuation amounts of the TS and EL are obtained. The variation of the annealing temperature change per 20 ° C (ΔTS, ΔEL), and (2) the conditions other than the average holding temperature T ₂ until the plating bath immersion after cooling are the same, only until the plating bath is immersed after cooling. When the steel sheets having different temperatures T _{2 are} held in an average, the fluctuation amounts of TS and EL are investigated, and the fluctuation amount of the average holding temperature change until the plating bath is immersed after cooling is measured from the fluctuation amount of TS and EL (ΔTS, ΔEL), and the TS variation amount (ΔTS) and the EL variation amount (ΔEL) per 20 ° C were evaluated for each temperature change.

Further, the hole-expanding property (stretch flangeability) of the hot-dip galvanized steel sheet obtained as described above was measured. The hole expandability (stretch flangeability) is carried out in accordance with the Japan Iron and Steel Federation specification JFST1001. After the obtained steel sheets were cut into 100 mm × 100 mm, the thickness of the plate was 2.0 mm or more and the gap was 12% ± 1%, and the thickness of the plate was less than 2.0 mm. The gap was 12% ± 2%, and the punching diameter was 10 mm. Then, using a mold with an inner diameter of 75 mm, a 60° conical punch is pressed into the hole while the pressing force of 9 ton is pressed, and the diameter of the hole at which the crack occurs is determined, and the limit is obtained from the following formula. The hole expansion ratio λ (%) was evaluated from the value of the ultimate hole expansion ratio.

Ultimate hole expansion ratio λ(%)={(D _f -D ₀ )/D ₀ }×100

Among them, D _f is a pore diameter (mm) at which cracking occurs, and D ₀ is an initial pore diameter (mm).

Further, in the present invention, the case of λ ≧ 70 (%) is judged to be good.

According to the results obtained above, as shown in Table 5 to Table 7.

The high-strength hot-dip galvanized steel sheet according to the present invention has a TS of 540 MPa or more, a λ of 70% or more, and an excellent hole expandability, and TS × EL ≧ 19000 MPa ‧ %, and the balance between strength and rolling property is high, and it is known that it is processability. Excellent high strength hot-dip galvanized steel sheet. Further, the values of ΔTS and ΔEL were also small, and it was found that the high-strength hot-dip galvanized steel sheet having excellent material stability was obtained. On the other hand, in the comparative example, either one of the rolling property and the hole expandability was inferior, and the material stability was poor.

(industrial availability)

The high-strength hot-dip galvanized steel sheet according to the present invention has a tensile strength TS of 540 MPa or more, and has high rolling property and high hole expandability, and is excellent in material stability. The high-strength hot-dip galvanized steel sheet according to the present invention can be used for, for example, an automobile structural member, and the fuel efficiency can be improved by the weight reduction of the vehicle body, and the industrial use value is extremely large.

Figure 1 is a graph showing the relationship between annealing temperature (T ₁ ) and TS.

Figure 2 is a graph showing the relationship between annealing temperature (T ₁ ) and EL.

Figure 3 is a graph showing the relationship between the cooling average holding temperature (T ₂ ) and TS.

Figure 4 is a graph showing the relationship between the cooling average holding temperature (T ₂ ) and EL.

Claims (7)

A high-strength hot-dip galvanized steel sheet having excellent material stability and workability, wherein the component composition is contained in an amount of C: 0.04% or more and 0.13% or less, and Si: 0.7% or more and 2.3% or less, and Mn. : 0.8% or more and 2.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, and the balance of Fe and unavoidable impurities; steel structure by area ratio It has a ferrite phase iron phase of 75% or more, a tough ferrite iron phase of 1.0% or more, and a bead iron phase of 1.0% or more and 10.0% or less, and the area ratio of the Matian iron phase is 1.0% or more. Less than 5.0%, and meet the area ratio of Ma Tian loose iron / (toughened ferrite grain area ratio + bead iron area ratio) ≦ 0.6. The high-strength hot-dip galvanized steel sheet having excellent material stability and workability according to the first aspect of the patent application, wherein the component composition further contains, by mass%, 0.05% or more and 1.0% or less, and V: 0.005% or more. Further, at least one element selected from the group consisting of 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less. The high-strength hot-dip galvanized steel sheet having excellent material stability and workability according to the first or second aspect of the patent application, wherein the component composition further contains, by mass%, from 0.01% to 0.1%, and Nb: 0.01. At least one element selected from the group consisting of % or more and 0.1% or less and B: 0.0003% or more and 0.0050% or less. Excellent material stability and processability as claimed in item 1 or 2 of the patent application In the high-strength hot-dip galvanized steel sheet, the component composition further contains at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. A high-strength hot-dip galvanized steel sheet having excellent material stability and workability according to the first or second aspect of the patent application, wherein the component composition is further contained in a range of Ta: 0.001% or more and 0.010% or less, and Sn: 0.002. At least one element selected from the group consisting of % or more and 0.2% or less. The high-strength hot-dip galvanized steel sheet having excellent material stability and workability according to the first or second aspect of the patent application, wherein the component composition further contains Sb: 0.002% or more and 0.2% or less by mass%. A method for producing a high-strength hot-dip galvanized steel sheet having excellent material stability and workability, characterized in that a steel preform having the composition of any one of claims 1 to 6 is subjected to hot rolling, pickling, Or further cold rolling, and then heated to a temperature range of 650 ° C or higher according to an average heating rate of 5 ° C / s or more, and then maintained at 15 to 600 s in a temperature range of 750 to 900 ° C, and cooled to 450 to 550 ° C. After the temperature range, it is maintained at a temperature of 450 to 550 ° C for 10 to 200 s, and then subjected to hot-dip galvanizing, and in the temperature range of 500 to 600 ° C, the alloying treatment of galvanizing is carried out according to the conditions satisfying the following formula. Thereby, the high-strength hot-dip galvanized steel sheet is produced; the steel structure of the high-strength hot-dip galvanized steel sheet has a ferrite phase iron phase of more than 75%, a ductile ferrite iron phase of 1.0% or more, and 1.0%. Above and below 10.0% of the bead iron phase, and the area ratio of the Mita iron phase is 1.0% or more and less than 5.0%, and satisfies the Ma Tian iron Area ratio / (toughened ferrite grain area ratio + bead iron area ratio) ≦ 0.6; 0.45 ≦ exp [200 / (400 - T)] × ln (t) ≦ 1.0 where T: 500 ~ 600 ° C temperature The average holding temperature (°C) in the domain t: the holding time (s) exp(X) and ln(X) of the temperature range of 500 to 600 °C refer to the exponential function and the natural logarithm of X, respectively.