PROCESS FOR PRODUCING A HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET WITH EXCELLENT MATERIAL STABILITY AND PROCESSABILITY

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to a high strength galvanized steel sheet, which is suitable for a member used in industrial fields of automobile, electricity, and the like and which has excellent formability and stability of mechanical properties, and a method for manufacturing the same.

2. Description of the Related Art

[0002] In recent years, enhancement of fuel economy of the automobile has become an important issue from the viewpoint of global environmental conservation. Consequently, there is an active movement afoot to reduce the thickness through increases in strength of car body materials, so as to reduce the weight of a car body itself.

[0003] However, an increase in strength of a steel sheet causes reduction in elongation, that is, reduction in formability. Therefore, development of materials having high strength and high formability in combination has been desired under the present circumstances.

[0004] Furthermore, in forming of the high strength steel sheet into a complicated shape, e.g., an automobile component, occurrences of cracking and necking in a punch stretch portion or a stretch flange portion cause large issues. Therefore, a high strength steel sheet which can overcome the issues on occurrences of cracking and necking and which has high elongation and high stretch flangeability in combination has also been required.

[0005] Moreover, the shape fixability is degraded by an increase in strength and thickness reduction of a steel sheet significantly. In order to cope with this, in press forming, it has been widely performed that changes in shape after release from a mold is predicted and the mold is designed in expectation of the amount of change in shape. However, if the tensile strength (TS) of a steel sheet is changed, deviation from the expected amount, in which these are assumed to be constant, becomes large and odd shapes occur. Consequently, rework, e.g., sheet-metal working of the shape on a one-by-one basis, becomes necessary after press-forming, and the efficiency in mass production is degraded significantly. Therefore, it is required that variations in TS of the steel sheet are minimized.

[0006] Regarding an improvement of formability of the high strength steel sheet, heretofore, various multi phase high strength galvanized steel sheets, e.g., a ferrite-martensite dual-phase steel and a TRIP steel taking the advantage of the transformation induced plasticity of retained austenite, have been developed.

[0007] For example, Japanese Unexamined Patent Application Publication No. 2001-140022 has proposed a steel sheet having excellent elongation by specifying the chemical components and specifying the volume ratios of retained austenite and martensite and methods for manufacturing the same. Moreover, Japanese Unexamined Patent Application Publication No. 04-026744 has proposed a steel sheet having excellent elongation by specifying the chemical components and, furthermore, specifying a special method for manufacturing the same. Japanese Unexamined Patent Application Publication No. 2007-182625 has proposed a steel sheet having excellent elongation by specifying the chemical components and specifying the volume ratios of ferrite, bainitic ferrite, and retained austenite phases. In addition, Japanese Unexamined Patent Application Publication No. 2000-212684 has proposed a method for manufacturing a high strength cold rolled steel sheet in which variations in elongation in the sheet width direction have been improved. Further, CA 2714117 A1 discloses a method for manufacturing a high strength galvanized steel sheet with excellent formability and a martensite content of 5-40%.

CIATION LIST

PATENT LITERATURE

[0008] 

[PTL 1] Japanese Unexamined Patent Application Publication No. 2001-140022

[PTL 2] Japanese Unexamined Patent Application Publication No. 04-026744

[PTL 3] Japanese Unexamined Patent Application Publication No. 2007-182625

[PTL 4] Japanese Unexamined Patent Application Publication No. 2000-212684

SUMMARY OF THE INVENTION

[0009] However, in Japanese Unexamined Patent Application Publication Nos. 2001-140022, 04-026744, and 2007-182625, an improvement in elongation of the high strength thin steel sheet is the main purpose. Therefore, the stretch flangeability is not taker, into consideration. In Japanese Unexamined Patent Application Publication No. 2000-212684, only variations in the total elongation EL in the sheet width direction are described, and variations in mechanical properties due to the component composition and the production condition are not taken into consideration. Consequently, development of a high strength galvanized steel sheet having high elongation and high stretch flangeability in combination and, in addition, having excellent stability of mechanical properties becomes an issue.

[0010] In consideration of the above-described circumstances, it is an object of the present invention to provide a method for manufacturing a high strength galvanized steel sheet having high tensile strength TS of 540 MPa or more and having excellent stability of mechanical properties and formability (high elongation and high stretch flangeability).

[0011] The present inventors performed intensive research to obtain a high strength galvanized steel sheet having high tensile strength TS of 540 MPa or more and, in addition, having excellent stability of mechanical properties and formability (high elongation and high stretch flangeability) and found the following.

[0012] By virtue of intentional addition of Si, an improvement of elongation due to an improvement of a work hardening property of ferrite, ensuring of strength due to solution hardening of ferrite, and an improvement of stretch flangeability due to relaxation of hardness difference from a secondary phase became possible. Furthermore, by making the most of bainitic ferrite and pearlite, the hardness difference between soft ferrite and hard martensite was able to be relaxed and the stretch flangeability was able to be improved. Moreover, if much hard martensite was present in a final microstructure, a large hardness difference occurred at a different phase interface of the soft ferrite phase, so that the stretch flangeability was degraded. Then, untransformed austenite, which was transformed to martensite finally, was converted to pearlite, and a microstructure including ferrite, bainitic ferrite, pearlite, a small amount of martensite was formed and, thereby, the stretch flangeability was able to be improved while high elongation was maintained. In addition, the area ratio of each of the above-described phases was controlled appropriately and, thereby, the stability of mechanical properties was able to be ensured.

[0013] The present invention has been made on the basis of the above-described findings and is described in the appended claim.

[0014] In this regard, in the present specification, every % indicating a component of a steel is on a percent by mass basis. Furthermore, in the present invention, "high strength galvanized steel sheet" refers to a galvanized steel sheet having a tensile strength TS of 540 MPa or more.

[0015] Moreover, in the present invention, regardless of whether an alloying treatment is performed or not, steel sheets in which a zinc coating is applied to a steel sheet by galvanization are generically called galvanized steel sheets. However, the galvanized steel sheets in the present invention are subjected to an alloying treatment.

[0016] According to the present invention, a high strength galvanized steel sheet, which has a tensile strength TS of 540 MPa or more, which has excellent formability because of high elongation and high stretch flangeability and, furthermore, which has excellent stability of mechanical properties, is obtained. In the case where the high strength galvanized steel sheet according to the present invention is applied to, for example, an automobile structural member, enhancement of fuel economy due to weight reduction of a car body can be facilitated. Therefore, an industrial utility value is very large.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] 

Fig. 1 is a diagram showing the relationship between the annealing temperature (T₁) and TS;

Fig. 2 is a diagram showing the relationship between the annealing temperature (T₁) and EL;

Fig. 3 is a diagram showing the relationship between the cooling average keeping temperature (T₂) and TS; and

Fig. 4 is a diagram showing the relationship between the cooling average keeping temperature (T₂) and EL.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present invention will be described below in detail.

[0019] In general, regarding a two-phase structure of soft ferrite and hard martensite, it is known that although the elongation can be ensured, a sufficient stretch flangeability is not obtained because the hardness difference between ferrite and martensite is large. Then, the present inventors further performed research on utilization of bainitic ferrite and pearlite, and performed detailed research taking note of the possibility of improvement in characteristics of multi phases including ferrite, bainitic ferrite, pearlite, and martensite (a part of the multi phases include retained austenite).

[0020] As a result, Si was added intentionally for the purpose of solution hardening of ferrite and an improvement of a work hardening property of ferrite, a microstructure including ferrite, bainitic ferrite, pearlite, a small amount of martensite was formed, a hardness difference between different phases was reduced, and furthermore, the area ratios of the multi phases were optimized, so that it was made possible to ensure the compatibility between high elongation and high stretch flangeability and ensure the stability of mechanical properties.

[0021] The present invention has been completed on the basis of the above-described technical features. Then, the present invention is characterized in that a component composition contains 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, P: 0.1% or less, S: 0.01% or less, A1: 0.1% or less, N: 0.008% or less, and the remainder composed of Fe and incidental impurities on a percent by mass basis, wherein a steel microstructure includes 75% or more of ferrite phase, 1.0% or more of bainitic ferrite phase, and 1.0% or more, and 10.0% or less of pearlite phase on an area ratio basis, the area ratio of martensitic phase is 1.0% or more, and less than 5.0%, and the area ratio of martensitic phase/(area ratio of bainitic ferrite phase + area ratio of pearlite phase) ≤ 0.6 is satisfied.

(1) Initially, the component composition will be described.

C: 0.04% or more, and 0.13% or less

[0022] Carbon is an austenite forming element and is an element indispensable for strengthening a steel. If the amount of C is less than 0.04%, it is difficult to ensure desired strength. On the other hand, if the amount of C exceeds 0.13% and, therefore, addition is excessive, a welded zone and a heat-affected zone are hardened significantly, and the mechanical characteristics of the welded zone are degraded, so that the spot weldability, the arc weldability, and the like are degraded. Therefore, C is specified to be 0.04% or more, and 0.13% or less.

Si: 0.7% or more, and 2.3% or less

[0023] Silicon is a ferrite forming element and is also an element effective in solution hardening. In this regard, 0.7% or more of addition is necessary to ensure good elongation due to an improvement in work hardening property of the ferrite phase. Furthermore, 0.7% or more of addition is also necessary to ensure a desired area ratio of bainitic ferrite phase and ensure good stretch flangeability. However, excessive addition of Si causes degradation of surface quality due to an occurrence of red scale and the like and degradation of deposition and adhesion of the coating. Therefore, Si is specified to be 0.7% or more, and 2.3% or less, and preferably 1.2% or more, and 1.8% or less.

Mn: 0.8% or more, and 2.0% or less

[0024] Manganese is an element effective in strengthening a steel. Furthermore, Mn is an element to stabilize austenite and an element necessary for adjusting the ratio of a secondary phase. For this purpose, addition of 0.8% or more of Mn is necessary. On the other hand, if addition is excessive and exceeds 2.0%, the area ratio of martensitic phase in the secondary phase increases and it becomes difficult to ensure the stability of mechanical properties. Moreover, an increase in cost is brought about because an alloy cost of Mn has increased in recent years. Therefore, Mn is specified to be 0.8% or more, and 2.0% or less, and preferably 1.0% or more, and 1.8% or less.

P: 0.1% or less

[0025] Phosphorus is an element effective in strengthening a steel. However, if addition is excessive and exceeds 0.1%, embrittlement is caused by grain boundary segregation, and the crashworthiness is degraded. Furthermore, if 0.1% is exceeded, an alloying speed is reduced significantly. Therefore, P is specified to be 0.1% or less.

S: 0.01% or less

[0026] Sulfur forms inclusions, e.g., MnS, to cause degradation in crashworthiness and cracking along a metal flow of a welded zone and, therefore, is minimized, although S is specified to be 0.01% or less from the viewpoint of production cost.

A1: 0.1% or less

[0027] If Al exceeds 0.1%, coarse Al₂O₃ is generated and the mechanical properties are degraded. In the case where Al is added for deoxidation of a steel, it is preferable that the amount of addition is specified to be 0.01% or more because if the amount is less than 0.01%, a large number of coarse oxides of Mn, Si, and the like are dispersed in the steel to degrade the mechanical properties. Therefore, the amount of Al is specified to be 0.1% or less, and preferably 0.01% to 0.1%.

N: 0.008% or less

[0028] Nitrogen is an element which degrades the aging resistance of a steel to a greatest extent and preferably is minimized. If 0.008% is exceeded, degradation of the aging resistance becomes significant. Therefore, N is specified to be 0.008% or less.

[0029] The remainder is composed of Fe and incidental impurities. However, besides these elements, at least one type selected from the following elements can be added, as necessary.

At least one type selected 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

[0030] Chromium, vanadium, and molybdenum have a function of improving the balance between the strength and the elongation and, therefore, can be added as necessary. The effect thereof is obtained when Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more are employed. However, if Cr, V, and Mo are added in such a way as to exceed Cr: 1.0%, V: 0.5%, and Mo: 0.5%, respectively, the secondary phase ratio becomes too large, and a significant increase in strength and the like may occur. Furthermore, an increase in cost is brought about. Therefore, in the case where these elements are added, the individual amounts thereof are specified to be Cr: 1.0% or less, V: 0.5% or less, and Mo: 0.5% or less.

[0031] Nickel and copper are elements effective in strengthening a steel and there is no problem in use for strengthening the steel within the bounds of the specification of the present invention. Furthermore, there is a function of facilitating internal oxidation so as to improve adhesion of the coating. In order to obtain these effects, it is necessary that each of Ni and Cu is 0.05% or more. On the other hand, if both Ni and Cu, each exceeding 1.0%, are added, the formability of the steel sheet is degraded. Moreover, an increase in cost is brought about. Therefore, in the case where Ni and Cu are added, the amount of addition of each of them is specified to be 0.05% or more, and 1.0% or less.

At least one type selected from Ti: 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

[0032] Titanium and niobium are effective in precipitation hardening of a steel. The effect is obtained when each of them is 0.01% or more and, therefore, there is no problem in use for strengthening the steel within the bounds of the specification of the present invention. However, if each of them exceeds 0.1%, the formability and the shape fixability are degraded. Furthermore, an increase in cost is brought about. Therefore, in the case where Ti and Nb are added, the amount of addition of Ti is specified to be 0.01% or more, and 0.1% or less and Nb is specified to be 0.01% or more, and 0.1% or less.

[0033] Boron has a function of suppressing generation and growth of ferrite from austenite grain boundaries and, therefore, can be added as necessary. The effect is obtained when B is 0.0003% or more. However, if 0.0050% is exceeded, the formability is degraded. Furthermore, an increase in cost is brought about. Therefore, in the case where B is added, B is specified to be 0.0003% or more, and 0.0050% or less.

At least one type selected from Ca: 0.001% or more, and 0.005% or less and REM: 0.001% or more, and 0.005% or less

[0034] Calcium and REM are elements effective in spheroidizing the shape of a sulfide to improve an adverse influence of the sulfide on the stretch flangeability. In order to obtain this effect, it is necessary that each of Ca and REM is 0.001% or more. However, excessive addition causes increases in inclusions and the like so as to cause surface and internal defects. Therefore, in the case where Ca and REM are added, the amounts of addition of each of them is specified to be 0.001% or more, and 0.005% or less.

At least one type selected from Ta: 0.001% to 0.010% and Sn: 0.002% to 0.2%

[0035] It is believed that tantalum has effects of not only contributing to an increase in strength by forming alloy carbides and alloy carbonitrides, but also stabilizing contribution of precipitation hardening to the strength by partially making solid solution with Nb carbide and Nb carbonitride to form complex precipitates, e.g., (Nb,Ta)(C,N), and thereby, suppress coarsening of precipitates significantly in the same manner as Ti and Nb. Consequently, in the case where Ta is added, it is desirable that the content thereof is specified to be 0.001% or more. However, if addition is excessive, not only the above-described precipitation stabilizing effect is saturated, but also an alloy cost increases. Therefore, in the case where Ta is added, it is desirable that the content thereof is specified to be 0.010% or less.

[0036] Tin can be added from the viewpoint of suppressing nitriding and oxidation of a steel sheet surface or decarbonization of several ten micrometers of region of a steel sheet surface layer generated through oxidation. Suppression of such nitriding and oxidation prevents reduction in the amount of generation of martensite on the steel sheet surface and improves the fatigue resistance and the aging resistance. From the viewpoint of suppression of nitriding and oxidation, in the case where Sn is added, it is desirable that the content thereof is specified to be 0.002% or more, and it is desirable that the content thereof is specified to be 0.2% or less because if 0.2% is exceeded, reduction in toughness is brought about.

Sb: 0.002% to 0.2%

[0037] In the same manner as Sn, Sb can be added from the viewpoint of suppressing nitriding and oxidation of a steel sheet surface or decarbonization of several ten micrometers of region of a steel sheet surface layer generated through oxidation. Suppression of such nitriding and oxidation prevents reduction in the amount of generation of martensite on the steel sheet surface and improves the fatigue resistance and the aging resistance. From the viewpoint of suppression of nitriding and oxidation, in the case where Sb is added, it is desirable that the content thereof is specified to be 0.002% or more, and it is desirable that the content thereof is specified to be 0.2% or less because if 0.2% is exceeded, reduction in toughness is brought about.

[0038] (2) Next, a steel microstructure will be described.

Area ratio of ferrite phase: 75% or more

[0039] In order to ensure good elongation, it is necessary that a ferrite phase is 75% or more on an area ratio basis.

Area ratio of bainitic ferrite phase: 1.0% or more

[0040] In order to ensure good stretch flangeability, that is, in order to relax a hardness difference between the soft ferrite and the hard martensite, it is necessary that the area ratio of bainitic ferrite phase is 1.0% or more.

Area ratio of pearlite phase: 1.0% or more, and less than 10.0%

[0041] In order to ensure good stretch flangeability, the area ratio of pearlite phase is specified to be 1.0% or more. In order to ensure desired balance between the strength and the elongation, the area ratio of pearlite phase is specified to be 10.0% or less.

Area ratio of martensitic phase: 1.0% or more, and less than 5.0%

[0042] In order to ensure desired balance between the strength and the elongation, the area ratio of martensitic phase is specified to be 1.0% or more. In order to ensure good stability of mechanical properties, it is necessary that the area ratio of martensitic phase having a large influence on the tensile characteristics (TS, EL) is specified to be 5.0% or less.

Area ratio of martensitic phase/(area ratio of bainitic ferrite phase + area ratio of pearlite phase) ≤ 0.6

[0043] In order to ensure good stability of mechanical properties, it is necessary that regarding the phase configuration of the secondary phase, the amount of martensite, which causes variations in mechanical properties, is reduced and the amount of bainitic ferrite and pearlite softer than martensite are increased, that is, the area ration of martensitic phase/(area ration of bainitic ferrite phase + area ratio of pearlite phase) ≤ 0.6 is satisfied.

[0044] Meanwhile, retained austenite, tempered martensite, and carbides, e.g., cementite, may be generated besides ferrite, bainitic ferrite, pearlite, and martensite. However, the purpose of the present invention can be achieved insofar as the above-described area rations of ferrite, bainitic ferrite, pearlite, and martensitic phases are satisfied.

[0045] In this regard, the area ratios of ferrite, bainitic ferrite, pearlite, and martensitic phases refer to proportions of the areas of the individual phases constituting an observation area.

[0046] The high strength galvanized steel sheet produced according to the present invention includes the steel sheet having the above-described component composition and the above-described steel microstructure and serving as a substrate steel sheet and a coating film subjected to an alloying treatment after
galvanization on the substrate steel sheet.

[0047] (3) Next, production conditions will be described.

[0048] The high strength galvanized steel sheet according to the present disclosure is produced by subjecting a steel slab having the component composition conforming to the above-described component composition range to hot rolling and pickling, or hot rolling, pickling, and cold rolling, performing heating to a temperature range of 650°C or higher at an average heating range of 5°C/s or more, followed by keeping in a temperature range of 750°C to 900°C for 15 to 600s, performing cooling to a temperature range of 450°C to 550°C, followed by keeping in the temperature range of 450°C To 550°C for 10 to 200s, and performing galvanization.

[0049] After the galvanization is performed, the alloying treatment of zinc coating is performed in a temperature range of 500°C to 600°C under the condition satisfying the following formula,

where

T: average keeping temperature (°C) in a temperature range of 500°c to 600°C,

t: keeping time (s) in a temperature range of 500°C to 600°C, and

exp(X) and In(X) represent an exponential function and natural logarithm, respectively, of X.

Detailed explanation will be made below.

[0050] A steel having the above-described component Composition is melted, is made into a slab through roughing or continuous casting, and is made into a hat rolled sheet through hot rolling by a known method. In performing hot rolling, it is preferable that the slab is heated to 1,100°C to 1,300°C, hot rolling is performed at a final finishing temperature of 850°C or higher, and steel sheet is coiled at 400°C to 650°C. In the case where the coiling temperature exceeds 650°C, carbides in the hot-rolled sheet may become coarse, and required strength cannot be obtained in some cases because such coarse carbides are not melted completely during soaking in annealing. Subsequently, a pickling treatment is performed by a known method. Alternatively, after pickling is performed, cold rolling is further performed. In performing the cold rolling, the condition thereof is not necessarily specifically limited, although it is preferable that the cold rolling is performed under the cold reduction ratio of 30% or more. This is because if the cold reduction ratio is low, in some cases, recrystallization of ferrite is not facilitated, unrecrystallized ferrite remains, and the elongation and the stretch flangeability are degraded.

[0051] The pickled hot rolled sheet or the cold rolled steel sheet is subjected to annealing described below and, then, cooling and galvanization are performed.

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

[0052] If the average heating rate in heating to the temperature range of 650°C or higher is less than 5°C/s, a fine uniformly dispersed austenite phase is not generated during annealing, the area ratio of martensitic phase in the final microstructure increases and it is difficult to ensure good stretch flangeability. Furthermore, a furnace longer than a usual furnace is necessary and, thereby, an increase in cost associated with large energy consumption and reduction in production efficiency are brought about. It is preferable that a direct fired furnace (DFF) is used as a furnace. This is because an internal oxide layer is formed through rapid heating by the DFF and, thereby, concentration of oxides of Si, Mn, and the like on the outermost layer of the steel sheet is prevented so as to ensure good wettability of the coating.

Keeping in temperature range of 750°C to 900°C for 15 to 600 s

[0053] Annealing, which is keeping in a temperature range of 750°C to 900°C, specifically in a single phase region of austenite or in a two-phase region of austenite and ferrite, for 15 to 600 s is performed. In the case where the annealing temperature is lower than 750°C or the annealing time is less than 15 s, hard cementite in the steel sheet is not dissolved sufficiently, so that the stretch flangeability is degraded, and furthermore, a desired area ratio of martensitic phase is not obtained, so that the elongation is degraded. On the other hand, if the annealing temperature exceeds 900°C, austenite particles grow significantly, it becomes difficult to ensure bainitic ferrite due to bainite transformation which occurs in the keeping after cooling, so that the stretch flangeability is degraded. Moreover, the area ratio of martensitic phase/(area ratio of bainitic ferrite phase + area ratio of pearlite phase) exceeds 0.6, so that good stability of mechanical properties are not obtained. In addition, if the keeping time exceeds 600 s, austenite becomes coarse, it becomes difficult to ensure desired strength, and an increase in cost associated with large energy consumption may be brought about.

Keeping in temperature range of 450°C to 550°C for 10 to 200 s

[0054] After the above-described annealing is performed, cooling to a temperature range of 450°C to 550°C is performed, followed by keeping in the temperature range of 450°C to 550°C for 10 to 200 s. If the keeping temperature exceeds 550°C or the keeping time becomes less than 10 s, bainite transformation is not facilitated, and the area ratio of bainitic ferrite phase becomes less than 1.0, so that desired stretch flangeability is not obtained. If the keeping temperature becomes lower than 450°C or the keeping time exceeds 200 s, most of the secondary phase is converted to austenite and bainitic ferrite, which are generated through facilitation of bainite transformation and which contain large amounts of carbon in solid solution, so that a desired area ratio of perlite phase of 1.0% or more is not obtained. Furthermore, the area ratio of hard martensitic phase becomes 5.0% or more, so that good stretch flangeability and stability of mechanical properties are not obtained.

[0055] Thereafter, the steel sheet is dipped into a coating bath at a usual bath temperature so as to be galvanized, and the amount of deposition of coating is adjusted through gas wiping or the like, followed by cooling, so that a high strength galvanized steel sheet having a coating layer not subjected to alloying is obtained.

[0056] After the galvanization is performed, the alloying treatment of zinc coating is further performed in a temperature range of 500°C to 600°C under the condition satisfying the following formula,

where

T: average keeping temperature (°C) in a temperature range of 500°C to 600°C,

t: keeping time (s) in a temperature range of 500°C to 600°C, and

exp(X) and ln(X) represent an exponential function and natural logarithm, respectively, of X.

[0057] If exp[200/(400 - T)] × ln(t) is less than 0.45, much martensite is present in a steel microstructure after the alloying treatment, the above-described hard martensite adjoins hard ferrite to cause a large hardness difference between different phases, so that the stretch flangeability is degraded. Furthermore, the area ratio of martensitic phase/(area ratio of bainitic ferrite phase + area ratio of pearlite phase) exceeds 0.6 and, thereby, the stability of mechanical properties is impaired. Moreover, deposition of the galvanization layer is degraded.

[0058] If exp[200/(400 - T)] × ln(t) exceeds 1.0, most of untransformed austenite is transformed to cementite or pearlite and, as a result, desired balance between the strength and the elongation is not ensured.

[0059] In the temperature of lower than 500°C, alloying of the coating layer is not facilitated, and it is difficult to obtain a galvannealed steel sheet. Meanwhile, in the temperature range exceeding 600°C, most of the secondary phase is converted to pearlite, so that a desired area ratio of martensitic phase is not obtained and the balance between the strength and the elongation is reduced.

[0060] Alloying of the coating layer can be performed in the scope of the present invention, in which the temperature is in the range of 500°C to 600°C and the above-described condition of exp[200/(400 - T)] × ln(t) is satisfied, without problems.

[0061] By the way, regarding a series of heat treatments in the manufacturing method according to the present invention, the keeping temperature is not necessary constant insofar as the temperature is in the above-described range. Furthermore, even in the case where the cooling rate is changed during cooling, the present invention is not impaired insofar as the rate is in the specified range. Moreover, the steel sheet may be subjected to a heat treatment by any equipment insofar as only the heat history is satisfied. In this regard, in the present invention, it is assumed that a steel is produced through usual steps of steel making, casting, and hot rolling. However, for example, the steel may be produced through thin wall casting or the like, where a part of or whole hot rolling step is omitted.

[0062] Fig. 1 and Fig. 2 are diagrams showing the organized relationships between TS and the annealing temperature (T₁) and between EL and the annealing temperature (T₁) with respect to Nos. 15, 16, and 17 of Steel A, which are invention examples, (Table 2 and Table 5) and Nos. 18, 19, and 20 of Steel H, which are comparative examples, (Table 2 and Table 5) in Examples described later. As is clear from Fig. 1 and Fig. 2, regarding Steel A of the invention example, variations in TS and EL associated with changes in annealing temperature are small, whereas variations in TS and EL are large regarding Steel H of the comparative example.

[0063] Fig. 3 and Fig. 4 are diagrams showing the organized relationships between TS and the average keeping time (T₂) in cooling after annealing and between EL and the average keeping time (T₂) with respect to Nos. 21, 22, and 23 of Steel A, which are invention examples, (Table 2 and Table 5) and Nos. 24, 25, and 26 of Steel H, which are comparative examples, (Table 2 and Table 5) in Examples described later. As is clear from Fig. 3 and Fig. 4, regarding Steel A of the invention example, variations in TS and EL associated with changes in average keeping time are small, whereas variations in TS and EL are large regarding Steel H of the comparative example.

Examples

[0064] A steel having a component composition shown in Table 1, where the remainder was composed of Fe and incidental impurities, was melted with a converter, and a slab was produced by a continuous casting method. The resulting slab was heated to 1,200°C, hot rolling to a sheet thickness of 3.2 mm was performed at a finish temperature of 870°C to 920°C, and coiling was performed at 520°C. Subsequently, the resulting hot-rolled sheet was pickled. A part of the resulting hot-rolled sheets were served as pickled hot-rolled steel sheets, and a part of the hot-rolled sheets were subjected to cold rolling, so as to produce cold-rolled steel sheets. Then, the hot-rolled steel sheet (after pickling) and the cold-rolled steel sheet obtained as described above were subjected to an annealing treatment and a galvanizing treatment with a continuous galvanization line under the production condition shown in Tables 2 to 4. Furthermore, an alloying treatment of the plating layer was performed, so as to obtain a galvannealed steel sheet. The amount of deposition of coating was specified to be 30 to 50 g/m² on one surface basis. Regarding a part of steel sheets, galvanized steel sheets, which were not subjected to an alloying treatment after being galvanized, were also produced.

Table 2

No.	Steel type	With or without cold rolling	Heating temperature	Average heating rate	Annealing temperature T1	Annealing time	Cooling average keeping time T2	Cooling keeping time	Alloying treatment average keeping temperature T	Alloying treatment keeping time t	exp(200/(400-T))× ln(t)	Remarks
			°C	°C/s	°C	s	°C	s	°C	s
1	A	with	750	11	855	160	495	60	570	15	0.835	Invention example
2	B	with	740	11	855	160	495	60	570	15	0.835	Invention example
3	C	with	740	11	855	160	495	60	570	15	0.835	Invention example
4	D	with	750	11	855	160	495	60	570	15	0.835	Invention example
5	E	with	730	11	855	160	495	60	570	15	0.835	Invention example
6	F	with	760	11	855	160	495	60	570	15	0.835	Invention example
7	G	with	730	11	855	160	495	60	570	15	0.835	Invention example
8	H	with	720	9	830	160	495	60	520	15	0.511	Comparative example
9	I	with	730	9	830	160	495	60	520	15	0.511	Comparative example
10	J	with	740	9	830	160	495	60	520	15	0.511	Comparative example
11	K	with	750	9	830	160	495	60	520	15	0.511	Comparative example
12	L	with	740	9	830	160	495	60	520	15	0.511	Comparative example
13	M	with	730	9	830	160	495	60	520	15	0.511	Comparative example
14	N	with	740	9	830	160	495	60	520	15	0.511	Comparative example
15	A	with	750	11	850	160	495	60	570	15	0.835	Invention example
16	A	with	740	11	800	160	495	60	570	15	0.835	Invention example
17	A	with	740	11	750	160	495	60	570	15	0.835	Invention example
18	H	with	730	9	850	160	495	60	520	15	0.511	Comparative example
19	H	with	740	9	800	160	495	60	520	15	0.511	Comparative example
20	H	with	730	9	750	160	495	60	520	15	0.511	Comparative example
21	A	with	740	11	850	160	530	60	570	15	0.835	Invention example
22	A	with	750	11	850	160	500	60	570	15	0.835	Invention example
23	A	with	730	11	850	160	470	60	570	15	0.835	Invention example
24	H	with	720	9	830	160	530	60	520	15	0.511	Comparative example
25	H	with	750	9	830	160	500	60	520	15	0.511	Comparative example
26	H	with	740	9	830	160	470	60	520	15	0.511	Comparative example
27	A	with	760	15	860	120	500	40	580	10	0.758	Invention example
28	A	with	740	15	780	120	500	40	580	10	0.758	Invention example
29	A	with	680	10	840	280	530	100	555	25	0.886	Invention example
30	A	with	660	10	840	280	470	100	555	25	0.886	Invention example
31	A	with	730	13	840	180	480	120	-	-	-	Comparative example
32	A	with	710	13	780	160	480	120	-	-		Comparative example
33	O	with	750	13	850	165	520	60	565	15	0.806	Invention example
34	O	with	740	13	850	165	470	60	565	15	0.806	Invention example
35	O	with	730	2	800	160	495	50	545	15	0.682	Comparative example
36	O	with	720	12	650	180	500	60	555	15	0.745	Comparative example
37	O	with	730	14	935	230	485	65	570	15	0.835	Comparative example

Underlined portion: out of the scope of the present invention

Table 3

No.	Steel type	With or without cold rolling	Heating temperature	Average heating rate	Annealing temperature T1	Annealing time	Cooling average keeping time T2	Cooling keeping time	Alloying treatment average keeping temperature T	Alloying treatment keeping time t	exp(200/(400-T) In(t)	Remarks
			°C	°C/s	°C	s	°C	S	°C	s
38	P	with	750	14	860	180	490	55	575	12	0.792	Invention example
39	P	with	740	14	780	180	490	55	575	12	0.792	Invention example
40	P	with	750	15	830	850	505	55	580	12	0.818	Comparative example
41	P	with	760	13	840	5	495	45	570	12	0.766	Comparative example
42	P	with	740	12	860	160	600	45	570	12	0.766	Comparative example
43	P	with	720	16	810	170	130	60	560	12	0.712	Comparative example
44	Q	with	700	11	845	190	520	70	565	18	0.860	Invention example
45	Q	with	690	10	830	200	480	4	555	18	0.795	Comparative example
46	Q	with	700	12	845	180	510	410	560	18	0.828	Comparative example
47	Q	with	710	10	840	200	510	55	570	40	1.138	Comparative example
48	Q	with	680	11	845	170	520	60	510	6	0.291	Comparative example
49	Q	with	680	13	810	190	490	70	660	18	1.339	Comparative example
50	Q	with	670	11	820	210	485	65	470	18	0.166	Comparative example
51	R	with	700	10	860	230	495	90	555	22	0.851	Invention example
52	R	with	680	9	820	230	495	90	555	22	0.851	Invention example
53	R	with	680	10	790	220	495	85	555	22	0.851	Invention example
54	S	with	700	11	840	200	495	75	560	20	0.858	Invention example
55	T	with	740	15	840	100	500	45	575	11	0.765	Invention example
56	U	with	700	11	840	190	520	65	555	18	0.795	Invention example
57	U	with	690	11	810	190	520	70	555	18	0.795	Invention example
58	U	with	700	10	780	200	520	65	555	18	0.795	Invention example
59	V	with	660	9	825	260	510	110	550	26	0.859	Invention example
60	W	with	750	16	840	110	540	40	580	9	0.723	Invention example
61	X	with	740	13	850	170	495	60	570	14	0.814	Invention example
62	X	with	730	12	820	180	495	60	570	14	0.814	Invention example
63	X	with	740	13	790	180	495	60	570	14	0.814	Invention example
64	Y	with	730	13	870	160	490	55	570	15	0.835	Comparative example
65	Y	with	750	13	800	160	490	55	570	15	0.835	Comparative example
66	Y	with	740	13	750	160	490	55	570	15	0.835	Comparative example
67	Z	with	730	14	860	180	540	60	560	15	0.776	Comparative example
68	Z	with	750	14	860	180	500	60	560	15	0.776	Comparative example
69	Z	with	740	14	860	180	470	60	560	15	0.776	Comparative example
70	AA	with	750	15	850	210	500	45	575	12	0.792	Comparative example
71	AA	with	740	15	800	210	500	45	575	12	0.792	Comparative example
72	AA	with	730	15	750	210	500	45	575	12	0.792	Comparative example
73	A	without	660	11	850	160	495	60	555	25	0.886	Invention example
74	A	without	660	11	800	160	495	60	555	25	0.886	Invention example
75	A	without	660	11	750	160	495	60	555	25	0.886	Invention example

Underlined portion: out of the scope of the present invention

Table 4

No.	Steel type	With or without cold rolling	Heating temperature	Average heating rate	Annealing temperature T1	Annealing time	Cooling average keeping time T2	Cooling keeping time	Alloying treatment average keeping temperature T	Alloying treatment keeping time t	exp(200/(400-T))x In(t)	Remarks
			°C	°C/s	°C	S	°C	S	°C	S
76	AB	with	700	8	850	140	490	50	540	17	0.679	Invention example
77	AB	with	700	8	770	140	490	50	540	17	0.679	Invention example
78	AC	with	690	9	850	150	500	60	535	15	0.616	Invention example
79	AC	with	690	9	770	150	500	60	535	15	0.616	Invention example
80	AD	with	680	8	850	130	480	55	540	14	0.632	Invention example
81	AD	with	680	8	770	130	480	55	540	14	0.632	Invention example
82	AE	with	700	8	850	150	495	70	545	16	0.698	Invention example
83	AE	with	700	8	770	150	495	70	545	16	0.698	Invention example
84	A	with	680	9	850	160	490	60	540	15	0.649	Invention example
85	A	with	680	9	770	160	490	60	540	15	0.649	Invention example
86	A	with	660	7	850	210	500	85	555	21	0.838	Invention example
87	A	with	660	7	770	210	500	85	555	21	0.838	Invention example

[0065] Regarding the resulting galvanized steel sheet, the area ratios of ferrite, bainitic ferrite, pearlite, and martensitic phases were determined by polishing a sheet thickness cross-section parallel to a rolling direction of the steel sheet, followed by corroding with 3% nital, and observing 10 visual fields with a scanning electron microscope (SEM) under a magnification of 2,000 times through the use of Image-Pro of Media Cybernetics, Inc. At that time, it was difficult to distinguish martensite and retained austenite. Therefore, the resulting galvanized steel sheet was subjected to a tempering treatment at 200°C for 2 hours, the microstructure of a sheet thickness cross-section parallel to the rolling direction of the steel sheet was observed by the above-described method, and the aria ratio of tempered martensitic phase determined by the above-described method was taken as the aria ratio of martensitic phase. Furthermore, the volume ratio of retained austenite phase was determined on the basis of integrated intensity of ferrite and austenite peaks of a face at one-quarter sheet thickness, where the steel sheet was polished up to the one-quarter face in the sheet thickness direction. Regarding the incident X-rays, X-ray diffractometer using Co-Kα was used, the intensity ratios were determined with respect to all combinations of integrated intensities of peaks of {111}, {200}, {220}, and {311} faces of retained austenite phase and {110}, {200}, and {211} faces of ferrite phase, and the average value of them was taken as the volume ratio of retained austenite phase.

[0066] Moreover, a tensile test was performed on the basis of JIS Z2241 by using JIS No. 5 test piece, where sample was taken in such a way that a tensile direction becomes in the direction orthogonal to the rolling direction of the steel sheet, and the tensile strength (TS) and the total elongation (EL) were measured. In this regard, in the present invention, the case of TS × EL ≥ 19,000 MPa·% was evaluated as good elongation.

[0067] Regarding the stability of mechanical properties, (a) amounts of variations in TS and EL were examined with respect to steel sheets, where only the annealing temperatures T₁ were different and the conditions other than the annealing temperature T₁ were the same, and the amounts of variations (ΔTS and ΔEL) relative to 20°C of change in the annealing temperature were determined from the resulting amounts of variations in TS and EL, (b) amounts of variations in TS and EL were examined with respect to steel sheets, where only the average keeping temperatures T₂ from completion of the cooling to the dipping into a coating bath were different and the conditions other than the average keeping temperatures T₂ from completion of the cooling to the dipping into a coating bath were the same, and the amounts of variations (ΔTS and ΔEL) relative to 20°C of change in the average keeping temperature T₂ from completion of the cooling to the dipping into a coating bath were determined from the resulting amounts of variations in TS and EL, and the evaluation was performed on the basis of each of the amounts of variations in TS (ΔTS) and the amounts of variations in EL (ΔEL) relative to the 20°C of temperature change.

[0068] In addition, regarding the galvanized steel sheet obtained as described above, the hole expansion property (stretch flangeability) was measured. The hole expansion property (stretch flangeability) was measured on the basis of the Japan Iron and Steel Federation Standard JFST1001. Each of the resulting steel sheets was cut into 100 mm × 100 mm, and a hole having a diameter of 10 mm was punched with a clearance of 12% ± 1% when the sheet thickness was 2.0 = or more and with a clearance of 12% ± 2% when the sheet thickness was less than 2.0 mm. Thereafter, a 60° cone punch was pushed into the hole while being held with a blank holder pressure of 9 ton by using a dice having an inside diameter of 75 mm, a hole diameter at the limit of occurrence of cracking was measured, a critical hole expansion ratio λ (%) was determined from the following formula, and the stretch flangeability was evaluated on the basis of the value of the resulting critical hole expansion ratio,
critical hole expansion ratio λ (%) = {(D_f - D₀) / D₀} × 100
where D_f represents a hole diameter (mm) when cracking occurred and D₀ represents an initial hole diameter (mm). In this regard, in the present invention, the case of λ ≥ 70 (%) was evaluated as good.

[0069] The results obtained as described above are shown in Table 5 to Table 7.

Table 5

No.	Steel type	Sheet thickness (mm)	Area ratio ot F	Area ratio of M	Area ratio of BF	Area ratio of P	Volume ratio of RA	M/(BF+P)	TS	EL	A	TS×EL	ΔT1/Δ20°C		ΔT2/Δ20°C		Remarks
			(%)	(%)	(%)	(%)	(%)		(MPa)	(%)	(%)	(MPa-%)	ΔTS	ΔEL	ΔT5	ΔEL
1	A	1.4	87.6	2.2	3.6	4.4	1.2	0.28	626	32.9	102	20595	-	-	-	-	Invention example
2	B	1.4	84.1	3.5	4.2	5.1	2.2	0.38	645	32.4	89	20898	-	-	-	-	Invention example
3	C	1.4	88.9	1.7	3.2	4.1	0.8	0.23	611	33.2	111	20285	-	-	-	-	Invention example
4	0	1.4	88.8	1.8	4.1	3.2	1.2	0.25	632	33.2	98	20982	-	-	-	-	Invention example
5	E	1.4	86.2	3.0	3.6	4.8	0.9	0.36	623	33.1	104	20621	-	-	-	-	Invention example
6	F	1.4	85.7	3.8	3.6	3.2	2.2	0.56	645	32.4	88	20898	-	-	-	-	Invention example
7	G	1.4	88.2	1.5	4.0	4.9	0.7	0.17	609	33.4	110	20341	-	-	-	-	Invention example
8	H	1.4	83.9	13.2	0.8	0.7	0.7	8.80	624	27.6	53	17222	-	-	-	-	Comparative example
9	I	1.4	82.5	14.8	0.5	0.4	0.8	16.4	689	25.2	44	17363	-	-	-	-	Comparative example
10	J	1.4	86.8	10.7	0.7	0.3	0.7	10.7	589	29.5	60	17376	-	-	-	-	Comparative example
11	K	1.4	84.5	13.0	0.8	0.7	0.5	8.67	630	27.8	49	17514	-	-	-	-	Comparative example
12	L	1.4	83.4	14.7	0.6	0.4	0.5	14.70	618	27.8	52	17180	-	-	-	-	Comparative example
13	M	1.4	81.9	15.2	0.9	0.4	1.1	11.69	691	26.0	45	17966	-	-	-	-	Comparative example
14	N	1.4	84.4	12.4	0.9	0.6	0.6	8.27	601	28.6	55	17189	-	-	-	-	Comparative example
15	A	1.4	87.6	2.4	3.6	5.1	0.9	0.28	612	33.6	110	20563	3.6	0.12	-	-	Invention example
16	A	1.4	87.4	2.3	3.7	5.0	1.1	0.26	621	33.2	105	20617			-	-	Invention example
17	A	1.4	875	2.2	3.6	4.8	1.3	0.26	630	33.0	103	+20790			-	-	Invention example
18	H	1.4	84.9	12.2	0.8	0.8	0.5	7.63	608	28.1	58	17085	16.0	0.64	-	-	Comparative example
19	H	1.4	83.8	13.2	0.7	0.8	0.6	8.80	649	26.8	50	17393			-	-	Comparative example
20	H	1.4	82.8	14.1	0.6	0.6	0.7	11.75	688	24.9	42	17131			-	-	Comparative example
21	A	1.4	87.6	2.4	3.6	4.7	1.2	0.29	632	33.1	99	20919	-	-	3.3	0.16	Invention example
22	A	1.4	87.9	2.1	3.8	4.8	1.0	0.24	627	33.3	103	20879	-	-			Invention example
23	A	1.4	87.4	2.0	4.1	4.7	1.3	0.23	622	33.6	112	20899	-	-			Invention example
24	H	1.4	83.1	139	0.7	0.7	0.7	9.93	661	26.5	48	17517	-	-	20.6	0.86	Comparative example
25	H	1.4	84.1	13.2	0.8	0.5	0.9	10.15	628	28.2	54	17710	-	-			Comparative example
26	H	1.4	84.8	12.3	0.8	0.6	1.0	8.79	599	29.1	61	17431	-	-			Comparative example
27	A	0.8	85.6	3.8	4.9	2.7	2.1	0.50	648	31.5	89	20412	3.6	0.26	-	-	Invention example
28	A	0.8	84.5	4.2	5.2	2.6	2.5	0.54	659	30.8	86	20297			-	-	Invention example
29	A	2.3	86.1	2.1	4.8	5.8	0.5	0.20	606	35.2	111	21331	-	-	2.0	0.20	Invention example
30	A	2.3	85.9	1.8	5.4	6.0	0.4	0.16	600	35.8	123	21480	-	-			Invention example
31	A	1.4	83.8	4.2	6.4	1.8	3.1	0.51	654	34.1	87	22301	2.3	0.03	-	-	Comparative example
32	A	1.4	84.2	4.4	6.5	1.7	2.8	0.54	661	34.2	84	22606			-	-	Comparative example
33	O	1.4	84.9	3.5	5.4	4.9	0.8	0.34	648	32.4	92	20995	-	-	2.6	0.13	Invention example
34	O	1.4	83.9	3.7	5.8	5.1	1.1	0.34	640	32.8	97	20992	-	-			Invention example
35	O	1.4	85.1	6.8	2.2	3.2	1.8	1.26	628	30.6	67	19217	-	-	-	-	Comparative example
36	O	1.4	84.6	0.2	2.9	3.2	0.8	0.03	620	29.7	64	18414	-	-	-	-	Comparative example
37	O	1.4	88.9	4.7	0.8	3.7	0.3	1.04	615	30.1	65	18512	-	-	-	-	Comparative example

Underlined portion: out of the scope of the present invention F: ferrite, M: martensite, BF: bainitic ferrite, P: peartite, RA: retained austenite M/(BF+P): Area ratio of M/(Area ratio of BF + Area ratio of P)

Table 6

No.	Steel type	sheet thickness (mm)	Area ratio oi F	Area ratio of M	Area ratio of BF	Area ratio ot P	Volume ratio of RA	M/(BF+P)	TS	EL	A	TS×EL	ΔT1/Δ20°C		ΔT2/Δ20°C		Remarks
			(%)	(%)	(%)	(%)	(%)		(MPa)	(%)	(%)	(MPa·%)	ΔTS	ΔEL	ΔTS	ΔEL
38	P	1.2	87.8	2.7	4.1	3.8	0.7	0.34	618	32.8	101	20270	1.3	0.05	-	-	Invention example
39	P	1.2	85.7	2.8	4.5	3.7	0.8	0.34	623	32.6	104	20310			-	-	Invention example
40	P	1.2	86.2	0.4	3.2	6.2	0.1	0.04	562	31.6	90	17759	-	-	-	-	Comparative example
41	P	1.2	85.2	0.3	2.1	2.8	0.5	0.06	603	31.2	62	18814	-	-	-	-	Comparative example
42	P	1.2	86.1	3.5	0.6	7.6	0.5	0.43	621	27.8	89	17264	-	-	-	-	Comparative example
43	P	1.2	85.1	8.1	1.2	3.2	1.5	1.84	645	26.4	65	17028	-	-	-	-	Comparative example
44	Q	1.6	87.8	2.1	4.1	4.4	0.6	0.25	620	32.8	100	20336	-	-	-	-	Invention example
45	Q	1.6	86.2	4.8	0.2	7.2	0.2	0.65	640	26.9	69	17216	-	-	-	-	Comparative example
46	Q	1.6	80.1	0.6	6.4	10.3	0.1	0.04	538	30.1	85	16194	-	-	-	-	Comparative example
47	Q	1.6	84.1	0.5	4.5	10.4	0.3	0.03	592	31.2	85	18470	-	-	-	-	Comparative example
48	Q	1.6	79.1	6.3	8.4	1.6	3.8	0.63	654	31.2	50	20405	-	-	-	-	Comparative example
49	Q	1.6	84.2	0.3	4.4	10.5	0.2	0.02	595	31.6	87	18802	-	-	-	-	Comparative example
50	Q	1.6	79.1	6.5	8.4	1.4	4.1	0.66	650	31.2	55	20280	-	-	-	-	Comparative example
51	R	2.0	87.8	1.6	3.6	5.2	0.8	0.18	615	34.0	112	20910	1.4	0.22	-	-	Invention example
52	R	2.0	87.6	1.7	3.7	5.0	1.2	0.20	617	33.8	108	20855			-	-	Invention example
53	R	2.0	87.5	1.8	3.8	5.1	1.3	0.20	620	33.2	102	20584			-	-	Invention example
54	S	1.8	87.6	2.0	3.2	5.0	1.0	0.24	626	33.1	108	20721	-	-	-	-	Invention example
55	T	1.0	86.4	3.8	4.3	3.2	1.9	0.51	631	32.4	95	20444	-	-	-	-	Invention example
56	U	1.6	87.2	2.4	3.6	4.8	1.4	0.29	628	32.9	102	20661	2.3	0.23	-	-	Invention example
57	U	1.6	87.3	2.5	3.6	4.8	1.2	0.30	631	32.6	100	20571			-	-	Invention example
58	U	1.6	87.5	2.6	3.5	4.9	1.3	0.31	635	32.2	99	20447			-	-	Invention example
59	V	2.3	86.8	1.6	4.0	6.2	0.6	0.16	613	35.1	121	21516	-	-	-	-	Invention example
60	W	0.8	85.2	4.2	5.2	2.8	2.3	0.53	640	32.4	92	20736	-	-	-	-	Invention example
61	X	1.4	87.4	2.1	3.5	4.9	1.1	0.25	625	32.9	99	20563	1.6	0.16	-	-	Invention example
62	X	1.4	87.2	2.2	3.6	5.0	1.2	0.26	627	32.6	100	20440			-	-	Invention example
63	X	1.4	87.0	2.4	3.7	5.2	1.3	0.27	630	32.4	95	20412			-	-	Invention example
64	Y	1.4	81.4	14.3	0.6	0.2	2.2	17.88	596	31.6	60	18834	17.3	0.86	-	-	Comparative example
65	Y	1.4	83.1	12.8	0.7	0.3	2.5	12.80	652	29.1	45	18973			-	-	Comparative example
66	Y	1.4	85.2	10.7	0.8	0.5	2.6	8.23	698	26.4	38	18427			-	-	Comparative example
67	Z	1.4	84.4	12.1	0.4	0.4	0.7	15.13	645	29.0	41	18705	-	-	14.0	0.31	Comparative example
68	Z	1.4	86.4	10.3	0.6	0.6	0.9	8.58	621	29.8	50	18506	-	-			Comparative example
69	Z	1.4	87.6	8.9	0.8	0.3	1.2	8.09	596	30.1	57	17940	-	-			Comparative example
70	AA	1.2	88.2	6.2	0.8	0.7	2.6	4.13	609	30.5	62	18575	15.0	0.52	-	-	Comparative example
71	AA	1.2	85.4	8.2	0.8	0.6	3.4	5.86	641	29.4	48	18845			-	-	Comparative example
72	AA	1.2	82.1	10.4	0.7	0.6	3.8	8.00	684	27.9	40	19084			-	-	Comparative example
73	A	2.3	87.8	2.0	3.2	5.6	1.2	0.23	610	34.8	120	21228	1.4	0.04	-	-	Invention example
74	A	2.3	87.9	2.2	3.0	5.2	1.2	0.27	606	35.0	115	21210			-	-	Invention example
75	A	2.3	87.6	1.9	3.6	5.0	1.2	0.22	603	34.9	114	21045			-	-	Invention example

Underlined portion: out of the scope of the present invention F: ferrite, M: martensite, BF: bainitic ferrite, P: pearlite, RA: retained austenite M/(BF+P): Area ratio of M/(Area ratio of BF + Area ratio of P)

Table 7

No.	Steel type	Sheet thickness (mm)	Area ratio of F	Area ratio of M	Area ratio of BF	Area ratio of P	Volume ratio of RA	M/(BF+P)	TS	EL	λ	TS×EL	ΔT1/Δ20°C		ΔT2/Δ20°C		Remarks
			(%)	(%)	(%)	%	(%)		(MPa)	(%)	(%)	(MPa·%)	ΔTS	ΔEL	ΔTS	ΔEL
76	AB	1.4	87.7	2.3	3.6	4.5	1.2	0.28	626	32.9	102	20595	3.5	0.05	-	-	Invention example
77	AB	1.4	86.9	2.4	3.9	5.2	0.8	0.26	612	33.1	83	20257			-	-	Invention example
78	AC	1.4	87.8	2.2	3.6	4.7	1.2	0.27	626	32.9	102	20595	2.0	0.05	-	-	Invention example
79	AC	1.4	86.7	2.6	3.8	5.2	0.7	0.29	618	33.1	85	20456			-	-	Invention example
80	AD	1.4	87.9	2.3	3.4	4.8	1.4	0.28	623	33.2	104	20684	1.0	0.03	-	-	Invention example
81	AD	1.4	86.9	2.5	3.9	5.3	0.9	0.27	619	33.1	81	20489			-	-	Invention example
82	AE	1.4	87.7	2.3	3.6	4.5	1.2	0.28	626	32.9	106	20595	3.8	0.05	-	-	Invention example
83	AE	1.4	86.9	2.4	3.9	5.2	0.8	0.26	611	33.1	85	20224			-	-	Invention example
84	A	1.4	87.9	3.7	2.8	4.0	1.2	0.54	636	32.2	78	20479	6.5	0.15	-	-	Invention example
85	A	1.4	86.9	2.3	3.8	5.1	0.7	0.26	610	32.8	112	20008			-	-	Invention example
86	A	2.3	87.3	2.5	3.7	4.8	0.8	0.29	607	35.7	105	21670	0.8	0.02	-	-	Invention example
87	A	2.3	87.4	2.8	3.4	4.7	0.9	0.35	604	35.8	100	21623			-	-	Invention example

F: ferrite, M: martensite, BF: bainitic ferrite, P: pearlite, RA: retained austenite M/(BF+P): Area ratio of M/(Area ratio of BF + Area ratio of P)

[0070] Every high strength galvanized steel sheet produced with a method according to the present invention has TS of 540 MPa or more and has λ of 70% or more so as to exhibit excellent stretch flangeability. Furthermore, TS × EL ≥ 19,000 MPa·% is satisfied and the balance between the strength and the elongation is high. Therefore, it is clear that a high strength galvanized steel sheet having excellent formability is obtained. Moreover, the values of ΔTS and ΔEL are small and, therefore, it is clear that a high strength galvanized steel sheet having excellent stability of mechanical properties is obtained. On the other hand, regarding comparative examples, at least one of the elongation and the stretch flangeability is poor, or the stability of mechanical properties is not favorable.

[0071] The high strength galvanized steel sheet produced according to the present invention has a tensile strength TS of 540 MPa or more, exhibits high elongation and high stretch flangeability, and has excellent stability of mechanical properties. In the case where the high strength galvanized steel sheet according to the present invention is applied to, for example, an automobile structural member, enhancement of fuel economy due to weight reduction of a car body can be facilitated. Therefore, an industrial utility value is very large.

Claims

1. A method for manufacturing a high strength galvanized steel sheet having excellent formability and stability of mechanical properties, comprising the steps of subjecting a steel slab having the component composition consisting of 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, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, and optionally at least one type of element selected 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, Cu: 0.05% or more, and 1.0% or less, Ti: 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, Ca: 0.001% or more, and 0.005% or less, REM: 0.001% or more, and 0.005% or less, Ta: 0.001% or more, and 0.010% or less, Sn: 0.002% or more, and 0.2% or less, and Sb: 0.002% or more, and 0.2% or less and the remainder composed of Fe and incidental impurities on a percent by mass basis, to hot rolling and pickling, or hot rolling, pickling, and cold rolling, performing heating to a temperature range of 650°C or higher at an average heating rate of 5°C/s or more, followed by keeping in a temperature range of 750°C to 900°C for 15 to 600 s, performing cooling to a temperature range of 450°C to 550°C, followed by keeping in the temperature range of 450°C to 550°C for 10 to 200 s, and performing galvanization, wherein after the galvanization is performed, an alloying treatment of zinc coating is performed in a temperature range of 500°C to 600°C under the condition satisfying the following formula, 0.45 ≤ exp [200/(400 - T)] x ln(t) ≤ 1.0
where
T: average keeping temperature (°C) in a temperature range of 500°C to 600°C,
t: keeping time (s) in a temperature range of 500°C to 600°C, and
exp(X) and ln(X) represent an exponential function and natural logarithm, respectively, of X.

Ansprüche

1. Verfahren zur Herstellung eines hochfesten verzinkten Stahlblechs mit ausgezeichneter Formbarkeit und Stabilität der mechanischen Eigenschaften, das die Schritte umfasst, in denen eine Stahlbramme, deren Zusammensetzung nach Bestandteilen auf Masseprozent-Basis besteht aus: 0,04 % oder mehr und 0,13 % oder weniger C, 0,7 % oder mehr und 2,3 % oder weniger Si, 0,8 % oder mehr und 2,0 % oder weniger Mn, 0,1 % oder weniger P, 0,01 % oder weniger S, 0,1 % oder weniger Al, 0,008 % oder weniger N und optional wenigstens einen Typ von Element, ausgewählt aus 0,05 % oder mehr und 1,0 % oder weniger Cr, 0,005 % oder mehr und 0,5 % oder weniger V, 0,005 % oder mehr und 0,5 % oder weniger Mo, 0,05 % oder mehr und 1,0 % oder weniger Ni, 0,05 % oder mehr und 1,0 % oder weniger Cu, 0,01 % oder mehr und 0,1 % oder weniger Ti, 0,01 % oder mehr und 0,1 % oder weniger Nb, sowie 0,0003 % oder mehr und 0,0050 % oder weniger B, 0,001 % oder mehr und 0,005 % oder weniger Ca, 0,001 % oder mehr und 0,005 % oder weniger REM, 0,001 % oder mehr und 0,010 % oder weniger Ta, 0,002 % oder mehr und 0,2 % oder weniger Sn, sowie 0.002 % oder mehr und 0.2 % oder weniger Sb und wobei der Rest aus Fe und zufälligen Verunreinigungen besteht, Warmwalzen und Beizen oder Warmwalzen, Beizen und Kaltwalzen, Durchführen von Erwärmen auf einen Temperaturbereich von 650°C oder darüber mit einer durchschnittlichen Erwärmungsgeschwindigkeit von 5°C/s oder mehr, gefolgt von Halten in einem Temperaturbereich von 750°C bis 900°C für 15 bis 600 s, Durchführen von Abkühlen auf einen Temperaturbereich von 450°C bis 550°C, gefolgt von Halten in dem Temperaturbereich von 450°C bis 550°C für 10 bis 200 s und Durchführen von Verzinkung unterzogen wird, wobei, nachdem die Verzinkung durchgeführt worden ist, eine Legierungsbehandlung der Zinkbeschichtung in einem Temperaturbereich von 500°C bis 600°C unter der Bedingung durchgeführt wird, die die folgende Formel erfüllt, d.h. 0,45 ≤ exp [200/(400 - T)] x In(t) ≤ 1,0,
wobei
T: durchschnittliche Haltetemperatur (°C) in einem Temperaturbereich von 500 °C bis 600 °C,
t: Haltezeit (s) in einem Temperaturbereich von 500 °C bis 600 °C, und
exp(X) und ln(X) eine Exponentialfunktion bzw. einen natürlichen Logarithmus von X repräsentieren.

Revendications

1. Procédé de fabrication d'une tôle en acier galvanisé à haute résistance présentant d'excellentes propriétés mécaniques de formage et de stabilité, comprenant les étapes de soumission d'une plaque d'acier dont la composition consiste en C : supérieur ou égal à 0,04 % et inférieur ou égal à 0,13 %, Si : supérieur ou égal à 0,7 % et inférieur ou égal à 2,3 %, Mn : supérieur ou égal à 0,8 % et inférieur ou égal à 2,0 %, P : inférieur ou égal à 0,1 %, S : inférieur ou égal à 0,01 %, Al : inférieur ou égal à 0,1 %, N : inférieur ou égal à 0,008 %, et éventuellement au moins un élément sélectionné parmi Cr : supérieur ou égal à 0,05 % et inférieur ou égal à 1,0 %, V : supérieur ou égal à 0,005 % et inférieur ou égal à 0,5 %, Mo : supérieur ou égal à 0,005 % et inférieur ou égal à 0,5 %, Ni : supérieur ou égal à 0,05 % et inférieur ou égal à 1,0 %, Cu : supérieur ou égal à 0,05 % et inférieur ou égal à 1,0 %, Ti : supérieur ou égal à 0,01 % et inférieur ou égal à 0,1 %, Nb : supérieur ou égal à 0,01 % et inférieur ou égal à 0,1 %, et B : supérieur ou égal à 0,0003 % et inférieur ou égal à 0,0050 %, Ca : supérieur ou égal à 0,001 % et inférieur ou égal à 0,005 %, terres rares : supérieur ou égal à 0,001 % et inférieur ou égal à 0,005 %, Ta : supérieur ou égal à 0,001 % et inférieur ou égal à 0,010 %, Sn : supérieur ou égal à 0,002 % et inférieur ou égal à 0,2 %, et Sb : supérieur ou égal à 0,002 % et inférieur ou égal à 0,2 %, le reste étant composé de Fe et d'impuretés inévitables, en pourcentage massique, à un laminage à chaud et décapage, ou à un laminage à chaud, décapage et laminage à froid, en mettant en œuvre une chauffe jusqu'à une plage de température supérieure ou égale à 650 °C à une vitesse de chauffe moyenne supérieure ou égale à 5 °C/s, suivie par un maintien dans une plage de température de 750 °C à 900 °C durant 15 à 600 s, en mettant en œuvre un refroidissement dans une plage de température de 450 °C à 550 °C, suivi par un maintien dans la plage de température de 450 °C à 550 °C durant 10 à 200 s, et en mettant en œuvre une galvanisation, dans lequel, après la mise en œuvre de la galvanisation, un traitement d'alliage du revêtement de zinc est mis en œuvre dans une plage de température de 500 °C à 600 °C sous la condition répondant à la formule suivante :

où

T est la température moyenne de maintien (°C) dans une plage de température de 500 °C à 600 °C,

t est le temps de maintien (s) dans une plage de température de 500 °C à 600 °C, et

exp(X) et ln(X) représentent respectivement la fonction exponentielle et le logarithme naturel de X.

Drawing