JP2005220417A - High strength hot dip galvanized steel sheet having excellent stretch flange formability, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot dip galvanized steel sheet having an extremely excellent stretch flange formability satisfying λ≥80%, and to provide its production method. <P>SOLUTION: The high strength hot dip galvanized steel sheet having excellent stretch flange formability has chemical components containng, by mass, 0.03 to 0.20% C, 1.5 to 3.0% Mn, ≤0.05% P, ≤0.01% S, ≤0.15% Al, ≤0.01% N and 0.05 to 0.25% Ti, and in which the atomic ratio between Ti* defined by Ti*=Ti-(48/14)N-(48/32)S and C, (Ti*/48)/(C/12) satisfies 0.15 to 0.80, and further, A defined by A=(Ti*/2C)+1.7-Mn satisfies A≥0, and the balance substantially iron, and has a steel sheet structure consisting of low temperature transformation phases of ferrite and austenite. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet having excellent stretch flange formability, which is mainly used for automobile structural parts such as members and lockers, and a method for producing the same.

In recent years, the collision safety performance of automobiles has been improved (when a car collides with an object while traveling, the energy absorption capacity of the member against impact is increased, and the impact load on the occupant is reduced, thereby improving the safety of the occupant's life) For this reason, in order to reduce the weight of the vehicle body for the purpose of improving the fuel efficiency associated with exhaust gas regulations, the application of high-strength steel sheets to various automobile parts such as members and lockers is being promoted.
On the other hand, with the globalization of the automobile market, rust prevention performance is also required in areas where the corrosive environment is more severe, so application of surface-treated steel sheets, mainly hot-dip galvanized steel sheets, is required. Development of high-strength hot-dip galvanized steel sheets has been underway, and steel sheets having a tensile strength of 590 MPa class have been commercialized and are increasingly being applied to automobile parts.

  However, for steel plates of 590 MPa class or higher, degradation of stretch flangeability due to lowering of ductility and further hole expandability is an issue, and is particularly important at 780 MPa class or higher. However, although ferrite single phase steel is excellent in stretch flange formability, it is difficult to increase the strength of 590 MPa class or higher. This is because even if the solid solution strengthening element is increased, the plating property is deteriorated, and even if the precipitation strengthening amount is increased, the press formability is decreased due to the increase in yield strength.

  On the other hand, since the composite structure steel has a mixed structure of a soft phase and a hard phase, the yield strength is low, but between the two phases tends to be a starting point of cracks at the time of stretch flange molding, and the stretch flange formability is poor. Therefore, it is very difficult to develop a high-strength hot-dip galvanized steel sheet that is excellent in stretch flange formability in composite structure steel.

  As a knowledge for improving stretch flangeability in a composite structure steel, Patent Document 1 discloses a method of increasing the ferrite fraction and refining the structure, and Patent Document 2 discloses a method of uniformly refining the structure. Patent Documents 3 and 4 disclose a technique for refining cementite in bainite by adding Si, and Patent Document 5 discloses a technique for refining martensite in Dual Phase steel.

However, in any of these techniques, the hole expansion ratio λ, which is an index indicating stretch flangeability, is about 80%, which is not a remarkable effect.
JP-A-4-350 JP-A-4-173946 JP-A-4-293758 Japanese Patent Laid-Open No. 5-78753 JP 2003-213369 A

  Therefore, in the present invention, by making effective use of Ti and Mn and optimizing the balance of Ti, C, and Mn and the annealing temperature, the stretch flange formability can be achieved while having good press formability with the composite structure steel. An excellent high-strength hot-dip galvanized steel sheet and a method for producing the same have been found.

  The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a high-strength hot-dip galvanized steel sheet having a very excellent stretch flange formability of λ ≧ 80% and a method for producing the same.

The above problems are solved by the following invention.
(1) C: 0.03 to 0.20%, Mn: 1.5 to 3.0%, P: ≤ 0.05%, S: ≤ 0.01%, Al: ≤ Ti containing 0.15%, N: ≦ 0.01%, Ti: 0.05-0.25%, and further defined by Ti ^* = Ti− (48/14) N− (48/32) S ^* And C atomic ratio (Ti ^* / 48) / (C / 12) satisfy 0.15 to 0.80,
Furthermore, A defined by A = (Ti ^{* /} 2C ⁾ + 1.7-Mn satisfies A ≧ 0,
A high-strength hot-dip galvanized steel sheet excellent in stretch-brown formability, characterized in that the balance is substantially made of iron and the steel sheet structure is made of a low-temperature transformation phase of ferrite and austenite.

(2) Ti ^* and Nb defined as T ^* = Ti- (48/14) N- (48/32) S, further containing Nb: ≦ 0.25% by mass% as a chemical component The atomic ratio of C (Nb / 93 + Ti ^* / 48) / (C / 12) satisfies 0.15 to 0.80, and A defined by A = (48Nb + 93Ti ^* ) / 186C + 1.7-Mn is A ≧ 0 The high-strength hot-dip galvanized steel sheet having excellent stretch flange formability as described in (1) above.

  (3) The high-strength hot-dip galvanized steel sheet having excellent stretch-flange formability as described in (1) or (2) above, wherein the chemical component further contains Si: ≦ 0.5% by mass. .

  (4) The high strength melt excellent in stretch flange formability according to any one of the above (1) to (3), wherein the chemical component further contains B: ≦ 0.0015% by mass%. Galvanized steel sheet.

(5) Further, as a chemical component, by mass%, Cr: 0.05 to 0.5%, V: 0.05 to 0.5%, Mo: 0.05 to 0.5%, or 1 or 2 The above (1), wherein A is defined as A = (48Nb + 93Ti ^* ) / 186C + 1.7-Mn ′ in Mn ′ = Mn + 2.5Mo + 1.07Cr + 2.5V, satisfying A ≧ 0. The high-strength hot-dip galvanized steel sheet having excellent stretch flange formability according to any one of (1) to (4).

  (6) After hot-rolling a slab having any of the components described in (1) to (5) above, winding it at 650 ° C. or lower to obtain a hot-rolled steel sheet, and pickling and cold-rolling The temperature is (0.7Ac3 + 0.3Ac1) ° C to (1.4Ac3−0.4Ac1) ° C, and is subjected to recrystallization annealing for 5 to 240 seconds, and then cooled at a rate of 10 ° C / s or less. And a method of producing a high-strength hot-dip galvanized steel sheet having excellent stretch flange formability, comprising a step of subjecting to an alloying treatment after hot-dip galvanizing.

  This invention was made based on the knowledge found as a result of extensive studies to obtain a high-strength hot-dip galvanized steel sheet having excellent stretch flange formability. That is, the present inventors effectively utilize Ti and Mn, optimize the balance of Ti, C, Mn, and the annealing temperature, thereby strengthening the precipitation of ferrite with fine carbides in the composite structure steel, and at the same time the second phase. It has been found that the hardness difference between the ferrite and the second phase can be effectively reduced by using bainite as a main component and making it softer than the second phase (martensite) of so-called Dual Phase steel. As a result, the present inventors have found knowledge about a high-strength hot-dip galvanized steel sheet that is excellent in stretch flange formability and a method for manufacturing the same, and discloses the result here.

In the high-strength hot-dip galvanized steel sheet according to the present invention, the following constituent elements are essential.
Below, the reason for the addition of the steel component of the present invention, the component limitation range, the structure morphology, the tensile properties, and the limitation of the production conditions will be described. In addition, the following% shows the mass%.

(1) Range of steel components
C: 0.03-0.20%
C is an element effective for strengthening steel. It contributes to solid solution strengthening and also contributes to precipitation strengthening by combining with Ti, Nb and the like to form carbides. In the present invention, steel is strengthened by further utilizing precipitation strengthening, and 0.03% or more of C is required to obtain sufficient strengthening ability. However, if the amount of C exceeds 0.20%, the decrease in cross tensile strength in spot welding becomes significant, so the amount of C is set to a range of 0.03 to 0.20%. However, in order to obtain the effect of improving the stretch flangeability of the present invention, it is necessary to satisfy the atomic ratio of Ti and Nb described below: 0.15 to 0.80 at the same time.

Mn: 1.5-3.0%
Mn is an element effective for strengthening the quenching of steel. Here, when the amount of Mn is less than 1.5%, hardenability is lowered, and in a hot dip galvanizing line having a relatively slow cooling rate, pearlite that deteriorates ductility is easily formed in the cooling stage. Moreover, when the amount of Mn exceeds 3.0%, when the molten steel is cast into a slab, cracks are likely to occur on the slab surface and corner portions. Furthermore, surface defects become apparent in the steel sheet obtained by hot rolling the slab and further cold rolling and annealing. For this reason, the amount of Mn shall be 1.5 to 3.0% of range. However, in order to obtain the effect of improving the stretch flangeability of the present invention, it is necessary to satisfy A ≧ 0 described below at the same time.

P ≦ 0.05%
P is an element effective for strengthening steel and can be appropriately added. However, if the amount of P exceeds 0.05%, the alloying reactivity after hot dip galvanization is lowered, causing a poor surface appearance called burn unevenness or spot weldability. Therefore, the amount of P is made 0.05% or less.

Al ≦ 0.15%
Al has the effect of suppressing the formation of Mn and Si-based oxides on the surface layer that hinders plating properties during annealing and improving the appearance of the plating surface. In terms of material, the Ac3 transformation point is raised, and the ferrite + austenite two-phase region is expanded, so that the proper annealing temperature range can be expanded. However, if the amount of Al exceeds 0.15%, the Ac3 point increases too much, making it impossible to produce in a continuous hot dip galvanizing line. Therefore, the Al content is set to a range of 0.15% or less.

S ≦ 0.01%
When S is excessively present in the steel, it segregates at the grain boundaries of austenite during slab heating, and red hot brittleness tends to occur from the steel sheet surface layer during hot rolling. In particular, when the amount of S exceeds 0.01%, this adverse effect is a concern. For this reason, the S amount is 0.01% or less, but is preferably 0.005% or less from the viewpoint of securing the cross tensile strength in spot welding.

N ≦ 0.01%
When N is excessively present in the steel, cracks are generated on the surface of the slab during casting, and the ductility of the steel sheet after galvanization deteriorates when hot dip galvanizing is performed. However, if it is 0.01% or less, such an adverse effect is not exerted. For this reason, N amount shall be 0.01% or less.

Ti: 0.05-0.25%, (Ti ^{* /} 48) / (C / 12): 0.15-0.8
Ti is an important element in the present invention. Ti forms fine carbides in the steel, thereby strengthening the steel by precipitation strengthening of ferrite. In order to obtain the effect, 0.05% or more is necessary. On the other hand, if the Ti content exceeds 0.25%, the surface quality of the steel sheet deteriorates, so the Ti content is 0.05 to 0.25%.
By the way, although Ti carbide is generated at the time of hot rolling, it is dissolved by high temperature annealing after cold rolling, and solid solution C is concentrated to austenite. It is necessary that the second phase becomes a bainite-based structure by the subsequent cooling, hot dipping, and alloying steps. However, for that purpose, (Ti ^* / 48) / (C / 12) needs to be 0.15 or more, and conversely, if it exceeds 0.8, solid solution C decreases and it is difficult to form such a structure. become. Therefore, (Ti ^* / 48) / (C / 12) is in the range of 0.15 to 0.8. Since Ti in steel forms TiN and TiS prior to carbide in a relatively high temperature range of slab heating to hot rolling, Ti ^* that can be consumed for carbide formation is Ti ^* =
It is defined by Ti- (48/14) N- (48/32) S.

A ≧ O
A is represented by A = (Ti ^{* /} 2C) + 1.7-Mn ′ when Ti is added, and A: (48Nb + 93Ti ^* ) / 186C + 1.7-Mn ′ when Ti + Nb is added. It is an important item. Mn ′ is an index indicating the ease of quenching of steel. Mn ′ = Mn + 2.5Mo + 1.07Cr + 2.5V when Cr, V, and Mo are added, and Mn ′ = Mn when Cr, V, and Mo are not added. It is.

When A is less than 0, the effect of ferrite precipitation strengthening decreases, and conversely, the structure strengthening due to the formation of martensite becomes significant, increasing the strength difference between the ferrite and the second phase and degrading stretch flangeability. To do. Therefore, the range is A ≧ 0.
Moreover, in this invention, Nb, Si, B, Cr, V, and Mo can be added in the following ranges as needed.

Nb ≦ 0.25%, (Nb / 93 + Ti ^* / 48) / (C / 12) ^: 0.15 to 0.8
Nb, like Ti, is a carbide-forming element. In addition to solid solution strengthening and precipitation strengthening, Nb is also an element that contributes to structural strengthening by enhancing hardenability, and even if added, the effect of Ti of the present invention is hindered. There is no. However, if the Nb content exceeds 0.25%, the recrystallization temperature of ferrite rises during annealing, so that the processed structure tends to remain in the steel sheet structure after annealing, and the ductility of the obtained steel sheet deteriorates significantly. Therefore, when Nb is added, the amount added is in the range of 0.25% or less.

In addition, (Nb / 93 + Ti ^* / 48) / (C / 12) is required to be 0.15 or more for the formation of the second phase mainly composed of bainite effective in the present invention. Molten C decreases, and it becomes difficult to form the second phase mainly composed of bainite effective in the present invention. Therefore, (Nb / 93 + Ti ^* / 48) / (C / 12) is in the range of 0.15 to 0.8.

Si ≦ 0.5%
Si is an element effective for improving the strength and ductility balance of steel, and can be added as appropriate. However, if the amount of Si exceeds 0.5%, the occurrence of non-plating in hot dip galvanizing and the decrease in reactivity during alloying treatment are promoted, so the surface properties and rust prevention performance deteriorate. Therefore, the Si amount is 0.5% or less.

B: ≦ 0.0015%
B is a grain boundary segregation element and segregates at the austenite grain boundary during annealing, and suppresses austenite grain growth and simultaneously suppresses ferrite precipitation during cooling, so that a fine structure can be finally obtained. As a result, it contributes to the improvement of stretch flangeability by increasing the strength of steel and finely dispersing the second phase. However, if the amount of B exceeds 0.0015%, not only the fine graining effect is saturated, but also the ferrite precipitation temperature is lowered, so that TiC is precipitated before transformation into ferrite, and sufficient precipitation strengthening is achieved. It can no longer be obtained. Furthermore, the alloying reactivity after hot dip galvanization is lowered, causing a surface quality defect called uneven burning. For this reason, when adding B, this addition amount shall be 0.0015% or less of range.

Cr: 0.05-0.5%
Cr is an extremely effective element for strengthening the quenching of steel, particularly in processes where the cooling rate after annealing is slow and martensite is difficult to form, such as in a continuous hot dip galvanizing line. In order to obtain this effect, addition of 0.05% or more is required. However, if the Cr content exceeds 0.5%, this effect is saturated, while the surface quality is significantly reduced. For this reason, the Cr content is in the range of 0.05 to 0.5%.

V: 0.05-0.5%
V is an element effective for strengthening steel, and the nitride formed with V contributes to the refinement of the annealed plate structure. In order to obtain these effects, V needs to be added in an amount of 0.05% or more. However, when the added amount of V exceeds 0.5%, these effects are saturated. For this reason, when adding V, this addition amount shall be the range of 0.05-0.5.

Mo: 0.05-0.5%
Mo is an element effective for strengthening the quenching of steel, and 0.05% or more of addition is required to obtain this effect. However, this effect is saturated when the Mo content exceeds 0.5%. For this reason, when adding Mo, the amount of addition shall be 0.05 to 0.5% of range.
In addition, the chemical components other than the steel components are not particularly impaired unless they are added excessively. In the present invention, the balance being substantially iron means that other alloy elements or unavoidable impurities may be contained as long as the target characteristics of the present invention are not adversely affected.

(2) Steel plate structure The steel plate structure of the steel of the present invention is composed of a low-temperature transformation phase of austenite whose main phase is ferrite and the second phase is bainite. The area fraction of the second phase is 5 to 98%, preferably 10 to 90%, in order to ensure the strength and ductility balance. In addition, the bainite main body means that it is composed of a volume fraction of 50% or more, preferably 70% or more in the second phase in order to ensure stretch flangeability. In addition to bainite, martensite, pearlite, It may contain bainitic ferrite and retained austenite.

(3) Steel plate manufacturing method
In the hot rolling process in which the steel having the above-mentioned chemical composition is melted and cast, and in the hot rolling process in which casting and rough rolling and finish rolling are performed, if the coiling temperature exceeds 650 ° C., the precipitated carbide becomes coarse and is cooled. It remains in a coarse state without being completely dissolved or reduced during annealing. Therefore, sufficient precipitation strengthening cannot be obtained, causing not only a decrease in stretch flangeability but also a decrease in strength. Therefore, the winding temperature is set to 650 ° C. or less.
In the annealing process after cold rolling, during the transformation from ferrite to austenite that occurs in the temperature rising and soaking process, carbides generated during hot rolling are dissolved or reduced. And it is thought that it exists as a fine carbide as a result by reprecipitation or slight Ostwald growth in the transformation from austenite to ferrite during cooling.
If the annealing temperature is less than (0.7Ac3 + 0.3Ac1) ° C, the transformation to austenite does not proceed sufficiently and fine carbides do not precipitate sufficiently. The annealing temperature is in the range of (0.7Ac3 + 0.3Ac1) ° C. to (1.4Ac3−0.4Ac1) ° C. in order to promote the formation of pearlite or the like that deteriorates the ductility due to insufficient concentration.
In addition, the residence time in the temperature range is less than 5 seconds, and recrystallization does not occur, so that sufficient ductility cannot be obtained. On the other hand, even if it exceeds 240 seconds, the productivity is reduced. It was 240 seconds. Preferably it is 15 to 120 seconds. Furthermore, with respect to the cooling rate after annealing, if it exceeds 10 ° C./s, the time required for carbide formation is shortened, the amount of precipitates is reduced, and sufficient precipitation strengthening cannot be obtained. did.
Then, it manufactures by a normal hot dip galvanizing process. In addition, you may pass through surface cleaning processes, such as pickling and a degreasing process, before an annealing process, and you may perform an alloying process after hot dip galvanization. In that case, it is preferable to carry out such that the Fe content in the alloyed plating layer is 9 to 12%.
In addition, the steel strip after galvanization may be subjected to temper rolling of 10% or less in order to adjust the shape correction, surface roughness, etc., and surface treatment such as chemical conversion treatment may be applied to the obtained steel plate. Even if applied, the desired properties are not adversely affected. Through the above manufacturing process, a hot-dip galvanized steel sheet excellent in stretch flange formability intended by the present invention can be manufactured.

According to the present invention, the chemical composition of the steel is defined, and the atomic ratios (Ti ^* / 48) / (C / 12) and (Nb / 93 + Ti ^* / 48) / (C / 12) are 0.15 to 0. .80, A = (Ti ^{* /} 2C) + 1.7-Mn ′ and (48Nb + 93Ti ^* ) / 186C + 1.7-Mn ′ are 0 or more, and the coiling temperature during hot rolling is 650 ° C. or less and after cold rolling Is annealed at a temperature of (0.7Ac3 + 0.3Ac1) ° C. to (1.4Ac3−0.4Ac1) ° C. for a time of 5 to 240 seconds, and then cooled at a rate of 10 ° C./s or less. By hot-dip galvanizing and subsequent alloying treatment, it is possible to stably produce a high-strength hot-dip galvanized steel sheet with excellent stretch-flange formability, in which the steel sheet structure consists of a composite structure of low-temperature transformation phases of ferrite and austenite. Therefore, in the automobile industry The utility value is high.

  In the following, the results of research that is the basis of the present invention will be described. The steel plate used in the experiment has a chemical composition of mass%, C: 0.04 to 0.12%, Mn: 1.69 to 3.39%, P: 0.006%, S: 0.001%, Al : Slab of 0.04 to 0.10%, N: 0.0028 to 0.0062%, Nb: 0 to 0.05%, Ti: 0 to 0.15%, B: 0 to 0.0006% After cold rolling a hot-rolled sheet (thickness of 3.2 mm) obtained by subjecting (thickness: 30 mm) to rough rolling and finish rolling to a thickness of 1.6 mm, the test material is actually used in a salt bath. Heat treatment equivalent to annealing, hot dip galvanizing, and alloying was performed by a heat cycle simulating a continuous hot dip galvanizing line. The contents are annealed at a maximum temperature of 850 ° C. for 180 seconds, then cooled at a rate of 5 ° C./s, then held at 550 ° C., which is a temperature equivalent to hot dip galvanizing and alloying treatment, for 90 seconds, and then cooled by air cooling. To do.

For steel components, the atomic ratio (Nb / 93 + Ti ^* / 48) / (C / 12) is calculated for Ti ^* calculated by Ti ^* = Ti− (48/14) N− (48/32) S. Calculated. As an index indicating the hardenability, Mn ′ = Mn + 2.5Mo + 1.07Cr + 2.5V was calculated. However, in the above test steel, Cr, V, and Mo were not added, so Mn ′ = Mn.

  Furthermore, the yield strength and tensile strength of the steel sheet were measured by a tensile test, and the ratio (yield strength / tensile strength × 100) was determined as the yield ratio YR. When YR becomes high, the dimensional accuracy at the time of press forming decreases, and the press formability deteriorates due to the decrease in surface accuracy due to wrinkles and the like, which is particularly noticeable at 90% or more.

In addition, as an index representing stretch flange formability, a hole expansion rate λ was obtained by a hole expansion test. The test was conducted by the method defined in the following Steel Federation standards. That is, after punching a 10mmφ hole with a clearance of 12.5% on the test piece and setting the burr side on the surface, the punched hole is pushed and expanded from the back with a conical punch with a tip angle of 60 ° while holding the test piece. Stops when a through crack occurs at the edge. For the hole diameter Dh at that time, the hole expansion ratio λ1 from the original hole diameter is defined as λ1 = {(Dh−10) / 10) × 100. The test was performed three times, λ1, λ2,
λ3 was determined and the average value was taken as λ.

These results are shown below. FIG. 1 shows the relationship between the atomic ratio (Nb / 93 + Ti ^* / 48) / (C / 12) and YR. It can be seen that YR increases as the atomic ratio increases, and increases significantly when the atomic ratio exceeds 0.8. This is thought to be due to the fact that the structure becomes a single phase of ferrite and the effect of suppressing grain growth by increasing the pinning effect of TiC and NbC, ie, the refinement of the structure. Therefore, it has become clear that (Nb / 93 + Ti ^* / 48) / (C / 12 ⁾ ≦ 0.8 is necessary.

In FIG. 2, in the relationship between the atomic ratio (Nb / 93 + Ti ^* / 48) / (C / 12) and Mn ′, λ ≧ 100% is ◎, 100%> λ ≧ 80% is ◯, and λ <80% is ×. The plotted result is shown. As a result, with the straight line represented by Mn ′ = (48Nb + 93Ti ^* ) / 186C + 1.7 as the boundary line, the lower region is λ ≧ 80% and the upper region is λ <80%, that is, A = (48Nb + 93Ti ^* ) It became clear that λ ≧ 80% under the condition of 186C + 1.7-Mn ′ ≧ 0. Furthermore, it was also found that at the atomic ratio <0.15, all λ <80%. Collectively, it became clear that the atomic ratio (Nb / 93 + Ti ^* / 48) / (C / 12) needs to be in the range of 0.15 to 0.80.
The present invention has been made based on the above findings.

Examples of the present invention are shown below.
First, steels (steel numbers 1 to 46) having the components shown in Table 1 below were melted in a laboratory and then cast to prepare a slab having a thickness of 50 mm. Next, this slab was subjected to mass rolling to a plate thickness of 30 mm, and then heated at 1270 ° C. for 1 hr in an atmospheric furnace and subjected to hot rolling. Subsequently, a hot-rolled sheet having a thickness of 4.0 mm was produced through rough rolling and finish rolling. The finishing temperature was 860 ° C. After rolling, the steel sheet was cooled at an average cooling rate of 20 ° C./s and subjected to a heat treatment equivalent to winding at 575 to 680 ° C. × 1 hr. Next, this hot-rolled sheet was pickled and cold-rolled to a thickness of 1.6 mm. Thereafter, the sample material was subjected to heat treatment corresponding to annealing, hot dip galvanizing, and alloying treatment by a heat cycle in accordance with an actual continuous hot dip galvanizing line in a salt bath. The content is that after annealing at a maximum temperature of 780 to 920 ° C. for 2 to 250 seconds, cooling at a rate of 5 to 15 ° C./s, and then holding for 90 seconds at 550 ° C., which is a temperature equivalent to hot dip galvanizing and alloying treatment. Cooling by air cooling.

The obtained steel sheet was temper-rolled at an elongation of 0.5%, and by tensile test and hole expansion test, yield strength (YP), tensile strength (TS), yield ratio (YR), total elongation (EI) ) And the hole expansion rate (λ) was measured. The tensile test was performed using No. 5 test piece described in Japanese Industrial Standard JISZ2201 in the rolling direction. The hole expansion test was performed by the method described above, and the hole expansion ratio λ was obtained. In addition, the microstructure of the cross section parallel to the rolling direction of the specimen was observed by SEM, and the breakdown of the alloy phase was indicated by ferrite (F), bainite (B), martensite (M), and pearlite (P). It was. Table 2 below shows production conditions and test results.

  It can be seen that the hot-dip galvanized steel sheet obtained according to the present invention has excellent stretch flange formability of λ ≧ 80%. It can also be seen that YR ≦ 90% and the press formability is excellent.

The figure which shows the relationship between atomic ratio and YR. The figure which shows (lambda) in the relationship between atomic ratio and Mn '.

Claims (6)

C: 0.03 to 0.20%, Mn: 1.5 to 3.0%, P: ≦ 0.05%, S: ≦ 0.01%, Al: ≦ 0.15 in terms of mass% %, N: ≦ 0.01%, Ti: contains 0.05 to 0.25 percent, still ^{Ti * = Ti- (48/14) N-} (48/32) defined by S Ti ^* and C Atomic ratio
(Ti ^* / 48) / (C / 12) satisfies 0.15 to 0.80,
Furthermore, A defined by A = (Ti ^{* /} 2C ⁾ + 1.7-Mn satisfies A ≧ 0,
A high-strength hot-dip galvanized steel sheet excellent in stretch-brown formability, characterized in that the balance is substantially made of iron and the steel sheet structure is made of a low-temperature transformation phase of ferrite and austenite. Further, as a chemical component, Nb: ≦ 0.25% by mass%, and Ti ^* and atomic ratio of Nb and C defined by T ^* = Ti- (48/14) N- (48/32) S (Nb / 93 + Ti ^* / 48) / (C / 12) satisfies 0.15 to 0.80, and A defined by A = (48Nb + 93Ti ^* ) / 186C + 1.7-Mn satisfies A ≧ 0. The high-strength hot-dip galvanized steel sheet having excellent stretch flange formability according to claim 1. The high-strength hot-dip galvanized steel sheet having excellent stretch flange formability according to claim 1 or 2, further comprising Si: ≤ 0.5% by mass as a chemical component. The high-strength hot-dip galvanized steel sheet having excellent stretch flange formability according to any one of claims 1 to 3, further comprising, as a chemical component, B: ≤ 0.0015% by mass%. Further, as a chemical component, one or more of Cr: 0.05 to 0.5%, V: 0.05 to 0.5%, Mo: 0.05 to 0.5% are contained by mass%. 5, and A defined by A = (48Nb + 93Ti ^* ) / 186C + 1.7−Mn ′ at Mn ′ = Mn + 2.5Mo + 1.07Cr + 2.5V satisfies A ≧ 0. A high-strength hot-dip galvanized steel sheet excellent in stretch flangeability as described in 1. After hot-rolling the slab having the component according to any one of claims 1 to 5 and winding it at 650 ° C or lower to obtain a hot-rolled steel sheet, pickling and cold-rolling, the temperature is (0. 7Ac3 + 0.3Ac1) ° C to (1.4Ac3-0.4Ac1) ° C, after recrystallization annealing for 5 to 240 seconds, cooling at a rate of 10 ° C / s or less, and hot dip galvanization A method for producing a high-strength hot-dip galvanized steel sheet having excellent stretch flange formability, characterized by comprising a step of applying an alloying treatment after the application.

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