JP2015063710A - Galvanized steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength galvanized steel sheet having tensile strength of 1270 MPa or more and excellent in bendability and delayed fracture resistance and a manufacturing method therefor.SOLUTION: There is provided a galvanized steel sheet having a galvanized layer on a surface of the steel sheet having a chemical composition containing, by mass%, C:0.08 to 0.20%, Si:0.001 to 0.35%, Mn:2.55 to 3.50%, P:0.1% or less, S:0.01% or less, sol.Al:0.001 to 0.5%, N:0.02% or less, Ti:0.02 to 0.30%, V:0.02 to 0.50%, B:0.0021 to 0.0150%, Nb:0 to 0.30%, Cr:0 to 2.0%, Mo:0 to 2.0%, Cu:0 to 2.0%, Ni:0 to 2.0%, Ca:0 to 0.01%, REM:0 to 0.1%, Bi:0 to 0.05% and the balance Fe with inevitable impurities and a micro structure having the volume fraction of retained austenite of 7% or less with tensile strength in a direction perpendicular to a rolling angle direction of 1270 MPa or more and yield ratio of 0.65 to 0.90.

Description

  The present invention relates to a hot-dip galvanized steel sheet and a method for producing the same, and particularly to a high-strength hot-dip galvanized steel sheet having a tensile strength of 1270 MPa or more and a method for producing the same. In the present invention, the galvanized steel sheet includes an alloyed galvanized steel sheet.

  In recent years, in the automobile field, there has been a demand for improved fuel efficiency from the viewpoint of protecting the global environment, and improved shock absorption from the viewpoint of ensuring the safety of passengers. For example, a tensile force of 1270 MPa or more is applied to an automobile part such as a rocker reinforcement. There is a movement to apply a high strength steel plate having strength by roll forming.

  A steel sheet applied to such a site requires high bendability and corrosion resistance in addition to high strength, so it has a tensile strength of 1270 MPa or more and has high bendability and high bendability. Strong demand for steel sheets.

  For a high-strength hot-dip galvanized steel sheet having a tensile strength of 1270 MPa or more, for example, Patent Document 1 discloses a method of utilizing Si as a strengthening element. Patent Document 2 discloses a method for ensuring strength by containing a large amount of Mn, Cr, or Mo. Furthermore, Patent Document 3 discloses a method for ensuring strength by containing a large amount of V.

JP 2010-126770 A JP 2009-120878 A JP 2006-183140 A

  However, in the technique described in Patent Document 1, it is difficult to obtain good plating properties because Si easily generates an oxide on the surface layer, and residual austenite tends to remain, so that deterioration of toughness and occurrence of delayed fracture occur. There's a problem.

  Moreover, the steel plate described in Patent Document 2 is not sufficient in bendability and delayed fracture resistance. In addition, since Mn, Cr, or Mo is contained in a large amount, there is a problem that the cost is increased.

  Furthermore, in the technique described in Patent Document 3, since the utilization of B for increasing the strength is insufficient, it is necessary to increase the C content to obtain a high strength of 1270 MPa or more, and the weldability deteriorates. There is a problem.

  An object of the present invention is to provide a high-strength hot-dip galvanized steel sheet having a tensile strength of 1270 MPa or more and excellent in bendability and delayed fracture resistance, and a method for producing the same.

  In the manufacturing method of hot dip galvanized steel sheet, in order to ensure the flatness of the steel sheet and make the coating amount uniform, the cooling after annealing is not a water cooling but a cooling method such as gas jet cooling with a relatively slow cooling rate. Used. Since it is difficult to obtain high strength when the cooling rate is slow, it is necessary to contain a large amount of elements such as C, Si, Mn, etc. in order to ensure strength, and as a result, good delayed fracture resistance and toughness can be ensured. It becomes difficult.

  As a result of intensive studies to solve the above problems, the present inventors have obtained the following new knowledge.

  Among the elements that enhance the hardenability, B has a smaller effect of reducing delayed fracture resistance than other elements. Further, by optimizing the contents of Mn, Ti and V, and by containing a large amount of B of 21 ppm or more, it becomes possible to further increase the strength while suppressing the deterioration of delayed fracture resistance. .

  Normally, the effect of increasing the strength accompanying an increase in the B content is saturated with a very small amount of B. However, the austenite grains at the time of annealing are refined by the action of Ti or V, so that the strength and toughness are improved, the interfacial area of the austenite grains where B segregates is increased, and B has a high concentration of 21 ppm or more. Even in the case where it is contained, the effect of increasing the strength as the B content increases is exhibited.

  Here, in order to sufficiently exhibit the effect of improving the hardenability by B in the austenite that has been made fine by the above mechanism, the Mn content needs to be 2.55% or more.

  Moreover, when only Ti is contained, the hot-rolled steel sheet is excessively hardened, and there is a possibility that problems such as breakage may occur when cold rolling is performed. While suppressing excessive hardening, it is possible to develop the effect of increasing the strength by containing a large amount of B described above. Therefore, it is necessary to contain Ti and V.

  As described above, when Ti and V are contained, precipitates generated in the steel by these actions become hydrogen trap sites, and the delayed fracture resistance is improved.

  Furthermore, the fine structure of the microstructure and precipitates increases the number of hydrogen trap sites and improves delayed fracture resistance. Refinement of microstructure and precipitates is realized by increasing the heating rate in the two-phase region in the annealing heating process of the hot dip galvanizing process, and this state is realized by measuring the yield ratio of the final product. It can be judged whether it is done. Specifically, the yield ratio increases as the microstructure and precipitates become finer, and the delayed fracture resistance is judged to be good when the yield ratio is 0.65 or more. The yield ratio is calculated by dividing the yield strength value by the tensile strength value. Yield strength means the yield point, and means 0.2% proof stress for a steel plate that yields continuously.

  The present invention has been made on the basis of the above-mentioned knowledge, and the gist thereof is the following hot-dip galvanized steel sheet and a method for producing the same.

(1) A hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet,
The said steel plate is the mass%, C: 0.08-0.20%, Si: 0.001-0.35%, Mn: 2.55-3.50%, P: 0.1% or less, S : 0.01% or less, sol. Al: 0.001 to 0.5%, N: 0.02% or less, Ti: 0.02 to 0.30%, V: 0.02 to 0.50%, B: 0.0021 to 0.0150 %, Nb: 0 to 0.30%, Cr: 0 to 2.0%, Mo: 0 to 2.0%, Cu: 0 to 2.0%, Ni: 0 to 2.0%, Ca: 0 -0.01%, REM: 0-0.1%, Bi: 0-0.05%, balance: Fe and chemical composition as impurities,
Having a microstructure with a volume fraction of retained austenite of 7% or less,
A hot-dip galvanized steel sheet having a tensile strength in the direction perpendicular to the rolling of 1270 MPa or more and a yield ratio of 0.65 to 0.90.

(2) The chemical composition is mass%,
Nb: 0.001 to 0.30%
The hot dip galvanized steel sheet according to (1) above, containing

(3) The chemical composition is mass%,
Cr: 0.001 to 2.0% and Mo: 0.001 to 2.0%
The hot-dip galvanized steel sheet according to (1) or (2) above, which contains one or two selected from:

(4) The chemical composition is mass%,
Cu: 0.001 to 2.0% and Ni: 0.001 to 2.0%
The hot-dip galvanized steel sheet according to any one of (1) to (3), which contains one or two selected from:

(5) The chemical composition is mass%,
Ca: 0.0001 to 0.01% and REM: 0.0001 to 0.1%
The hot-dip galvanized steel sheet according to any one of (1) to (4) above, which contains one or two selected from:

(6) The chemical composition is mass%,
Bi: 0.0001-0.05%
The hot-dip galvanized steel sheet according to any one of (1) to (5), containing

  (7) The steel plate having the chemical composition according to any one of (1) to (6) above is heated to 780 to 950 ° C. with a heating rate in the temperature range of 720 to 780 ° C. being 2 ° C./second or more, Subsequently, it cools to 600 degrees C or less with the average cooling rate of 0.2-80 degrees C / second, and also manufactures the hot dip galvanized steel sheet which performs hot dip galvanization with the penetration | invasion temperature to the plating bath of this steel sheet being 400-500 degreeC. Method.

  According to the present invention, a hot dip galvanized steel sheet having a high tensile strength of 1270 MPa or more and excellent in bendability and delayed fracture resistance can be obtained. Therefore, the hot dip galvanized steel sheet according to the present invention is optimal for use in structural parts of automobiles such as rocker reinforcement.

  Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition of steel sheet The reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.08 to 0.20%
C is an element having an effect of increasing the strength of the steel sheet. If the C content is less than 0.08%, it is difficult to ensure a tensile strength of 1270 MPa or more. Therefore, the C content is 0.08% or more. On the other hand, if the C content exceeds 0.20%, the toughness and weldability are significantly reduced. Therefore, the C content is 0.20% or less. The C content is preferably 0.12% or more, and preferably 0.18% or less.

Si: 0.001 to 0.35%
Si has an effect of increasing the strength of the steel sheet, and is an element effective for strengthening ferrite, homogenizing the structure, and improving workability. If the Si content is less than 0.001%, it is difficult to obtain the effect by the above action. Therefore, the Si content is 0.001% or more. On the other hand, if the Si content exceeds 0.35%, the occurrence of non-plating in hot-dip plating becomes a problem, and the toughness, weldability, and delayed fracture resistance are significantly reduced. Therefore, the Si content is set to 0.35% or less.

Mn: 2.55-3.50%
Mn is an element having an effect of increasing the strength of the steel plate. Furthermore, in this invention, it has the important effect | action which promotes the high strengthening of a steel plate by interaction with B mentioned later. If the Mn content is less than 2.55%, it is difficult to obtain the effect by the above action. Therefore, the Mn content is 2.55% or more. On the other hand, if the Mn content exceeds 3.50%, the toughness, weldability, and delayed fracture resistance deteriorate significantly. Therefore, the Mn content is 3.50% or less. The Mn content is preferably 2.60% or more, and more preferably 2.70% or more.

P: 0.1% or less P is an element that is generally contained as an impurity and deteriorates toughness. When the P content exceeds 0.1%, the toughness is remarkably reduced. Therefore, the P content is 0.1% or less. The P content is preferably 0.02% or less.

S: 0.01% or less S is an element that is generally contained as an impurity, forms MnS in steel, and deteriorates delayed fracture resistance and hole expansibility. When the S content exceeds 0.01%, the delayed fracture resistance and the hole expandability deteriorate significantly. Therefore, the S content is 0.01% or less. The S content is preferably 0.005% or less, and more preferably 0.0012% or less.

sol. Al: 0.001 to 0.5%
Al is an element having an action of deoxidizing steel. sol. If the Al content is less than 0.001%, it is difficult to obtain the effect by the above action. Therefore, sol. The Al content is 0.001% or more. On the other hand, sol. If the Al content exceeds 0.5%, inclusions increase and workability deteriorates. Therefore, sol. The Al content is 0.5% or less.

N: 0.02% or less N is generally contained as an impurity, and forms a nitride during continuous casting, causing cracks in the slab. If the N content exceeds 0.02%, the slab will be cracked significantly. Therefore, the N content is 0.02% or less.

Ti: 0.02 to 0.30%
Ti is an element having an action of preventing N from bonding with N in steel to form a nitride, and B becoming a nitride to deteriorate the effect of B described later. Moreover, it has the effect | action which refines | miniaturizes the austenite at the time of annealing. When the Ti content is less than 0.02%, it is difficult to obtain the effect by the above action. Therefore, the Ti content is 0.02% or more. On the other hand, if the Ti content exceeds 0.30%, the hot-rolled steel sheet is excessively hardened, and the cold rolling property is hindered when cold rolling is performed. Therefore, the Ti content is set to 0.30% or less. The Ti content is preferably 0.08% or less.

V: 0.02-0.50%
V is an element that has the effect of forming precipitates in the steel and refining the microstructure. In addition, the precipitate formed in the steel becomes a hydrogen trap site, and has an effect of improving delayed fracture resistance. Furthermore, the increase in the strength of the hot-rolled steel sheet accompanying the increase in the content is moderate, and when cold rolling is performed, the cold rolling property is not hindered so much. When the V content is less than 0.02%, it is difficult to obtain the effect by the above action. Therefore, the V content is 0.02% or more. On the other hand, if the V content exceeds 0.50%, the effect of the above action is saturated and disadvantageous in cost. Therefore, the V content is 0.50% or less. The V content is preferably 0.10% or more, and preferably 0.30% or less.

B: 0.0021 to 0.0150%
B is an element having an effect of increasing the strength of the steel sheet by suppressing the nucleation of ferrite from the austenite grain boundary and enhancing the hardenability. Further, B is an element effective in obtaining a tensile strength of 1270 MPa or more with a trace amount contained compared to other elements and ensuring good delayed fracture resistance. When the B content is less than 0.0021%, it is difficult to sufficiently obtain the effect of the above action. Therefore, the B content is 0.0021% or more. On the other hand, if the B content exceeds 0.0150%, the effect of the above action is saturated, which is disadvantageous in cost. Therefore, the B content is 0.0150% or less. The B content is preferably 0.0025% or more, and more preferably 0.0030% or more. Further, the B content is preferably 0.0060% or less.

Nb: 0 to 0.30%
Nb, like V, is an element that forms precipitates in steel and has the effect of refining the microstructure. Therefore, you may contain Nb as needed. However, if the Nb content exceeds 0.30%, the hot-rolled steel sheet is excessively hardened, and cold-rollability is hindered when cold rolling is performed. Therefore, the Nb content in the case of inclusion is 0.30% or less. In order to obtain the effect of the above action more reliably, the Nb content is preferably set to 0.001% or more.

Cr: 0 to 2.0%
Mo: 0 to 2.0%
Cr and Mo are elements that have the function of promoting transformation strengthening by stabilizing austenite in the same manner as Mn, and the function of increasing the strength of the steel sheet. Therefore, you may contain 1 type or 2 types selected from these elements as needed. However, when it contains exceeding the said upper limit, there exists a possibility of causing the fall of chemical conversion treatment property in any case. Therefore, the content of Cr and Mo when contained is set to 2.0% or less. In order to obtain the effect of the above action more reliably, the content of at least one of the elements is preferably 0.001% or more.

Cu: 0 to 2.0%
Ni: 0 to 2.0%
Cu and Ni are elements that have a corrosion-inhibiting action and have an action to improve delayed fracture resistance by concentrating on the surface and suppressing the penetration of hydrogen. Therefore, you may contain 1 type or 2 types selected from these elements as needed. However, even if the content exceeds the upper limit, the effect of the above action is saturated and disadvantageous in cost. Therefore, the content of Cu and Ni when contained is set to 2.0% or less. In order to obtain the effect of the above action more reliably, the content of at least one of the elements is preferably 0.001% or more.

Ca: 0 to 0.01%
REM: 0 to 0.1%
Ca and REM have the effect | action which improves local ductility by couple | bonding with S in steel and making a sulfide spherical. Therefore, you may contain 1 type or 2 types selected from these elements as needed. However, even if the content exceeds the upper limit, the effect of the above action is saturated and disadvantageous in cost. Therefore, when Ca is included, the Ca content is 0.01% or less, and the REM content is 0.1% or less. In order to obtain the effect of the above action more reliably, the content of at least one of the elements is preferably 0.0001% or more. Note that REM means 17 elements in which Y and Sc are added to 15 elements of lanthanoid.

Bi: 0 to 0.05%
When Mn and the like are microsegregated, a band structure with non-uniform hardness develops and the workability decreases. Bi is an element that has the effect of concentrating on the solidification interface to narrow the dendrite interval and reduce the solidification segregation. Therefore, Bi may be included as necessary. However, if the Bi content exceeds 0.05%, the surface properties may deteriorate. Therefore, the Bi content in the case of inclusion is 0.05% or less. The Bi content is preferably 0.01% or less, and more preferably 0.0050% or less. In order to obtain the effect by the above action more reliably, the Bi content is preferably 0.0001% or more, and more preferably 0.0003% or more.

  The steel sheet of the present invention has a chemical composition comprising the above-described elements from C to Bi, the balance Fe and impurities.

  Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means something.

(B) Microstructure of steel sheet Residual austenite volume ratio: 7% or less Residual austenite has the effect of transforming into martensite during processing to release hydrogen and worsening delayed fracture resistance. If the volume fraction of retained austenite exceeds 7%, it is difficult to ensure good delayed fracture resistance under a high tensile strength of 1270 MPa or more. Therefore, the volume ratio of retained austenite is 7% or less. The microstructure other than the volume fraction of retained austenite is not particularly defined, but the volume fraction of polygonal ferrite is preferably set to 15% or less in order to prevent the bendability from being deteriorated due to the non-uniformity of the microstructure.

(C) Mechanical properties of hot-dip galvanized steel sheet Tensile strength in the direction perpendicular to rolling: 1270 MPa or more If the tensile strength in the direction perpendicular to rolling is less than 1270 MPa, the impact absorbability is insufficient and it is not possible to meet the recent demand for higher strength. . Therefore, the tensile strength in the direction perpendicular to the rolling is 1270 MPa or more. Note that the direction perpendicular to the rolling means a direction perpendicular to the rolling direction.

Yield ratio in the direction perpendicular to rolling: 0.65 to 0.90
If the yield ratio in the direction perpendicular to the rolling is less than 0.65, the bendability and delayed fracture resistance deteriorate significantly due to the formation of coarse grains or coarsening of precipitates. Therefore, the yield ratio in the direction perpendicular to rolling is set to 0.65 or more. On the other hand, when the yield ratio exceeds 0.90, the delayed fracture resistance is remarkably deteriorated due to an increase in non-recrystallized grains. Therefore, the yield ratio in the direction perpendicular to rolling is set to 0.90 or less. The yield ratio in the direction perpendicular to the rolling is preferably 0.70 or more.

(D) Manufacturing conditions The manufacturing method of the hot dip galvanized steel sheet according to the present invention is not particularly limited. For example, it can be manufactured by a manufacturing method described below.

  The steel plate having the above chemical composition is heated to 780 to 950 ° C. at a heating rate in the temperature range of 720 to 780 ° C. of 2 ° C./second or more, and then 600 ° C. with an average cooling rate of 0.2 to 80 ° C./second. The steel sheet is cooled to the following temperature, and hot dip galvanization is performed at a penetration temperature of the steel sheet into the plating bath of 400 to 500 ° C.

  The temperature range of 720 to 780 ° C. is a temperature range including a two-phase region of ferrite and austenite. When the heating rate in the temperature range is less than 2 ° C./second, the austenite previously precipitated grows to form coarse grains. In some cases, the bendability and delayed fracture resistance may be deteriorated due to the formation or coarsening of precipitates. Therefore, the heating rate in the above temperature range is preferably 2 ° C./second or more. The heating rate in the temperature range is more preferably 3 ° C./second or more, and further preferably 5 ° C./second or more. The upper limit of the heating rate in the temperature range does not need to be specified in particular, but it is not preferable to make the heating rate in the temperature range higher than 50 ° C./second because excessive equipment is required. Therefore, the heating rate in the above temperature range is preferably 50 ° C./second or less.

  When the maximum heating temperature is less than 780 ° C., austenitization may be insufficient and the strength may be insufficient. In addition, when the steel sheet to be hot dip galvanized is a cold-rolled steel sheet, a large number of non-recrystallized grains remain in the final product, and elongation may be lowered and delayed fracture resistance may be deteriorated. Therefore, the maximum heating temperature is preferably 780 ° C. or higher, and more preferably 820 ° C. or higher. On the other hand, when the maximum heating temperature exceeds 950 ° C., austenite becomes coarse and deteriorates delayed fracture resistance. Therefore, the heating temperature is preferably 950 ° C. or less, and more preferably 900 ° C. or less.

  If the average cooling rate from the maximum heating temperature to 600 ° C. is less than 0.2 ° C./second, residual austenite may remain excessively and good delayed fracture resistance may not be obtained. In addition, a sufficient hard phase may not be obtained, or carbides may be excessively precipitated, which may make it difficult to ensure the desired high strength. Therefore, the average cooling rate from the maximum heating temperature to 600 ° C. is preferably 0.2 ° C./second or more, and more preferably 5 ° C./second or more.

  On the other hand, if the average cooling rate from the maximum heating temperature to 600 ° C. exceeds 80 ° C./second, the flatness may deteriorate and unevenness in the amount of plating may occur, or the plate may flutter and the plate passing property may deteriorate. There is. Therefore, the average cooling rate from the maximum heating temperature to 600 ° C. is preferably 80 ° C./second or less, and more preferably 60 ° C./second or less.

  If the cooling stop temperature from the maximum heating temperature exceeds 600 ° C., the fine fracture becomes insufficient or the precipitate becomes coarse, resulting in deterioration of delayed fracture resistance. Therefore, the cooling from the maximum heating temperature is preferably performed to at least 600 ° C. The lower limit of the cooling stop temperature from the maximum heating temperature does not need to be specified in particular, but it is better in terms of suppressing the formation of tempered martensite and ensuring high strength more reliably, or suppressing the coarsening of carbides. From the standpoint of ensuring the delayed fracture resistance, it is preferably 150 ° C. or higher.

  In order to make the zinc adhesion amount uniform, isothermal holding may be performed after cooling is stopped. If the isothermal holding time is too long, excessive austenite remains and good delayed fracture resistance may not be obtained. Therefore, the isothermal holding time is preferably within 400 seconds.

  Although hot dip galvanization is performed after cooling from the maximum heating temperature, if the steel sheet temperature when immersed in the plating bath is less than 400 ° C., the zinc adhesion amount tends to vary. Therefore, it is preferable that the temperature of the steel sheet when immersed in the hot dip galvanizing bath is 400 ° C. or higher. On the other hand, if the steel plate temperature when immersed in the hot dip galvanizing bath exceeds 500 ° C., alloying at the interface between the steel plate and the plating layer may proceed excessively and the plating adhesion may deteriorate. Therefore, it is preferable that the steel plate temperature at the time of being immersed in the hot dip galvanizing bath is 500 ° C. or less.

  After being immersed in a hot dip galvanizing bath, the zinc adhesion amount is controlled to a predetermined value by gas wiping or the like, and after alloying as necessary, it is cooled to room temperature.

  Furthermore, there is no problem even if a skin pass and / or leveler is applied for flattening and surface roughness adjustment, and an oil coating or a film having a lubricating action may be applied.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steel having the chemical composition shown in Table 1 was melted in an experimental furnace to produce a slab having a thickness of 40 mm. Further, after hot rolling at a heating temperature of 1250 ° C. and a finishing temperature of 910 ° C., water spray cooling of about 30 ° C./second was performed, and the coiling temperature was set to 660 ° C. to produce a hot-rolled steel plate having a thickness of 2.6 mm. Winding is simulated by water cooling to the coiling temperature and then charging into the furnace, holding at the coiling temperature for 60 minutes, and then cooling the furnace to 200 ° C or less at a cooling rate of 20 ° C / hour. did. Thereafter, the scale was removed by pickling and cold rolling was performed to a plate thickness of 1.2 mm. A test material was sampled from the steel sheet thus obtained and subjected to heat treatment simulating a continuous hot-dip galvanized steel sheet manufacturing process. Annealing was heated to 700 ° C. at 5 ° C./second, then heated to the annealing temperature (maximum heating temperature) under the conditions shown in Table 2, and then subjected to primary cooling. Some test materials were kept isothermally and then cooled to 460 ° C., which was the hot-dip bath temperature, at 5 ° C./second and subjected to hot dip galvanizing treatment. Thereafter, some of the test materials were further alloyed and cooled to room temperature at 10 ° C./second.

  In the tensile test, No. 5 tensile test piece defined in Japanese Industrial Standard JIS Z 2241 was collected from the obtained galvanized steel sheet so that the direction perpendicular to the rolling direction was the tensile direction, and the yield strength (YS ), Tensile strength (TS) and total elongation (El). The yield ratio (YR) was determined by dividing YS by TS. The amount of retained austenite was measured by performing an X-ray diffraction test at a 1/4 depth position of the thickness of the steel plate as the base material from the boundary between the steel plate as the base material and the plating layer. Moreover, the hole expansion test was done according to JIS Z 2256.

  Next, a U-notch with a depth of 0.2 mm was given to a test piece having a plate thickness of 1.0 mm by double-side grinding, and fixed in a 4-point bending state where the surface distortion was 1.4% when there was no notch. Then, it was immersed in a 0.1% ammonium thiocyanate aqueous solution at 35 ° C. for 7 days to determine the presence or absence of cracks, and the delayed fracture resistance was evaluated. Furthermore, in accordance with JIS Z 2248, No. 3 bending test piece having the direction perpendicular to the rolling direction as the longitudinal direction was sampled and subjected to a bending test by the V block method. The radius of the tip of the pressing metal was changed at a pitch of 0.5 mm within a range of 2.0 to 6.0 mm. After the bending test, the top of the test piece was observed using a microscope, and the minimum radius of the press fitting in which no crack was observed was defined as the limit bending R.

  In the present invention, the case where El was 7% or more was judged to be excellent in ductility. Moreover, when the hole expansion rate in the hole expansion test was 30% or more, it was judged that the hole expansion property was excellent. And it was judged that the case where the limit bending R in a bending test was 3.0 mm or less was excellent in bendability. The above results are shown in Table 3.

  Test No. which is an example of the present invention. The galvanized steel sheets 1 to 13 and 19 to 30 exhibited high strength of 1270 MPa or more in tensile strength and good delayed fracture resistance. Also, the bendability, ductility and hole expandability were good.

  On the other hand, test No. in which B is not added. 16 and test No. with low Mn content. No. 14 has a low tensile strength and contains a large amount of Si. 15, Test No. to which V was not added. Test No. 17 with no addition of 17 and Ti. No. 18 had a low YR and was inferior in delayed fracture resistance and bendability.

  In addition, Test No. Nos. 31 and 35 had low tensile strength, high YR, and inferior delayed fracture resistance, bendability, ductility and hole expandability. Test No. Nos. 32 and 37 had low tensile strength and low YR, resulting in inferior delayed fracture resistance, bendability and hole expandability. Test No. 33, 34, 36, 38 and 39 had low YR, resulting in poor delayed fracture resistance, bendability and hole expandability.

  According to the present invention, a hot dip galvanized steel sheet having a high tensile strength of 1270 MPa or more and excellent in bendability and delayed fracture resistance can be obtained. Therefore, the hot dip galvanized steel sheet according to the present invention is optimal for use in structural parts of automobiles such as rocker reinforcement.

Claims (7)

A hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet,
The said steel plate is the mass%, C: 0.08-0.20%, Si: 0.001-0.35%, Mn: 2.55-3.50%, P: 0.1% or less, S : 0.01% or less, sol. Al: 0.001 to 0.5%, N: 0.02% or less, Ti: 0.02 to 0.30%, V: 0.02 to 0.50%, B: 0.0021 to 0.0150 %, Nb: 0 to 0.30%, Cr: 0 to 2.0%, Mo: 0 to 2.0%, Cu: 0 to 2.0%, Ni: 0 to 2.0%, Ca: 0 -0.01%, REM: 0-0.1%, Bi: 0-0.05%, balance: Fe and chemical composition as impurities,
Having a microstructure with a volume fraction of retained austenite of 7% or less,
A hot-dip galvanized steel sheet having a tensile strength in the direction perpendicular to the rolling of 1270 MPa or more and a yield ratio of 0.65 to 0.90. The chemical composition is mass%,
Nb: 0.001 to 0.30%
The hot-dip galvanized steel sheet according to claim 1 containing The chemical composition is mass%,
Cr: 0.001 to 2.0% and Mo: 0.001 to 2.0%
The hot-dip galvanized steel sheet according to claim 1 or 2, which contains one or two selected from. The chemical composition is mass%,
Cu: 0.001 to 2.0% and Ni: 0.001 to 2.0%
The hot-dip galvanized steel sheet according to any one of claims 1 to 3, which contains one or two selected from: The chemical composition is mass%,
Ca: 0.0001 to 0.01% and REM: 0.0001 to 0.1%
The hot-dip galvanized steel sheet according to any one of claims 1 to 4, which contains one or two selected from: The chemical composition is mass%,
Bi: 0.0001-0.05%
The hot-dip galvanized steel sheet according to any one of claims 1 to 5, comprising:   A steel plate having the chemical composition according to any one of claims 1 to 6 is heated to 780 to 950 ° C at a heating rate of 2 ° C / second or more in a temperature range of 720 to 780 ° C, A method for producing a hot dip galvanized steel sheet, wherein the steel sheet is cooled to 600 ° C. or lower at an average cooling rate of 2 to 80 ° C./second, and further hot dip galvanized at a penetration temperature of 400 to 500 ° C. of the steel sheet.