JP2007262553A - Hot dip galvanized steel sheet and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot dip galvanized steel sheet which has a strength satisfying a tensile strength (TS) of ≥590 MPa and a fatigue strength (FL) of ≥200 MPa in a state of having a blanked fracture. <P>SOLUTION: The hot dip galvanized steel sheet has a composition comprising, by mass, 0.05 to 0.30% C, 0.8 to 3.00% Mn, 0.003 to 0.100% P, ≤0.010% S, 0.10 to 2.50% Al, 0.03 to 0.50% Cr and ≤0.007% N, and the balance substantially Fe with inevitable impurities, has a structure comprising a ferrite phase, a retained austenite phase and a low temperature transformed phase, and in which the fraction of the ferrite phase is ≤97% by volume ratio, and also, AlN is precipitated in a region to 1 μm from the steel surface except a plated layer, and also has the characteristics of a tensile strength (TS) satisfying ≥590 MPa and a fatigue strength (FL) satisfying ≥200 MPa in a state of having a blanked fracture. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a hot-dip galvanized steel sheet used in industrial fields such as automobiles and electricity and a method for producing the same, and in particular, fatigue strength (FL) in a state where a tensile strength (TS) is 590 MPa or more and a punched fracture surface is present ( The present invention relates to a hot-dip galvanized steel sheet having a high strength of 200 MPa or more and a manufacturing method thereof. In addition to the above properties, the present invention further provides a high strength and high fatigue strength in which the formability index, ie, the product of strength (TS) and total elongation (El), (TS) × (El) is 21000 MPa ·% or more Further, the present invention relates to a hot-dip galvanized steel sheet having high formability and a method for producing the same.

  In recent years, in order to achieve both improvement in fuel efficiency of automobiles and improvement in safety at the time of automobile collision, the strength and thinning of body materials have been attempted. For this purpose, high-strength steel plates are used as car body materials, but automobiles are subject to repeated vibrations as they travel, so car body materials are required to have high fatigue strength in addition to high strength. It has been. Especially in China, India, Russia, etc., where motorization has been active recently, there are still many unpaved roads, and the vibration applied when driving a car is large, so the fatigue strength for these areas is higher than before. There is a need for high strength steel sheets.

  Further, in the manufacturing process for automobiles, the steel sheet is formed while being punched, and often assembled with a punched end face, but in a state having such a punched end face, it inevitably occurs on the punched end face, Alternatively, fatigue strength decreases due to remaining cracks and the like. For this reason, automobile steel plates, particularly parts in the lower part of the vehicle body, are required to have high strength and fatigue strength in a state having a punched end face, that is, high fatigue strength while being punched. This punched fatigue strength is governed by the propagation suppression force of microcracks that already exist. Fatigue strength evaluated using conventional machined specimens with no microcracks on the surface ( This is fundamentally different from the fact that "machining fatigue strength" is largely governed by resistance to fatigue crack initiation. Incidentally, a typical hot dip galvanized steel sheet has a machining fatigue strength of about 500 MPa, while the punched fatigue strength may be about 150 MPa.

  Furthermore, because the lower part of the car body is exposed to corrosive environments containing rainwater and salt for winter snow melting, etc., in addition to the above, corrosion resistance is required, and at the same time, thinning is required to reduce the car body weight. For example, a hot dip galvanized steel sheet having a thickness of 2.3 mm or less is often used. Such a hot dip galvanized steel sheet is also required to have a high fatigue strength while being punched as well as a high strength steel sheet.

  Needless to say, it is desired that the steel plate having high fatigue strength while being punched with high strength also has high formability and high workability.

  In Patent Document 1, the microstructure is composed of three phases of ferrite, martensite, and retained austenite, and the martensite space factor is 1% or more and 20% or less, and the residual austenite space rate of 5 μm or less is 1% or more. A high-strength hot-rolled steel sheet for processing excellent in fatigue durability of 10% or less is disclosed.

  In Patent Document 2, the steel sheet structure is composed of a composite structure composed of ferrite as a main phase, retained austenite with a volume ratio of 3% or more, and a low-temperature transformation phase, and 70% or more of the retained austenite has an aspect ratio. A high-tensile hot-dip galvanized steel sheet that is excellent in strength-elongation balance and fatigue properties in which a hot-dip galvanized layer is formed on the surface of steel having a ratio of 0.2 to 0.4 is disclosed.

  In Patent Document 3, the steel sheet structure has a composite structure composed of ferrite, tempered martensite, retained austenite, and low-temperature transformation phase, and the ferrite is in a volume ratio of 30% or more, and the tempered martensite is in volume. High-tensile galvanized steel sheet having excellent ductility and stretch flangeability, including 20% or more in terms of percentage, 2% or more of the retained austenite in terms of volume percentage, and having an average crystal grain size of the ferrite and tempered martensite of 10 μm or less Is disclosed.

Patent Document 4 has a composite structure composed of a main phase of ferrite and a second phase, and the second phase includes bainite: 80 vol% or more, and the balance is martensite, residual austenite, and pearlite. The ratio of the average length in the sheet thickness direction to the average length in the rolling direction of the second phase is 0.7 or more, using the hot rolled sheet as a raw material, cold rolling the hot rolled sheet, Hold the primary heat treatment for 5s or more in the temperature range above the Ac ₁ transformation point, then cool to a temperature below the Ms point at a cooling rate of 10 ° C / s or more, and further, the Ac ₁ transformation point or primary heating After performing the secondary heat treatment that is held for 5 to 120 seconds in the temperature range of the temperature, it is cooled to a temperature of 500 ° C. or less at a cooling rate of 5 ° C./s or more, and then subjected to hot dip galvanizing treatment. Excellent ductility and fatigue resistance for cooling to 300 ° C at a cooling rate of ℃ / s or more Method for producing a high-tensile galvanized steel sheet has been disclosed.

  In Patent Document 5, the steel sheet structure is composed of a composite structure composed of ferrite as a main phase, retained austenite with a volume ratio of 3% or more, and a low-temperature transformation phase, and the ratio of martensite in the low-temperature transformation phase. Is 20% or less and the hardness ratio of bainite in the low-temperature transformation phase and ferrite as the main phase is 2.6 or less, and 70% or more of the retained austenite has an aspect ratio of 0.2 to 0.4 on the surface of the steel sheet. A high-tensile hot-dip galvanized steel sheet having excellent fatigue characteristics and hole expansibility formed by forming a hot-dip galvanized layer is disclosed.

  Patent Document 6 discloses a steel sheet for surface treatment having a steel composition containing Al: 0.1% or more and less than 3% by mass% and having an AlN precipitated phase in the surface layer portion.

Japanese Patent Laid-Open No. 06-65677 JP 2004-256836 A JP 2001-192768 A Japanese Patent Laid-Open No. 2003-326441 JP 2004-292891 A JP 2004-162163 A

  The above Patent Documents 1 to 5 disclose the composition and structure condition of a steel plate having high strength and excellent fatigue durability, but the target fatigue durability is the machining fatigue strength, and the present invention is intended. The fatigue strength is not shown as punched. On the other hand, Patent Document 6 discloses a steel sheet for surface treatment having an AlN precipitated phase in the surface layer portion, but does not disclose improvement in fatigue strength while being punched.

  Therefore, the first object of the present invention is specifically a high strength with a tensile strength (TS) of 590 MPa or more and a fatigue strength (fatigue strength without punching) (FL) in a state having a punched fracture surface of 200 MPa or more. An object of the present invention is to provide a hot-dip galvanized steel sheet having high fatigue strength in a state having a punched end face. In addition to the above characteristics, the second purpose is to provide a high formability and high workability, in other words, a workability index expressed by the product of strength (TS) and total elongation (El), (TS) X (El) is to provide a hot-dip galvanized steel sheet of 21000 MPa ·% or more. Furthermore, the third object of the present invention is to provide a method for producing a hot dip galvanized steel sheet having the above characteristics.

  As a result of scrutinizing the relationship between the fatigue strength as punched and the precipitation state of AlN precipitated in the steel sheet, the present inventors have precipitated AlN over at least 1 μm from the steel sheet surface excluding the plating layer. Was found to contribute to the improvement of fatigue strength while being punched, and the present invention was completed.

  The hot dip galvanized steel sheet according to the present invention is, by mass ratio, C: 0.05 to 0.30%, Mn: 0.8 to 3.00%, P: 0.003 to 0.100%, S: 0.010% or less, Al: 0.10 to 2.50%, Cr : 0.03-0.50%, N: 0.007% or less, the balance being substantially composed of Fe and inevitable impurities, including ferrite phase, residual austenite phase and low-temperature transformation phase, the ferrite phase fraction is 97 by volume %, And has a structure in which AlN is deposited in the region from the steel surface excluding the plating layer to 1 μm, and the tensile strength (TS) is 590 MPa or more. Has characteristics of 200 MPa or more.

  In the above invention, Si can be contained in the range of 1.40% or less, V: 0.005-2.0%, Mo: 0.005-2.0%, Ni: 0.0O5-2.0%, Cu: 0.005-2.0% Furthermore, one or more elements selected from one or more elements, Ti: 0.01 to 0.20%, Nb: 0.005 to 0.10%, and B: 0.0002 to 0.005% can be contained.

  In the above inventions, one or two elements selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% can be contained.

  The hot dip galvanized steel sheet according to the present invention preferably has a retained austenite phase fraction of 3% or more by volume ratio.

  In addition, the hot-dip galvanized steel sheet according to the present invention desirably has a product of tensile strength (TS) and total elongation value (El), (TS) × (El) of 21000 MPa ·% or more.

The hot-dip galvanized steel sheet according to the invention is a hot-rolled and cold-rolled slab having any one of the compositions described above to obtain a cold-rolled steel sheet,
The obtained cold-rolled steel sheet is subjected to primary heating to be heated to 300 ° C. or higher and lower than 570 ° C. in an atmosphere having an air ratio of 1.0 to 2.0, and further heated to a maximum heating temperature of 900 ° C. or lower. And after maintaining in the maximum heating temperature region for 15 s or more and 600 s or less, the product is cooled to a hot dip galvanizing temperature and subjected to a hot dip galvanizing process.

  In the above production method, the maximum heating temperature is preferably 750 ° C. to 900 ° C., and cooling from the maximum heating temperature region is rapid cooling at a cooling rate of 5 ° C./s or more, and the rapid cooling stop temperature Is preferably set to 550 ° C. or lower and up to a hot dip galvanizing temperature. Moreover, it is preferable that the air ratio of the said secondary heating shall be 0.8-1.1.

  The hot-dip galvanized steel sheet according to the present invention has the characteristics that the tensile strength (TS) is 590 MPa or more, and the fatigue strength is 200 MPa or more while being punched.For example, to extend the service life of an automobile traveling on an unpaved road. Contribute. Moreover, since the hot dip galvanized steel sheet according to the present invention is excellent in workability, a complicated automobile part is suitable as a forming material.

  Hereinafter, the composition, steel structure and characteristic values of the hot dip galvanized steel sheet according to the present invention will be described.

(composition)
C: 0.05 to 0.30% (mass%, hereinafter the same unless otherwise specified)
C is an element that stabilizes austenite, and is an element necessary for increasing the strength of the steel sheet by generating phases other than ferrite. Further, when C is concentrated in the retained austenite by a certain amount or more, the formability of the steel sheet is improved by the TRIP effect. If the C content is less than 0.05%, it is difficult to ensure the strength. If the C content exceeds 0.30%, the welded part and the heat-affected zone are significantly hardened and the weldability deteriorates. Therefore, the C content is in the range of 0.05 to 0.30%, preferably 0.08% to 0.20%.

Mn: 0.8 ~ 3.00%
Mn is an element effective for strengthening steel. Further, it is an element that stabilizes austenite, and is an element that is necessary for increasing the volume of phases other than ferrite to increase TS to 590 MPa or more. This effect is obtained when Mn is 0.8% or more. However, if Mn exceeds 3.00%, the second phase fraction becomes excessive, the strength increases due to solid solution hardening, and the moldability decreases.

P: 0.003-0.100%
P is an element effective for strengthening steel, and this effect is obtained at 0.003% or more. However, if it exceeds 0.100% and is added excessively, it causes embrittlement due to grain boundary segregation and degrades impact resistance. Therefore, the P content is limited to 0.003% to 0.100%.

S: 0.010% or less
S is a product element of non-metallic inclusions such as MnS, and it is better to be as low as possible because it causes deterioration of impact resistance and cracks along the metal flow of the weld. However, it is 0.010% from the viewpoint of manufacturing cost. The following.

Al: 0.10-2.50%
Al is a component element indispensable for generating AlN. If A1 is less than 0.10%, the amount of AlN produced is insufficient in the annealing process described later, and the fatigue strength cannot be improved as it is, which is the object of the present invention. On the other hand, when the Al content is more than 2.50%, inclusions in the steel sheet, for example, Al ₂ O ₃ increases, and ductility is deteriorated. Therefore, the Al content is set to 0.10 or more and 2.50% or less. In addition, Al has the effect | action which suppresses deterioration of the plating property and plating surface property by Si, and this effect is acquired at 0.30% and is saturated at 1.2%. From this standpoint, the amount of Al added is preferably 0.3 to 1.20%.

Cr: 0.03-0.50%
Cr is an effective element to stably produce phases other than ferrite by stabilizing austenite during cooling after annealing and galvanizing treatment, and efficiently increases the strength of steel sheet to 590 MPa or more. Is effective. The effect is manifested at 0.03% or more, and at 0.1% or more, the effect of the cooling rate on the strength during heat treatment in the manufacturing process described later is reduced, and it greatly contributes to the stability of product characteristic values. Therefore, it is preferable to contain 0.10% or more. However, if it exceeds 0.50%, the galvanizing property deteriorates. Therefore, the Cr content is 0.03-0.50%, preferably 0.1-0.5%.

N: 0.007% or less
When the N content exceeds 0.007%, coarse AlN inside the steel sheet increases, and the fatigue characteristics deteriorate rapidly. Therefore, the N content is 0.007% or less.

  The balance composition excluding the above composition components substantially consists of Fe and inevitable impurities. However, it is possible to contain the following elements according to the required characteristics of steel, such as strength improvement.

Si: 1.40% or less
Si has the effect of solid-solution strengthening the steel matrix, and is a ferrite-forming element, and has the effect of promoting the formation of retained austenite by suppressing the concentration of C in the austenite and the formation of carbides. It is a preferred additive element for the production of high-strength steel containing residual austenite. The solid solution strengthening action and residual austenite formation action of the steel matrix is stronger than Al, and a part of Al can be replaced with Si, and the total amount of Al + Si is 0.3% or more, so good elongation is achieved. Characteristics can be obtained. However, when the Si content is excessive, the formability and toughness deteriorate due to an increase in the amount of solid solution in ferrite, and the surface properties due to the occurrence of red scale deteriorate. In addition, when hot-dip plating is performed, plating adhesion and adhesion are deteriorated. Therefore, the upper limit of Si is 1.40%, and preferably 0.2 to 1.00% can be added and contained.

V: 0.005-2.0%, Mo: 0.005-2.0%, Ni: 0.005-2.0% and Cu: 0.005-2.0%
These elements have the effect of suppressing the formation of pearlite during cooling from the annealing temperature and facilitating the formation of phases other than ferrite. The effect can be obtained at 0.005% or more, but if it exceeds 2.0%, the effect is saturated, which causes a cost increase. Accordingly, these elements can be added or contained within the above ranges alone or in combination.

Ti: 0.01-0.20% and Nb: 0.005-0.10%
These elements are effective for strengthening steel mainly by precipitation effects. In the case of Ti, the effect is obtained at 0.01% or more, and in the case of Nb, it is obtained at 0.005% or more. However, the effect is saturated at 0.2% in the case of Ti and 0.1% in the case of Nb, and addition exceeding this causes a cost increase. Accordingly, these elements can be added or contained within the above ranges alone or in combination.

B: 0.0002 to 0.0050%
B has the effect of suppressing the formation of ferrite from the austenite grain boundaries and increasing the strength. The effect is obtained at 0.0002% or more. However, addition exceeding 0.0050% causes a cost increase. Therefore, these B can be added and contained within a range of 0.0002 to 0.0050%.

Ca: 0.001 to 0.005% and REM: 0.001 to 0.005%
These elements have an effect of contributing to improvement of elongation, that is, improvement of moldability by improving local ductility. In the case of Ca, the effect is obtained at 0.001% or more, and is saturated at 0.005%. On the other hand, REM is obtained at 0.001% or more and saturates at 0.005%. Accordingly, these elements can be added or contained within the above ranges alone or in combination.

(Steel structure)
The steel of the present invention is a composite structure steel containing a ferrite phase, a retained austenite phase, and a low-temperature transformation phase. Among these, the constituent ratio (ferrite phase fraction) of the ferrite phase is limited to 97% or less. If the ferrite phase fraction exceeds 97%, the tensile strength (TS) cannot be increased to 590 MPa or more. On the other hand, if the ferrite phase fraction is less than 20%, sufficient workability cannot be obtained, for example, the total elongation (El) is 10% or less. Therefore, the ferrite phase fraction is preferably 20% or more.

  By limiting the upper limit of the ferrite phase fraction to 97% or less, the tensile strength (TS) can be increased to 590 MPa or higher, but the high formability index (TS) x (El) is 18000 MPa ·% or higher. In order to obtain the properties, the retained austenite phase fraction needs to be 3% or more. In order to obtain higher workability, for example, a formability index of 21000 MPa ·% or more, it is preferable to set the retained austenite phase fraction to 5% or more. Is preferable.

  In addition, the ferrite phase fraction is a 1000 times image taken using SEM for the position of the thickness of the product galvanized steel sheet 1/4, and measured by image processing, the residual austenite phase fraction is The surface obtained by grinding the product from the surface to a 1/4 position of the plate thickness and then polishing by 0.1 mm by chemical polishing, using Mo Kα ray with an X diffraction diffractometer, (200), ( 220), (311) plane, and (200) plane, (211) plane, and (220) plane integral strength of bcc iron were measured, and based on these, the retained austenite phase fraction was obtained.

  In the present invention, in order to impart fatigue strength with high punching in such high-strength steel, it is assumed that AlN is precipitated over at least a region (depth) from directly below the steel surface to 1 μm. Here, AlN refers to a precipitate in which both N and Al are detected by observing the cross section of the steel sheet with EPMA at five positions at a width of 1/4 of the hot-dip galvanized steel sheet, and its maximum precipitation depth. This is defined as “AlN precipitation depth”. Since AlN exists from directly under the steel sheet surface, the “AlN precipitation depth” means that AlN is precipitated from directly under the steel sheet surface to that depth. In the present invention, it is necessary that the AlN defined as described above is deposited in the range of at least 1 μm from the steel sheet surface, in other words, the AlN deposition depth is 1 μm or more. The steel sheet surface refers to the surface of the steel sheet excluding the plated layer of the hot dip galvanized steel sheet to which the present invention is applied, in other words, the boundary surface between the plated layer of the hot dip galvanized steel sheet and the base plate.

  FIG. 1 is a graph showing the relationship between the AlN precipitation depth measured by the above method and the fatigue strength as punched. As shown in FIG. 1, when the AlN precipitation depth is 1 μm or more, the fatigue strength is 200 MPa or more while being punched.

  The hot-dip galvanized steel sheet according to the present invention is a slab having the above-described composition, for example, manufactured by continuous casting, hot-cold and cold-rolled to obtain a cold-rolled steel sheet, After subjecting the rolled steel sheet to primary heating from 300 ° C to less than 570 ° C in an atmosphere with an air ratio of 1.0 or more and 2.0 or less, heating is continued until the maximum heating temperature of 750 ° C to 900 ° C or less. The secondary heating is performed, and is maintained at 15 to 600 s in the maximum heating temperature range of the secondary heating, and then cooled to a hot dip galvanizing temperature and then hot dip galvanized. it can. Here, the air ratio is the ratio of the amount of air consumed (Vc) during operation to the amount of air (Vt) sufficient to completely burn the gaseous fuel when continuously heating the steel sheet using a direct-fired heating furnace ( Vc / Vt (volume ratio)).

  In the above, heating to 300 ° C or more and less than 570 ° C in an atmosphere with an air ratio of 1.0 or more and 2.0 or less is so that AlN is generated in a region from the steel plate surface to 1 µm, in other words, a region very close to just below the surface. It is to do. In addition, the heating temperature in an atmosphere with an air ratio of 1.0 or higher is set to 300 ° C. or higher and lower than 570 ° C. The oxide generated by oxidation of the steel sheet surface due to the oxidizing atmosphere at temperatures of 570 ° C. or higher. This is because there is a possibility that surface defects may occur due to being picked up by the roll. From this point of view, the heating temperature of the primary heating is less than 570 ° C, preferably less than 500 ° C.

  The steel sheet on which AlN has been generated as described above is then heated to a temperature range of 750 ° C. to 900 ° C. (secondary heating) and held at that temperature for a period of 15 s to 600 s. . The heating temperature of the secondary heating is set to 750 ° C. to 900 ° C. A composite having good elongation characteristics by reducing the ferrite phase in order to obtain high strength, recrystallizing the cold rolled structure, and precipitating austenite. This is to form an organization. The higher the heating temperature, the lower the ferrite phase and thereby the higher the strength. However, since the effect is saturated at 900 ° C., the upper limit of the heating temperature of the secondary heating is 900 ° C. On the other hand, the lower limit is required to be 750 ° C. or higher in order to make residual austenite exist and to improve the elongation characteristics. In the secondary heating, it is preferable to set the air-air ratio to 0.8 or more because the generation of AlN is further promoted. However, when the air ratio exceeds 1.1, the reduction of Fe oxide on the steel sheet surface is not sufficiently performed, so that the plating property is deteriorated. Therefore, when heating to the temperature range of 750 ° C. to 900 ° C., it is preferable that the annealing atmosphere has an air ratio of 0.8 to 1.1.

  The holding time at the heating temperature is 15 s or more and 600 s or less. If the holding time is less than 15 s, the concentration of C into the austenite phase is insufficient, and stable residual austenite is hardly generated, and sufficient total elongation cannot be obtained. On the other hand, the effect of lowering the ferrite phase fraction is saturated even if the holding time exceeds 600 s.

  The steel plate on which the necessary austenite phase has been generated by the above heating is then rapidly cooled and hot dip galvanized. This rapid cooling is performed by setting a cooling rate of 5 ° C./s or more in a temperature range of at least 550 ° C. from the maximum heating temperature range. The upper limit of the quenching stop temperature for quenching is set to 550 ° C. in order to prevent the pearlite transformation from proceeding and to obtain a high total elongation as a structure including a residual austenite phase and a hard phase. Therefore, the quench stop temperature is determined by well-known means so as to obtain the necessary strength and total elongation.

  Hot dip galvanization is performed by a well-known means, and what is called an alloying process is performed as needed.

  A slab having the composition shown in Table 1 is produced by continuous casting, and hot rolling and cold rolling are performed on the slab to obtain a 1.6 mm thick cold rolled sheet. Table 2 shows the obtained cold rolled sheet. The product was annealed and hot dip galvanized under the conditions shown in.

The obtained product was subjected to the following tests to measure various characteristics.
(1) Ferrite phase fraction: A 1000-magnification image was taken using a SEM at a position where the plate thickness was 1/4 and measured by image processing.
(2) The fraction of retained austenite phase: The surface selected by grinding the product from the surface to a thickness of 1/4 position and then polishing by 0.1 mm by chemical polishing, using Mo Kα rays with an X-refraction diffractometer Measure the integrated intensity of the (200), (220), (311) face of fcc iron and the (200), (211) face and (220) face of bcc iron, and based on these, measure the residual austenite phase The rate was determined.
(3) Deposition depth of AlN: The cross section of the product (C direction cross section) is etched with 3% nital solution to reveal the structure, and the surface thickness of the plate thickness is 10000 times using a scanning electron microscope (SEM), 5 fields of view Observed and identified AlN by EPMA to determine its precipitation depth.
(4) Tensile strength (TS), total elongation (EL) and (TS) x (EL): Cut a JIS No. 5 test piece in the C direction at a position of 1/4 the plate width, and in accordance with JIS Z 2241 A tensile test was performed, TS (tensile strength) and El (total elongation) were measured, and (TS) × (EL) which is a value of strength-elongation balance was obtained as a product of these.
(5) Fatigue strength with punching: A fatigue strength test piece with the shape shown in FIG. 2 was cut in the C direction at a position of 1/4 the plate width. The punched hole had a diameter of 10 mm and was punched with a clearance of 6.25%. Here clearance C is defined by:
C = {(die diameter-hole diameter) / 2} × 100 / plate thickness
= {(Die diameter-10) / 2} x 100 / plate thickness (%)
For other plate thicknesses, punching is performed with a clearance C of 7.25 ± 2.5%.
(6) The tensile fatigue test was performed with a single swing constant load control (0 tension), the stress amplitude frequency was 15 kHz, and the stress ratio was 0.05. The maximum stress when the fracture was not repeated at the number of repetitions of 10 ⁷ was regarded as the fatigue strength while being punched.

  The measurement results are summarized in Table 2. As is clear from these results, it can be seen that a steel sheet that satisfies the requirements defined in the present invention has a high value of fatigue strength and strength-elongation balance (TS) × (EL) while being punched.

It is a graph which shows the relationship between AlN precipitation depth and fatigue strength with punching. It is a top view which shows the shape dimension of a fatigue strength test piece with being punched.

Claims (11)

By mass ratio, C: 0.05 to 0.30%, Mn: 0.8 to 3.00%, P: 0.003 to 0.100%, S: 0.010% or less, Al: 0.10 to 2.50%, Cr: 0.03 to 0.50%, N: 0.007% or less The balance substantially consists of Fe and inevitable impurities,
A structure including a ferrite phase, a residual austenite phase, and a low-temperature transformation phase, wherein the ferrite phase fraction is 97% or less by volume, and AlN is precipitated in a region from the steel plate surface excluding the plating layer to 1 μm. Have and
A hot-dip galvanized steel sheet characterized by a tensile strength (TS) of 590 MPa or more and a fatigue strength (FL) of 200 MPa or more with a punched fracture surface.   The hot dip galvanized steel sheet according to claim 1, wherein the Si content is 1.40% or less.   Furthermore, it contains one or more elements selected from V: 0.005 to 2.0%, Mo: 0.005 to 2.0%, Ni: 0.0O5 to 2.0%, Cu: 0.005 to 2.0%, Item 3. The hot-dip galvanized steel sheet according to item 1 or 2.   Furthermore, the molten ball lead plating in any one of the Claims 1-3 which contain the 1 type (s) or 2 or more types of element chosen from Ti: 0.01-0.20% and Nb: 0.01-0.10%. steel sheet.   The hot-dip galvanized steel sheet according to any one of claims 1 to 4, further comprising B: 0.0002 to 0.0050%.   The hot-dip galvanized steel sheet according to any one of claims 1 to 5, further comprising one or two elements selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005%.   The hot dip galvanized steel sheet according to claim 1, wherein the retained austenite phase fraction is 3% or more by volume ratio.   The hot-dip galvanized steel sheet according to claim 6 or 7, wherein the product of tensile strength (TS) and total elongation value (EL), (TS) x (EL) is 21000 MPa ·% or more. Hot rolling and cold rolling are performed on the slab having the composition according to any one of claims 1 to 6 to obtain a cold rolled steel sheet,
The obtained cold-rolled steel sheet is heated to 300 ° C. or more and lower than 570 ° C. in an atmosphere with an air ratio of 1.0 or more and 2.0 or less, and then further heated to a maximum heating temperature of 900 ° C. or less. A method for producing a bath-dip galvanized steel sheet, which is held in a heating temperature region for 15 s or more and 600 s or less, then cooled to a hot dip galvanizing temperature and subjected to a hot dip galvanizing treatment.   The method for producing a hot-dip galvanized steel sheet according to claim 9, wherein the maximum heating temperature is 750C to 900C. The method for producing a hot dip galvanized steel sheet according to claim 10, wherein the cooling rate from the maximum heating temperature region to 550 ° C or lower and the hot dip galvanizing temperature is 5 ° C / s or higher.

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