JP2005187947A - Hot dip galvanized steel sheet having excellent strain age hardening property, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot dip galvanized steel sheet having excellent strain age hardening properties, and suitable for an automobile structural member, and to provide its production method. <P>SOLUTION: A steel stock having a composition comprising 0.04 to 0.12% C, ≤0.4% Si, 1.0 to 3.0% Mn, ≤0.05% P, 0.001 to 0.1% Al and 0.005 to 0.02% N, and further comprising one or more kinds of metals selected from ≤0.1% Ti, ≤0.1% Nb and ≤0.1% V is hot-rolled, or is further cold-rolled so as to be a steel sheet. The steel sheet is thereafter subjected to treatment so as to be heated to a temperature region of the temperature T(°C) defined by T=860-250C-150N+45Si-30Mn+700P+400Al-15Ni-10Cr+30Mo+400Ti+80Nb or higher, and subsequently, be cooled to 550°C at an average cooling rate of 5 to 50°C/s, and is then subjected to hot dip galvanizing treatment. In this way, excellent strain age hardening properties satisfying ΔTS:≥80MPa can be obtained. Further, pretreatment of heating to the T°C or higher and pickling treatment of removing a component-concentrated layer on the surface can be performed in order before the heating treatment. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a hot-dip galvanized steel sheet suitable for use in automobile structural members, leg members, etc., and particularly has good workability, and the yield strength and tensile strength are both increased by baking coating, and the member strength is increased. The present invention relates to a hot-dip galvanized steel sheet having excellent strain age hardening characteristics and a method for producing the same.
In addition, the hot dip galvanized steel sheet as used in this invention shall also contain an alloyed hot dip galvanized steel sheet.

In recent years, there has been a demand for improvement in fuel efficiency of automobiles from the viewpoint of conservation of the global environment. In addition, in order to protect passengers in the event of a collision, it is also required to improve the safety of automobile bodies.
For this reason, the weight reduction of the automobile body and the reinforcement of the automobile body are being actively promoted. It is said that it is effective to increase the strength of component materials in order to satisfy the weight reduction and strengthening of the automobile body at the same time. Recently, high-strength steel sheets have been actively used for automobile parts. Further, there is a strong demand for high-strength hot-dip galvanized steel sheets for automobile members (such as structural members and leg members) that require corrosion resistance.

Since many automobile parts made of steel plates are processed by press working, excellent press formability is also required for hot dip galvanized steel sheets for automobile members. However, increasing the strength of steel sheets generally has the problem of degrading press formability.
In response to such problems, a high-strength hot-dip galvanized steel sheet using so-called strain age hardening, which is easy to work during press formation and increases in strength by baking during coating, has been disclosed.

For example, Patent Document 1 and Patent Document 2 include C: 0.01 to 0.08%, Si: 0.005 to 1.0%, Mn: 0.01 to 3.0%, Al: 0.001 to 0.1%, N: 0.0002 to 0.01%, An alloy hot-dip galvanized steel sheet containing one or more of W, Cr, and Mo in a total of 0.05 to 3.0% and having a structure mainly composed of ferrite or ferrite and having a heat-treating ability to increase strength after forming is disclosed.
However, in the hot dip galvanized steel sheet described in Patent Document 1 and Patent Document 2, coating baking is performed at a temperature of 200 to 450 ° C. higher than the conventional coating baking temperature (about 170 ° C.) in order to increase the strength after forming. It is necessary to perform processing. For this reason, in addition to the economical disadvantage of heating to a high temperature, there is a problem that it is necessary to make a separate process from the conventional process.

Patent Document 3 includes C: 0.005 to 0.15%, Mn: 0.3 to 3.0%, Mo: 0.05 to 1.0%, Al: 0.005 to 0.02%, N: 0.005 to 0.02%, and N / Al: 0.3 Thus, a high-tensile hot-dip galvanized steel sheet that has a composite structure of ferrite and martensite and has excellent strain age hardening characteristics is disclosed. However, in order to satisfy a desired high member strength as an automobile part, it has been desired to further increase the strain age hardening amount.
Japanese Patent Laid-Open No. 10-310824 JP-A-10-310847 JP 2001-247946

  The present invention advantageously solves the above-mentioned problems of the prior art, can greatly improve the strength of the member after processing, and is suitable for use as an automotive structural member. An object is to provide a manufacturing method. The hot dip galvanized steel sheet intended by the present invention has excellent workability during press forming, high yield strength and tensile strength after forming-paint baking, and high tensile strength with excellent strain age hardening characteristics. It is a hot-dip galvanized steel sheet. In the present invention, “high tensile hot dip galvanized steel sheet having excellent strain age hardening characteristics” has a tensile strength TS of 490 MPa or more and a strength-elongation balance TS × El of 17000 MPa% or more. % Strain pre-straining and then applying a baking process at 170 ° C for 20 minutes. In the strain age hardening process, the difference in tensile strength before and after the baking process, ΔTS (strain age hardening characteristic) is 80 MPa or higher. It shall mean a galvanized steel sheet.

  In order to achieve the above-mentioned problems, the present inventors have conducted intensive research on various factors affecting strain age hardening characteristics. As a result, in order to obtain a hot-dip galvanized steel sheet with excellent strain age hardening characteristics, the heating temperature and subsequent cooling conditions are appropriately controlled before the hot-dip galvanizing treatment, and the steel sheet structure is changed to the polygonal ferrite phase. It was found that it was effective to make a composite structure containing a certain amount of low-temperature transformation-generated ferrite phase.

First, the experimental results on which the present invention is based will be described.
0.08 mass% C-0.3 mass% Si-2.3 mass% Mn-0.01 mass% P-0.002 mass% S-0.015 mass% Al-0.015 mass% N-based steel (steel A), 0.05 mass% C-0.01 mass% Si -1.3 mass% Mn-0.01 mass% P-0.002 mass% S-0.02 mass% Al-0.017 mass% N-0.42 mass% Mo-based steel (steel B), 0.1 mass% C-0.2 mass% Si-1.7 mass% Mn-0.01 mass% P-0.002 mass% S-0.02 mass% Al-0.017 mass% N-0.015 mass% Nb steel (Steel C) Each was made into a slab. These slabs were heated to 1180 ° C, and then rolled into hot-rolled sheets (2.0 mm thick) by rough rolling and finish rolling with a rolling end temperature of 850 ° C. The hot-rolled sheet was cooled at 20 ° C./s or more after rolling and wound at 600 ° C.

  The obtained hot-rolled sheet is pickled and then heated in a hot dip galvanizing line for 40 seconds in the range of 700 ° C to 900 ° C, and then cooled at a rate of 500 ° C at 20 ° C / s. The steel sheet was then cooled to ℃, then hot dip galvanized bath was immersed in a hot dip galvanizing bath, and then hot dip galvanized. . In addition, about the steel A and the steel C, before the heat processing, the pretreatment which performs cooling after heating for 30 s at 860 degreeC in the continuous annealing line, and the pickling process after that were given.

The conditions for the hot dip galvanizing treatment were as follows.
・ Plate temperature: 475 ℃
・ Plating bath: 0.13% Al-Zn
・ Bath temperature: 475 ℃
・ Immersion time: 3s
・ Weight per unit: 45g / m ² (per side)
From the obtained galvannealed steel sheet, a JIS 13B tensile test piece was taken with the direction perpendicular to the rolling direction as the longitudinal direction, and a tensile test was conducted in accordance with the provisions of JIS Z 2241. Tensile properties (TS) were determined.

Further, 8% tensile pre-strain was applied to a tensile test piece taken from an alloyed hot-dip steel sheet, and then unloading was performed, and a strain age hardening treatment was performed in which heat treatment was performed at 170 ° C. for 20 minutes. Subsequently, the tensile test was again performed on the test pieces after the strain age hardening treatment, and the tensile properties (TS _HT ) after the strain age hardening treatment were obtained. Then, the amount of increase in tensile strength ΔTS by strain age hardening treatment is expressed by the following equation: ΔTS = TS _HT −TS
Calculated with TS is the tensile strength of the plated steel sheet.

The relationship between the obtained ΔTS and the heating temperature of the heat treatment is shown in FIG.
From FIG. 1, it can be seen that a large strain age hardening of ΔTS: 80 MPa or more can be obtained only when the heating temperature of the heat treatment is set to a temperature higher than a specific temperature for each steel type. As a result of various studies, the present inventors have found that the heating temperature T of the heat treatment that provides a large strain age hardening of ΔTS: 80 MPa or more, which is unique to each steel type, is expressed by the following equation (1) T = 860-250 C-150N + 45Si-30Mn + 700P + 400Al-15Ni-10Cr + 30Mo + 400Ti + 80Nb (1)
(Here, T: heating temperature (° C.), C, N, Si, Mn, P, Al, Ni, Cr, Mo, Ti, Nb: content of each element (mass%))
It was found that Note that T (A), T (B), and T (C) in FIG. 1 are temperatures T calculated from the formula (1) using the element contents of Steel A, Steel B, and Steel C, respectively. Represents.

  In addition, the present inventors conducted an experiment in the same manner when the pretreatment conditions for steel A before the heat treatment were set to 760 ° C. × 30 s, and found the relationship between ΔTS and the heating temperature of the heat treatment. Compared with the case where the processing conditions are 860 ° C. × 30 s, FIG. The temperature T ° C. in Steel A is 795 ° C., and when the pretreatment condition is 860 ° C., as described above, heat treatment is performed to heat to a temperature equal to or higher than the temperature T ° C. defined by Equation (1). .DELTA.TS: Excellent strain age hardening characteristics of 80 MPa or more. However, when the pretreatment conditions are 760 ° C x 30 s, even if the heating temperature of the heat treatment is changed in the range of 700 ° C to 900 ° C, excellent strain age hardening characteristics of ΔTS: 80 MPa or more are not obtained. .

From the results of FIG. 2, it can be seen that there is an appropriate pretreatment heating temperature in order to obtain a steel sheet having excellent strain age hardening characteristics of ΔTS: 80 MPa or more. As a result of further investigation based on such knowledge, the present inventors have determined that the heating temperature of the pretreatment is equal to or higher than the temperature T ° C. defined by the formula (1), similarly to the heating temperature of the heating treatment. It was found that it is preferable.
The present invention has been completed based on the above-described findings and further studies. That is, the gist of the present invention is as follows.
(1) A hot dip galvanized steel sheet having a hot dip galvanized layer on the surface layer of the steel sheet, wherein the steel sheet is in mass%, C: 0.04 to 0.12%, Si: 0.4% or less, Mn: 1.0 to 3.0 %, P: 0.05% or less, Al: 0.001 to 0.1%, N: 0.005 to 0.02%, Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less Containing a seed or two or more types, the balance consisting of Fe and inevitable impurities, a polygonal ferrite phase and a low-temperature transformation-forming ferrite phase with an area ratio of 10% to 50%, and an average grain size of 8 μm A hot-dip galvanized steel sheet having excellent strain age hardening characteristics, which is a steel sheet having the following structure.
(2) In (1), in addition to the above composition, the composition further contains one or more selected from Cr: 1.0% or less, Mo: 1.0% or less, and Ni: 1.0% or less A hot-dip galvanized steel sheet with excellent strain age hardening characteristics.
(3) A hot-dip galvanized steel sheet having excellent strain age hardening characteristics, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer in (1) or (2).
(4) By mass%, C: 0.04 to 0.12%, Si: 0.4% or less, Mn: 1.0 to 3.0%, P: 0.05% or less, Al: 0.001 to 0.1%, N: 0.005 to 0.02%, Further, hot rolling or further cold rolling is applied to a steel material having a composition containing one or more selected from Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less. After applying the steel sheet, the following (1) formula T = 860-250C-150N + 45Si-30Mn + 700P + 400Al-15Ni-10Cr + 30Mo + 400Ti + 80Nb (...) 1)
(Where, T: temperature (° C.), C, N, Si, Mn, P, Al, Ni, Cr, Mo, Ti, Nb: content of each element (mass%))
Strain aging characterized in that it is heated to a temperature range equal to or higher than the temperature T defined by, then cooled at an average cooling rate of up to 550 ° C. at 5 to 50 ° C./s, and then hot dip galvanized. A method for producing a hot-dip galvanized steel sheet having excellent hardening characteristics.
(5) In (4), in addition to the above composition, the steel material is, in addition to mass%, selected from Cr: 1.0% or less, Mo: 1.0% or less, and Ni: 1.0% or less. The manufacturing method of the hot-dip galvanized steel plate excellent in the strain age hardening characteristic characterized by containing the seed | species or more.
(6) In (4) or (5), before the heat treatment, the pretreatment for heating to a temperature range equal to or higher than the temperature T defined by the equation (1) and the surface component concentrated layer are removed. A method for producing a hot-dip galvanized steel sheet having excellent strain age hardening characteristics, characterized by sequentially performing pickling treatment.
(7) In any one of (4) to (6), a method for producing a hot-dip galvanized steel sheet having excellent strain age hardening characteristics, wherein an alloying treatment is further performed after the hot-dip galvanizing treatment.

  In addition, as a steel material, it is preferable that the remainder other than the said composition shall consist of Fe and an unavoidable impurity.

  According to the present invention, a high-tensile hot-dip galvanized steel sheet excellent in strain age hardening characteristics, which is particularly suitable for structural members of automobiles, can be easily and inexpensively produced, and has a remarkable industrial effect.

The hot dip galvanized steel sheet of the present invention is a steel sheet having a hot dip galvanized layer on the surface layer of the plating material (steel sheet), and is characterized by the composition and structure of the steel sheet as the plating material. A hot-rolled steel plate or a cold-rolled steel plate is suitable for the steel plate used as the plating material.
First, the reasons for limiting the composition of the steel sheet that is the plating material of the hot dip galvanized steel sheet of the present invention will be described. Hereinafter, the mass% in the composition is simply expressed as%.

C: 0.04-0.12%
C has a high solid solution strengthening ability or a high structure strengthening ability, and is an important element for strengthening steel. It also contributes effectively to strain age hardening. Such an effect is recognized at a content of 0.04% or more, while a content exceeding 0.12% deteriorates weldability. For this reason, C was limited to 0.04 to 0.12%. In addition, Preferably it is 0.05 to 0.10%.

Si: 0.4% or less
Si is an element that increases the strength, and can be contained depending on the desired strength. However, if it exceeds 0.4%, the strain age hardening property is lowered. For this reason, Si was limited to 0.4% or less. In addition, Preferably, it is 0.005-0.4%.
Mn: 1.0-3.0%
Mn is an element that combines with S to prevent hot embrittlement due to S as MnS and strengthens the steel by solid solution strengthening or transformation strengthening. In the present invention, in particular, Mn contributes effectively to the formation of low-temperature transformation-generated ferrite (bainitic ferrite). Such an effect is recognized when the content is 1.0% or more. On the other hand, if the content exceeds 3.0%, workability is deteriorated. For this reason, Mn was limited to 1.0 to 3.0%. In addition, Preferably it is 1.5 to 2.5%.

P: 0.05% or less P is an element that reinforces steel, and the content is preferably adjusted according to the desired strength. However, if it exceeds 0.05%, weldability is deteriorated and plating properties are increased. Reduce. For this reason, P was limited to 0.05% or less. Preferably, it is 0.005% to 0.03%.
Al: 0.001 to 0.1%
Al is an element that acts as a deoxidizer, but if it is less than 0.001%, its effect is poor, while even if it contains more than 0.1% in a large amount, not only an effect commensurate with the content can be expected, On the contrary, the surface properties are deteriorated. For this reason, Al was limited to 0.001 to 0.1%.

N: 0.005 to 0.02%
N is an element that improves strain age hardening characteristics, and is an extremely important element in the present invention. Such an effect is recognized when the content is 0.005% or more. On the other hand, if the content exceeds 0.02%, moldability is deteriorated. For this reason, N was limited to 0.005 to 0.02%. In addition, Preferably it is 0.008 to 0.018%.

In the present invention, in addition to the above-described components, one or more selected from Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less are further contained.
One or more selected from Ti: 0.1% or less, Nb: 0.1% or less, V: 0.1% or less
Ti, Nb, and V all form carbides and nitrides with C and N, respectively, and contribute effectively to improving strength and toughness. Such effects become prominent when the content is 0.001% or more. However, when the content exceeds 0.1%, the amount of solid solution C and solid solution N required for strain age hardening cannot be secured, and strain age hardening characteristics are obtained. Decreases. For this reason, it was limited to Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less.

In the present invention, in addition to the basic composition described above, one or more selected from Cr: 1.0% or less, Mo: 1.0% or less, and Ni: 1.0% or less can be contained.
One or more selected from Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less
Cr, Mo, and Ni are elements that strengthen the steel by solid solution strengthening and strengthen the steel by structural strengthening, and improve the hardenability and effective low-temperature transformation-generated ferrite phase for improving strain age hardening characteristics. In the present invention, it can be contained if necessary. Such an effect becomes remarkable when Cr, Mo, and Ni are each contained in an amount of 0.1% or more. However, when each content exceeds 1.0%, plating properties, formability, and spot weldability are deteriorated. For this reason, Cr, Mo, and Ni are each preferably limited to 1.0% or less.

The balance other than the above components is Fe and inevitable impurities. In addition, as an inevitable impurity, S: 0.0050% or less is acceptable.
The hot-dip galvanized steel sheet of the present invention has the above-described composition, and further includes a polygonal ferrite phase and a low-temperature transformation-generated ferrite phase having an area ratio of 10% or more, and has a structure with an average crystal grain size of 8 μm or less. .

The “low temperature transformation generated ferrite” phase α _B in the present invention is a ferrite generated in a low temperature range (approximately 500 ° C. or less), which is different from the ferrite in the usual sense (“polygonal ferrite” phase α _P ). It means a structure composed of bainitic ferrite and / or upper bainite. This “low temperature transformation-generated ferrite” phase α _B is an important structure that contributes to the development of excellent strain aging characteristics. In low-temperature transformation-generated ferrite, the dislocation density is originally high, so that C and N in steel, which is an interstitial solid solution element, adhere to these dislocations, and these adhering dislocations become resistance to the movement of dislocations during plastic deformation. , It exhibits excellent strain age hardening characteristics.

In the present invention, the low temperature transformation-generated ferrite phase α _B has a structure containing 10% or more in area ratio. When the low-temperature transformation-generated ferrite phase is less than 10%, excellent strain age hardening characteristics of ΔTS: 80 MPa or more cannot be stably obtained. In addition, it is necessary from a viewpoint of workability that the low temperature transformation generation ferrite phase is 50% or less.
Other than the above-described low-temperature transformation-generated ferrite phase, it is substantially composed of a polygonal ferrite phase, but a part of the martensite phase or pearlite phase may be mixed. If the amount of martensite phase or pearlite phase mixed is large, it is difficult to obtain the desired effect. Therefore, the mixing of these phases is preferably 10% or less in terms of area ratio. For this reason, it is preferable that the total of the low-temperature transformation generation ferrite phase and the polygonal ferrite phase is 90% or more in terms of area ratio.

The structure including the polygonal ferrite phase and the low-temperature transformation-generated ferrite phase having an area ratio of 10% to 50% has an average crystal grain size of 8 μm or less. If the average crystal grain size exceeds 8 μm and the structure becomes coarse, a significant increase in tensile strength due to strain aging cannot be expected.
In addition, the present inventors consider the reason why the strain aging characteristics are improved by making the structure having an average crystal grain size of 8 μm or less as follows.

By adjusting the average crystal grain size to 8 μm or less, the crystal grain boundaries increase, but because of the increase in crystal grain boundaries, even when the same strain amount is processed, mobile dislocations are distributed at high density, which is excellent. It is considered that the strain age hardening characteristics are exhibited.
Further, when the crystal grains are refined, the grain interface area where the solid solution N exists is increased, and diffusion of the solid solution N at room temperature is suppressed, so that there is an effect that room temperature aging is suppressed.

Next, the manufacturing method of the hot dip galvanized steel sheet of this invention is demonstrated.
It is preferable that the molten steel adjusted to the above composition is melted by a known melting method such as a converter and used as a steel material by a known casting method such as a continuous casting method. Whether this steel material is charged into a heating furnace at a high temperature or without cooling to room temperature, or after cooling to room temperature and then into a reheating furnace, hot rolling is performed to obtain a hot-rolled sheet. Alternatively, the hot-rolled sheet is preferably pickled and then cold-rolled to form a cold-rolled sheet. In the present invention, the hot rolling conditions or further cold rolling conditions do not need to be particularly limited, and may be generally known conditions that can be a predetermined size and shape.

These hot-rolled plates or cold-rolled plates are then subjected to heat treatment. The heat treatment is preferably performed in a continuous hot dip galvanizing line together with the subsequent hot dip galvanizing treatment. In the heat treatment, a hot-rolled sheet or a cold-rolled sheet having the above composition is expressed by the following formula (1): T = 860-250C-150N + 45Si-30Mn + 700P + 400Al-15Ni-10Cr + 30Mo + 400Ti + 80Nb ... (1)
(Here, T: heating temperature (° C.), C, N, Si, Mn, P, Al, Ni, Cr, Mo, Ti, Nb: content of each element (mass%))
It is set as the process which cools as the average cooling rate to 550 degreeC to 5-50 degreeC / s after heating to the temperature range more than the temperature T defined by (5). It is preferable that the steel plate heated to a temperature range equal to or higher than the temperature T (° C.) defined by the formula (1) is held at that temperature for 10 to 300 seconds. If the heating and holding time is less than 10 seconds, the structure of the steel sheet before the heat treatment remains, and the structure may be nonuniform, which is likely to lead to a decrease in workability. On the other hand, if the holding time is longer than 300 s, crystal grains may be coarsened.

By setting the heating temperature to be equal to or higher than the temperature T (° C.) defined by the formula (1), a steel plate having excellent strain age hardening characteristics can be obtained. The present inventors consider this mechanism as follows.
That is, when the heating temperature is in the temperature range below the temperature T (° C.) defined by the equation (1) and in the (α + γ) two-phase region, the amount of the solid solution N distributed to the γ phase is large. Therefore, the presence of the solid solution N becomes non-uniform. Furthermore, since elements such as C and Mn are simultaneously distributed to the γ phase, the γ phase tends to be a low-temperature transformation phase such as martensite having a high dislocation density in the subsequent cooling process. The solid solution N distributed in the γ phase is bound by dislocations in the low temperature transformation phase, and the diffusion of the solid solution N is not sufficiently performed during the subsequent strain age hardening treatment, and a sufficiently high strain age hardening amount is obtained. Absent. Further, when the heating temperature is an α single phase, the steel sheet structure becomes non-uniform, and the ductility is significantly reduced.

On the other hand, when the heating temperature is in the temperature range equal to or higher than the temperature T (° C.) defined by the equation (1), the distribution of each element to the γ phase is slight even in the case of the (α + γ) two-phase region. Become. Further, when the heating temperature is in the γ single phase region, uneven distribution of each element does not occur.
For this reason, it is not a low-temperature transformation phase like martensite by heating to a temperature in which the heating temperature is equal to or higher than the temperature T (° C.) defined by the formula (1) and adjusting the subsequent cooling. It is easy to obtain a low-temperature transformation-generated ferrite phase such as a bainitic ferrite phase or an upper bainite phase that can exhibit excellent strain age hardening characteristics.

In addition, although the upper limit of the heating temperature in heat processing is determined depending on the heating capability of a continuous hot dip galvanization equipment, generally it is about 950 degreeC.
After being heated to a temperature range equal to or higher than the temperature T (° C.) defined by the equation (1), the steel sheet is then cooled at an average cooling rate up to 550 ° C. of 5 to 50 ° C./s. In the present invention, the cooling rate after heating is controlled so that a bainitic ferrite phase effective for strain age hardening characteristics is easily generated. When the cooling rate is less than 5 ° C / s, the formation of polygonal ferrite becomes remarkable during cooling, and the formation of bainitic ferrite phase is small. On the other hand, when the cooling rate is 50 ° C / s or more, the formation of bainitic ferrite phase occurs. Becomes noticeable, and the ductility is significantly reduced.

Cooling with an average cooling rate of 5 to 50 ° C./s is performed to 550 ° C. or less. When the cooling stop temperature is higher than 550 ° C., the formation of polygonal ferrite phase and pearlite phase becomes remarkable, the bainitic ferrite phase is hardly formed, and excellent strain age hardening characteristics cannot be obtained.
In the present invention, prior to the heat treatment described above, in order to further improve the plating property, the heat treatment is performed by a pretreatment for heating to a temperature range equal to or higher than the temperature T (° C.) defined by the equation (1), and the pretreatment. It is preferable to perform a pickling treatment to remove the component concentrated layer on the surface of the steel plate. In addition, it is preferable to perform pre-processing in a continuous annealing line. Further, the pickling treatment is preferably performed in a continuous hot dip galvanizing line.

By such pretreatment and subsequent pickling treatment, elements that hinder plating properties existing on the surface layer are removed, and plating such as adhesion and uniformity of the hot dip galvanized layer formed by subsequent hot dip galvanizing treatment Improved.
In the pretreatment, when the heating temperature is lower than the temperature T (° C.) defined by the equation (1) and the heating temperature is in the (α + γ) two-phase region, the solid solution N is transformed into the γ phase. The amount to be distributed increases, and the presence of solute N becomes uneven. Such uneven distribution of solid solution N does not show a sufficiently high strain age hardening characteristic because it remains without being wiped off even when the heating temperature is set to a temperature range of T ° C. or higher in the subsequent hot dip galvanizing process. Further, when the heating temperature is the α single phase, the effect of improving the plating property by the pretreatment cannot be obtained.

Therefore, when pretreatment is performed, the heating temperature is higher than the temperature T (° C.) defined by the equation (1), preferably higher than the heating temperature of the subsequent heat treatment. It is preferable from the viewpoint of improving the plating property. The upper limit of the heating temperature in the pretreatment is determined depending on the heating capacity of the continuous annealing equipment, but is generally about 950 ° C.
Further, the pickling treatment following the pretreatment may be performed by a generally known method such as immersion in an aqueous hydrochloric acid solution as long as the component concentrated layer on the surface can be removed.

The steel plate (plating material) subjected to the above heat treatment or pretreatment-pickling treatment-heat treatment is then subjected to hot dip galvanizing treatment.
The hot dip galvanizing treatment is preferably the same as the conditions performed in a normal hot dip galvanizing line.
The hot dip galvanizing treatment may be performed by immersing the steel sheet in a hot dip galvanizing bath adjusted to a temperature range of about 450 to 550 ° C. to form a hot dip galvanized layer on the steel sheet surface layer. The hot dip galvanizing bath is preferably a Zn bath containing 0.10 to 0.15% Al. Further, wiping for adjusting the basis weight may be performed as necessary after the plating process.

  Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method. After heating these steel materials to 1180 ° C, hot rolling by rough rolling and finish rolling at a finish rolling finish temperature of 850 to 920 ° C, cooling is performed at a cooling rate of 20 ° C / s or more, Winding was performed at a temperature of 650 ° C. or lower. Subsequently, these hot-rolled sheets were pickled and a part was further cold-rolled to obtain cold-rolled sheets. These hot rolled plates or cold rolled plates were used as plating materials.

  These plating materials are subjected to pretreatment and pickling treatment in the continuous annealing line under the conditions shown in Table 2, and then heat treatment, hot dip galvanizing treatment, and alloy in the continuous hot dip galvanizing line under the conditions shown in Table 2. Was applied. Some steel plates omitted pretreatment in the continuous annealing line or alloying treatment in the continuous hot dip galvanizing line. In the hot dip galvanizing treatment, the steel sheet was immersed in a hot dip galvanizing bath, and then the steel sheet was pulled up and the basis weight was adjusted by gas wiping. The conditions for the hot dip galvanizing treatment are as follows.

Plate temperature: 475 ℃
Plating bath: 0.13% Al-Zn
Bath temperature: 475 ℃
Immersion time: 3s
Weight per unit area: 45g / m ² (per side)
The surface of the obtained hot-dip galvanized steel sheet was visually observed, and the presence or absence of non-plating defects was determined to evaluate the plating properties. In addition, the evaluation is ◯ when there is no unplating defect (good plating property), △ when there is some unplating defect (slightly good plating property), and many unplating defects occur (bad plating property) ) Was marked with x.

Further, JIS 13B tensile test specimens were collected with the direction perpendicular to the rolling direction from the obtained hot-dip galvanized steel sheet as the longitudinal direction. Using these tensile test pieces, a tensile test was conducted at a strain rate of 10 ⁻³ / s in accordance with the provisions of JIS Z 2241, and the tensile properties (yield stress (YS), tensile strength ( TS) and elongation (El)).
Moreover, the test piece for structure | tissue observation was extract | collected from the obtained hot-dip galvanized steel plate, and the kind of steel plate structure | tissue was identified from the optical microscope structure | tissue photograph which observed the structure | tissue with the optical microscope about the rolling direction cross section. Moreover, the value calculated by the quadrature method prescribed in ASTM from the obtained optical microscopic structure photograph (magnification 400 times) by an image analyzer was used as the average crystal grain size of the structure.

Moreover, about the obtained hot dip galvanized steel plate, N content in a steel plate and N amount which exists as a precipitate were measured by chemical analysis, and the difference was made into the solid solution N amount.
In addition, JIS 13B tensile test specimens were collected from the obtained hot-dip galvanized steel sheet, and after applying a tensile pre-strain of 8% to these tensile test specimens, they were once unloaded and subjected to heat treatment at 170 ° C. for 20 minutes. Then, a strain age hardening treatment was performed. Subsequently, these tensile test pieces were again subjected to a tensile test to determine tensile properties (YS _HT , TS _HT ) after strain age hardening. From the tensile strength TS _HT after strain age hardening treatment and the tensile strength TS of the hot-dip galvanized steel sheet, the following formula ΔTS = TS _HT -TS
Thus, ΔTS was calculated as an increase in tensile strength due to the strain age hardening treatment. Further, before the heat treatment of the yield stress YS _HT and age hardening treatment after strain aging hardening treatment and a pre-deformation stress YS, the following formula BH amount = (yield stress after age hardening treatment) over (before the heat treatment of age hardening treatment Pre-deformation stress)
Thus, the amount of BH was calculated.

  The obtained results are shown in Table 3.

  Each of the inventive examples has a tensile strength of 490 MPa or more, a high strength-elongation balance TS × El of 17000 MPa% or more, an excellent strain age hardening property of ΔTS: 80 MPa or more, and a high BH amount: 100 MPa or more. It is a hot-dip galvanized steel sheet exhibiting BH properties. Furthermore, all of the examples of the present invention are hot-dip galvanized steel sheets that are free from non-plating defects and have excellent plating properties.

  On the other hand, the comparative example which is out of the scope of the present invention sufficiently satisfies the target characteristics because the strength-elongation balance is low, ΔTS is small, the amount of BH is small, or the plating property is poor. It is not a hot dip galvanized steel sheet.

It is a graph which shows the relationship between the raise amount (DELTA) TS of the tensile strength by a strain age hardening process, and heating temperature. It is a graph which shows the influence of pre-processing temperature which has on the relationship between (DELTA) TS and heating temperature.

Claims (7)

A hot dip galvanized steel sheet having a hot dip galvanized layer on the surface layer of the steel sheet, wherein the steel sheet is in% by mass,
C: 0.04 to 0.12%, Si: 0.4% or less,
Mn: 1.0 to 3.0%, P: 0.05% or less,
Al: 0.001 to 0.1%, N: 0.005 to 0.02%
And a composition comprising one or more selected from Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less, the balance Fe and inevitable impurities, Excellent strain age hardening characteristics characterized by being a steel sheet having a structure in which the average crystal grain size is 8 μm or less, including a null ferrite phase and a low-temperature transformation-generated ferrite phase having an area ratio of 10% to 50%. Hot dip galvanized steel sheet.   In addition to the above composition, the composition further comprises, in mass%, one or more selected from Cr: 1.0% or less, Mo: 1.0% or less, and Ni: 1.0% or less. The hot-dip galvanized steel sheet having excellent strain age hardening characteristics according to claim 1.   The hot dip galvanized steel sheet having excellent strain age hardening characteristics according to claim 1 or 2, wherein the hot dip galvanized layer is an alloyed hot dip galvanized layer. % By mass
C: 0.04 to 0.12%, Si: 0.4% or less,
Mn: 1.0 to 3.0%, P: 0.05% or less,
Al: 0.001 to 0.1%, N: 0.005 to 0.02%
A steel material having a composition containing one or more selected from Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less, or After cold rolling into a steel plate, the steel plate is heated to a temperature range equal to or higher than the temperature T defined by the following formula (1) as a heat treatment, and then the average cooling rate up to 550 ° C. is 5 to 5 ° C. A method for producing a hot dip galvanized steel sheet having excellent strain age hardening characteristics, characterized by performing a cooling treatment at 50 ° C./s and then performing a hot dip galvanizing treatment.
T = 860-250C-150N + 45Si-30Mn + 700P + 400Al-15Ni-10Cr + 30Mo + 400Ti + 80Nb (1)
Where T: temperature (° C.)
C, N, Si, Mn, P, Al, Ni, Cr, Mo, Ti, Nb: Content of each element (mass%)   The steel material further contains one or more selected from Cr: 1.0% or less, Mo: 1.0% or less, and Ni: 1.0% or less in mass% in addition to the composition. The manufacturing method of the hot dip galvanized steel plate excellent in the strain age hardening characteristic of Claim 4.   Before the heat treatment, a pretreatment for heating to a temperature range equal to or higher than the temperature T defined by the formula (1) and a pickling treatment for removing the component concentrated layer on the surface are sequentially performed. The manufacturing method of the hot dip galvanized steel plate excellent in the strain age hardening characteristic of Claim 4 or 5.   The method for producing a hot-dip galvanized steel sheet having excellent strain age hardening characteristics according to any one of claims 4 to 6, wherein an alloying treatment is further performed after the hot-dip galvanizing treatment.