JP2003321714A - Method of producing galvanized cold rolled steel sheet having excellent strain aging hardening property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a galvanized cold rolled steel sheet which has excellent fatigue properties and safety against collision, e.g. as automobile structural parts by increasing the yield strength and tensile strength owing to strain aging in a relatively low strength steel sheet having the tensile strength of <590 MPa. <P>SOLUTION: A steel stock having a componential composition containing, by mass, 0.01 to 0.2% C, ≤0.4% Si, 0.2 to 2.0% Mn, ≤0.05% P, 0.001 to 0.1% Al and 0.005 to 0.02% N, and the balance Fe with inevitable impurities is hot- rolled, and is then cold-rolled to obtain a cold rolled steel sheet containing solid solution N of ≥50 ppm. The cold rolled steel sheet is subjected to heating- cooling treatment so as to be heated to a temperature range of 650 to 900°C, and to be cooled at a mean cooling rate of 5 to 50°C/s at least in a temperature range from the heating temperature to 650°C. Successively, plating treatment where a plating layer is formed on the surface of the steel sheet is performed. In the fundamental process, the time of the heating-cooling treatment in a temperature range of ≥650°C is regulated in the case the content of the solid solution N is less than the prescribed value. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is suitable for use in automobile structural parts, foot parts and the like, and is increased in yield strength and tensile strength after baking processing after processing, resulting in fatigue characteristics and collision resistance. The present invention relates to a method for producing a hot-dip galvanized cold-rolled steel sheet having a tensile strength of less than 590 MPa, which has improved safety and excellent rust prevention.

[0002]

2. Description of the Related Art In recent years, for the purpose of ensuring the safety of passengers of an automobile, an automobile body having excellent collision resistance has been developed, and therefore, the strength of automobile steel sheet has been further enhanced. Also, from the perspective of environmental issues related to CO ₂ emissions, it is necessary to reduce the weight of the vehicle body in order to improve fuel efficiency.
It is desired to reduce the thickness of steel sheets.

By the way, there is a problem that increasing the strength of a steel sheet makes press forming difficult. In particular, the dimensional accuracy strongly depends on the strength of the steel sheet. Above all, at tensile strength levels of 590 MPa or higher, it becomes extremely difficult to achieve dimensional accuracy. What has characteristics is desired.

Here, as a strength-enhancing technique that achieves both moldability and vehicle strength, a technique utilizing so-called strain age hardening, which is easy to process during molding and increases strength by baking during coating, is known. There is. For example, Japanese Patent Laid-Open No. 10-31
In 0824 and Japanese Patent Laid-Open No. 10-310847, C:
0.01-0.08mass%, Si: 0.005-1.0mass%, Mn:
0.01-3.0mass%, Al: 0.001-0.1mass%, N: 0.0
An alloy containing 002 to 0.01 mass% and further containing one or more of W, Cr and Mo in a total amount of 0.05 to 3.0 mass% and having a structure of ferrite or ferrite as a main component and having heat treatment performance for increasing strength after forming. A galvanized steel sheet and a method for manufacturing the same are disclosed.

However, in this technique, the coating baking process is performed at a temperature of 200 to 450, which is higher than the conventional temperature (about 170 ° C.).
Since it needs to be performed at a temperature of ℃, it is not compatible with the conventional process, and there are problems that it is economically disadvantageous due to the high temperature and that a new process must be added.

Further, in Japanese Patent Laid-Open No. 2001-247946, C:
0.005 to 0.15 mass%, Mn: 0.3 to 3.0 mass%, Mo: 0.
005 to 0.02 mass%, Al: 0.005 to 0.02 mass%, N:
Disclosed are a high-strength hot-dip galvanized steel sheet containing 0.005 to 0.02 mass% and N / Al: 0.3 or more, having a composite structure of ferrite and martensite and excellent in strain age hardening characteristics, and a method for producing the same. However, this technique has a cost problem because expensive elements such as Mo, Cr, and Ni are used.

Furthermore, Japanese Patent Laid-Open No. 2001-303180 discloses that
C: 0.20mass% or less, Si: 2.0mass% or less, Mn: 3.0
mass% or less, P: 0.08mass% or less, S: 0.02mass%
Below, Al: 0.02mass% or less, N: 0.0050 to 0.0250mass
s%, Nb: 0.005 to 0.50 mass% is included, and N / Al
Is 0.3 or more and has a structure containing a ferrite phase with an average grain size of 10 μm or less in an area ratio of 50% or more.
A high-yield ratio type high-strength hot-dip galvanized steel sheet having a tensile strength of 0.7 or more and a tensile strength of 440 MPa or more and a method for producing the same are disclosed.

[0008] By this technique, a hot dip galvanized steel sheet having excellent strain aging characteristics and tensile characteristics can be obtained.
Stable acquisition of these characteristics is essential for industrial-scale production, and this point has not been fully studied.

[0009]

SUMMARY OF THE INVENTION The present invention is to solve the above problems, and in a steel sheet having a relatively low tensile strength of less than 590 MPa, yield strength and tensile strength due to strain aging are remarkably increased. It is an object of the present invention to propose a method for producing a hot-dip galvanized cold-rolled steel sheet that can be raised and can exhibit excellent fatigue characteristics and collision resistance as a structural part of an automobile, for example.

[0010]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the inventors of the present invention have made extensive studies on heat treatment in a plating treatment step performed after a cold rolling step, and found that the heat treatment at a temperature higher than a specific temperature is performed. It was found that the desired properties can be achieved by controlling the time within an appropriate range determined by the composition of the steel sheet and the manufacturing method. The present invention is based on the above knowledge, and its gist is as follows.

(A) C: 0.01 to 0.2 mass%, Si: 0.4 m
ass% or less, Mn: 0.2 to 2.0 mass%, P: 0.05 mass%
Below, Al: 0.001 to 0.1 mass% and N: 0.005 to 0.0
A cold rolled steel sheet containing 2 mass% and the balance being Fe and unavoidable impurities, obtained by hot rolling and then cold rolling, and having a solid solution N content of 50 ppm or more. Is heated to a temperature range of 650 ℃ to 900 ℃,
At least in the temperature range from the heating temperature to 650 ° C, cooling is performed at an average cooling rate of 5 to 50 ° C / s, heating-cooling treatment is performed, and then a plating layer is formed on the surface of the steel sheet. As a basic process, if Ns1 represented by the following formula (1) is Ns1 <0.005, 65
A method for producing a hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics, characterized in that the time of the heating-cooling treatment in a temperature range of 0 ° C. or higher is regulated within tg 1 second represented by the following formula (2). Note Ns1 = [N]-(14/27) [Al] ... (1) Log (tg1) = 0.000075 × (Tg-800) ² + tO ... (2) Where, Tg: Heating- Heating temperature in cooling process (° C) t0 = 2.0 × Pr1 −0.008 × CR +0.85 However, CR: Cold reduction rate (%) Pr1 = ([N] −0.005) / [N] [N], [Al] ] Is the content of N and Al elements (mass%)

(B) In addition to the basic step described in (A) above, prior to the heating-cooling treatment, a pretreatment of heating above the heating temperature in the heating-cooling treatment and then pickling is added. If Ns1 <0.005 shown in the following formula (1), the pretreatment time in the temperature range of 650 ℃ or higher is regulated within tc seconds shown in the following formula (3). Of the hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics, which is characterized in that the time of the heating-cooling treatment in the temperature range of 650 ° C. or higher is regulated within tg2 seconds represented by the following formula (4). Production method. Note Ns1 = [N] − (14/27) [Al] ... (1) Log (tc) = 0.000075 × (Tc−800) ² + tOc ... (3) where Tc: pretreatment Heating temperature (℃) tOc = 2.0 × Pr1 −0.008 × CR +0.85 However, CR: Cold reduction rate (%) Pr1 = ([N] −0.005) / [N] Log (tg2) = 0.000075 × ( Tg −800) ² + tOg ··· (4) where Tg: Heating temperature in heating-cooling process (° C) t0g: Log (tc) <0.000075 × (Tc-800) ² -0.008 × CR + 0.85
When t0g = 2.0 × Pr1 + 0.85 Log (tc) ≧ 0.000075 × (Tc-800) ² -0.008 × CR + 0.85, t0g = 2.0 × Pr1 + 0.85- {Log (tc) -0.000075 × (Tc −
800) ² } However, CR: Cold reduction (%) Pr1 = ([N] -0.005) / [N] [N], [Al] is the content of N and Al elements (mass%)

(C) The cold rolled steel sheet is made of a steel material of 1000 to 1300.
After heating in the temperature range of ℃, rough rolling, and then finish rolling, if Ns1 ≧ 0.005, wind at 650 ℃ or less and Ns
If 1 <0.005, hot-rolled steel sheet produced by winding at 550 ° C. or lower is cold-rolled, and hot-dip galvanizing having excellent strain aging characteristics as described in (A) or (B) above. Manufacturing method of cold rolled steel sheet.

(D) C: 0.01 to 0.2 mass%, Si: 0.4 m
ass% or less, Mn: 0.2 to 2.0 mass%, P: 0.05 mass%
Below, Al: 0.001 to 0.1 mass% and N: 0.005 to 0.0
2 mass% is contained, and any one of Ti: 0.001 to 0.1 mass% and Nb: 0.001 to 0.1 mass% or 2
Seed, [N] ≧ (14/93) [Nb] + (14/48) [Ti] +
Cold rolling with a content of solid solution N of 50 ppm or more obtained by hot rolling and then cold rolling a steel material containing less than 0.005 and the balance being Fe and inevitable impurities. After heating the steel plate in the temperature range of 650 ℃ to 900 ℃,
At least in the temperature range from the heating temperature to 650 ° C, cooling is performed at an average cooling rate of 5 to 50 ° C / s, heating-cooling treatment is performed, and then a plating layer is formed on the surface of the steel sheet. Basically, if Ns2 represented by the following equation (5) is Ns2 <0.005, the heating-cooling time in the temperature range of 650 ° C or higher is within tg1 second represented by the following equation (2). A method for producing a hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics, which is characterized in that Note Ns2 = [N]-(14/27) [Al]-(14/93) [Nb]-(14/48) [Ti] ... (5) Log (tg1) = 0.000075 × (Tg- 800) ² + tO ··· (2) Where, Tg: Heating temperature (℃) t0 = 2.0 × Pr2 −0.008 × CR + 0.85 However, CR: Cold reduction rate Pr2 = {[N]- (14/93) [Nb]-(14/48) [Ti] -0.005} / {[N]-(14 /
93) [Nb]-(14/48) [Ti]} [N], [Al], [Nb] and [Ti] are the contents of N, Al, Nb and Ti (mass%), respectively.

(E) In addition to the basic process described in (D) above, a pre-treatment of adding a pre-treatment of heating to a heating temperature or higher in the heating-cooling treatment and then pickling is added prior to the heating-cooling treatment. If Ns2 <0.005 expressed by the following formula (5), the pretreatment time in the temperature range of 650 ° C or higher is expressed by the following formula (3).
The time of the heating-cooling treatment in the temperature range of 650 ° C or higher, which is regulated within seconds, is represented by the following formula (4).
A method for producing a hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics, characterized by being regulated within 2 seconds. Note Log (tc) = 0.000075 × (Tc −800) ² + tOc ··· (3) where Tc: Heating temperature in pretreatment (° C) tOc = 2.0 × Pr2 −0.008 × CR +0.85 However, CR : Cold reduction (%) Pr2 = {[N]-(14/93) [Nb]-(14/48) [Ti] -0.005} / {[N]-(14 /
93) [Nb]-(14/48) [Ti]} Log (tg2) = 0.000075 × (Tg −800) ² + tOg ··· (4) Where, Tg: Heating temperature in heating-cooling process (° C ) t0g: Log (tc) <0.000075 × (Tc-800) ² -0.008 × CR + 0.85
, T0g = 2.0 × Pr2 + 0.85 Log (tc) ≧ 0.000075 × (Tc-800) ² -0.008 × CR + 0.85, t0g = 2.0 × Pr2 + 0.85- {Log (tc) -0.000075 × (Tc −
800) ² } However, CR: Cold reduction (%) Pr2 = {[N]-(14/93) [Nb]-(14/48) [Ti] -0.005} / {[N]-(14 /
93) [Nb]-(14/48) [Ti]} [N], [Al], [Nb] and [Ti] are the contents of N, Al, Nb and Ti (mass%), respectively.

(F) The cold rolled steel sheet is made of a steel material of 1000 to 1300.
After heating in the temperature range of ℃, rough rolling, then finish rolling, if Ns2 ≧ 0.005, wind it at 650 ℃ or less and Ns
In the case of 2 <0.005, hot-rolled steel sheet produced by winding at 550 ° C. or lower is cold-rolled, and hot-dip galvanizing having excellent strain aging characteristics as described in (D) or (E) above. Manufacturing method of cold rolled steel sheet.

(G) A hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics as described in any one of (A) to (F) above, which is characterized by further performing a heat alloying treatment after forming the plated layer. Production method.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Experimental results leading to the present invention will be described in detail below. That is, C: 0.1 m
ass%, Si: 0.01 mass%, Mn: 1.2 mass%, P: 0.0
17mass%, S: 0.0020mass% and Al: 0.018mass
%, N is changed in the range of 0.0050 to 0.0190 mass%, and various steels having the composition as the balance iron and unavoidable impurities are melted in a converter and continuously cast. Made into a slab. These were heated at 1180 ° C, subjected to rough rolling and finish rolling, and then wound at 500 ° C to obtain hot rolled steel sheets. After pickling these hot rolled sheets, the rolling rate is 30%
Cold-rolled. Next is the plating process
The temperature is raised at a rate of 5 ° C / s in a hot dip galvanizing line to 750
Heat treatment at ℃ was carried out for a holding time of 5 to 185 seconds, cooled to 500 ℃ at a cooling rate of 10 ℃ / s, the steel sheet was immersed in a hot dip galvanizing bath, and plating treatment was performed under the following conditions. After that, an alloying treatment was performed at 520 ° C. for 25 seconds. Plate temperature: 475 ° C Plating bath: 0.13 mass% Al- balance Zn Bath temperature: 475 ° C Immersion time: 3s Unit weight (per side): 45g / m ²

The plated steel sheet thus obtained is JIS 13
No. B tensile test pieces were processed and the mechanical properties were investigated. Also,
After applying a pre-strain of 5% to the plated steel sheet,
Once unloaded, heat treatment was carried out at 170 ° C. for 20 minutes, and then a tensile test was conducted again to investigate its mechanical properties. Then, from the tensile strength obtained here, subtract the tensile strength of the plated steel sheet as described above (that is, the tensile strength of the plated steel sheet before strain aging treatment) to obtain the tensile strength by strain aging treatment. The amount of increase (hereinafter referred to as ΔTS) was calculated. The relationship between the amount of N contained and the holding time of heat treatment is shown in FIG.

As can be seen from this figure, the N content is 0.01
In the region of 4 mass% or more, good strain aging characteristics were exhibited under all conditions. In addition, the content of N is 0.014 mass%
It was also found that there is a region exhibiting good strain aging characteristics even in the region below.

Furthermore, among the above steels, the N content is 0.015 m
Using ass% steel, the heating temperature is 750 ° C, the holding time at the heating temperature is 30 seconds, and the cooling rate is 2 to 20 ° C / s.
A plated steel sheet was manufactured by the same process as above, which was changed to. For plated steel sheets, ΔTS was determined by the same method as above. Figure 2 shows the results of an investigation of the relationship between ΔTS and the cooling rate at this time. As shown in the figure,
Even in the region where the content of N is 0.014 mass% or more, the cooling rate is 5
It can be seen that the desired strain age hardening characteristics cannot be obtained when the temperature is lower than ° C / s.

Next, the chemical composition is C: 0.1 mass%, S
i: 0.01 mass%, Mn: 1.2 mass%, P: 0.017 mass
%, S: 0.0020 mass%, Al: 0.018 mass% and N:
Steel A containing 0.0135 mass% and remaining iron and unavoidable impurities, C: 0.07 mass%, Mn: 0.7 mass%, P:
0.010mass%, S: 0.002mass%, Al: 0.020mass
%, N: 0.0165 mass%, Nb: 0.015 mass% and Ti
: Steel B containing 0.01 mass% and remaining iron and inevitable impurities, and Steel C having the same composition as Steel A except that N: 0.0180 mass% were melted in a converter. A slab was made by a continuous casting method. These were heated at 1180 ° C, subjected to rough rolling, and then wound at 500 ° C to obtain hot rolled steel sheets. After subjecting these hot-rolled sheets to pickling, the rolling reduction was 3
Cold rolling was performed at 0 to 90%. Next, the temperature is raised at a temperature of 5 ° C / s in the hot dip galvanizing line, heat treatment is performed for 70 seconds while changing the temperature in the range of 625 to 925 ° C, and the temperature is cooled to 500 ° C at a cooling rate of 15 ° C / s. Then, the steel sheet was immersed in a hot dip galvanizing bath to carry out a plating treatment under the following conditions, and then an alloying treatment was carried out at 520 ° C. for 25 seconds. Plate temperature: 475 ° C Plating bath: 0.13mass% Al- balance Zn Bath temperature: 475 ° C Immersion time: 3s Unit weight (on one side): 45g / m ²

With respect to the plated steel sheet thus obtained, ΔTS was determined by the same method as described above. The results are shown in Fig. 3 in the case where the cold rolling reduction is 50%. As can be seen from this figure, ΔTS shows a temperature dependence such that 800 ° C. is minimized. Steels A and B show good strain age hardening characteristics on the high temperature side and low temperature side, and in Steel C, Good strain age-hardening characteristics were exhibited even at the temperature. Moreover, such temperature dependence was recognized in the temperature range of 650 ° C or higher. Further, Fig. 4 shows the result when Steel A was heated to 725 ° C. From this figure, it can be seen that ΔTS decreases with cold reduction.

From the results shown in FIG.
It was considered that the holding time in the temperature range above ℃ affected the strain age hardening characteristics. Therefore, using the steels A and B described above, plated steel sheets are manufactured through the same process in which the heating temperature is 675 to 925 ° C. and various heating and holding times are changed, and these steel sheets are manufactured by the same procedure as above. ΔTS was calculated. FIG. 5 shows the relationship between this result and the treatment time in the temperature range of 650 ° C. or higher, that is, the holding time in the temperature range of 650 ° C. or higher. As you can see from this figure, 650 ° C
When the holding time in the above temperature range exceeds a certain threshold value, the strain age hardening characteristics deteriorate, and the threshold value is the shortest at a heating temperature of 800 ° C, and the higher the heating temperature or the lower, the longer the threshold value becomes. It's time.

Further, ΔTS measured by the same procedure as above
6 shows the relationship between the heating temperature and the holding time in the temperature range of 650 ° C. or higher, where ◯ is 60 MPa or more and x is less than 60 MPa. As you can see from the figure, the heating temperature is 8
In the range of around 50 ℃ and around 00 ℃, the appropriate holding time was short, and the higher or lower the heating temperature was, the longer the suitable holding time was. It was also found that the thresholds of O and X at this time existed in a parabolic shape between the temperature and the logarithm of the retention time in the temperature range of 650 ° C or higher.

As described above, it has been newly found that ΔTS is improved by properly operating the chemical composition, heating temperature, reduction rate and holding time in the temperature range of 650 ° C. or higher. However, even if these conditions are properly controlled, the product quality may still fluctuate, and there is still room for further improvement, especially on the premise of industrial scale production.

Then, the present inventors re-examined the above experimental facts in detail, and found that the total content of Al was the value obtained by subtracting the N content necessary for forming a precipitate from the N content. When Ns1 or Nb and Ti are further contained, the total content of Nb and Ti is the value obtained by subtracting the N content necessary for forming precipitates in addition to the N content of Al content Ns2, That is, the following formula (1) or (5) Ns1 = [N] − (14/27) [Al] ... (1) Ns2 = [N] − (14/27) [Al]-(14/93 ) [Nb]-(14/48) [Ti] ··· (5) is 0.005 or more, it is necessary to take the cooling rate into consideration, and when Ns1 or Ns2 is less than 0.005. Newly found that, in addition to this, it is necessary to regulate the holding time in the temperature range of 650 ° C or higher during the heating-cooling treatment. [] In the above formula indicates the mass content of the elements described therein, and this is the same in the following formulas.

The process leading to the above new knowledge will be described in detail below. First, the inventors examined the contents of Figure 1 in detail based on the composition of the cold-rolled steel sheet used here, and
It was judged that solid solution N of m or more is necessary for strain age hardening characteristics.
Based on this amount of solute N, it was further found that the strain age hardening characteristics can be controlled based on an equation that takes into account the precipitation process of N 2.

That is, the suitability condition (tg1) for the holding time in the temperature range of 650 ° C. or higher is changed like a parabola that has a minimum value at 800 ° C. when the logarithm of time is taken, and the parabola is At a heating temperature of 800 ° C, the maximum holding time (t0) in the longest allowable temperature range of 650 ° C or more because ΔTS exceeds 60 MPa, and the maximum precipitation that can secure solid solution N of 50 ppm or more
It was found that the ratio can be expressed as the ratio of N content to the total precipitation amount (Pr1 or Pr2) and cold reduction rate (CR),
(2) was obtained as an empirical formula.

Note Log (tg1) = 0.000075 × (Tg −800) ² + tO ··· (2) Where, Tg: Heating temperature in heating-cooling process (° C) t0 = 2.0 × Pr1 −0.008 × CR +0 .85 However, CR: Cold rolling reduction (%) Pr1 = ([N] -0.005) / [N] or when Nb and / or Ti contain one or two, Pr2 below instead of Pr1 Pr2 = {[N]-(14/93) [Nb]-(14/48) [Ti] -0.005} / {[N]-(14 /
93) [Nb]-(14/48) [Ti]}

Here, the result of rearranging the contents shown in FIG. 1 by the holding time in the temperature range of 650 ° C. or more is shown in FIG. 7, and in FIG. )} Is used to calculate tg1 (or tg2, which will be described later) indicated by a solid line. The results are shown in FIG. As can be seen from these figures, by appropriately setting and controlling the conditions of the plating treatment step according to the above formula (2), it is possible to stably manufacture a plated steel sheet having high strain age hardening characteristics. .

Next, the chemical composition is C: 0.1 mass%, S
i: 0.01 mass%, Mn: 1.2 mass%, P: 0.017 mass
%, S: 0.0020 mass%, Al: 0.018 mass% and N:
Steel A containing 0.0135 mass% and remaining iron and unavoidable impurities, C: 0.07 mass%, Mn: 0.7 mass%, P:
0.010mass%, S: 0.002mass%, Al: 0.020mass
%, N: 0.0145 mass% and Nb: 0.022 mass%, and the balance iron and steel B which becomes unavoidable impurities were melted in a converter and formed into cast pieces by a continuous casting method. These were heated at 1180 ° C, subjected to rough rolling and finish rolling, and then wound at 500 ° C to obtain hot rolled steel sheets. These hot-rolled sheets were pickled and then cold-rolled at a rolling reduction of 60%. Next, for steel A, the heating temperature is 680 in the continuous annealing line.
A pretreatment consisting of heat treatment and pickling treatment was performed under three conditions of ℃, 730 ℃ and 780 ℃. At this time, the plate passing speed was kept constant and the holding time at 650 ° C or higher was set to 50, 65 and 80 seconds, respectively. Further, Steel B was not pretreated.

Subsequently, at the hot dip galvanizing line 66
Heat treatment at 0 ℃, cool to 500 ℃ at a cooling rate of 20 ℃ / s, immerse the steel sheet in a hot dip galvanizing bath, and perform the plating treatment under the following conditions, then alloy at 520 ℃ for 25 seconds Chemical treatment was performed. Plate temperature: 475 ° C Plating bath: 0.13 mass% Al- balance Zn Bath temperature: 475 ° C Immersion time: 3s Unit weight (per side): 45g / m ²

The steel sheet thus obtained was processed into a JIS 13 B tensile test piece and examined for mechanical properties. Further, after applying a tensile prestrain of 5% to the plated steel sheet, the steel sheet was once unloaded, heat-treated at 170 ° C. for 20 min, and then subjected to a tensile test again to investigate the mechanical properties. The amount of increase in TS due to strain aging, ΔTS, was determined by subtracting the TS of the as-plated steel sheet from the tensile strength (TS) at this time. The results are shown in FIG.

As is clear from this figure, remarkably large strain age hardening can be obtained by appropriately controlling the holding time at 650 ° C. or higher in the pretreatment step and the hot dip galvanizing step. That is, even when this pretreatment is performed, when Ns1 or Ns2 is less than 0.005, the holding time of 650 ° C. or higher is represented by the following formula (3) as in the case of the above formula (2). Must be regulated according to (tc).

Log (tc) = 0.000075 × (Tc−800) ² + t0c (3) where Tc: heating temperature in pretreatment (° C.) tOc = 2.0 × Pr1−0.008 × CR + 0.85 However, CR: Cold rolling reduction (%) Pr1 = ([N] -0.005) / [N] In addition, as in the case of considering the holding time of 650 ° C or more during heating-cooling treatment, Nb and Ti When either one or two are contained, the following Pr2 Pr2 = {[N]-(14/93) [Nb]-(14/48) [Ti] -0.005} / {[N ]-(14/
93) [Nb]-(14/48) [Ti]}

When performing the pretreatment, the above equation (2) is replaced with the following equation (4) to control the heating and holding time. Note Log (tg2) = 0.000075 × (Tg-800) ² + t0g ··· (4) Where, Tg: Heating temperature in heating-cooling process (℃) t0g: Log (tc) <0.000075 × (Tc-800 ) ² -0.008 × CR + 0.85
, T0g = 2.0 × Pr1 + 0.85 Log (tc) ≧ 0.000075 × (Tc-800) ² -0.008 × CR + 0.85, t0g = 2.0 × Pr1 + 0.85- {Log (tc) -0.000075 × (Tg −
800) ² } However, CR: Cold rolling reduction (%) Pr1 = ([N] -0.005) / [N] or Pr2 = {[if one or two of Nb and Ti are contained. N]-(14/93) [Nb]-(14/48) [Ti] -0.005} / {[N]-(14 /
93) [Nb]-(14/48) [Ti]}

The present invention has been completed by conducting more detailed studies in addition to the above experimental facts. Each condition of the present invention will be described in detail below. First, the reasons for limiting the steel components will be specifically described. C: 0.01 to 0.2 mass% C is an important element for strengthening steel, has a high solid solution strengthening ability, and is also effective for strain age hardening. If the content is less than 0.01 mass%, sufficient strength cannot be obtained, while if it exceeds 0.2 mass%, the weldability deteriorates. Therefore, the C content should be 0.01 to 0.2 mass%.

Si: 0.4 mass% or less Si is an element having a high solid solution strengthening ability, and is added according to the desired strength, but if it exceeds 0.4 mass%, the strain age hardening characteristic is deteriorated. Therefore, the Si content is 0.4 mass.
% Or less.

Mn: 0.2 to 2.0 mass% Mn is an element added to prevent hot embrittlement and to secure strength. If it is less than 0.2 mass%, its effect is poor.
If it exceeds mass%, the workability is deteriorated. Therefore Mn
The content is 0.2 to 2.0 mass%.

P: 0.05 mass% or less P is an element having a high solid solution strengthening ability, and is an element to be added according to the desired strength. However, if it exceeds 0.05 mass%, the weldability is deteriorated and the plating property is increased. Reduce sex.
Therefore, the P content should be 0.05 mass% or less.

Al: 0.001 to 0.1 mass% Al is an element necessary for deoxidizing steel, but 0.001 mass
If it is less than 0.1%, the effect is poor, and even if it is added in a large amount exceeding 0.1mass%, no further effect can be expected and the surface properties are deteriorated, and N necessary for strain age hardening is fixed as a precipitate. , This will deteriorate it.
Therefore, the Al content is 0.001 to 0.1 mass%.

N: 0.005 to 0.02 mass% N is an extremely important element in the present invention. A content of 0.005 mass% or more is necessary to improve the strain age hardening characteristics. On the other hand, if it exceeds 0.02 mass%, the formability is deteriorated. Therefore, the N content is 0.005 to 0.02mass.
%. Further, as will be described later, when Nb and Ti are contained, the N content is [N] ≧ (14/93) [Nb] + (14/48
) Adjust so that the range is [Ti] +0.005. still,
Here, [N], [Nb], and [Ti] are the contents (mass%) of each element. If the N content deviates from this range, the desired strain age hardening characteristics cannot be obtained because N is precipitated and fixed by Ti and Nb during the hot rolling process.

Although the basic components have been described above, in the present invention, in addition, Ti: 0.001 to 0.1 mass%, Nb: 0.001 to
One or two kinds selected from 0.1 mass% can be appropriately contained. Ti: 0.001 to 0.1 mass%, Nb: 0.001 to 0.1 mass% Ti and Nb both form carbonization, nitriding, and sulfides with C, N, and S to effectively contribute to the improvement of strength and toughness. If the addition amount is less than 0.001 mass%, a sufficient effect cannot be obtained, while if it exceeds 0.1 mass%, C necessary for strain age hardening is obtained.
, N amount is fixed as a precipitate, which lowers it. Therefore, the content of these elements is 0.001
~ 0.1mass% Furthermore, Ti and Nb are [N] ≧
The content shall be (14/93) [Nb] + (14/48) [Ti] +0.005. If it is out of this range, sufficient solid solution N cannot be secured even if the processing step conditions described later are satisfied, and the strain aging characteristics deteriorate.

Next, the reasons for limiting the manufacturing conditions in the present invention will be specifically described. The steel material with the above composition is
It is preferable that the molten steel melted by a known melting method is cast by a known continuous casting method, an ingot making method, or the like to be provided in the shape of a slab or the like.

This steel material is heated by a known device such as a heating furnace. Here, in order to secure a desired amount of solid solution N in the hot-rolled sheet or the cold-rolled sheet, it is necessary to dissolve N during heating, and in order to satisfy this, the heating temperature is set to 1000
It is preferable to set the temperature range to ˜1300 ° C. That is,
If the heating temperature is lower than 1000 ° C, it becomes difficult to leave a desired amount of nitrogen in a solid solution state because precipitation of N progresses.
It is difficult to set the amount of solute N in the cold rolled steel sheet to 50 ppm or more. On the other hand, when the temperature exceeds 1300 ° C, the structure of the hot-rolled sheet before cold rolling becomes coarse, and it becomes difficult to obtain desired strain aging characteristics even if the conditions described later are satisfied.

It is preferable to wind the heated steel material in a temperature range of 650 ° C. or less when the above Ns1 or Ns2 is 0.005 or more after rough rolling and finish rolling. On the other hand, if Ns1 or Ns2 is less than 0.005, 55
It is preferable to wind in a temperature range of 0 ° C. or lower. That is, when the coiling temperature exceeds 650 ° C., the structure of the hot rolled sheet before cold rolling becomes coarse, and it becomes difficult to obtain the desired strain aging characteristics even if the conditions described later are satisfied. Furthermore, when Ns1 or Ns2 is less than 0.005 and the coiling temperature exceeds 550 ° C, N is fixed as a precipitate, so that it becomes difficult to secure a desired amount of solid solution N 2.

The hot-rolled steel sheet after winding is preferably subjected to usual pretreatments such as pickling and degreasing, and then cold-rolled at a cold reduction of 20% or more. That is, if the cold reduction ratio is less than 20%, it is not possible to obtain a uniform structure in the hot-dip galvanized steel sheet of the final product, resulting in a decrease in formability. Further, when the solid solution N existing in the steel sheet at the stage of the original plate (cold rolled sheet) is less than 50 ppm, the desired strain age hardening characteristics cannot be obtained even if the hot dip galvanizing treatment described later is performed. Can not.

The cold rolled steel sheet thus manufactured is first subjected to heat treatment. This heat treatment is preferably performed in a continuous hot dip galvanizing line together with the subsequent plating treatment. In the heat treatment process, Ns1 or Ns2 is 0.0
In the case of 05 or more, after heating in the temperature range of 650 ℃ to 900 ℃, the average cooling rate from the heating temperature to 650 ℃ is cooled to 5 to 50 ℃ / s. On the other hand, N
If s1 or Ns2 is less than 0.005, 650 ℃ or more 900
650 ℃ from the heating temperature after heating in the temperature range below ℃
When cooling at an average cooling rate of up to 5 to 50 ° C / s, the time of processing in the temperature range of 650 ° C or higher, that is, the holding time in the temperature range of 650 ° C or higher, It is important to carry out the heating-cooling step shown in (2) within tg 1 second.

When the heating temperature is less than 650 ° C.,
Sufficient workability cannot be obtained, and the plateability is impaired. On the other hand, if the heating temperature exceeds 900 ℃,
The final product, hot-dip galvanized steel sheet, coarsens the ferrite crystal grain size and reduces strain aging characteristics. Furthermore, heating at such a high temperature is not realistic from an industrial viewpoint.

After the heat treatment, the steel sheet is cooled. In this cooling process, the average cooling rate up to 650 ° C is 5 ° C /
If it is less than s, the crystal grains become coarse and the strain age-hardening property deteriorates.In particular, when Ns1 or Ns2 is less than 0.005, precipitation of N becomes remarkable and the desired strain age-hardening property can be satisfied. Absent. Further, if the average cooling rate up to 650 ° C exceeds 50 ° C / s, it hardens due to the formation of a low temperature transformation phase and reduces ductility. Therefore, the average cooling rate from the heating temperature to 650 ° C is 5 to 50 ° C / s.

Further, when Ns1 or Ns2 is less than 0.005, the holding time in the temperature range of 650 ° C. or higher is tg1 second or less shown by the above formula (2) even if the above condition is satisfied. There is a need. If this exceeds tg 1 second, the desired strain age hardening characteristics cannot be obtained. This is because N forms Al or further Ti and Nb precipitates in this temperature range, and reduces the amount of solid solution N in the steel. The rate of precipitation of these precipitates varies depending on the chemical composition and temperature, so if Ns1 or Ns2 is less than 0.005, make the amount of precipitation less than the amount that can secure the required solid solution N. It is necessary to control the holding time at high temperature. That is, as is clear from the above experimental results, excellent strain age hardening characteristics can be obtained by controlling according to the above formula (2), which indicates the precipitation rate determined by the chemical composition and temperature.

When Ns1 or Ns2 is 0.005 or more, even if a precipitate is formed, a sufficient amount of solid-dissolved N 2 can remain, so that the control of the holding time at high temperature is not considered. May be.

Next, the plating treatment according to the present invention is as follows.
Usually, similar to the conditions performed in hot dip galvanizing line,
It is immersed in a zinc bath in the temperature range of 450 to 550 ° C to form a hot dip galvanized layer on the surface of the steel sheet. The zinc bath may be a commonly used Zn bath. Further, after the plating treatment, wiping for adjusting the basis weight may be performed if necessary. An alloying treatment may be performed after the plating treatment. The alloying treatment may be performed by a usual method.

Further, in the present invention, in order to further improve the plating property, prior to the series of treatment steps in the above-mentioned continuous galvanizing line, the heat treatment for heating to a temperature range higher than the heating temperature in the heating-cooling treatment. When,
It is possible to perform a pretreatment, which is a subsequent pickling treatment for removing the component-rich phase on the surface of the steel sheet. Even when this pretreatment is added, if Ns1 or Ns2 is less than 0.005, the holding time in the temperature range of 650 ° C or higher in the pretreatment step is within tc seconds shown by the above formula (3). In addition, by keeping the holding time in the temperature range of 650 ° C. or higher in the heating-cooling treatment in the above continuous galvanizing line within tg2 seconds shown by the above formula (4), the desired strain age hardening characteristics can be obtained. Can be obtained.

This is because, for the same reason as described above, the amount of solid solution N 2 can be secured by controlling the holding time at high temperature in the whole series of processes including the pretreatment process and the plating process.

When the pretreatment is performed, it is necessary to consider the amount of precipitation in the pretreatment step in controlling the holding time in the temperature range of 650 ° C. or higher in the heating-cooling treatment in the subsequent plating treatment step. . That is, if the pretreatment is completed during the precipitation-free incubation period, that is, log (tc) <0.0000
When 75 × (Tc-800) ² +0.85, the influence of the pretreatment need not be taken into consideration. On the other hand, when precipitation occurs during pretreatment, that is, when log (tc) ≥ 0.000075 × (Tc-800) ² +0.85, the holding time in the temperature range of 650 ° C or higher in the plating process depends on the amount of precipitation. Is adjusted to the short side. However, the effect of cold rolling does not appear in the plating process because the strain is released during the pretreatment process. Therefore, the term of CR is unnecessary in the calculation formula of tg2.

[0058]

Example 1 A steel having the chemical composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. These cast pieces were hot-rolled under the conditions shown in Table 2 to obtain hot-rolled steel sheets. These hot rolled steel sheets are pickled and cold rolled under the conditions shown in Table 2 to obtain cold rolled steel sheets,
Next, heat treatment was performed in the continuous annealing line under the conditions shown in Table 2, and further, pickling treatment was performed using a pretreatment facility in the continuous hot-dip galvanizing line to perform pretreatment.
Subsequently, in the continuous hot-dip galvanizing line, heating-cooling treatment according to the conditions shown in Table 2 was performed, and then plating treatment and alloying treatment were performed. Here, the plating treatment was performed by immersing the steel sheet in a hot dip galvanizing bath, pulling up the immersed steel sheet, and adjusting the basis weight by gas wiping. The plating treatment conditions are as follows. Plate temperature: 475 ° C Plating bath: 0.13mass% Al-Remainder Zn Bath temperature: 475 ° C Immersion time: 3s Unit weight (on one side): 45g / m ²

[0059]

[Table 1]

[0060]

[Table 2]

The above-mentioned pretreatment or alloying treatment was omitted for some steel sheets. The plated steel sheet thus obtained is processed into a JIS No. 13B tensile test piece and subjected to a tensile test at a strain rate of 10 ^-3 / s to obtain mechanical properties {yield strength (YS)
, Tensile strength (TS) and total elongation (El)} were investigated. In addition, the N content in the steel sheet and the N content present as precipitates were measured by chemical analysis at the stage of the cold-rolled steel sheet before treatment, and the difference was taken as the solid solution N content. The results are shown in Tables 2 and 3.

The plated steel sheet was subjected to a tensile prestrain of 5%, was then partially removed, heat-treated at 170 ° C. for 20 minutes, and then subjected to a tensile test again to investigate the mechanical properties. The TS increase amount ΔTS due to strain aging treatment was obtained by subtracting the TS of the as-plated steel sheet described above from the TS at this time. In addition, after applying a 5% tensile pre-strain to the plated steel sheet after plating, it is temporarily unloaded,
The amount of BH was obtained by subtracting the stress when the prestrain of 5% was applied from the yield strength observed when the tensile test was conducted again after heat treatment at 170 ° C. for 20 min. The results are shown in Table 3. From Table 3, it can be seen that the plated steel sheet according to the present invention exhibits high strain age hardening.

The plating property was determined by visually observing the surface of the steel sheet and determining the presence of non-plating defects. And, there are no non-plating defects (good plating property), ◯, some non-plating defects occur (slightly good plating property), and many non-plating defects occur (poor plating property)
Was designated as x.

[0064]

[Table 3]

Example 2 Steel having a chemical composition according to Steel a in Table 1 was melted in a converter and made into a slab by a continuous casting method. These slabs were hot-rolled and cold-rolled under the same conditions as in Example 1 to manufacture a plurality of cold-rolled steel sheets as coils. Next, heating temperature 720 ℃, average cooling rate from heating temperature to 650 ℃ 20 ℃ / s, plating plate temperature 4
Heat-cooling treatment, plating treatment and alloying treatment were performed in a continuous hot dip galvanizing line so that the temperature was 75 ° C., the alloying temperature was 520 ° C., and the alloying time was 20 seconds. The plating treatment was performed by immersing the steel sheet in a hot dip galvanizing bath, pulling up the immersed steel sheet, and adjusting the basis weight by gas wiping. The plating treatment conditions are as follows. Plate temperature: 475 ° C Plating bath: 0.13mass% Al-Remainder Zn Bath temperature: 475 ° C Immersion time: 3s Unit weight (on one side): 45g / m ²

Here, for one coil of cold rolled steel sheet,
650 ℃ according to the variation of heating temperature according to the method of this invention
By changing the passing speed, the holding time above is
It was changed to be smaller than tg1 calculated by the heating temperature (Invention Example). Further, as a comparison, one coil has a constant holding time of 100 seconds at 650 ° C. or higher over the entire length of the coil (comparative example).

The plated steel sheet thus obtained is JIS 13
It was processed into a No. B tensile test piece and examined for mechanical properties. Further, with respect to the same plated steel sheet, after applying a 5% tensile pre-strain, the load was once removed, heat treatment was carried out at 170 ° C. for 20 minutes, and then a tensile test was conducted again to investigate the mechanical properties. From the TS at this time, the TS of the plated plate as described above
By subtracting the amount of increase in TS due to strain aging treatment ΔTS
I asked. FIG. 10 (invention example) and FIG. 11 (comparative example) show changes in heating temperature, tg1, holding time of 650 ° C. or more, and ΔTS in the coil length direction at this time. As can be seen from these figures, by controlling the holding time at 650 ° C. or higher, it is possible to suppress variations in strain age hardening characteristics.

[0068]

According to the present invention, it is possible to manufacture a high-strength hot-dip galvanized steel sheet having excellent strain aging characteristics, which is particularly suitable for use in automobile structural parts.

[Brief description of drawings]

FIG. 1 is a diagram showing a preferable range of a content N content and a holding time at a heating temperature with respect to ΔTS.

FIG. 2 is a diagram showing a relationship between ΔTS and a cooling rate.

FIG. 3 is a diagram showing the relationship between ΔTS and heating temperature.

FIG. 4 is a diagram showing a relationship between ΔTS and a cold reduction rate.

FIG. 5 is a diagram showing a relationship between ΔTS and a holding time in a temperature range of 650 ° C. or higher.

FIG. 6 is a diagram showing a preferable range of a holding time in a temperature range of 650 ° C. or higher and a heating temperature with respect to ΔTS.

FIG. 7 is a diagram showing a suitable range of ΔN of content N amount, Ns amount and holding time at heating temperature.

FIG. 8 is a diagram showing a preferable range of ΔT for holding time and heating temperature in a temperature range of 650 ° C. or higher.

FIG. 9 is a diagram showing a relationship between ΔTS and a holding time in a temperature range of 650 ° C. or higher.

[Fig. 10] Heating temperature, tg1, holding time of 650 ° C or higher,
FIG. 6 is a diagram showing variations in ΔTS in the coil longitudinal direction.

FIG. 11: Heating temperature, tg1, holding time of 650 ° C. or higher,
FIG. 6 is a diagram showing variations in ΔTS in the coil longitudinal direction.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] May 16, 2002 (2002.5.1)
6)

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 4

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

(D) C: 0.01 to 0.2 mass%, Si: 0.4 m
ass% or less, Mn: 0.2 to 2.0 mass%, P: 0.05 mass%
Below, Al: 0.001 to 0.1 mass% and N: 0.005 to 0.0
2 mass% is contained, and any one of Ti: 0.001 to 0.1 mass% and Nb: 0.001 to 0.1 mass% or 2
Seed, [N] ≧ (14/93) [Nb] + (14/48) [Ti] +
Cold rolled steel containing less than 0.005, the balance being Fe and unavoidable impurities, obtained by hot rolling and then cold rolling, and having a solute N content of 50 ppm or more. After heating the steel plate in the temperature range of 650 ℃ to 900 ℃,
At least in the temperature range from the heating temperature to 650 ° C, cooling is performed at an average cooling rate of 5 to 50 ° C / s, heating-cooling treatment is performed, and then a plating layer is formed on the surface of the steel sheet. Basically, if Ns2 represented by the following equation (5) is Ns2 <0.005, the heating-cooling time in the temperature range of 650 ° C or higher is within tg1 second represented by the following equation (2). A method for producing a hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics, which is characterized in that Note Ns2 = [N]-(14/27) [Al]-(14/93) [Nb]-(14/48) [Ti] ... (5) Log (tg1) = 0.000075 × (Tg- 800) ² + tO ··· (2) Where, Tg: Heating temperature in heating- cooling process (℃) t0 = 2.0 × Pr2 −0.008 × CR + 0.85 However, CR: Cold reduction rate (%) Pr2 = {[N]-(14/93) [Nb]-(14/48) [Ti] -0.005} / {[N]-(14 /
93) [Nb]-(14/48) [Ti]} [N], [Al], [Nb] and [Ti] are the contents of N, Al, Nb and Ti (mass%), respectively.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Saiji Matsuoka             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. (72) Inventor Kei Sakata             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. F-term (reference) 4K037 EA01 EA05 EA06 EA15 EA18                       EA19 EA23 EA27 EA31 EB06                       EB08 FA02 FA03 FB00 FE01                       FE02 FG00 FJ04 FJ05 FJ06                       FK03 GA05 HA01 JA01

Claims (7)

[Claims] 1. C: 0.01 to 0.2 mass%, Si: 0.4 mass% or less, Mn: 0.2 to 2.0 mass%, P: 0.05 mass% or less, Al: 0.001 to 0.1 mass% and N: 0.005 to 0.02 mass%. A cold-rolled steel sheet containing 50 ppm or more of solute N, obtained by subjecting a steel material having the composition of Fe and unavoidable impurities to the rest, hot rolling, and then cold rolling,
After heating to a temperature range of 650 ℃ or more and 900 ℃ or less, at least the temperature range from the heating temperature to 650 ℃ is cooled at an average cooling rate of 5 to 50 ℃ / s, subjected to heat-cooling treatment, and subsequently steel sheet The basic step is to form a plating layer on the surface, and to perform a plating treatment. If Ns1 represented by the following formula (1) is Ns1 <0.005, the above heating-cooling treatment in a temperature range of 650 ° C or higher. The method for producing a hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics, characterized in that the time is controlled within tg 1 second represented by the following formula (2). Note Ns1 = [N]-(14/27) [Al] ... (1) Log (tg1) = 0.000075 × (Tg-800) ² + tO ... (2) Where, Tg: Heating- Heating temperature in cooling process (° C) t0 = 2.0 × Pr1 −0.008 × CR +0.85 However, CR: Cold reduction rate (%) Pr1 = ([N] −0.005) / [N] [N], [Al] ] Is the content of N and Al elements (mass%) 2. The basic process according to claim 1, wherein
Prior to performing the cooling treatment, the heating-addition of a pretreatment of heating after the heating temperature in the cooling treatment or higher and pickling, based on a series of steps, Ns represented by the following formula (1)
When 1 <0.005, the pretreatment time in the temperature range of 650 ° C or higher is regulated within tc seconds represented by the following formula (3), and the heating temperature in the temperature range of 650 ° C or higher-
A method for producing a hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics, characterized in that the cooling treatment time is restricted within tg2 seconds represented by the following formula (4). Note Ns1 = [N]-(14/27) [Al] ... (1) Log (tc) = 0.000075 × (Tc-800) ² + tOc ... (3) where Tc: pretreatment Heating temperature (℃) tOc = 2.0 × Pr1 −0.008 × CR +0.85 However, CR: Cold reduction rate (%) Pr1 = ([N] −0.005) / [N] Log (tg2) = 0.000075 × ( Tg −800) ² + tOg ··· (4) Where, Tg: Heating temperature in heating-cooling process (℃) t0g: Log (tc) <0.000075 × (Tc-800) ² -0.008 × CR + 0.85
When t0g = 2.0 × Pr1 + 0.85 Log (tc) ≧ 0.000075 × (Tc-800) ² -0.008 × CR + 0.85, t0g = 2.0 × Pr1 + 0.85- {Log (tc) -0.000075 × (Tc −
800) ² } However, CR: Cold reduction (%) Pr1 = ([N] -0.005) / [N] [N], [Al] is the content of N and Al elements (mass%) 3. A cold rolled steel sheet is produced by heating a steel material in a temperature range of 1000 to 1300 ° C., performing rough rolling, and then finish rolling, and then Ns.
If 1 ≥ 0.005, wind at 650 ℃ or less, Ns1 <0.
In the case of 005, the hot-rolled steel sheet produced by winding at 550 ° C. or lower is cold-rolled, and the production of hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics according to claim 1 or 2. Method. 4. C: 0.01 to 0.2 mass%, Si: 0.4 mass% or less, Mn: 0.2 to 2.0 mass%, P: 0.05 mass% or less, Al: 0.001 to 0.1 mass% and N: 0.005 to 0.02 mass%. In addition, one or two of Ti: 0.001 to 0.1 mass% and Nb: 0.001 to 0.1 mass% are added to [N] ≧ (14/93).
[Nb] + (14/48) [Ti] + contained under 0.005, balance F
e and steel materials with inevitable impurity component composition,
After cold-rolled steel sheet containing 50 ppm or more of solute N obtained by hot rolling and then cold rolling is heated to a temperature range of 650 ° C to 900 ° C, at least from the heating temperature.
In the temperature range up to 650 ° C, cooling is performed at an average cooling rate of 5 to 50 ° C / s, heating-cooling treatment is performed, and then a plating layer is formed on the steel sheet surface. Ns2 shown in the equation (5) is Ns2 <0.00
In the case of 5, the above heating in the temperature range of 650 ° C or higher
A method for producing a hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics, characterized in that the cooling treatment time is restricted to tg 1 second represented by the following formula (2). Note Ns2 = [N]-(14/27) [Al]-(14/93) [Nb]-(14/48) [Ti] ... (5) Log (tg1) = 0.000075 × (Tg- 800) ² + tO ··· (2) Where, Tg: Heating temperature (℃) t0 = 2.0 × Pr2 −0.008 × CR + 0.85 However, CR: Cold reduction rate Pr2 = {[N]- (14/93) [Nb]-(14/48) [Ti] -0.005} / {[N] (14/9
3) [Nb]-(14/48) [Ti]} [N], [Al], [Nb] and [Ti] are the contents of N, Al, Nb and Ti (mass%), respectively. 5. The basic process according to claim 4, wherein
Prior to performing the cooling treatment, the heating-addition of a pretreatment of heating after heating to a heating temperature in the cooling treatment and then pickling, based on a series of steps, further represented by the following formula (5) Ns
When 2 <0.005, the pretreatment time in the temperature range of 650 ° C or higher is regulated within tc seconds represented by the following formula (3), and the heating temperature in the temperature range of 650 ° C or higher-
A method for producing a hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics, characterized in that the cooling treatment time is restricted within tg2 seconds represented by the following formula (4). Note Log (tc) = 0.000075 × (Tc −800) ² + tOc ··· (3) where Tc: Heating temperature in pretreatment (° C) tOc = 2.0 × Pr2 −0.008 × CR +0.85 However, CR : Cold reduction (%) Pr2 = {[N]-(14/93) [Nb]-(14/48) [Ti] -0.005} / {[N]-(14 /
93) [Nb]-(14/48) [Ti]} Log (tg2) = 0.000075 × (Tg −800) ² + tOg ··· (4) Where, Tg: Heating temperature in heating-cooling process (° C ) t0g: Log (tc) <0.000075 × (Tc-800) ² -0.008 × CR + 0.85
, T0g = 2.0 × Pr2 + 0.85 Log (tc) ≧ 0.000075 × (Tc-800) ² -0.008 × CR + 0.85, t0g = 2.0 × Pr2 + 0.85- {Log (tc) -0.000075 × (Tc −
800) ² } However, CR: Cold reduction (%) Pr2 = {[N]-(14/93) [Nb]-(14/48) [Ti] -0.005} / {[N]-(14 /
93) [Nb]-(14/48) [Ti]} [N], [Al], [Nb] and [Ti] are the contents of N, Al, Nb and Ti (mass%), respectively. 6. A cold rolled steel sheet is produced by heating a steel material in a temperature range of 1000 to 1300 ° C., performing rough rolling, and then finish rolling, and then Ns.
If 2 ≥ 0.005, wind at 650 ℃ or less, Ns2 <0.
In the case of 005, the hot-rolled steel sheet produced by winding at 550 ° C. or lower is cold-rolled, and the production of hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics according to claim 4 or 5. Method. 7. The method for producing a hot-dip galvanized cold-rolled steel sheet having excellent strain aging characteristics according to claim 1, further comprising heat alloying treatment after forming the plating layer.