JPH06306478A - Production of high strength galvannealed hot rolled steel sheet excellent in elongation flanging property - Google Patents

Abstract

PURPOSE:To provide a method for producing a high strength galvannealed hot rolled steel sheet excellent in elongation flanging properties and used for automotive parts in which working is severe or the like. CONSTITUTION:A steel slab having a compsn. contg. <=0.0l% C, <=0.3% Si, 0.1 to 2% Mn, <=0.1% Al, <=0.01% S and <= 0.1% P and furthermore contg. 0.8 to 2.0% Cu, and the balance Fe with inevitable impurities is used, and the drafts of rough rolling and finish rolling at the time of subjecting it to rolling in a hot rolling line immediately as it is or after low temp. heating and the cooling conditions after the rolling are prescribed, by which the texture is controlled to improve its elongation flanging properties. Moreover, skinpass with 1 to 5% elongation percentage is executed, and the sheet passing temp. in a reducing atmosphere in a continuous galvanizing line is prescribed to produce the high strength galvannealed hot rolled steel sheet in which the strength is increased by utilizing the precipitation strengthening by Cu and having excellent elongation flanging properties and >=540MPa tensile strength. Furthermore, Ni may be added thereto by 0.3 to 1.0%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled galvanized hot-rolled steel sheet having excellent stretch flangeability, which is used for mechanical structural members such as automobiles and general processing.

[0002]

2. Description of the Related Art In recent years, there has been a strong demand for reducing the plate thickness and increasing the strength in order to save energy and reduce fuel consumption. In response to this requirement, so-called Dual Phase steel and the like have been developed, but in the current situation, the members to which stretch flange processing is severe are cracked even in these high-strength hot-rolled steel sheets, so that the applicable members are limited. , A steel plate with high strength and good workability is required.

In order to overcome such a situation, a method for producing a hot rolled steel sheet having excellent stretch flangeability is disclosed in Japanese Patent Publication No. 2860/1990.
No. 8 and Japanese Patent Publication No. 51-44508. However, the method of adding Si for high strength as disclosed in JP-B-2-48608 has a problem of non-plating due to generation of Si scale.

Further, the method disclosed in Japanese Patent Laid-Open No. 51-44508 is economically disadvantageous because it requires the addition of Cr, and is intended for a high strength steel plate having a tensile strength of 540 MPa or more as in the present invention. Not what I did. Further, in these patent publications, there is no disclosure of alloy galvanized. The hot dip galvanizing is performed in a continuous hot dip galvanizing line, but since it is reheated in the line, it is very difficult to obtain a desired quality.
For example, if the crystal grains become coarse due to reheating, the strength will be lower than that of the as-hot-rolled material.

It is necessary to increase the amount of alloy in the steel in order to compensate for this decrease in strength to obtain the desired strength, but this increase in the amount of alloy causes problems such as poor plating adhesion and non-plating. Therefore, as an attempt to produce a hot-dip galvanized high-workability high-strength steel sheet, Japanese Patent Laid-Open No. 62-
No. 133059 discloses a high-strength hot-dip galvanized steel sheet having a tensile strength of 440 MPa or more in a steel component system containing Nb, Ti, and V, but no stretch flangeability has been examined.

[0006]

In order to improve stretch flangeability, it is effective to use an ultra low carbon system without a second phase such as carbides as a base. Therefore, the inventors of the present invention have conducted extensive studies to improve the stretch flange formability of the ultra low carbon hot rolled steel sheet, and as a result, the stretch flange formability and r in each direction are obtained in the ultra low carbon type which is not affected by the second phase. It was found that there is a strong correlation with the value. That is, in the hole expansion test for evaluating stretch flangeability, the test ends when the crack penetrates the plate thickness, but the crack occurs at this time in the direction of low r value.

To improve the minimum r value, it is effective to reduce the difference between the characteristic values in each direction, that is, to reduce the in-plane anisotropy. For that purpose, it is important to randomize the texture of the hot rolled steel sheet, that is, to promote the dynamic recrystallization of γ grains. However, Cu-added steel easily forms a strong texture and has large in-plane anisotropy. Therefore, the minimum r value was small, and it was difficult to improve the hole expansibility.

[0008] Therefore, the present inventors, in the hot rolling line,
By strictly determining the manufacturing conditions based on the following items, the in-plane anisotropy was reduced, the minimum r value was improved, and the stretch flangeability was improved. (1) Making the γ grains fine by rough rolling facilitates dynamic recrystallization of the γ grains.

(2) Dynamic recrystallization of γ grains is promoted by rolling under a large pressure after finishing rolling and finishing rolling at a high finishing temperature. Further, it is also promoted by air cooling after the completion of rolling. (3) Do not add Nb and Ti, which have a strong tendency to cause non-recrystallization of γ grains. Next, the present inventors investigated the influence of press-kind pass on the precipitation strengthening of Cu.

Hot rolling was performed using the test steels shown in Table 1.
The hot-rolled sheet was subjected to a skin pass while changing the elongation rate to 0, 1, and 2%, and then subjected to Cu precipitation treatment at 600 ° C.-3 mi.
n, and the change in tensile strength was investigated. The experimental procedure is shown in FIG. Further, FIG. 2 shows the relationship between the skin pass rate and the tensile strength. As described above, it can be seen that the skin strength after hot rolling improves the tensile strength as compared with the case of heat treatment under the same conditions. It is presumed that this is because the dislocation introduced by the skin path becomes a Cu precipitation site. By utilizing the effect of the skin pass after hot rolling, a steel sheet having higher strength can be obtained even with the same Cu content.

[0011]

[Table 1]

Further, the present invention regulates the temperature in the reducing atmosphere when passing through a continuous hot-dip galvanizing line and utilizes the precipitation strengthening of Cu to enhance the strength without sacrificing the hole expansibility. This enables galvannealing with excellent spot weldability. As described above, in view of the above problems, it is an object of the present invention to provide a method for producing a high-strength hot-dip galvanized hot-rolled steel sheet having excellent stretch-flangeability without impairing the appearance properties and economy. It is a thing.

[0013]

The gist of the present invention is as follows. (1) Mass%, C: 0.01% or less, Si: 0.
3% or less, Mn: 0.1 to 2%, Al: 0.1% or less,
S: 0.01% or less, P: 0.1% or less, and C
u: A steel containing 0.8 to 2.0% with the balance Fe and unavoidable impurities is made into a slab, and immediately or after heating to 1000 to 1200 ° C. and hot rolling,
In rough rolling, rolling is performed at a total reduction of 70% or more, and in finish rolling, reduction is effective strain [εeff = final pass reduction (%).
+1/2 (1st stage pass reduction before final (%)) + 1/4
Rolling (second pre-final second pass reduction (%))] is 30% or more, rolling is completed at a finishing temperature of (Ar ₃ points + 20 ° C.) to 950 ° C., and air cooling is performed for 1 second or more after rolling. Then, it is cooled at an average cooling rate of 10 ° C./sec or more and wound at 750 ° C. or less to form a hot rolled steel strip, and the obtained hot rolled steel strip has an elongation of 1
After performing a skin pass of ˜5%, it is passed through a continuous hot dip galvanizing line in a reducing atmosphere at a temperature of 550 to 680 ° C., and after cooling, it is immersed in a hot dip galvanizing bath to apply hot dip galvanizing, A method for producing a high-strength hot-dip galvanized hot-rolled steel sheet having a tensile strength of 540 MPa or more, which is excellent in stretch-flangeability and is characterized by heating again and alloying at 500 to 600 ° C. (2) Mass%, C: 0.01% or less, Si: 0.
3% or less, Mn: 0.1 to 2%, Al: 0.1% or less,
S: 0.01% or less, P: 0.1% or less, and C
Steel containing u: 0.8 to 2.0% and Ni: 0.3 to 1.0% and the balance Fe and unavoidable impurities was formed into a slab, and immediately or heated to 1000 to 1250 ° C. When performing hot rolling, the total rolling reduction is 70 in rough rolling.
% Or more, and in finish rolling, the reduction is effective strain [εe
ff = final pass reduction (%) + 1/2 (first final stage pass reduction (%)) + 1/4 (previous second stage pass reduction (%))] to 30% or more And finish rolling at a finishing temperature of (Ar ₃ points + 20 ° C.) to 950 ° C., air cool for 1 second or more after rolling, and then average cooling rate 10 ° C./sec.
Cooled as above, rolled up at 750 ° C or lower to form a hot rolled steel strip,
The hot-rolled steel strip thus obtained is subjected to a skin pass with an elongation of 1 to 5%, and then it is passed through a continuous hot dip galvanizing line in a reducing atmosphere at a temperature of 550 to 680 ° C., and after cooling, it is put in a hot dip galvanizing tank. A high-strength hot-dip galvanized hot-rolled steel sheet having a tensile strength of 540 MPa or more, which is excellent in stretch-flange formability, characterized by being immersed in hot-dip galvanized steel and then heated again to be alloyed at 500 to 600 ° C. Manufacturing method.

[0014]

Next, the reasons for limiting the constituents of the present invention will be described in detail. C is 0.01% or less. If it exceeds this, carbides are generated, and stretch flangeability is deteriorated. Mn is an element that imparts strength, and is added in the range of 0.1 to 2%. If it is less than the lower limit, the target strength cannot be obtained, and if it exceeds 2%, C is picked up in production and the upper limit of C cannot be satisfied.

Since Si causes Si scale and causes plating failure, the content of Si is set to 0.3% or less. Al is necessary as a deoxidizing agent, but if it exceeds 0.1%, alumina-based inclusions increase and the stretch flangeability of steel is impaired. S increases the amount of A-type inclusions extending in the rolling direction, and cracks propagate from that point as the starting point, so the hole expandability decreases. Therefore, the upper limit value is set to 0.01%.

P is an element that imparts corrosion resistance. However, if it exceeds 0.1%, the ductility decreases and the stretch flangeability decreases. Cu is an important element in the present invention. That is, according to the present invention, a predetermined strength can be obtained by precipitating Cu during winding and subsequent slow cooling.
If Cu is less than 0.8%, there is no effect, and if it exceeds 2.0%, the effect is saturated and a surface defect called Cu heggling may occur during hot rolling.

Ni is preferably added to prevent Cu hegging, but if it is less than 0.3%, it has no effect, and if it exceeds 1.0%, the effect is saturated and the economy is impaired. Next, the hot rolling conditions will be described. The above-mentioned steel is usually melted in a converter and made into a slab by continuous casting. After the converter is melted, various secondary refining may be performed.

This slab is inserted into a heating furnace as it is as a cold piece, a hot piece or a hot piece. The heating temperature at this time is 1000-
The temperature is 1200 ° C. The lower limit was set to the range where the current continuous hot rolling equipment can operate without lowering productivity. The upper limit is 1200
℃. If it exceeds this, Cu heggling occurs and the surface quality deteriorates. Further, when Ni is added within the above range, the upper limit of the heating temperature is set to 1250 ° C. The upper limit is Ni
Although it is improved by the addition, it is unavoidable that Cu heggling is generated when the amount exceeds this. Similarly, the lower limit value is a value that can be taken by the current equipment.

The rolling reduction in the hot rolling process is an important condition in the present invention. The total rolling reduction in rough rolling is 70% or more. If it is less than this range, the γ grains become coarse, dynamic recrystallization of γ is less likely to occur, and a strong texture is formed. as a result,
The in-plane anisotropy increases, the minimum value of the r value in each direction decreases, and the stretch flangeability decreases. In finish rolling, the reduction ratio in the post-finishing stage is particularly effective. Therefore, εeff = final pass reduction ratio (%) + 1/2 (first final stage pass reduction ratio (%)) + 1/4 (final front second stage) The effective strain defined by the pass reduction rate (%) is set to 30% or more. Below this γ
Dynamic recrystallization of grains is less likely to occur and a strong texture is formed. As a result, the in-plane anisotropy increases, and r in each direction
The minimum value decreases, and stretch flangeability decreases.

The finishing temperature is also a very important condition in the present invention. That is, according to the present invention, the finishing temperature is regulated and Cu
By randomizing the texture of the added hot rolled steel sheet,
It is intended to reduce the in-plane anisotropy. Therefore, the finishing temperature is (Ar ₃ points + 20 ° C.) to 950 ° C. If it is less than the lower limit value, a strong texture is formed, the in-plane anisotropy increases, the minimum value of the r value in each direction decreases, and the stretch flangeability decreases. The upper limit is a value that can be manufactured on an actual machine in consideration of the heating temperature.

After finish rolling, air cooling is performed for 1 second or more. If it is less than this, unrecrystallized γ grains are likely to remain, and a texture develops,
Stretch-flangeability deteriorates. Average cooling rate after air cooling is 1
0 ° C./sec or more. If it is less than this range, the α-grains become coarse and the ductility decreases. The winding temperature is 750 ° C or lower.
If it exceeds this, the α-grains become coarse and ductility decreases. The skin pass conditions after hot rolling are important in the present invention. As mentioned above, C in the continuous hot dip galvanizing line by skin pass
The precipitation strengthening amount of u can be improved. The condition is an elongation rate of 1 to 5%. If it is less than the lower limit value, there is no effect, and if it exceeds the upper limit value, work hardening is large and ductility deteriorates.

Next, the continuous hot dip galvanizing conditions will be described in detail. In the present invention, the temperature in the reducing atmosphere when passing through a continuous hot-dip galvanizing line is important. That is, it is intended to obtain the target strength by passing through the plate under the temperature condition according to the present invention and precipitating Cu or strengthening the cluster. The temperature is 550 to 680 ° C. Cu exceeds the upper limit
Becomes a solid solution state, and the target strength cannot be obtained. If it is less than the lower limit, an oxide film remains on the surface of the steel sheet, and the plating adhesion decreases.

Since the amount of Cu precipitation or cluster strengthening varies depending on the coiling temperature, the tensile strength of the hot-rolled steel strip changes. Therefore, it is necessary to adjust the reducing atmosphere temperature in the continuous hot-dip galvanizing line by the strength of the hot-rolled steel strip so as to reach the target tensile strength in the final product. Especially when the strength of the hot rolled steel strip is lower than the target strength, the reducing atmosphere temperature in the continuous hot dip galvanizing line is 570 to 6
Set to 30 ° C.

The heating temperature after immersion in the hot dip galvanizing bath is 5
The temperature is set to 00 to 600 ° C. The upper limit was set to a value that is possible with current equipment. Below the lower limit, alloying will be insufficient. The temperature is preferably 520 to 560 ° C.

[0025]

Example Steels having the components shown in Table 2 were tapped in a converter and subjected to secondary refining such as vacuum degassing to obtain slabs. Table 2
The steels indicated by the symbols A to E are within the scope of the present invention, and F
The steels indicated by L are outside the scope of the present invention. For F steel, C exceeds the upper limit, G
For steel, Si exceeds the upper limit, and for H steel, P, Al, and S exceed the upper limit. In the I steel, Cu exceeds the upper limit, in the J steel, Ni is less than the lower limit, the K steel contains Ti, and the L steel contains Nb and Ti.

Starting from these steels, Table 3 and Table 4
An alloyed hot-dip galvanized hot-rolled steel sheet was produced under the hot-rolling conditions and hot-dip galvanizing conditions shown in Table 3 (continued). Table 4 shows the characteristic values of the obtained steel sheet. Tensile test is JIS Z
Using JIS No. 5 test piece according to 2201, JIS Z 2
According to the method described in 241.

The r-value was measured in the rolling direction, 90 ° direction and 45 ° direction after applying 10% strain. Since the r value in the rolling direction was the minimum in the steel sheets shown in the examples, it is shown in Table 4. The stretch flangeability was evaluated by the hole expansion ratio in the punching hole expansion test. As the test piece, a 250 mm square steel plate having a diameter of d0 (= die diameter) punched out by a punch having a diameter of 20 mm and a die having a clearance (one side) of 10% of the plate thickness was used. In the hole expansion test, the above test piece is punched out by a press tester, and it is expanded with a 30 ° conical punch from the surface side without hole burrs (opposite to burrs) (At this time, there is no inflow of material into the expansion part. A 60 ton wrinkle retainer is applied to the flange), and it is stopped when the crack penetrates the plate thickness, and the hole diameter at this time (d)
The ratio (d / d0) of the original hole diameter (d0) was defined as the hole expansion ratio.

The plating adhesion was evaluated by an impact test. The method is a hemispherical punch (diameter: 12.7 mm) on a steel plate.
φ) was dropped, a tape was attached to the formed circular recess, and the amount of plating adhered to the tape after peeling was visually determined. The case where the entire surface was peeled was defined as bad, and the others were regarded as good. In Tables 3 and 4, No. 1-No. 5, No.
11 and No. Nos. 23 and 24 are steels of the present invention examples, which have the strength and the good hole expansion ratio which are the objects of the present invention, do not generate Si scale and Cu heggling, and have good plating adhesion and alloying degree. It was

No. 6-No. 10. No. 12 to N
o. 22 and No. 22. 25-No. 26 is a comparative example steel. No. In No. 6, since the heating temperature was too high, Cu heggies were generated and the surface properties were deteriorated. No. In No. 7, since the total rolling reduction of the rough rolling in the hot rolling process was too low, the dynamic recrystallization of γ was insufficient, the texture of the hot rolled steel sheet developed, and the in-plane anisotropy became large. As a result, the r value in the L direction decreased, and the hole expansion ratio decreased. No. In No. 8, since the effective strain in hot finish rolling was too small, the dynamic recrystallization of γ was insufficient, the texture of the hot-rolled steel sheet developed, and the in-plane anisotropy became large. The r value of was decreased, and the hole expansion ratio was decreased.
No. In No. 9, unrecrystallized γ remained because the air-cooling time in the hot rolling process was short, the texture of the hot rolled steel sheet developed, and the in-plane anisotropy increased, and the r value in the L direction accordingly increased. It became smaller and the hole expansion ratio decreased. No. In No. 10, since the cooling rate in the hot rolling step was too slow, the ferrite grains became coarse and the ductility decreased. No. In No. 12, the winding temperature in the hot rolling step was too high, so the ferrite grains became coarse and the ductility decreased. No. In No. 13, since C was too high, carbide was generated and the hole expansion ratio was lowered. No. In No. 14, since Si was too high, Si scale was generated and the surface quality was deteriorated. N
o. In No. 15, since Al and S were too high, inclusions were generated and the hole expansion ratio decreased. No. 16 is C because Cu is too high
u heggies are generated and No. No. 17 was Cu because Ni was too low
Baldness was generated and the surface quality was deteriorated. No. 1
In No. 8, the unrecrystallized texture of γ developed due to the addition of Ti, the in-plane anisotropy increased, and along with this, the r value in the L direction decreased and the hole expansion ratio decreased. No. 19 is N
Due to the addition of b and Ti, a non-recrystallized texture of γ developed and the in-plane anisotropy increased, and along with this, the r value in the L direction decreased and the hole expansion ratio decreased. No. In No. 20, the temperature in the reducing atmosphere in the continuous hot-dip galvanizing line was too low, so the plating adhesion was reduced. No. In No. 21, since the temperature in the reducing atmosphere in the continuous hot-dip galvanizing line was too high, Cu was in a solid solution state, precipitation of Cu or cluster strengthening was not obtained, and the target strength was not obtained.
No. In No. 22, the alloying temperature in the continuous hot-dip galvanizing line was too low, so Fe (%) in the plating layer was lowered.
No. In No. 25, the skin pass ratio was more than the upper limit, so that work hardening was large and ductility was lowered. No. In No. 26, the skin pass ratio was less than the lower limit, so the amount of precipitation strengthening of Cu was insufficient and the strength was reduced.

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

As described above, according to the present invention, it is possible to provide an alloyed hot-dip galvanized hot-rolled steel sheet having excellent stretch flangeability and corrosion resistance without impairing the appearance and economical efficiency. Thus, by applying the steel sheet of the present invention to a member that requires severe stretch-flange processing, it is possible to easily reduce the weight of, for example, an automobile, improve fuel efficiency, save resources, and the like, and have an extremely high industrial value.

[Brief description of drawings]

FIG. 1 is a diagram showing an experimental procedure of Cu precipitation treatment for investigating the influence of presskin pass on Cu precipitation strengthening.

FIG. 2 is a diagram showing the influence of the elongation rate of the skin path obtained in the experiment of FIG. 1 on the tensile strength.

Continuation of front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI technical display location // C22C 38/00 301 T 38/16 (72) Inventor Atsushi Itami 1 Kimitsu, Chiba, Japan Made in Japan Kimitsu Steel Works, Ltd.

Claims (2)

[Claims] 1. Mass%, C: 0.01% or less, S
i: 0.3% or less, Mn: 0.1 to 2%, Al: 0.1
%, S: 0.01% or less, P: 0.1% or less, Cu: 0.8 to 2.0%, and the balance Fe and unavoidable impurities after slab Immediately or at the time of performing hot rolling by heating to 1000 to 1200 ° C., rolling is performed at a total reduction ratio of 70% or more in rough rolling, and effective reduction in final rolling is [εeff = final pass reduction ratio (% ) +1/2 (1st stage pass reduction before final (%)) + 1/4 (2nd stage pass reduction before final (%))]
Rolling to 30% or more (Ar ₃ points + 20 ° C)
Rolling was completed at a finishing temperature of up to 950 ° C, air-cooled for 1 second or more after rolling, subsequently cooled at an average cooling rate of 10 ° C / sec or more, and rolled at 750 ° C or less to obtain a hot-rolled steel strip, which was obtained. A hot-rolled steel strip is skin-passed with an elongation of 1 to 5%, and then subjected to 550 in a reducing atmosphere in a continuous hot-dip galvanizing line.
After passing through the plate at a temperature of up to 680 ° C., cooling, dipping in a hot dip galvanizing bath to apply hot dip galvanizing, and then heating again, 5
A method for producing a high-strength hot-dip galvanized hot-rolled steel sheet having a tensile strength of 540 MPa or more, which is excellent in stretch-flangeability and is characterized by alloying at 00 to 600 ° C. 2. Mass%, C: 0.01% or less, S
i: 0.3% or less, Mn: 0.1 to 2%, Al: 0.1
% Or less, S: 0.01% or less, P: 0.1% or less, and Cu: 0.8 to 2.0%, Ni: 0.3 to 1.
Immediately after the steel containing 0% and the balance Fe and unavoidable impurities was made into a slab, or 1000 to 125
When heating to 0 ° C. and performing hot rolling, rolling is performed at a total reduction ratio of 70% or more in rough rolling, and effective strain [εeff = final pass reduction ratio (%) + 1/2 ( Rolling is performed so that the first-stage pre-final pass reduction (%)) + 1/4 (the second-stage pre-final second-pass reduction (%)) is 30% or more,
Rolling is completed at a finishing temperature of (Ar ₃ points + 20 ° C.) to 950 ° C., air cooling is performed for 1 second or more after rolling, and then an average cooling rate of 10
After cooling at ℃ / sec or more and winding at 750 ° C or less to form a hot rolled steel strip, the obtained hot rolled steel strip is skin-passed with an elongation of 1 to 5%, and then a reducing atmosphere is applied in a continuous hot dip galvanizing line. The sheet is stretched at a temperature of 550 to 680 ° C., cooled, immersed in a hot dip galvanizing bath for hot dip galvanizing, then heated again, and alloyed at 500 to 600 ° C. 540 with excellent tensile strength
A method for producing a high-strength hot-dip galvanized hot-rolled steel sheet having a pressure of MPa or more.