JPS63243225A - Production of cold rolled steel sheet having excellent resistance to cracking by brazing - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet having excellent resistance to cracking by brazing by hot rolling a steel slab contg. specific ratios of C, Si, Mn, SolAl, P, S, N, O, Ti, and B and subjecting the rolled slab to cold rolling and annealing under specific conditions. CONSTITUTION:The slab of the steel contg., by weight %, 0.001-0.01% C, <=0.1% Si, 0.05-0.50% Mn, 0.01-0.10% SolAl, <=0.03% P, <=0.015% S, <=0.007% N, <=0.01% O, >=4XC% (effective Ti content) conforming to the formula and <=0.30% Ti, 0.0004-0.0015% B, and the balance Fe and inevitable impurities is hot rolled and is coiled at about 600-750 deg.C. After the rolled slab is pickled, the slab is subjected to cold rolling at >=50% draft and annealing at the temp. above the recrystallization temp. and <=900 deg.C. The cold rolled steel sheet which maintains ductility and deep drawability and has the excellent resistance to cracking by brazing is thereby produced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing a cold rolled steel sheet having good ductility and deep drawability and excellent brazing cracking resistance.

[Conventional technology]

For example, cold-rolled steel sheets for automobiles are required to have ductility and deep drawability depending on their use.

After the parts are formed, soldering may be performed, and resistance to soldering cracking may be required. In response to the requirements for ductility and deep drawability, various cold-rolled steel sheets have already been proposed that have developed a recrystallized texture with a (111) plane orientation that is effective in improving deep drawability. However, as will be explained later, there is no effective preventive measure against solder joint cracking2. Braze joint cracking is caused by a type of liquid metal embrittlement0
For example, the Bulletin of the Japan Institute of Metals 8 (1969) ρ235 describes the embrittlement development, which occurs when a solid metal is subjected to tensile stress in the presence of a liquid metal.

It is said that this is embrittlement development that occurs when liquid metal penetrates into grain boundaries. That is, it is thought that brazing cracks occur when the periphery of the brazing part, which is locally heated during soldering, receives tensile stress during cooling, and the molten solder acts thereon, causing intergranular cracking.

Further, the degree of cracking in the solder joint varies depending on the type of solder, and is most noticeable in brass solder, and is somewhat reduced in the case of silver solder. No effective means to prevent cracking of brass solders has been found so far, and when cracking of brass solders occurs, it is recommended to solder with silver solder to reduce the cracking. However, even if silver solder is used, soldering cracks cannot be completely prevented.

Using silver solder in this way reduces solder joint cracking.

Although the reason is not clear, it is thought that the degree of embrittlement differs depending on the combination of liquid metal and solid metal.

[Problem that the invention seeks to solve]

If expensive silver solder is used instead of brass solder to prevent solder joint cracking, the R construction cost will become too high and the incidence of solder joint cracking will be lower than with brass solder, but There is a problem that cracks in the solder joint still remain.

It is an object of the present invention to solve one or more of the problems of the prior art and to provide a method for producing a cold rolled steel sheet having good ductility and deep drawability, and excellent resistance to brazing cracking.

[Means for solving problems]

As a result of intensive research into the development of cold-rolled steel sheets with excellent blast weld cracking properties, the present inventors have discovered that the addition of B is effective in improving the blast weld cracking properties.

[Structure of the invention]

The present invention In weight%.

C: 0.001-0601%.

Si: 0.1% or less.

Mn: 0.05-0.50%.

Sol, AJ! : 0.01-0.10%.

P: 0.03% or less.

S: 0.015% or less.

N: 0.007% or less.

0: 0.01% or less.

Ti: [Effective Ti amount] according to the following formula (1) is 4×C% or more and 0.30% or less.

B: 0.0004 to 0.000%, with the essential condition of containing oois%, Nb: 0.03 to 0.10%.

Cr: 0.06-0.20%.

One or more of these may be contained as necessary, and the remainder: Fe and unavoidable impurities.

After hot rolling a steel slab consisting of
Provided is a method for producing a cold rolled steel sheet with excellent brazing cracking resistance, which comprises cold rolling as described above and then annealing at a temperature of not less than the recrystallization temperature and not more than 900°C.

That is, the present invention provides a method for producing cold-rolled steel sheets with excellent brazing cracking resistance. Steel to which 0.0004 to 0.0015% B is added is used, and in this case, the ductility is increased by reducing the amount of C and Ti to below a predetermined value, and a predetermined amount of B is added in combination. The basic feature is to prevent solder cracking, and thereby a method for manufacturing a cold rolled steel sheet with excellent solder cracking resistance that solves the above problem has been obtained.0 Advantages of the present invention The characteristics will be specifically shown in Examples below, but the reasons for limiting the chemical component values are summarized as follows.

The lower the content of C, the better it is for improving the ductility of the cold-rolled steel sheet, and if the amount exceeds 0.01%, a large amount of carbonitride-forming elements are required and carbonitride precipitation occurs. As the amount increases, press formability deteriorates. On the other hand, in a practical-scale steelmaking furnace, it is difficult to reduce the C content to less than o,oot%. For this reason, the C content is set to o, ooi to 0.01%.

Mn is added for the purpose of preventing hot embrittlement of steel, but if it is less than 0.05%, this purpose will not be achieved, and if it is too much, it will reduce the ductility and deep drawability, so the Mn content should be reduced to 0. .05 to 0.50%.

Si is added for the purpose of deoxidizing molten steel.

If added in too large a quantity, the ductility will be reduced, so the content is set to 0.1% or less.

AQ is added for the purpose of deoxidizing molten steel, but if the amount is less than 0.01% of Sol and Aρ (acid soluble AR) in steel, this purpose cannot be achieved. Sol
If the amount of Sol and All exceeds 0.10%, the effect will be saturated and non-metallic inclusions will increase, causing surface flaws.
10%.

If too much P is contained, it will increase the yield strength and tensile strength, and in ultra-low C steel, it will segregate to the grain boundaries and cause secondary work cracking, so the upper limit of its content should be set. is set to 0.03%.

The smaller the amount of N, the smaller the amount of Ti added, but if it is too large, it reduces the [effective amount of Ti] and deteriorates the press formability of the final product, so it is set to 0.007% or less.

When S and 0 become too large, the [effective Ti amount] decreases, and the total Ti amount for ensuring the [effective Ti amount] increases. In addition, S, O
are S≦0.015% and O≦0.010%, respectively.

By fixing C and N, Ti has the effect of increasing ductility and causing the generated TiC to generate a recrystallized texture with a (111) plane orientation that is effective for improving deep drawability. For this purpose, the [effective Ti amount] shown by the above-mentioned formula (1) needs to be 4×C% or more. However, 0.3
When it exceeds 0%, T becomes solid solution in ferrite.
The amount of i increases, resulting in an increase in yield strength and a decrease in ductility. It also increases manufacturing costs. Therefore, Ti has an [effective Ti amount] of 4×C% or more and 0.30
% or less.

B is an important additive element for preventing copper solder cracking in the steel of the present invention, but if it is less than 0.0004%, the purpose cannot be fully achieved.
If it exceeds 5%, the brazing cracking resistance will improve, but the ductility and deep drawability will deteriorate. Therefore, the amount of B is 0.00
04 to 0.0015%.

It is currently unclear why the addition of a small amount of B prevents solder joint cracking, but it is thought that B segregated at the grain boundaries of steel weakens the grain boundary embrittlement effect caused by molten wax during soldering. .

When Nb is contained in an amount of 0.03% or more, it has the effect of improving the in-plane true orientation of the r value of a cold rolled steel sheet, but excessive Nb causes deterioration of ductility. 03～
Add in a range of 0.10%.

Cr has the effect of improving deep drawability and stretchability by adding Ti or in combination with Ti and Nb, and to achieve this purpose, 0.06% or more of Cr is required, but 0.06% or more of Cr is required. Even if Cr is added in an amount exceeding 20%, this effect will be saturated and the manufacturing cost will only increase. Therefore, Cr is added as necessary in the range of 0.06 to 0.20%.

In this way, the present invention improves ductility and deep drawability by adding a predetermined amount of B in Ti-added steel or steel in which a predetermined amount of one or more of Nb and Cr is added to Ti-added steel as necessary. The objective is to produce a cold-rolled steel sheet that has excellent brazing cracking resistance without degrading the soldering properties, but it is preferable to manufacture this cold-rolled steel sheet under the following conditions.

First, it is desirable to melt steel in a steelmaking furnace and perform vacuum degassing treatment before ingot making or continuous casting.This reduces the C2O content in the steel as described above and also reduces the composition range as described above. ti4m can be advantageously carried out with high yield. When performing this vacuum degassing treatment, AQ can also be added for deoxidation treatment. After the vacuum degassing treatment, a slab is manufactured by ingot formation and blooming rolling or by continuous casting, and after performing slab care as necessary, hot rolling is performed.

Alternatively, after continuous casting, the hot slab may be charged into a heating furnace and hot rolled.

When carrying out hot rolling, from the perspective of improving deep drawability,
The hot rolling finishing temperature is set to Ar, as desired. Moreover, the hot rolling coiling temperature is preferably in the range of 600 to 750°C.

After pickling, hot-rolled steel sheets are cold-rolled.This cold-rolling requires more than 50% of the total cold rolling to develop a recrystallized texture with a (111) orientation that is advantageous for deep drawability. Perform at rolling rate.

Next, annealing is performed, and this annealing may be either batch annealing or continuous annealing, and the temperature is 90°C above the recrystallization temperature of the cold rolled steel sheet.
Excellent press formability can be obtained by carrying out the process at a temperature range of 0°C or lower. In this way, according to the present invention, a cold-rolled steel sheet that maintains ductility and deep drawability and has excellent brazing cracking resistance can be economically provided.

[Specific disclosure of the invention]

Example 1 Steel having the chemical composition values shown in Table 1 was melted using an 80-ton converter and degassing equipment, each molten steel was made into slabs by continuous casting, and each slab was heated to a temperature of 1,200 yen.
- 1,230℃, hot rolling finishing temperature 900-930℃, hot rolling winding temperature 700-730℃ sheet thickness 4,0
A hot-rolled coil with a thickness of 1.5 mm was obtained, and after pickling, it was cold rolled to a plate thickness of 1.2III1 at a cold rolling rate of 70%. Each coil was subjected to batch annealing at an annealing temperature of 780° C. and an annealing time of 2 hours. Table 2 shows the mechanical property values and braze cracking resistance index of the obtained cold rolled steel sheet.

In the brazing cracking resistance test, as shown in Figure 1, a steel plate was processed into a size of 1.2 x 25 x 135 mm, and a U notch with a width of 2 mm, a depth of 2n++a, and a radius of 11 at the end was inserted in the longitudinal center. Use commercially available brass solder (Cu: 60.7%, Sn: 1.35%,
(Si: 0.2%, Zn: balance) was placed and heated and maintained at a test temperature of 1030°C, which is higher than the melting point of brass solder. The distance between the chucks of the test piece was 40m+a. After that, a load was applied at a displacement rate of 0.05 m5+/sec, and as shown in Figure 2, the time t from the start of the load to the highest load point at which cracks were determined to occur was measured. This was used as an index of crackability. Figure 2 also shows the load-hour curve when no solder is placed. As a result of investigating the correlation between 70 and the brazing cracking resistance of actual molded parts, it was found that to was 3.
When it was longer than 0 seconds, no cracking occurred in the soldering, so
The soldering cracking resistance was determined to be acceptable in 23.0 seconds.

As for deep drawability evaluation, total elongation (T, lQ) was 48%.
If the above and the g value were 1.60 or more, it was judged as good.

As is clear from Table 2, Ti-
Comparative steel A, which is a Cr-Nb-added steel, and comparative steel B, which is a Ti-Cr-added steel, both have a total elongation of 48.3% or more and a T value of 2.1.
0 or more, which indicates good deep drawability, but the time t1 from the start of load to the highest load point at which cracks occur is 2.8.
It is short, less than seconds, and has poor brazing cracking resistance.

Invention steel C with composite addition of 0.0006% B to Ti-added steel, invention steel C with 0.0012% B added to Ti-Cr-Nb-added steel, and 0.0007% B to Ti-Cr addition steel.
0.0 to inventive steel G and Ti-Nb added steel.
Both of the steels of the present invention with 0.006% B added were 48.8
% or more, an F value of 1.61 or more, good deep drawability, and a tl of 4.0 seconds or more, and excellent brazing cracking resistance.

Comparative steel E, in which 0.0020% B was added to a Ti-Cr-Nb steel, and comparative steel F, in which 0.0030% B was added to a Ti-added steel, showed a tl of 6.4 seconds or more, and the durability was high. It has excellent brazing cracking properties, but the total elongation is less than 48%, the F value is small at 1.46 or less, and the deep drawability is low.

〔Effect of the invention〕

According to the invention, Ti-added steel or optionally T
By adding 0.0004 to 0.0015% B to i-added steel with a predetermined composite addition of one or more of Nb and Cr, excellent brazing resistance can be achieved while maintaining good ductility and deep drawability. A cold-rolled steel sheet having tangential crackability is obtained.

[Brief explanation of drawings]

Figure 1 is a front view of the test piece for the braze cracking resistance test, and Figure 2 shows the time from the start of load to the highest load point at which cracking is determined, which is an index of the braze cracking resistance. t8
It is a figure which shows an example of the load one hour curve for measuring.

Claims (1)

[Claims] 1. In weight%, C: 0.001 to 0.01%, Si: 0.1% or less, Mn: 0.05 to 0.50%, Sol. Al: 0.01 to 0.10%, P: 0.03% or less, S: 0.015% or less, N: 0.007% or less, O: 0.01% or less, Ti: the following formula (1) A steel slab containing [effective Ti amount] of 4×C% or more and 0.30% or less, B: 0.0004 to 0.0015%, and the balance: Fe and unavoidable impurities is hot rolled. A method for producing a cold-rolled steel sheet with excellent brazing cracking resistance, which comprises cold rolling at a reduction rate of 50% or more, and then annealing at a temperature of not less than the recrystallization temperature and not more than 900°C. [Effective Ti amount]: Total Ti amount - [N% (48/14) + S
%(48/32)+0%×(48/16)×1/2]・
...(1) 2. In weight%, C: 0.001 to 0.01%, Si: 0.1% or less, Mn: 0.05 to 0.50%, Sol. Al: 0.01 to 0.10%, P: 0.03% or less, S: 0.015% or less, N: 0.007% or less, O: 0.01% or less, Ti: the following formula (1) [Effective Ti amount] is 4×C% or more and 0.30% or less, B: 0.0004 to 0.0015%, Cr: 0.06 to 0.20%, the balance: Fe and unavoidable A steel slab consisting of impurities is hot-rolled, then cold-rolled at a reduction rate of 50% or more, and then annealed at a temperature above the recrystallization temperature and below 900°C. Manufacturing method for cold rolled steel sheets. [Effective Ti amount] = Total Ti amount - [N% (48/14) + S
%(48/32)+0%×(48/16)×1/2]・
...(1) 3. In weight%, C: 0.001 to 0.01%, Si: 0.1% or less, Mn: 0.05 to 0.50%, Sol. Al: 0.01 to 0.10%, P: 0.03% or less, S: 0.015% or less, N: 0.007% or less, O: 0.01% or less, Ti: the following formula (1) [Effective Ti amount] is 4×C% or more and 0.30% or less, B: 0.0004 to 0.0015%, Nb: 0.03 to 0.10%, the balance: Fe and unavoidable A steel slab consisting of impurities is hot-rolled, then cold-rolled at a reduction rate of 50% or more, and then annealed at a temperature above the recrystallization temperature and below 900°C. Manufacturing method for cold rolled steel sheets. [Effective Ti amount]: Total Ti amount - [N% (48/14) + S
%(48/32)+0%×(48/16)×1/2]・
...(1) 4. In weight%, C: 0.001 to 0.01%, Si: 0.1% or less, Mn: 0.05 to 0.50%, Sol. Al: 0.01 to 0.10%, P: 0.03% or less, S: 0.015% or less, N: 0.007% or less, O: 0.01% or less, Ti: the following formula (1) [Effective Ti amount] is 4×C% or more and 0.30% or less, B: 0.0004 to 0.0015%, Nb: 0.03 to 0.10%, Cr: 0.06 to 0.20 %, with the remainder being Fe and unavoidable impurities, is hot-rolled, then cold-rolled at a rolling reduction of 50% or more, and then annealed at a temperature above the recrystallization temperature and below 900°C. A method for producing cold-rolled steel sheets with excellent brazing cracking resistance. [Effective Ti amount] = Total Ti amount - [N% (48/14) + S
%(48/32)+0%×(48/16)×1/2]・
...(1)