JPH0741903A - Hot rolled steel sheet for deep drawing excellent in workability and having small anisotropy and its manufacture - Google Patents

Abstract

PURPOSE:To provide a hot rolled steel sheet for deep drawing excellent in workability and having small anisotropy by hot-rolling a dead soft steel slab in which the content of impurity components is controlled under a specified condition. CONSTITUTION:After hot-rolling the dead soft steel slab having a composition which contains, by wt.%, <=0.0025% C, <=0.005% P, <=0.003% N, <=0.004% S, in which the total amount of them is controlled in the range of 30-900ppm and which contains 0.05-0.20%, Mn, 0.005-0.07% Al and <=0.03% Si at a finishing temp. of >=910 deg.C and the total finishing draft of >=90%, cooling is started within 1sec and the slab is cooled to <=750 deg.C at a cooling rate of >=30 deg.C/sec and successively coiled at 650-750 deg.C. The hot-rolled steel sheet whose grain size of ferrite is >= No. 6.0 of GSNO (grain size number), which has excellent workability and deep drawability, small anisotropy and properties better than that of a cold-rolled steel sheet can be manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet, which is a material having a small anisotropy and is required to have a drawability at the time of molding such as a compressor container, and can be used as a substitute for a cold-rolled steel sheet, and a method for producing the hot-rolled steel sheet. It is about.

[0002]

2. Description of the Related Art Conventionally, hot-rolled steel sheets are inferior in deep drawability to cold-rolled steel sheets, but it is difficult to build workability in cold-rolled steel sheets because it is difficult to build workability in cold-rolled steel sheets. Was used. However, there is a limit to improving the drawability of the hot-rolled steel sheet by the conventional method, so that it cannot be used for high working products. Further, even in the case of supplementing a hot-rolled steel sheet having a high ductility, when the crystal grain size is large, roughening occurs after working, which is a problem. Furthermore, since secondary processing cracks may occur during molding, B was added to strengthen the grain boundaries and prevent cracking.

A problem after the molding becomes possible is to reduce the waste caused by the molding and improve the yield. That is, when a material with large anisotropy is used when molding a compressor cover or the like, earrings occur, the amount of cutting and cutting increases, and a wide steel plate is required to allow for blanking. There was waste. However, since the material to which B is added basically raises the earring, it is difficult to achieve both secondary processing crack prevention and small anisotropy.

To solve these problems, Japanese Unexamined Patent Publication No. 2-20
In Japanese Patent No. 9424, compatibility is achieved by setting extremely low P,
The crystal grain size is controlled by setting the winding temperature to 550 to 650 ° C. However, at present, the requirements for materials are becoming more stringent, and we are looking for materials that have high ductility and deep drawability, and that do not cause surface defects such as rough skin. Absent.

[0005]

SUMMARY OF THE INVENTION The present invention provides a hot-rolled steel sheet that compensates for the above drawbacks, is excellent in secondary workability, has small anisotropy, does not cause rough skin, and has excellent deep drawability. At the same time, it is an object of the present invention to provide a method for manufacturing the steel sheet with the current equipment without major equipment modification.

[0006]

In the present invention, a steel sheet for achieving the above object is (1) wt%, C ≦ 0.
0025%, P ≦ 0.005%, N ≦ 0.003%, S
≦ 0.004%, Mn: 0.05 to 0.20%, Al:
0.005-0.07%, Si ≦ 0.03%, including other unavoidable elements, 40 ppm ≦ C + N + P + S ≦ 1
Satisfies 10 ppm and the ferrite grain size is GS
It is a hot-rolled steel sheet for deep drawing having NO (grain size number) of 6.0 or more, excellent workability and small anisotropy, and its manufacturing method is as follows: Temperature ≧ 91
The method of manufacturing a hot-rolled steel sheet for deep drawing, which is excellent in workability and has small anisotropy, which is characterized in that hot rolling is performed at 0 ° C. and winding is performed at 650 to 750 ° C., and (3) finishing temperature ≧
Hot rolling at 910 ° C with a total reduction of ≧ 90%,
Then, winding at 650 to 750 ° C, (4) hot rolling at a finishing temperature ≧ 910 ° C, starting cooling within 1 second after completion of rolling, and winding at 650 to 750 ° C, (5) finishing temperature Hot rolling is performed at ≧ 910 ° C. and a finishing reduction ratio of ≧ 90%, cooling is started within 1 second after the completion of rolling, and 650 to 7
Winding at 50 ° C., (6) hot rolling at a finishing temperature ≧ 910 ° C., then cooling to 750 ° C. or less at a cooling rate of 30 ° C./S or more, and then winding at 650 to 750 ° C.,
(7) Finishing temperature ≧ 910 ° C., finishing total rolling reduction ≧ 90
%, Hot rolling at a cooling rate of 30 ° C./S or more to 750 ° C. or less, and then winding at 650 to 750 ° C.,
(8) Hot rolling is performed at a finishing temperature ≧ 910 ° C., cooling is started within 1 second after the completion of rolling, and 75% at a cooling rate of 30 ° C./S or more.
Cool to 0 ° C. or lower, then wind at 650 to 750 ° C., (9) finish rolling ≧ 910 ° C., finish rolling reduction ≧ 90% hot rolling, and start cooling within 1 second after rolling. , A method of manufacturing a hot-rolled steel sheet for deep drawing with excellent processability and having small anisotropy, characterized by cooling to 750 ° C. or lower at a cooling rate of 30 ° C./S or more and then winding at 650 to 750 ° C. Is.

[0007]

In the present invention, the inventors have set a total elongation of 58% or more and an average r value of 1.
GSNO as a ferrite grain size number that is 0 or more and the Δr value is 0.1 or less and does not cause rough skin
Was set to 6.0 or more, and the experiment was started with the goal of not causing secondary work cracks. The grain size number for preventing rough skin is preferably 7.0 or more in a normal hot rolled steel sheet, but the average r value, anisotropy,
In the above materials having good ductility, GSNO is 6.
No. 0 has sufficient product value.

As a first experiment, various experiments were repeated in order to obtain high ductility, and the influence of each element was investigated. According to it, it was found that the ductility was improved as the content of each element was reduced. Among these, it was found that the effects of C, N, P, and S were particularly large, and at the same time, the crystal grain size was also greatly affected (see FIG. 1). However, it has been found that, in a steel material containing extremely low components, the total elongation cannot exceed 58% and the GSNO of ferrite cannot exceed 6.0 or more under ordinary hot rolling conditions.

Therefore, as a second experiment, the relationship between the crystal grain size and the elongation was investigated by adding the conditions of large reduction rolling and quenching immediately after rolling to the first experimental condition. As a result, it was confirmed that by adding these two conditions, it is possible to obtain a crystal grain size that does not cause rough skin while substantially ensuring ductility.

Further, in the case of high-purity steel, it forms a solid solution with C and N.
Is lessened, and the role of strengthening the grain boundary is lost, so that secondary work cracking is likely to occur. However, when rearranging many experimental results, the steel materials with reduced P are
It was found that the occurrence of secondary processing cracks was small. Further, as shown in FIG. 2, the influence on S has a great influence on the anisotropy, and it is necessary to regulate the amount of S in the steel in order to obtain a Δr value of 0.1 or less based on high-purity steel. I knew it was.

Based on the above experimental results, the constitution range of the present invention was determined. The detailed configuration conditions are as follows. C has a great influence on the ductility. Any of the generated cementite, solid solution C in ferrite crystal grains, etc. reduces ductility. Therefore, in the present invention, the amount of C is 25
It was set to ppm or less. This is to improve the ductility as much as possible, and if the C content is higher than this, the target high elongation, 5
This is because 8% or more cannot be stably obtained. N also has a great influence on the ductility, and nitrides, solute N, etc., which are generated similarly to C, deteriorate the ductility. Therefore, the amount of N is regulated to 30 ppm or less. Regarding P, there are reasons for improving the ductility and reducing the occurrence of secondary work cracks. Particularly, in order to prevent secondary work cracking, it is necessary to set the content to 50 ppm or less.

S is one of the most important requirements in the present invention. As shown in FIG. 3, in the extremely low C, N, P within the above range, if the amount of S is high, the anisotropy is increased. Becomes higher and the Δr value needs to be 40 ppm or less in order to be 0.1 or less. Further, Mn is another important component in the present invention. As a result of detailed investigation of the influence of C, N, and P on the material, the inventors of the present invention, when these elements are within the ranges shown above, as shown in FIG.
It has been discovered that the average r-value increases within a certain Mn addition range. The range is Mn: 0.05 to 0.20%
Is. This is a major feature of the present invention. That is, when C, N, and P are used as trace components, the average r value is improved within a certain Mn range, and even in the hot-rolled steel sheet, the average r value is increased.
This is a point where a value of 1.0 or more can be obtained.

When Si is added in a large amount, it not only deteriorates the ductility, but also causes Si-Mn-based inclusions to be harmful. Therefore, in the present invention, Si ≦ 0.03 is set as the condition that the ductility is not affected and no harmful inclusion is generated.
%. Further, Al is important as a deoxidizing element and is necessary for reducing inclusions in steel. In particular, 0.
If it is less than 005%, a large amount of inclusions are generated due to insufficient deoxidation. However, if the amount is too much, there is a concern that the cost may increase and the precipitates generated in the rolling process may be adversely affected. Therefore, the range was made into Al: 0.005-0.07%.

In order to build a material having high ductility as in the present invention, it is necessary to comprehensively regulate a small amount of elements that have a great influence. In the present invention, as shown in FIG. 1, it was confirmed that regulation of a trace amount of 4 elements exerts a great effect on ductility. That is, as a condition for improving ductility, 30 ppm
≦ C + N + P + S ≦ 90 ppm is required. At this time,
As shown in FIG. 2, the reason for setting the content to 30 ppm or more is that if it is less than this, even if a large pressure reduction and a rapid cooling are not performed, a grain size of 6.0 or more cannot be obtained with GSNO, and the steelmaking cost is high. This is because The upper limit is 90ppm or less,
This is because, as is clear from FIG. 1, the target total elongation of 58% or more cannot be obtained above this range.

Although the mechanism by which the average r value is improved in the component system of the present invention is unknown, in the present discovery, it occurs in a state where C, N and P, which have a great influence when they exist at the grain boundaries, are reduced. It is a phenomenon. The crystal grains forming the texture are greatly affected by the state of crystal grain boundaries in both transformation and recrystallization. It is well known that the influence of C, N, and P, which is a constituent factor of the present invention, on the grain boundary is as follows. First, regarding C and N, it is known that in rolling in the α region, the recrystallized ferrite generated from the grain boundaries of the high-purity steel has a texture excellent in drawability,
The presence of solute C and N is said to adversely affect the texture.

Regarding the effect of P on secondary work cracking, it is known that P tends to segregate at grain boundaries, which contributes to work cracking. In addition, when the texture of the invention steel sheet was investigated, the center part was almost random and the surface layer part had an average r.
There is a texture that is advantageous for improving the value. From these things, the precipitates present in the purified crystal grain boundaries are
By controlling the crystal orientation that could not be considered before,
It may have contributed to the improvement of the average r value.

The finishing temperature in finish rolling is 910 ° C.
What is done is to finish rolling at the Ar ₃ transformation point or higher,
This is to keep the transformed ferrite grains generated thereafter in a sized state. Further, the reason why the winding temperature is regulated to 650 to 750 ° C. is that the strain of the surface layer portion is recovered or removed by recrystallization to stably obtain an elongation of 58% or more, and it does not coarsen more than GSNO 6.0. This is because.

However, in the composition system close to that of high-purity steel as in the present invention, the transformation point is very high and the growth of crystal grains is also very fast. In this case, it may not be possible to stably obtain fine particles of 6.0 or more by GSNO only by controlling the winding condition. In such a case, it is preferable to prevent coarsening by utilizing rolling and cooling and combining conditions. As a result of detailed investigation of the conditions, it is effective to finish rolling with a total rolling reduction of 90% or more, to start cooling within 1 second immediately after rolling, and to cool to 750 ° C or less at a cooling rate of 30 ° C / S or more. Needless to say, it is most effective to use both at the same time. The steel sheets under the conditions of the present invention described above were hot-rolled steel sheets with a high average r value, little anisotropy, high workability, and no secondary work cracks.

[0019]

[Examples] Table 1 shows examples of steel composition, and Table 2 shows examples of rolling conditions and material results. No. manufactured under the conditions of the present invention. 1
To No. Among the test materials up to 9, A, B, C and D in the composition range of the present invention, the material of the steel sheet after rolling keeps the target grain size number, elongation, average r value and Δr value, Two
Subsequent processing cracks did not occur either. However, F containing a large amount of Mn
For the material I and the material with a large amount of C + N + P + S, the elongation did not reach the target. Further, in the G material having a large amount of P, secondary work cracking occurred. In the H material in which the amount of C + N + P + S was too small, the elongation was high, but the crystal grains were too large, and roughening occurred after processing. In addition, E, G, and I materials with large amounts of S and P have Δr
The value did not reach the target. Further, the average r value was low for the F and H materials in which the Mn content was not appropriate, the G material with a large P content, and the I material with a large C and N content, and did not reach the target of 1.0.

Next, using the same A material, No. No. 10 to No. In No. 15, the finishing temperature, which is an important requirement, did not satisfy the conditions of the present invention. 1
0, the winding temperature was too high. In No. 14, the crystal grains were too large and rough skin was generated. The winding temperature was too low. 1
No. 6 was not satisfied with the growth. No. 11, 12, 13
Since the finishing finish temperature and the winding temperature were satisfied, the grain size number 6.0 was managed to be managed and the target could be achieved even if the reduction ratio, the cooling start time, and the cooling rate were not satisfied. No. which satisfied all the conditions. 15, and No. 15 In No. 1, a very good material was obtained.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

The steel sheet according to the present invention is a hot-rolled steel sheet having high elongation and deep drawability, small anisotropy, and can be manufactured at a low cost with properties superior to those of the cold-rolled steel sheet. Also,
Since the composition is mainly regulated, the load on hot rolling is small, and it is possible to manufacture without trouble.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the C + N + S + P content in steel and the total elongation.

FIG. 2 is a diagram showing the relationship between the S content in steel and the Δr value.

FIG. 3 is a diagram showing the relationship between the Mn content in steel and the average r value.

Claims (9)

[Claims] 1. By weight%, C ≦ 0.0025%, P ≦ 0.005%, N ≦ 0.003%, S ≦ 0.004%, Mn: 0.05 to 0.20%, Al: 0.005-0.07%, Si ≦ 0.03%, including other unavoidable elements, 30 ppm ≦ C + N + P +
A hot-rolled steel sheet for deep drawing, which satisfies S ≦ 90 ppm and has a ferrite grain size of 6.0 or more in GSNO (grain size number), excellent workability and small anisotropy. 2. In% by weight, C ≤ 0.0025%, P ≤ 0.005%, N ≤ 0.003%, S ≤ 0.004%, Mn: 0.05 to 0.20%, Al: 0.005-0.07%, Si ≦ 0.03%, including other unavoidable elements, 30 ppm ≦ C + N + P +
A steel piece satisfying S ≦ 90 ppm is finished at a finishing temperature ≧ 910.
A method for producing a hot-rolled steel sheet for deep drawing, which is excellent in workability and has small anisotropy, which comprises hot rolling at ℃ and then winding at 650 to 750 ℃. 3. By weight%, C ≤ 0.0025%, P ≤ 0.005%, N ≤ 0.003%, S ≤ 0.004%, Mn: 0.05 to 0.20%, Al: 0.005-0.07%, Si ≦ 0.03%, including other unavoidable elements, 30 ppm ≦ C + N + P +
A steel piece satisfying S ≦ 90 ppm is finished at a finishing temperature ≧ 910.
A method for producing a hot-rolled steel sheet for deep drawing, which is excellent in workability and has small anisotropy, which comprises hot-rolling at 0 ° C and a total rolling reduction ≧ 90%, and then winding at 650 to 750 ° C. 4. In% by weight, C ≤ 0.0025%, P ≤ 0.005%, N ≤ 0.003%, S ≤ 0.004%, Mn: 0.05-0.20%, Al: 0.005-0.07%, Si ≦ 0.03%, including other unavoidable elements, 30 ppm ≦ C + N + P +
A steel piece satisfying S ≦ 90 ppm is finished at a finishing temperature ≧ 910.
Hot rolling at ℃, start cooling within 1 second after rolling,
A method for producing a hot-rolled steel sheet for deep drawing, which is excellent in workability and has small anisotropy, which comprises winding at 650 to 750 ° C. 5. By weight%, C ≦ 0.0025%, P ≦ 0.005%, N ≦ 0.003%, S ≦ 0.004%, Mn: 0.05 to 0.20%, Al: 0.005-0.07%, Si ≦ 0.03%, including other unavoidable elements, 30 ppm ≦ C + N + P +
A steel piece satisfying S ≦ 90 ppm is finished at a finishing temperature ≧ 910.
℃, finish rolling reduction ≧ 90%, hot rolling, start cooling within 1 second after rolling, and take up at 650 to 750 ° C. Excellent workability, small depth of anisotropy Manufacturing method of hot-rolled steel sheet for drawing. 6. In% by weight, C ≦ 0.0025%, P ≦ 0.005%, N ≦ 0.003%, S ≦ 0.004%, Mn: 0.05 to 0.20%, Al: 0.005-0.07%, Si ≦ 0.03%, including other unavoidable elements, 30 ppm ≦ C + N + P +
A steel piece satisfying S ≦ 90 ppm is finished at a finishing temperature ≧ 910.
After hot rolling at ℃, 750 ℃ at a cooling speed of 30 ℃ / S or more
A method for producing a hot-rolled steel sheet for deep drawing, which is excellent in workability and has small anisotropy, which comprises cooling to the following temperature and then winding at 650 to 750 ° C. 7. By weight%, C ≦ 0.0025%, P ≦ 0.005%, N ≦ 0.003%, S ≦ 0.004%, Mn: 0.05-0.20%, Al: 0.005-0.07%, Si ≦ 0.03%, including other unavoidable elements, 30 ppm ≦ C + N + P +
A steel piece satisfying S ≦ 90 ppm is finished at a finishing temperature ≧ 910.
30 after hot rolling at ℃, finishing total reduction ≧ 90%
Cool down to 750 ° C or lower at a cooling rate of ℃ / S or higher, then 6
A method for producing a hot-rolled steel sheet for deep drawing, which is excellent in workability and has small anisotropy, which comprises winding at 50 to 750 ° C. 8. In% by weight, C ≦ 0.0025%, P ≦ 0.005%, N ≦ 0.003%, S ≦ 0.004%, Mn: 0.05 to 0.20%, Al: 0.005-0.07%, Si ≦ 0.03%, including other unavoidable elements, 30 ppm ≦ C + N + P +
A steel piece satisfying S ≦ 90 ppm is finished at a finishing temperature ≧ 910.
Hot rolling at ℃, start cooling within 1 second after the completion of rolling, and cool to 750 ° C or less at a cooling rate of 30 ° C / S or more,
Then, the method for producing a hot-rolled steel sheet for deep drawing which is excellent in workability and has small anisotropy, which is characterized by winding at 650 to 750 ° C. 9. By weight%, C ≦ 0.0025%, P ≦ 0.005%, N ≦ 0.003%, S ≦ 0.004%, Mn: 0.05 to 0.20%, Al: 0.005-0.07%, Si ≦ 0.03%, including other unavoidable elements, 30 ppm ≦ C + N + P +
A steel piece satisfying S ≦ 90 ppm is finished at a finishing temperature ≧ 910.
℃, finish rolling reduction ≧ 90% hot rolling, start cooling within 1 second after rolling, cool to 750 ° C. or less at a cooling rate of 30 ° C./S or more, and then at 650 to 750 ° C. A method for producing a hot-rolled steel sheet for deep drawing, which has excellent workability and is small in anisotropy, which is characterized by winding.