JPH1150194A - Soft cold-rolled steel sheet excellent in structural stability and method for producing the same - Google Patents

Abstract

[PROBLEMS] To provide a B-added soft cold-rolled steel sheet having excellent structure stability in high-temperature annealing and a method for producing the same. SOLUTION: In weight%, C: 0.01 to 0.03%;
Si ≦ 0.1%, Mn ≦ 0.5%, P ≦ 0.03%
And S ≦ 0.03%, N ≦ 0.0035%, and B ≦
0.0035%, and B / N =
0.6-1.5 and Al ≦ 0.035 × √ (B / N ×
0.6), and the balance is Fe and inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a soft cold-rolled steel sheet having excellent structural stability in continuous annealing particularly suitable for automobiles and home electric appliances, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, in the production of cold-rolled steel sheets for processing produced by continuous annealing, high-temperature winding is performed at the time of hot rolling in order to soften and increase the r-value, and precipitation of AlN and coarsening of carbides are performed. Is promoting. However, when high-temperature winding is performed, there is a problem that the scale thickness is increased at both ends of the coil where oxygen can be easily supplied, and the pickling property is deteriorated. Therefore, as a method of reducing the winding temperature by using softening by adding B, Japanese Patent Application Laid-Open No. Hei 2-263939 discloses a method of heating a B-added steel having a specified Mn / S to 1000 to 1200 ° C. 5
A method for producing a cold-rolled steel sheet for deep drawing in which winding is performed at 60 to 650 ° C and continuous annealing is performed at a relatively high temperature of 730 to 880 ° C is disclosed. Various methods have been proposed for obtaining good workability by high-temperature continuous annealing with low-temperature winding while utilizing good grain growth properties of the B-added steel. For example, Japanese Unexamined Patent Publication
Japanese Unexamined Patent Publication No. 9-3550 discloses a method for producing a cold-rolled steel sheet for processing, characterized in that B-added steel is wound at 600 to 700 ° C. and annealed at 740 to 930 ° C. The B-added steel is wound at 630 to 720 ° C.,
Japanese Patent Application Laid-Open No. 56-156720 discloses a method for producing a cold-rolled steel sheet for working, which comprises annealing at 880 ° C.
A method for producing a cold-rolled steel sheet having excellent workability by performing low-temperature winding at 650 ° C. or less and defining high-temperature annealing by defining the relationship between N and N is disclosed. Further, in order to obtain high workability, B / N, additional elements and slab heating temperature are specified as disclosed in
No. 15327, the slab heating temperature of steel to which B is added in an amount equal to or more than N is specified, and the steel is wound at 550 to 700 ° C.
A method of annealing at 50 to 850 ° C .;
No. 56, 0.10 to 0.30% of Cr is added,
550-700 ° C for steel with a / N ratio of 0.5-2.0
A cold-rolled steel sheet excellent in press formability and wound at around 800 ° C. is disclosed.

[0003]

However, when B-added steel excellent in grain growth is subjected to high-temperature annealing at 780 ° C. or more, there is a problem that a mixed grain structure is often formed and the surface quality during processing is deteriorated. In recent years, high quality has been required for the surface properties even more than before, and the deterioration of the surface properties due to such a mixed grain structure, which did not lead to a big problem until now, is becoming a problem Nevertheless, the above-mentioned prior art does not show any measures for improving the surface quality due to the mixed grain structure generated by annealing at 780 ° C. or more.

[0004] As described above, at present, there has been no method for improving the structural stability of B-added steel in continuous annealing to prevent the occurrence of mixed grains.

An object of the present invention is to provide a B-added soft cold-rolled steel sheet having excellent structure stability in high-temperature annealing and a method for producing the same.

[0006]

In order to solve the above-mentioned problems and achieve the object, the present invention uses the following means.

(1) The steel sheet of the present invention has a C:
0.01-0.03%, Si ≦ 0.1%, Mn ≦
0.5%, P ≦ 0.03%, S ≦ 0.03%, N
≦ 0.0035% and B ≦ 0.0035%,
And B / N = 0.6-1.5 and Al ≦ in stoichiometric ratio
0.035 × √ (B / N × 0.6), the remainder being F
e) and a soft cold-rolled steel sheet having excellent structural stability, characterized by being inevitable impurities.

(2) The steel sheet of the present invention further comprises Cu ≦ 0.5%, Ni ≦ 0.5%, Cr
≦ 0.5%, Sn ≦ 0.5%, Ca ≦ 0.1%, and O
Soft cold-rolling excellent in tissue stability according to the above (1), characterized by containing one or more selected from the group of ≦ 0.05% in a total range of 2% or less. It is a steel plate.

(3) The production method of the present invention is a method for producing a steel sheet having the composition described in (1) or (2) above, wherein the slab obtained by continuous casting is subjected to a temperature range of at least Ar ₃ points. Finish rolling and winding at a temperature lower than 650 ° C, and cold rolling of the hot-rolled steel sheet at a heating rate of 10 ° C
/ Step and continuous annealing at a soaking temperature of 780 ° C or more.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted intensive studies in order to obtain a B-added soft cold-rolled steel sheet having excellent structural stability during high-temperature annealing and a method for producing the same, and as a result, have obtained the following findings. Reached.

[0011] Since the B-added steel has excellent grain growth properties, the structure tends to be mixed grains when subjected to high temperature annealing. FIG. 1 shows an example, where C = 0.015%, Al = 0.023%, and B = 0.
When a steel of 0007%, N = 0.020% and B / N = 0.45 is wound at 600 ° C. and annealed at 800 ° C., coarse ferrite grains are partially generated.

The inventors of the present invention have conducted intensive studies on the cause of the occurrence of such mixed grains during high-temperature annealing. As a result, when high-temperature annealing is performed in a state where some solute N remains, AlN precipitates non-uniformly. In particular, it has been found that local coarse grains are easily generated in a B-added steel having excellent grain growth properties. Then, in order to suppress this mixed grain, first, the B / N ratio is defined to reduce the amount of solute N in the hot-rolled sheet, and further, the equation (1) Al ≦ 0.035 × √ (B / N N × 0.6) By reducing the amount of Al in accordance with B / N according to the relationship shown in (1), the start of precipitation of AlN during annealing may be delayed, and during recrystallization during high-temperature annealing, It has been found that a soft cold-rolled steel sheet having excellent structural stability can be manufactured without locally inhibiting grain growth. The following is the experiment that became the basis of this finding. C = about 0.015%, Mn = 0.20%,
P = about 0.011%, S = about 0.008%, Al = about 0.010%, B ≦ 0.0035%, N ≦ 0.0035
%, And the B / N ratio is changed, and the material is heated to 1200 ° C., and finish-rolled at a temperature of ₃ points or more of Ar.
Wound at 00 ° C. After pickling and cold rolling, 20
The temperature was raised at a rate of ℃ / sec and annealed at a temperature of 800 ° C.
Was prepared, and the EL was measured using a cross-sectional structure observation and a JIS No. 5 tensile test piece. The results are shown in FIG. B / N
With a rise in the EL, the EL slightly increases, and the softening effect conventionally known is recognized, but the B / N is 0.2%.
No remarkable effect is recognized above. However, the maximum grain size (the average of the 10 largest crystal grains among the crystal grains having a thickness of 1 mm in the sectional structure) is significantly increased when the B / N is in the range of 0.2 to 0.6. However, mixed grains were generated instead of normal grain growth. Also, B / N
Exceeds 1.5, the EL was reduced due to the effect of solid solution B on fine particles and solid solution strengthening. Next, C = about 0.015%,
Mn = about 0.20%, P = about 0.011%, S = about 0.1%.
A material containing 008%, B / N = about 1, and changing the amount of Al was heated to 1200 ° C., subjected to finish rolling at a temperature of ₃ points or more of Ar, and wound at 600 ° C. Subsequently, after pickling and cold rolling, the temperature was raised at a rate of 20 ° C./sec and annealing was performed at 800 ° C. to produce an annealed sheet having a thickness of 1.2 mm. did. The results are shown in FIG. The change in EL with the change in Al content is gradual, but the maximum particle size increases sharply above the Al content (0.027%) calculated by the above equation (1), resulting in a mixed grain structure. You can see that it is.

On the basis of the above findings, the present inventors controlled the B / N ratio and the amount of Al added to the B-added steel within a certain range, and further optimized the hot rolling and annealing conditions. Thus, a B-added soft cold-rolled steel sheet having excellent structure stability in high-temperature annealing and a method for producing the same have been found, and the present invention has been completed.

The reasons for adding the components, the reasons for limiting the components, and the reasons for limiting the production conditions of the present invention are described below.

(1) Component composition range C: 0.01 to 0.03% When C is added in excess of 0.03%, a large amount of carbides precipitates, lowering the r value and EL, and reducing the formability. Because of the inhibition, the upper limit is 0.03%. On the other hand, if it is less than 0.01%, the driving force for precipitation of carbides becomes small during overaging of continuous annealing, and the aging resistance is deteriorated. Therefore, the lower limit is 0.01%. Si ≦ 0.1% Si is excessively added to increase the strength and deteriorate the formability. Mn ≦ 0.5% Mn fixes S in the form of MnS and has a function of improving hot ductility. Therefore, it is desirable to add 0.05% or more of Mn. In this case, the upper limit is 0.5%.

P ≦ 0.03% P is a solid solution strengthening element, and the addition of more than 0.03% causes hardening of steel, so the upper limit is 0.03%.

S ≦ 0.03% Since S is an element that inhibits hot ductility and formability, M
Fixed as nS. Therefore, lower is desirable.
Since the addition exceeding 0.03% leads to an increase in the amount of Mn and lowers workability, the upper limit is 0.03%.

N ≦ 0.0035% In the present invention, N is fixed as BN. However, if the amount of BN is large, the workability is reduced.
5%.

B ≦ 0.0035% B is an element effective for softening, but the excessive addition of B increases the deformation resistance, so the upper limit is 0.0035%.
%.

B / N ratio: 0.6 to 1.5 In the present invention, the B / N ratio is extremely important. B / N is 0.
If it is less than 6, the precipitation amount of fine AlN increases and the steel becomes hard, so the lower limit of B / N is 0.6. Further, when B / N exceeds 1.5, B in the steel is generated and the steel becomes hard, so the upper limit of B / N is 1.5.

Sol. Al ≦ 0.035 × √ (B / N × 0.6) (1) Since Al is used as a deoxidizing agent, Al is included to some extent, but in the present invention, precipitation of fine AlN during annealing is performed. The addition range is important because it affects the starting time. Conventionally, a large amount of Al has been added for the purpose of completely fixing N, but in the present invention, it is necessary to keep the amount low. Precipitation of AlN during annealing is related to the amount of Al and the amount of solute N. A
Since the precipitation of 1N starts early in the unrecrystallized portion where the driving force is large, if the solute N is in an appropriately low range as in B-added steel, N is consumed in the precipitation of the unrecrystallized portion and other Precipitation is difficult in some parts, and the precipitation becomes non-uniform. Recrystallization and grain growth are suppressed in the portion where AlN is precipitated, but grain growth progresses in other portions, and once the grain size difference is obtained, the difference further increases in the growth process, resulting in a mixed grain structure. In contrast, A
By defining the amount of l in accordance with the above formula (1), the precipitation of AlN is delayed, the precipitation in the non-recrystallized portion can be avoided, and the generation of mixed grains can be suppressed.

Further, according to the present invention, Cu ≦ 0.5%, Ni
≦ 0.5%, Cr ≦ 0.5%, Sn ≦ 0.5%, Ca ≦
One or more selected from the group of 0.1% and O ≦ 0.05% may be contained in a total range of 2% or less.

Since Cu, Ni, Cr, Sn, Ca, O and the like do not impair the structural stability intended in the present invention, they can be added in appropriate amounts with the same concept as ordinary steel. That is, the corrosion resistance is improved by adding Cu, Ni, Cr, and Sn in the above ranges. Ca
When added in the above range, it promotes carbide aggregation and improves aging resistance. O exists in the form of oxides in steel, and M
It serves as precipitation nuclei for nS and BN, and promotes their precipitation.

By adjusting to the above-mentioned component composition range, it becomes possible to obtain a B-added soft cold-rolled steel sheet having excellent structure stability in high-temperature annealing.

The steel sheet having such characteristics can be manufactured by the following manufacturing method.

(2) Steel Sheet Manufacturing Process (Manufacturing Method) A steel adjusted to the above-described composition range is melted, and a slab obtained by continuous casting is finish-rolled in a temperature range of Ar ₃ points or more, and 650 ° C. Take up with less than. next,
The rolled hot-rolled steel sheet is cold-rolled and continuously annealed at a heating rate of 10 ° C./sec or more and a soaking temperature of 780 ° C. or more.

In the present invention, the temperature of each step has an important meaning, and if any one of them is applied, the effect of the present invention is reduced.

A. Finishing Temperature In the present invention, the finishing temperature is at least Ar ₃ points. When finishing temperature is Ar less than ₃ points, since the texture to reduce the r value will developed, the lower limit is ₃ points Ar.

B. Winding Temperature The upper limit of the winding temperature is 650 ° C. from the viewpoint of pickling properties. However, if the temperature is lower than 200 ° C., the shape of the coil becomes unstable.

C. Annealing Heating Rate In the present invention, the heating rate is important. In the present invention,
Although Al and B / N are specified to delay the precipitation of AlN from recrystallization, if the temperature rise rate is less than 10 ° C./sec, AlN is likely to precipitate, and AlN is not recrystallized before recrystallization is completed. In order to partially suppress the recrystallization and grain growth, the structure becomes a mixed grain. Therefore, the lower limit of the heating rate is 10 ° C./sec.

D. Annealing temperature When the annealing temperature is lower than 780 ° C, the softening is not sufficiently performed. Therefore, the lower limit of the annealing temperature is 780 ° C. However, 900
If the annealing is performed at a temperature higher than 900C, the texture formed by cold rolling becomes random.

The slab heating temperature is not particularly limited, but is preferably 1050 ° C. or higher from the viewpoint of rolling load and finishing temperature. In addition, there is no problem even if direct rolling, in which rolling is performed directly without cooling the continuous casting slab, is performed. Even if the finish rolling is performed by heating or holding after the rough rolling, the effect of the present invention is not lost. After the rough rolling, there is no problem even if the rough bars are joined and the finish rolling is continuously performed. Further, even if a thin slab is used, the effect of the present invention does not change.
As for cold rolling after pickling, the rolling ratio is preferably 30 to 90% from the viewpoint of workability, particularly deep drawing. There is no limitation on the condition of the temper rolling, but if it exceeds 2%, the EL is drastically reduced.

For the composition adjustment of the steel of the present invention, both a converter and an electric furnace can be used.

[0034]

【Example】

(Example 1) Steel having the components shown in Table 1 was hot-rolled at a temperature of _three or more points of Ar and wound at a winding temperature shown in Table 1.
Subsequently, pickling and cold rolling were performed, continuous annealing was performed under the annealing conditions shown in Table 1, and then temper rolling was performed at a pressure control rate of 1.2% to produce a sheet having a sheet thickness of 0.7 mm. Invention Example No. 1
-4, 6-9, 11-14, 16, 17;
5, 10, 15).

The tissue stability was evaluated by observation of the tissue.
The maximum grain size (the average of the 10 largest crystal grains among the crystal grains having a thickness of 1 mm in the sectional structure) was measured. Regarding the formability, tensile properties were examined using JIS No. 5 tensile test pieces. The evaluation results are also shown in Table 1.

From Table 1, it can be seen that Example No. 1-4, 6-
9, 11, 14, 16, and 17 all have good texture stability and moldability.

On the other hand, in Comparative Example No. In Comparative Example No. 5, the B / N ratio was lower than the range of the present invention. In Comparative Example No. 10, the amount of Al exceeded the range of the present invention. Sample No. 15 is inferior in microstructure stability as compared with the examples of the present invention because the annealing temperature raising rate is below the range of the present invention. As described above, according to the present invention, a steel sheet having a stable structure can be obtained even when high-temperature annealing at 780 ° C. or more is performed.

[0038]

[Table 1]

[0039]

[Table 2] (Example 2) Without cooling steel having the components shown in Table 2,
After the production as it is, hot rolling is performed at a temperature of Ar ₃ points or more,
It was wound at the winding temperature shown in Table 2. Subsequently, pickling and cold rolling were performed, continuous annealing was performed under the annealing conditions shown in Table 2, and then temper rolling was performed at a pressure control rate of 0.8% to obtain a sheet thickness of 1.6 mm.
(Examples of the present invention Nos. 1-4, 6-9, 11)
To 14, 16, 17 and Comparative Example Nos. 5, 10, 15).

The tissue stability was evaluated by observation of the tissue.
The maximum grain size (the average of the 10 largest crystal grains among the crystal grains having a thickness of 1 mm in the sectional structure) was measured. Regarding the formability, tensile properties were examined using JIS No. 5 tensile test pieces. The evaluation results are also shown in Table 2.

From Table 2, it can be seen that Example No. 1-4, 6-
9, 11, 14, 16, and 17 all have good texture stability and moldability.

On the other hand, in Comparative Example No. 5 is B content and B / N
The ratio is beyond the range of the present invention, and the moldability is inferior to the examples of the present invention. Comparative Example No. In Comparative Example No. 10, the amount of Al exceeded the range of the present invention. Sample No. 15 is inferior in microstructure stability as compared with the examples of the present invention because the annealing temperature raising rate is below the range of the present invention. As described above, according to the present invention, a steel sheet having a stable structure can be obtained even when high-temperature annealing at 780 ° C. or more is performed.

[0043]

[Table 3]

[0044]

[Table 4]

[0045]

According to the present invention, by specifying the steel composition and the production conditions, it is possible to provide a B-added soft cold-rolled steel sheet having excellent structural stability in high-temperature continuous annealing and a production method thereof.

[Brief description of the drawings]

FIG. 1 is a micrograph showing a cross-sectional structure of a B-added steel in which coarse ferrite grains are partially generated by high-temperature annealing.

FIG. 2 is a diagram showing a relationship between a B / N ratio, EL, and maximum particle size according to the embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the amount of Al, the EL, and the maximum particle size according to the embodiment of the present invention.

Claims (3)

[Claims] C: 0.01 to 0.03% by weight
, Si ≦ 0.1%, Mn ≦ 0.5%, and P ≦ 0.0
3%, S ≦ 0.03%, N ≦ 0.0035%, B
≦ 0.0035% and B / N in stoichiometric ratio
= 0.6-1.5 and Al ≦ 0.035 × √ (B / N ×
0.6), wherein the balance is Fe and inevitable impurities, and the soft cold-rolled steel sheet has excellent structural stability. 2. The steel component further includes Cu ≦
0.5%, Ni ≦ 0.5%, Cr ≦ 0.5%, Sn ≦
It is characterized by containing one or more selected from the group of 0.5%, Ca ≦ 0.1%, and O ≦ 0.05% in a total range of 2% or less. 2. A soft cold-rolled steel sheet having excellent structural stability according to 1. 3. The method for producing a steel sheet having the composition according to claim 1 or 2, wherein the slab obtained by continuous casting is finish-rolled in a temperature range of _three or more points of Ar and wound at a temperature of less than 650 ° C. And cold annealing the rolled hot-rolled steel sheet, and continuously annealing at a heating rate of 10 ° C./second or more and a soaking temperature of 780 ° C. or more. Manufacturing method of excellent soft cold rolled steel sheet.