JPH08311540A - Production of thin hot rolled steel sheet for working small in anisotropy - Google Patents

Abstract

PURPOSE: To provide a method for producing a thin hot rolled steel sheet for working small in anisotropy. CONSTITUTION: At the time of subjecting a steel to hot rolling, after rough rolling, bending is executed in such a manner that the radius of curvature is regulated to <=1.5m, the steel sheet is coiled at <=1000 to >=850 deg.C into a coil shape, the holding time t (sec) satisfies the relationship of the same temp. T (K) and t>=0.08exp (6000/T), also, the upper limit is regulated to <=30min, and thereafter, recoiling is executed. After that, in finish rolling, hot rolling is executed so as to regulate the total draft in the temp. range of the Ar3 transformation point or below to >=750 deg.C to 75 to 98%, and treatment of reducing solid solution N is executed to produce the thin hot rolled steel sheet for working small in anisotropy. The compsn. of the steel is the one contg., by weight, 0.01 to 0.06% C, <=0.01% N, <=1% Si and 0.005% to 1% Al.

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 thin soft hot-rolled steel sheet having small anisotropy.

[0002]

2. Description of the Related Art As a manufacturing condition that the workability of a steel sheet is significantly deteriorated, it is known that finish rolling is performed at an Ar ₃ transformation point or lower during hot rolling. However, when a thin hot-rolled sheet is produced, the sheet temperature rapidly decreases during rolling, and therefore a large amount of austenite former elements such as C and Mn that lower the equilibrium transformation temperature Ae ₃ cannot be added from the viewpoint of workability. The working steel sheet may be partially rolled at a temperature not higher than the Ar ₃ transformation point.

Although Ae ₃ is lowered, Cr is used as an additive element that does not increase the strength and can secure workability, and a technique capable of producing a thin hot-rolled steel sheet having a uniform structure without rolling in the two-phase region even at a relatively low temperature is special. It is disclosed in Kaihei 2-104637. However, in this technique, since Cr addition is indispensable, deterioration of pickling properties and chemical conversion treatability, which are harmful effects associated with Cr addition, cannot be avoided, and a relatively large amount of Cr is added to lower the equilibrium transformation temperature Ae _3. It has to be added, which has the drawback of increasing alloy cost.

If anything, Cr is an element that lowers the equilibrium transformation temperature Ae ₃ itself and also lowers the Ar ₃ transformation point.
On the other hand, the equilibrium transformation temperature Ae ₃ itself does not change so much, but there is an element that delays the transformation kinetically and lowers the Ar ₃ transformation point. The representative element is B. Regarding the manufacturing method of the hot-rolled steel sheet for processing containing B, Japanese Patent Application No. 6-47309.
No. 6 / 143,224, Japanese Patent Laid-Open No. 63-1
It is disclosed in Japanese Patent No. 43225, Japanese Patent Laid-Open No. 63-216925 and the like.

The effect of B is generally explained to be due to the delay of the nucleation of ferrite due to the segregation at the austenite grain boundaries. However, unless special limitation is applied during hot rolling, even if B is added, a mixed grain structure is partially generated at a finishing temperature of 850 ° C. or less, and nonuniform mechanical properties appear.

Japanese Patent Application No. 6-47309 shows that a soft hot-rolled steel sheet can be produced even at a finishing temperature of 785 ° C. by limiting the cooling rate after hot rolling. However, when the rolling temperature is low, the rolling force is reduced. In many cases, a predetermined cooling rate cannot be obtained unless the inter-stand cooling is performed due to the rise in temperature and processing heat generation. In that case, it is difficult to avoid local unevenness of the material due to uneven cooling.

On the other hand, recently, a number of techniques for manufacturing thin hot-rolled steel sheets have been disclosed which do not cause deterioration in workability even when finish rolling at a temperature below the Ar ₃ transformation point during hot rolling (for example, JP-A-54).
-109022, Japanese Patent Publication No. 01-038855, etc.). However, the hot-rolled steel sheet manufactured in the ferrite region has a large anisotropy.

Therefore, technological development has been carried out to achieve reduction of anisotropy in hot rolling in the ferrite region, and technology for reducing anisotropy by increasing the strain rate has been developed (Japanese Patent Publication No. 2-57129). Publication, Japanese Patent Publication No. 2-57130
Japanese Patent Publication No. 2-59846, Japanese Patent Publication No. 3-33
767, JP-A-63-60231).

However, when hot-rolling a hot strip, normally, the strip tip greatly bounces until the strip tip is caught in the winding device and the strip passing property is poor, so the strip passing speed is limited. Must. Therefore, the strain rate during that time is lower than 300 (1 / sec), and the anisotropy cannot be improved. Japanese Unexamined Patent Publication No. 63-60231 discloses that the sheet bar is held for a certain period of time after rough rolling to perform precipitation treatment, which is advantageous for deep drawability of the hot rolled steel sheet. No mention is made, and there is no disclosure of a technique for reducing anisotropy.

[0010]

DISCLOSURE OF THE INVENTION The present invention provides a method for producing a thin soft hot-rolled steel sheet having a small anisotropy without being limited by the strain rate during hot rolling.

[0011]

Means for Solving the Problems The present inventors have studied in detail the relationship between the workability of a thin soft hot-rolled steel sheet and the hot-rolling conditions and obtained the following results. 1) The material which is finish-rolled at the Ar ₃ transformation point or higher has excellent workability and relatively small anisotropy. However, when the plate thickness becomes thin, a part is rolled at a temperature not higher than the Ar ₃ transformation point, resulting in significant variation in characteristics.

2) Even in the material hot-rolled below the Ar ₃ transformation point, the workability is improved when the total reduction ratio is increased. However, as the total rolling reduction increases, the anisotropy is slightly improved, but it is larger than that of the material finish-rolled at the Ar ₃ transformation point or higher.

3) The material having a reduced strip-passing speed in finish rolling showed a small anisotropy. As a result of considering the cause, the texture is randomized by causing recrystallization between passes,
It was speculated that the anisotropy became smaller. However, lowering the strip running speed means lower productivity, so alternative methods were investigated.

4) After the rough rolling, it was revealed that the anisotropy is reduced by performing bending with a certain curvature or more and holding it for a short time. The mechanism of improving the anisotropy by performing such thermomechanical treatment before finish rolling is not clear, but the rapid diffusion phenomenon utilizing the dislocations introduced by the bending process promotes coarsening of precipitates, for example. By doing so, it is presumed that recrystallization easily occurs between the passes of finish rolling.

Further, the bending process facilitates the recrystallization and grain growth of grains in a specific orientation, and as a result, the formation of a texture that weakens the anisotropy can reduce the anisotropy of the hot-rolled sheet. Sexuality is also considered. In particular, as a phenomenon that supports the latter reasoning, it is pointed out that the improvement of anisotropy due to the thermomechanical treatment does not clearly appear in the ultra low carbon steel whose grain growth behavior is significantly different from that of the low carbon steel.

Based on the above experimental results, a manufacturing technology of thin hot-rolled steel sheet for working having small anisotropy was established. The gist of the present invention is as follows. 1) C: 0.01% by weight or more and 0.06% or less by weight,
After rough rolling of steel containing N: 0.01% or less, Si: 1% or less, and Al: 5N% or more and 1% or less, the radius of curvature is 1.5 m.
The following bending process is performed, and the steel sheet is wound into a coil at a temperature T of 1000 ° C or lower and 850 ° C or higher, and a holding time t (second)
Is the temperature T (K) and t ≧ 0.08exp (6000 /
After satisfying the relationship of T) and setting the upper limit to 30 minutes or less,
Rewinding and then hot rolling in finish rolling so that the total rolling reduction in the temperature range of Ar ₃ transformation point or lower and 750 ° C. or higher is 75% or higher and 98% or lower, 600 ° C. or higher, 7% or higher.
A method for producing a thin hot-rolled steel sheet for working having small anisotropy, which comprises winding at a temperature of 50 ° C. or lower.

2) C: 0.01% or more by weight, 0.0
6% or less, N: 0.01% or less, Si: 1% or less, A
1: 0.005% or more, 1% or less, B: 11N / 14-
After rough rolling of steel containing so as to satisfy the condition of 0.001 ≦ B ≦ 11N / 14 + 0.002, bending is performed with a radius of curvature of 1.5 m or less, and a coil shape is formed at a temperature T of 1000 ° C. or less and 850 ° C. or more. The steel sheet is wound on the sheet, and the holding time t (sec) is the above temperature T (K) and t ≧ 0.08exp (6000 / T)
Satisfying the above relation and having an upper limit of 30 minutes or less, rewinding, and then finish rolling at 750 ° C. or higher, in which case A
Manufacture of thin hot-rolled steel sheet for working with small anisotropy, which is characterized by hot rolling such that the total rolling reduction in the temperature range of r ₃ transformation point or lower and 750 ° C. or higher is 75% or more and 98% or less. Method.

3) C: 0.01% or more by weight, 0.0
6% or less, N: 0.01% or less, Si: 1% or less, A
1: 0.005% or more, 1% or less, Ti: 48 N / 14
After rough rolling of steel containing so as to satisfy the condition of −0.005 ≦ Ti ≦ 48 N / 14 + 0.01, the radius of curvature is 1.5 m
The following bending process is performed, and the steel sheet is wound into a coil at a temperature T of 1000 ° C or lower and 850 ° C or higher, and a holding time t (second)
Is the temperature T (K) and t ≧ 0.08exp (6000 /
After satisfying the relationship of T) and setting the upper limit to 30 minutes or less,
Rewinding, and then finish rolling at 750 ° C. or higher, at which time hot rolling is performed so that the total reduction ratio in the temperature range of Ar ₃ transformation point or lower and 750 ° C. or higher becomes 75% or more and 98% or less. A method for producing a thin hot-rolled steel sheet having a small anisotropy for processing.

[0019]

The present invention will be described in detail below. In the present invention, the lower limit of the amount of C is 0.01% because
This is because the amount cannot reduce the anisotropy even if it is produced under the process conditions within the range of the present invention. This suggests that ultra low carbon steel behaves differently from low carbon steels in recrystallization and grain growth. Further, the upper limits of C and N are C: 0.06% or less and N:
0.01% or less means that C, N exceeds these amounts.
This is because the workability is deteriorated by adding.

Since Si and Al are elements that increase the transformation point, the addition of these elements is effective in increasing the temperature range of hot rolling in the ferrite region and promoting recrystallization during rolling and after rolling. . However, addition of a large amount not only has the risk of suppressing recrystallization, but also deteriorates the workability, so both upper limits were made 1%. Further, Al is a minimum amount of 0.1 that remains in the steel when deoxidation is sufficiently performed.
The lower limit was 005%.

In the case of precipitating N with Al to improve workability, the lower limit of the amount of Al added was set to 5 times the amount of nitrogen. A
Since the precipitation treatment of N with l is performed at the time of winding, it is necessary to wind in a temperature range in which Al can sufficiently diffuse, so the lower limit of the winding temperature was set to 600 ° C. or higher. Moreover, the upper limit was set to 750 ° C. in view of the pickling property.

The precipitation treatment of N is also possible with B and Ti. In this case, the lower limit of the amount of addition of B and Ti is 11N / 14-0.001≤ in order that almost no solid solution nitrogen remains.
B or 48N / 14-0.005 ≦ Ti. In addition, the upper limit is B ≦ so that workability does not deteriorate.
11N / 14-0.002 or Ti ≦ 48N / 14
Limited to +0.01.

Since the precipitation treatment of N with B or Ti occurs during heating and hot rolling, it is not necessary to limit the winding temperature. Further, in the present invention, as other components of steel, components normally contained in hot-rolled steel sheets for working, that is, Mn ≦ 1 wt%,
You may contain P <= 0.1 wt% and S <= 0.02 wt%.

After the rough rolling, bending with a radius of curvature of 1.5 m or less is performed, and the steel sheet is wound into a coil at a temperature of 1000 ° C. or less and 850 ° C. or more, and at least t ≧ 0.08exp
It is the most important point of the present invention to maintain the relationship of (6000 / T) for a time longer than that. The reason why the bending radius of curvature after rough rolling is limited to 1.5 m or less is that the anisotropy is not improved even if the bending radius of curvature exceeds this limit.

Further, the holding temperature is set to 1000 ° C. or less because if the holding temperature after rough rolling is more than 1000 ° C., it will take a long time for the temperature to drop if a sufficient finish rolling is performed below the Ar ₃ transformation point. This is because the productivity is low and the productivity is poor. For the same reason, the upper limit of holding time was set to 30 minutes.

The holding temperature is set to 850 ° C. or higher because the anisotropy cannot be sufficiently improved at a temperature lower than this temperature. For the same reason, the lower limit of the holding time is the holding temperature T (K) and the holding time t (second) is 0.08exp (6000 /
T) It is limited to the above. t = 0.08exp (60
The 00 / T) relational expression suggests that the same phenomenon occurs rapidly in a short time when the temperature rises, and the coefficient is often used when the phenomenon occurs during the thermal activation process.

Combined with the above-mentioned thermomechanical treatment after rough rolling,
There is a condition of finish hot rolling in which the anisotropy becomes small, and the anisotropy becomes small when the total rolling reduction in the temperature range of Ar ₃ transformation point or lower and 750 ° C. or higher is 75% or more and 98% or less. Further, if the total rolling reduction is less than 75%, the workability also deteriorates. In claims 2 and 3, which do not limit the winding temperature, the finishing temperature is 750.
When the temperature is lower than 0 ° C, the workability deteriorates and the anisotropy also increases.

[0028]

EXAMPLES Steels having the chemical compositions shown in Table 1 were used as examples. Ar ₃ is Ar ₃ (° C.) = 910-509.
C (wt%)-64Mn (wt%) + 23Si (wt%) + 1
It was calculated by the formula of 9 Al (wt%). Table 2 shows manufacturing conditions and respective characteristic values. Standard conditions are slab thickness: 250m
m, heating temperature HT: 1200 ° C., hot rolled finish plate thickness: 1.
It was set to 0 mm.

The anisotropy of the characteristics depends on the rolling direction and the rolling direction.
The r value and elongation in the direction of 5 degrees and the direction perpendicular to the rolling direction are
The value was expressed as an absolute value of Δr = (r ₀ + r ₉₀ −2r ₄₅ ) / 2, and the elongation was expressed as ΔE1 = maximum value−minimum value in each direction. In general, the anisotropy is an absolute value of Δr of 0.
A value of 4 or less and a ΔEl of 4% or less are judged to be good.

The bending process of this example was carried out by a method using a roll bender and a method in which a coarse bar was run and bumped against a wall having an inclination. In the latter case, the radius of curvature can be changed by changing the tilt angle. In the experiment for obtaining the present embodiment, the upper limit of the strain rate is set to 300 (1 / se
Less than c).

[0031]

[Table 1]

[0032]

[Table 2]

Experiment Nos. 1 and 2 satisfying the scope of the present invention,
The materials of 3, 5, 9, 10, 12, 14, 15, 20 are
Both the absolute value of Δr value and ΔEl are small, and excellent anisotropy resistance is exhibited. The material of Experiment No. 19 in which the winding temperature after bending of the rough bar is lower than the range of the present invention, and the material of Experiment No. 6 in which the winding time is short show a large Δr value and ΔEl. The material of Experiment No. 21, which had a large radius of curvature during bending, had large anisotropy.

The materials of Experiment Nos. 11, 13, and 17, which were subjected to finish rolling without being subjected to bending after rough rolling, had greater anisotropy than the materials of the present invention. Experiment number 1 using comparative steel
Nos. 6, 18, and 22 have not only poor ductility but also large anisotropy. Particularly, in Experiment No. 16, it is considered that TiC may have been finely precipitated during rolling, recrystallization of the hot-rolled sheet was significantly suppressed, and the workability was significantly deteriorated.

In Experiment No. 4 in which the coiling temperature of the aluminum-killed steel is low, the ductility is deteriorated due to the residual solid solution nitrogen. In this case, the anisotropy also deteriorated at the same time. The total rolling reduction is 70% in the temperature range of 750 ° C or higher and below the Ar ₃ transformation point.
In Experiment No. 7, which is low, the ductility and anisotropy are poor. Also,
The material of Experiment No. 8 using the extremely low carbon steel did not improve the anisotropy.

[0036]

Industrial Applicability According to the present invention, a steel sheet having a small anisotropy and excellent workability can be produced as hot-rolled, which is an industrially valuable invention.

Claims (3)

[Claims] 1. A steel containing C: 0.01% or more and 0.06% or less, N: 0.01% or less, Si: 1% or less, and Al: 5N% or more and 1% or less by weight ratio. After rolling, bending is performed with a radius of curvature of 1.5 m or less, and the steel sheet is wound into a coil at a temperature T of 1000 ° C. or less and 850 ° C. or more,
The holding time t (second) is equal to the temperature T (K) and t ≧ 0.08.
After satisfying the relationship of exp (6000 / T) and setting the upper limit to 30 minutes or less, rewinding, and then A in the finish rolling.
r ₃ below transformation point, 750 total rolling reduction at ℃ above temperature range more than 75%, hot rolled to be less than 98%, 600
A method for producing a thin hot-rolled steel sheet for working having small anisotropy, which comprises winding at a temperature of from ℃ to 750 ° C. 2. A weight ratio of C: 0.01% or more, 0.06% or less, N: 0.01% or less, Si: 1% or less, Al: 0.005% or more, 1% or less, B: 11N / 14-0.001 ≦ B ≦ 11N / 14 +
After rough rolling the steel containing so as to satisfy the condition of 0.002,
Bending with a radius of curvature of 1.5 m or less is performed at 1000 ° C
Hereinafter, the steel sheet is wound into a coil shape at a temperature T of 850 ° C. or higher, and the holding time t (seconds) is equal to the temperature T (K) and t ≧ 0.
Satisfies the relationship of 08exp (6000 / T) and has an upper limit of 3
After setting to 0 minutes or less, rewinding, and then finish rolling at 750 ° C. or higher, at which time the total rolling reduction in the temperature range of Ar ₃ transformation point or lower and 750 ° C. or higher becomes 75% or more and 98% or less. A method for producing a thin hot-rolled steel sheet for working having small anisotropy, which is characterized by hot rolling. 3. By weight ratio, C: 0.01% or more, 0.06% or less, N: 0.01% or less, Si: 1% or less, Al: 0.005% or more, 1% or less, Ti: 48N / 14-0.005 ≦ Ti ≦ 48N / 14
After rough rolling the steel containing so as to satisfy the condition of +0.01,
Bending with a radius of curvature of 1.5 m or less is performed at 1000 ° C
Hereinafter, the steel sheet is wound into a coil shape at a temperature T of 850 ° C. or higher, and the holding time t (seconds) is equal to the temperature T (K) and t ≧ 0.
Satisfies the relationship of 08exp (6000 / T) and has an upper limit of 3
After setting to 0 minutes or less, rewinding, and then finish rolling at 750 ° C. or higher, at which time the total rolling reduction in the temperature range of Ar ₃ transformation point or lower and 750 ° C. or higher becomes 75% or more and 98% or less. A method for producing a thin hot-rolled steel sheet for working having small anisotropy, which is characterized by hot rolling.