JP3339341B2 - Manufacturing method of soft cold rolled steel sheet with small coil end property - Google Patents

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 which is particularly used for automobiles and home electric appliances, is suitable for processing such as press forming, and has a small coil end property.

[0002]

2. Description of the Related Art High formability is required for steel sheets used for automobiles and home electric appliances, and softness and high r-value are being vigorously promoted. When producing such a steel sheet by continuous annealing, it is necessary to fix C and N, which cause a decrease in the r value, as precipitates by hot rolling of hot rolling. However, when high-temperature winding is performed, since the cooling speed of the coil tip wound around the mandrel at room temperature and the rear end of the coil that is in direct contact with air is fast (coil end property), the tip of the coil with respect to the center, Material deterioration of the rear end becomes severe. Furthermore, when the scale is formed to be thick, the pickling property also decreases. In response to this, measures such as cutting both ends in the coil longitudinal direction have been taken. However, this causes a sharp decrease in yield and significantly impairs productivity, and various methods have been proposed to secure the material at both ends in the coil longitudinal direction. Conventionally, the cause of the coil end property is considered to be AlN. For controlling the form of AlN, for example, Japanese Patent Publication No. Sho 63-72829 discloses a coil end property that presupposes high-temperature winding. A reduction method is disclosed, and it is shown that the fine dispersion of AlN is suppressed by reducing the heating temperature of the hot-rolled slab and winding it at a high temperature. However, in this method, in order to coarsely precipitate AlN and obtain a good r-value, it is necessary to wind an ultra-high temperature of 700 ° C. to 800 ° C., and inevitably there is a large difference between the cooling rates of both ends of the coil and the middle part. In fact, it is in fact that the improvement is not sufficiently made and the pickling property is remarkably deteriorated. Further, Japanese Patent Publication No. 55-36051 discloses a method of winding both ends at a high temperature by winding both ends of the strip without water injection, and Japanese Patent Application Laid-Open No. 5-43946 discloses a method of winding at both ends. A method of controlling the cooling rate of the entire coil by raising the temperature and winding tightly is disclosed. However, in these methods, it is difficult to control the coiling temperature, and since the coiling temperature is increased at both ends of the coil where oxygen can be easily supplied, the pickling property of the coil is greatly deteriorated. There is. Further, an IF steel in which carbon and nitride forming elements such as Ti are added to a steel in which C and N in the steel are extremely reduced, and an IF steel in which no solid solution C and N is present has been used. Steelmaking costs required to reduce
It will be expensive. On the other hand, if low-temperature winding is performed, the coil end property can be reduced to some extent, but there is a problem that the material is deteriorated. Therefore, as a method of obtaining a high r value by increasing the grain growth property on the premise of low-temperature winding, Japanese Patent Application Laid-Open No. 51-1385 discloses a method.
No. 16 discloses a method of heating a slab of B-added low carbon steel at a low temperature and winding the slab at 600 to 700 ° C. to convert most of the nitride to BN, thereby improving the grain growth and improving the material quality. It has been disclosed. However, even in this method,
Although the material at the center of the coil is improved, the material at the coil end is about the same as the conventional one, and at present, the coil end property is large.

[0003]

As described above, various inventions have been made to improve the coil end property. However, in any of the inventions, the coil end property is reduced while maintaining the material so as not to cause a problem. Less than.

An object of the present invention is to provide a method of manufacturing a soft cold-rolled steel sheet having a small coil end property, which fundamentally solves the problem of the coil end property and does not impose a load on a subsequent step such as pickling. It is in.

[0005]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) The method for producing a cold-rolled steel sheet according to the present invention is as follows.
0.03%, Mn ≦ 0.5%, Si ≦ 0.1%,
P ≦ 0.025%, S ≦ 0.03%, Sol. Al
≦ 0.1%, N ≦ 0.0035%, and B ≦ 0.003
%, And the atomic ratio of B / N = 0.6 to 1.5 is satisfied, and the balance is Fe and unavoidable impurities.
A step of roughly rolling to a rough bar, a step of heating a rough bar of 950 ° C. or less to 980 ° C. or more, performing a finish rolling at _three or more points of Ar, and winding the same at 650 ° C. or less, And cold rolling and annealing the steel sheet.

(2) The method for producing a cold-rolled steel sheet according to the present invention is the same as the method for producing a steel sheet having the composition described in (1) above, except that the continuously cast steel is inserted into a heating furnace at a temperature of 750 ° C. or more and 1150. C. or less, and rough rolling to a coarse bar, and heating a coarse bar of 950 ° C. or less to 980 ° C. or more.
r Rolling at a temperature not lower than ₃ points and winding at 650 ° C. or lower, and cold rolling and annealing the rolled hot-rolled steel sheet; This is a method for producing a cold-rolled steel sheet.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The inventor of the present invention aims to fundamentally solve the problem of the coil end property and to obtain a soft cold-rolled steel sheet having a small coil end property without imposing a load on a subsequent step such as pickling. In addition, we investigated the cause of coil end properties when conventional B-added aluminum-killed steel was heated at low temperature, and conducted intensive research on a manufacturing method that reduces fine precipitates that hinder grain growth at both ends of the coil during finish rolling. Was piled up.

As a result, the present inventors have discovered the following grain growth inhibitory factors at the coil end. First,
Even if it is heated at a low temperature, a certain amount of MnS forms a solid solution. Second, since the amount of solid solution MnS is smaller than that when heated at a high temperature, the precipitation driving force is small, precipitation starts at the time of rough rolling at the time of hot rolling, and precipitation is continued finely until finish rolling. thing. Third, BN precipitates at the time of finish rolling. Part of the BN precipitates at the core of MnS already precipitated at the time of rough rolling, but at the start of finish rolling, the amount of fine MnS is too small and BN alone precipitates finely. That there is. Fourth, MnS precipitated at the time of finish rolling does not become BN precipitation nuclei. Since MnS finely precipitated during this finish rolling and BN finely precipitated alone suppress grain growth, grain growth does not occur sufficiently at both ends of the coil, and coil end properties occur. Therefore, the inventors have conducted intensive studies and found that once the coarse bar after the rough rolling is once reduced to 950 ° C. or less, the fine bar can be completely precipitated before the finish rolling by heating to 980 ° C. or more. Accordingly, fine MnS precipitated at the time of finish rolling is eliminated, and all fine MnS become nuclei of BN to become coarse composite precipitates and can be rendered harmless. That good grain growth was ensured over a long period of time. The experimental results on which this is based are shown below.

By weight%, C: about 0.02%, Si: about 0.02%, Mn: about 0.16%, P: about 0.012
%, S: about 0.008%, Sol. Al: about 0.015
%, N: about 0.0017%, B: about 0.0012%, melted steel, heating temperature was 1100 ° C, and after rough rolling, when the rough bar reached 900 ° C, coarse bar heating was performed by induction heating. The hot rolling was performed by changing the rough bar heating conditions under the basic conditions of a finishing temperature of 880 ° C. and a winding temperature of 600 ° C. After pickling the obtained hot rolled sheet, cold rolling was performed at a cold pressure ratio of 75%, and annealing was performed at 750 ° C. Further pressure regulation ratio 0.8
%, And the r-value of 3 m of the coil tip (T section), the center (M section), and the rear end (B section) 3 m was measured. FIG. 1 shows a plot of the difference between the lower r value at the both ends of the coil and the r value at the center with respect to the rough bar heating temperature. From FIG. 1, it is understood that heating the rough bar to 980 ° C. or more after the rough rolling significantly reduces the coil end property.
Although the reason for this phenomenon has not yet been completely elucidated, MnS can be obtained by cooling the coarse bar to 950 ° C. or lower.
It is considered that MnS is completely precipitated after heating the coarse bar because heating is performed in a state where the precipitation driving force is increased. Here, since the temperature is low even when the temperature is maintained at 950 ° C. or lower, precipitation of fine MnS hardly occurs unless there is a large distortion due to finish rolling. BN precipitates in the nucleus fine MnS completely precipitated after heating the coarse bar during finish rolling,
Does not adversely affect the grain growth because it becomes a coarse composite precipitate, and there is no single fine precipitation of BN. The good grain growth obtained in this way is inherited even after cold rolling and annealing, so that the cold rolled sheet also has good grain growth. Therefore, even in the low-temperature winding in the present invention, it is possible to secure the same material as the conventional one.

It has also been found that by controlling the atomic ratio of B / N to a certain range, it is possible to suppress the hardening of steel due to excessive addition of B and to secure a material equivalent to that of the prior art. When the B / N ratio is 1 or less, AlN precipitates, but since the amount is small, there is no adverse effect on the grain growth.

Further, the slab temperature after continuous casting is 750.
When inserted into a heating furnace at a temperature of not less than ℃, the slab is kept at a high temperature without precipitation of AlN, that is, in a state where there is a sufficient amount of solute N. It can realize a good grain growth. BN in slab
The same effect can be expected by increasing the slab heating time, but there is a problem that if the slab heating time is increased, the surface quality is reduced due to grain boundary oxidation of the slab surface and energy cost is required. Thus, if the slab temperature after continuous casting is inserted into the heating furnace at a temperature of 750 ° C. or higher, the effect of the present invention can be further increased.

[0012] Based on the above findings, the present inventor:
The B / N atomic ratio of the B-added low carbon steel is controlled within a certain range,
In order to reduce fine precipitates that hinder grain growth at both ends of the coil during finish rolling, the steel (slab) heating temperature,
By controlling the heating temperature, the finish rolling temperature, and the winding temperature of the rough bar after the rough rolling, a method for producing a soft cold-rolled steel sheet having a small coil end property of the present invention was found, and the present invention was completed.

That is, according to the present invention, the steel composition and the production conditions are limited to the following ranges, thereby fundamentally solving the problem of the coil end property, and also providing no load to the next step such as pickling. A soft cold-rolled steel sheet having a small end property can be obtained. 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 ≦ 0.03% If C is too large, a large amount of carbides precipitates out, lowering the r value and elongation (EL) and impairing the formability.
03% or less.

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. The upper limit is 0.5% in order to bring about hardening and to deteriorate the formability.

S ≦ 0.03% Since S is an element that inhibits hot ductility and moldability, M
Fixed as nS. Therefore, lower is desirable.
Further, if the amount of MnS is too large, the EL is lowered, so the upper limit is 0.03%.

Si ≦ 0.1% Since excessive addition of Si increases the strength and deteriorates the formability, the content of Si is 0.1% or less.

P ≦ 0.025% P is a solid solution strengthening element, and the upper limit is 0.025% since excessive addition causes hardening of steel.

Sol. Al ≦ 0.1% Since Al is used as a deoxidizing agent, Al is included to some extent. In the present invention, a considerable amount of N is fixed as BN by the addition of B, so the amount of AlN precipitated is small, but if added excessively, AlN precipitates during annealing after cold rolling.
Is excessively finely precipitated and hinders grain growth, so the upper limit is 0.1%.

N ≦ 0.0035% If N is finely precipitated as AlN, it causes an increase in the coil end property. In the present invention, it is fixed as BN. However, if the amount of BN is large, the workability is reduced. Therefore, the upper limit is 0.0035%.

B ≦ 0.003% B is an element that plays an important role in the present invention. B
If MnS or AlN is not added, MnS or AlN will be dissolved to some extent even at low temperature heating, and fine MnS or AlN
N precipitates. B precipitates as BN with fine MnS as a nucleus, becomes a coarse composite precipitate, and fixes N to suppress the precipitation of fine AlN. As a result, in the present invention, unprecedented grain growth is realized. But BN
Is excessively present, the workability is reduced. Therefore, the upper limit of the addition amount is 0.003%.

B / N (atomic ratio) = 0.6 to 1.5 When B is excessively added to N, the amount of solid solution B increases and the steel becomes harder. Is 0.6 ~
1.5. At other atomic ratios, good workability (r
Value) cannot be obtained. This has been clarified by the following experiments of the present invention.

By weight%, C: about 0.02%, Si: about 0.01%, Mn: about 0.15%, P: about 0.01%,
S: about 0.008%, Sol. Al: about 0.015%,
N: about 0.002%, hot rolling was performed using a material in which the addition amount of B was variously changed. Slab heating temperature 112
After the rough rolling, the coarse bar at 940 ° C. was heated to 1050 ° C., and the winding temperature was 600 ° C. The obtained hot rolled sheet was pickled, cold rolled at a cold pressure ratio of 80%, and then continuously annealed at 720 ° C. The r value of the obtained material was measured. Sampling positions are as follows: 3 m at the T section (the leading end, based on the rolling direction during hot rolling), M section (the center in the longitudinal direction of the coil), and B section (based on the rolling direction at the time of hot rolling). (End) 3 m. The results are shown in FIG. The measured value is shown for the M part, and the difference from the M part is shown for the T and B parts. When the B / N ratio was 0.6 or more, the difference between the M part and the T and B parts became 0.2 or less. From this, it is understood that the effect of the present invention is exhibited when the B / N ratio is 0.6 or more, and the coil end property is reduced, and that when the B / N ratio exceeds 1.5, the material is significantly deteriorated due to excess B. Further, B / N is 0.8 to 1.2.
This range is preferable because the effect is large.

The effect of the present invention is not lost even if Cu, Ni, Cr, Sn, Mo, Pb or the like is added to the steel to be used in the present invention for various purposes. Where T
When elements that form fine nitrides such as i, V, Nb, and Zr are added, these fine precipitates impair the grain growth. Therefore, the addition or mixing of these elements is 0.01% or less. It is desirable.

By adjusting to the above component range, the problem of the coil end property is fundamentally solved, and a soft cold-rolled steel sheet with a small coil end property is obtained without applying a load to the next step such as pickling. It becomes possible.

The steel sheet having such characteristics can be manufactured by the following manufacturing method. (2) Steel plate manufacturing process (2-1) Manufacturing conditions of mode 1 (Manufacturing method) After the steel adjusted to the above-described composition range is melted in a converter, it is slab-formed by continuous casting, and 1150 ° C.
Heated below, rough rolled to rough bar, then 950 ° C
The following coarse bar is heated to 980 ° C. or more, finish-rolled at _three or more Ar points, and wound at 650 ° C. or less. Next, the rolled hot-rolled steel sheet is cold-rolled and annealed. a. Slab heating temperature The slab is heated to 1150 ° C. or lower and roughly rolled to form a rough bar. In the present invention, the heating temperature of the slab plays an important role. In the present invention, MnS is coarsened to eliminate the coil end property. Therefore, for MnS coarsely precipitated in the slab, low-temperature heating is intended to keep the coarseness as much as possible. If the heating temperature is too high, a large amount of MnS forms a solid solution at the time of heating and fine MnS precipitates excessively, so that the effect of coarsening by BN is reduced. Therefore, the heating temperature is 1150 ° C. or lower. Also,
If the rolling load is too low, the rolling load becomes excessive.
C. or higher is preferred.

B. Coarse Bar Heating Temperature The coarsely rolled coarse bar is heated to 980 ° C. or higher after the temperature of the coarse bar is reduced to 950 ° C. or lower. Coarse bar heating is the core of the present invention, such as promoting the precipitation of MnS and eliminating the temperature difference and skid marks at both ends of the coarse bar. In order to finely precipitate MnS, the upper limit of the temperature before heating the coarse bar is 950 ° C. or less.
As for the temperature before heating the coarse bar, A
When 1N is precipitated, the temperature is preferably 750 ° C. or more because recrystallization is delayed and the crystal grains become fine by becoming recrystallization nuclei. More preferably, when the temperature before heating the coarse bar is lower than the Ar ₃ point, the γ particle size after the heating of the coarse bar becomes slightly smaller due to the γ / α transformation than when the temperature is not cooled below Ar ₃ , resulting in hot rolling. The temperature before heating the coarse bar is preferably Ar ₃ or more because the plate grain size becomes slightly smaller.

If the heating temperature of the coarse bar is lower than 980 ° C., fine MnS cannot be completely precipitated before the finish rolling, and the effect of the present invention cannot be obtained. Therefore, the lower limit of the heating temperature is 980 ° C. or higher. On the other hand, if the coarse bar heating temperature is high, MnS precipitated finely dissolves again,
Since the effect of the present invention is lost, the heating temperature of the rough bar is desirably equal to or lower than the slab heating temperature, and more desirably 1050 ° C. or lower.

The heating method is not particularly limited.
Induction heating, electric resistance heating, or the like that can rapidly and uniformly heat while maintaining the nS precipitation driving force is desirable. Also, if the coarse bar before the coarse bar heating is wound once into a coil shape and then rewound and then coarse bar heating is performed, the driving force for precipitation of MnS is improved. good.

C. Finish Rolling Temperature In the present invention, the finish rolling temperature is at least Ar ₃ points. When the finish rolling temperature is less than ₃ points Ar, since the texture to reduce the r value will developed, the lower limit is at least ₃ points Ar.

D. Winding temperature The hot-rolled steel sheet subjected to finish rolling is wound at 650 ° C. or lower. The winding temperature is important in the present invention. In order to eliminate the coil end property, it is necessary to make the difference in cooling rate between the center and both ends of the coil as small as possible. Therefore, the upper limit of the winding temperature is 650 ° C. However, if the temperature is lower than 300 ° C., the grains do not grow sufficiently after winding, and the softening effect is saturated.

(2-2) Manufacturing conditions of embodiment 2 (Manufacturing method) After smelting the steel adjusted to the above-mentioned component composition range in the converter, it is made into a slab by continuous casting, and the slab having a temperature of 750 ° C. or more is heated in a heating furnace. And heated to 1150 ° C or lower, and rough-rolled to form a coarse bar. Thereafter, the coarse bar at 950 ° C or lower is heated to 980 ° C or higher, finish-rolled at _three or more points of Ar, and wound at 650 ° C or lower. . Next, the rolled hot-rolled steel sheet is cold-rolled and annealed. a. Slab heating furnace insertion temperature When the slab temperature after continuous casting is 750 ° C or higher, the slab is inserted into the heating furnace.

After the continuous casting, when the slab temperature becomes lower than 750 ° C., AlN precipitates in the slab. When AlN precipitates, N in the slab is consumed and coarsening of BN is not promoted. Therefore, in order to suppress the precipitation of AlN and promote the coarse precipitation of BN, the lower limit of the heating furnace insertion temperature of the slab after continuous casting. Is 750 ° C. or higher at which AlN starts to precipitate.
Although not particularly specified, coarse cooling of MnS and BN is delayed unless cooled to 1000 ° C. or lower. Therefore, it is preferable to insert the heating furnace at 1000 ° C. or lower. Thus, if the slab temperature after continuous casting is inserted into the heating furnace at a temperature of 750 ° C. or higher, the effect of the present invention can be further increased.

B. Slab heating temperature Same as the manufacturing condition of the first embodiment. c. Coarse bar heating temperature Same as the manufacturing condition of the first embodiment.

D. Finishing rolling temperature Same as the manufacturing conditions of aspect 1. e. Winding temperature Same as the manufacturing conditions of aspect 1.

As described above, in the present invention, the temperatures in the respective steps of Embodiments 1 and 2 have an important meaning, and if any one of them is applied, the effect of the present invention cannot be obtained. The cold rolling after pickling is preferably performed at a rolling ratio of 30 to 90% from the viewpoint of workability, particularly deep drawing. Annealing is also performed at 600 ° C. or more for softening and 900 ° C. or less for suppressing coarse grains. The annealing method is continuous annealing. However, there is no problem even if the box annealing is performed. There are no restrictions on the conditions of the temper rolling, but if it is too high, the EL will be drastically reduced. For adjusting the composition of the steel of the present invention, both a converter and an electric furnace can be used. Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention.

[0037]

【Example】

(Example 1) Steels having the components shown in Table 1 were melted and manufactured under the manufacturing conditions (Examples of the present invention: Nos. 1 to 19, Comparative Examples: No. 1) shown in the table.
20 to 25). The heating of the coarse bar is performed by induction heating, and the time required for raising the temperature is within 5 seconds. Further, the obtained hot-rolled sheet was cold-rolled at a cold-pressure ratio of 75%, and annealed at 750 ° C. by a continuous annealing heat cycle. The plate thickness is 0.75 mm. A JIS No. 5 tensile test piece was sampled from the front end portion (T portion) 3 m, the center portion (M portion), and the rear end portion (B portion) 3 m of the manufactured coil at the time of hot rolling and the tensile strength (TS ), Elongation (EL)
And the average r value were measured.

The average r value was determined by taking a JIS No. 5 tensile test specimen in the rolling direction, the direction perpendicular to the rolling direction, and the 45 ° direction of the rolling of the steel sheet, and applying 15% strain by tension.
From the values, r ₀ , r ₉₀ , and r ₄₅ , mean-r = (r ₀ + 2r ₄₅ +
r ₉₀ ) / 4.

Table 2 shows the above measurement results. The measured value is shown for the M part, and the difference from the M part is shown for the T and B parts. As for EL, the present invention example No. In Nos. 1 to 19, the M part is 40% or more, and the difference between the M part and the T and B parts is 3% or less, and the r value of the M part is 1.4 or more and the M part is And the difference between the T and B parts was 0.2 or less.

On the other hand, in Comparative Example No. In the case of 20 to 25, any one of the heating temperature, the rough bar heating temperature, the winding temperature and the B amount was applied, and no reduction in the coil end property of at least one of EL and r value was observed. Or the value is low. Comparative Example No. In the case of Example No. 24 of the present invention, B was excessively added, and although the coil end property was reduced, the values of EL and r were low. 1
It can be seen that it is inferior to # 19. From the above, the materials of the T and B portions of the cold-rolled steel sheet manufactured according to the present invention are equivalent to the M portion.

[0041]

[Table 1]

[0042]

[Table 2]

(Example 2) Steels having the components shown in Table 3 were melted and inserted into a heating furnace at the temperature shown in the table, and the production conditions shown in the table (Examples of the present invention: Nos. 1 to 19, comparative examples) : No. 20
２５25). Further, the obtained hot-rolled sheet was cold-rolled at a cold pressure rate of 60%, and annealed at 680 ° C. by a continuous annealing heat cycle. The plate thickness was 1 mm. TS and EL at the longitudinal end (T section) 3 m, the center (M section) and the rear end (B section) 3 m of the manufactured coil at the time of hot rolling.
And the average r value were measured in the same manner as in Example 1.

Table 4 summarizes the measurement results. For the M part, the measured values of the TS, EL, and r values are shown, and for T and B, the difference between the EL and the M part in the r value is shown. As for EL, the present invention example No. In 1 to 19, M part is 40%
The difference between the M part and the T and B parts is 2% or less, and the r value of the M part is 1.4 or more and the difference between the M part and the T and B parts is 0.1 or less. Became.

On the other hand, Comparative Example No. In the case of 20 to 25, any one of the heating temperature, the rough bar heating temperature, the winding temperature, and the B amount is applied.
No reduction or low value of the coil end property of at least one of the values is observed. Comparative Example No. In the case of Example No. 24 of the present invention, B was excessively added, and although the coil end property was reduced, the values of EL and r were low. 1 to
It turns out that it is inferior to 19. From the above, the materials of the T and B portions of the cold-rolled steel sheet manufactured according to the present invention are equivalent to the M portion.

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

According to the present invention, by specifying the steel composition and the manufacturing conditions, the material in the longitudinal direction is uniform without cutting the both ends in the longitudinal direction of the coil, and the soft chill with excellent workability is obtained. It is possible to manufacture a rolled steel sheet at low cost.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a rough bar heating temperature and M according to an embodiment of the present invention.
The figure which shows the relationship between a part and the difference of r value of T, B part.

FIG. 2 shows the B / N ratio and r of the M part according to the embodiment of the present invention.
The figure which shows the relationship between a value and the difference of r value of M part and T, B part.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Sawada 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Yasuhide Ishiguro 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Naoshi Murayama 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/10 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. The method according to claim 1, wherein C ≦ 0.03% and Mn ≦
0.5%, Si ≦ 0.1%, P ≦ 0.025%,
S ≦ 0.03%, Sol. Al ≦ 0.1%, N ≦
0.0035% and B ≦ 0.003%, and the atomic ratio of B / N = 0.6 to 1.5 is satisfied, and the balance is Fe
And a process for preparing steel consisting of unavoidable impurities, the steel was heated to 1150 ° C. or less, a step of the rough rolling to a crude bar, heated 950 ° C. or less coarse bar above 980 ° C., Ar ₃
Soft rolling with a small coil end property, comprising: a step of performing finish rolling at a temperature of not less than 650 ° C. and winding at 650 ° C. or less; and a step of cold rolling and annealing the rolled hot rolled steel sheet. Steel plate manufacturing method. 2. The method for producing a steel sheet having the composition according to claim 1, wherein the continuously cast steel is inserted into a heating furnace at 750 ° C. or higher.
A step of heating to 50 ° C or lower and rough rolling to form a rough bar; a step of heating a coarse bar of 950 ° C or lower to 980 ° C or higher, performing finish rolling at _three or more Ar points, and winding at 650 ° C or lower; Cold rolling and annealing the rolled hot-rolled steel sheet; and a method for producing a soft cold-rolled steel sheet having low coil end properties.