JP3339342B2 - Manufacturing method of soft hot rolled steel sheet with small coil end property - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention is particularly used for automobiles and home electric appliances, and is suitable for processing such as bending and press molding.
The present invention relates to a soft hot-rolled steel sheet having low coil end properties.

[0002]

2. Description of the Related Art High formability is required for steel sheets used in automobiles and home electric appliances, and softness and ductility have been vigorously promoted. In order to make a hot-rolled steel sheet soft and highly ductile,
It is essential that the precipitates be coarsened by high-temperature winding to improve the grain growth. However, when performing high-temperature winding, the cooling speed of the coil tip wound around the mandrel at room temperature and the rear end of the coil that comes into direct contact with the air is fast, so the material of the coil tip and rear ends relative to the center is Deterioration becomes severe (coil end property). 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. Japanese Patent Application Laid-Open No. 58-144417 discloses that the amount of Al and N is reduced to reduce the amount of fine AlN deposited, and A method for obtaining a soft hot-rolled steel sheet that is uniform in a coil is disclosed.In this method, the material is uniform in the width direction, but material variation between a coil front end, a rear end, and a center is avoided. It is not possible at present. Japanese Patent Application Laid-Open No. 58-207335 discloses a method in which B is added to fix N as BN to prevent precipitation of fine AlN and to reduce the winding temperature. However, even in this method, the material is uniform in the width direction, but at present, the material difference between both ends and the center of the coil cannot be said to be sufficient.

[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 for manufacturing a soft hot-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 hot-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 form a rough bar, a step of heating a rough bar of 950 ° C. or less to 980 ° C. or more, performing finish rolling at _three or more Ar points, and winding it at 650 ° C. or less. This is a method for producing a soft hot-rolled steel sheet having a small coil end property.

(2) The method for producing a hot-rolled steel sheet according to the present invention is the method for producing a steel sheet having the composition described in (1) above, wherein 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,
A method of producing a soft hot-rolled steel sheet having a small coil end property, comprising a step of performing finish rolling at _three or more points of Ar and winding at 650 ° C. or less.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor fundamentally solves the problem of the coil end property and obtains a soft hot 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 the coil end property that occurs when conventional B-added aluminum-killed steel was heated at low temperature, and conducted diligent research on a method of reducing fine precipitates that hinder grain growth at both ends of the coil during finish rolling. Stacked.

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.

[0009] The experimental results on which this is based are shown below.
By weight%, C: about 0.02%, Si: about 0.02%, M
n: about 0.16%, P: about 0.012%, S: about 0.0
08%, Sol. Al: about 0.015%, N: about 0.0
017%, B: steel containing about 0.0012% is melted, and at a heating temperature of 1100 ° C., when the temperature reaches 920 ° C. after the rough rolling, the coarse bar is heated to a predetermined temperature at about 20 ° C./sec. Hot rolling was performed at a temperature of 600C and a winding temperature of 600C. The obtained hot-rolled sheet having a thickness of 2.3 mm is pickled and temper-rolled at a pressure control rate of 0.8%.
m, JIS from 3m at the center (M) and 3m at the rear end (B)
A No. 5 tensile test piece was collected, and its EL was measured. FIG. 1 shows the difference between the center of the coil, the front end, and the rear end plotted against the rough bar heating temperature. Coarse bar heating temperature is 980
The difference in EL between both ends of the coil becomes 3% or less at a temperature equal to or higher than 0 ° C., and it can be seen that the coil end property is improved by rough bar heating. Although the reason for this phenomenon has not yet been completely elucidated, Mn was reduced by cooling the coarse bar to 950 ° C. or lower.
It is considered that MnS is completely precipitated after heating the coarse bar because the heating is performed with the driving force for the precipitation of S 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. According to the good grain growth obtained in this way, in the present invention, even in low-temperature winding, the same material as the conventional material can be secured.

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 hot-rolled steel sheet having small coil end properties 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 hot-rolled steel sheet with low end properties 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 precipitate and elongation (EL)
And lowers the moldability, so that the content is 0.03% or less.

Mn ≦ 0.5% Mn: Mn fixes S in the form of MnS and has a function of improving hot ductility, so it is desirable to add 0.05% or more. The upper limit is 0.5% because it hardens steel and deteriorates formability.
It is.

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% because excessive addition causes hardening of steel.

S ≦ 0.03% S: S is an element that inhibits hot ductility and formability, and is fixed as MnS. Therefore, lower is desirable. On the other hand, if the amount of MnS is too large, the EL is lowered and the stretch flangeability is lowered, so the upper limit is 0.03%.

Sol. Al ≦ 0.1% Since Al is used as a deoxidizing agent, Al is included to some extent. In the present invention, N is fixed as BN by adding B, but if added in excess of 0.1%, AlN is also finely precipitated simultaneously with BN, so the upper limit is 0.1.
%.

N ≦ 0.0035% In the present invention, N is fixed as BN.
If added in excess of 0035%, the amount of nitrides increases and the workability decreases, so the upper limit is 0.0035%.

B ≦ 0.003% B is an element that plays an important role in the present invention. B
Precipitates as BN with fine MnS as a nucleus, becomes a coarse composite precipitate, renders fine MnS harmless, fixes N, and suppresses precipitation of fine AlN. As a result, in the present invention, unprecedented grain growth is realized, and the coil end property is reduced. However, if BN is excessively present, the workability is reduced. Therefore, the upper limit of the added 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, the elongation (EL) decreases and the coil end property is not improved.

This has been made clear 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
The temperature was set to 0 ° C, and when the temperature reached 940 ° C after the rough rolling, coarse bar heating was performed to 1050 ° C, and the winding temperature was 600 ° C. The obtained hot-rolled sheet was pressure-controlled at a pressure control rate of 1% to obtain a hot-rolled steel sheet having a thickness of 1.6 mm. A JIS No. 5 tensile test piece was sampled from the leading end (T section) 3 m, the longitudinal center part (M section), and the trailing end (B section) 3 m of the obtained coil in the rolling direction, and a tensile test was performed. The difference between the EL of the M part and the EL of the M part and the EL of the T and B parts (Δ
EL) plotted against the B / N ratio is shown in FIG. When the B / N ratio is 0.6 or more, ΔEL becomes 3% or less,
It can be seen that the coil end property is improved, and that when the B / N ratio exceeds 1.5, the excess B lowers the EL. Particularly, since the softening effect is remarkable and the coil end property is extremely reduced, the B / N ratio is desirably 0.8 to 1.2.

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 can be fundamentally solved, and a soft hot rolled steel sheet having a small coil end property can be obtained without imposing a load on a subsequent 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. 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 plays an important role. In the present invention, MnS is coarsened to eliminate the coil end property. Therefore, M which is coarsely precipitated in the slab
As for nS, low-temperature heating is aimed at to keep as coarse 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. On the other hand, if the temperature is too low, the rolling load becomes excessive.
The above 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.

When 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 workability would be residual coarse grains and processed structure is observed is reduced, 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 grain growth does not sufficiently occur after winding and the material does not soften.

(2-2) Manufacturing conditions of embodiment 2 (Manufacturing method) After smelting a steel adjusted to the above-described composition range in a converter, converting the steel into a slab by continuous casting, 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 obtain a coarse bar, and then the coarse bar of 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. .

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 continuous casting, when the slab temperature is 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. Also,
Although not specified, coarse precipitation of MnS or BN is delayed unless cooled to 1000 ° C. or less.
It is desirable to insert into a heating furnace at 000 ° C. or lower.

Thus, the slab temperature after continuous casting is 7
The effect of the present invention can be further increased by inserting into a heating furnace at 50 ° C. or higher. 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 effects of the present invention cannot be obtained.

The performance of the hot-rolled steel sheet according to the present invention does not change regardless of whether it is an acid-washed material or black scale. In addition, there is no problem if hot-dip galvanizing is performed after pickling or with black scale without pickling. There are no restrictions on the conditions of the temper rolling, but if it is too high, the EL will drop sharply.
2% or less is desirable. Furthermore, even if the material is adjusted by performing continuous annealing after the temper rolling, the coil end property is not affected at all. 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.

[0039]

【Example】

(Example 1) Steel having the components shown in Table 1 (steel of the present invention: No. 1)
To 19, Comparative steel: No. 20 to 24), and the production conditions shown in Table 2 (Examples of the present invention: Nos. 1 to 19, Comparative Examples: N)
o. 20 to 24). The heating of the coarse bar is performed by induction heating, and the time required for raising the temperature is within 10 seconds. Further, the obtained hot-rolled sheet was pickled and adjusted at a pressure adjustment rate of 1% to form a coil. A JIS No. 5 tensile test piece was sampled in the rolling direction from 3 m in the longitudinal direction of the manufactured coil (T section), 3 m in the center (M section), and 3 m in the rear end (B section) during hot rolling, and the tensile strength was obtained. (TS) and elongation (EL) were measured.

The results are summarized in Table 2. The measured value is shown for the M part, and the difference from the M part is shown for the T and B parts.
Invention Example No. In 1 to 19, for TS, M
Part, the difference between the T and B parts is 20 N / mm ² or less, and for EL, the M part is 40% or more, and the difference between the M part, the T and B parts is 3% or less, and the coil end property is improved. ing.

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

[0042]

[Table 1]

[0043]

[Table 2]

(Example 2) Steels having the components shown in Table 3 (inventive steels: Nos. 1 to 19, comparative steels: Nos. 20 to 24) were melted, and the hot pieces after casting were subjected to temperatures shown in Table 4. And inserted into the heating furnace. Subsequently, the manufacturing conditions (Example of the present invention: No.
1 to 19, Comparative Example: No. 20 to 24). The heating of the coarse bar is performed by induction heating, and the time required for raising the temperature is within 10 seconds. Further, the obtained hot rolled sheet was subjected to pressure regulation at a pressure regulation rate of 1% to form a coil. 3m in the longitudinal direction at the time of hot rolling of the manufactured coil (T part) and 3m at the center (M part)
Furthermore, from the rear end (part B) 3m, JIS in the rolling direction
A No. 5 tensile test piece was collected, and TS and EL were measured.

The results are summarized in Table 4. The measured value is shown for the M part, and the difference from the M part is shown for the T and B parts.
Invention Example No. In 1 to 19, for TS, M
Part, the difference between the T and B parts is 20 N / mm ² or less, and for EL, the M part is 40% or more, and the difference between the M part, the T and B parts is 2% or less, and the coil end property is improved. ing.

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

[0047]

[Table 3]

[0048]

[Table 4]

[0049]

According to the present invention, by specifying the steel composition and the manufacturing conditions, a soft material having a uniform workability in the longitudinal direction without cutting the both ends in the longitudinal direction of the coil and having excellent workability can be 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 the part and the difference of EL of T and B part.

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

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasuhide Ishiguro 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Naoshi Murayama 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Incorporated (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
A process of finishing rolling at a temperature not lower than the temperature and winding at 650 ° C. or lower. 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.
Heating to 50 ° C. or lower and rough rolling to obtain a coarse bar; heating a coarse bar at 950 ° C. or lower to 980 ° C. or higher to obtain Ar ₃
A process of finishing rolling at a temperature not lower than the temperature and winding at 650 ° C. or lower.