JP3231174B2 - Hot-rolled high-strength steel sheet with good drawability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet having excellent drawability and high strength, and a method for producing the same, and is an inexpensive alternative to a cold-rolled steel sheet having both workability and strength as a material for automobiles or industrial machines. It is intended to provide a hot rolled steel sheet.

[0002]

2. Description of the Related Art Conventionally, a cold-rolled steel sheet has been used as a drawing steel sheet. However, when a thicker material is required or when cost reduction and productivity improvement are intended, the use of a hot-rolled steel sheet is used. Have been considered. In general, hot-rolled steel sheet is inferior in deep drawability compared to cold-rolled steel sheet.
Is being improved. For example, Japanese Patent Application Laid-Open No. 2-1457
No. 48 discloses that a high-ductility steel sheet having a strength of 35 kgf / mm ² or more and a strength of 40% or more can be obtained by performing hot rolling of a low-carbon steel sheet at a temperature of at least 50% in a ferrite recrystallization temperature range. ing. However, in this method, the drawability of the manufactured hot-rolled steel sheet is insufficient, and the surface state of the steel sheet is different from that of the cold-rolled steel sheet.

[0003]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a hot-rolled steel sheet having an excellent deep drawability with an increase in steel sheet strength, and a hot-rolled steel sheet having a surface property comparable to that of a cold-rolled steel sheet. , And a method of manufacturing the same.

[0004]

Means for Solving the Problems The constitution of the present invention is as follows.
0.003 ≦ C ≦ 0.04%, Si ≦ 0.03%,
1.5 ≦ Mn ≦ 3.0%, P ≦ 0.1%, S ≦ 0.01
%, N ≦ 0.005%, 0.03 ≦ Ti ≦ 0.2%, and 48 (C / 12 + N / 14) ≦ Ti, and heated at 950 to 1100 ° C. including other unavoidable elements After finishing the rough rolling at 900 to 950 ° C.,
≤800 ° C, each pass 15-30% in coefficient of friction μ ≦
After rolling while lubricating to 0.15, 650-750 ° C. by self-annealing or heat treatment
And then rolled up, pickled and then rolled down with a skin pass at 1-3% to obtain a surface roughness Ra.
≦ 2μm, the structure is ferrite single phase and crystal grain size ≦ 20μ
m can be obtained. Further, the ductility can be improved by holding the temperature at 900 ° C. or more for 5 minutes or more between the rough rolling and the finish rolling.

[0005]

The purpose of the steel sheet is determined depending on the part to be formed. In general, when the degree of molding is large, workability, that is, elongation and drawability are required, but strength is reduced instead. In addition, when it has a high strength, the workability is reduced. In the production of conventional hot-rolled steel sheets for processing, there are many methods of improving workability by increasing the purity of iron, and extremely soft hot-rolled steel sheets have been obtained by rolling this in a γ region or an α region. However, the original properties of iron require some strength. Therefore, if a hot-rolled steel sheet with increased strength and maintainable workability is made, its use will be expanded.

[0006] The present inventors have repeatedly studied for this purpose. The aim is to increase strength (TS ≧ 35kgf / m
m ² ), improvement in drawability (r average value ≧ 1.2), and improvement in surface texture (Ra ≦ 2 μm). First, with regard to the improvement of the drawability of a hot-rolled steel sheet having a low characteristic value, it was studied to improve the drawability by lubricating rolling in the α region. Conventionally, a high r average value has been obtained by this method in ultra-low carbon steel. This is because a {110} plane that deteriorates the drawability is generated on the extreme surface layer, but this layer disappears due to the lubrication rolling, and a {111} plane that is the same as the center portion is generated, thereby improving the drawability. However, on the high-strength side, workability has been degraded, so that sufficient studies have not been made. Therefore, a study was conducted based on this method. It was also found that the ductility was improved by maintaining the temperature in an appropriate temperature range between the rough rolling and the finish rolling.

[0007] Next, to increase the strength of the steel, strengthening by a precipitate formed by adding Ti in order to eliminate solid solution C and solid solution strengthening were studied. As a result, it has been found that when reinforced with P and Mn, the strength can be increased without impairing the drawability obtained by α region lubricating rolling. The structure in this case was a ferrite single phase and fine grains of 20 μm or less, and a steel sheet suitable for workability was obtained. When the particle size exceeds 20 μm as shown in FIG. 4, the surface of the part becomes rough after molding, and the commercial value is reduced. For this reason, it is necessary to suppress the crystal grain size of the steel sheet to 20 μm or less.

Further, it was found that by applying a skin pass immediately after pickling, a roughness of 2 μm or less comparable to that of a cold-rolled steel sheet can be obtained, and a surface property which does not change in appearance can be obtained. Usually, even if the coil after pickling is skin-passed, such a surface property cannot be obtained, and it is important to apply a skin-pass in a pickling line. The reason for this is not clear, but the steel sheet is pickled and a new surface appears on the surface.
It is considered that the effect of reducing the pressure immediately without surface oxidation or contamination is that the pressure is reduced immediately.

[0009] It has been determined on the basis of such effects,
The conditions for limiting the scope of the invention will be described in detail below. C is an element to be reduced as much as possible for improving the drawability, but is an element necessary for securing the strength. The contribution to strength is
It depends on the precipitate formed with Ti. 0.00 as the lower limit
The reason for setting it to 3% is that if it is lower than this, a strength of 35 kgf / mm ² or more cannot be secured even by solid solution strengthening by the addition of Mn and P. Further, the upper limit is set to 0.04%. If the upper limit is not less than 0.04%, Ti added to eliminate solid solution C is used.
This is because the amount increases and the cost increases, and the precipitation strengthening allowance stagnates due to coarsening of TiC.

N is an element to be reduced as much as possible to improve drawability, and the upper limit is made 0.005%. The amount of Ti is
In order to eliminate solid solution C and N, it is substantially determined by the amounts of C and N, and it is sufficient to satisfy 48 (C / 12 + N / 14) ≦ Ti. The lower limit of 0.03% is the minimum required amount for precipitation strengthening, and the upper limit of 0.2% or more is costly and cannot be expected to have any further effect on strength. Si can be used as a strengthening element, while Si scale and Mn-S
Harmful due to i-based inclusions. In the present invention, as a condition that does not affect ductility and does not generate harmful inclusions, Si ≦
0.03%.

S is also an element that easily forms harmful inclusions,
In order to prevent an adverse effect on ductility, the content was regulated to S ≦ 0.01%. In the present invention, Mn is an effective strengthening element capable of increasing the strength without impairing the drawability, as shown in FIGS. To obtain a 35 kgf / mm ² or more,
1.5% or more is required even when P and precipitates are used in combination. However, at 3.0% or more, the elongation is remarkably reduced as shown in FIG. P can also increase the strength without impairing the drawability. However, the addition of a large amount has an adverse effect on toughness and weldability, so the upper limit was regulated to 0.1% to avoid this effect.

In hot rolling, it does not matter whether a reheated slab or a directly rolled material is used. Further, a thin CC may be used during continuous casting. It is effective to heat at 950 to 1100 ° C. to fix C and N as precipitates. A time of 15 minutes or more is required for the precipitation. In the rough rolling after heating, it is necessary to roll in the γ low temperature range. This is because the γ crystal grains become fine grains and the α area after transformation becomes finer. This is effective for increasing the number of crystal orientation {111} planes effective for drawability. Therefore, 900 just above Ar ₃
The rough rolling is completed at ９950 ° C.

After that, the finish rolling is performed. The finish rolling may be performed immediately after the rough rolling is completed.
By maintaining the temperature at 00 ° C. or higher for 5 minutes or longer, improvement in ductility is also realized. This temperature maintenance is easily achieved by using a so-called coil box. The finish rolling needs to be started in the α region, and for this purpose, the steel sheet may be cooled between the coarse and the finish in consideration of production efficiency. Performing the finish in the α range is that when transformation is performed from γ to α during normal hot rolling of tandem, the rolling operation becomes unstable because the deformation resistance changes significantly, which is likely to cause trouble. Therefore, by preventing this, and by starting the α-range rolling from the beginning, the accumulated strain in the α-region is increased, so that recrystallization of ferrite is easily caused. On the other hand, if it is too low, the roll load becomes too large, so that regulation is required. For this purpose, the finish rolling temperature is set at 600 to 800 ° C.

In order to improve the drawability, in finish rolling, it is necessary to perform lubricating rolling by a method such as spraying a high oil concentration solution onto a roll or applying oil to the roll surface. If the friction coefficient at this time is μ ≦ 0.15, the lubrication effect is sufficient and the shear layer on the surface layer can be removed. However, when the friction coefficient is low, troubles such as biting slip occur, and in order to keep rolling at a low friction coefficient, the rolling reduction in each rolling mill needs to be 15 to 30%.

The structure of the steel sheet after the α-region rolling is a processed structure. For this purpose, it is necessary to recrystallize the processing α. Therefore, it is sufficient to set the temperature to a recrystallization temperature of 650 to 800 ° C. or higher in the wound state. This is heating between finishing and winding, self-annealing method using a proximity coiler,
There is a method of performing reheating by a coil box.

Next, after the scale formed by the hot rolling is peeled off in an pickling line, the scale is rolled under a skin pass pressure. This step is important for obtaining a surface state comparable to a cold-rolled steel sheet. At this time, the roll surface is transferred at a rolling reduction of 1% or more, and as shown in FIG. 3, a surface roughness state comparable to that of a cold-rolled steel sheet is obtained. However, if it exceeds 3%, the deterioration of elongation becomes remarkable, so the content is restricted to the range of 1 to 3%.
By the above production, the surface roughness Ra ≦ 2 μm, the structure is a ferrite single phase, and the crystal grain size ≦ 20 μm is 35 kgf / mm ^2.
The above hot-rolled high-strength steel sheet can be obtained.

[0017]

EXAMPLES In order to know the influence of the components, a steel material having the components shown in Table 1 was rolled under the conditions shown in Table 2. Cuddle 5149 was used as a lubricating rolling oil, mixed with water and sprayed onto the surface of a rolling roll. After the completion of the rolling, a winding treatment was performed, and then a pickling-skin pass was performed. Then, a sample was collected and a material inspection was performed. In the investigation, a surface roughness, a tensile test (JIS No. 5), and a crystal grain size were measured. Table 2 shows the results.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

[0021]

[Table 4]

[0022]

[Table 5]

Sample No. Conditions 1, 2, 3, 4, and 5 after hot rolling fall within the scope of the present invention. However, No. 3 using the D material having a small Mn content was used. Sample No. 4 had low strength and did not reach 35 kgf / mm ² . Also, the crystal grain size was large and exceeded 20 μm.

Table 3 shows the steel types A,
It is a result of examining hot rolling conditions using C. Sample No.
In Nos. 1 and 2, the coarse-grained ferrite before finishing was not obtained because the rough rolling temperature was deviated, the crystal grain size of the product plate was increased, and the r average value was decreased. No. In Sample No. 3, the finishing temperature was too high, the amount of strain added to the ferrite was small, and recrystallization was unlikely to occur. Thus, coarse recovery grains were obtained, and the strength, elongation, and r average value were poor.

No. Sample No. 4 had a high finish reduction ratio and could not be rolled because it was not bitten by lubrication rolling. No. No. 7 is an example in which the film was rolled up after pickling and then skin-passed. However, the surface roughness was slightly rough and a surface property comparable to that of cold rolling could not be obtained. Also,
No. No. 8 is No. 8 when the skin pass rolling reduction is high. 1
0 is too low, but if too high, the decrease in elongation is remarkable, and if too low, the roughness is coarse. No. No. 12 indicates that when the temperature at the time of winding was low, No. 14 was the case where the lubrication was insufficient, but the recrystallization was insufficient or the texture of the surface layer was insufficiently improved, and the r average value was low.

No. 15 is 950 ° C between rough rolling and finish rolling
, And the improvement of the r average value and the ductility were simultaneously achieved. No. No. 16 is a slab manufactured by thin continuous casting and rolled through a DR process (the slab temperature is not lowered to 900 ° C. or less), and the holding at 950 ° C. for 5 minutes between the rough rolling and the finish rolling is performed. It was done. This is No. As in the case of No. 15, improvements in the r average value and ductility were simultaneously achieved. Other material No. produced under the conditions of the present invention. 6,
Good material properties were also obtained in 9, 11, and 13.

[0027]

According to the present invention, a hot-rolled steel sheet having a high drawability comparable to that of a cold-rolled steel sheet and a high strength can be obtained by regulating the component conditions and the rolling conditions, and the economical effect can be obtained. Very high.

[Brief description of the drawings]

FIG. 1 is a chart showing the effect of the amount of Mn on the average value of r.

FIG. 2 is a chart showing the effect of Mn content on TS and El.

FIG. 3 is a table showing the effect of skin pass reduction on surface roughness.

FIG. 4 is a table showing the relationship between the crystal grain size and the occurrence of rough skin.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00-8/04 C21D 9/46-9/48

Claims (4)

(57) [Claims] 1. In% by weight, 0.003 ≦ C ≦ 0.04%, Si ≦ 0.03%, 1.5 ≦ Mn ≦ 3.0%, P ≦ 0.1%, S ≦ 0.01 %, N ≦ 0.005%, 0.03 ≦ Ti ≦ 0.2%, and 48 (C / 12 + N / 14) ≦ Ti, including other unavoidable elements, and having a surface roughness of Ra
A hot-rolled high-strength steel sheet having a structure of ≦ 2 μm, a ferrite single phase and a crystal grain size of ≦ 20 μm. 2. In% by weight, 0.003 ≦ C ≦ 0.04%, Si ≦ 0.03%, 1.5 ≦ Mn ≦ 3.0%, P ≦ 0.1%, S ≦ 0.01 %, N ≦ 0.005%, 0.03 ≦ Ti ≦ 0.2%, and 48 (C / 12 + N / 14) ≦ Ti, and 950 to 1
After heating at 100 ° C., after rough rolling at 900 to 950 ° C., each pass between 600 and 800 ° C. is 15 to 30%.
After performing rolling while lubricating so that the friction coefficient becomes μ ≦ 0.15 at 65 ° C., self-annealing or heat treatment is performed.
A method for producing a hot-rolled high-strength steel sheet, comprising: winding at 0 to 750 ° C .; 3. In% by weight, 0.003 ≦ C ≦ 0.04%, Si ≦ 0.03%, 1.5 ≦ Mn ≦ 3.0%, P ≦ 0.1%, S ≦ 0.01 %, N ≦ 0.005%, 0.03 ≦ Ti ≦ 0.2%, and 48 (C / 12 + N / 14) ≦ Ti, and 950 to 1
After heating at 100 ° C., finishing the rough rolling at 900 to 950 ° C., holding at 900 ° C. or more for 5 minutes or more,
≤800 ° C, each pass 15-30% in coefficient of friction μ ≦
After rolling while lubricating to 0.15, 650-750 ° C. by self-annealing or heat treatment
3. The method for producing a hot-rolled high-strength steel sheet according to claim 2, wherein the roll is wound after being pickled, and then pickled and then rolled down by 1 to 3% by a skin pass. 4. In% by weight, 0.003 ≦ C ≦ 0.04%, Si ≦ 0.03%, 1.5 ≦ Mn ≦ 3.0%, P ≦ 0.1%, S ≦ 0.01 %, N ≦ 0.005%, 0.03 ≦ Ti ≦ 0.2%, and 48 (C / 12 + N / 14) ≦ Ti, and continuously and thinly cast a slab containing other unavoidable elements. After the rough rolling at 900 to 950 ° C. is completed without lowering the temperature of the slab to 900 ° C. or less, the steel slab is held at 900 ° C. or more for 5 minutes or more. After rolling while lubricating so that the coefficient of friction μ ≦ 0.15, the temperature is adjusted to 650 to 750 ° C. by self-annealing or heat treatment, and then rolled up. 3. The hot-rolled high-strength steel sheet according to claim 2, wherein the steel sheet is wound. Construction method.