JPH05140655A - Manufacture of hot rolled steel sheet excellent in deep drawability - Google Patents

Abstract

PURPOSE:To provide the method for manufacturing a hot rolled steel sheet excellent in deep drawability. CONSTITUTION:Steel contg. <=0.005% C, <=0.5% Si, <=0.5% Mn, <=0.05% P, <=0.01% S, <=0.05% Al and <=0.01% N, furthermore contg. Nb alone or both Nb and Ti in the range of <=0.1% Nb and <=0.1% Ti satisfying the relationship of {(C/12)+(N/14+(S/32)}<{(Ti/48)+(Nb/93)} and the balance Fe with inevitable impurities is heated to the Ar3 point or above to <=1200 deg.C, is subjected to rough rolling at the Ar3 point or above and is subjected to rolling in the temp. range of the Ar3 point or below to >=600 deg.C at 50 to 96% total draft to form its structure immediately after the rolling into a one in which a structure having >=80% recrystallizing ratio occupies by 5 to 30% in the sheet thickness direction from the surface, and after that, is subjected to coiling treatment and is then subjected to recrystallization annealing treatment. This rolling is a non- lubricated ferrite range rolling, therefor, easy manufacture is attained without any operational problems.

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 hot-rolled steel sheet having excellent deep drawability, which is used for automobile steel sheets and the like.

[0002]

2. Description of the Related Art Conventionally, the problems to be solved by the invention
Cold-rolled steel sheets having excellent formability were generally used for the processing thin steel sheets used for automobile steel sheets and the like, but recently, in various fields, for the purpose of cost reduction, etc.,
The use of hot-rolled steel sheets instead of cold-rolled steel sheets is being considered.

However, in general, hot-rolled steel sheets are inferior in deep drawability to cold-rolled steel sheets. The deep drawability of the steel sheet increases as the number of crystallographic {111} planes parallel to the plate surface increases,
The smaller the {100} plane is, the better it is, and it is generally judged by the Rankford value (r value).

That is, a hot rolled steel sheet has conventionally been transformed at a transformation temperature A.
rBecause it is manufactured by finish rolling at a temperature of ₃ points or more,
The texture becomes random due to the austenite-ferrite transformation, and as a result, the hot-rolled steel sheet is inferior in deep drawability as compared with the cold-rolled steel sheet, as described above.

Therefore, recently, several methods have been proposed as a method for producing a hot-rolled steel sheet having an excellent deep drawability. For example, Japanese Examined Patent Publication No. 1-38885 discloses that by performing lubrication rolling with a total rolling reduction of 50% or more in a temperature range of 500 ° C. or more and Ar ₃ points or less, or performing lubrication rolling with a friction coefficient of 0.2 or less, A method for producing a hot-rolled steel sheet having a property of r value ≧ 1.42 is shown. However, in this method, since the friction coefficient is as low as 0.2 or less, there is a problem in operation such as slippage of the steel sheet during hot rolling and defective biting. Further, when hot rolling is performed at ₃ Ar points or less without lubrication, {110} develops on the surface layer of the thin steel sheet due to the shear stress between the roll and the steel sheet, and it is difficult to obtain a high r value.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a method by which a hot-rolled steel sheet excellent in deep drawability can be easily obtained.

[0007]

DISCLOSURE OF THE INVENTION The inventors of the present invention have diligently researched a method for obtaining a hot rolled steel sheet having excellent deep drawability without high lubrication rolling in order to solve such a problem.
As a result of repeated investigations, the addition of Nb and Ti, especially the essential addition of Nb, reduced the solid solution C and hot rolled in the ferrite region,
By controlling the temperature and the reduction rate and, if necessary, controlling the heat treatment conditions after hot rolling, it was found that excellent deep drawability can be obtained without lubrication, and operation can be facilitated. The present invention has been made.

That is, according to the present invention, C ≦ 0.005%,
Si ≤ 0.5%, Mn ≤ 0.5%, P ≤ 0.05%, S ≤ 0.5.
01%, Al ≦ 0.05%, N ≦ 0.01%, and Nb ≦ 0.1%, Ti ≦ 0.1%, and
{(C / 12) + (N / 14 + (S / 32)} <{(Ti / 48) + (Nb / 9
3)} is contained in an amount of Nb alone or both Nb and Ti, and the balance is Fe and inevitable impurities. The steel is heated to a temperature of _{3 to} 1200 ° C., and at ₃ or more Ar. By performing rough rolling and rolling at a total rolling reduction of 50 to 96% in a temperature range of Ar ₃ points or less and 600 ° C. or more,
Immediately after rolling, after the structure having a recrystallization rate of 80% or more occupies 5% to 30% in the plate thickness direction from the surface,
The gist is a method for producing a hot-rolled steel sheet having excellent deep drawability, which is characterized by performing a winding treatment and then performing a recrystallization annealing treatment.

The present invention will be described in more detail below.

[0010]

[Action]

First, the reasons for limiting the chemical components in the present invention will be explained.

C: C is an element having a great influence on the deep drawability, and the lower the content, the higher the deep drawability obtained.
To obtain the desired high deep drawability, the C content is 0.005%.
Must be:

Si, Mn, P: Si, Mn, and P are all strength-improving elements, and are effective additive elements for obtaining high strength. In the present invention, which is intended to improve deep drawability, On the contrary, is an unfavorable element. Si, Mn, P
Are above 0.5%, 0.5% and 0.05% respectively,
The workability deteriorates and the purpose of improving the deep drawability cannot be fulfilled. Therefore, the Si content is less than 0.5% and the Mn content is 0.5%.
Hereinafter, the P content is suppressed to 0.05% or less.

S: S combines with various elements to form impurity inclusions and adversely affects the ductility, so the smaller the content, the better. However, if it is less than 0.01%, practically no harm occurs.
The amount of S is 0.01% or less.

Al: Al may be added for deoxidation and denitrification of steel materials, but if it is too much, the steel material becomes brittle and ductility is significantly impaired, so it must be suppressed to 0.05% or less. ..

The reason why the N: N amount is limited to 0.01% or less is that the workability is deteriorated and the addition amount of Nb and Ti is increased and the cost is increased if the N content is contained more than this amount.

Nb, Ti: Nb and Ti are particularly important as additive elements for improving deep drawability. Nb and Ti are N
By precipitating bC and TiC, the amount of dissolved C in steel can be made lower than the limit reduced by the steelmaking technology. Also, N
The addition of b and Ti also has an important role of preventing the coarsening of the crystal grains due to the pinning effect of the precipitate and raising the recrystallization temperature of ferrite. Is an essential element.

Specifically, {(C / 12) + (N / 14 + (S / 3
2)} <{(Ti / 48) + (Nb / 93)} Nb that satisfies the conditional expression
The r value after the recrystallization annealing can be increased by adding them alone or by adding Nb and Ti. However, considering economic efficiency, the upper limits of Ti and Nb are each 0.1%.

Next, the reasons for limiting the manufacturing conditions will be described.

The manufacturing conditions of the present invention are, in short, 600 ° C.
In hot rolling in the temperature range of Ar ₃ points or less, the shear stress between the roll and the steel plate is used to change the equivalent strain applied to the steel plate in the plate thickness direction, and The crystal temperature can be changed. Utilizing this phenomenon, a structure having a recrystallization rate of 80% or more immediately after hot rolling is recrystallized at 5% or more and 30% or less from the plate surface to form an ultrafine and random structure, thereby forming a thin steel sheet surface layer. Further, it is possible to suppress the development of the {110} texture peculiar to hot rolling of unlubricated unrecrystallized ferrite region and avoid the adverse effect on the r value peculiar to the {110} texture. On the other hand, it is possible to leave a rolling texture in the central portion and develop a recrystallized {111} texture obtained from it.

First, the steel having the above chemical composition is melted and cast by a conventional method, but the slab heating temperature is aimed at refining the crystal grain size after rough rolling and suppressing the coarsening of the austenite crystal grains. The upper limit is 1200 ° C, and the lower limit is Ar in order to refine the grains by austenite-ferrite transformation.
_The rough rolling temperature is set to ₃ points or more for Ar. Considering the refinement of crystal grains before rolling and cost reduction, the slab heating temperature and the rough rolling are preferably 1100 ° C. or lower.

Next, rolling with a total reduction of 50 to 96% is carried out in a temperature range of Ar ₃ point or lower and 600 ° C. or higher. Although rolling in the initial stage may be carried out in the recrystallization temperature range, The total rolling reduction in the non-recrystallization temperature range is 50% or more, or even in the recrystallization temperature range, the strain amount immediately after the final pass is 5%.
Do so that it is 0% or more. Further, in this rolling, due to the shear stress from the roll near the surface of the steel sheet, the equivalent strain amount is high, and the recrystallization temperature is lower than in the center portion of the plate thickness that is not affected by the shear stress. It is possible to control the temperature above the recrystallization temperature in the vicinity of this surface and below the recrystallization temperature in the central portion. As a result, the crystal ratio is 80%
5% or more, 30% from the surface in the thickness direction from the surface
It is possible to create an organization (see Fig. 1) that occupies the following.

The reason for rolling under the above conditions is that
This is because the operation is facilitated by the low lubrication rolling and the deep drawability of the steel sheet is improved, and the reason is as follows.

That is, when rolled under the above conditions, a rolling texture belonging to the ND // <111> and RD // <110> orientation groups develops in the non-recrystallized portion excluding the vicinity of the steel sheet surface subjected to the shear stress of the roll. Then, the subsequent annealing treatment or high temperature winding gives a recrystallized structure in which the ND // <111> orientation group is developed. The ferrite having many <111> orientations causes a high r value in the hot rolled steel sheet. On the other hand, in the vicinity of the surface of the steel sheet, ND // <110> orientation groups, which have a shearing stress of the roll and adversely affect the r value, develop, which may cause a high r value not to be obtained. However, due to the shear stress of the roll, the amount of strain is high and the recrystallization temperature is low. By utilizing this, by recrystallizing the vicinity of the surface of the steel sheet subjected to roll shear stress at the time of rolling to make it fine and random, it is possible to suppress the development of <110> texture and suppress the decrease of r value.

On the other hand, deep drawability is better when the rolling temperature is lower, but when rolling at a temperature lower than 600 ° C., recrystallization near the surface becomes difficult and the structure cannot be randomized, which adversely affects the r value. A high r value cannot be obtained because the ND / <110> direction group exerting influence develops. Also, if the Ar is higher than ₃ points, austenite-ferrite transformation occurs and ND // <111>
And it is difficult to develop the rolling texture belonging to the RD <110> orientation group in the central portion of the plate thickness.

The reason why low lubrication rolling is carried out is to prevent operational problems such as slippage and bite failure of the steel sheet.

Further, the reason why the recrystallization annealing is performed after the hot rolling (after the winding treatment) is that by this heat treatment, crystal grains in which the <111> orientation is developed are obtained from the unrecrystallized region and This is because a hot rolled steel sheet having excellent drawability can be obtained. The conditions for recrystallization annealing are not particularly limited, but it is desirable that the annealing temperature is 700 ° C. or higher and Ac ₃ transformation point or lower. The recrystallization annealing method includes not only continuous annealing and batch annealing but also heat treatment before the dipping step in the hot dip galvanizing line. In this hot dip galvanizing line, for example, once from 500 to 850 ° C., from the viewpoint of uniform adhesion of hot dip zinc and temperature control during zinc adhesion.
Temperature is raised to 10 to 30 seconds and then 4 to 50 ° C /
It is cooled to 300 to 500 ° C. in a second and immersed in molten zinc. Further, reheating treatment may be performed at 550 to 650 ° C. for about 10 to 20 seconds, alloying treatment may be performed, and corrosion resistance may be improved.

Next, examples of the present invention will be described.

[0029]

【Example】

[Table 1] After smelting the sample steel with the chemical composition shown in, make a slab with a thickness of 20 mm.

[Table 2] Under the conditions shown in, heating, rolling, winding, and annealing are performed.
The value was calculated. The results are also shown in Table 2.

As is clear from Table 2, the inventive examples can obtain a high r value of 1.1 or more as compared with the comparative examples, and can obtain excellent deep drawability. As shown in the cross-sectional photograph of the material of the present invention in FIG. 1, the crystal ratio is 80% or more in the portion of 5 to 30% in the plate thickness direction from the surface. FIG. 2 shows the relationship between the ratio of the structure having a recrystallization rate of 80% or more in the plate thickness direction from the surface and the r value (average value) for the material of the present invention (steel type A). A high r value (average value) is obtained in the range of 5 to 30% in the plate thickness direction from the surface of the structure having a rate of 80% or more.

The r value (average value) was calculated by the following equation. r (average value) = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 where r ₀ : value measured in the rolling direction r ₄₅ : value measured at 45 ° to the rolling direction r ₉₀ : to the rolling direction Value measured at 90 °

[0032]

As described in detail above, according to the present invention,
Along with adjusting the chemical composition, the transformed ferrite was refined by rough rolling in the austenite region, followed by rolling in the ferrite region without lubrication, and then recrystallization annealing treatment, so a steel sheet with excellent deep drawability. There is an effect that it can be easily manufactured at low cost by using hot rolling equipment.

[Brief description of drawings]

FIG. 1 is a photograph showing a cross section (metal structure) of the material of the present invention obtained in an example, in which 5 to 30% of the portion in the plate thickness direction from the surface has a crystallization rate of 80% or more. ..

FIG. 2 shows the material of the present invention (steel type A) obtained in Examples,
It is a figure which shows the relationship between the ratio which a structure with a recrystallization rate of 80% or more occupies in the plate thickness direction from a surface, and r value (average value).

Claims (1)

[Claims] 1. In weight% (hereinafter, the same), C ≦ 0.005
%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.05%, S
≤0.01%, Al≤0.05%, N≤0.01%, and Nb≤0.1%, Ti≤0.1%, and {(C / 12) + (N / 14 + (S / 32)} <{(Ti / 48) + (Nb
/ 93)} containing Nb alone or both Nb and Ti with the balance being Fe and unavoidable impurities, and heating it to a temperature not lower than Ar ₃ points and not higher than 1200 ° C. and not lower than Ar ₃ points. Rough rolling is performed, and the total rolling reduction is performed in the temperature range of 600 ° C. or more at an Ar ₃ point or less, and the total rolling reduction is 50 to 96%. A method for producing a hot-rolled steel sheet having excellent deep drawability, which comprises performing a winding treatment and then a recrystallization annealing treatment after forming a structure that occupies 5% or more and 30% or less in a thickness direction.