JPH03140417A - Production of hot rolled steel plate excellent in deep drawability - Google Patents

Abstract

PURPOSE:To obtain a hot rolled steel plate having deep drawability equal to that of a cold rolled steel sheet while obviating the necessity of a cooling or cold rolling-annealing stage by specifying steel components and rolling conditions, particularly, roll diameter at the time of finish rolling, initial plate thickness, and friction coefficient. CONSTITUTION:A steel having a composition which consists of, by weight, <=0.008% C, <=0.5% Si, <=1.0% Mn, <=0.15% P, <=0.02% S, 0.010-0.10% Al, <=0.008% N, and the balance Fe and in which the additive quantity of Ti and/or Nb satisfies the relation in an inequality I is used. This steel is subjected to rolling at a temp. in the region between a temp. lower than the Ar3 transformation point and 600 deg.C under the conditions where roll radius R(mm), plate thickness t(mm) before rolling using the roll, and friction coefficient (mu) satisfy the relations in an inequality II so that the total rolling reduction at a temp. lower than the Ar3 transformation point is regulated to >=60%. Then, the resulting rolled steel plate is coiled under the conditions where rolling finishing temp. (FDT) and coiling temp. (CT) satisfy the relations in inequalities III.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application 1] The present invention relates to a method for manufacturing a hot rolled steel sheet with excellent deep drawability used for automobile steel sheets and the like.

[Prior Art 1] Conventionally, deep drawing thin steel sheets used for automobile steel sheets and the like are required to have a high Lankford value (r value) and high ductility (EI2). Such deep drawing steel sheets are generally produced by cold rolling 1tili, which is produced by completing hot rolling at the A r 3 transformation point or higher, then cold rolling to a thin sheet with the final thickness, and then recrystallization annealing. It was used.

In recent years, with the aim of reducing costs, there has been an increasing demand for hot-rolled steel plates to replace members that conventionally used cold-rolled steel plates.

However, in order to ensure workability, especially ductility, in conventional hot-rolled steel sheets for processing, rolling was completed at the Ar3 transformation point or higher to avoid the formation of an unrecrystallized ferrite structure. Therefore, in -M, the texture becomes random during the γ→α transformation, so the deep drawability of the hot-rolled steel sheet was significantly inferior to that of the cold-rolled steel sheet.

Several methods for producing hot rolled steel sheets with excellent deep drawability have been disclosed. For example, in JP-A-59-226149, C10,002%, 5i10.02%, M n /
0.23%, Plo, 009%, S10, 008%, A
ε10.025%, N10.0021%, T i /
0.10% low carbon Aff quilt steel at 500-900℃
By applying lubricating oil to a copper strip with a thickness of 1.6 mm by 76% rolling, a thin J
An example of manufacturing using ii is shown. However, since strong lubrication must be applied during hot rolling, operational difficulties such as poor biting and slippage of the steel plate occur.

Moreover, in Japanese Patent Application Laid-open No. 192539/1983, C10,0
08%, 5i10.04%, Mn/1.53%, Plo
, 015%, 510-004%, T110, 068%,
Low carbon AI2 quilt steel with 10.024% Nb
An example of manufacturing a thin steel sheet having a property of r=1.41 by performing 92% rolling at 3 to A r 3 + 150° C. is shown. However, the above method finishes hot rolling in the γ region and utilizes the transformed texture resulting from the subsequent γ-α transformation.

Inevitably, the anisotropy of the r value becomes large, and Δr=−1, 2, which is very large, and furthermore, there is a limit to the r value that can be obtained.

[Problem to be Solved by the Invention 1] In the present invention, the cold rolling process or the cold rolling-annealing process is achieved by appropriately regulating the steel composition and rolling conditions, especially the roll diameter, initial plate thickness, and friction coefficient during finish rolling. Skip the process and
It is an object of the present invention to provide a method for manufacturing a thin steel sheet having deep drawability comparable to that of conventional cold-rolled steel sheets.

[Means for solving the problem] As a result of intensive research, the inventors of the present invention have found that by regulating the manufacturing conditions as shown below, it is possible to manufacture hot-rolled steel sheets with excellent deep drawability. I found it. What is the gist?

C: 0.008% by weight or less, Si: 0.5% by weight or less.

Mn: 1.0% by weight or less, P: O, 15% by weight or less, S: 0.02% by weight or less, Aff: 0.010-0.10% by weight, N: 0.008
% by weight or less, and the amount of one or both of Ti and Nb added is 12
A steel having the relationship (C/t 2 + N/14) ≦ (T i / 48 + N b / 93), the balance being Fe and unavoidable impurities is used. This steel was processed in a temperature range above Ar3 transformation point 'rf4600°C, roll radius: R (mm), plate thickness before rolling by the roll: t (mm), and friction coefficient 2μ of U≦-0, After rolling the steel plate under lubrication conditions that satisfy the relationship 212o g (R/t) +0.55 and with a total reduction of 60% or more below the Ar3 transformation point, the steel plate is heated to the finishing temperature (FDT). and the winding temperature are (FDT)-(CT)≦100°c, and (CT)≧
5! A hot-rolled steel sheet with excellent deep drawability that is characterized by being rolled under conditions that satisfy the relationship: 600'C! It is a method of sending.

Also, using the same steel as above, this steel is below Ar3 transformation point 5
In a temperature range of 00'C or higher, the roll radius R (mm), the plate thickness before rolling by the roll t (mm), and the friction coefficient 2μ are μ≦−0, 2flog (R/tl +0. Hot rolling with excellent deep drawability, characterized by performing recrystallization annealing after rolling under lubrication conditions that satisfy the relationship 55 and with a total reduction of 60% or more below the Ar3 transformation point. This is a method for manufacturing steel plates.

In addition, in the above steel, B:O,0OO1 to 0.0020% by weight
If any of the above treatments is carried out using steel to which .

Furthermore, in addition to any of the above steel components, Sb: 0.00
Provided is a method for producing a hot-rolled steel sheet with excellent deep drawability, characterized by containing 1 to 0.020% by weight.

[Function] The research results that served as the basis for the numerical limitations of the present invention will be described below.

C-0,002 weight%, Si: 0.02% by weight, Mn: 0.1% by weight.

A hot-rolled sheet with the following composition: P: 0.011% by weight, S: 0.013% by weight, N: 0.002% by weight, Ti: 0.04% by weight, Nb 0.013% by weight, at 700°C. Rolling was performed by 60% in one bath, followed by winding and self-annealing at 700° C. for 1 hour.

At this time, roll radius: 300 mm, plate thickness = 3 mm,
The friction coefficient can be adjusted from 0.1 to 0 by varying the lubrication conditions.
．． It was varied within a range of 25.

Figure 1 shows the influence of the friction coefficient on the r value of hot rolled sheets.

When Rog (R/t) is constant, the r value strongly depends on the friction coefficient, and when εo g (R/t) = 2.0, it was significantly improved by setting μ≦0.15. , by changing the roll radius and plate thickness, ffog (
R/t) was varied. Figure 2 shows the influence of 20 g (R/t) on the r value of the hot rolled sheet after annealing. When the friction coefficient μ is constant, the r value is strongly (
Depends 1. When =0.15, Rog(R/t)≦2.
A significant improvement was achieved by setting the value to 0.

Based on the above experimental results, the scope of the present invention was limited as follows.

(1) Steel composition Steel composition is important in this invention. C・o,　o　os　s weight% or less.

Si: 0.5% by weight or less, Mn: 1.0% by weight or less, P: 0.15% by weight or less, S: 0-02% by weight or less, A2・o, oio~0.10% by weight, N: 0.008% by weight or less, and 1+ of Ti and Nb! Or for addition of 2 groups or 1
．． 2 (C/12+N/14) ≦ (T i / 48 + N b / 93). Furthermore, B: 0.0001 to 0.0020% by weight to improve fFI secondary processing brittleness, and sb 0.001 to 0.0% to prevent nitriding during batch annealing.
Unless the 0-tone component that needs to be added satisfies the above relationship, excellent deep drawability cannot be obtained.

The reasons for the limitations on each component will be shown below.

(a) C: 0.008% by weight or less The smaller the amount of C, the better the deep drawing [quality will be improved, so it is preferable, but if the content is less than 0.008% by weight, it will not have much of a negative effect. It was limited to % by weight or less.

(b) Si: 0.5% by weight or less Si has the effect of strengthening steel, and is added in the necessary amount depending on the desired strength, but if the amount added exceeds 0.5% by weight, deep drawability will deteriorate. Since it has an adverse effect, it is limited to 0.5% by weight or less.

(c) Mn: 1.0% by weight or less Mn has the effect of strengthening steel, and is added in the necessary amount depending on the desired strength, but if the amount added exceeds 1.0% by weight, deep drawability deteriorates. Since it has an adverse effect, it is limited to 1.0% or less.

(d) P: 0.15% by weight or less P has the effect of strengthening steel and is added in the required amount depending on the desired strength, but if the amount added exceeds 0.15% by weight, deep drawability will deteriorate. Since it has an adverse effect, it is limited to 0.15% by weight or less.

(e) S: 0.02% by weight or less S is preferable because the less it is, the better the deep drawability is, but if its content is 0.02% by weight or less, it does not have much of an adverse effect, so it is 0.02% by weight or less. limited to.

(fl Al1: 0.010-0.10 wt% AI2
．． is added as necessary to deoxidize and improve the yield of carbonitride-forming elements, but if it is less than 0.010% by weight it has no effect (on the other hand, if it is added more than 0.1% by weight) 0.010~
It was limited to 0.10% by weight.

(g) N: 0.008% by weight or less The smaller the amount of N, the better the deep drawability is, so it is preferable, but if the content is less than 0.008% by weight, it does not have much of an adverse effect, so O,OO8% by weight % or less.

(h) Ti: 0.01-0.20% by weight Ti is a carbonitride-forming element, reduces solid solution (C, N) in steel, and gives priority to (111) orientation, which is advantageous for deep drawability. It is added to form a chemical compound. If the amount added is less than 0.01% by weight, there will be no effect, while if it is added in excess of 0.20% by weight, no further effect will be obtained, and conversely it will lead to deterioration of the surface properties of the steel sheet. The content was limited to 0.01 to 0.20% by weight.

(i) Nb: 0.001 to 0.40% by weight Nb is a carbide-forming element and is effective in reducing solid solution C in steel and in refining the structure before finish rolling. In other words, even if there is no solid solution (C,N) in the steel, if the finish rolling surface structure is coarse, the (111) orientation is preferentially formed because the strain introduced during rolling is not accumulated. , deep drawability is improved. Furthermore, it has been revealed that solid solution Nb has the effect of accumulating strain during rolling. As a result, the (l 11) orientation was preferentially formed, improving deep drawability.6 It was also revealed that solid solution Nb has the effect of accumulating strain during rolling. Its content is o, oot
Less than 0.001 to 0.04% by weight has no effect, while more than 0.040% by weight increases the recrystallization temperature.
It was limited to 0% by weight.

(j) 1.2 (C/1 2+N/14)≦ (T
i/48+Nb/93) If no solid solution (C,N) exists before finish rolling, the (l 11) orientation is preferentially formed after rolling and annealing, improving deep drawability. In the present invention, 1.2 (C/t 2+N/14
+S/32)≦(Ti/48+Nb/93) and C and N
It has been found that by adding Tj or Nb in an amount equal to or more than the amount of Tj or Nb, solid solution (C, N) no longer exists before finish rolling. Furthermore, it was revealed that at that time, the r value improved. Therefore, 1.2 (C/t 2+N/1
4)≦(T i / 48 + N b / 93)
limited to.

(k) B: 0.0001 to O, OO 20% by weight B is effective in improving secondary processing brittleness 6 Its addition amount is 0.0
Less than 0.001% by weight has no effect; on the other hand, O,OO20
If it exceeds 0.0001% by weight, the deep drawability will deteriorate.
~O,OO was limited to 20% by weight.

(I2) Sb: 0.001 to 0.020% by weight sb
is added to prevent nitriding during batch annealing. If the content is less than 0.001% by weight, there is no effect;
．． If added in excess of 0.02% by weight, the surface properties of the steel sheet would deteriorate, so it was limited to 0.001 to 0.020% by weight.

(2) Rolling process The rolling process is important in the present invention, in which the roll radius R (m
m), the plate thickness before rolling by the roll: and the coefficient of friction:
After rolling where μ satisfies the relationship μ≦−0,29,og (R/t) +0.55 and the total reduction ratio below the A r 3 transformation point is 60% or more, hot rolling finishing is performed. Temperature (FD
Hot rolling is performed so that T) = (CT) satisfies the following relationships: (FDT) - (CT) 5100°C and (CT) 2600°C, or A r
3 Pervert point not yet! In the temperature range of 4500°C or higher, the radius R (mm) of the roll and the plate thickness before rolling with this roll:
t (mm) and friction coefficient: μ is μ≦−〇, 212o
g (R/t) + 0.55, and A
After rolling with a total reduction ratio of 60% or more below the r3 transformation point, it is necessary to perform recrystallization annealing.

Furthermore, to further improve deep drawability, rough rolling to 950
It is preferable that the hot rolling start temperature (FET) be 800° C. or lower, and the hot rolling start temperature (FET) should be 800° C. or lower. In other words, when rough rolling is completed in a temperature range of 950°C or below and above the A r 3 transformation point, the finish rolling surface structure becomes fine, so the strain introduced during finish rolling is likely to accumulate, resulting in (111) orientation is preferentially formed, and deep drawability is improved. In addition, since the rolling reduction during rough rolling becomes high, 50
% or higher is desirable.6 Furthermore, when the FET is set to 800°C or lower, the rolling reduction rate in the low temperature range increases, which increases the amount of strain on the (111) oriented grains introduced during rolling, which reduces recrystallization. After annealing, the (1111 orientation) is preferentially formed.

Furthermore, if the finish rolling is finished in a temperature range above the Ar3 transformation point, the texture becomes random due to the γ-α transformation, making it impossible to obtain excellent deep drawability. on the other hand.

Even if the finishing temperature is lowered to 500°C or lower, further improvement in deep drawability cannot be expected and the rolling load only increases, so the rolling temperature was set to 500°C or higher below the Ar3 transformation point.

In addition, unless the total rolling reduction below the Ar3 transformation point is 60% or more, the (111) orientation will not be formed during rolling.
Poor deep drawability.

Furthermore, it is necessary that the roll radius, the plate thickness before rolling, and the friction coefficient satisfy μ≦−0, 2nog (R/t)+0.55. That is, below the Ar3 transformation point μ > −0,2
When rolling is performed under the conditions of βog (R/t) + 0.55,
Due to the frictional force between the rolls and the steel plate, additional shearing force is exerted on the mi surface layer, and as a result, the (11) orientation, which is unfavorable for deep drawability, is preferentially formed on the surface layer of the steel plate.Therefore, the deep drawability is 6 However, μ≦−0,2120g (R/t) + C1
55, it was found that the (ito) orientation of the surface layer of the steel sheet decreased, and the (l 11) orientation also increased.

Therefore, it was limited to μ≦0.2.9og (R/t)+0.55. Note that this effect of the roll radius and the thickness before rolling is considered to be due to changes in the deformation mode and deformation mechanism during rolling.

In addition, for finished middle or rear stands where the plate thickness is relatively thin, the roll radius is the normal size (2300 mm).
In this case, the coefficient of friction must be made extremely small, which tends to cause operational troubles such as slipping. Therefore, for finishing middle or rear stands, the roll radius should be set to 25.
The thickness is preferably 0 mm or less, preferably 200 mm or less.

Note that the effects of roll radius and initial plate thickness in the present invention are effective only in normal rolling formats, and are not applicable to rolling methods that have a different deformation pattern compared to normal rolling, such as planetary mills. , has no effect.

Note that in a rolled self-annealed material that is not subjected to recrystallization annealing after rolling, recrystallization is not completed unless the coiling temperature is 600°C or higher, so CT was set to 600°C or lower. Further, in order to improve deep drawability, it is better to lower the rolling temperature, and the higher the coiling temperature is, the more it costs $11. Therefore, the rolling finishing temperature (F
DT) and the winding temperature (CT) are (FDT) - (CT)5
It is necessary to perform rolling under conditions that satisfy the condition of 100°C.

Note that for products that undergo recrystallization annealing after hot rolling, rolling self-annealing is not necessary, so the hot rolling end temperature is set at 500°C.
In addition to the above, the winding temperature may also be low.

Recrystallization annealing after hot rolling may be either continuous annealing or box-shaped annealing. A range of 550°C to 950°C is suitable for the annealing temperature. Also, the heating rate is 10”C/hr to 5
It may be in the range of 0°C/S.

The steel of the present invention can be applied to various surface-treated thick plates such as hot-dip galvanizing.

[Example] A steel slab with a composition shown in Table 1 was heated and soaked at 1150°C, rough rolled, and then finished rolled. Rough rolling end temperature (RDT) and finish rolling start temperature (FET) at this time
, Finish rolling temperature (FDT), Coiling temperature (CT), (
FDT) (CT), roll radius (R) of each stand, rolled plate thickness (t), Z = -0,2f2 o g
(R/t) +0.55 and the friction coefficient (μ) are shown in Table 2. Note that the finished plate thickness is 1.2 mm.

Table 2 shows the material properties of the hot rolled sheets subjected to pickling or post-pickling recrystallization annealing. The tensile properties were measured using a JIS No. 5 tensile test piece. In addition, the r value was determined at 1ill by a three-point method after applying 15% tensile strain, and was determined in the L direction (rolling direction).
, the r values in the D direction (45% direction in the rolling direction) and the C direction (90 degrees direction in the rolling direction) are rl, r2. r3
and the average value 〒 is r = (r l'+ 2 r 2 + r 3 )
/4.

It can be seen that the hot-rolled m-plate produced within the scope of the present invention has superior deep drawability compared to the comparative example.

[Effects of the Invention] According to the present invention, it is possible to manufacture a hot-rolled steel sheet with excellent deep drawability equivalent to that of a cold-rolled steel sheet, and a significant cost reduction can be achieved compared to the production of conventional cold-rolled steel sheets.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the effect of the friction coefficient μ on the i value, and FIG. 2 is a graph showing the effect of R/t on the T value.

Claims (1)

[Claims] 1 C: 0.008% by weight or less, Si: 0.5% by weight or less, Mn: 1.0% by weight or less, P: 0.15% by weight or less, S: 0.02% by weight. Hereinafter, Al: 0.010 to 0.10% by weight, N: 0.008% by weight or less, and the amount of one or both of Ti and Nb added is 1.2 (C/12 + N/14) ≦( A steel having the following relationship (Ti/48+Nb/93) with the balance consisting of Fe and unavoidable impurities is heated in a temperature range of 600°C or higher below the Ar_3 transformation point,
Under lubrication conditions where the roll radius: R (mm), the plate thickness before rolling by the roll: t (mm), and the friction coefficient: μ satisfy the relationship μ≦−0.2log(R/t)+0.55. and after rolling with a total reduction of 60% or more below the Ar_3 transformation point, the finishing temperature (FDT) and the winding temperature (C
A method for producing a hot-rolled steel sheet with excellent deep drawability, characterized in that T) is rolled under conditions that satisfy the following relationships: (FDT)-(CT)≦100°C and (CT)≧600°C. 2. When the steel according to claim 1 is heated in a temperature range of 500° C. or higher below the Ar_3 transformation point, the roll radius: R (mm), the plate thickness before rolling by the roll: t (mm), and the friction coefficient: μ are μ≦ -0.2log(R/t)+0.55 Under lubrication conditions that satisfy the relationship, and after rolling with a total reduction of 60% or more below the Ar_3 transformation point, recrystallization annealing is performed. A method for producing hot-rolled steel sheets with excellent deep drawability. 3 In addition to the steel components, B: 0.0001 to 0.0020
3. The method for producing a hot-rolled steel sheet with excellent deep drawability according to claim 1 or 2, characterized in that a steel containing % by weight is used. 4. Production of a hot-rolled steel sheet with excellent deep drawability according to any one of claims 1 to 3, characterized in that the steel further contains Sb: 0.001 to 0.020% by weight. Method.