JPH0225518A - Production of hot-rolled steel sheet having excellent deep drawability - Google Patents

Abstract

PURPOSE:To omit cold-rolling process or cold-rolling and annealing process and to produce a thin steel sheet having deep drawability without any inferiority to the ordinary cold-rolled steel sheet by suitably regulating steel components and rolling condition. CONSTITUTION:The steel containing <=0.008wt.% C, <=0.5% Si, <=1.0% Mn, <=0.15% P, <=0.02% S, 0.010-0.10% Al, <=0.008% N, 0.035-0.20, Ti, <=0.001-0.015% Nb and satisfying the inequality of C, N and S contents and added contents of Ti and Nb, is used. After completing the rolling to this steel in the range of <=950 deg.C and >=Ar3 transformation point, while applying lubrication in the temp. range of <=Ar3 transformation point and >=600 deg.C, the rolling is executed at >=80% rolling reduction ratio. After that, the steel sheet is coiled under condition satisfying the relation (FDT)- (CT)<=100 deg.C and (CT)>=600 deg.C between hot rolling finished temp. (FDT) and cooling temp. (CT).

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing a hot-rolled steel sheet with excellent deep drawability used for automobile steel sheets and the like.

(Prior Art) 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 (Ef). Such deep-drawing steel sheets are generally cold-rolled steel sheets that are manufactured by completing hot rolling at an Ar3 transformation point or higher, then cold-rolling the sheet into a thin sheet with the final thickness, and then recrystallizing and annealing the sheet. was. However, in recent years, for the purpose of cost reduction, there has been an increasing demand for replacing members that conventionally used cold-rolled steel sheets with hot-rolled steel sheets.

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 -i, the assembled fibers become 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 manufacturing hot-rolled conditioned plates with excellent deep drawability have been disclosed. For example, in JP-A-59-226149, C: 0.002 wt% (hereinafter simply expressed as %), S
i: 0.02%, Mn: 0.23%, P:
0.009%, S: 0.008%, Al: 0.02
5%, N: 0.0021%, Ti: 0.10% low carbon Al killed steel was rolled at 500~900''C with lubricating oil at 76% to form a steel strip with a thickness of 1.6 ms. An example of manufacturing a thin steel plate having the characteristics of r=1.21 is shown in Japanese Patent Laid-Open No. 62-192539, where c: o, ooa%, Si: 0.04.
%, Mn: 1.53%, P: 0.015%, S
: 0.004%, Ti: 0.068%, Nb:
0.024% low carbon At killed steel at Ar3~Arz
By applying 92% rolling at +150 °C, r =
An example of manufacturing a thin steel plate having a characteristic of about 1.41 is shown.

(Problems to be Solved by the Invention) However, in the method described in Japanese Patent Application Laid-open No. 59-226149, the r value obtained is as low as 1.21 at most when no annealing treatment is performed. Furthermore, even if recrystallization annealing is performed after hot rolling, the r value obtained is at most 1.
．． 51, and it cannot be said that the deep drawability is sufficiently satisfied. In addition, in the method described in JP-A-52-192539, hot rolling is finished in the T region and the transformation texture resulting from the subsequent T→α transformation is utilized, so the r
The difference in value becomes large, Δr=-1.2, which is very large, and there is also a limit to the r value that can be obtained, at most 1.41.
It is difficult to say that the deep drawability is fully satisfied.

This invention provides a method for producing thin steel sheets that have deep drawability comparable to conventional cold-rolled steel sheets by omitting the cold rolling process or the cold rolling-annealing process by appropriately regulating steel components and rolling conditions. The purpose is to provide

(Means for solving the problem) First, the research results that formed the basis of this invention will be described. C:
0.001-0.008%, Si: 0.01%,
Mn: 0.1-0.6%, P: 0.008-0.0
1.5%, S: 0.002-0.02%, N:
0.001-0.008%, Ti: 0.01-0.
115 steel with a composition of 20%, Nb:O ~ 0.007%
After heating and soaking at 0°C, rough rolling was performed, followed by finish rolling at a total rolling reduction of 90%. At this time, the hot rolling finish temperature was kept constant at 700° C. by adjusting the finish rolling start temperature. Subsequently, the coiled material was subjected to self-annealing treatment at 700° C. for 1 hour. Note that the finish rolling was performed using lubricated rolling.

Figure 1 shows the influence of steel components on the r value of hot rolled sheets.

The r value strongly depends on the steel composition, and is 1.2 (C/12+N/14+S/32) < (Ti
/48+Nb/93) and was significantly improved by adding 1007% of Nb=O.

Also, C: 0.002%, Si: 0.01%,
Mn: 0.1%, P: 0.01%, S; 0
．． 010%, N: 0.002%, Ti: 0.055
%, Nb: 0.006% steel at 1150°C
After heating and soaking, rough rolling is performed, followed by total rolling reduction: 9
0% finish rolling was performed. At this time, by adjusting the rough rolling start temperature, the rough rolling end temperature (RDT) is set to 105
The temperature was varied from 0 to 880°C. In addition, for finish rolling, by adjusting the finish rolling start temperature, the hot rolling finishing temperature can be adjusted to 700.
The temperature was kept constant at °C. Subsequently, a self-annealing treatment was performed at 700° C. for 1 hr. Note that the finish rolling was performed using lubricated rolling.

FIG. 2 shows the influence of the rough rolling end temperature on the r value of the hot rolled sheet. The r value strongly depends on the RDT, and was significantly improved by setting the RDT≦950°C.

Also, CF 0.002%, Si: 0.01%, M
n: 0.1%, P: 0.01%, S: 0; 008%
, N: 0.002%, Ti 0.065%, Nb
: Heating steel with a composition of 0.007% at 1150"C.
After soaking, rough rolling was performed, followed by finish rolling at a total rolling reduction of 90%. At this time, the hot rolling finishing temperature was varied from 680°C to 750°C by adjusting the finishing rolling start temperature. Subsequently, lh at a temperature range of 650℃ to 750℃
A self-annealing treatment was applied to the rolled material.

Note that the finish rolling was performed using lubricated rolling.

FIG. 3 shows the influence of the winding temperature on the r' value of the hot rolled sheet. The r value strongly depends on (FDT) − (CT), and (
FDT) - (CT)≦ioo'c, it was significantly improved.

Based on the above experimental results, the present inventors conducted subsequent research and found that it is possible to produce hot-rolled steel sheets with excellent deep drawability by regulating the steel composition and manufacturing conditions as shown below. I found that. The gist is: 1. C: 0.008% or less, Si: 0.5%
Below, Mn: 1.0% or less, P: 0.15% or less, S: 0.02% or less, Al: 0.010~
0.10%, N: 0.008% or less, Ti: 0
．． 035-0.20%, FJb: 0.001-0.
015% and C,N.

The amount of S and the amount of Ti and Nb added are 1.2 (C/1
2±N/14+S/32) < (Ti/48+Nb/
93) to the steel that satisfies the relationship: 950'C or less Ar,
After finishing rolling in the temperature range above the transformation point, below the Art transformation point 6
While applying lubrication in the temperature range of 00°C or higher, the reduction rate is 80°C.
% or more, and then the hot rolling finishing temperature (FDT) and coiling temperature (CT) are (FDT) - (CT) 5100°C and (CT) ≧ 60
characterized by winding under conditions that satisfy the relationship of 0°C;
A method for producing hot-rolled steel sheets with excellent deep drawability.

2. C: 0.008% or less, Si: 0.5%
Below, Mn: 1.0% or less, P: 0.15% or less, S: 0.02% or less, At: 0.010~
0.10%, N: 0.008% or less, Ti: 0
．． 035-0.20%, Nb: 0.001-0.0
15% and B: 0.0001 to 0.00%, and the amounts of C, N, and S and the amounts of Ti and Nb added are 1.2 (C/12 + N/14 + S/32)
< (Ti/48 + Nb/93) After rolling in Ar below 950°C in a temperature range above the transformation point, while applying lubrication in a temperature range above 600°C below Ar = transformation point. , rolling is performed with a reduction ratio of 80% or more, and then the hot rolling finishing temperature (FDT) and coiling temperature (CT) are (FDT) - (CT) 5100°C and (CT) ≧ 60
A method for producing a hot-rolled steel sheet with excellent deep drawability, characterized by winding under conditions satisfying the relationship of 0''C.

3. C: 0.008% or less, Si: 0.5
% or less, Mn: 1.0% or less, P: 0.15% or less, S: 0.02% or less, Al: 0.010 to 0
．． 10%, N: 0.008% or less, Ti: 0.
035-0.20%, and Nb: 0.001-0
．． 015% and the amount of CN, S and Ti and N
The amount of b added is 1.2 (C/12+N/14+s/3
2) Steel that satisfies the relationship < (Ti/48+Nb/93) is rolled under Ar below 950°C and at a temperature above the transformation point, and then lubricated at a temperature above 500°C below the Ar3 transformation point. while rolling with a reduction rate of 80% or more,
A method for producing a hot-rolled steel sheet with excellent deep drawability, the method comprising subsequently performing recrystallization annealing.

4. C: 0.008% or less, Si: 0.5%
Below, Mn: 1.0% or less, P: 0.15%
Below, S: 0.02% or less, Al: 0.010~
0.10%, N: 0.008% or less, Ti: 0
．． 035-0.20%, Nb: 0.001-0.0
15% and B: 0.0001 to 0.00%, and the amounts of C, N, and S and the amounts of Ti and Nb added are 1.2 (C/12+N/14+s/32) < (Ti
/48+Nb/93), 950
``After rolling in the temperature range of C or lower and Ar1 transformation point or higher, Ar
, while applying lubrication at a temperature range of 500°C or higher below the transformation point,
A method for producing a hot-rolled steel sheet with excellent deep drawability, the method comprising rolling with a rolling reduction of 80% or more, followed by recrystallization annealing.

It is.

(for production) This invention will be explained in detail below.

(1) Steel composition The steel composition is important in this invention, C: 0.0
08% or less, Si: 0.5% or less, Mn: 1.
0% or less, p: o, ts% or less, S: 0.02% or less, Al: 0.010 to 0.10%, N: 0.
008% or less, Ti: 0.035-0.20%, N
b: 0.001 to 0.015%, and C, N,
The amount of S and the amount of Ti and Nb added are 1.2 (C/12+
N/14 +S/32) < (Ti/48+Nb/9
3) Must be. Furthermore, it is necessary to add B: 0.0001 to 0.0010% in order to improve resistance to secondary work brittleness and anisotropy of r value.

Unless the steel composition satisfies the above conditions, excellent deep drawability cannot be obtained. Hereinafter, the reasons for limiting each component will be explained.

(a) C: 0.008% or less C is preferable because the less it is, the better the deep drawability is, but if its content is 0.008% or less, it will not have much of a negative effect, so it should be kept at o, oos% or less. Limited.

(b) Si: 0.5% or less Si has the effect of strengthening steel, and is added in the required amount depending on the desired strength, but if the amount added exceeds 0.5%, it has a negative effect on deep drawability. Therefore, it was limited to 0.5% or less.

(C) Mn: 1.0% 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 is 1.0% or less, Mn has the effect of strengthening steel. If it exceeds Owt%, it will have an adverse effect on deep drawability, so it is limited to 1.0wt% or less.

(d) P: 0.15% or less P has the effect of strengthening steel, and is added in the necessary amount depending on the desired strength, but if the amount added exceeds 0.15%, it has a negative effect on deep drawability. Therefore, it was limited to 0.15% or less.

(e) S: 0.02% or less S is preferable because the smaller the content, the better the deep drawability is.However, if the S content is 0.02% or less, it does not have much of a negative effect, so it should be kept at 0.02% or less. Limited.

At: 0.010 to 0.10%Al is added as necessary to deoxidize and improve the yield of carbonitride-forming elements, but if it is less than 0.010%, the addition effect will be low. On the other hand, even if it is added in an amount exceeding 0.10%, a further deoxidizing effect cannot be obtained, so it is limited to 0.010 to 0.10%.

(Powder N: 0.008% or less The smaller the N content, the better the deep drawability is, so it is preferable, but if the content is 0.008% or less, it does not have much of a negative effect, so it is limited to 0.008% or less. did.

(h) Ti: 0.035-0.20% Ti is a carbonitride forming element, reduces solid solution (C9N) in steel, and preferentially forms the (111) orientation, which is advantageous for deep drawability. However, if the amount added is less than 0.035%, it has no effect (on the other hand, if it is added more than 0.20%, no further effect can be expected, and on the contrary, it may cause a decrease in surface quality. Therefore, it was limited to 0.035 to 0.20%.

(i) Nb: 0.001 to 0.015% Nb is a carbide-forming element and is effective in reducing solid solution C in steel, as well as in refining the structure before finish rolling. In other words, even if there is no solid solution (C,N) in the steel,
If the structure before finish rolling is coarse, the strain introduced during rolling will not accumulate, making it difficult to form the (111) orientation.

On the other hand, if the structure before finish rolling is fine, strain is likely to accumulate, and as a result, the (111) orientation is preferentially formed, improving deep drawability. Furthermore, it has been revealed that solid solution Nb has the effect of accumulating strain during rolling.

If the content is less than 0.001%, there is no effect (on the other hand, 0.001%
．． If it exceeds 0.15%, the recrystallization temperature will rise, so o, o
It was limited to oi~0.015%.

(j) B: 0.0001 to 0.0010% B is effective in improving the resistance to secondary work brittleness and is also effective in improving the anisotropy of the r value. That is, when Nb and B coexist, the crystal grains become finer than in the Nb-added material, and as a result, the anisotropy of the r value (Δr) becomes smaller. If the amount added is less than 0.0001%, there is no effect;
．． If it exceeds 0.010%, deep drawability deteriorates, so 0.0
It was limited to 0.001% to 0.0010%.

(Capital) 1.2 (C/12+N/14 +S/32) <
(Ti/48+Nb/93) Solid solution (C,
When N) is not present, the (111) orientation is preferentially formed after rolling and annealing, and deep drawability is improved. In this invention, 1.2 (C/12+N/14+S/32) < (Ti
/4B+Nb/93) and by adding Ti and Nb in an amount equivalent to or more than C and N, it was found that 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/12+N/14+S/32) < (
Ti/4B+Nb/93).

(2) Rolling process The rolling process is important in this invention, and rough rolling is
Ar
= While applying lubrication at a temperature range of 600℃ or higher below the transformation point,
The total rolling reduction is 80% or more, and the hot rolling finishing temperature (FDT
) and the winding temperature (CT) are (FDT) - (CT)
Rolling is carried out under conditions that satisfy the relationship: ≦100℃ and (CT)2600℃, or after rough rolling is completed in a temperature range of 950℃ or below Ar=transformation point or above, a temperature of 500℃ or above Ar3 transformation point or below After rolling with a total reduction of 80% or more while applying lubrication in the area, it is necessary to perform recrystallization annealing.

If rough rolling is completed in a temperature range of 950°C or higher,
After rough rolling, that is, before finish rolling, the structure becomes coarse;
Strain introduced during finish rolling is less likely to accumulate,
As a result, the (111) orientation becomes difficult to form. Also,
If the process is finished in a temperature range below the Ar3 transformation point, the deep drawability will deteriorate because a (1001 orientation) is formed during rough rolling. When the process is completed, the structure before finish rolling becomes finer, so the strain introduced during finish rolling is more likely to be accumulated, and as a result (1111 orientation is preferentially formed, and deep drawability is improved. , 950'' C-Ar3 transformation point range is desirably 50% or more in order to make the fibers finer.

Furthermore, if finish rolling is completed in a temperature range above the transformation point, the texture becomes random due to T→α transformation, and excellent deep drawability cannot be obtained. Meanwhile, set the finishing temperature to 500'C.
Even if it is lowered below, further improvement in deep drawability cannot be expected.
Since the rolling load only increases, the rolling temperature is changed to Ar,
The temperature was 500'C or more below the transformation point.

Further, unless the total rolling reduction during finish rolling is 80% or more, the (111) orientation will not be formed during rolling, resulting in poor deep drawability.

Furthermore, if lubrication rolling is not performed during finish rolling, additional shearing force will act on the surface layer of the steel sheet due to the frictional force between the rolls and the steel sheet, which will result in poor deep drawability of the surface layer of the steel sheet (110). Since the orientation is preferentially formed, deep drawability deteriorates. Therefore, lubricated rolling is necessary.

In addition, in a coiled 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≧600'C was set. Furthermore, in order to improve deep drawability, it is advantageous to have a low rolling temperature and a high winding temperature. Therefore, the hot rolling finishing temperature (FDT
) and the winding temperature (CT) are (FDT) - (CT)5
It is necessary to perform rolling under conditions that satisfy 100°C.

In addition, for those that undergo recrystallization annealing after hot rolling,
Since winding and self-annealing is not required, the hot rolling finish temperature is set at 500.
℃ or more, and the winding temperature may also be low.

Recrystallization annealing after hot rolling may be continuous annealing or box annealing. The annealing temperature is suitably in the range of 550 to 950'C. The heating rate is also 10'C/hr to 50'C/hr.
A range of S is sufficient.

(Example) A steel slab having the composition shown in Table 1 was heated at 1150℃ and after soaking, the rolling reduction was 85% in the temperature range of 950℃-Ar3 transformation point.
After performing rough rolling, finish rolling was performed. Rough rolling end temperature (RDT) and finishing rolling end temperature (FDT) at this time
Table 2 also shows the rolling reduction, coiling temperature (CT), presence or absence of lubrication, and material properties of the hot-rolled sheet after pickling in the temperature range of 600°C or more below the Arz transformation point.The tensile properties are J
Measured using IS No. 5 tensile test piece, and r value is 1
After applying 5% tensile prestrain, measurements were made using a three-point method, and the average values in the L direction (rolling direction), D direction (45° direction to the rolling direction), and C direction (90° direction to the rolling direction) and Anisotropy r-(rL+2 re +rc )/4, Δr-(r+
-2ro +rc) /2. For evaluation of secondary processing brittleness resistance, a cylindrical sample processed at a critical drawing ratio of 3.8 was cooled to -50°C, and then a crushing test was performed.
Evaluation was made based on the presence or absence of brittle cracking.

It can be seen that the hot rolled steel sheet manufactured according to the present invention has excellent deep drawability and secondary work brittleness resistance compared to the comparative example.

Further, steel slabs having the compositions shown in Table 1 were heated and soaked at 1150°C, rough rolled in the same manner as above, and then finished rolled. At this time, the rough rolling end temperature (RDT), finish rolling end temperature (FDT), 500 below the Arz transformation point
Table 3 shows the rolling reduction rate, coiling temperature (CT), and presence or absence of lubrication in the temperature range of .degree. C. or higher. After pickling, the rolled plate is N1111~
No. 15 was subjected to rapid heating annealing at 830°C for 60 seconds, and Nos. 16 to 20 were subjected to box-type annealing at 750°C for 5 hours.

Table 3 also shows the material properties of the hot rolled sheet after annealing.

It can be seen that the hot rolled steel sheet manufactured according to the present invention has excellent deep drawability and secondary work brittleness resistance compared to the comparative example.

(Effect of the invention) According to the present invention, even if the cold rolling process or the cold rolling-annealing process is omitted, it is possible to manufacture a hot rolled steel sheet with excellent deep drawability equivalent to that of a cold rolled steel sheet, and Significant cost reductions can be achieved compared to manufacturing rolled steel sheets.

[Brief explanation of the drawing]

Figure 1 is a graph showing the influence of steel components on the r-value of hot-rolled steel sheets, Figure 2 is a graph showing the influence of the rough rolling end temperature on the r-value of hot-rolled steel sheets, and Figure 3 is , is a graph showing the influence of the winding temperature on the r value of a hot rolled steel sheet. Figure 1 12 (Le pIth) - ("Asu9 meeting) Figure 2 kDT (τ)

Claims (1)

[Claims] 1. C: 0.008wt% or less, Si: 0.5wt% or less, Mn: 1.0wt% or less, P: 0.15wt% or less, S: 0.02wt% or less, Al: 0.010-0.1
0wt%, N: 0.008wt% or less, Ti: 0.03
5-0.20wt% and Nb: 0.001-0.01
5 wt%, and the amount of C, N, S and Ti and N
The amount of b added is 1.2 (C/12+N/14+S/3
2) For steel that satisfies <(Ti/48+Nb/93), 9
After rolling in a temperature range of 50°C or lower and Ar_3 transformation point or higher,
While applying lubrication in a temperature range of 600°C or higher below the Ar_3 transformation point, rolling is performed at a rolling reduction of 80% or more, and then the hot rolling finishing temperature (FDT) and coiling temperature (CT) are (FDT) - ( CT)≦100℃ and (CT)≧600
A method for producing a hot-rolled steel sheet with excellent deep drawability, which is characterized by winding under conditions that satisfy the relationship: °C. 2, C: 0.008 wt% or less, Si: 0.5 wt% or less, Mn: 1.0 wt% or less, P: 0.15 wt% or less, S: 0.02 wt% or less, Al: 0.010 to 0. 1
0wt%, N: 0.008wt% or less, Ti: 0.03
5-0.20wt%, Nb: 0.001-0.015w
t% and B: 0.0001 to 0.0010 wt%, and the amounts of C, N, and S and the amounts of Ti and Nb added are 1.2 (C/12 + N/14 + S/32) < (Ti/ 4
8+Nb/93), Ar_
After finishing rolling in a temperature range of 3 transformation point or higher, the reduction rate is 8 while applying lubrication in a temperature range of 600°C or higher below Ar_3 transformation point.
0% or more rolling, then hot rolling finishing temperature (FD)
and the winding temperature (CT) are (FDT)-(CT)≦100°C and (CT)≧60
A method for producing a hot-rolled steel sheet with excellent deep drawability, characterized by winding under conditions that satisfy the relationship of 0°C. 3. C: 0.008 wt% or less, Si: 0.5 wt% or less, Mn: 1.0 wt% or less, P: 0.15 wt% or less, S: 0.02 wt% or less, Al: 0.010 to 0. 1
0wt%, N: 0.008wt% or less, Ti: 0.03
5-0.20wt% and Nb: 0.001-0.01
5 wt%, and the amount of C, N, S and Ti and N
The amount of b added is 1.2 (C/12+N/14+S/3
2) For steel that satisfies <(Ti/48+Nb/93), 9
After rolling in a temperature range of 50°C or lower and Ar_3 transformation point or higher,
Manufacture of a hot-rolled steel sheet with excellent deep drawability, characterized by rolling with a rolling reduction of 80% or more while applying lubrication in a temperature range of 500°C or more below the Ar_3 transformation point, followed by recrystallization annealing. Method. 4, C: 0.008 wt% or less, Si: 0.5 wt% or less, Mn: 1.0 wt% or less, P: 0.15 wt% or less, S: 0.02 wt% or less, Al: 0.010 to 0. 1
0wt%, N: 0.008wt% or less, Ti: 0.03
5-0.20wt%, Nb: 0.001-0.015w
t% and B: 0.0001 to 0.0010wt%, and the amounts of C, N, and S and the amounts of Ti and Nb added are 1.2(C/12+N/14+S/32)<(Ti /4
8+Nb/93), Ar_
After finishing rolling in a temperature range of 3 transformation point or higher, the reduction rate is 8 while applying lubrication in a temperature range of 500°C or higher below Ar_3 transformation point.
A method for producing a hot-rolled steel sheet with excellent deep drawability, the method comprising rolling at 0% or more and then recrystallizing annealing.