JPH02173216A - Manufacture of cold rolled steel sheet for deep drawing having excellent secondary working brittleness resistance - Google Patents

Abstract

PURPOSE:To manufacture the steel sheet having excellent deep drawability and secondary working brittleness resistance by subjecting a steel having specified relationship of the amounts of C, N and S and the amt. of Ti to be added to hot rolling, holding it to the area of specific temp. and thereafter subjecting the steel to cold rolling and continuous annealing. CONSTITUTION:A steel contg., by weight, <=0.005% C, <=0.5% Si, <=1.0% Mn, <=0.025% P, <=0.02% S, <=0.005% N, 0.01 to 0.1% Al and 0.01 to 0.10% Ti and satisfying the relationship of (Ti/48)>(C/12+N/14+S/32) is subjected to hot rolling. Then, the steel is held to the temp. area of 600 to 800 deg.C for 30min to 30hr and is thereafter subjected to cold rolling and continuous annealing. If required, 0.0001 to 0.0020% B and 0.001 to 0.02% Nb are incorporated into the above steel to satisfy the relationship of (Ti/48+Nb/93)>(C/12+N/14+S/32).

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for manufacturing a cold-rolled steel sheet for deep drawing, which is used for automobile steel sheets and the like and has excellent resistance to secondary work brittleness.

<Prior Art> Cold-rolled steel sheets used for automobile panels and the like are required to have excellent deep drawability. In order to improve deep drawability, a high Lankford value (F (straight)) and high ductility (Effi) are required as mechanical properties of the steel sheet.

By the way, conventionally, automobile bodies are assembled by spot welding a large number of press parts, but recently there has been a demand to reduce the number of parts and welds by making some of these parts larger and more compact. has been increasing.

For example, due to the complicated shape of automobile oil pans, they are currently completed by welding, but there is a strong demand from automobile manufacturers for integral molding. As car designs become more complex, there are an increasing number of parts that are difficult to form using conventional sheet steel. In order to meet these demands, a cold-rolled steel sheet with far superior deep drawability is required, and what are its characteristics? ≧2.4. Fl≧55% is required.

By the way, it is known that the greater the degree of deformation of pressed parts that have undergone severe deep drawing, the more easily brittle fracture occurs during subsequent secondary processing, and improvement of this secondary processing brittleness is important. This is an important issue along with improving deep drawability.

Several methods have been proposed for producing cold-rolled steel sheets with excellent deep drawability. For example, iron and steel (shio1.61°1975
, P2350), C: 0.046%, Mn
: 0.30%P: 0.004%, S: 0
．． 005%, N: 0.0016%, Ti: 0
．． After hot rolling steel with a composition of 21%, the temperature is 600-900℃.
This shows that reheating and annealing in a temperature range of 1 hr coarsens the precipitates and improves the F value and ductility. However, the obtained 1 value and elongation are at most F
= 1.8. E j! -45%, it is hard to say that it satisfies the deep drawability.
In Publication No. 93221, C: 0.0048%, Si
: 0.02%, Mn: 0.17%, P: 0.0
17%, S: 0.010%, N: Q, 004
7%, Ti: 0.024%, Nb: 0.0
25%.

B: By subjecting steel with a composition of 0.0010% to hot rolling-cold rolling and continuous annealing, F=2.3. E
The present invention discloses a method for manufacturing a cold-rolled steel sheet with l=52% and excellent resistance to secondary work brittleness and chemical conversion treatment. However, it cannot be said that the deep drawability is sufficiently satisfied. Further, in Japanese Patent Application Laid-Open No. 63-86818, C: 0.0015%.

Si: 0.01%, Mn: 0.08%,
P: 0.012%, S: 0.005%, N
: 0.0018%, Ti:O. 065%, f
ib: 0.006%.

A steel having a composition of Sb: 0.007% was hot-rolled and then cold-rolled, and then subjected to box annealing to obtain F=2.5. E
Discloses a method for manufacturing a cold-rolled steel sheet that has a heat resistance of -58% and excellent resistance to secondary work brittleness. However, the final annealing must be performed by box annealing, which requires higher costs than continuous annealing, and is also inferior to continuous annealing in terms of uniformity of the material inside the coil.

<Problems to be Solved by the Invention> The present invention solves the above problems and uses a continuous annealing method to
2.4. It is an object of the present invention to provide a method for producing a cold rolled steel sheet having extremely excellent deep drawability with Effi≧55% and excellent resistance to secondary work brittleness.

Means for Solving the Problems> The inventors have completed the present invention as a result of intensive research to solve the above problems.

That is, in the first invention, in terms of weight ratio, c: 0.005% or less, Si: 0.5% or less, Mn
: 1.0% or less, P: 0.025% or less S:
0.02% or less, N: 0.005% or less Aj! :0.
01 to 0.1%, Ti: 0.01 to 0.10%, and the addition amount of Ti in the world of C, N, S is (Ti
/48)> (C/12+N/14+S/32) After hot rolling, the steel is held in a temperature range of 600 to 800°C for 30 min to 30 hr, and then cold rolled and continuously annealed. The second invention is a method for producing a cold-rolled steel sheet for deep drawing with excellent resistance to secondary work brittleness, and the second invention is a method for producing a cold-rolled steel sheet for deep drawing, which has a weight ratio of C: 0.005% or less, Si: 0.5% or less, Mn:
1. 0% or less, P: 0.025% or less S:
0.02% or less, N: 0.005% or less810
01-0.1%, Ti: 0.01-0.10%B
: Contains 0.0001 to 0.0020% and C
, N, and S and the amount of Ti added are (Ti/48)> (
C/120N/14+S/32) After hot rolling, the steel was rolled at a temperature of 600 to 800°C for 30 sin to 3
A third invention is a method for producing a cold-rolled steel sheet for deep drawing with excellent resistance to secondary work brittleness, characterized by performing cold rolling and continuous annealing after holding for 0 hr, and the third invention is a method for producing a cold-rolled steel sheet for deep drawing, which is characterized by performing cold rolling and continuous annealing after being held for 0 hr, and the third invention is a method for producing a cold-rolled steel sheet for deep drawing, which is characterized by performing cold rolling and continuous annealing. 0.005% or less, Si+0.5% or less Mn:
1.0% or less, P i O, 025% or less S:
0.02% or less, N: 0.005% or less Aff
i: 0.01-0.1%, Ti: 0.01-0.1
0% Nb: Contains 0.001 to 0.02%, and the amounts of C, N, and S and the amounts of Ti and Nb added are (Ti/48+Nb/93) > (C/12+
After hot rolling, a steel with a relationship of N/14+S/32) was heated to
A fourth invention is a method for producing a cold-rolled steel sheet for deep drawing with excellent resistance to secondary work brittleness, characterized by performing cold rolling and continuous annealing after holding for hr, and the fourth invention is a method for producing a cold-rolled steel sheet for deep drawing, which is characterized by subjecting it to cold rolling and continuous annealing. .005% or less, Si: 0.5% or less Mn: 1
．． 0% or less, P: 0.025% or less S: 0.0
2% or less, N: 0.005% or less^j! :0.
01-0.1%, Ti: 0.01-0.10%Nb
: 0.001 to 0.02% B: 0.0001 to 0.0020%, and the amounts of C, N, and S and the amounts of Ti and Nb added are (Ti/
48+Nb/93) > (C/12+N/14+ S
/32) After hot rolling, the steel with the relationship: 600 to 800
This is a method for producing a cold-rolled steel sheet for deep drawing with excellent resistance to secondary work brittleness, characterized by performing cold rolling and continuous annealing after holding in a temperature range of 30 min to 30 hr in a temperature range of °C.

Next, the reasons for limiting the steel components in the present invention are as follows.

■ C≦0.005% The smaller the amount of C, the more advantageous it is to improving deep drawability, but if its content is less than 0.01%, it will not have much of a negative effect on deep drawability, so it is limited to 0.01% or less. did.

■ St≦0.5% 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 will have a negative effect on deep drawability, so It was limited to .5% or less.

■ Mn≦1.0% Mn has the effect of strengthening steel, and is added in the required amount depending on the desired strength, but if the amount added exceeds 1.0%, it will have a negative effect on deep drawability. .0% or less.

■ P≦0.025% 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 0.025%, it will adversely affect deep drawability. Furthermore, since it leads to deterioration of resistance to secondary work brittleness, it is limited to 0.025% or less.

(2) S So, 02% The smaller the amount of S, the better the deep drawability is, so it is preferable, but the content is limited to 0.02% or less because it does not have much of an adverse effect if the content is 0.02% or less.

(2) N≦0.005% The smaller the N content, the better the deep drawability is, so it is preferable, but if the content is less than o, oos%, it does not have much of an adverse effect, so it is limited to 0.005% or less.

■Affi: 0.010 to 0.10%^2 is added as necessary to deoxidize and improve the yield of carbonitride-forming elements, but if it is less than o, oto%, the addition effect will be ineffective. On the other hand, even if it is added in excess of 0.10%, no further deoxidizing effect can be obtained, so it is limited to o, oio to 0.10%.

■ Ti: 0.01-0.10% Ti contributes to obtaining excellent deep drawability by precipitating and fixing C, N, S, etc.
By precipitating and fixing P as an iP compound, it is essential to prevent grain boundary segregation of P and improve secondary work brittleness. If the content is less than 0.01%, there is no addition effect; on the other hand, if it is added in excess of 0.10%, no further effect can be obtained, and conversely it leads to deterioration of the surface properties of the steel sheet. It was limited to 0.10%.

■ Nb: o, oot~0.02%Nb contributes to obtaining excellent deep drawability by precipitating and fixing C, N, etc., and also refines the crystal grains of hot rolled sheets and improves cold rolling and annealing. This has the effect of improving the subsequent anisotropy. The amount added is 0.00
If it is less than 1%, there is no effect of addition, while if it is added in excess of 0.02%, the recrystallization temperature will increase and the ductility will decrease, so it was limited to 0.001 to 0.02%.

@B 70.0001 to 0.002% B increases grain boundary strength by segregating at grain boundaries in a solid solution state and contributes to improving secondary work brittleness resistance.

If the amount added is less than 0.0001%, there is no effect of addition,
On the other hand, if it is added in excess of 0.002%, there will be no further effect and, on the contrary, it will lead to deterioration of deep drawability.
It was limited to ~0.002%.

■(Ti/48) > (C/12+N/14+ S/
32) Ti contributes to obtaining excellent deep drawability by precipitating and fixing C, N, and S. The amount added is (Ti/
48) < (C/120N/14+S/32),
Deep drawability is poor because solid solution (C, N) remains in the steel.

On the other hand, (Ti/48)> (C/12÷N/14+S
/32), the solid solution (C, N) in the steel can be precipitated and fixed as Ti carbon and nitride, thereby improving deep drawability. Furthermore, in the present invention, the addition of Ti is a solid solution (C,
This has the effect of not only reducing N) but also preventing grain boundary segregation of P by precipitating and fixing P as a FeTi P compound and improving secondary work brittleness. Therefore, (Ti/48) > (C/
120N/14+S/32) and FeT in the steel.
It is necessary to add excess Ti from which iP compounds can be formed. Furthermore, to further improve the secondary work brittleness resistance (T
i/48) >(C/12+ N/14+ S/32
) + 1/2 (P /31) is preferable.

@ (Ti/48+sb/93)>(C/12+N/
14+S/32) Ti and Nb contribute to obtaining excellent deep drawability by precipitating and fixing C, N, and S.

The amount added is (Ti/48+Nb/93) < (C/
12+N/14+S/32), solid solution (C,
N) remains, resulting in poor deep drawability. On the other hand, (Ti/4
8+Nb/93) > (C/12+N/14÷S/
32), the hardness t8(C,N) in the steel becomes T
Since i and Nb can be precipitated and fixed as carbon and nitride, deep drawability is improved. In addition, in the present invention, the addition of Ti not only reduces the hardness f\i (C, N) in steel, but also replaces P with FeTl.
By precipitating and fixing P as a P compound, it has the effect of preventing grain boundary segregation of P and also improving secondary work brittleness.

Therefore, to improve secondary processing brittleness, (Ti/48+N
b/93)> (C/12+N/14+s/ 32), and it is necessary to add surplus Ti that can form a FeTi P compound in the steel. Furthermore, to further improve secondary work brittleness, (Ti/48+Nb/93) > (C
/120N/14+S/32)+1/2(P/31) is preferable.

Figure 1 shows the effects of (Ti/48+Nb/93) −(C/12
+N/14+S/32), the steel composition is C: 0.002% sl: 0.01%, Mn: 0
．． 10%, PjO, 010%, S: 0.010
%, N: 0.002%, ^10.04%, Ti
n 0.01 to 0.10%, Nb: 0.02% or less or no additive, and the hot rolled sheet was heated at 700°C for 5 hours.
After soaking, the cold rolling reduction was 77% and the plate thickness was 0.8mm.
Continuous annealing at 0° C. for 20 seconds was performed. Further, precipitated Pi was determined by extraction analysis. As is clear from Figure 1, (
Ti/48÷Nb/93) > (C/12+N/14
+s/32), excellent deep drawability is exhibited, and almost all of the P in the steel is precipitated, resulting in excellent secondary work brittleness resistance.

In addition, in the present invention, St+ is 0.0 as another additive element.
This sb addition, which may be added in an amount of 01 to 0.010%, is
Effective in preventing nitriding during heat treatment of hot rolled sheets.

Next, heat treatment of the hot rolled sheet, which is particularly important in the present invention, will be explained.

In conventional manufacturing methods, precipitation occurs during hot rolling and coiling, and precipitates are formed depending on the amounts of C, N, S, Ti, and Nb added. However, its morphology and distribution are
Relatively fine precipitates are densely distributed. This precipitate is coarsely and coarsely distributed, and further P in the steel is reduced to FeTi.
The purpose of the hot-rolled sheet heat treatment is to precipitate and fix as a P compound, and it is necessary to reheat and annealing the hot-rolled sheet in a temperature range of 600 to 800° C. for 30+win to 30 hr.

1 value in Figure 2, Ejl! The w4 components are C: 0.002%, Si: 0.01%,
Mn: 0°11%, P: 0.010%, S
: 0.011%, N: 0.002%, Al4
:0.04%, Tin: 0.052%. The hot-rolled plate annealing time was 5 hr, and the heating and cooling rate was 20 hr.
゛C/hr. It can be seen that excellent deep drawability and secondary work brittleness can be obtained by setting the heat treatment temperature to 600 to 800"C. This is because when the annealing temperature is less than 600"C, This is because the diffusion rate of P is slow, so coarsening of the precipitates is difficult to occur, and furthermore, FeTLP compounds are not formed.On the other hand, when the annealing temperature is 80
When the temperature exceeds 0°C, the FeTi p compound melts, resulting in deterioration in secondary work brittleness, and furthermore, some precipitates begin to melt, resulting in deterioration in deep drawability.

The annealing time was 30 m to ensure sufficient coarsening of the precipitates.
1n or more is required. On the other hand, if it is carried out for more than 30 hours, there is no further effect, and it causes an increase in cost and a decrease in productivity, so it was set to be less than 30 hours. In the present invention, the hot rolled sheet annealing may be a heating or cooling process as long as it is 600°C or higher, and the residence time in the temperature range of 600 to 800°C may be 30 m1n or more.

In cold rolling, the reduction rate is 50 to ensure deep drawability.
It is desirable that it is ~95%, more preferably 65%.
~85% is a preferred range. Further, for continuous annealing, an annealing temperature of 700 to 950°C and a heating rate of 1 to 50°C/S are suitable.

Note that the steel plate after annealing can be subjected to 0.2 to 5% lil rolling.

<Example> Examples of the present invention will be described below.

Steel slabs with compositions shown in Table 1 were manufactured by continuous casting in a converter furnace, and then hot rolled. Slab heating temperature is 1250℃
1 Hot rolling finishing temperature is 880℃, coil winding temperature is 40℃
The temperature is 0 to 700°C. Subsequently, the hot rolled coils were batch annealed under the conditions shown in Table 2. Heating and cooling rates are 20°C/hr. After pickling the hot-rolled steel plate, it was cold-rolled at a rolling reduction of 77% to a thickness of 0.8, and then heated at 850℃-20
Continuous annealing of s was performed.

Table 2 shows the material properties of cold rolled steel sheets.The tensile properties are JI3
Measurement was performed using a No. 5 tensile test piece. Also, the r value is 15%
After applying tensile prestrain, measurement was performed using the three-point method, and the L direction (
rolling direction), D direction (45° direction to rolling direction) and C
Average value and anisotropy in the direction (90” direction in the rolling direction) F−(rL + 2 ra + re )/4Δr−(
It was determined as rL 2 re +rc)/2. In addition, to evaluate the resistance to secondary work brittleness, a cylindrical sample processed at a critical drawing ratio of 3.8 was cooled and then subjected to a pressure immersion test, and evaluated at the temperature at which knee cracking occurred (brittle transition temperature). did.

If the brittle transition temperature is 160° C. or lower, there is no problem with respect to secondary work brittleness.

It can be seen that the cold rolled steel sheet manufactured within the scope of the present invention has superior deep drawability and secondary work brittleness compared to the comparative example.

<Effects of the Invention> The present invention makes it possible to manufacture cold-rolled steel sheets with extremely excellent deep drawability and resistance to secondary work brittleness, and also enables press forming of parts that were conventionally impossible to press. The problem of secondary processing brittleness, which had been a problem, can also be solved.

[Brief explanation of the drawings] Figure 1 shows the i value, Ej! W, brittle transition temperature and precipitated Pg
A graph showing the influence of steel components on the f-value,
It is a graph showing the influence of hot-rolled sheet heat treatment temperature on Ei value, brittle transition temperature, and amount of precipitated P.

Claims (4)

[Claims] (1) Weight ratio: C: 0.005% or less, Si: 0.5% or less Mn: 1.
0% or less, P: 0.025% or less, S: 0.02% or less,
N: 0.005% or less Al: 0.01-0.1%, Ti
:0.01 to 0.10%, and the amounts of C, N, and S and the amount of Ti added are (Ti/48)>(C/12+N/
14+S/32) after hot rolling, 600
After being maintained at a temperature range of ~800°C for 30 min ~ 30 hr,
A method for producing a cold-rolled steel sheet for deep drawing that has excellent resistance to secondary work brittleness, which comprises performing cold rolling and continuous annealing. (2) Weight ratio: C: 0.005% or less, Si: 0.5% or less Mn: 1.
0% or less, P: 0.025% or less, S: 0.02% or less,
N: 0.005% or less Al: 0.01-0.1%, Ti
:0.01~0.10%B:0.0001~0.002
0%, and the amounts of C, N, S and added amount of Ti are (T
i/48)>(C/12+N/14+S/32) After hot rolling, the steel is held in a temperature range of 600 to 800°C for 30 min to 30 hr, and then cold rolled and continuously annealed. A method for producing cold-rolled steel sheets for deep drawing, which is characterized by its excellent resistance to secondary work brittleness. (3) Weight ratio: C: 0.005% or less, Si: 0.5% or less Mn: 1.
0% or less, P: 0.025% or less, S: 0.02% or less,
N: 0.005% or less Al: 0.01-0.1%, Ti
: 0.01 to 0.10% Nb: 0.001 to 0.02%, and the amounts of C, N, and S and the amounts of Ti and Nb added are (Ti/48+Nb/93)>(C /12+N/14
+S/32) After hot rolling, steel with the relationship 600~8
A method for producing a cold-rolled steel sheet for deep drawing with excellent resistance to secondary work brittleness, which comprises holding the steel sheet in a temperature range of 00° C. for 30 min to 30 hr, followed by cold rolling and continuous annealing. (4) Weight ratio: C: 0.005% or less, Si: 0.5% or less Mn: 1.
0% or less, P: 0.025% or less, S: 0.02% or less,
N: 0.005% or less Al: 0.01-0.1%, Ti
: 0.01~0.10% Nb: 0.001~0.02% B: 0.0001~0.0020%, and the amounts of C, N, and S and the amounts of Ti and Nb added are (Ti/48+Nb/93)>(C/12+N/14
+S/32) After hot rolling, steel with the relationship 600~8
A method for producing a cold-rolled steel sheet for deep drawing with excellent resistance to secondary work brittleness, which comprises holding the steel sheet in a temperature range of 00° C. for 30 min to 30 hr, followed by cold rolling and continuous annealing.