JPH0452230A - Production of cold rolled high tensile strength steel sheet for working - Google Patents

Abstract

PURPOSE:To produce a cold rolled high tensile strength steel sheet excellent in workability by using a low oxygen and dead-soft carbon steel containing large amounts of P and also specifying strain-giving conditions at the time of cold rolling. CONSTITUTION:A steel which has a composition consisting of, by weight, <0.02% C, <1.0% Si, <2.0% Mn, 0.01-0.10% Ti, 0.0010-0.0100% Nb, 0.0002-0.0020% B, 0.03-0.20% P, <0.03% S, 0.010-0.100% Al, <0.008% N, <0.0045% O, and the balance iron with inevitable impurities and further satisfying Ti>(48/12)Cwt.%+(48/14)Nwt.% is cast, hot-rolled, and then subjected to cold rolling where sheet temp. is regulated to <=300 deg.C and the relationship between the sheet temp. T( deg.C) and the strain rate epsilon'(s<-1>) satisfies TXepsilon<=50,000 deg.Cs<-1> and, further, the sum of reductions of area at the time of respective reductions is regulated to >=50%, followed by continuous annealing.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing high-strength cold-rolled steel sheets for processing, which is suitable for press forming, stretch forming, etc. for outer panels, and is also applicable to deep drawing. It is something.

In recent years, particularly in cold-rolled steel sheets for automobiles, the demand for high-strength steel sheets has increased significantly in order to reduce the weight of automobiles to reduce fuel consumption and to protect the safety of passengers.

Since recent high-strength cold-rolled steel sheets are used not only for the inner panels of automobiles but also for the outer panels of hoods, trunks, fenders, etc., excellent workability is required.

(Prior art) In order to improve the workability of cold-rolled steel sheets, technologies have been proposed that take measures such as reducing the amount of carbon and adding carbonitride-forming elements. Publication No. 317648 discloses that Ti and Nb are added to ultra-low carbon steel.

A cold-rolled steel sheet containing B and having excellent workability has been proposed and disclosed.

Furthermore, proposals have been made to increase the amount of reinforcing elements such as P and Mn in the above-mentioned composition system, and Japanese Patent Publication No. 11294/1983 describes a steel with increased amounts of P and Japanese Patent Publication No. 1-28817 discloses a steel with increased amounts of P and Mn. A method for manufacturing a high-strength cold-rolled steel sheet with excellent workability is disclosed, in which continuous annealing is performed after cold rolling.

However, even with these techniques, deterioration in workability due to increased strength is still unavoidable, and further increases in strength and workability are required.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for manufacturing a high-strength cold-rolled steel sheet for processing that uses ultra-low carbon steel and has better workability.

(Means for Solving the Problems) The present invention has the following features: C: 0.02 wt% or less i: 1.0 wt% or less n: 2. Oht%t% or less i: 0.01 wt%t% or less, 10 wt%t
% or less, and further Ti>(48/12) Cwt%+(4B/14) N
Nb: 0.0010 st%t% or less, 0100
wt% t% or less: 0.0002% 1t% or more, 0.
0020-t% or less, P: 0.03 wt% or more,
0.20 wt%t% or less: 0.03 wt%t%
Below I! : 0.010 wt%t% or less, 100w
t% t% or less: 0.008% 1t% or less and 0
: 0.0045 wt% t% or less, the balance consists of iron and unavoidable impurities, and after casting and hot rolling, the plate temperature is 300°C or less, and the plate temperature T (°C) and the strain rate i in rolling are The relationship between (s-1) is T×■≧50,000°C s −1 The sum of the rolling reductions of each rolling is 50% or more, and then continuous annealing is performed. This is a method for manufacturing high-strength cold-rolled steel sheets for processing.

Here, the sheet temperature T (° C.) is determined by measuring the temperature of the steel sheet immediately adjacent to the exit side of each cold rolling stand using an infrared temperature needle, and this value is used, and the strain rate ε during rolling is calculated using the following formula.

Here, n: Roll circumferential speed (RP haze) H0 Thickness on the two-man side (Kaku) T: Reduction ratio R: Low J radius (-) (Function) This invention is for processing high-tensile cold-rolled steel sheets for processing. As a result of repeated research on improving properties, we have found that ultra-low carbon steels containing a large amount of P and reduced oxygen have excellent processing properties by devising strain imparting conditions during cold rolling. It was discovered that high tensile strength cold rolled steel sheets can be obtained.

Based on the above research results, we will first discuss the reason for limiting the sum of the rolling reductions of each rolling where the product of plate temperature and strain rate is 50,000° C. s −1 or more to be 50% or more.

First, A, which is melted in a converter and has the component composition shown in Table 1,
Using continuous cast slabs of B and C3 steel, the continuous cast slabs were
Heating to 50°C, rough rolling with a reduction rate of 88%, rolling reduction rate of 88%
% hot finish rolling (hot rolling end temperature: 880°C, coil winding temperature: 500°C) to form a hot coil with a thickness of 4.0 mm, and then cold rolling at a reduction rate of 82.5%. After making the plate thickness 0.7 m, it was continuously annealed at a temperature of 810°C and subjected to skin pass rolling with a rolling reduction of 0.8% to obtain a rolled plate.

Here, in cold rolling, the plate temperature is within the range of 30°C to 300°C, and the rolling rate, that is, the strain rate ε, is 108-
The range was varied from 1 to 2,0 OOs-'.

In addition, the sheet temperature was adjusted by the sheet temperature at the start of cold rolling and the amount of cooling water.

The cold-rolled sheet thus obtained was measured for its lower value, elongation, tensile strength, and truncated cone forming height using a truncated cone forming test to find workability closer to actual forming.

Note that the truncated cone forming test conditions are as follows.

Q punch diameter = 8011Ilφ ・Dice diameter: 140 mmφ This shows the relationship between the sum of the rolling reduction rates of each rolling reduction.

As is clear from this figure, low acid material A with a high P content
The steel has a tensile strength higher than B steel with a low P content, and the product of plate temperature and strain rate is 50,000℃S-1 or more.If the sum of the rolling reductions of each rolling is 50% or more, , elongation, and lower values only increased slightly, but the truncated cone forming height, that is, the actual forming processability, was significantly improved and reached a value comparable to that of Steel B, which has low tensile strength.

In addition, in C steel which has a large P content but a large oxygen content,
No significant improvement in processability is observed.

Therefore, in manufacturing high-strength cold-rolled steel sheets with excellent workability, a low-acid material with a high P content is used.
It is necessary that the sum of the reduction rates of each reduction, which is 0,000°C S -j or more, be 50% or more.

Here, in the conventional cold rolling of cold rolled steel sheets, the sum of the rolling reductions of each rolling where the product of plate temperature and strain rate is so, ooo°Cs -j or more is usually 30%. In order to make the sum of this reduction ratio 50% or more, it is necessary to raise the sheet temperature at the start of rolling by increasing the rolling speed, controlling the amount of cooling water in the line, continuing the process from the previous process (usually the pickling line), etc.
Such measures are required.

In this way, by using a low acid material with a high P content and specifying the strain imparting conditions during cold rolling, excellent workability can be obtained, but the reason for this is not clear.

However, in general, when the P content is high, a unique segregation band is seen in the central part of the sheet in the thickness direction when microscopically observed, but most of the segregation bands are not seen in the steel sheet manufactured by the method of this invention. This shows that the cold rolling conditions of this invention gave some changes to the segregation zones that are different from those of the conventional method.Secondly, according to the cold rolling conditions of this invention, the plate thickness This may be due to the fact that the segregation zone was subjected to a larger rolling process due to more uniform working in the direction.

Although these segregation bands do not have much of a negative effect on the elongation or lower value in the tensile test, they cause material non-uniformity in the thickness direction, which may lead to deterioration of workability in actual forming.

Therefore, it is believed that according to the method of this invention, the uniformity in the thickness direction of the plate was improved by favorable processing of the segregation bands through cold rolling, resulting in improved formability under conditions close to actual conditions. It will be done.

Furthermore, when a large amount of 0 exists, a large amount of inclusions becomes an obstacle, and the amount of cold rolling to the segregation zone decreases, making it impossible to obtain the above-mentioned effect of improving workability.

Next, the reason for limiting the range of chemical components in the steel composition of this invention will be described.

C: A reinforcing element, and is particularly effective as a reinforcing element that does not deteriorate workability when added together with Ti. However, if it exceeds 0.02 wt%, good workability cannot be obtained even if the amount of Ti added is increased. Therefore, the amount of C is 0
．． The upper limit is 0.02 wt%. Note that in order to obtain better workability, the content is preferably 0.006 wt% or less.

St: If more than 1.0 wt%, elongation and drawability deteriorate, so the upper limit is set at 1.0 wt%.

Mn is effective in increasing strength without deteriorating drawability, but excessive addition deteriorates elongation and drawability.
．． The upper limit is 0wt%.

Ti: An important element that fixes C and N in steel and prevents material deterioration due to solid solute C, and also plays a role in inhibiting the formation of BN and preventing a decrease in effective solid solute B. therefore,
C equivalent: (48/12) Cwt% and N equivalent: (4
8/14) It is necessary to add more than the sum of Nwt%.

On the other hand, if it is less than 0.01 wt%, the distribution in the steel becomes too thin and the effect cannot be exhibited, and if it is added in excess of 0.10 wt%, the strength will decrease. Therefore, 0.01wt% or more and 0.10wt% or less, and (
48/12) C%1t%+(48/14) Nwt
% or more.

Nb: An essential element for improving the lower value and increasing strength by co-adding with B, but at 0.0010wt%
If it is less than 0.0100 wt %, the effect will be weak, and if it is added in excess of 0.0100 wt %, workability will decrease and the balance between strength and workability will deteriorate. Therefore, the content should be O,0O10i1t% or more and 0.0100wt% or less, but considering the use of deep drawing, it is preferably 0.0075wt% or less.

B: It has secondary processing brittleness resistance and is an essential element to increase strength by co-adding with Nb, but 0,0002
If it is less than 0.002 wt%, the effect is poor, and
If added in excess of 0 wt%, material deterioration will be significant. Therefore, 0.0002wt% or more, 0.0020wt
% or less, preferably 0,0012 wt% or less.

P: An important strengthening element, its effect is 0.03et%
The above is remarkable. However, if it exceeds 0.20 eyt%, the balance between strength and workability deteriorates, and the adverse effect on brittleness cannot be ignored. Therefore, 0.035wt
% or more and 0.20 wt% or less. A more preferable range is 0.04 wt% or more and 0.15 wt% or less.

S: Reducing the amount of S in steel is necessary to improve deep drawability, but if the content is 0.03wt% or less, it will not have a significant adverse effect on workability, so 0.03w
The upper limit is t%.

AQ: It is necessary to deoxidize and improve the yield of carbonitride-forming elements, as well as to avoid surface flaws on the steel sheet due to the formation of Ti01, but if it is less than 0.010 wt%, it has no effect. Moreover, even if it is added in an amount exceeding 0.10 wt%, further deoxidizing effect cannot be obtained, and moreover, A12
0.01 because surface flaws on the steel plate caused by zOz become a problem.
It should be at least 0.10 t% by weight.

N: In addition to deteriorating deep drawability, if it is not fixed with Ti, it combines with B and causes a decrease in secondary work brittleness, so the larger the amount, the more Ti is required, which is uneconomical. Therefore, the upper limit is set at 0.008 wt%, but preferably 0.006 wt% or less.

0: As described above, the content must be reduced in order to improve processability, which is a requirement of this invention. When the content exceeds 0.0045 wt%, as mentioned above, a large amount of inclusions becomes an obstacle and the amount of cold rolling to the segregation zone decreases. In addition to deteriorating, the effect on brittleness cannot be ignored. Therefore, the upper limit is set to 0.0045 wt%.

Next, conditions and desirable conditions for producing a material and a steel plate having the composition of the present invention will be described.

First, steel manufacturing and hot rolling may be carried out according to conventional methods, and the present invention does not require any limitations on the conditions.

The coiling temperature during hot rolling is usually around 400°C to 700°C, which is sufficient to obtain a material of excellent quality.
High temperature winding is not required. Rather, in order to reduce costs and prevent softening, winding at a low temperature of 550°C or less is desirable.

Regarding cold rolling, the above-mentioned conditions of the present invention must be met, that is, the product of plate temperature and strain rate is 50,000'' (S-1 or more, and the sum of the rolling reductions of each rolling reduction is 50% or more. For example, there is no particular restriction on the total rolling reduction, and it may be within the range of common sense.

Regarding sheet temperature, as the rolling temperature increases, shear deformation concentrates on the steel sheet surface, making it difficult to process the center segregation area.
Although the temperature limit is up to 00°C, it is preferably 150°C or less considering the burden of overheating on cold rolling equipment.

In annealing after cold rolling, if a box annealing method is used, the component composition of this invention tends to cause softening, so a continuous annealing method is used, but the annealing temperature of this continuous annealing is not the usual recrystallization temperature T. The maximum heating temperature is preferably τ* + 100° in order to minimize softening.
It is desirable to set it to C.

The temper rolling after annealing may be carried out within a common sense range (approximately % of the plate thickness (arin)) for the purpose of straightening the plate shape or the like.

(Example) Compatible Example 7 steel type of this invention and Comparative Example 3ti4, which were melted in a converter and cast by continuous casting, and had the chemical components shown in Table 2.
Continuously cast slabs of 10 steel types were hot-rolled to a thickness of 3.
After forming a hot coil of 0+c++, it was cold rolled to a plate thickness of 0.72wa+, subjected to continuous annealing, and then subjected to temper rolling of 0.7% except for the steel type Nα3 to obtain a cold rolled coil.

The diameter of the roll used for cold rolling was 600III.
+, Rolling speed is 1500-2500 at the exit side of the final stand
m/sin.

Among the above IO steel types, steel types kl and nu 2 were heated to N113 under three conditions each with different cold rolling conditions and continuous annealing conditions.
were manufactured under two conditions, and other steel types were manufactured under one condition each.

The hot rolling conditions and continuous annealing conditions are shown in Table 3, the cold rolling conditions are shown in Table 4, and the results of the material quality investigation of the cold rolled steel sheet thus obtained are shown in Table 5.

From Table 6, sample 111cL2. 5. 6. 9.13
．． Comparative Examples No. 14 and No. 15 show values of truncated cone molding height from 2 mm to 35 mm, which give properties close to actual workability, whereas examples adapted to the present invention show values from 45 mm to 55 mm. This shows that the actual processability is excellent.

In addition, the sample! 1k13 is a suitable example in which continuous annealing was replaced with alloyed hot-dip galvanizing treatment, and this alloyed hot-dip galvanized steel sheet also exhibits excellent workability similar to that of cold-rolled steel sheets.

Also, even if the product of plate temperature and strain rate is 50.000℃s −
Even if the sum of the reduction ratios of each reduction of 7 or more is 50% or more,
When the plate temperature exceeds 300° C., the height of the truncated cone molding is as low as 20 m, as in sample magnet 6, and the workability is not improved.

(Effects of the Invention) This invention uses a low oxygen, ultra-low carbon steel containing a large amount of P, and by specifying strain imparting conditions during cold rolling,
A method for manufacturing high-strength cold-rolled steel sheets with excellent workability has been established, and cold-rolled steel sheets manufactured by the method of this invention are suitable for use in processing such as press forming, stretch forming, and deep drawing. .

[Brief explanation of the drawings] Figure 1 shows the properties of cold rolled steel sheets such as T value, elongation, tensile strength, and truncated cone forming height, and the product of sheet temperature and strain rate during cold rolling, so, oooCs. It is a graph showing the relationship between the sum of the rolling reduction rates of each rolling reduction and the sum of the rolling reduction ratios.
tt34jinoiiai/)/ishito4sheenh(i≦,)
Item 1 of the procedural amendment, page 5, line 11 of the specification is corrected as follows. July 1990 Mr. Satoshi Uematsu, Commissioner of the Japan Patent Office■, Display of the case Patent application No. 159856 of 1990 2. Name of the invention Method for manufacturing high-strength cold-rolled steel sheets for processing 3. Relationship with the amended person case Patent application Person (125) Kawasaki Steel Co., Ltd. 4, agent month 210,000 days "Detailed description of invention" column of statement

Claims (1)

[Claims] 1. C: 0.02 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, Ti: 0.01 wt% or more and 0.10 wt% or less,
Furthermore, Ti>(48/12)Cwt%+(48/14)Nwt
%, and Nb: 0.0010 wt% or more and 0.0100 wt% or less, B: 0.0002 wt% or more, 0.0020 wt%
Hereinafter, P: 0.03 wt% or more, 0.20 wt% or less,
S: 0.03 wt% or less, Al: 0.010 wt% or more, 0.100 wt% or less,
N: 0.008 wt% or less, O: 0.0045 wt% or less, and after casting and hot rolling, the steel consisting of iron and unavoidable impurities has a plate temperature of 300°C or less, and The relationship between the temperature T (℃) and the strain rate ■ (s^-^1) in rolling satisfies T A method for producing a high-strength cold-rolled steel sheet for processing, which comprises cold rolling and then continuous annealing.