JPS6119739A - Preparation of high tensile steel plate having good drawing property by continuous annealing - Google Patents

Abstract

PURPOSE:To prepare a high tensile steel plate having a good drawing property, by limiting the composition of steel consisting of C, Si, Mn, P, Al, N and Fe, the wind-up temp. in hot rolling and the draft in cold rolling while controlling continuous annealing in connection with the content of C and P. CONSTITUTION:A steel slab containing 0.010-0.040% C, 1.0% or less Si, 1.0% or less Mn, 0.030-0.15% P, 0.01-0.10% Al and 0.0060% or less N on a wt. basis and comprising the remainder of Fe and inevitable impurities is hot rolled and wound up at a temp. range of 600-730 deg.C. Subsequently, the hot rolled coil is subjected to cold rolling at a draft of 50% or more while the rolled coil is continuously annealed at holding temp. T within a range shown by formula I (wherein [C] is % of C and [P] is % of P). After the above mentioned annealing, the annealed steel coil is gradually cooled to a quenching start temp. TQ represented by formula II at a cooling speed of 0.5-5 deg.C/sec and subsequently quenched from the temp. TQ to a temp. region of 300-500 deg.C at a cooling speed of 30-300 deg.C/ sec and further subjected to over ageing treatment for 10sec or more in a temp. region of 300-500 deg.C to inexpensively obtain the titled high tensile steel plate.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing a high tensile strength steel plate with good drawing properties by continuous annealing.

Recently, there has been a marked increase in the use of high-strength steel sheets for the purpose of improving vehicle safety and reducing vehicle weight. In particular, for parts that require severe deep drawing, such as fenders, soft steel plates with a tensile strength of TS = 30 to 35 mm/-2s have been used, but recently these parts have been made with higher strength. As progress has been made, high-strength steel plates for drawing p of the TS = 35 to 45 kf/- class are being used, and the present invention is utilized for manufacturing high-strength steel plates for drawing.

[Conventional technology]

The above-mentioned high-strength steel is a low-carbon Al-killed steel with Pi as an essential strengthening element and Mn as necessary.

S], etc., and is manufactured by the box annealing method.

The reason why the continuous annealing method is not used in manufacturing is that in the box annealing method, high strength is achieved while improving the drawability by adding P, but in the continuous annealing method, which has good production efficiency, the addition of P deteriorates the drawability. This is because there is a strong tendency to cause the steel sheet to oxidize, and this has been the main reason why the inexpensive high-strength steel sheet for drawing with Pk added cannot be manufactured using the fully continuous annealing method.

As a manufacturing technology for high-strength steel sheets for deep drawing by continuous annealing using Pi-added steel, for example, Japanese Patent Publication No. 55-514
10 and "Tetsu to Hagane" No. 68 + 1982) No. 9, pages 1355 to 1361 are known. The former uses Pk-added steel sheet A c 1 transformation point ~ 9
After heating to 00℃, it is rapidly cooled from the same temperature in jet water and then heated to 2
Kongo is a continuous annealing cycle in which tempering is performed at 00 to 500°C, but with this method, the Lankford value T value, which is a measure of drawability, is only about 1.3 at most, and when using the box annealing method, It was difficult to obtain a value of 7, 1, 5 or higher.

On the other hand, the latter indicates that it is necessary to add t-addition, which is expensive, in order to achieve a high 7 value in continuous annealing using P-added steel.

[Problem that the invention seeks to solve]

An object of the present invention is to solve the problems of the prior art's r value and the use of expensive elements, and to provide a complete method for manufacturing a high-strength steel sheet with good drawability through continuous annealing.

[Means and effects for solving the problem] The inventors of the present invention have found that even if inexpensive elements, mainly Pk, are used in the continuous annealing method, the heating holding temperature of the continuous annealing can be adjusted by changing the heating temperature of the steel, especially C, By controlling the amount of P, the γ transformation rate during heating and holding is adjusted to an appropriate level, and then by controlling cooling to the cooling start temperature controlled by the amount of C9P, it is possible to improve the drawability of high-strength steel sheets, that is, to increase the number of 7 values. We found seven things that could be accomplished and were able to complete the present invention.

The gist of the present invention is as follows.

That is, in terms of weight ratio, C: 0.010 to 0.040%, Si: 1.0% or less, Mn + 1. Otl) Below,
P: 0,030~0,15chi, A/-:0.01~0.
10chi, N: 0. All units remaining below OO60% are F
a step of hot-rolling a steel slab comprising e and unavoidable impurities at a temperature range of 600 to 730°C; a step of cold-rolling the coiled hot-rolled coil at a total reduction rate of 50% or more; A step of continuously annealing the rolled coil at a heating and holding temperature T in the temperature range shown by the following formula (1), and after the annealing, the following (
2) 0.5-b to the quenching start temperature TQ shown by the formula 30-b from the quenching start temperature TQ after the slow cooling to a temperature range of 300 to 500°C The quenching stage and the temperature of 300 to 500°C after the quenching A method for producing a high-strength steel sheet with good drawability by continuous annealing, comprising the step of performing an over-aging treatment for 10 seconds or more in a continuous annealing process.

1.2oxlO'[C)"-6,88x103[C)+
550CP: ]+856≦T (C)≦1.73 X
10'CC)"-8,25X 10"CC':J+
500CPE+960...(1)670≦TQ(℃)
≦-t, moxto"[c)-+-6so[P)+77
4...(2) However, (:C): Weight % of C CP): Weight % of P The reason for limiting the components of the present invention will be explained.

C: C is effective in increasing strength, but by 0.040%
If it exceeds k, there will be a significant deterioration of the drawability, that is, a decrease in the lower value, so the upper limit k is set at 0.040%, and the deterioration rate of -7,000 is set as 0.040%.
Although it is advantageous for increasing the value, if it is less than 0.010S, solid solution C remains even if all the overaging treatment during continuous annealing is performed, and the aging property is poor, so the lower limit is 0.010S.

3i, Mn: 83, Mn are all important elements for increasing overall strength. However, due to P, the target tensile strength TS = 35
It is not necessary if ~45kF/- can be obtained, and 1
If the content is too large, the 7 value will deteriorate, so both were limited to 1.0% or less.

P: P is an element that can increase the strength without causing a total change in seven values when combined with the heat cycle of continuous annealing. To exhibit this effect, a content of 0.030% or more is required; on the other hand, a content exceeding 0.15% makes the steel brittle and particularly deteriorates weldability, so the content should be in the range of 0.030 to 0.15%. Limited.

Incidentally, P and C have an important influence on the overall control of the heat cycle of continuous annealing as described later.

Al: Al fixes Nk, which is harmful to the aging properties of steel plates.
0.01% or more is required, but if it exceeds 0.10%, the N fixed effect will become saturated and become a factor of cost increase.
．． The range was limited to 0.01 to 0.1o.

N: N is 0.0060 because if it exceeds 60% k, it will dissolve into the steel and completely deteriorate the aging properties of the steel, but it will completely inhibit the grain growth during continuous annealing and cause the seven-value ratio to deteriorate. Limited to less than H.

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

First, regarding the rolling conditions, the winding temperature of hot rolling must be 600° C. or higher in order to achieve a high seven-value steel sheet. But 7
If the temperature exceeds 30'C'41m, the pickling properties after hot rolling will deteriorate significantly, so the hot rolling winding temperature is limited to a range of 600 to 730°C [7]. Further, the cold rolling reduction ratio in cold rolling is required to be 50% or more in order to achieve a high T value.

Additionally, continuous annealing is an important requirement in the present invention, and the present invention improves the drawability of the steel sheet by limiting the heating holding temperature T and the rapid cooling start temperature TQ after slow cooling according to the amounts of C and P. It is based on a completely new idea that has never existed before.

That is, the heating holding temperature T is the content of C and P [C], [
It is necessary to control according to the following equation (1) according to P'3%.

1.20X10'[C]"-6,88X10"[C]+
550 CP:]+856≦T(℃)≦1.73x 1
0'[C:]"-&25x10'CC]+500[P:
l+960-(1) Equation (1) means that the total γ transformation rate of the steel is controlled within the range of 10 to 90% at the end of heating and holding, and the subsequent K O,5 to b. As a result of repeated experiments using a large number of steels, the present inventors found that the γ transformation rate of steel differs greatly depending on the C and P contents, and that in order to achieve a high 7 value by subsequent slow cooling, it is necessary to heat At the end of retention, the γ transformation rate is 1
It has been found that 0 to 90% gold is required. In addition, in order to obtain a γ transformation rate of 10% or more at the end of heating and holding, (1
) The temperature must be higher than that shown on the left side of the equation, and the temperature must be higher than 90°C.
It has been found that in order to obtain a γ transformation rate of less than 1, it is necessary to maintain the temperature at or below the right-hand side of equation (1).

(1) When the temperature is lower than the left side of equation (1), that is, when the γ transformation rate is less than 1°, not only the development of (11.11 million) during heating and holding is weak, but also the development due to slow cooling is small. The γ transformation rate exceeds 90'Xf, and random orientation develops during heating and holding, both of which are disadvantageous for drawability.

Next, the slow cooling rate to the rapid cooling start temperature TQ is 0.5~
b The (111) texture is quenched starting temperature T, 5℃/
By performing slow cooling for less than seconds, γ→α transformation (1
11) Preferential growth of oriented crystal grains is completely suppressed, generation of random α grains is completely suppressed, and drawability is improved. This effect is more effective as the annealing rate is made slower, but slowing down requires either extending the line length of the continuous annealing furnace or slowing down the threading speed, which is disadvantageous in terms of production costs. The lower limit of speed is limited to 0.5°C/sec.

Further, the quenching start temperature TQ was limited to the range expressed by the following equation (2).

670≦TQ(℃)≦-1,80X10”[:C]+6
80[P]+774...(2) Rapid cooling start temperature T
The lower limit of Q is set in order to fully achieve the effect of reducing solid solution (l), which is disadvantageous to aging properties, during the subsequent overaging treatment, that is, to sufficiently achieve the supersaturation degree of solid solution C by rapid cooling from T67
A temperature of 0°C or higher is required. On the other hand, the upper limit of TQ is the (2
) It is necessary to limit the temperature to the temperature shown on the right side of the equation.

During heating and holding, the
However, the TQ, that is, the temperature at which slow cooling is terminated, is preferably a temperature below the temperature at which the preferential growth of the (111) orientation due to the γ→α transformation terminates, that is, the temperature on the right side of equation (2).

Next, with regard to rapid cooling and overaging, the rapid cooling rate from the rapid cooling start temperature TQ is important in order to complete the precipitation treatment of solid solution C in the subsequent overaging treatment process, but it is important to achieve high tensile strength, which is the purpose of the present invention. There is no direct relationship with improving the drawability of steel sheets. However, the drawing steel plate must also have good aging properties. In order to precipitate solid solution C in a short time during the overaging process, a rapid cooling rate of 30° C./second or more is required.

Aging properties will improve if the rapid cooling rate is increased to 300℃.
If the speed exceeds 00° C./sec, fine carbides (FesC) will precipitate, and the ductility will deteriorate due to precipitation strengthening, so the upper limit was limited to 00° C./sec.

In addition, the overaging treatment temperature after quenching was limited to 10 seconds or more in the temperature range of 300 to 500℃.
When the temperature exceeds 0°C or lower than 300°C, a large amount of solid solute C remains even after a short overaging temperature, which significantly deteriorates not only the aging property but also the ductility. Further, holding the temperature in this temperature range for less than 10 seconds is not sufficient time to reduce the solid solution C during overaging treatment.

After rapid cooling to t-300 to 500°C, the over-aging treatment as described above was carried out at that temperature for 10 seconds or more, thereby reducing solid solution C and completely improving aging properties.

〔Example〕

Using the steel slab shown in Table 1 that was tapped in a converter and continuously cast, it was hot rolled at 880-910°C and finished at 670°C.
I wound it up. After pickling, it was finished into a cold rolled sheet of 0.8 m with a cold rolling reduction of 75% and subjected to continuous annealing. Continuous annealing conditions were as shown in Table 2 for heating holding temperature T1 slow cooling rate V and rapid cooling start temperature TQ, and other conditions were the same. That is, heating rate 5~b quenching cooling rate Fi from quenching start temperature TQ to 380°C
The overaging treatment was carried out at 80° C./second for 120 seconds, etc. at 380° C. After continuous annealing, all 1.0 inch pieces were subjected to temper rolling, and their mechanical properties were investigated and are shown in Table 2.

In addition, in Tables 1 and 2, items that do not satisfy all of the limiting conditions of the present invention are underlined.

From Table 2, the examples of the present invention have extremely good drawing properties, and if the yield point elongation after being kept at 30°C for one month is 1 inch or less, the aging property is said to be good, and the strength-elongation balance is excellent. It can be seen that it has excellent mechanical properties that are equivalent to or better than box-annealed materials.

〔Effect of the invention〕

As is clear from the above examples, the present invention completely limits the composition, hot rolling coiling temperature, and cold rolling reduction ratio, continuous annealing,
By controlling the P content, it is possible to produce high-strength steel sheet metal with good drawability through continuous annealing without using expensive elements.

Claims (1)

[Claims] (1) Weight ratio: C: 0.010-0.040%, S
i: 1.0% or less, Mn: 1.0% or less, P: 0.03
0-0.15%, Al: 0.01-0.10%, N: 0
．． 0060% or less and the balance is Fe and unavoidable impurities. After hot rolling, the steel slab is coiled at a temperature range of 600 to 730°C, and the coiled hot rolled coil is cold rolled at a reduction rate of 50% or more. a step of rolling, a step of continuously annealing the cold-rolled coil at a heating holding temperature T in a temperature range shown by the following equation (1), and a step of continuously annealing the cold-rolled coil at a heating holding temperature T in a temperature range shown by the following equation (2), and a step of 0.5 to a rapid cooling start temperature T_Q shown by the following equation (2) after the annealing. A step of slow cooling at a cooling rate of ~5° C./sec, and a step of slow cooling at a cooling rate of 300° C. from the rapid cooling start temperature T_Q after slow cooling.
The process comprises the steps of: rapidly cooling to a temperature range of ~500°C at a cooling rate of 30 to 300°C/sec; and performing overaging treatment for 10 seconds or more in a temperature range of 300 to 500°C after the rapid cooling. A method for producing a high-strength steel sheet with good drawability by continuous annealing, characterized by: 1.20 x 10^5 [C]^2-6.88 x 10^3 [
C]+550[P]+856≦T(℃)≦1.73×1
0^5 [C]^2-8.25×10^3 [C]+500
[P] +960...(1) 670≦T_Q(℃)≦-
1.80×10^3 [C] + 680 [P] + 774...
・(2) However, [C]: Weight % of C [P]: Weight % of P