CA1142069A

CA1142069A - Process for producing two-phase high tension cold rolled steel sheet having improved workability

Info

Publication number: CA1142069A
Application number: CA000365677A
Authority: CA
Inventors: Osamu Akisue; Teruaki Yamada
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1979-11-27
Filing date: 1980-11-27
Publication date: 1983-03-01
Also published as: IT1134491B; FR2470163B1; JPS5677329A; SE8008247L; SE441278B; NL184790C; FR2470163A1; NL184790B; DE3044339A1; GB2066852B; GB2066852A; DE3044339C2; NL8006404A; BE886350A; BR8007714A; SE441278C; IT8026281A0

Abstract

ABSTRACT

A two-phase high tension cold rolled steel sheet having improved workability is produced by hot rolling a steel containing 0.01 to 0.12% C, not more than 0.1% Si, 1.0 to 1.8%
Mn, 0.01 to 0.10% soluble Al, with the balance being Fe and unavoidable impurities, cold rolling the hot rolled steel sheet and subjecting the cold rolled steel sheet to a continuous annealing. The continuous annealing is performed by soaking at a temperature ranging from 730 to 800°C for a period rang-ing from 20 seconds to 2 minutes and cooling the sheet to 250°C
or below at a cooling rate ranging from 30 to 300°C/sec. The steel sheet obtained in accordance with the invention is parti-cularly useful for automobile car bodies.

Description

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This invention relates to a process for producing a two-phase, cold rolled steel sheet ha~ing a low yield point low yield ratio and high ductility, yet having a tensile strength rangin~ from 40 to 80 kg/mm2. Steel strip as well as cut steel sheets are collectively referred to herein as "sheet'.
Recently, in the automobile industry, much effort has been made to reduce the weight o~ car ~odies, mainly for the purpose of lowering the car fuel consumption. Since wei~ht reduction necessitates a thickness reduction of the body steel sheet, it is essential then to use high strength steel sheets for the above purpose as well as for assurance of safety.
Presently, the use of high tension steel sheets i5 more common in the automobile industry.
To meet such demands in the automobile industry, it is necessary for the steel industry to provide steel sheets having improved properties over conventional high strength cold rolled steel sheets at lower prices.
A conventional two-phase high tension cold rolled steel sheet and a production process thereof is disclosed~
for example, in .Tapanese Laid-Open Patent Application Sho-50-98419, according to which a steel containing carbon and manganese and if necessary, 0.1 to 0.7% of silicon, is heated to the two-phase ( ~ +y ) coeY.isting zone as determined in the phase equilibrium diagrams, then relatively rapidly cooled to 500C at an average cooling rate of from 0.5 to 30C/sec. to obtain a steel structure mainly composed of ferrite and the transformation product produced by the rapid cooling and partially containing the residual austenite.
It is an object of the present invention to provide a process for producing at low cost a two-phase high tension ~4~

cold rolled steel sheet having improved workability.
In accordance with the invention, there is provided a process for producing a two-phase high tension cold rolled steel sheet having improved workability, comprising the steps of:
hot rolling a steel containing 0.01 to 0.12% C, no more than 0.1% Si, 1.0 to 1.8% Mn, 0.01 to O~l~/o soluble Al, with the balance being Fe and unavoidable impurities, cold rolling the hot-rolled steel sheet, and subjecting the cold rolled steel sheet to a continuous annealing by soaking at a temperature ranging from 730 to 800C
for a period ranging from 20 seconds to 2 minutes and cooling the sheet to 250C or below at a cooling rate ranging from 30 to 300C/sec.
One of the main features of the present invention is to apply a very high cooling rate such as 30 to 300C/sec. for the cooling to a temperature not higher than 250C in a conti-nuous annealing process. In order to achieve the desired object of the present invention with such a rapid cooling, the starting steel material mu~t satisfy the following condi-~ions with respect to its chemical composition.
When the carbon content is less than 0.01%, the structure which can be produced by the rapid cooling is inade-quate, but on the other hand when the carbon content is beyond 0.12%, the amount of the transformation product due to the rapid cooling is excessive so that it is difficult to obtain a two-phase structure having high ductility as desired by the present invention. In this case, the transformation product is composed of martensite and non-transformed austenite.

Manganese is effective to increase the hardenability of the y -phase, to promote the rapid cooling of the transforma-0~3 ..

tion product during the cooling step, and to strengthen the ferrite matrix, hence increasing the ductility~ However, manganese contents of less than 1.0% are not enough for obtaining the desired hardenability. On the other hand, mangenese contents exceeding 1.8% are not desirable because the weldability is de~eriorated thereby and certain economical disadvantages are brought about.
Aluminum is essential for the deoxidation of the steel, but less than 0.01% Al is not enough for this purpose.
On the other hand, aluminum contents of no more than 0.10%
are enough for the purposev Silicon is not essential, and no more than 0.1% of silicon is enough, but for improving the ductility of the two-phase structure, it may be added in an amount not exceed-ing 1.2%. Silicon contents beyond 1.2% will produce adverse effects on the paintability and corrosion resistance of the r~sultant steel sheets. Therefore, the upper limit of the silicon content should not exceed 1.2%.
For further improving the strength, chromium, copper and nickel, each in an amount of no more than 1%, may be added, and for improving the bending workability, calcium, rare earth metal (REM) and zirconium, each in an amount of no more than 0.1%, may be added.
The starting material having a chemical composition as defined above is processed into slabs by a conventional continuous casting process or ingot-breaking process, then the slabs are hot rolled and cold rolled into cold rolled sheets, and the cold rolled sheets thus obtained are subjected to a continuous annealing treatment according to the present inven-tion.
The soaking in the continuous annealing process ~l~ZC~

should be performed at a temperature range of ~rom 730 to 800Cfor a perlod ranging from 20 seconds to 2 minutes ~or the following reasons.
Soaking at a temperature lower than 730C and for a period shorter than 20 seconds will not produce enough arnount of the y -phase and enough concentration of carbon, ~langanese, etc. in the y -phase necessary for obtaining a desired resi-dual austenite~ and even if a rapid cooling is performed, a desired two-phase structure cannot be obtained and thus it is impossible to obtain a steel sheet having a well balanced strength and ductility. On the other hand, soaking at a temperature exceeding 800C will produca an excessive amount of the y -phase and the manganese content in the y -phase will be diluted, so that even with a rapid cooling a desired amount of residual austenite cannot he produced, and rather the amount of hard martensite increases. Thus, the resultant steel sheet is not well balanced between strength and ductili-ty. The upper limit of the soaking time is generally set at

2 minutes, although a longer soaking time may be applied.
However, a longer time requires a longer length of the furnace and thus is not desirable from the points of capital cost and economy.
The cooling subsequent to the above soaking is performed at a cooling rate ranging from 30 to 300C/secO~
preferably 100 to 300C/sec. to 250C. ~he reason why the terminal point of the rapid cooling is set at 250C is that if the rapid cooling is terminated at temperatures above 250C, the residual austenite is transformed into martensite so that a two-phase structure cannot be produced.
Regarding the cooling rate, it is noted that when the cooling rate is less than 30C/sec~, it is impossible to obtain a two-phase structure with the low alloy steel composi-tion defined by the present invention, and when the cooling rate exceeds 300C/sec. the ductility of the resultant steel is low. So far as the ductility is concerned, it does not substantially change if the cooling rate is within the range of from 30 to 300C/sec. A cooling rate controlled within the above range is difficult to obtain by liquid cooling or cooling ~y immersion in water, but can be easily obtained by vapour-liquid cooling by blowing a mixture of vapour and liquid onto the steel sheet~ An additional advantage of this vapour-liquid cooling is that a uniform cooling effect across the width of the steel strip can be achieved, and hence a uniform quality of the material can be achieved.
The steel sheets after the continuous annealing process are subjected to straightening normally with a reduc-tion of about 10%.
The steel sheet obtained by the process according to the present invention shows a low yield point and yet a tensi~
le strength ranging from 40 to 80 kg/mm and a high elonga-tion. As a result, steel sheets produced according to thepresent invention has the following advantages.
The yield point is related to the "spring-back"
phenomenon of the steel sheet when subjected to the press forming. Therefore, the low yield point (low yield ratio) ensures a better fitting of the steel to the press mold and hence a better formed shape, so that loads on the press form-ing machine can be substantially reduced. The present inven-tion can contribute to meet the demands in the automobile industry for high tension steel sheets at lower prices because a two-phase high strength steel sheet can be produced at low production cost with lower amounts of alloying elements.

The high ductility of the steel sheet obtained according to the process of the present invention ensures successful high degree o~ working. In this point, the present invention is markedly advantageous.
On the other hand, the steel sheets conventionally used for the outer layer of automobile car bodies have a thickness of about 0.8 mm. However, as mentioned hereinbefore, ; many efforts have been made to reduce the weight of the car bodies, and one way is to reduce the thickness of the steel sheets.
However, in the effort for reducing the steel sheet thickness, a problem has arised concerning the dent resistance, namely resistance to the local denting of the sheet. The dent resistance depends on the thickness and the strength of the steel sheet. The very reason for using a high strength thin gauge steel sheet for the outer skin of the automobile car bodies is to improve this dent resistance. Therefore, the present invention has a remarkable industrial advantage in that a steel sheet having an improved dent resistance with a high tensile strength while maintaining a satisfactory work-ability with a low yield point and high elongation can be obtained, and the present invention has a great commercial significance in that the steel sheet obtained by the present invention can well meet to the demands of the automobile industry for substitution of the conventional soft steel sheets used as the outer skin of automobile car odies.
The invention will now be illustrated hy the following non limiting examples and with reference to the appended drawings, in which:
Figs. l(a), (b) and (c) are diagrams respectively showing the relation of a two-phase structure formation relative to various contents of carbon and manganese, at a ~2(~

certain cooling rate.
Steel slabs having various steel compositions as shown in Table 1 were prepared and hot xolled into hot rolled steel strips of 2.5 mm in thickness, and further cold rolled into cold rolled steel strip of 0~7 mm in thickness. These cold rolled steel strips were subjected to a continuous annealing whereby they were soaked at 770C for ~0 seconds, and cooled to ordinary temperatures at various cooling rates ranging from 10~C to about 1000C/sec. (water quenching).
The properties o~ the resultant ~teel sheets are also shown in Table 1.
According to the conventional continuous annealing, the cooling subsequent to the soaking is carried out at a rate of about 10C/sec. With such a slow cooling rate, it is impossible to obtain a two-phase steel sheet having a low yield point and low yield xatio unless the amount of alloying elements (M~) is large as in Steel D. Further, this conven-tional treatment has a disadvantage that as the manganese content increases the weldability deteriorates and the produc-tion cost increases.
When the cooling rate is maintained within therange of from 30 to 300C/sec. as defined in the pre~ent invention, a satisfactory two-phase structure can be obtained even with a smaller amount of alloying elements, and the resultant ductility does not substantially change, as illus-trated by Steels A to C and E to I.
On the other hand, when the cooling rate is further increased to 350C/sec. (water spray) or 1000C/sec. (immersion in water), the resultant ductility is remarkably deteriorated and the desired object of the present invention cannot be achieved~

:

. In Figs. l(a), (b) and (c) showing the relation ` between the cooling rate and the formation of the two-phase . structure at various contents of carbon and manganese, the numerical references represent the yield ratio in per cent, and the yield ratio of 50% is set as the border line for the two-phase structure.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a two-phase high tension cold rolled steel sheet having improved workability, comprising the steps of:
hot rolling a steel containing 0.01 to 0.12% C, no more than 0.1% Si, 1.0 to 1.8% Mn, 0.01 to 0.10% soluble Al, with the balance being Fe and unavoidable impurities, cold rolling the hot rolled steel sheet, and subjecting the cold rolled steel sheet to a conti-nuous annealing by soaking at a temperature range of from 730 to 800°C for a period ranging from 20 seconds to 2 minutes and cooling the sheet to 250°C or below at a cooling rate ranging from 30 to 300°C/sec.

2. A process according to claim 1, in which the steel contains silicon in an amount of 0.1 to 1.2%.

3. A process according to claims 1 or 2, in which the cooling rate is from 100 to 300°C/sec.

4. A process according to claims 1 or 2, in which the cooling is carried by contacting the sheet with a vapour-liquid mixture.

5. A process according to claims 1 or 2, in which the cooling is carried out by contacting the sheet with a vapour-water mixture.