JPH0372033A - Production of cold rolled steel sheet excellent in press formability - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet excellent in press formability by applying, in succession, respective treatments of cutting, hot rolling, coiling, cold rolling, continuous annealing, and overageing to a continuously cast slab with a specific composition under respectively specified conditions. CONSTITUTION:A cast slab which has a composition consisting of, by weight, 0.008-0.030% C, <=0.10% Si, 0.05-0.40% Mn, <=0.015% S in the range where Mn/S is 20-40, <=0.02% P, 0.001-0.040% solAl, <=0.0030% N, B so that B/N is 0.6 to 1.0, further one or more kinds among REM, Ca, and Zr by the amounts in the ranges where REM/S, Ca/S, and Zr/S are regulated to 1-3, 1-3, and 2-10, respectively, and the balance Fe with inevitable impurities is cut at >=900 deg.C surface temp. in continuous casting. Subsequently, after the lapse of >=10min, hot rolling is started and finished at a temp. of the Ar3 transformation point or above, and the resulting plate is coiled at >=600 deg.C, descaled, and cold rolled at >=60% rolling reduction, and further, the resulting sheet is subjected to continuous annealing where the sheet is heated up to >=700 deg.C and soaked and rapid cooling is started from a temp. not exceeding 680 deg.C at 50-200 deg.C/sec cooling rate and then the sheet is subjected to overageing treatment at <=400 deg.C for 1-10min.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for manufacturing cold plate steel sheets with excellent press formability, and in particular to a method for manufacturing M-killed steel that combines direct rolling and continuous annealing. be.

(Prior Art) In the production of cold steel plates with excellent press formability, efforts have been made to develop continuous processes and omit processes in order to improve production efficiency, and these efforts have been put into practical use one after another. One example is continuous annealing equipment that combines the conventional electrical cleaning, box annealing, temper rolling, and inspection processes into a single process, and the continuous annealing method has become rapidly popular today. On the other hand, as for the manufacturing method of the steel billets used as these materials, continuous casting has mostly been applied instead of the ingot blooming method, but recently continuous casting and hot rolling have been directly linked. The so-called direct rolling method is being put into practical use (for example, JP-A-58-100629 and JP-A-59-123720).

However, when comparing the material quality of continuously annealed cold steel plates produced by the conventional method of reheating and hot rolling a slab and the direct rolling method, the material produced by the direct rolling method is inferior. Generally, when producing cold steel sheets with excellent press formability by continuous annealing, carbonitrides, etc. are coarsely precipitated in the hot rolling process, and the grains after cold rolling and recrystallization annealing are It is necessary to facilitate growth. To achieve this, there are two methods: lowering the heating temperature of the slab to reduce the re-melting of already coarse precipitates (Japanese Patent Application Laid-Open No. 138516/1982), and raising the coiling temperature to increase the growth and coarsening of precipitates. A method (Japanese Unexamined Patent Publication No. 113411/1983) is adopted. However, this method reduces pickling properties and also degrades plane strain formability. On the other hand, in the direct rolling method, precipitation and growth of sulfides and carbonitrides hardly progress from the solidification process to the start of hot rolling, and fine precipitation occurs during the rolling to coiling process. For this reason, grain growth after cold rolling and recrystallization annealing is inhibited, resulting in poor deep drawability, and formability against plane strain deformation, which is often seen in bead passage areas during actual press forming, is reduced. Incidentally, if the above-described method of increasing the winding temperature is adopted, although the deep drawability is improved, the plane strain formability may actually deteriorate further.

(Problems to be Solved by the Invention) Therefore, the present invention directly connects continuous casting and hot rolling, and improves deep drawability, plane strain formability, etc. by continuous annealing after cold rolling even in low-temperature coiling. The purpose is to manufacture cold plate steel sheets with excellent press formability.

(Means for solving problems) The gist of the present invention is as follows.

(1) C: 0.008-0°030%, Si: 0
．． 10% or less, Mn: 0.05-0.40%, S: 0
．． 015% or less, and Mn/S=20-40, P:
0.02% or less, sol, kl: 0.001~
0.040%, N: 0.0030% or less, B to B/
N is 0.6 to 160. Furthermore, REM/S: 1-3
, Ca/S: L ~ 3, Zr/S: 2 to 10 or more, with the remainder consisting of iron and unavoidable impurities. After cutting at a surface temperature of 900 ° C. or higher in continuous casting, the slab is left as it is. Alternatively, start hot rolling after 10 minutes or more including auxiliary heating, finish at Ar3 transformation point or higher, coil at 600°C or higher, descale, and then cold roll at a reduction rate of 60% or higher, Heat to 700℃ or higher and after soaking, heat from 50 to 200'' (/sec) from a temperature not exceeding 680℃.
A method for producing a cold steel sheet with excellent press formability, characterized by cooling at 400° C. and overaging for 1 to 10 minutes at 400° C. or lower.

(2) Continuous annealing is heated to 700"C or higher and after soaking 68
50-200℃/sec from 0”C to below 300℃
The method for producing a cold steel plate with excellent press formability as described in the preceding item 1, characterized in that the temperature is increased by 50°C or more up to a maximum of 450°C, and then cooled to 250°C within 1 to 10 minutes. .

The present invention will be explained in detail below.

C deteriorates deep drawability and plane strain formability, so 0.
030% or less. On the other hand, if the content is too small, the precipitation of solute carbon during over-aging will not progress and aging properties will become a problem, so the content should be at least 0.00 s%.

o, oto to 0.025% is preferred.

Si hardens the W4 plate and deteriorates press formability, so the content should be 0.10% or less. It is preferably 0.03% or less.

If there is too little Mn, the precipitation rate of MnS during the direct rolling process will be slow and fine precipitation will occur during the hot rolling process, hardening the steel plate and deteriorating the press formability, so 0.05% or more is required, but on the other hand, too much Mn 0.40 because it also deteriorates formability.
% or less. Further, in order to stably ensure moldability, the ratio of Mn to S (Mn/S) is set to 20 to 40. M
The lower limit of n/S is regulated to promote precipitation of MnS, and the upper limit is regulated to prevent deterioration of formability.

Too much S deteriorates plane strain formability, so it should be set at 0.
0.015% or less.

P hardens the @ temporary and deteriorates deep drawability, but it also has the harmful effect of causing hot embrittlement cracking when it is directly fed and rolled, so it should be kept at 0.02% or less. Preferably 0.010%
The following is good.

M is 3o1. for deoxidation. It contains 0.001% or more of N, but on the other hand, it only increases the cost, so the content is set to 0.040% or less.

N is fixed as nitride BN by B, but if there is too much nitride, plane strain formability deteriorates, so 0.003
0% or less. It is preferably 0.0020% or less.

B is an element that effectively fixes N as relatively coarse N even in thermal history such as direct rolling, and the ratio of B to N (B/N) is required to be 0.6 or more, but excessive B of
is set to 1.0 or less since it deteriorates moldability.

REM+Ca and Zr are necessary to control the form of sulfide in the direct rolling process so as not to hinder recrystallization and grain growth, and one or more of them are included. In order to obtain the above effects, each of REM, Ca, and Zr has a quantitative relationship with S such that RtiM/S: 1 to 3. Ca/S: 1~
3. Zr/S: It is necessary to satisfy 2 to 1O.

The remainder becomes iron and unavoidable impurities, but Cr + Cu
If the total amount of elements such as , Ni, etc. is 0.10% or less, the object of the present invention is not impaired.

Steel having the above composition is subjected to molten metal pretreatment, converter melting, light degassing treatment, etc. to adjust its composition, continuous casting, and after cutting, it is hot-rolled as it is or after supplementary heating. At this time, the surface temperature of the slab during cutting is set to 900° C. or higher to ensure the finishing temperature during hot rolling. Also,
After cutting the slab, it takes at least 10 minutes to start hot rolling (rough rolling) in order to adjust the morphology of sulfides to facilitate grain growth during annealing. Note that as means for auxiliary heating, a gas edge heater or a short-time heating furnace (an insulating furnace is also acceptable) may be used.

In hot rolling, the finishing temperature is set to Ar=transformation point or higher to ensure deep drawability. The winding temperature is set to 600° C. or higher in order to improve plane strain formability and deep drawability. However, if the temperature is too high, the plane strain formability deteriorates and the pickling properties are significantly reduced, so the temperature is preferably 700°C or less. In order to make the material uniform, it is effective to make the top and bottom of the hot rolled coil relatively hotter than the middle part (50°C or higher).

Next, in cold rolling after descaling, a reduction ratio of 60% or more is adopted in order to obtain deep drawability. Preferably it is in the range of 80 to 95%. Continuous annealing is performed by heating to 700℃ or higher, and after soaking, the temperature is 50 to 20℃ from a temperature not exceeding 680℃.
Start quenching at 0℃/sec and 1 to 1 at a temperature of 400℃ or less.
Performs 0 minute overage processing.

The reason why the annealing temperature is set to 700° C. or higher is to recrystallize and further grow grains to improve press formability. In addition, in the cooling process of annealing, rapid cooling is started from a temperature not exceeding 680°C at a rate of 50°C/second or more to prevent precipitation of large carbides in the high temperature range and to contain as much solid solution carbon as possible. This is because it is necessary to improve plane strain formability by reducing solid solution carbon in overaging treatment. on the other hand,
The upper limit of the cooling rate should be set at 200"C/s, as even if it is too fast, carbides will precipitate into extremely fine particles, deteriorating the plane strain formability.
Let it be ee. A cooling rate of 70 to 120°C/sec is preferable from the viewpoint of improving plane strain formability, and the overaging treatment is
The reason why the temperature is 00° C. or lower for 1 to 10 minutes is to reduce the rapidly cooled supersaturated solid solution carbon and improve plane strain formability.

In order to more efficiently reduce solute carbon and improve plane strain formability, it is necessary to
Cool at 200°C/see, raise the temperature again by 50°C or more up to a maximum of 450°C, then increase the temperature by 1 to 10°C to 250°C.
Adopt a method that cools down within minutes.

use

After annealing, temper rolling is usually performed to eliminate elongation at yield point and adjust surface roughness.

Note that the manufacturing method according to the present invention can be applied not only to cold steel sheets but also as a method for manufacturing original sheets of surface-treated steel sheets to which various types of plating are applied.

(Example) After casting the steel composition shown in Table 1, it was directly rolled under the conditions also shown in Table 1. Then, hot rolled under the conditions shown in Table 2,
After pickling, the sample was cooled to 0.8 mm at a cooling rate of 80%. In the next continuous annealing, after soaking at 800°C, slow cooling to 680°C was performed, and two types of cooling/overaging treatments were performed. One is 68
Cool from 0°C to 350°C at 90° (: /sec,
Thereafter, an overaging treatment was performed while gradually cooling to 250° C. for 10 minutes (referred to as type A). The other one is 680
℃ to 200° C. at a rate of 90° C./sec, then heated to 380° C., and then subjected to an overaging treatment in which the sample was cooled to 250° C. for 5 minutes (referred to as type B). And 1
．． Taking into consideration the time it takes for the sheet to be used by a customer after 0% temper rolling, the characteristics of the sheet as a product were investigated after one month had elapsed.

The results are shown in Table 2. Note that the plane strain formability was evaluated by forming a rectangular specimen of 127 mm x 178 mm using a 100 mm diameter ball-head punch and evaluating the forming height under plane strain deformation.

As is clear from the properties of the product sheet, by employing the method of the present invention, the deep drawability and plane strain formability of the steel sheet subjected to direct rolling and continuous annealing are improved, and good characteristics can be obtained even when coiled at a low temperature.

(Effects of the invention) Plane strain formability and deep drawability have been achieved through a new manufacturing process that pursues process-savings, centered on the direct rolling method that directly connects continuous casting and hot rolling, and the continuous annealing method that serializes the annealing process. Since cold steel sheets with excellent properties can be manufactured from N-killed steel, the energy saving effect of direct rolling and the use of expensive
Switching from rsttial free steel to N-killed steel has great significance in that it can reduce manufacturing costs.

Claims (2)

[Claims] (1) C: 0.008-0.030%, Si: 0.10
% or less, Mn: 0.05-0.40%, S: 0.015
% or less, and Mn/S=20 to 40, P: 0.02% or less, sol. Al: 0.001-0.040%, N: 0
．． 0030% or less, B in B/N 0.6 to 1.0, further REM/S: 1 to 3, Ca/S: 1 to 3, Zr/S:
In continuous casting, a slab containing one or more of 2 to 10, with the remainder consisting of iron and unavoidable impurities, has a surface temperature of 9.
After cutting at 00℃ or higher, start hot rolling as it is or after 10 minutes or more including supplementary heating, finish at Ar_3 transformation point or higher, coil at 600℃ or higher, and after descaling, 60% or higher. Cold rolled at a rolling reduction rate, heated to 700℃ or higher, and after soaking, the temperature does not exceed 680℃ to 50~20℃.
A method for producing a cold steel sheet with excellent press formability, characterized by cooling at 0°C/sec and overaging at 400°C or less for 1 to 10 minutes. (2) Continuous annealing is heated to 700℃ or higher and after soaking 680℃
2. Cooling at a rate of 50 to 200° C./sec from 0.degree. A method for producing cold plate steel sheets with excellent press formability.