JPS63238241A - Cold-rolled steel sheet excellent in workability and its production - Google Patents

Abstract

PURPOSE:To manufacture a cold-rolled steel sheet excellent in workability in high efficiency, by subjecting a continuously cast steel slab with a specific composition consisting of C, Mn, Al, N, Ti, and Fe to specific hot rolling, cold rolling, and continuous annealing. CONSTITUTION:A continuously cast slab of a steel having a composition which consists of, by weight, <=0.0030% C, 0.03-0.80% Mn, 0.04-0.12% sol.Al, 0.0050-0.0120% N, 0.002-0.025% Ti, and the balance Fe with inevitable impurities and in which Ti<=(N-0.0030)48/14 is satisfied is hot-rolled at 700-970 deg.C finishing temp. Then, the hot-rolled steel plate is wound up at a temp. as low as <=560 deg.C to prevent the precipitation of fixed AlN. The resulting hot-rolled plate is descaled and then cold-rolled at 65-85% draft to improve r45. Subsequently, the cold-rolled steel sheet is continuously annealed under the conditions of 15-300sec temp.-rise time up to 400-700 deg.C and 700-900 deg.C maximum heating temp. to accelerate AlN precipitation and recrystallization. In this way, the cold-rolled steel sheet excellent in workability and having high r45 can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a cold-rolled steel sheet that has excellent workability, particularly deep drawability, and is used after being subjected to press forming or other processing, and a method for manufacturing the same.

(Prior art and its problems) Excellent workability is required for cold-rolled steel sheets that are widely used for automobile exterior panels, home appliances, furniture, kitchen products, and the like. Conventionally, such cold-rolled steel sheets have C: about 0.04%^l
It is manufactured by cold rolling killed steel and then box annealing (batch annealing), and the r value, which is an index of deep drawability, is 1.
The above-mentioned cold-rolled low carbon killed steel sheet which was annealed in a zero-box and had a grain size of 5 to 2.0 has an elongated grain structure with a grain size of usually 40μ or more, which has a high r value. That's the reason.

In recent years, continuous annealing processes, which are highly effective in streamlining production and reducing costs, have become popular in place of box annealing.

However, when a steel plate made of the same material that was conventionally subjected to box annealing is subjected to a continuous annealing process, the rapid heating rate causes the crystal grains to become fine and equiaxed, making them hard and F(11
1.0 to 1.5, resulting in poor workability.

As one means for obtaining a cold-rolled steel sheet with an r value of 1.5 or more even after continuous annealing, it is known to convert the raw material steel into a 2-killed steel with a carbon content of 0.01% or less. An example of this is the invention disclosed in Japanese Patent Publication No. 48-25609. Cold-rolled steel sheets manufactured by this method have r values in three directions of 0", 45°, and 95° with respect to the rolling direction (respectively r
,,ras,r,. ), that is, the average value f of (
re +2 ras + r*o)/4 is high, but ras is extremely low at 1.1 or less, r4. is low, which means that
When deep drawing into a cylindrical shape, ears tend to appear and the yield decreases, and in general deep drawing, the mold and shape are designed to reduce the flow of the plate in the 45° direction during forming. This means that if you do not design or cut out the plates, there is a high possibility that cracks will occur in that direction.

The present invention aims to obtain a cold-rolled steel sheet with excellent workability comparable to box-annealed materials while using a continuous annealing process, and in particular, greatly improves the rts, which is a drawback of conventional continuously annealed materials. The present invention provides a cold-rolled steel sheet and a method for manufacturing the same.

(Means for Solving the Problems) The gist of the present invention lies in the following cold-rolled steel sheet and its manufacturing method.

(1) In weight%, C: 0.0030% or less, Mn
: 0.03-0.80%, sol, Al: 0
．． 04-0.12%, N: 0.0050-0.012Q
%, Tl: 0.002-0.025%, the balance consists of Fe and unavoidable impurities, Ti (%)≦(N (%') -0,003034
Cold rolled steel sheet with excellent workability of 8/14.

(2) CF 0.0030% or less, Mn in weight%
: 0.03-0.80%, sol, A j! :
0.04-0.12%, N: 0.0050-0.01
20%, Ti: 0.002-0.025%, balance F
Ti (%)≦(N (%) −0,0030) 48
/14 Continuously cast slab of steel finished at a temperature of 700 to 9
After hot rolling at 70°C, coiling at a temperature of 560°C or lower, and cold rolling at a rolling reduction of 65 to 85%,
Heating time to 0℃: 15-300 seconds, maximum heating temperature: 70℃
A method for producing a cold-rolled steel sheet with excellent workability, characterized by continuous annealing under conditions of 0 to 900°C.

(Function) In order to achieve the above-mentioned object, the present inventor has accumulated the following basic knowledge through numerous test studies. That is, in order to increase the r value of the continuously annealed material, especially 14%, it is necessary to (1) reduce the grain size of the hot rolled steel sheet to 50 μm or less before cold rolling;
Three requirements are important: making the reduction ratio in cold rolling 65% or more, and (1) finely precipitating AIN during heating during continuous annealing to control the recrystallized texture and create an elongated grain structure. . Although just one of these three requirements is effective, it is even more effective if two or three requirements are combined, and ras is 1.3.
It is also possible to manufacture the above cold-rolled steel sheets.

However, it is not easy to achieve the above three requirements using existing low carbon M-killed steel.

For example, in order to satisfy the condition (■), the hot-rolled steel sheet must be made thicker before cold rolling, but if this happens, the cooling rate after rolling will be slow and the grains will become larger, and the condition (■) will not be satisfied. In order to satisfy the conditions (2), it is necessary to raise the heating temperature of the hot rolling to sufficiently dissolve MN in solid solution, and then the condition (2) will no longer be satisfied.

The present inventor has confirmed that the contradictory conditions described above can be achieved in an extremely rational manner by utilizing nitride precipitates in the steel material and by setting consistent conditions from hot rolling to continuous annealing. .

First, the steel used as the material is ultra-low carbon AQ killed steel, and is characterized by containing a small amount of Tt and relatively high amounts of AOL, AQ, and N. When such steel is made into a slab by continuous casting and hot rolling is completed at a finishing temperature of 700 to 950°C, the resulting hot rolled steel plate will be TiN even if the plate thickness is thick or the heating temperature is high. The grains become finer due to the fine precipitation of . Such a hot rolled steel plate is rolled at a rolling reduction ratio of 65 to 85.
% and then subjected to continuous annealing under specific conditions, AQN precipitated during the heating process controls the recrystallized texture,
? An expanded grain structure with high value and ras is obtained. That is,
F165 or higher and RA 31.3 or higher can be obtained.

Each condition defined in the present invention will be explained in detail below.

First 1. The reasons for limiting the composition of the steel plate are as follows.

C: In order to obtain excellent workability even if continuous annealing is performed, it is necessary to have an extremely low carbon content of 30% or less. That is, if the amount of C exceeds 0.0030%, the crystal grains after continuous annealing become fine (and the steel sheet becomes hard), and strain aging is likely to occur.

Mn: Mn has the effect of preventing hot embrittlement of steel and promoting the formation of AQH. In order to have these effects, 0.
However, since it is also a component that lowers the r value, the upper limit is kept at 0.80%.

sol, to Q: 0.04% more than normal ^2 killed steel to form AfllN with N below.
The content ranges from 0.12% to 0.12%. 0.12%
If it exceeds this value, the steel plate will become hard and workability will deteriorate.

N: Must be contained in an amount sufficient to form AQN and TiN. If it is less than o, ooso%, it is insufficient to precipitate the required amount of ampholytic compound. However, 0.0
When the amount exceeds 120%, the total amount of nitrides becomes too large and the steel sheet becomes hard. The ideal distribution of N is N7 as 8QN! l(0,006%, N as TiN is 0.002%. N as MN is 0.003%
%, sufficient AIIN is not formed to control recrystallization assembly &11Va.

Ti: An essential component to precipitate as TiN and refine the grains of hot rolled steel sheets. TiN is 0.000 in terms of N.
The content must be 6% or more, that is, 0.002% or more as T1. On the other hand, when Ti exceeds 0.025%, Tic and TiS are partially generated, reducing the elongation of the product cold-rolled steel sheet.

It is also important to determine the Tk content in relation to the N content. The relational expression is Ti (%)≦(N (%)-0,003] 48/1
It is 4. This formula shows that N forming AQN is 0.003%
If Ti is more than the right side of the above equation, AQN formed during recrystallization will be insufficient, the recrystallized texture will not be controlled, and an elongated grain structure will not be obtained.

The attached figure shows the relationship between the above-mentioned N and Ti contents, and in the figure, the shaded area is the appropriate range of N and Ti contents.

In addition to the above components, the remainder is Fe and unavoidable impurities. Typical impurities, P and S, may be kept to the same level as ordinary cold-rolled steel sheets, that is, P≦0.03% and S≦0.02%.

Next, each condition of the manufacturing method will be explained.

The material used is continuous casting slab. In theory, a blooming slab from an ingot can be used, but since the cold-rolled steel sheet that is the product is intended for mass production, it is rational to use a slab made by continuous casting, which is highly productive. It is also possible to use the so-called honcharging technique, in which the hot slab coming from the casting line is placed directly into a heating furnace, and the direct rolling technique, in which the slab is rolled without heating.

The slab heating temperature and hot rolling start temperature may be the same as those in normal low carbon AQ killed steel. A rough guideline is a heating temperature of 1100°C to 1300°C and a rolling start temperature of 1000°C or higher.

The finishing temperature of hot rolling is important. If the finishing temperature is lower than 700° C., a texture develops that has an unfavorable effect on the workability of the final product, reducing the r value of the product cold rolled steel sheet.

When the finishing temperature exceeds 970°C, the effect of grain refinement due to TiN precipitation cannot be expected, and the grains of the hot-rolled sheet become coarse, and even if the subsequent processes are carried out under the conditions of the present invention, the r value, especially r , s cannot be improved.

Winding after hot rolling is performed at 560°C or lower. The reason is,
This is to prevent AQN dissolved in solid solution during hot rolling from precipitating during slow cooling after winding. Therefore, after finishing hot rolling in the above temperature range, it is rapidly cooled and coiled at the lowest possible temperature of 560° C. or lower.

However, there are problems with coiler capacity and coil rust caused by residual cooling water, so the lower limit of the realistic winding temperature is 1.
It will be around 00℃.

The hot-rolled steel sheet obtained as described above has fine crystal grains due to the effect of TiN precipitation, and is in an ideal state in which most of the MN is dissolved in solid solution. This is subjected to treatments such as descaling using conventional methods, and then sent to the next cold rolling process.

Cold rolling is performed at a rolling reduction of 65 to 85%. If the rolling reduction is less than 65%, the improvement in r and s will be small. On the other hand, if you try to obtain a rolling reduction of more than 85%, the hot-rolled sheet must be made thicker. As a result, the quenching effect after hot rolling is lost, leading to a decrease in r4S.

In continuous annealing, the initial heating rate and maximum heating temperature must be strictly controlled.
This condition, in which the temperature is raised from 1 to 700° C. in 15 to 300 seconds, is necessary to sufficiently precipitate AIN before completion of recrystallization. That is, at a temperature lower than 400°C, substantially no precipitation of An occurs, and at a temperature exceeding 700°C, recrystallization proceeds, so the temperature increase rate must be controlled between 400 and 700°C. Among these, the temperature between 500 and 650°C requires particular control from a metallurgical point of view.

This is because the rate of AQN precipitation and recrystallization is high in this temperature range.

15 to raise the temperature by 300℃ from 400℃ to '700℃
In a short period of less than seconds, the temperature increase rate is too fast and sufficient precipitation of AfN before recrystallization does not occur. On the other hand, 30
At a slow heating rate of more than 0 seconds, the productivity, which is a characteristic of continuous annealing, is reduced to I! It ends up being 4.

In other words, the temperature increase conditions above are from 400℃ to 700℃
This means that the temperature is raised at an average rate of 1 to 20°C/sec. Even in processes where the temperature is not raised at a constant rate but in stages with one or more holding zones in the middle, the
The time required to raise the temperature to 0°C may be 15 to 300 seconds, for example, rapid heating to 550°C in 2 seconds,
Raise the temperature from 550℃ to 600℃ over 20 seconds, then 600℃.
If the temperature is raised from 00 to 700°C in 3.3 seconds, the average temperature increase rate from 400°C to 700°C falls within the above range. In actual operation, the above average temperature increase rate can be controlled by measuring the heating zone inlet temperature and soaking zone inlet temperature of the annealing furnace and performing appropriate heat transfer calculations.

The maximum heating temperature during annealing is set to 700° C. or higher so that recrystallization is completed in a short time. If it is lower than this, some unrecrystallized grains may remain. When the maximum heating temperature exceeds 900°C, an austenite phase appears and the r value decreases.

The cooling conditions from the maximum heating temperature are not particularly limited, and may be slow cooling or rapid cooling, or may be a U heat pattern including overaging treatment.

iI for cold rolled steel plate after annealing! It is also optional to perform surface treatments such as rough rolling and plating. When performing hot-dip galvanizing or tin plating, continuous annealing may be performed under the above-mentioned conditions using an annealing equipment within the plating line.

Hereinafter, the effects of the present invention will be specifically explained using Examples.

(Example 1) C: 0.0020%, Mn: 0.18%, 5so1.
AQ: 0.082%, N: 0.0QT5%, Ti:
0.010%, P: 0.015%, S: 0.004
%, balance Fe, a continuously cast slab of steel is cast hot (approximately 95%
After charging into a heating furnace at 1150°C for 1 hour, it was hot-rolled to a thickness of 3.2 mm at a finishing temperature of 910°C. After hot rolling, it is immediately cooled with water, rolled up at 500°C, and then heated to 350°C.
and then rapidly cooled to room temperature.

The hot-rolled sheet was descaled and cold-rolled to a thickness of 0.81 at a rolling reduction of 75%, and then continuously annealed under various conditions shown in Section 1'*. After annealing, the material was cooled at an average rate of 30° C./second, temper rolled at room temperature with an elongation rate of 0.5%), and a tensile test was conducted.

Table 1 shows the r4s of the cold rolled steel sheet obtained with the above continuous annealing conditions and the average value (P) of the r values in three directions.
In A3, A5 and A7, where the temperature increase rate is too high from 400 to 700°C, ras is particularly low, and 88, which has a low maximum heating temperature, has extremely low P and r4s of 1°.On the other hand, under the conditions of the present invention described above, Psr 4% for those that meet
Both of these are greatly improved.

(Example 2) Table 2 shows the composition of the steel used and the hot rolling and cold rolling conditions. The heating temperature of all slabs is 1200℃, and the thickness of hot-rolled sheets is 3.
．． It is a 2'm seagull.

The conditions for continuous annealing were a temperature increase of 8°C for all cases except for fragrance B14.
/second constant rate heating, 400-700℃ heating in 100 seconds,
The maximum heating was kept constant at 800°C x 40 seconds. Incense B14
The temperature was raised at a constant rate of 20°C/second, and the temperature was raised from 400°C to Too°C in 15 seconds, and the maximum heating was 800°C x 40 seconds as in the other cases.

After annealing, skin pass rolling with an elongation rate of 0.5% was performed, and a tensile test was conducted.

The test results are also listed in Table 2.

Looking at the test results in Table 2, the composition of the product steel sheet is F, r4
%. That is, Fsra
B4, B5, and B6, which are all low in s, have too much C or Ti content, and the same <88 has insufficient sol and AQO content.
B12, in which the winding temperature of the hot rolled sheet is too high, also has low F and r4s. When the crystal grains of these steel sheets were observed, they all had a fine equiaxed structure.

In addition, B7 has too much N content, so the elongation is small,
B9 has too little T1 content, so P is large but r4s
is small. Furthermore, BI3 has a low rolling reduction ratio in cold rolling, so it is an unsatisfactory value of '5rJs.

In contrast, 81 to B3.8 according to the embodiment of the present invention
What about 10.814? The crystal structures of these materials corresponding to the examples of the present invention, which not only have a large value but also show a remarkable improvement in rss, are all elongated grain structures.

The reason why the method of the present invention particularly improves r4S is considered to be due to the following reasons.

In other words, by adding a small amount of TI and controlling the hot rolling conditions, the grain size of the hot rolled sheet becomes smaller, and as a result, in the cold rolled sheet, the area parallel to the sheet surface (1101) decreases, and after recrystallization, the grain size of the hot rolled sheet decreases. The development of (110) oriented grains that lower the ras decreases.
10) There is an effect of suppressing the development of oriented grains, and due to these comprehensive effects, the (1101 oriented grains in the recrystallized annealed sheet are reduced compared to conventional steel sheets, r4S is increased, and f is also increased as a result.

(Effects of the Invention) According to the present invention, a cold rolled steel sheet is provided which has excellent workability comparable to box annealed material while using a highly efficient process of continuous annealing. In particular, the steel sheet of the present invention having a high r4s greatly contributes to streamlining the forming process during use and improving yield.

[Brief explanation of the drawing]

The attached diagram shows the appropriate range of Ti and N content in the steel sheet of the present invention.

Claims (2)

[Claims] (1) In weight%, C: 0.0030% or less, Mn: 0.
03-0.80%, sol. Al: 0.04-0.12
%, N: 0.0050-0.0120%, Ti: 0.0
02 to 0.025%, the balance being Fe and unavoidable impurities, and having excellent workability in which Ti (%) ≦ [N (%) - 0.0030] 48/14. (2) In weight%, C: 0.0030% or less, Mn: 0.
03-0.80%, sol. Al: 0.04-0.12
%, N: 0.0050-0.0120%, Ti: 0.0
A continuously cast slab of steel, consisting of 02 to 0.025%, the balance Fe and unavoidable impurities, with Ti (%) ≦ [N (%) - 0.0030] 48/14, is hot-cast at a finishing temperature of 700 to 970 °C. Rolled and coiled at a temperature of 560°C or less, reduction rate 65
After cold rolling at ~85%, heating time from 400℃ to 700℃ 15-300 seconds, maximum heating temperature 700-90℃
A method for producing a cold-rolled steel sheet with excellent workability, characterized by continuous annealing at 0°C.