JPH0770650A - Production of cold rolled steel sheet extremely excellent in deep drawability - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet having deep drawability exceedingly excellent compared to that of the conventional one by specifying the componental compsn. of steel and the producing conditions in finish rolling, cooling, coiling or the like. CONSTITUTION:Steel stock contg., by weight, <=0.0015% C, <=0.1% Si, <=0.5% Mn, <=0.1% Al, <=0.05% P, <=0.01% S and <=0.003% N, contg. <=0.1% Ti and/or <=0.05% Nb, contg., at need, 0.0001 to 0.0030% B, and the balance Fe is subjected to hot rolling. At this time, finish rolling is completed at the Ar3 transformation point or above in such a manner that the cumulative draft in three passes on the outlet side in the finish rolling is regulated to >=50%. Within 5sec from the completion of the finish rolling, cooling is started, and it is cooled in the temp. range from the cooling starting temp. to (the Ar3 transformation point -60 deg.C) at 50 to 400 deg.C/sec cooling rate and is thereafter coiled in the temp. range of 800 to 550 deg.C. After that, this steel sheet is subjected to cold rolling and is next subjected to continuous annealing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of cold-rolled steel sheets having excellent deep drawability, which are useful for applications such as automobile steel sheets.

[0002]

2. Description of the Related Art Cold-rolled steel sheets used for automobile panels and the like are required to have excellent deep drawability because they are pressed. In order to obtain excellent deep drawability, high r value (Rankford value) and elongation (E
l) is required.

Therefore, an ultra-low carbon IF (Interstitial Free) steel having excellent deep drawability has been developed by an annealing process consisting of heating and cooling in a short time typified by continuous annealing and has been widely used. . On the other hand, due to the recent advances in steelmaking degassing technology and the widespread use of degassing equipment,
Steel with a C and N content of 20 ppm or less has been manufactured efficiently and in large quantities.

In such an ultra-low carbon steel, although the elongation can be improved, the crystal grains are likely to be coarsened during hot rolling, which deteriorates the deep drawability of the product after cold rolling and annealing. I will end up. That is, when the amount of C, which is known to be effective in improving the deep drawability, is reduced in order to produce a cold-rolled steel sheet having further excellent deep drawability, if the amount of C in the steel is extremely lowered, , Coarse crystal grains are generated in the hot rolled sheet after hot rolling, and the coarse crystal grains are reduced rather than the deep drawability of the cold rolled steel sheet, so that the deep drawability cannot be improved. It was. Therefore, higher r than ever before
In order to obtain the value, a technique for reducing the crystal grain size of the hot rolled sheet is important.

A steel sheet having a good deep drawability has a C content of 0.005 wt.
A method for producing steel from less than 1% is already known. For example, in Japanese Patent Laid-Open No. 61-276930, ultra low carbon Ti-Nb-based steel with adjusted components is used for hot rolling finishing temperature, cooling rate and cooling range,
A method for producing a cold-rolled steel sheet having excellent elongation and deep drawability by combining conditions such as a coiling temperature, a cold rolling reduction rate, and continuous annealing in a predetermined heat cycle is disclosed. This method, the α transformation while suppressing the growth of γ grains by starting the quenching immediately after hot rolling, aims at the refinement of the structure of the hot rolled sheet, and by extension, a cold rolled steel sheet excellent in deep drawability. It is manufactured, and it can be said that the characteristic point is the average cooling rate after hot rolling. However, in this method, even if it is said that the hot rolled material is cooled at an average cooling rate of 10 ° C / s or more, the description of specific numerical values is limited to 30 ° C / s in Examples, and thus the hot rolled sheet obtained There is also a limit to the α grain size, and a cold rolled steel sheet having a very high deep drawability has not been manufactured.

As a method for improving the deep drawing property, that is, the problem left over in Japanese Patent Laid-Open No. 61-276930, Japanese Laid-Open Patent Application No. 1-177322 discloses an ultra-low carbon steel having C: 15 ppm or less. It is disclosed that a cold-rolled steel sheet having excellent deep drawability is obtained by hot-rolling and rapidly cooling. This method is characterized in that by increasing the cooling rate after hot rolling to 110 to 400 ° C./s, the growth and recovery of γ grains are further suppressed and fine α grains are generated. However, since the amount of C was reduced to 15 ppm or less and the amounts of Si, Mn, P and S were also considerably reduced, the growth and recovery of γ grains were fairly fast, and the rapid cooling after hot rolling alone resulted in an r value of 2.48. Was the highest, and there was still a limit to improving the deep drawability.

[0007]

SUMMARY OF THE INVENTION The present invention advantageously solves the above problems, and by controlling the steel composition and manufacturing conditions, a cold-rolled steel sheet having deep drawability far superior to conventional ones. It is an object of the present invention to propose a method capable of manufacturing.

[0008]

As a result of intensive studies to improve the deep drawability, the inventors of the present invention have excellent deep drawability by limiting the steel composition and manufacturing conditions as follows. It has been found that a cold rolled steel sheet can be manufactured.

The gist of the present invention based on the above knowledge is as follows. (1) C: 0.0015 wt% or less, Si: 0.1 wt% or less, Mn: 0.5
wt% or less, Al: 0.1 wt% or less, P: 0.05 wt% or less, S:
0.01 wt% or less and N: 0.003 wt% or less, and Ti:
Steel containing at least one of 0.1 wt% or less and Nb: 0.05 wt% or less, and the balance being Fe and inevitable impurities is used as a material, and when this steel material is hot-rolled, it is The cumulative rolling reduction in the 3 passes on the delivery side is 50% or more
Finish rolling is completed at the Ar ₃ transformation point or higher, and cooling is started within 0.5 seconds after the finish rolling is completed.
₃ transformation point -60 ℃) at a cooling rate of 50 ~ 400 ℃ / s, and then wound at a temperature in the range of 800 ~ 550 ℃,
After that, cold rolling and then continuous annealing are performed, which is a method for producing a cold rolled steel sheet having excellent deep drawability (first
invention).

(2) C: 0.0015 wt% or less, Si: 0.1 wt% or less, Mn: 0.5 wt% or less, B: 0.0001 to 0.0030 wt%, Al:
0.1 wt% or less, P: 0.05 wt% or less, S: 0.01 wt% or less and N: 0.003 wt% or less, and at least one of Ti: 0.1 wt% or less and Nb: 0.05 wt% or less Then
The balance is made of steel consisting of Fe and unavoidable impurities,
When hot rolling this steel material, finish rolling at the Ar ₃ transformation point or higher with a cumulative rolling reduction of 50% or more in the exit 3 passes in finish rolling, and cool within 0.5 seconds after the finish rolling. From the cooling start temperature (Ar ₃ transformation point −60
(C) is cooled at a cooling rate of 50 to 400 ° C / s, then wound at a temperature in the range of 800 to 550 ° C, then cold rolled and then continuously annealed. A method for producing a cold rolled steel sheet having excellent deep drawability (second invention).

The results of the research on which the present invention is based will be described below. C: 0.0008 wt%, Si: 0.008 wt%, M
n: 0.1 wt%, P: 0.012 wt%, S: 0.001 wt%, Al:
0.05wt%, N: 0.0018wt%, Ti: 0.01wt%, Nb: 0.003
A steel slab having a composition of wt% and B: 0.0005 wt% is heated and soaked at 1150 ° C, then hot rolling is completed at a hot rolling finishing temperature of 920 ° C, and 0.3 seconds after the completion of rolling, immediately cooling to 860 ° C is completed. gave. Subsequently, the hot rolled sheet thus obtained was wound at 750 ° C., cold rolled at a rolling reduction of 80%, and then recrystallized at 860 ° C. for 20 seconds.

FIG. 1 shows the effects of the exit side three-pass cumulative rolling reduction in finish rolling and the cooling rate after hot rolling on the r value after cold rolling and annealing. From FIG. 1, the r value after cold rolling-annealing depends on the final reduction rate during finish rolling and the cooling rate after hot rolling, and the finish reduction rate is 50% or more, preferably the cumulative reduction rate in the three passes on the outlet side, preferably. It was found that the crystal grain size of the hot-rolled sheet was made finer and a high r value was obtained by increasing the cooling rate after hot rolling to 60% or more.

[0013]

As a result of repeated studies based on the above experimental results, the inventors have limited the scope of the invention as follows. (1) Steel material composition C: 0.0015 wt% or less, Si: 0.1 wt% or less, Mn: 0.5 wt%
Below, Al: 0.1 wt% or less, P: 0.05 wt% or less, S: 0.01
wt% or less and N: 0.003 wt% or less and Ti: 0.1
At least one of wt% or less and Nb: 0.05 wt% or less is further contained, and if necessary, B: 0.0001 to 0.0030 wt%
A steel that contains Fe and the unavoidable impurities as the balance. The reasons for limitation of each component are shown below.

(A) C: 0.0015 wt% or less The smaller the content of C, the better the deep drawability and the ductility, and the lower the content of C, the smaller the amount of Ti and Nb added for fixing the same. Since the amount of carbides generated is small, extremely excellent deep drawability can be obtained. The upper limit of the C content is 0.0015 wt% as an allowable range in the ultra-low carbon steel that can be achieved by the recent refining technology.

(B) Si: 0.1 wt% or less Since Si has the effect of hardening the steel and adversely affects the deep drawability and surface properties, it is limited to 0.1 wt% or less. (c) Mn: 0.5 wt% or less Mn also has the effect of hardening the steel and adversely affects the deep drawability, so it was limited to 0.5 wt% or less. (d) P: 0.05 wt% or less P also acts to harden the steel and adversely affects deep drawability and brittleness, so it was limited to 0.05 wt%.

(E) S: 0.01 wt% or less S combines with Mn to form MnS and precipitates. If a large amount of MnS precipitates, it hardens the steel and reduces press formability.
Therefore, the smaller the S content, the better the deep drawability, so the content was limited to 0.01 wt% or less. In order to further improve the deep drawability, it is preferably 0.005 wt% or less.

(F) Al: 0.1 wt% or less Al is added in order to deoxidize molten steel and improve the yield of Ti and Mb. However, excessive addition of Al impairs the press formability of the steel sheet, so it was limited to 0.1 wt% or less.

(G) N: 0.003 wt% or less The smaller the amount of N, the better the deep drawability, and the lower the amount of N, the smaller the amount of Ti and Nb added, and the less the nitrides produced. 0.003 wt
Limited to less than or equal to%. In order to further improve the deep drawability, it is preferably 0.002 wt% or less.

(H) Ti: 0.1 wt% or less Ti is a component useful for obtaining a good press formability by precipitating and fixing solute C and solute N in steel as carbon and nitride to reduce them. Is. Therefore, in the present invention, Ti alone or
Add with Nb. The amount of Ti added should be 0.1 wt%, and even if added in excess of 0.1 wt%, no further effect can be obtained, and conversely it leads to deterioration of ductility, so it was limited to 0.1 wt% or less. The lower limit is preferably about 0.005 wt% or more so that solid solution C and solid solution N are sufficiently precipitated and fixed.

(I) Nb: 0.05 wt% or less Nb is a component useful for obtaining a good press formability by precipitating and fixing solid solution C in the steel as a carbide to reduce it. Therefore, in the present invention, Add Nb alone or with Ti. The amount of Nb added is sufficient to be 0.05 wt%.
%, No further effect can be obtained and, conversely, it leads to deterioration of ductility, so it was limited to 0.05 wt% or less. The lower limit is 0.003 wt in order to precipitate and fix solid solution C sufficiently.
% Or more is preferable.

(J) B: 0.0001 to 0.003 wt% B is added in the second invention for improving the secondary processing brittleness resistance. If the added amount is less than 0.0001 wt%, there is no effect, while if added over 0.003 wt%, no further effect is obtained and conversely the deep drawability deteriorates.
Limited to 3wt%.

(2) Manufacturing process applied to steel material When hot rolling is performed, finishing rolling is completed at an Ar ₃ transformation point or higher with a cumulative reduction of 50% or more in 3 passes on the exit side in finishing rolling. Cooling is started within 0.5 seconds after finishing rolling, and the temperature range from the cooling start temperature to (Ar ₃ transformation point −60 ° C) is cooled at a cooling rate of 50 to 400 ° C / s, and then 800 to 550.
Winding is carried out at a temperature in the range of ° C, followed by cold rolling and then continuous annealing. The reasons for limiting each manufacturing process will be described below.

(I) Hot Rolling Step The hot rolling step is the most important in the present invention, and the cumulative rolling reduction is achieved in the final three passes in the finish rolling after the rough rolling.
Rolling is performed to 50% or more, hot rolling is completed at the Ar ₃ transformation point or higher, and within 0.5 seconds after the hot rolling is completed, the temperature range from the cooling start temperature to (Ar ₃ transformation point −60 ° C.) 50 to 400
It is necessary to cool at a cooling rate of ° C / s.

The metallurgical change of the steel slab in the manufacturing process of the hot rolled steel sheet from heating of the steel slab to rough rolling, finish rolling, cooling and coil winding is caused by the rough rolling of coarse austenite (γ) grains during the heating of the slab. During the finish rolling, deformation-recrystallization is repeated to form fine grains, and in the subsequent cooling process, the fine grains γ are transformed into a structure mainly composed of ferrite (α) grains. The γ → α transformation from the recrystallized γ grains begins with the formation of α nuclei at the γ grain boundaries, proceeds with the growth of the α nuclei, and ends when the α grains collide with each other.
Therefore, the smaller the γ grains before transformation are, the more the α nuclei are generated, so that the distance until they hit each other becomes shorter and the α grains become finer. Heretofore, such a recrystallization has been aimed at miniaturization of γ grains, and further aimed at improvement of deep drawability by miniaturization of α grains.

By the way, in the latter stage of the finish rolling which is the state before the γ → α transformation, the recrystallization of the γ grains is difficult to occur immediately after the rolling because the rolling temperature is low, and the state remains the processed state.
Such processed γ-grains have a larger grain boundary area per unit area because they are grains that are longer than recrystallized grains. In addition, a processed structure called a deformation zone is formed in the grain and becomes a place for α nucleation. Therefore, the processed γ grains are simply recrystallized γ described above.
Since there are significantly more α-nucleation sites than the grains, the α-grains transformed from the processed γ-grains are finer than the α-grains from the recrystallized γ-grains.

The point of the present invention is to accumulate a large amount of γ grains in the above-mentioned processed state, and to utilize the transformation from the processed γ grains to make α grains finer. That is, in the normal process, the rolling temperature is high, so that the processing is easy and the re-crystallization immediately occurs where recrystallization is high. However, the reduction rate is low. Therefore, in the conventional process, even if processing is performed by hot finish rolling, the γ-grains are recrystallized afterwards due to the high rolling temperature at which strong working is performed, and after all, a small amount of γ-grains are processed at the end of rolling. Only accumulated. Moreover, since there is a non-water cooling zone for several seconds before the steel sheet reaches the water cooling equipment from the exit side of the finish rolling equipment, reworking and recovery (disappearance of deformation zone) of the processed γ-grains progresses during that time, and the processed state The transformation from the γ-grains could not be expected.

On the other hand, in the present invention, the reduction ratio in the latter stage of finish rolling having a low rolling temperature is increased to accumulate a large amount of processed γ grains in the steel, and in addition, the steel sheet temperature is immediately reduced after the finish rolling. By shortening the residence time in the γ region of the steel sheet, both the recrystallization temperature and the recovery rate are rapidly reduced, so it is possible to reach the α transformation while maintaining the processing state, and it is possible to achieve finer grain refinement than before. It became. Due to the α grain refinement mechanism of the present invention, the cumulative rolling reduction in the final three passes in finish rolling is set to 50% or more.

The finish rolling finish temperature is not lower than the Ar ₃ transformation point. If the end temperature is lower than the Ar ₃ transformation point, coarse grains will be generated in the hot rolled sheet or the work structure will remain, impairing the deep drawability of the steel sheet after cold rolling and annealing. Further, cooling can be started from an appropriate temperature above the Ar ₃ transformation point. That is, as long as the cooling start temperature is at or above the Ar ₃ transformation point, the cooling start timing does not have to be immediately after finish rolling, and cooling can be started at an appropriate position on the runout table. Therefore, in the present invention, even in a normal rolling mill in which the strip thickness gauge and the thermometer are arranged after the finishing rolling mill, the strip thickness and temperature of the rolled material are measured and managed without being affected by steam or the like due to cooling. , Controllable.

(Ii) Cooling Step Next, the cooling rate after finish rolling in the present invention is such that the temperature range from the cooling start temperature to (Ar ₃ transformation point −60 ° C.) is 50 to 400 ° C./s. . That is, the cooling start temperature is Ar ₃ transformation point or higher, the cooling end temperature is (Ar ₃ transformation point −60 ° C.) or lower, and from such cooling start temperature (Ar ₃ transformation point −60 ° C.)
It is necessary to cool the temperature range up to 50 to 400 ° C / s. By applying this cooling method to the material finished under high pressure by increasing the reduction rate in the latter stage of finish rolling as described above,
It has made it possible to enhance the deep drawability of the steel sheet after cold rolling and annealing. Specifically, after the finish rolling, the conventional method in which only the cooling conditions were specified (Japanese Patent Laid-Open No. 1-177322) had a maximum r value of 2.5 for the cold-rolled / annealed sheet. By setting the cumulative pressure ratio of the final 3 passes to 50% or more and cooling this material under the above cooling conditions, it was possible to obtain an r value of 2.7 for the cold rolled and annealed sheet.

If the cooling is started from the Ar ₃ transformation point or lower or the cooling rate is 100 ° C./s or lower, the residence time in the γ region becomes long, during which the γ grains are recrystallized and recovered ( Disappearance of deformation zone) progresses, and transformation from γ grains in the processed state cannot be expected. Therefore, in the present invention, by cooling immediately after hot rolling, the processed γ grain state is frozen (state in which rolling strain is accumulated), and the α grain is refined by the subsequent γ → α transformation. At this time, it is desirable that the cooling device is arranged as close as possible to the finish rolling mill within a range that does not hinder the operation of the thermometer and the plate thickness gauge that are usually arranged after the finish rolling mill. This is because cooling starts from the Ar ₃ transformation point or higher. The cooling method may be either water cooling or gas cooling.

(Iii) Winding Step Next, the winding temperature in the present invention is 800 ° C to 550 ° C.
The effect at the time of winding is to completely precipitate the precipitate and further to increase the growth coarseness of the precipitate. In the present invention, a large amount of AlN is precipitated before winding, and it can be sufficiently deposited and fixed even at low temperature winding, but if the winding temperature is too low such that the winding temperature falls below 550 ° C, the winding shape will deteriorate. Or
Cannot improve deep drawability. Further, when winding at a temperature higher than 800 ° C, the pickling property of the hot rolled sheet deteriorates due to the generation of scale, so 800 ° C was set as the upper limit. The hot rolling slab heating temperature is not particularly limited, but is 1000 ° C or higher.
If the temperature is 1300 ° C or lower, a good material can be obtained. The hot-rolled slab can be used immediately after continuous casting or after being cooled once.

(Iv) Cold rolling process The conditions for cold rolling are not particularly specified, but a cold rolling reduction ratio of 50 to 95%, preferably 70 to 90%, results in a particularly excellent deep drawing. A cold-rolled steel sheet having properties is obtained.

(V) Annealing Step The cold rolled steel strip that has undergone the cold rolling step needs to be subjected to recrystallization annealing. The annealing method may be either a box-type annealing method or a continuous-type annealing method. The annealing temperature is not too high and the temperature below recrystallization is not preferable, and it is preferably 700 ℃ ~
The range of 900 ℃ is good. The annealed steel strip may be subjected to temper rolling of 5% or less in order to correct the shape and adjust the surface roughness. The cold-rolled sheet obtained by the present invention can also be applied to an original sheet of a worked surface-treated steel sheet. The surface treatment includes zinc plating (including alloy system), tin plating, enamel and the like.

[0034]

EXAMPLE A steel slab having the composition shown in Table 1 is shown in Table 2
Finish rolling was performed under the hot rolling conditions shown in. The obtained hot-rolled sheet was pickled, cold-rolled to 80% to form a cold-rolled steel strip with a thickness of 0.7 mm, and then recrystallized in a continuous annealing facility at 860 ° C for 20s. And then a 0.7% temper rolling. Table 2 also shows the results of an examination of the r value of the cold rolled steel sheet thus obtained and the grain size of the hot rolled sheet. The r value is measured by the 3-point method after applying a 15% tensile prestrain using JIS No. 5 tensile test piece, and L direction (rolling direction), D direction (45 ° direction to rolling direction) and C direction. The average value (in the direction of 90 degrees in the rolling direction) was determined as r = (r _L + 2r _D + r _C ) / 4.

[0035]

[Table 1]

[0036]

[Table 2]

As shown in Table 2, a steel slab having a chemical composition within the scope of the present invention was used, and the finish rolling rate in hot rolling, the rolling end temperature, the cooling start temperature under the conditions according to the present invention,
It can be seen that by performing hot rolling at the cooling end temperature and the cooling rate, it is possible to manufacture a cold rolled steel sheet having extremely excellent deep drawability.

[0038]

According to the present invention, by limiting the steel composition and manufacturing conditions, it is possible to manufacture a cold-rolled steel sheet having a deep drawability that is far superior to conventional ones.

[Brief description of drawings]

FIG. 1 is a graph showing influences of a delivery-side three-pass cumulative reduction rate in finish rolling and a cooling rate after hot rolling on an r value after cold rolling-annealing.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Sakata 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Technical Research Division, Kawasaki Steel Corporation (72) Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba Kawasaki Steel Corporation Technical Research Division

Claims (2)

[Claims] 1. C: 0.0015 wt% or less, Si: 0.1 wt% or less, Mn: 0.5 wt% or less, Al: 0.1 wt% or less, P: 0.05 wt% or less, S: 0.01 wt% or less and N: 0.003. containing at least one of Ti: 0.1 wt% or less and Nb: 0.05 wt% or less, with the balance being Fe and unavoidable impurities as a raw material. When rolling, finish rolling at the Ar ₃ transformation point or higher with a cumulative rolling reduction of 50% or more on the exit side 3 passes in finishing rolling, and start cooling within 0.5 seconds after the completion of finishing rolling and start cooling. The temperature range from temperature to (Ar ₃ transformation point -60 ° C) is 50 ~
After cooling at a cooling rate of 400 ° C / s, it is wound at a temperature in the range of 800 to 550 ° C, then cold rolled, and then continuously annealed. Production method. 2. The method according to claim 1, wherein the steel material is C: 0.0015 wt% or less, Si: 0.1 wt% or less, Mn: 0.5 wt% or less, Al: 0.1 wt% or less, B: 0.0001 to 0.0030 wt. %, P: 0.05 wt% or less, S: 0.01 wt% or less and N: 0.003 wt% or less, and at least one of Ti: 0.1 wt% or less and Nb: 0.05 wt% or less, and the balance Is a steel consisting of Fe and unavoidable impurities, which is a method for producing a cold-rolled steel sheet with excellent deep drawability.