JP2526122B2 - Manufacturing method of cold-rolled steel sheet for deep drawing by strip casting - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a cold-rolled steel sheet for deep drawing by strip casting.

2. Description of the Related Art Steel sheets used for forming automobile doors, oil pans, etc. are required to have excellent deep drawability. Conventionally, such a steel sheet is obtained by cutting a completely solidified continuously cast slab, cooling it, and then charging it in a soaking furnace heated and held at a temperature of about 1200 ° C to soak it and hot-roll it into a hot-rolled coil. Further, it is manufactured by further performing pickling, cold rolling and annealing.

However, recently, as the improvement of the surface property of the slab has progressed, energy saving by omitting the process has been studied, and a technique of continuously hot rolling without reheating the slab as cast, or even as cast. Has developed a technology for producing a slab having a plate thickness equivalent to that of a hot rolled steel plate. In particular,
The latter technique is called strip casting, and it enables significant reduction of steps in the manufacturing process of cold-rolled steel sheet, so that further technological development is expected in the future. However, this technique has a problem that after cold rolling and annealing, the deep drawing formability of the steel sheet is significantly inferior to that of the conventional steel sheet.

By the way, when a steel obtained by adding Ti to an ultra-low C steel is used as a cold-rolled steel sheet according to the conventional method, a hot-rolled coil in which C in the steel is fixed by ti or Nb is obtained. It is already well known that cold rolling and recrystallization annealing can impart extremely excellent deep drawability. Further, by fixing C in the steel with Ti, Nb, etc., and then performing ferrite area rolling, and further performing recrystallization annealing,
It is already known that the deep drawing formability of a hot rolled steel sheet can be improved.

Usually, in order to fix C in steel in the state of hot rolled coil, a method of winding the coil at a high temperature to some extent, for example, about 700 ° C. is adopted, and C in steel before the ferrite rolling is adopted. In order to fix, the method of holding the rough bar billet at a temperature of 800 to 900 ℃ for a long time is adopted. Therefore,
If cold-rolled steel sheet is manufactured by cold-rolling and annealing a thin-walled slab by strip casting, it is very inferior in deep drawing formability because the thin-walled slab is solidified in the steel. It seems that one of the causes is the presence of a relatively large amount of dissolved C.

The above can also be understood from the conventional techniques described below. For example, in Japanese Patent Laid-Open No. 61-96031, in the production of a cold-rolled steel sheet for press working using a thin casting strip, in the case of a quenching casting strip, C,
Since it is impossible to fix N and S, the total amount of N and S is regulated to improve the r value.
No. 133324 discloses a method for producing a thin steel sheet having excellent formability by using a thin slab, increasing the winding temperature after casting, or heating the slab after casting, and the degree of aggregation of precipitates. It is described that the ductility of the steel sheet is increased by increasing the.

On the other hand, in JP-A-59-43823 and JP-A-59-43825, C in the steel of a rapidly cast thin cast piece is cold-rolled and annealed without being fixed by Nb, Ti or the like. However, it is described that a high r value can be obtained.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, as described above, heating for the purpose of fixing the solid solution C of the slab and holding for a certain period of time at a predetermined temperature in the cooling process of the slab are performed in recent manufacturing processes. This is contrary to simplification and energy saving, and eventually leads to an increase in manufacturing cost. Therefore, it is desirable to sufficiently fix the solid solution C while the thin cast piece is manufactured.

Under the technical background as described above, the inventors of the present invention, by appropriately controlling the chemical composition of the thin slab and the cooling after casting, in the process of cooling the slab to room temperature, Solid solution C
The present invention has been made by finding that Ti can be sufficiently fixed with Ti.

That is, an object of the present invention is to provide a method for producing a cold rolled steel sheet for deep drawing by fixing solid solution C in the as-manufactured thin cast piece and cold rolling it.

Means for Solving the Problems The method for producing a cold-rolled steel sheet for deep drawing by strip casting according to the present invention is (a) C 0.001 to 0.01%, Mn 0.10 to 0.30%, and N 0.005% by weight. % Or less, and (b) the weight ratio of at least one element selected from the group consisting of Ti 0.02 to 0.10% and Nb 0.02 to 0.05%.
Ti ^* / C ^* ≥ 4.0 (in the formula, Ti ^* = Ti + (48/93) Nb (wt%), C ^* = C + (12/14) N (wt%)) is contained so that the balance the thin cast steel strip consisting of iron and unavoidable impurities continuous casting, shall apply in Ar ₃ to Ar following average cooling rate of 15 ° C. / sec between ₁ and was cooled to Ar ₁ point is 830 ° C. or less rear,
It is characterized by performing cold rolling and recrystallization annealing.

 First, the present invention will be described based on experimental facts.

C 0.0030%, Si 0.01%, Mn 0.22%, P 0.017%, S 0.005%, Al 0.027%, Ti 0.088%, N 0.0025% have chemical components, and stoichiometrically enough C in steel is contained. An ultra-low C-Ti steel made by adding a fixed amount of Ti was vacuum-melted and cast between steel molds with a gap of 8 mm.
After being rapidly cooled to ℃ and solidified, it was cooled to room temperature at various cooling rates. After that, a 4 mm thick tensile test piece was obtained by grinding the front and back surfaces, the cast piece was given cold working of 10%, and further subjected to strain aging treatment by heating at 170 ° C. for 10 minutes. And the test piece yield point thus obtained,
Difference from the yield point of the as-rolled test piece (ΔYP (kgf / m
The degree of m ² )) was evaluated as a measure of the amount of C dissolved in the cast piece as a solid solution. Therefore, it is indicated that the larger ΔYP is, the larger the amount of C dissolved in the thin cast piece is. In general, ΔYP is
When it is 1 kgf / mm ² , the amount of solid solution C is said to be 2 to 5 ppm. The measurement result of ΔYP is shown in FIG.

As can be seen in Fig. 1, ΔYP greatly depends on the continuous cooling rate from about 1000 ° C, and the continuous cooling rate is about 20%.
ΔYP is extremely small at ℃ / sec or less. About 10 after casting
At the time of quenching to 00 ° C, the solubility product of Ti and C
Since C is assumed to be in a non-precipitated state, approximately 20 ° C /
The extremely small ΔYP obtained by cooling with a cooling rate of sub-second or less indicates, surprisingly, that TiC precipitation occurs actively during the continuous cooling process from about 1000 ° C.

That is, although it has been conventionally known that the precipitation reaction of Ti and C is promoted in the ferrite rather than in the austenite, the reduction reaction in the high temperature range of the austenite or the ferrite causes the precipitation reaction in the material. It is understood that Ti and C do not precipitate in the continuous cooling process when dislocations and deformation zones are not introduced.

Thus, the present inventors conventionally, when rapidly cooling the thin slab after casting to near room temperature, while no precipitation of TiC was observed, after casting, the temperature of about 1000 ℃. By cooling at a relatively slow continuous cooling rate from
It has been found that precipitation of TiC can occur. When the cooling rate is higher than about 20 ° C / sec, ΔYP is large because Tic
It is considered that this is because sufficient time for precipitation was not secured in the high temperature region necessary for precipitation of.

Next, the present inventors investigated the TiC precipitation mechanism in the continuous cooling process from about 1000 ° C. as described above. That is,
After casting, when continuously cooled at about 1000 ℃ to 10 ℃ / sec,
The precipitation conditions of TiC were investigated by ΔYP at different temperatures. Results are shown in FIG.

In Fig. 2, the precipitation of TiC became active around 820 ° C, and at around 750 ° C, the precipitation almost finished and ΔYP was 1 kg.
It is about f / mm ² . The reason why the precipitation of TiC rapidly progresses in such a temperature range is not clear, but Ar
₃ points and Ar ₁ point seem to play an important role.
That is, most of the precipitation phenomena, it is the behavior between the Ar ₃ to Ar ₁ point, and shall apply in Ar ₁ point is relatively high, and the influence that the deposition reaction of TiC is advanced easy temperature It seems that it is. However, as the Ar ₁ point is at high temperatures, nor those TiC precipitation proceeds, as described below, Ar ₁ point is 800
It is important that the temperature is below ℃ to accelerate the precipitation of TiC.

The present invention has been made based on such findings.

Next, the chemical composition of the slab used in the present invention will be described.

The lower the amount of C added, the better the deep-draw formability of the cold-rolled steel sheet obtained, but when it is less than 0.001%, it is extremely difficult to melt the steel, while when it exceeds 0.01%. When added, a large amount of Ti or Nb needs to be added to fix C, resulting in an increase in manufacturing cost. Therefore,
In the present invention, the C content is in the range of 0.001 to 0.01%.

Mn is added in an amount of 0.10% or more in order to prevent hot brittleness of steel and to optimize the Ar ₁ point.
Since the Ar ₃ point and Ar ₁ point are extremely low, precipitation of TiC is suppressed, the steel is deteriorated due to deep drawing, and the increase in steel strength becomes excessive, leading to deterioration of ductility, so 0.30% is the upper limit. And

The smaller the N content, the more effectively Ti or Nb will be used to fix C. On the other hand, when it exceeds 0.005%, the deep drawability deteriorates. Therefore, the upper limit of N amount is 0.005%
And

Ti and Nb have a strong affinity with the solid solution C of steel and are essential elements for the precipitation of carbonitrides. However, for any element, when the added amount is less than 0.02%,
When C is insufficiently fixed, on the other hand, when Ti exceeds 0.10% and when Nb exceeds 0.05%, the respective effects are saturated, resulting in an increase in manufacturing cost.

Particularly, according to the present invention, when Ti ^* = Ti + (48/93) Nb (weight%) and C ^* = C + (12/14) N (weight%), Ti ^* / C ^* ≧ 4.0 is satisfied. By doing so, C in steel can be more reliably fixed in actual operation.

Generally, the cast slab contains P and S that are not necessarily impurities. Therefore, in the present invention, it is allowed to contain P and S in the above-mentioned amounts, and further, regarding Si, Cr, Mo, Ni and Cu, respectively,
0.1% or less, V0.02% or less, B0.001% or less, Zr, REM, Ca
It is also permissible to contain 0.02% or less for Ce and Ce, respectively.

According to the present invention, continuous casting cast steel strip of thin having such chemical components, shall apply in Ar ₃ to Ar following average cooling rate of 15 ° C. / sec between ₁ and, Ar ₁ point is 830 ° C. or less After cooling so that by performing cold rolling and recrystallization annealing,
A cold rolled steel sheet having excellent deep drawing formability can be obtained by strip casting.

In the present invention, the average cooling rate after the production of the thin cast steel strip is, as described above, between Ar ₃ and Ar ₁ , 15 ° C./sec or less in order to actively perform the precipitation of TiC, and , Ar
As mentioned above, _one point is required to be 830 ° C or lower. This is because precipitation of TiC is considered to be significantly delayed when the Ar ₁ point exceeds 830 ° C. In addition to the cooling rate, the Ar ₁ point is affected by the chemical composition, the state of the austenite structure, etc. In particular, the austenite grain size greatly affects the Ar ₁ point, and refining of recrystallized austenite by strong rolling in the austenite state may increase the Ar ₁ point to 830 ° C. or higher. It is recommended that the processing of (1) be approximately 50% or less at the total rolling reduction. The austenite grain size immediately before transformation is difficult to indicate exactly, but considering the experimental results, it is preferably about 100 μm or more.

Next, the slab may be cooled to room temperature after cooling it to a temperature of Ar ₁ point or lower, or may be wound into a coil. For the purpose of increasing the precipitation of TiC as much as possible, it is desirable to wind the coil at 700 ° C. or higher, but on the other hand, such high temperature winding makes it difficult to remove the scale in the subsequent step of pickling. Generally, the range of 600 to 700 ° C is preferable.

After that, cold rolling and recrystallization annealing may be performed according to a conventional method. The cold rolling rate is preferably 50 to 85%. The recrystallization annealing may be batch annealing or continuous annealing, but 600 to 750 ° C for batch annealing and 700 to 900 for continuous annealing.
It is desirable to heat to a temperature in the range of ° C.

The steel sheet according to the present invention can be used not only as a cold-rolled steel sheet but also as an original plate for CG and EG plating steel sheets.

EFFECTS OF THE INVENTION As described above, according to the method of the present invention, a thin cast steel strip made of extremely low C-Ti and / or Nb is continuously cast, and this is cast at a predetermined average cooling rate and Ar ₁ After cooling so that the point becomes 830 ° C. or less, by cold rolling and recrystallization annealing, by strip casting, it is possible to obtain a cold-rolled steel sheet excellent in deep drawability. it can.

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

As shown in Table 1, steels having various chemical compositions were melted by the above-mentioned method and cast into 8 mm thick slabs. About 100
The sample was cooled to room temperature at an average cooling rate of 0 ° C to 10 ° C / sec. After that, by grinding the front and back surfaces to make a 4 mm thick steel plate,
A 1 mm thick steel plate was obtained by cold rolling. Subsequently, 850 ℃
After 90 seconds of continuous annealing, the mechanical properties were examined. The results are shown in Table 1.

Comparative steel 2 has an average cooling rate between 900 and 700 ° C after casting.
Comparative steel 3 which was cooled to room temperature at 70 ° C / sec.
After casting, it was cooled to room temperature between 900 and 700 ° C at an average cooling rate of 70 ° C / sec, and then reheated at 700 ° C for 3 hours.

In Table 1, invention steels 1, 4 and 5 satisfying the conditions specified in the present invention show excellent elongation and r value. On the other hand, in Comparative Steel 2, as described above, since the slab was rapidly cooled to room temperature, a large amount of solute C remained in the steel at the time of cold rolling, resulting in an extremely low r value. However, as shown in Comparative Steel 3, even if the slab is rapidly cooled to room temperature,
A high r-value was obtained by reheating at 700 ° C.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the continuous cooling rate from about 1000 ° C. and ΔYP after casting a slab, and FIG. 2 is a graph showing the relationship between continuous cooling rate of about 1000 ° C. and 10 ° C./sec. 6 is a graph showing the relationship between temperature and ΔYP.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/14 C22C 38/14

Claims (1)

(57) [Claims] 1. By weight% (a) C 0.001 to 0.01%, Mn 0.10 to 0.30%, and N 0.005% or less, and (b) Ti 0.02 to 0.10% and Nb 0.02 to 0.05%. Weight ratio of at least one element selected from the group
Ti ^* / C ^* ≥ 4.0 (in the formula, Ti ^* = Ti + (48/93) Nb (wt%), C ^* = C + (12/14) N (wt%)) is contained so that the balance the thin cast steel strip consisting of iron and unavoidable impurities continuous casting, shall apply in Ar ₃ to Ar following average cooling rate of 15 ° C. / sec between ₁ and was cooled to Ar ₁ point is 830 ° C. or less rear,
A method for producing a cold-rolled steel sheet for deep drawing by strip casting, which comprises performing cold rolling and recrystallization annealing.