JPH0317233A - Manufacture of cold rolled steel sheet for deep drawing by strip casting - Google Patents

Abstract

PURPOSE:To manufacture the cold rolled steel sheet having excellent deep drawability by subjecting a thin cast steel strip contg. specified ratios of C, Mn, N, Ti and Nb to continuous casting, cooling it under specified conditions and executing cold rolling and recrystallization annealing. CONSTITUTION:A thin cast steel strip contg., by weight, 0.001 to 0.01% C, 0.10 to 0.30% Mn and >=0.005% N, contg. at least one kind of elements selected from 0.02 to 0.10% Ti and 0.02 to 0.05% Nb so as to satisfy, by weight, Ti*/C*>=4.0 [Ti*=Ti+(48/93)Nb(wt.%) and C*=C+(12/14)N(wt.%)] and the balance iron with inevitable impurities is subjected to continuous casting. Next, the cast steel strip is cooled so that the average cooling rate from Ar3 to Ar1 is regulated to <=15 deg.C/sec and the Ar1 point to <=830 deg.C and is thereafter subjected to cold rolling and recrystallization annealing. In this way, solid soln. C is fixed in the thin slab as manufactured to obtain the cold rolled steel sheet having excellent deep drawability.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a cold-rolled hollow plate for deep drawing by strip casting.

Steel plates used for forming automobile doors, oil vans, etc. are required to have extremely good deep drawing properties.

Conventionally, such steel sheets have been produced by cutting completely solidified continuously cast slabs, cooling them, charging them into a soaking furnace heated and maintained at a temperature of about 1200°C, soaking them, and hot-rolling them into hot-rolled coils. , and is further produced by cold rolling and annealing after pickling.

However, as improvements in the surface properties of slabs have recently progressed, energy savings through the omission of processes have been considered, and technology has been developed to continuously hot-roll slabs as cast without reheating them, and even more. A technology has been developed to produce slabs with the same thickness as hot-rolled steel sheets. In particular, the latter technique is called strip casting, and because it enables a significant reduction in process steps in the manufacturing process of cold-rolled steel sheets, further technological development is expected in the future. However, this technology includes cold rolling,
After annealing, there is a problem in that the deep drawing properties of the steel sheet are significantly inferior to those made using conventional methods. By the way, extremely low Cfi
When steel made by adding Ti to L is made into cold rolled steel sheets according to the conventional method, a hot rolled coil is obtained in which C in the steel is fixed with Tt, Nb, etc., and this is cold rolled and recrystallized. It is already well known that extremely excellent deep drawability can be imparted by annealing. Also, C in steel
It is already known that the deep drawing formability of hot rolled steel sheets can be improved by fixing them with Ti, Nb, etc., rolling them in the ferrite region, and then recrystallizing them. Normally, to fix C in the rope in a hot-rolled coil state,
A method of winding the coil at a certain high temperature, for example, around 700°C, is adopted, and in order to fix the C in the steel before ferrite rolling, the rough bar steel piece has a temperature of 800 to 900°C.
The method used is to maintain the temperature at ℃ for a long time. Therefore, if cold-rolled IiIvi is produced by cold-rolling and annealing a thin-walled slab by strip casting, it will have very poor deep drawing formability because it will harden into steel in the form of a thin-walled slab. One of the reasons seems to be that there is a relatively large amount of dissolved C.

The above can also be understood from the conventional techniques shown below. For example, in Japanese Patent Application Laid-open No. 61-96031, in the production of press-formed cold-rolled steel sheets using thin cast strips, in the quenched cast strips,
Since it is impossible to fix C, N, and S, the total amount of them is regulated to improve the r value. In a method for manufacturing a thin steel sheet with excellent formability, the ductility of the steel sheet can be increased by increasing the coiling temperature after casting or heating the slab after casting to increase the degree of agglomeration of precipitates. It is written. On the other hand, JP-A-59-43823 and JP-A-59-4
Publication No. 3825 discloses that carbon in rapidly cooled thin slab steel is replaced by Nb.
It is described that a high r value can be obtained even if cold rolling and annealing are performed without fixing with STi or the like.

However, as mentioned above, heating for the purpose of fixing solid solution C in the slab and holding the slab at a predetermined temperature for a certain period of time during the cooling process have changed in recent manufacturing processes. This goes against simplification and energy saving, and also increases manufacturing costs, so it is preferable to sufficiently fix the solid solution C in the thin cast slab as it is manufactured.

Based on the above-mentioned technical background, the present inventors have succeeded in improving the hardness of steel during the process of cooling the slab to room temperature by appropriately controlling the chemical composition of the thin slab and the cooling after casting. ? 8C
The present invention was developed based on the discovery that it is possible to sufficiently fix the temperature at Tt.

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

Goho Gakuin Lt4 Dai! ■The method for producing cold rolled steel sheets for deep drawing by strip casting according to the present invention is expressed in weight% (arc
0.0 0 1 to 0.0 1%, Mn 0.10-0
．． (b) Ti 0.02 to 0.10%, and Nb O
. 02 to 0.05% at a weight ratio of at least one element selected from the group consisting of
'+(48/93)Nb (weight%>, c''=c+
(t2/14) N (wt%)), the balance was iron and unavoidable impurities, and the thin cast steel strip was continuously cast, and the average cooling rate between Ar+ and Ar was 15°C/sec. and Ar. It is characterized by cold rolling and recrystallization annealing after cooling to a temperature of 830° C. or lower.

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

C 0.0030%, Si 0.01%, Mn 0.22%, P 0.017%, so. oos%, Al 0.027%, Ti 0.088%, N 0.0025%, and Ti is added in an amount that can sufficiently fix C in the steel stoichiometrically. Ultra-low C-Ti steel was vacuum melted and cast between steel molds with a gap of 8 fl, which was rapidly cooled to approximately 1000°C to solidify, and then cooled to room temperature at various cooling rates. .

Thereafter, the front and back surfaces were ground to obtain a tensile test piece with a thickness of 4fl, and this slab was subjected to 10% cold working, and then
Strain aging treatment was performed by heating at 70° C. for IO minutes. Then, the difference (ΔY P (kgf/m
m”)) was evaluated as a measure of Cl dissolved in the slab as it is. Therefore, the larger ΔYP is, the greater the amount of C dissolved in the thin slab.Generally, ,
When ΔYP is I kgf/mm, solid solution Cfi
It is said that t is present in an amount of 2 to 5 ppm. The measurement results of ΔYP are shown in FIG.

As seen in FIG. 1, ΔYP is highly dependent on the continuous cooling rate from about 1000° C., and when the continuous cooling rate is about 20° C./sec or less, ΔYP is extremely small. After casting, it is assumed that TiC is in an unprecipitated state from the solubility product of Ti and C when it is rapidly cooled to about 1000°C. The small ΔYP indicates that TiC precipitation surprisingly occurs actively during the continuous cooling process from about 1000°C.

In other words, although it has been known that the precipitation reaction between Ti and C is more accelerated in ferrite than in austenite, dislocations and This is because it was understood that if no deformation zone is introduced, Ti and C will not precipitate during the continuous cooling process.

In this way, the present inventors discovered that, while conventionally, when a cast thin slab was rapidly cooled to around room temperature, no precipitation of TiC was observed, It has been found that precipitation of TiC can occur by cooling at a relatively slow continuous cooling rate. Approximately 20
The reason why ΔYP is large when the cooling rate is higher than °C/sec is considered to be because sufficient time for precipitation was not secured in the high temperature range necessary for Tie precipitation.

Next, the present inventors investigated the precipitation mechanism of Tic during the continuous cooling process from about 100° C. as described above. That is, when continuous cooling was performed at 10° C./sec from about 1000° C. after casting, the state of precipitation of TiC was investigated by ΔYP at various temperatures. The results are shown in Figure 2.

In Fig. 2, the precipitation of TiC becomes active from around 820°C, and at around 750°C, the precipitation is almost completed and Δ
YP is about 1 kgf/mm''.The reason why TiC precipitation progresses rapidly in such a temperature range is not necessarily clear, but it is believed that the Ar. point and the Ar point play an important role. In other words, most of the precipitation phenomenon occurs between points Ar3 and Ar.
r. This seems to be due to the relatively high temperature at which the TiC precipitation reaction tends to proceed. However, the higher the temperature of the Ar point, the more Tic precipitation progresses.
It is important that the temperature be below 00°C in order to promote the precipitation of TiC.

The present invention has been made based on such knowledge.

Next, the chemical composition of the slab used in the present invention will be explained. The smaller the amount of C added, the better the deep drawing formability of the resulting cold rolled steel sheet will be. However, if it is less than 0.001%, it will be extremely difficult to form the shape. 01%
When adding more than 1, it becomes necessary to add a large amount of Ti or Nb to fix C, resulting in an increase in manufacturing costs. Therefore, in the present invention, cl is 0.001 to
The range is 0.01%.

Mn prevents hot embrittlement of steel and Ar. In order to optimize the Ti point, 0.10% or more is added, but if too much is added, the Ar3 point and Ar point become extremely low, and the Ti
In addition to suppressing the precipitation of C and deteriorating the steel due to deep drawing, the increase in steel strength becomes excessive, leading to deterioration of ductility.
The upper limit is 0.30%. The smaller N is, the more Ti or Nb will be effectively used for fixing C, and on the other hand,
When it exceeds 0.005%, deep drawing formability deteriorates. Therefore, the upper limit of the N amount is set to 0. 0 0 5%.

Ti and Nb have a strong affinity with solid solution C in steel and are essential elements for precipitation of carbonitrides. However, for any element, when the amount added is less than 0.02%, it is insufficient for fixing C;
% or when Nb exceeds 0.05%, the effects are saturated, leading to an increase in manufacturing costs.

In particular, according to the invention, Ti"-Ti+(48/93)
Nb (wt%) and C”=C+ (1 2/14)N
(% by weight) By satisfying Tiゝ/C''≧4.0, C in the steel can be more reliably fixed in actual operation.

In general, slabs contain a level of P that cannot necessarily be said to be an impurity.
and S. Therefore, in the present invention, it is permissible to include P and S in the above-mentioned amounts,
Furthermore, regarding S L Cr, Mo, Ni and Cu,
0.061% or less, V0.02% or less, 80.00 respectively
It is also permissible to contain 1% or less, and 0.02% or less of each of Zr, REM, Ca, and Ce.

According to the present invention, a thin-walled casting strip having such a chemical fraction is continuously cast, and an average cooling rate of 15° C. between Ar3 and Ar.
/second or less, and after cooling to an Ar point of 830°C or less, cold rolling and recrystallization annealing are performed to achieve excellent deep drawing shapeability in strip casting. You can get steel plates.

In the present invention, the average cooling rate after manufacturing the thin-walled cast steel strip is set to 15°C/sec or less in order to actively precipitate TiC between Ar3 and Ar1, as described above.
Furthermore, Ar. As mentioned above, the temperature must be 830° C. or lower. This is because when the Ar point exceeds 830°C, the precipitation of TiC seems to be significantly delayed.
The Ar point is influenced by the cooling rate, chemistry, the state of the austenite structure, and the like. In particular, the austenite grain size is Ar. Refinement of recrystallized austenite by intense rolling in the austenitic state can increase the Ar+ point to 830°C or higher.
During cooling, the processing into the slab should be approximately 50% or less in terms of total reduction. The austenite grain size just before transformation is
Although it is difficult to specify exactly, if experimental results are taken into consideration, it is preferable that it is approximately {OOμm or more.

Next, the cast slab was placed in an Ar. After being cooled to a temperature below a point, it may be left to cool to room temperature, or it may be wound into a coil. For the purpose of maximizing the precipitation of TiC, it is desirable to wind the coil at a temperature of 700°C or higher.
On the other hand, such high-temperature winding makes it difficult to remove scale in the post-process pickling, so the winding temperature is usually 600-700°C.
A range of is preferred.

Thereafter, it may be cold rolled and recrystallized annealed according to a conventional method. The cold rolling rate is preferably 50 to 85%.

Recrystallization annealing may be either batch annealing or continuous annealing, but the temperature is 600 to 750°C for batch annealing and 750°C for continuous annealing.
It is desirable to heat to a temperature in the range of 00-900°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 sheet for a copper sheet for CG and BG plating.

As described above, according to the method of the present invention, a thin cast steel strip made of ultra-low C-Ti and/or Nb is continuously cast, and this is cast at a predetermined average cooling rate, and , Ar, point is 830℃
After cooling as follows, cold rolling and recrystallization annealing can be performed to obtain a cold rolled steel sheet with excellent deep drawing shapeability by strip casting.

The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way.

As shown in Table 1, steels having various chemical components were melted by the method described above and cast into slabs of 8 wmM. Approximately 1
The sample was cooled from 000° C. to room temperature at an average cooling rate of 10° C./sec. Thereafter, a 4u thick steel plate was obtained by grinding the front and back surfaces, and a 1) 1 thick steel plate was obtained by cold rolling. Subsequently, it was subjected to continuous annealing at 850° C. for 90 seconds, and its mechanical properties were examined. The results are shown in Table 1.

Comparative Steel 2 was cooled to room temperature at an average cooling rate of 70"C/sec between 900 and 700°C after casting, and Comparative Steel 3 was cooled to room temperature at an average cooling rate of 900 to 700°C after casting. It was cooled to room temperature at a rate of 70°C/sec, and then reheated at 700°C for 3 hours.

In Table 1, invention steels 1, 4, and 5 that satisfy the conditions specified by the present invention exhibit 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 solid solution C remained in the steel at the time of cold rolling, and as a result, the r value was extremely low. However, the comparison fi
As shown in 13, even if the slab is rapidly cooled to room temperature, a high r value can be obtained if the slab is reheated at 700°C.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the relationship between the continuous cooling rate from about 1000°C and ΔYP after casting the slab, and Fig. 2 is a graph showing the relationship between the continuous cooling rate from about 1000°C and ΔYP.
It is a graph showing the relationship between temperature and ΔYP when cooling from 00° C. at a continuous cooling rate of IO° C./sec.

Claims (1)

[Claims] (1) Contains (a) 0.001 to 0.01% of C, 0.10 to 0.30% of Mn, and 0.005% or less of N in weight%, and (b) 0.02 to 0.10% of Ti. , and Nb0.02-0.05% at a weight ratio of Ti^*/C^*≧4.0 (wherein, Ti^*=T1+(
48/93) Nb (wt%), C^*=C+(12/1
4) Continuously cast a thin cast steel strip containing a satisfactory amount of N (wt%) and the remainder iron and unavoidable impurities, with an average cooling rate of 15°C/1 between Ar_3 and Ar_1.
A method for producing a cold-rolled steel sheet for deep drawing by strip casting, which comprises cooling the steel sheet for a temperature of 830° C. or less and then cold rolling and recrystallization annealing.