JP2938147B2 - Manufacturing method of cold rolled steel sheet by thin cast strip - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method of manufacturing a thin cast strip directly from molten steel, and omitting or simplifying a hot rolling step, and producing a ductile or cast product produced by cold rolling and annealing. The present invention relates to a method for producing a cold-rolled steel sheet which is excellent in deep drawability and does not cause surface roughness during forming.

(Prior art) In recent years, as a new sheet manufacturing process, a thin cast strip is cast directly from molten steel, and rough rolling in hot rolling or cold rolling is omitted, in order to greatly shorten the sheet manufacturing process. A method of manufacturing a steel plate has been proposed. However, in these methods, due to insufficient precipitation of precipitates before cold rolling and coarse crystal grains, which were not a problem in the conventional manufacturing process, the ductility after cold rolling and annealing was the same as that of the conventional process. There is a disadvantage that it deteriorates as compared with. Above all, in the step in which the hot rolling step is omitted, the structure of the thin cast strip is coarse, so that the cold-rolled steel sheet obtained in this step becomes rough when formed. In order to prevent this roughening and to improve ductility or deep drawability, it is necessary to refine the structure after casting.
207828, (2) JP-A-61-99630 and (3) JP-A-63-6630
It is disclosed in Japanese Patent Publication No. 2822. (1) uses CaO or CaS formed by adding Ca as crystallization nuclei of δ-ferrite to reduce the solidification structure. However, such solidification structure control is difficult to cast because there are many constraints such as the amounts of O and S in steel, the temperature of molten steel, and the timing of addition of additional elements. In addition, the structure of the slab is likely to be non-uniform, and as a result, the structure after cold rolling and annealing is made non-uniform, and ductility is deteriorated. On the other hand, (2) and (3) aim at miniaturization of the structure of the slab by in-line reheat treatment, but none of them regulate cooling conditions after solidification. In other words, this is different from the present invention which discloses a method of further reducing the structure of the slab by regulating the cooling conditions after solidification and obtaining excellent ductility or deep drawability after cold rolling and annealing.

(Problems to be Solved by the Invention) A process in which the hot rolling process is omitted or simplified for a cast piece cast directly into a thin cast strip in order to prevent surface roughness after forming and to improve ductility or deep drawability. However, it is an object of the present invention to establish a process for producing a cold-rolled steel sheet in which the structure before cold rolling is fine and uniform, and the structure after cold rolling and annealing is also uniform.

(Means for Solving the Problems) The present inventors have conducted intensive studies in view of the above-described circumstances,
After casting from molten steel to a thin cast strip, it is cooled from solidification to the ferrite region with a heat history as shown in Fig. 1 and then heated again to repeat one heat treatment with reverse transformation from ferrite to austenite and its initial stage. By utilizing intragranular ferrite as the microstructure, they have found that the microstructure before cold rolling can be made fine and uniform. That is, in the present invention, by appropriately setting the cooling rate from the austenite region to the ferrite transformation temperature region after casting, coarse austenite is once formed into a fine structure while being non-uniform due to intragranular transformation ferrite, and then austenitized again. Thus, a fine austenitic structure equivalent to that of a conventional hot-rolled material can be obtained in a short time. In addition, according to the present invention, by adding a reverse transformation, while eliminating the non-uniformity of the structure which has been a problem in the formation of the conventional intragranular ferrite structure, in order to reduce the conventional non-uniformity. It is possible to relax the component conditions and the rolling down conditions which are limited in production. In other words, by making the structure before cold rolling fine and uniform, the structure after cold rolling and annealing is made uniform, and as a result, ductility or deep drawability is improved, and the occurrence of surface roughness after forming is prevented. is there.

 That is, the present invention is configured as follows.

C: 0.02-0.2 wt% or less, Si: 2.0 wt% or less, Mn: 0.1-3.0
wt%, P: 0.15 wt% or less, S: 0.02 wt.% or less, Al: 0.01 to 0.1
After casting molten steel containing wt% and the balance of iron and unavoidable impurities into a thin steel strip by continuous casting, the average cooling rate from solidification to the austenite area is 30 ° C / s or less, and the average cooling rate from the austenite area : 5 ° C. / s or more (1) is cooled to temperatures T ₁ or less satisfying the formula and terminated allowed wick Goldman Gerhard Ten ferrite ferrite transformation, followed again by an average heating rate 5 ° C. / s or more heating rate After heating to the temperature range above the Ae ₃ transformation point and completely transforming it again to austenite, after cooling again, cold rolling is performed in the usual manner, and continuous annealing and temper rolling are performed to obtain a cold-rolled steel sheet. Production method.

T ₁ (° C.) = Ae ₃ −2000 × (Cwt%) (1) First, the reasons for limiting the chemical components in the present invention will be described.

C is an important element that determines the strength of the steel and is the most important element that determines the structural morphology of the transformation from austenite to ferrite. Excessive addition deteriorates the weldability, so the upper limit is made 0.2 wt%. Also,
In order to obtain Widmanstetten ferrite as an initial transformation structure as in the present invention, addition of 0.02% or more is necessary.

Si is added when increasing the strength without breaking the strength-ductility balance of steel, and is added to increase the transformation point and promote the precipitation of ferrite. However, excessive addition causes deterioration of ductility and surface properties, so the upper limit is 2.
0 wt%.

Like C, Mn is an element added for the purpose of increasing the strength of steel, and is an indispensable element for forming intragranular ferrite structures. That is, it forms MnS in steel and S, which will be described later, and precipitates on an oxide formed by unavoidable components in the steel to become a transformation nucleus of ferrite.
For this reason, the effect is clearly seen in the amount of addition of 0.1.
%, And the upper limit is set to 3.0% or less so that the cost of controlling the composition in the steel making process is kept low and the workability is not deteriorated.

P is also an element added to increase the strength of steel,
Excessive addition deteriorates ductility and weldability.
0.15wt%.

S is Mn, which plays an important role in the present invention, like Mn.
It is a constituent element of S. However, excessive addition causes hot cracking, so the content is set to 0.02 wt% or less. However, it is preferably 0.003 wt% or more because of problems such as an increase in desulfurization cost.

Al is necessary for deoxidation of steel, and 0.01 wt% or more is required. On the other hand, excessive addition increases the cost and leaves inclusions in the steel, so the upper limit is 0.1 wt%.
And

In the present invention, components other than those described above should be kept low in principle.On the other hand, for the purpose of increasing the strength of the product and the like, an appropriate amount of Ti, V, Nb,
Mo, B, etc. may be added. In particular, Ti, V, etc. may be added as an element for promoting intragranular ferrite transformation, but the present invention is not limited by this.

 Next, the manufacturing method will be described.

In the present invention, after the steel having the above-described composition is cast, the first ferrite transformation may be carried out after a working of 80% or less in total reduction in the austenite region. Here, the upper limit is set for the amount of processing because the further reduction is no different from the conventional hot rolling process from the viewpoint of metallurgical histology and production cost. on the other hand,
This includes light reduction of 15% or less for the purpose of adjusting the surface properties of the slab.

Next, regarding the most important cooling and heating conditions for the thin steel strip in the present invention, the austenite region after solidification, preferably
The temperature range immediately above the Ar ₃ transformation point cools relatively slowly,
It is necessary to sufficiently precipitate MnS, which is a transformation nucleus of intragranular Widzmanstetten ferrite. Therefore, the cooling rate in this range is set to 30 ° C./s or less. If the cooling rate is higher than this, the precipitation of MnS will be insufficient, so that ferrite will precipitate from the austenite grain boundaries in the next cooling process, and the precipitation of intragranular Widmanstätten ferrite will be small and uneven. Become an organization. Therefore, cold rolled
The structure after annealing also becomes non-uniform, leading to roughening of the surface during molding and deterioration of ductility. Subsequently, it is necessary to cool from the austenitic region, preferably just above the Ar ₃ transformation point, to an average cooling rate: 5 ° C./s or more and to a temperature T ₁ or less satisfying the expression (1).
If T ₁ is not cooled to a temperature below, as described below
Ae Polygonal ferrite cannot be obtained even when reheated and cooled to a temperature range of ₃ points or more. Limited by equation (1)
Temperature range from temperature to Ae ₃ that T ₁ is the temperature at which ferrite is precipitated from the austenite grain boundaries in the usual hot rolling step, during which the gradually cooling the temperature range below the cooling rate of 5 ° C. / s Coarse ferrite precipitates from the austenite grain boundaries, making it difficult to form a fine austenite structure even in the subsequent reverse transformation. For this reason, as described above, after the cold rolling and annealing, it causes the occurrence of surface roughness during the forming process, and also causes the deterioration of the ductility. Incidentally, efficient formation of intragranular Ui' Goldman Gerhard Te' Ten ferrite is readily accomplished by holding a few minutes after cooling below T ₁ temperature.

Next, in the present invention, when the ferrite structure thus formed is austenitized again, the temperature rising rate must be 5 ° C./s or more. This is because if the heating rate is low, the generated austenite becomes coarse. Although not limited, for the same reason, it is desirable that the heating temperature and the holding at that temperature be as low and short as possible within a range where the structure is completely austenitized. In addition, it is preferable to perform rolling in this austenite state because of finer grains, and therefore, does not impair the purpose of the present invention.

In this state, the austenite grain size is equal to the grain size at the end of rolling in the conventional hot rolling process, and the cooling conditions thereafter are not particularly different from those in the conventional process. That is, by appropriately selecting the cooling conditions here, not only a fine polygonal ferrite structure, but also a mixed structure of ferrite and bainite, martensite, pearlite, or the like can be formed as necessary.

The cold rolling and annealing steps may be performed in a usual manner,
In particular, there is no problem in performing an overaging treatment by box annealing or continuous annealing.

As described above, according to the method of the present invention, a structure equivalent to that of a conventional material can be separately formed even from a thin cast strip. Further, since a thin cast strip is used as a starting material, segregation is small, and a band-like non-uniform structure often generated in a conventional hot-rolled material is not formed at all, and a completely isotropic structure can be obtained.

(Example) C: 0.10 wt%, Si: 0.1 wt%, Mn: 1.2 wt%, P: 0.011 wt%, S:
A steel containing 0.009 wt%, Al: 0.025 wt%, balance Fe and unavoidable impurities was output from a converter and made into a thin cast strip by continuous casting. Next, with the heat history as shown in FIG. 1, rolling and cooling and heating as shown in Table 1 were performed, and the plate thickness of the slab was 4 mm,
After the final cooling was completed, the film was wound at 600 ° C. After the pickling, 80% cold rolling was performed, followed by continuous annealing at 750 ° C. for 1 minute, followed by 1% temper rolling. Thereafter, the test piece was processed into a JIS Z 2201, No. 5 test piece, and a tensile test was performed according to the test method described in 2421. Table 2 shows the results.

Reduction and heat history after casting are within the scope of the present invention
In Nos. 1, 2, 3, 8, 9 and 10, the structure before cold rolling (cast slab) is a uniform polygonal ferrite structure, and the structure after cold rolling and annealing is accordingly uniform. Therefore, even when a tensile test is performed, rough surface does not occur and the ductility is excellent. on the other hand,
In the cooling end temperature (T _f) is out higher than the T ₁ No.4,
Since it does not become Widmanstatten ferrite during cooling, the microstructure before cold rolling after final cooling becomes Widmanstatten ferrite, the microstructure after cooling annealing becomes uneven, the material becomes hard, and the ductility is low. Also, even in Nos. 5, 6, and 7 in which the cooling or reheating rate after casting is out of the range of the present invention, the structure before cold rolling is an uneven or coarse polygonal ferrite structure. Is uneven and causes deterioration of ductility. In particular, in Nos. 6 and 7, where the microstructure before cold rolling was coarse polygonal ferrite, the surface was roughened.

Example 2 Steel having the chemical composition shown in Table 3 was output from a converter and cast into a 3 mm thin cast strip by continuous casting. Then, it is cooled at 10 ° C / s to 900 ° C and then at 10 ° C / s to 500 ° C,
Heated to 1000 ° C at ° C / s. Further, it was cooled to 600 ° C. at 50 ° C./s and wound at that temperature. After cold rolling at a rolling reduction of 73% after pickling, continuous annealing was performed for 1 minute under the conditions shown in Table 3 and temper rolling of 1% was performed. A tensile test was performed. Table 4 shows the results.

A, B, C, D, E, F and G steels according to the method of the invention are:
Example: Excellent ductility or deep drawability without surface roughness after rolling and annealing. In H and I steels with low C or Mn content, the microstructure in the cast slab is non-uniform because it does not become a Widmanstätten ferrite microstructure during cooling, so the microstructure after cold rolling and annealing is also non-uniform. Becomes
Therefore, ductility and deep drawability are poor.

(Effects of the Invention) According to the present invention, even when cast into a thin cast strip, the microstructure before cold rolling becomes fine and uniform, and as a result, the microstructure after cold rolling and annealing becomes uniform, and the surface becomes rough during forming. At the same time, ductility or deep drawability can be ensured.
In other words, even in a step in which the hot rolling step is omitted, it is possible to manufacture a cold-rolled steel sheet equal to or more than the material in the conventional step, so that a significant cost reduction can be achieved as compared with the conventional step.

[Brief description of the drawings]

 FIG. 1 shows the heat history after casting.

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/06 C22C 38/06 (56) References JP-A-63-115654 (JP, A) JP-A-63-62822 (JP) , A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/00-11/22 C21D 9/48 C22C 38/00

Claims (1)

(57) [Claims] C: 0.20 to 0.2 wt%, Si: 2.0 wt% or less, Mn: 0.2 to 0.2 wt%.
1 to 3.0 wt%, P: 0.15 wt% or less, S: 0.02 wt% or less, Al: 0.0
After casting molten steel containing 1 to 0.1 wt% and the balance of iron and unavoidable impurities into a thin steel strip by continuous casting, the average cooling rate from solidification to the austenite region is 30 ° C / s or less, and from the austenite region average cooling rate: 5 ° C. / s or more (1) is cooled to temperatures T ₁ or less satisfying the formula and terminated allowed wick Goldman Gerhard Ten ferrite ferrite transformation, followed by an average heating rate 5 ° C. / s or more heating Heated again at a temperature above the Ae ₃ transformation point and completely transformed again to austenite, cooled again, cold-rolled in the usual way, continuous annealing and temper rolling Manufacturing method of rolled steel sheet. T ₁ (° C) = Ae ₃ -2000 × (Cwt%) (1)