JPH0421723A - Production of cold rolled steel sheet from thin cast slab - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet which has uniform structure and in which surface roughing is prevented by casting a thin slab from a molten steel with a specific composition, exerting cooling, heating, and recooling under respectively specified conditions, performing cold rolling, and then subjecting the resulting cold rolled sheet to continuous annealing and to temper rolling. CONSTITUTION:A molten steel having a composition consisting of, by weight, 0.02-0.2% C, <=2.0% Si, 0.1-3.0% Mn, <=0.15% P, <=0.02% S, 0.01-0.1% Al, and the balance Fe with inevitable impurities is continuously cast into a thin slab. Then, this thin cast slab is cooled from solidification to the austenite region at <=30 deg.C/sec average cooling rate and further cooled from the austenite region down to a temp. not higher than the temp. T1 satisfying an equation at >=5 deg.C/sec average cooling rate, by which ferrite transformation is finished. Subsequently, the above cast steel slab is heated again up to a temp. in the region not lower than the Ae3 transformation point at >=5 deg.C/ sec average temp.-rise rate to undergo transformation into austenite again, cooled again, cold-rolled by the ordinary method, and subjected to continuous annealing and temper rolling. By this method, the cold rolled steel sheet in which ductility and deep drawability are secured can be obtained.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is directed to casting a thin strip directly from molten steel, omitting or simplifying the hot rolling process, and then cold rolling and annealing it to produce a ductile or thin strip. The present invention relates to a method for producing a cold-rolled steel sheet that has excellent deep drawability and that does not cause surface roughness during forming.

(Conventional technology) In recent years, in order to significantly shorten the thin plate manufacturing process, a new thin plate manufacturing process has been developed in which thin strips are directly cast from molten steel.
Methods have been proposed in which cold rolled steel sheets are produced by rough rolling in hot rolling or by omitting hot rolling itself. However, these methods have problems with the ductility after cold rolling and annealing compared to the conventional process due to insufficient precipitation of precipitates and coarse grains before cold rolling, which did not pose problems in the conventional manufacturing process. It has the disadvantage of causing deterioration compared to . Among these, in processes that omit the hot rolling process, the structure of the thin cast strip is coarse.
The cold-rolled steel sheet obtained in this process becomes rough when formed. Prevent this rough skin and.

In order to improve ductility or deep drawability, it is necessary to refine the structure after casting.
I'm illusory. (1) is CaO formed by adding Ca.
This method attempts to refine the solidified structure by using δ ferrite and CaS as crystallization nuclei of δ ferrite.

However, such control of the solidification structure is difficult to cast because there are many restrictive conditions such as the amount of O and S in the steel, the temperature of the molten steel, and the timing of addition of additional elements. In addition, the structure of the slab tends to be non-uniform, and as a result, the structure after cold rolling and annealing becomes non-uniform and the ductility deteriorates. On the other hand, (2) and (3) aim to refine the structure of the slab by in-line reheating treatment, but in both cases there are no restrictions on the cooling conditions after solidification. In other words, this invention is different from the present invention, which discloses a method for further refining the structure of a 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 for omitting or simplifying the hot rolling process for cast slabs directly cast into thin T4$ in order to prevent surface roughness after forming and improve ductility or deep drawability. However, it is an object of the present invention to establish a manufacturing process for a cold rolled steel sheet that has a fine and uniform structure before cold rolling and also has a uniform structure after cold rolling and annealing.

(Means for Solving the Problems) As a result of intensive studies in view of the above-mentioned circumstances, the inventors of the present invention found that after casting molten steel into a thin strip, it is cooled from solidification to the ferrite region using a thermal history as shown in Figure 1. They discovered that the microstructure before cold rolling could be made fine and uniform by repeating the heat treatment once with the reverse transformation from ferrite to austenite by heating again, and by utilizing intragranular ferrite as the initial structure. It is. That is, in the present invention, by appropriately controlling the cooling rate from the austenite region to the ferrite transformation temperature region after casting, coarse austenite is once made into a fine structure by intragranular transformed ferrite, albeit unevenly, and
After that, by austenitizing again, a fine austenitic structure equivalent to that of conventional hot-rolled materials can be obtained in a short time. Furthermore, according to the present invention, by adding reverse transformation, it is possible to eliminate the non-uniformity of the structure which has been a problem in the formation of conventional intragranular ferrite structure, and to reduce the non-uniformity in the conventional structure. It is possible to relax the component conditions, rolling conditions, etc. that were limited in manufacturing. In other words, by making the structure fine and uniform before cold rolling, the structure after cold rolling and annealing is made uniform, and as a result, ductility or deep drawability is improved, and the occurrence of rough skin after forming is prevented. be.

That is, the present invention is configured as follows.

C: 0.02 to 0.2 wt% or less, Si: 2.
(ht% or less, Mn: 0.1-3.0wt%, P
: 0.15iit% or less, S: 0.02wt, %
Above, A Q: After casting molten steel containing 0.01 to 0.1 wt% and the remainder iron and unavoidable impurities into a thin cast band by continuous casting, the average cooling rate from solidification to austenite region = 30 ° C / s or less, and then further cooled from the austenite region to a temperature T that satisfies equation (1) at an average cooling rate of 5°C/s or more to complete the ferrite transformation, and then at an average heating rate of 5°C/s or more. Ae is heated again at a temperature increase rate to a temperature range above the transformation point to completely transform to austenite again, and after cooling again, cold rolling is performed in the usual manner, continuous annealing and temper rolling are performed. A method for producing cold-rolled steel sheets.

T, (''C) = Ae, -2000X (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 steel, and is also the most important element that determines the structural morphology in the transformation from austenite to ferrite. Excessive addition deteriorates weldability, so the upper limit is set to 0.2 wt%. Further, in order to obtain Wittmannstellaten ferrite as the initial transformed structure as in the present invention, it is necessary to add 0.02% or more.

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

Like C, Mn is an element added for the purpose of increasing the strength of steel, and is also an essential element for forming an intragranular ferrite structure. That is, this forms MnS in the steel together with S, which will be described later, and precipitates on the oxide formed by the inevitable components in the steel, thereby becoming a transformation nucleus of ferrite. Therefore, this effect clearly appears depending on the amount added.
It is set at 0.1% or more, and the upper limit is set at 3.0% or less so that the cost of composition control in the steel manufacturing process can be kept low and workability is not deteriorated.

P is also an element added to increase the strength of steel, but excessive addition deteriorates ductility and weldability, so the upper limit is set to 0.
．． It is set to 15 wt%.

Like Mn, S plays an important role in the present invention.
It is a constituent element of nS. However, excessive addition causes hot cracking, so the content should be 0.02 wt% or less, but it is preferably 0.003 wt% or more due to problems such as increased desulfurization cost.

Al1 is necessary for deoxidizing steel and is 0.01wt
% or more is required. On the other hand, since excessive addition increases costs and leaves inclusions in the steel, the upper limit is set to 0.1 wt%.

In the present invention, components other than those mentioned above should be kept to a low level in principle, but on the other hand, appropriate amounts of Ti, V,
Nb, Mo, B, etc. may also be added, and in particular, Ti, V, etc. may be added as elements to promote intragranular ferrite transformation, but the present invention is not limited by this.

Next, the manufacturing method will be described.

In the present invention, after casting the steel having the above-mentioned composition, the steel may be processed to a total reduction of 80% or less in the austenite region, and then the first ferrite transformation may be performed. The reason for setting an upper limit on the amount of processing is that if the rolling reduction is greater than this, there will be no difference from the conventional hot rolling process in terms of metallurgical structure and manufacturing cost. on the other hand,
This also includes light reduction of 15% or less, which is carried out for the purpose of adjusting the surface quality of the slab.

Next, regarding the cooling and heating conditions of the thin cast band, which are the most important in the present invention, the austenite region after solidification, preferably A
r, the temperature range up to just above the transformation point cools relatively slowly,
It is necessary to sufficiently precipitate MnS, which becomes the transformation nucleus of intragranular lyradmanstellate 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 M n S will be insufficient, so
In the next cooling process, ferrite begins to precipitate from the austenite grain boundaries, resulting in a non-uniform structure with less precipitation of intragranular lyradmanstellaten ferrite. Therefore, the structure after cold rolling and annealing becomes non-uniform, leading to rough skin and deterioration of ductility during forming. Subsequently, it is necessary to cool from the austenite region, preferably just above the Ar3 transformation point, to a temperature T2 or lower that satisfies equation (1) at an average cooling rate of 5° C./s or higher.

If it is not cooled to a temperature below T1, polygonal ferrite cannot be obtained even if it is reheated and cooled to an intended temperature range, Ae, as described later. The temperature range from temperature T1 to Ae, which is limited by formula (1), is the temperature at which ferrite precipitates from austenite grain boundaries in the normal hot rolling process, but this temperature range can be controlled by cooling below 5°C/s. If slow cooling is performed at a slow rate, coarse ferrite will precipitate from the austenite grain boundaries during this time, making it difficult to form a fine austenite structure even in the subsequent reverse transformation. Therefore, as described above, this causes surface roughness during forming after cold rolling and annealing, and also causes deterioration of ductility. Note that efficient formation of intragranular lyradmanstellate ferrite can be easily achieved by maintaining the temperature below T1 temperature for several minutes after cooling.

Next, in the present invention, when the ferrite structure thus formed is austenitized again, the temperature increase rate must be 5° C./s or more. This is because if the heating rate is slow, the austenite produced becomes coarse. Although not limited, for the same reason, it is desirable that the heating temperature and the holding time at that temperature be as low and as short as possible within the range where the structure is completely austenitized. Further, it is preferable to apply rolling in this austenite state because it makes the grain finer.

This does not detract from the spirit of the present invention.

In this state, the austenite grain size is equivalent to the grain size at the end of rolling in the conventional hot rolling process, and the subsequent cooling conditions are not particularly different from the conventional process. That is, by appropriately selecting the cooling conditions here, it is not only possible to form a fine polygonal ferrite structure, but also to form a mixed structure of ferrite, bainite, martensite, pearlite, etc., if necessary.

The cold rolling and annealing process may be performed by a conventional method,
In particular, there is no problem in annealing even if the overaging treatment is performed by box annealing or continuous annealing.

As described above, according to the method of the present invention, it is possible to create a structure equivalent to that of conventional materials even from a thin cast strip. Furthermore, since a thin cast strip is used as the starting material, there is little segregation, and the band-like heterogeneous structure that often occurs in conventional hot-rolled materials is not formed at all, making it possible to obtain a completely isotropic structure.

(Example) Example I C: 0.10wt%, Si: 0.1wt%, Mn
: 1.2vt%, P: 0.011wt%, S
: 0.009wt%, AQ: 0.025v
t%, the balance being Fe and unavoidable impurities was tapped from a converter and made into a thin cast strip by continuous casting. Next, with the thermal history shown in Fig. 1, rolling, cooling and heating were carried out as shown in Table 1, the thickness of the slab was reduced to 4 mm, and after final cooling,
It was wound up at 600°C. After pickling and 80% cold rolling, continuous annealing was performed at 750° C. for 1 minute, followed by 1% temper rolling. Thereafter, it was processed into a JIS Z 2201.5 test piece and subjected to a tensile test according to the test method described in JIS Z 2241. Table 2 shows the results.

The rolling reduction and thermal history after casting are in accordance with the scope of the present invention.
In Nos. 1, 2, 3, 8, 9, and 1o, the structure before cold rolling (the slab) becomes a uniform borigonal ferrite structure, and accordingly, the structure after cold rolling and annealing is also uniform. Therefore, even when subjected to a tensile test, no roughening occurs and the ductility is excellent. On the other hand, in case No. 4 in which the cooling end temperature (Tr) deviates higher than T1, the structure before cold rolling becomes lyradmannstellaten ferrite after final cooling because it does not become lyradmannstellaten ferrite during cooling, and the structure before cold rolling becomes lyradmannstellaten ferrite after the final cooling. The subsequent structure is non-uniform, the material becomes hard and has low ductility.

In addition, even for Na5, 6, and 7 whose cooling or reheating rate after casting is out of the range of the present invention, the structure before cold rolling is a nonuniform or coarse polygonal ferrite structure, so the structure after cold rolling and annealing is is uneven, leading to deterioration of ductility. Particularly in NO3 and NO7, which had a coarse polygonal ferrite structure before cold rolling, rough skin occurred.

Example 2 Steel having the chemical composition shown in Table 3 was tapped in a converter, and cast into three thin strips by continuous casting. Then 90 at lO°C/s
It was cooled to 0°C, then cooled to 500°C at 10°C/s, and immediately heated to 1000°C at 10°C/s. It was further cooled to 600°C at 50°C/s and wound up at that temperature.

After pickling, the material was cold rolled at a rolling reduction of 73%, then subjected to continuous annealing for 1 minute under the conditions shown in Table 3, followed by 1% temper rolling, and in the same manner as in Example 1. A tensile test was conducted. Table 4 shows the results.

A, B, C, D, E according to the method of the invention.

F and G steels exhibit excellent ductility or deep drawability without roughening after cold rolling and annealing.

In H and 1 steels with low C or Mn content, the structure in the slab is non-uniform due to the fact that it does not become a Windmannstellaten ferrite structure during cooling, so the structure after cold rolling and annealing is also non-uniform. Become.

Therefore, ductility and deep drawability are poor.

(Effects of the Invention) According to the present invention, even when a thin strip is cast, the structure before cold rolling becomes fine and uniform, and as a result, the structure after cold rolling and annealing becomes uniform, resulting in rough skin during forming. At the same time, it is possible to prevent
Ductility or deep drawability can be ensured. In other words, even in a process that omits the hot rolling process, it is possible to produce cold-rolled steel sheets that are equivalent to or better than those produced in the conventional process.
Significant cost reductions can be achieved compared to conventional processes.

[Brief explanation of drawings]

Figure 1 shows the thermal history after casting. Agent Patent Attorney Yoshishima Car box figure Dankong T 1000, 2, tP end E: LL

Claims (1)

[Claims] C: 0.02 to 0.2 wt%, Si: 2.0 wt% or less, Mn: 0.1 to 3.0 wt%, P: 0.15 wt% or less, S: 0.02 wt% Below, Al: 0.01 to 0.1
After casting molten steel containing wt% and the balance iron and unavoidable impurities into a thin cast band by continuous casting, the average cooling rate from solidification to the austenite region is 30 ° C / s or less, and then the average cooling rate from the austenite region : At 5℃/s or more (1
) to complete the ferrite transformation by cooling to a temperature below T_1 that satisfies the formula, and then heating again to a temperature range above the Ae_3 transformation point at an average heating rate of 5°C/s or more to completely transform into austenite again. After metamorphosis and cooling again,
A method for manufacturing cold-rolled steel sheets obtained by cold rolling using a normal method, followed by continuous annealing and temper rolling. T_1(℃)=Ae_3-2000×(Cwt%)(1
)