JPS6149762A - Production of continuously cast ingot - Google Patents

Abstract

PURPOSE:To prevent the time-crack of a steel ingot and the time-crack in a direct rolling process, etc. by subjecting the surface layer part of the ingot apprehensive for forming a surface flaw in the low temp. region of a required temp. range where the ingot is under cooling to working. CONSTITUTION:Rolls formed by providing projections to the surface of, for example, guide rolls are used to apply >=5% working strain to the ingot in the stage of continuous casting at >=2mm. depth in the surface layer part at >=1X 10<-2>S<-1> straining speed when the surface temp. is 900-500 deg.C. The ingot is subjected to >=1 times of a temp. decrease to the Ar3 point or below and is then recuperated to the Ac3 point or above during or after the above-mentioned process. The recuperated ingot is then passed through drawing rolls. The steel ingot obtd. in the above-mentioned manner is free from cracks and flaws and can be therefore subjected to direct hot working without reheating prior to the succeeding hot working.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for preventing hot cracking in continuously cast slabs, and a method for preventing hot cracking in a so-called direct rolling process or hot charge rolling process.

More specifically, the present invention provides medium-low carbon steel or AQ steel containing either or both of Si and Mn.
, Nb, TtsTa, V + B, etc., containing 1% or less of each alloying element, and a direct rolling process in which the steel is rolled without reheating immediately after continuous casting, or it is cooled to room temperature. The present invention relates to a method for preventing cracking of continuously cast slabs and steel slabs during hot rolling in a hot charge rolling process in which hot rolling is performed after reheating without any damage.

(Prior Art) When producing medium-low carbon steel or low alloy steel as described above, for example, by a continuous casting method using a curved continuous casting machine,
Surface cracks often occur in continuously cast slabs mainly due to bending stress applied during slab straightening and thermal stress generated by cooling, and this tendency is particularly noticeable in Nb-containing steel. Such cracks require a cleaning step before proceeding to the next step, so it is necessary to cool the product to around room temperature. Even in the case of the normal rolling process using cold slabs, the need for a care process complicates operations and increases costs.On the other hand, direct rolling and real charge rolling aim to reduce costs through energy and labor savings. The occurrence of such cracks is a significant hindrance.

Further, even if no flaws occur in the slab, cracks may occur during the direct rolling or hot charge rolling process, which also poses a significant obstacle to the practical application of these processes. Incidentally, the occurrence of such cracking is particularly remarkable in materials containing a high content of S as an impurity.

Therefore, in order to stably and inexpensively manufacture products using the direct rolling or bot charge rolling process, it is necessary to prevent the occurrence of defects in slabs during continuous casting, and to prevent the occurrence of defects during the subsequent process of direct rolling or hot charge rolling. It is desired to establish a method for completely preventing the occurrence of surface flaws. On the other hand, even when continuously cast slabs are once cooled, reheated, and hot-worked, if there are no defects in the resulting continuous cast slabs, the flaw removal process becomes unnecessary and the practical benefits are extremely large. However, there is a desire to establish a method that completely prevents the occurrence of defects during the production of continuously cast slabs.

First, as a method for preventing surface flaws occurring in such continuously cast slabs, Japanese Patent Laid-Open No. 58-1282S5 discloses a method of continuous impact of shot balls. however,
As stated on page 290.3 to 7 and below of the publication, the purpose of this method is to remove cracked pressure pipes and trapped foreign matter directly under the mold, and to prevent oxidation of the slab surface. Moreover, as is clear from FIG. 4 of the publication, this is just a process immediately below the mold and before entering the guide roll. Cracks will continue to occur after that, and as will be described later, this is not a complete preventive measure against the occurrence of cracks.

Furthermore, JP-A-54-155123 discloses a method of applying plastic strain to a slab, which involves adjusting the amount of plastic strain in the surface layer, slab temperature, and austenite grain size within a certain range. According to the findings of the present inventors, these conditions alone cannot completely prevent the occurrence of defects. Moreover, roll reduction, shot blasting, and laser pulses, which have been proposed as means for imparting this plastic strain,
In either case, sufficient effects cannot be obtained. That is, if a slab including an unsolidified portion is rolled down with a normal roll, the entire thickness of the solidified shell will simply be depressed, and strain cannot be imparted to the surface layer of the slab. Also, with shot blasting, the depth at which distortion can be applied is shallow (and the effect cannot be achieved,
In addition, there are many problems with the shot recovery method, making it impractical.

Furthermore, the method using laser pulses is based on the slab surface thickness number + μ.
The purpose of this method is to apply heat to the molded slab to create strain due to the temperature difference between it and the inside, and it is almost impossible in principle to apply such a method to hot slabs because the temperature difference is small.

Furthermore, since there is cooling water on the surface of the slab, the effect is further weakened, making it extremely difficult to apply it to an actual production line.

In addition, as a means to prevent the occurrence of defects during hot rolling in the direct rolling or real charge rolling process that follows continuous casting, the cooling rate during continuous bi casting is Countermeasures have been proposed, such as controlling the cooling rate, but it takes an extremely long time to complete cooling, so even though it is theoretically possible, it is difficult to apply it to actual operations. There are many problems.

(Problems to be Solved by the Invention) Thus, an object of the present invention is to completely prevent cracks as surface defects that occur during the production of continuously cast slabs and when they are subjected to direct rolling or hot charge rolling. The objective is to enable stable operation of such a process and to significantly reduce costs.

The present inventors have discovered that cracks as surface defects are caused by thermal stress and stress applied to continuously cast slabs in the low-temperature austenite (T) region during the cooling process, and in some cases in the coexistence region with ferrite (α). This occurs due to low strain rate deformation such as external stress applied to the slab during straightening of the slab in such a temperature range (Mat, Sci, En
H,, 62 (1984) p, 109-119, and Trans, JIM, 2s (1984) p.
160-167), and also discovered and announced that during hot rolling, this occurs due to high strain rate deformation in the relatively low temperature γ region, and that both are due to the destruction of γ grain boundaries.

The embrittlement of the material during low strain rate deformation is caused by A2N and NbC.
Alternatively, carbonitrides such as TaC, Tic, and VN may continuously precipitate at the γ grain boundaries during deformation, and may also precipitate finely within the grains, or even relatively soft ferrite (α) may be present at the grain boundaries.
precipitates in the form of a film, and the inside of the grain becomes relatively strengthened, and strain concentrates in the precipitate-free zone along the γ grain boundaries and the soft part of the α film, resulting in interfacial separation between the grain boundary precipitates and the matrix. (Mat, Sci, Eng
,,” L (1984) p. 109-119, T
Rans, JIM, 2s (1984) p.
, 160-167).

In addition, the embrittlement during high strain rate deformation observed during hot rolling is caused by continuous precipitation of (Fe, Mn)S at one grain boundary during deformation and intragranular strengthening due to fine precipitation within the grain. It occurs in the same way. In this case, if γ-grain boundary continuous precipitation and intragranular precipitation of carbonitrides occur before this high strain rate deformation,
(Fe=Mn) Ija formation by S is significantly promoted.

Therefore, in order to prevent embrittlement due to γ grain boundary cracking in both processes, the γ grains must be made finer to reduce the susceptibility to grain boundary embrittlement, or precipitates must be removed by the time of problematic deformation (for example, during slab straightening and rolling). It is sufficient to coarsen the grains to prevent γ grain boundary precipitation and intragranular fine precipitation during deformation. However, the reality is that sufficient countermeasures are not currently being taken due to equipment and commercial constraints. For example, coagulation and coarsening of precipitates during solidification can be achieved by reducing the cooling rate or maintaining constant temperature during cooling [For carbonitrides, Mat,
Sci. Eng., 62 (1984)
p, 109-L19, JP-A-58 for sulfides.
52442] is not practical because it takes an order of magnitude longer time for cooling and significantly impairs productivity. There have also been attempts to refine the grains by recrystallizing the γ grains (see JP-A-54-155123), but
Since the grains are extremely coarse, the area of the grain boundaries as recrystallization nuclei is extremely small, and it is necessary to apply large strain to make the grains finer. In order to form fine crystal grains of .1 mm or less, it is necessary to apply plastic strain of at least 40%, and it is extremely difficult to apply this to slabs containing unsolidified parts, and so far Not put into practical use. On the other hand, γ−α
Attempts have been made to use transformation to refine the γ grains, but carbonitrides such as NbC and VN precipitate at the T/α interface during transformation and significantly suppress the transformation, making refinement extremely difficult. This is difficult and has not been sufficiently effective.

Also, considering the embrittlement mechanism mentioned above, it is possible to suppress the occurrence of surface flaws by adjusting the chemical composition of the steel, but the chemical composition of the steel must be added to give the material the desired properties. There are many restrictions as there are some things that cannot be obtained, so it is not a fundamental countermeasure. For example, to prevent precipitation of A2N, AQ,
It is possible to improve ductility by reducing H or by adding Ti to fix N in the γ grains as TiN, but these reductions are accompanied by an increase in cost, and addition of Ti has many harmful effects such as impairing the toughness of the weld. . Further, addition of Nb, etc. is essential for ensuring product quality, and it is impossible to take countermeasures by changing it. Reducing S is also effective, but it is accompanied by an increase in cost and does not necessarily lead to a reduction in total cost.

(Means for resolving the problem) The present inventors discovered that after attempting intragranular precipitation of carbonitrides, γ-α
We have repeatedly investigated ways to achieve transformation, refine the structure, and coarsen sulfides within a practical time. It was discovered that the objective could be achieved by adding processing to one surface layer.

That is, the steel having the composition shown in Table 1 was used, a tensile test piece was taken from the punch, and the following experiment was conducted.

Table 1 and FIG. 1 are explanatory diagrams showing various processing and heat treatment conditions employed in this experiment. In the figure, the strain rate at the final deformation at 800°C is A for all cases ■, ■, and ■.
For steel, ε=10″″3S-1, and for B512I, ε-IQXs-'.

In other words, in order to simulate the processing applied to continuously cast slabs during the cooling process, in case ①, the material was solution-treated at 1350°C, and the temperature was first lowered to 800°C, and then cracks were removed during straightening of the slabs. For A steel, which is a problem, a −1
Steel B, which had a problem of cracking during subsequent direct rolling at 0-'s-', was tensile deformed at -108-1. In case ■, γ−
In order to cause α deformation, it was first cooled to 600°C, then reheated to 800°C, further cooled to 600°C, and then subjected to final deformation at 800°C.

In case ■, 20% at 700 °C = 20% at IQ -I S-1 before carrying out the treatment in such case ■
A tensile strain of up to゛In addition, the holding time at 600℃ and 800℃ in cases ■ and ■ is 3
It was a minute.

The aperture value (IIA) at the time of final deformation when the amount of pre-deformation strain is 10% obtained from these results is shown in a graph in FIG. From this, it can be seen that the ductility is significantly improved due to the effect of promoting T-α transformation after the surface layer of the slab is subjected to light working as in case ①. In addition, the amount of preliminary deformation strain is 500 to 900.
Although the temperature was varied within a range of 0.degree. C., almost no change in the RA value was observed. Similarly, Fig. 3+a) shows the relationship between the amount of pre-deformation strain at 700°C for case ①, and it shows that adding processing of about 5% or more can significantly improve ductility during subsequent deformation. I can see that it was done. In addition, Fig. 3 (bl) is a graph showing the change in the aperture value when 10% predeformation is applied to steel A by changing the strain rate of predeformation with the same temperature history as in case ■. It can be seen that the prestrain rate must be approximately 1xto-2s-2s-1 or higher to increase i to a value of 50% or more.

Here, the gist of the present invention is to apply a processing strain of 5% or more to a depth of 2 or more in the surface layer of a slab during continuous casting for 1 x 10-2 s when the surface temperature is 900 to 500°C. -2s
Ac3
This is a method for manufacturing a continuously cast slab, which is characterized in that the continuous cast slab is passed through a drawing roll after being subjected to a treatment for reheating the slab to a temperature above a point. Since the continuously cast slab obtained in this way does not develop cracks, it can be directly hot worked without reheating or cooled to room temperature (without reheating) prior to subsequent hot working. Hot processing may be added after heating.

Note that hot working means all hot working methods such as forging in addition to normal hot rolling.

(Function) Next, the reason for limiting the processing conditions in the present invention will be explained.

The reason why we limited the area where processing strain is applied to 2mm or more of the surface layer of the slab is based on the knowledge that flaws that occur in the area within 2III1 from the surface will remain as cracks or cracks in the subsequent process. The purpose is to add a predetermined processing to the area with a depth of up to.

The reason why the amount of processing strain is limited to 5% or more is based on the reason that unless the amount of processing is 5% or more, nucleation of precipitates is difficult. In addition, the reason for limiting the strain rate is that in the case of low strain rate deformation, plastic deformation concentrates near the γ grain boundaries, promoting the precipitation of carbonitrides at the γ grain boundaries, which may promote cracking. This is because is lXl0-J-,1. In addition, in order to accumulate strain at such high temperatures, precipitation nuclei must be generated before the introduced dislocations can recover.
It is sufficient if ε≧to-2s-'.

Next, in the present invention, the surface temperature of the slab is set to 900 to 500°C during processing as described above, and the Ar temperature is set to 3 or less at least once after or during the process.
If the temperature exceeds 'C, the precipitates will become coarser in the subsequent cooling process, making it unnecessary to use transformation to refine the grains of γ. On the other hand, force J processing at temperatures below 500°C is not practical. It is.

In the present invention, as a processing method for imparting processing strain as described above, for example, the use of a roll with protrusions on the surface of a guide roll, an air hammer, a special press, etc. can be considered, and the required processing strain and strain rate can be applied. Other methods may also be adopted depending on the situation, as long as they are practicable. Further, the processing point is not particularly limited as long as the processing is performed at a position prior to straightening to cause a predetermined transformation.

The applicable steel type of the present invention is not particularly limited, but AC! N, , NbC, TaC, TiC, BN, VN
Steel types that are susceptible to surface flaws that are thought to be caused by the precipitation of such materials as medium-low carbon steel containing either or both of Si, Mn, Nb, Ti,
Ta, V.

This is particularly effective for low alloy steels containing 1% or less of each alloying element such as B. On the other hand, in a component system in which carbonitrides are difficult to precipitate, a great effect can be obtained in preventing surface defects mainly caused by sulfide precipitation during direct rolling or pot charge rolling.

Using a curved continuous casting machine with a length of 12.5 m at a direct-drawing manufacturing factory, slabs with a cross section of 2s0 Dragon The degree of occurrence was visually evaluated. Figure 4 shows the ωJ force applied to the hydraulic cylinder that moves in the width direction of the slab between the rolls on the upper surface of the slab, which was used to apply processing strain to the surface layer of the slab. The slab striking device used as the source is shown together with the casting line.In the illustrated example, processing strain is applied to the slab 1 by the slab striking device 2 at a stage where there is some unsolidified R&H. The slab striking device is composed of an indenter 3 and a hydraulic cylinder 4 connected to the indenter 3, whose striking amount and the like are controlled by a controller 7 via a hydraulic unit 5 and a hydraulic pump unit 6.

FIG. 5 shows the condition of the surface layer of a slab which has been subjected to processing strain by an indenter attached to the tip of the slab striking device. Indenter spherical diameter 511m, depth of compression 3 dragons, number of strokes 180
A blow was applied at a cycle/minute, and the average strain amount of the three surface layer parts of the slab was 7% and the strain rate was 0.3 s-'. Table g32 shows the composition of the steel used in this example, and Table 3 shows the casting conditions and results.

As can be seen from these results, many cracks occurred in the slab cast by the conventional method, but no cracks occurred at all on the surface of the slab subjected to the surface treatment according to the present invention.

FIG. 6 shows the temperature pattern at this time.

Table 2 Table 3 Rin J1 Hada L A cast slab with a cross section of 2s0ma x 2100mm was constructed using a curved continuous casting machine with a radius of 12.5m at a manufacturing plant, and the chemical composition shown in Table 4 was as shown in Table 5. After casting under different conditions, the surface defects of the slabs after straightening were visually evaluated. In addition, as a method of applying processing strain to the surface layer of the slab at this time, as shown in Figure 7, the upper guide roll between 9 and 11 m from the course surface is replaced with a roll with protrusions shown in the same figure. The temperature pattern shown in Figure 8 was used. At this time, the protrusion having the shape shown in Fig. 7 is 0.06 to 0.0 mm on the surface of the solidified shell with a thickness of 72 to 79 mm.
It sinks in with a static iron pressure of 7 kg/mm2 as a reaction force, and the strain spreads as shown in Figure 9. From the following formula, it is possible to apply at least 7% strain to the depth of 5 mm in the slab surface area. did it. The strain rate was estimated to be 2×10−′ s−′.

H= CZ +0.5 ) -17(2X a
S = (1,8~2.2) x a T: Ali,
To give a minimum strain of 5%, a = 7mm, H = 3
nun was needed.

As shown in Table 5, according to the continuous casting machine equipped with the roll with protrusions according to the present invention, although impressions of the protrusions remained on the slab surface, no cracks or defects occurred at all. In the conventional method, many cracks and defects occur, and the effect of the present invention is clear.

Table 4 Table 5 Since it was found that this method could prevent the occurrence of surface flaws in continuously cast slabs, the effect of preventing cracks during direct rolling was further tested. After melting the steel shown in Table 6 and using the method described above, four sets of rolls with protrusions were placed so as to bite into both the upper and lower surfaces of the slab, and the slab was cast using the temperature pattern shown in Figure 10. The sample was cut and rolled in 5 passes to a thickness of 150 mm using a rolling roll with a diameter of 1300 mm, and the degree of occurrence of surface flaws was visually evaluated. The results are shown in the 7th section.
They are summarized in the table.

It is clear from the results shown in Table 7 that significant effects can be obtained by the present invention.

Table 6 Table 7

[Brief explanation of the drawing]

Figure 1 is a schematic diagram 2 showing various processing and heat treatment patterns;
Figure 3+a+ and Figure 3 (bl are graphs showing the results of a preliminary test considering the effects of pre-processing heat treatment, amount of strain at that time, and strain rate on ductility, respectively; Figures 4 and 3) Fig. 5 is a schematic explanatory diagram of a slab striking device as a means for applying processing strain; Fig. 6 is a graph showing the temperature pattern of the slab when it is reheated to Ar3 point or higher after processing; Fig. 7 is a schematic explanatory diagram when processing strain is applied using a roll with protrusions; Fig. 8 is a graph showing the temperature pattern of the slab when processing strain is applied using a roll with protrusions; A schematic explanatory diagram of the propagation of processing strain when using a roll with protrusions; and Fig. 10 shows the surface of the slab when the slab is cut after applying processing strain and further hot worked (rolled). It is a graph showing the temperature pattern. 1: Slab 2: Slab striking device 3: Indenter 4: Hydraulic cylinder 5: Hydraulic unit 6: Hydraulic pump 7: Controller Applicant Sumitomo Kinku Kogyo Co., Ltd. Agent Patent attorney Hiroshi Se Akira - Calculation A1UjJ Case ■ Case O Wide 2 Figure 3 Figure (a) Preliminary I is % (7I) Figure 3 (b) Figure 4 Hole 5 Figure 7 Figure 6 Figure 8 Menu input input Kauno L chicken 1 (m) Figure 9 Figure 10

Claims (3)

[Claims] (1) 5% on the surface layer depth of 2mm or more of slab during continuous casting
The above processing strain is applied at a strain rate of 1 x 10^-^2s^-^1 or more when the surface temperature is 900 to 500°C, and the temperature is lowered to below the Ar_3 point at least once during or after that process. A method for manufacturing a continuously cast slab, characterized by passing the cast slab through a drawing roll after performing a treatment for reheating it from A to Ac_3 points or higher. (2) 5% on the surface layer depth of 2 mm or more during continuous casting
The above processing strain is applied at a strain rate of 1 x 10^-^2s^-^1 or more when the surface temperature is 900 to 500°C, and the temperature is lowered to below the Ar_3 point at least once during or after that process. The continuous casting slab is heat worked by directly hot working the obtained continuous casting slab by passing it through a drawing roll after being reheated from to Ac_3 point or higher without reheating. Intermediate processing method. (3) 5% on the surface layer depth of 2 mm or more during continuous casting
The above processing strain is applied at a strain rate of 1 x 10^-^2s^-^1 or more when the surface temperature is 900 to 500°C, and the temperature is lowered to below the Ar_3 point at least once during or after that process. Continuous casting is characterized in that the continuous cast slab is passed through a drawing roll after being reheated from the temperature to Ac_3 points or higher, and the obtained continuous cast slab is reheated without cooling to room temperature, and then hot worked. Hot working method for cast slabs.