JPS63168260A - Hot working method for continuously cast billet - Google Patents

Abstract

PURPOSE:To prevent the development of surface defect as compared with conventional steel ingot, such as crack, etc., by rapidly cooling only the cast billet surface, so as to become the specific temp. at the time of cooling the surface temp. of cast billet to the specific temp. CONSTITUTION:When the surface temp. of cast billet composing of killed steel produced by continuous casting is cooled to the temp. at 150-50 deg.C higher than Ar3 transformation point, only the cast billet surface is cooled, so as to become the temp. at 100-400 deg.C lower than Ar1 transformation point under red heat state in the inner part of cast billet. In this way, crystal grain of the surface layer part of cast billet is made to fine and sensibility to the crack and the surface defect after rolling are reduced as compared with the conventional steel ingot.

Description

Detailed Description of the Invention [Industrial Application Field 1] The present invention relates to a hot working method for cast pieces produced by a continuous steel casting method. This prevents cracks that often occur in steel during hot rolling of cast pieces.

[Prior art 1] Conventionally, when a soaked steel ingot is bloomed and rolled to produce a steel billet, various cracks generally occur depending on the composition, structure, heating conditions, rolling conditions, etc. of the steel ingot. It is a well-known fact that the phenomenon of transverse cracking of steel ingots that occurs during the rolling process occurs especially in carbon steel containing low and medium carbon and low alloy steel that has been subjected to aluminum deoxidation. This obstructs the rolling operation, greatly impairs the rolling yield, and causes extremely large economic losses.

Originally, when a steel ingot is cut from a mold and inserted into a heating furnace for heating to perform blooming rolling, there are the following three processing methods in the manufacturing process between the cutting and insertion into a soaking furnace.

1) Not only the inside of the steel ingot but also the surface layer does not fall much.
When the steel ingot is inserted into a soaking furnace while being maintained at a temperature considerably higher than the A r 3 transformation point.

2) When inserting a steel ingot whose temperature at the surface layer of the steel ingot has fallen to just above the Ars point, that is, a steel ingot whose overall temperature has decreased compared to case 1> but which is still in an austenitic state.

3) Usually called a cold ingot, the die-cut steel ingot is slowly cooled to bring the temperature of not only the surface layer but also the entire steel ingot below the Ar+ point, sometimes to near room temperature, then inserted into a heating furnace and rolled to a rolling temperature. This is a case of soaking.

Rolling work for steel ingots is classified into one of the three methods mentioned above, depending on the weight, shape, temperature of the steel ingot, and time until insertion, but in method 1, there is a risk of cracking. Although the ratio is small, most of the horizontal cracks are concentrated in method 2), while in method 3) there is no tank cracking.

This shows statistically and experimentally that the phenomenon of VT cracking is caused by the process from die cutting to insertion into the heating furnace. In other words, horizontal cracking of steel ingots occurs during steel manufacturing.

There are very few factors related to heating and rolling, and it is largely controlled by the temperature drop of the steel ingot itself from punching to insertion into the heating furnace.

The invention described in Japanese Patent Publication No. 49-7771 was invented based on this knowledge of steel ingot rolling operations, and the gist of the invention is that ``a steel ingot is applied to a refrigerant in a container that can circulate or by an injection method. A steel ingot hot working method characterized by rapidly immersing or exposing the entire steel ingot so that the inside of the ingot is red-hot and only the surface layer is below the A1 transformation point, and then heated in a furnace and formed. It is.

The invention provides that the cause of transverse cracking in a steel ingot is that the columnar crystals themselves, which are the casting structure of the surface layer of the steel ingot, become extremely coarse during heating;
Alternatively, the grain boundaries of the austenite crystals in the surface layer may be oxidized, making the surface region brittle and fractured by blooming rolling. In order to achieve this, only the surface layer is rapidly cooled. The invention also provides that in aluminum quilt steel, so-called dissolved aluminum and nitrogen in the steel combine to form aluminum nitride, which exceeds the solid solubility limit during the process of temperature drop after solidification of the steel ingot, thereby forming an austenite grain boundary. Since aluminum nitride precipitates in the form of plate-like precipitates and causes cracks, the steel ingot was rapidly cooled to suppress the precipitation of aluminum nitride at the austenite grain boundaries, thereby preventing tank cracks. be.

[Problems to be solved by the invention] However, in the case of manufacturing a steel ingot using a normal ingot, the temperature at which the surface layer of the steel ingot starts to be rapidly cooled can be freely selected, and the surface layer of the steel ingot is relatively cool. It can be rapidly cooled from a high temperature and is extremely effective, but when obtaining slabs using a continuous casting machine, it is cast into a water-cooled mold, a solidified crust is formed on the surface by the cooling action from the mold, and then the slab is cast from below by pinch rolls. During drawing, cooling is rapid compared to ordinary steel ingots so that the solidified shell does not break. Therefore, in continuous casting, there is a large temperature difference between the surface and the inside, and distortion and transformation strain are likely to occur.Additionally, distortion due to the static pressure of molten steel and external distortion caused by the rolls during drawing and straightening of slabs are added, which is normal. Since cracks are more likely to occur than steel, in continuous casting, the surface part is actively cooled, so the temperature of the surface part drops to just above the Ar3 point, and the effects of the invention cannot be fully demonstrated. I couldn't do it.

The present invention has been made to solve the above-mentioned problems in the hot working method of continuously cast pieces.
Slab machine! It is an object of the present invention to provide a method for hot working a continuous cast piece in which the fI part can be rapidly cooled from a sufficiently high temperature and the generation of surface defects such as cracks in the steel ingot is prevented.

[Means for Solving the Problems] The present inventors have conducted extensive research on conventional hot working methods for continuously cast pieces. As a result, in the conventional method of immersing the steel ingot after cutting a continuous M cast piece in a water tank to rapidly cool the surface layer of the steel ingot, the temperature of the steel ingot drops to A, close to the transformation point, and
), it is not possible to refine the crystal grains in the surface layer of the steel ingot, and it is also not possible to reduce precipitates at grain boundaries, so the desired effect may not be obtained. found. Therefore, the inventors came up with the idea that by cooling the surface layer of the slab from a higher temperature, it would be possible to refine the crystal grains in the surface layer of the slab and reduce the precipitates at the grain boundaries, and thus completed the present invention. This is what we have come to.

The hot working method for continuously cast slabs of the present invention involves processing slabs made of killed steel manufactured by continuous casting so that the surface temperature thereof is Ar.
When cooled to a temperature 150 to 50℃ higher than the bad point, the inside of the slab was red hot due to the cooling medium, and the surface temperature was A.
The gist is that after rapidly cooling the slab to a temperature 100 to 400°C lower than the r+ transformation point, the slab is cut into a predetermined length, and then heated in a furnace to hot form.

[Function] In the present invention, the surface temperature of the slab is 150 to
When cooled to a temperature 50℃ higher, the inside of the slab remains as a red-hot core, and only the surface of the slab (preferably about 20 to 40m) is rapidly cooled to a temperature 100 to 400℃ lower than the Ar+ transformation point. This transforms the structure into bainite, applies compressive stress to the inside of the slab, and prevents the formation of structurally weak film-like ferrite at austenite grain boundaries. It also suppresses precipitation of AIN, etc., and prevents embrittlement. Furthermore, the thickness of the transformed structure is made uniform, the compressive stress is kept constant, and stress concentration is prevented.

The temperature at which rapid cooling of the slab starts with a cooling medium is such that the surface temperature of the slab is 150 to 50°C higher than the A r 323 point. At this temperature, the slab is in the austenite phase, and the rapid cooling This is the temperature at which the crystal grains in the surface layer of the slab can be sufficiently refined.

The reason why the quenching start temperature is limited to this range is that the surface temperature is A.
This is because if the temperature is 150°C or more higher than the r2 transformation point, sufficient rapid cooling will not be obtained and it will be difficult to completely transform the slab into Ar1. This is because the crystal grains in the surface layer become coarser and more cracks occur. Any cooling medium may be used, and any cooling method may be used; for example, walk spray, mist spraying, or a pond tank may be used. The surface temperature of the slab is reduced to Ar due to the cooling medium.
By being rapidly cooled to a temperature 1002400°C lower than the 1 transformation point, the surface layer of the slab completely completes the A r 1 transformation. The reason for limiting the cooling end temperature to the above range is as follows.
If the temperature is 400°C or more lower than the Ar + transformation point, cracking due to shrinkage will increase, which is undesirable. If the temperature lower than the API transformation point exceeds 100°C, Ar on the surface layer of the slab will increase.

This is because the metamorphosis is not fully completed. This prevents the refinement of the crystal grains in the surface layer of the slab and the precipitation of impurities at the grain boundaries. After that, it is preferable to stop the rapid cooling immediately after the transformation of the surface layer of the slab is completed, and then allow it to cool.This prevents the cracks that have formed in the slab from spreading due to unnecessary shrinkage of the slab. 0 The objects of the present invention are:
It is an aluminum quilt steel that undergoes Ar+ to Arz transformation.

[Examples] Examples of various aspects of the present invention will be shown to clarify the effects of the present invention.

(Example 1) JIS-3CR22 was melted in an electric furnace, aluminum was added and sufficiently deoxidized to make aluminum killed steel, and the steel was poured into a mold of a bending continuous casting machine and cast from below using pinch rolls. After pulling out the piece and passing the continuously cast piece through the pinch rolls, the surface temperature of the piece was measured at 920°C (A", transformation point +
When the temperature reaches 70℃, the surface temperature of the slab reaches 500℃ (A r + transformation point -200℃).
). After quenching, the slab is cut, immediately charged into a heating furnace, heated to a predetermined temperature, and then rolled to 130
A billet of X130X8000+*m was prepared and surface flaws were investigated. For comparison, a slab whose surface temperature after cutting was cooled to 1000℃ without being cooled after passing through the pinch rolls was charged into a water tank and rapidly cooled. As a reference example, a sample that was not cooled was similarly charged into a heating furnace and then rolled, and the surface flaws were investigated. The results of the zero surface flaw investigation are shown in Figure 1. The lengths are tallied individually, and the length ratio is calculated by dividing the total length of each surface flaw by the total length of the billet.
The surface flaw depth is plotted on the horizontal axis and the length ratio is plotted on the vertical axis. As can be seen from FIG. 1, the reference example had many surface flaws of 1.0 mm or less, and the comparative example had an increased number of surface flaws of 0.5 mm or less, whereas the present invention The example has significantly less surface flaws than these.

(Example 2) Continuously cast pieces were manufactured in the same manner as in Example 1, and the position and length of the cooling zone after correction with pinch rolls were variously changed.
After rapidly cooling the slab at the quenching start temperature and quenching end temperature shown in Table 1, immediately inserting it into a heating furnace and rolling it,
Inspected for surface flaws.

As can be seen from Table 1, in the comparative examples, the quenching end temperature was too high or the quenching start temperature was too low, and the flaw length ratio was poor in both cases. On the other hand, the quenching start temperature Ar2 is 150 to 50°C higher than the bad point, and the quenching end temperature is Ar2.
It was confirmed that the examples of the present invention, which are 100 to 400 degrees Celsius lower than the first transformation point, had extremely low flaw length ratios and significantly fewer surface defects than comparative examples outside this temperature range.

"Effect of the Invention J The hot working method for continuously cast slabs of the present invention, as explained above, processes slabs made of killed steel manufactured by continuous casting.
When the surface temperature was 150 to 50 degrees Celsius higher than the Ar3 transformation point, the inside of the slab was kept red hot by a cooling medium, and the surface temperature was rapidly cooled to 100 to 400 degrees Celsius lower than the Ar+ transformation point. After that, the slab is cut into a predetermined length, and then heated in a furnace for hot forming. 2. Due to cooling, the crystal grains in the surface layer of the slab become V & finer, and the precipitation of aluminum nitrides, etc. in the grain boundaries is suppressed, which significantly reduces the cracking susceptibility of the steel ingot, which has not been sufficiently effective in the past. It has the excellent effect of significantly reducing surface flaws after rolling of steel ingots obtained by continuous casting.

[Brief explanation of the drawing]

Figure 1 shows the surface flaw depth and length r after hot rolling of a steel ingot.
It is a figure showing Wi section.

Claims (1)

[Claims] (1) A cast slab made of killed steel manufactured by continuous casting has a surface temperature of 150 to 50°C above the Ar_3 transformation point.
When cooling to a high temperature, the inside of the slab is in a red-hot state due to the cooling medium, and the surface temperature is 100 to 40% higher than the Ar_1 transformation point.
1. A method of hot working a continuously cast piece, which comprises rapidly cooling the piece to a temperature lower than 00° C., cutting the piece into a predetermined length, and then heating it in a furnace to hot form it.