JP7156220B2 - Heavy steel plate with excellent toughness, its manufacturing method, and steel slab used as raw material for thick steel plate - Google Patents

Description

TECHNICAL FIELD The present invention relates to a thick steel plate suitable as a structural material for use in ships, marine structures, bridges, buildings, and the like, a method for producing the same, and a steel slab as a raw material for the thick steel plate. The present invention is particularly effective in improving the toughness of steel plates having a thickness of 50 to 150 mm and a tensile strength of 490 to 720 MPa, especially at extremely low temperatures of -45°C or lower.

In recent years, as structures such as ships, offshore structures, bridges, and buildings have become larger, there has been a demand for increased strength and thickness of thick steel plates used. From the perspective of ensuring the reliability and soundness of large structures, improving toughness to prevent brittle fracture is one of the most important issues. technology is being considered.

Generally, in high-strength steel plates, it is known that cracks are likely to occur starting from non-metallic inclusions or the like present in the structure of the steel plate (hereinafter referred to as the parent phase) (see Non-Patent Document 1). . In other words, fragile non-metallic inclusions cause cracks and lower toughness.

A detailed study of this phenomenon reveals that in order for a high-strength steel plate to exhibit excellent toughness, it is necessary to make the matrix phase uniform and free of non-metallic inclusions (such as MnS).

However, with conventional steel plate manufacturing technology, especially steel plate manufacturing technology with a thickness of 50 to 150 mm and a tensile strength of 490 to 720 MPa, center segregation (that is, alloying elements are Phenomenon of thickening in the center) occurs, promoting the formation of non-metallic inclusions, and the problem of differences in temperature and plastic deformation between the slab surface and the inside of the slab during the slab rolling process. remains, and no technology has been established to solve the problem.

In other words, with the conventional technology, it is not possible to prevent the formation of a concentrated region of alloying elements also in the central portion of the rolled steel plate in the plate thickness direction, and the concentrated alloying elements in that region cannot be prevented by non-metallic intervening. It precipitates as a substance and causes a decrease in the toughness of the steel plate.

Takanobu Murakami, "Metal Fatigue: Effect of Micro Defects and Inclusions," Yokendo Publishing, March 8, 1993

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art and to provide a thick steel plate having excellent toughness, a method for manufacturing the same, and a steel slab as a raw material for the thick steel plate.

The present inventor investigated the relationship between the center segregation that inevitably occurs in the process of continuously casting slabs and the concentrated region of alloying elements remaining in the steel plate obtained through the process of rolling the slabs. As a result, it was found that when the plastic deformation applied to the central part in the plate thickness direction in the slab rolling process is insufficient, the enriched region of the alloying elements due to the central segregation of the slab remains even in the steel plate. Then, it became clear that non-metallic inclusions are generated in regions where the alloying elements remaining in the steel plate are concentrated, and brittle cracks originate from the non-metallic inclusions. Therefore, in order to improve the toughness of the steel plate, it is necessary to eliminate the enriched regions of the alloying elements or reduce the size of the non-metallic inclusions in the rolling process.

Next, the present inventor conducted detailed research on rolling techniques for eliminating regions of concentrated alloying elements. As a result, plastic deformation during the rolling process is less likely to be applied to the central portion in the thickness direction of the steel plate. Found it.

However, when rolling a slab with a small thickness, that is, a slab whose center in the thickness direction is close to the surface of the slab, a phenomenon is observed in which the concentration of alloying elements in the center in the thickness direction of the resulting steel plate is reduced. was taken. This is because the shear strain imparted by the rolling rolls in the rolling process strongly acts on the central portion in the thickness direction of the slab, and plastic working is applied to the region where the alloying elements are concentrated.

In other words, even for a slab with a large plate thickness, by moving the center segregation that inevitably occurs in a continuously cast slab to a position away from the center in the plate thickness direction of the slab and then subjecting it to the rolling process, It was found that sufficient plastic deformation is applied to the alloying element enriched region in the rolling process, and the alloying element enriched region disappears.

The present invention has been made based on such findings.
That is, the present invention provides a steel plate manufactured by hot-rolling a slab having a thickness of T _CASTED (mm) obtained by continuous casting, the steel plate having two surfaces parallel to each other and facing each other in the thickness direction of the slab. This is a thick steel plate manufactured by hot rolling after reducing the thickness of the slab by removing a layer of a constant thickness ΔT (mm)≧T _CASTED /100 from one side of the slab.

In the thick steel plate of the present invention, the removal processing for removing the thickness ΔT is preferably cutting processing for cutting out the slab in layers, grinding processing for scraping the slab in layers, or cutting processing for melting and removing the slabs in layers. . The thick steel plate preferably has a thickness of 50 to 150 mm and a tensile strength of 490 to 720 MPa.

The present invention also provides a method for producing a thick steel plate by hot-rolling a slab having a thickness T _CASTED (mm) obtained by continuous casting to produce a thick steel plate. A removal process is performed to remove a constant thickness ΔT (mm) ≥ T _CASTED /100 in layers from one of the two opposing parallel surfaces, then the slab is heated, and the slab is hot rolled to form a thick steel plate. It is a method for manufacturing a thick steel plate to be used.

In the method for manufacturing a thick steel plate according to the present invention, as the removing process for removing the thickness ΔT, a cutting process for cutting the slab in layers, a grinding process for scraping the slab in layers, or a cutting process for melting and removing the slabs in layers is performed. is preferred. The thick steel plate preferably has a thickness of 50 to 150 mm and a tensile strength of 490 to 720 MPa.

Further, the present invention is a billet obtained by subjecting one side or both sides of a slab obtained by continuous casting to removal processing, wherein the segregation of the billet is separated from the center of the billet by ΔT/2 or more.

In addition, if the plate thickness of the slab after the removal processing is applied in the present invention is T _REMOVED (mm),
ΔT = T _CASTED - T _REMOVED
is.

According to the present invention, a thick steel plate having excellent toughness can be obtained. In particular, even steel plates with a thickness of 50 to 150 mm and a tensile strength of 490 to 720 MPa can be improved in toughness at extremely low temperatures of -45°C or lower, which is a significant industrial effect. In addition, as a secondary effect, an improvement in fatigue characteristics can also be expected.

FIG. 4 is a cross-sectional view showing a thickness T _CASTED of a slab obtained by continuous casting and a thickness T _PLANED of the slab after removal processing. It is a perspective view which shows the shape of a test piece.

Figure 1 shows the thickness T _CASTED (mm) of the slab obtained by continuous casting, removal processing (for example, cutting to cut the slab in layers, grinding to scrape the slab in layers, and melting and removing the slab in layers). FIG. 10 is a cross-sectional view showing the plate thickness T _REMOVED (mm) of the slab after machining, etc.).

As shown in FIG. 1, the plate thickness of the slab 1 obtained by continuous casting is T _CASTED . The thickness of the slab 1 becomes T _REMOVED by removing one of the two parallel surfaces facing each other in the thickness direction. In the following, the reference number of the slab after removal processing is 3.

Center segregation due to continuous casting occurs near the center line 2 in the plate thickness direction of the slab 1 . When the slab 1 is subjected to a rolling process, it is difficult for plastic deformation to occur in the vicinity of the center line 2 in the plate thickness direction. A thickened area remains.

However, if the slab 1 is removed before being subjected to rolling, the center line 4 in the plate thickness direction of the obtained slab 3 moves to a position different from the center line 2 where the center segregation remains. When the slab 3 is subjected to a rolling process, plastic deformation is hardly applied to the center line 4 in the plate thickness direction, but sufficient plastic deformation can be applied to the center line 2 where the center segregation remains.

Therefore, in the rolling process, it becomes possible to apply sufficient plastic deformation to the center segregation region, and to eliminate the region in which the alloy elements are concentrated. In this way, it is possible to prevent the formation of non-metallic inclusions and thus the occurrence of brittle cracks.

_If the difference ΔT (=T _CASTED −T _REMOVED ) between the thickness T _CASTED (mm) of slab 1 and the thickness T REMOVED (mm) of slab 3 is too small, the Since the center segregation will be present, it becomes difficult to apply plastic deformation to the center segregation site in the process of rolling the slab 3 . Therefore, ΔT needs to be T _CASTED /100 or more (ΔT≧T _CASTED /100).

On the other hand, if ΔT is too large, it takes a long time to process the removal, resulting in an increase in processing cost. Therefore, T _CASTED /10≧ΔT≧T _CASTED /100 is preferred.

The removal process for removing the surface of the slab 1 (thickness T _CASTED ) obtained by continuous casting in a layered manner may be performed on one of the two parallel surfaces facing each other in the thickness direction, or may be performed on both. good. When removal processing is applied to both sides, the center line 4 of the removed slab 3 (thickness T REMOVED ) is _{aligned with the center line 2 of the slab 1 (thickness T CASTED )} by providing a difference in the thickness of the layers to be removed. can be moved to different positions. That is, the removal processing is performed so that the segregation remaining in the steel slab obtained by performing the removal processing on both sides or one side of the slab 1 is separated from the center of the steel slab by ΔT/2 or more.

The steel slab, from which both sides or one side of the slab 1 has been removed in this manner, is subjected to hot rolling to manufacture a thick steel plate.

However, if both sides of the slab 1 are subjected to the removal process, the cost of the removal process is doubled, resulting in an increase in the manufacturing cost of the thick steel plate. Therefore, it is preferable to apply removal processing to one side of the slab 1 .

ADVANTAGE OF THE INVENTION According to this invention, the effect which improves the toughness of a thick steel plate is acquired regardless of the steel grade and the dimension of a thick steel plate. In particular, thick steel plates with a thickness of 50 to 150 mm and a tensile strength of 490 to 720 MPa (for example, C: 0.05 to 0.18 mass%, Mn: 0.9 to 2.0 mass%, Si: 0.10 to 0.55 mass%, P: 0.005 to 0.035% by mass, S: 0.001 to 0.035% by mass, and containing one or more of Cu, Cr, TI, and Ni), technologies to improve toughness, especially extreme temperatures below -45°C. Techniques for improving toughness at low temperatures have not been established, and the application of the present invention makes it possible to exhibit a great effect.

The effect of the invention was investigated using a 310 mm thick slab manufactured by continuous casting. The components of the slab are C: 0.06% by mass, Mn: 1.91% by mass, Si: 0.15% by mass, P: 0.005% by mass, S: 0.002% by mass, Cu: 0.35% by mass, Cr: 0.21% by mass, Ti: 0.01% % by mass.

Since the rolling mill used in this study was limited in the thickness that can be rolled, first, both sides of the slab were machined by 65 mm each to obtain a machined slab with a thickness of 180 mm. Subsequently, the processed slab was subjected to single-sided cutting as removal processing to obtain a material steel plate with a thickness of 120 mm. As a result, the center segregation of the slab could be shifted by 30 mm from the position of 60 mm in the plate thickness direction of the steel plate (that is, the center line in the plate thickness direction). The material steel plate was subjected to rolling to obtain a thick steel plate with a thickness of 60 mm. For rolling, the slab was heated to 1050° C. and soaked for 1 hour or more before rolling. The number of rolling passes was 13, and the reduction in each pass was 4 to 6%. After the final rolling pass, accelerated cooling by water cooling was performed to control the temperature of the steel sheet. Let this be an invention example.

For comparison, a slab (thickness: 310 mm) of the same composition was rolled by the conventional method to obtain a thick steel plate with a thickness of 70 mm. This is a comparative example.

V-notch Charpy impact test specimens (see FIG. 2) were taken from the center segregation regions of these steel plates, respectively, and V-notch Charpy impact tests were performed at -75°C. Table 1 shows the results. The unit of numerical values in FIG. 2 is mm.

As is clear from Table 1, the steel plates of the invention examples have higher absorbed energy and a lower brittle fracture surface ratio than the steel plates of the comparative examples, indicating that the steel plates have excellent toughness. In other words, it is considered that the plastic deformation applied to the centerline of the slab in the plate thickness direction affected the toughness of the central segregation region inside the steel plate.

1 Slab obtained by continuous casting 2 Center line in thickness direction of slab 1 3 Slab after removal processing 4 Center line in thickness direction of slab 3

Claims (4)

In a method for manufacturing a thick steel plate by hot-rolling a slab having a thickness T _CASTED (mm) obtained by continuous casting to manufacture a thick steel plate, after cooling the slab, the slabs facing each other in the thickness direction A removal process is performed to remove a constant thickness ΔT (mm) ≧ T _CASTED /100 from one of the two parallel surfaces in a layered manner, then the slab is heated, and the slab is hot-rolled to form the thick steel plate. A method for manufacturing a thick steel plate, characterized by: The removal process for removing the thickness ΔT is performed by cutting the slab in layers, grinding the slab in layers, or melting and removing the slab in layers. Item 1. A method for manufacturing a thick steel plate according to Item 1. 3. The method for producing a thick steel plate according to claim 1 , wherein the thick steel plate has a thickness of 50 to 150 mm and a tensile strength of 490 to 720 MPa. A method for producing a steel slab by removing a constant thickness ΔT (mm)≧T _CASTED /100 in layers from one side or both sides of a slab obtained by continuous casting, wherein the steel slab is produced. A method for producing a billet, characterized in that segregation is separated from the center of the billet by ΔT/2 or more.

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