JP4424189B2 - Billet billet manufacturing method with excellent internal quality - Google Patents

Description

  The present invention relates to a billet slab manufacturing method capable of manufacturing billet slabs with good internal quality at a low cost and without impairing productivity even in a steel type that easily generates porosity.

  Reduction of porosity (porosity) generated in the vicinity of the final solidification position during continuous casting is an important issue in improving the quality of steel products, and various improvement methods have been attempted. As a simple method, by increasing the value of the “aspect ratio” (meaning the ratio of long piece to short piece, also referred to as “flat ratio”) in the cross-sectional shape of the mold, the porosity at the slab stage is increased. It is possible to suppress the occurrence to some extent. However, in order to meet the demand for billets having a large cross section, it may be necessary to produce a slab having a thickness greater than a certain thickness. Even if it is increased, it is difficult to prevent the occurrence of porosity defects depending on the steel type.

The following techniques are known as methods for reducing porosity, and each has the following problems.
1) Adjustment of flattening ratio (aspect ratio) of mold cross section In Patent Document 1, in the manufacturing method of high chromium-containing rolling material steel containing 5% or more of chromium, the rolling material steel has a substantial aspect ratio. Discloses a method of continuous casting as 1.5 to 4.0 slabs. According to this method, by increasing the flatness ratio of the mold cross-section, the concentration of porosity at the center of the slab cross-section is reduced, but depending on the steel type, the effect is insufficient, and the center of the slab cross-section Porosity may remain in the part.

2) Unsolidified reduction of cast slab during continuous casting Many patent documents including Patent Document 2 disclose unsolidified reduction technology. For example, Patent Document 2 discloses a reduction by forging that is 1.5 times or more the unsolidified thickness of a slab in a region where the solid phase ratio at the thickness center portion of the continuous cast slab is 0.9 or more. In the case of slab, the center polo is added to the region of 20 to 40% of the bloom width in the central region in the width direction of the bloom, and in the case of slab, to the region of 70 to 90% of the slab width in the central region in the width direction of the slab. A method for producing a continuous cast slab without a city is disclosed.

  However, in these methods, it is necessary to set the unsolidified reduction zone at the end of solidification, and it is difficult to focus the reduction position at an appropriate position, and it is not easy to improve the slab quality stably.

3) Reduction after solidification of cast slab during continuous casting In Patent Document 3, in a method of producing a thick steel plate from a continuous cast material, a reduction device is installed on the outlet side of the continuous casting machine. There is disclosed a method for producing a continuous cast steel sheet that is subjected to dehydrogenation without being cooled at 500 ° C. or lower after being subjected to reduction so as to satisfy a predetermined relationship. However, the installation of a large reduction device in a continuous casting facility is greatly restricted in terms of securing an installation space and the installation in an existing facility is more difficult. Further, when a plurality of continuous casting facilities are provided, it is necessary to install a large reduction device for each continuous casting machine, and there is a tendency to restrict the facility cost.

4) Porosity detoxification by roll reduction Roll reduction has less penetration of the reduction force into the center of the slab than press reduction, and its effect is difficult to obtain. As a countermeasure against this, there is a method of intensively rolling down the center part in the width direction of the slab using a convex roll having a large diameter in the center part in the longitudinal direction of the roll.

  For example, in Patent Document 4, in the method of lightly reducing the center of a continuously cast slab generated after solidification in a continuous casting line by a pinch roll having a convex portion formed at the center, a subsequent stage of the drawing correction roll. A slab light reduction method is disclosed in which the pinch roll is disposed on the surface, the surface is cooled to 600 ° C. to 400 ° C. by a slab surface cooling facility provided immediately before the pinch roll, and then the slab is reduced by the pinch roll.

  However, in the method using the roll reduction as described above, a depression is generated in the center portion in the width direction of the slab, and thus, a piece of rolled material such as a piece is easily generated in a piecewise rolling process using a roll in a subsequent step.

5) Press reduction before partial rolling For example, Patent Document 5 discloses a continuous casting bloom in a reduction head that presses between a soaking furnace and a partial rolling mill and presses the center porosity. There is disclosed a reduction device that suppresses metal flow in the slab movement direction and improves the penetration of the press reduction force by providing a convex portion that is orthogonal to the movement direction. However, depending on conditions such as slab size and steel type, the effect may not always be exhibited.

6) Billet production only by pressing pressure Although the method of manufacturing a billet only by pressing pressure is performed, without performing the lump rolling by a roll, it is difficult to improve productivity.

  As mentioned above, even in the steel grades that easily generate porosity, it is still a problem to produce billet billets with good internal quality at a low cost and without compromising productivity. There are still problems to be solved.

Japanese Patent No. 2840103 (Claims and page 2, left column, lines 24 to 29)

Japanese Patent No. 2945060 (Claims and page 2, left column, lines 24 to 45) JP-A-55-114404 (claims and page 1, right column, line 20 to page 2, upper left column, line 14) Japanese Patent No. 2705414 (Claims and paragraph [0008]) Japanese Utility Model Laid-Open No. 5-28544 (claim for utility model registration and paragraph [0008]) Yasuhiro Ito, Masayuki Kawamoto, Fumio Kawato, Naoyoshi Yoshida, Makoto Kato: CAMP-ISI Vol. 9 (1966) -55

  The present invention has been made in view of the above problems, and the problem is that, even in a steel type in which porosity is likely to be generated, the flat ratio of the mold cross-section is specified and the solidification shrinkage rate specific to the steel type is determined. It is another object of the present invention to provide a billet billet manufacturing method that can produce billet billets with good internal quality at low cost without impairing productivity by defining the amount of press reduction.

  In order to solve the above-mentioned problems, the present inventor has studied a manufacturing method of billet billets with good internal quality based on the conventional problems, and has obtained the following findings (a) and (b) The present invention was completed.

  (A) For steel grades with a solidification shrinkage rate of 4% or more when solidifying from the liquid phase to the solid phase, the porosity at the bloom stage will not be excessive by optimizing the flatness ratio of the mold cross section. After controlling, depending on the size of the porosity, which varies depending on the steel type, by performing an appropriate amount of press rolling before roll rolling with a roll, while suppressing the reduction in production efficiency and cost increase, Quality can be improved.

(B) The above steel type (a) was continuously cast using a mold having a mold cross section thickness of 240 mm or more and a mold cross section flatness ratio of 1.5 to 4.0, and the following (1) When the press reduction is performed under the conditions satisfying the relationship represented by the formula, the porosity of the steel slab becomes sufficiently pressure-bonded, and a billet steel slab having good internal quality is obtained after roll rolling.
0.7 ≦ L / Lo + 0.03 × Δδ ≦ 1.0 (1)
Here, Lo is the distance (mm) between the pressed surfaces before pressing at the time of the maximum reduction by the press, L is the distance (mm) between pressing surfaces after the pressing at the maximum reduction by the press, and Δδ is the solidification shrinkage rate (%). ) Respectively.

The present invention has been completed based on the above findings, and the gist thereof lies in a method for manufacturing a billet billet as shown in the following (1) and ( 2 ).

(1) A steel melt having a solidification shrinkage rate of 4% or more when solidified from a liquid phase to a solid phase has a cross-sectional thickness of 240 mm or more and a width of 1.8 to 4.0 times the thickness. continuous casting using a mold, the obtained Bloom cast piece, the width direction, meets the following relationship (1), and a reduction rate over the entire length under conditions in the range of 100~300mm A method for producing a billet billet comprising performing roll rolling after pressing.

0.7 ≦ L / Lo + 0.03 × Δδ ≦ 1.0 (1)
Here, Lo is the distance (mm) between the pressed surfaces before pressing at the time of the maximum reduction by the press, L is the distance (mm) between pressing surfaces after the pressing at the maximum reduction by the press, and Δδ is the solidification shrinkage rate (%). ) Respectively.

(2 ) When performing the roll rolling after the press reduction, the billet slab according to (1 ) is continuously rolled in a hot state without heating the bloom slab after the press reduction. Production method.

In the present invention, “solidification shrinkage rate when solidifying from a liquid phase to a solid phase” means a volume shrinkage rate when molten steel is solidified from a liquid phase to a solid phase. It can be measured by the method described in 1. The typical values of solidification shrinkage for each steel type are 2.4% for SUS304 steel (0.06% C-18% Cr-8% Ni) and BBS steel (1.0% C-1.5). % Cr) is 3.7%, H50 steel (0.50% C) is 4.1%, M9S steel (0.1% C-9% Cr) is 4.7%, Super 13% Cr steel (0 .01% C-12% Cr) is 5.9%, and 13% Cr steel (0.2% C-13% Cr) is 6.4%.

  Further, “in the hot state” means that the temperature of the bloom slab remains in the range of 300 to 1000 ° C.

  According to the method of the present invention, even in a steel type in which porosity is likely to be generated, by adjusting the press reduction amount according to the flatness ratio of the slab cross section and the solidification shrinkage rate specific to the steel type to an appropriate range, the internal quality Can be manufactured at low cost without impairing productivity. Therefore, the present invention can greatly contribute to the improvement of the internal quality of the steel billet that is a raw material for pipe products and the like, and is extremely effective in preventing the occurrence of flaws on the inner surface and outer surface of the pipe product.

  As described above, according to the method of the present invention, a molten steel having a solidification shrinkage rate of 4% or more when solidified from a liquid phase to a solid phase is obtained by using a cross-sectional thickness of 240 mm or more and a width of 1.8 mm. After continuous casting using a ~ 4.0-fold mold, the resulting bloom cast slab was subjected to press reduction over the entire length under the condition satisfying the relationship of the above expression (1) in the width direction or the thickness direction. A method of manufacturing a billet steel slab characterized by performing roll rolling. Hereinafter, the method of the present invention will be described in more detail.

  Conventionally, billets were produced by casting a mold with a flatness ratio of 1.5 or more in a relatively large cross-section bloom having a cross-sectional thickness of 240 mm or more, and then rolling it with a roll. For materials with a high rate, the porosity could not be completely suppressed.

  Therefore, the present inventors installed a press facility in the roll smash factory, and performed billet production and pipe making tests by changing the solidification shrinkage rate, mold flatness ratio, press amount and press direction of the slab material. As a result, it was found that an appropriate press amount exists depending on the solidification shrinkage and the slab cross-sectional shape determined by the steel type.

  The reason why the present invention is limited to the above range and the preferable range of the present invention will be described below.

1) Solidification shrinkage (Δδ)
Continuous casting is performed using a mold having a mold size of 300 mm × 600 mm, and when the short side of the obtained slab is pressed down in the width direction of the slab, partial rolling is performed with a roll. The billet was 230 mm in diameter. This billet was formed into a steel pipe having an outer diameter of 275 mm and a wall thickness of 13 mm using a Piercer mill and a mandrel mill, and the inner surface was visually inspected to check for defects.

  FIG. 1 is a diagram showing a relationship between a solidification shrinkage rate and a pipe inner surface defect occurrence state. In the same figure, the occurrence of defects on the pipe inner surface is “good” means that no defects were found in the pipe inner surface after the mandrel mill pipe making, and “defect” means that the pipe inner surface was visually observed. The situation where one or more defects are recognized.

  From the results of FIG. 1, when the solidification shrinkage rate is less than 4%, the occurrence of inner surface defects of the pipe can be prevented by setting the flatness ratio to 2.0, which is 1.8 or more, without performing press reduction. It turns out that it becomes favorable. Therefore, in the present invention, a steel type having a solidification shrinkage rate of 4% or more was targeted. The upper limit of the coagulation shrinkage rate is not particularly defined, but it is preferable that the coagulation shrinkage rate is within 7% because the effect of the present invention can be obtained.

2) Mold cross-sectional thickness When the mold cross-sectional thickness is less than 240 mm, the time until the slab is completely solidified is short. Therefore, bridging that is caused by lowering the casting speed and causes center porosity can be easily suppressed. In addition, since a relatively large cooling rate can be secured even near the final solidification position, the transition region from the liquid phase to the solid phase can be narrowed, and there is little possibility that the porosity grows greatly.

  Therefore, the present invention is directed to a method for producing a billet billet by casting with a mold having a mold size having a mold cross section having a thickness of 240 mm or more and having a high porosity sensitivity (that is, porosity is easily generated). .

3) Ratio of long side to short side of mold (flat ratio of mold cross section)
When the flat ratio of the mold cross-section is less than 1.8, the effect when the manufacturing method of the present invention is used is small, so the flat ratio is set to 1.8 or more.

  Moreover, in order to investigate the appropriate upper limit of the flatness ratio of the mold cross section, the same investigation as described above was performed. That is, by changing the mold width based on a mold having a mold size of 300 mm × 600 mm, continuous casting using mold sizes having various flatness ratios is performed, and the long side of the obtained slab is placed on the long side of the slab. Roll rolling was performed for the case where the thickness method was pressed and the case where the short side was pressed in the width direction of the slab to obtain a billet having a diameter of 230 mm. This billet was formed into a steel pipe having an outer diameter of 275 mm and a wall thickness of 13 mm using a Piercer mill and a mandrel mill to obtain a 13% Cr steel pipe of API 5CT L-80. The outer surface was visually inspected for the presence of defects.

  FIG. 2 is a diagram showing the relationship between the flatness ratio of the mold cross section and the pipe outer surface flaw occurrence index. In the figure, the pipe outer surface flaw occurrence index refers to the length ratio (%) at which harmful flaws existed in the visual inspection when the billet is made without maintenance.

  When the flattening ratio of the mold cross section exceeds 4.0, the bloom becomes too thin. Therefore, the central part of the long side tends to be concave when the short side is reduced in roll rolling, and the long side surface has a ridge-like shape due to compression. Outer surface flaws are likely to occur.

  For the above reasons, the flattening ratio of the mold cross section is set in the range of 1.8 to 4.0.

  In addition, the range with a preferable flat ratio of a mold cross section is the range of 1.8-3.0.

4) Press reduction amount In order to investigate an appropriate press reduction amount according to the solidification shrinkage rate when steel solidifies from a liquid phase to a solid phase, the following investigation was conducted. Continuous casting is performed using a mold having a mold size of 300 mm × 600 mm, the short side of the obtained slab is pressed in the width direction of the slab, and then roll-rolling is performed to form a billet having a diameter of 230 mm. The billet was formed into a steel pipe having an outer diameter of 275 mm and a wall thickness of 13 mm using a Piercer mill and a mandrel mill, and the inner surface was visually inspected to check for defects.

FIG. 3 shows the ratio (L / Lo) of the solidification shrinkage rate and the distance (L) between the pressed surfaces after pressing at the time of maximum reduction by pressing and the distance (Lo) between the pressing surfaces before pressing at the maximum reduction by pressing. It is a figure which shows the relationship between a pipe inner surface flaw occurrence condition. In the figure, a circle indicates that there is no internal flaw, and a circle indicates that an internal flaw has occurred. In addition, the presence or absence of internal flaws was determined by visual observation of the pipe internal surface after mandrel mill pipe production, when visible flaws were observed, and when no visible flaws were observed, it was determined that there was no occurrence. did.
From the results in FIG. 3, the following matters were found. That is, when the value of the ratio (L / Lo) exceeds (1.0−0.03 × Δδ), the porosity is not sufficiently pressed. On the other hand, if the ratio (L / Lo) is less than (0.7−0.03 × Δδ), the reduction amount becomes excessive, resulting in an increase in cost due to unnecessary reduction power, and casting due to press reduction. The deformation of the piece becomes large, the billet is bent at the time of roll lump, and stable productivity is likely to be lost, and the corner portion is likely to be bent.

  Note that when the slab is pressed down until it reaches the final shape of the billet, which is an intermediate product, the efficiency is poor and the productivity is lowered. Therefore, it is appropriate to perform roll rolling after the pressing.

5) Pressing down the short side of the slab Whether the long side of the slab is pressed down in the thickness direction of the slab, or when the short side of the slab is pressed down in the width direction of the slab, As shown in FIG. 2, the same effect can be obtained for the suppression of the generation of porosity. However, if the short side of the slab is pressed down in the width direction of the slab, the thickness of the steel slab does not decrease, so even when casting using a mold of the same mold size, a billet with a larger cross section Can be manufactured. This is because the roll billet rolling after the press reduction cannot increase the thickness of the steel slab, and the maximum billet diameter that can be manufactured is determined by the thickness after the press reduction. Therefore, the press reduction on the short side is advantageous and advantageous in industrial production because a large range of billet diameters that can be manufactured can be secured even when the same mold size is used.

6) Roll reduction after press reduction It is preferable from the viewpoint of energy cost saving to continue rolling with a roll in a hot state without performing reheating after press reduction.

  In order to confirm the effect of the billet manufacturing method of the present invention, a billet steel slab manufacturing test is performed as follows, and a pipe making test is further performed using the billet steel slab to determine the quality of the pipe inner surface and the pipe outer surface. Evaluated.

(Test method)
Continuous casting is performed using molten steels of different steel types with different chemical compositions and solidification shrinkage rates, and the resulting slab is soaked to 1250 ° C in a soaking furnace, then pressed, and then rolled by rolls. To produce billet billets. Further, this billet steel slab was pierced and rolled by a piercer mill and a mandrel mill to produce a seamless steel pipe. By visually inspecting the inner and outer surfaces of the steel pipe thus obtained, the presence or absence of surface defects was investigated and the quality was evaluated.

For press reduction, a slab was reduced in the horizontal direction (horizontal direction) by installing a reduction mold having a height of 1 m and a width of 1 m in a hydraulic 3000T (2.94 × 10 ⁷ N) press facility. The bloom was pressed down over its entire length by a method in which pressing was repeated for every about 0.8 m of propulsion. The press reduction on the long side was performed by placing the bloom vertically, and the press reduction on the short side was performed by placing the bloom horizontally.

  The solidification shrinkage rate at the time of solidification of steel was investigated by the sessile drop method described in Non-Patent Document 1. That is, the experimental sample (diameter φ6 mm) was placed on a stage on the rotation axis in the furnace, and the inside of the furnace was replaced with Ar gas, and then the sample silhouette was photographed using a CCD camera for each measurement temperature, Images were recorded. In order to improve accuracy, the stage was rotated and measurements were taken from four directions to prevent errors due to the asymmetrical shape of the sample. Based on the obtained image data, the volume and density of the sample were calculated by the sectional quadrature method.

  The inner surface quality and outer surface quality after pipe making were as follows. That is, the quality of the inner surface was evaluated as good (◯) when the inner surface flaws were not generated and evaluated as poor (×) when the inner surface flaws were generated. As for the outer surface quality, the case where the length ratio of the outer surface defects was less than 1% was evaluated as good (◯), and the case where the length ratio of the outer surface defects was 1% or more was evaluated as defective (x).

(Test results)
Tables 1 and 2 collectively show test conditions and results.

  Test numbers 3 to 8, Test number 11, Test numbers 15 to 20, and Test number 23 are the solidification shrinkage rate, the slab cross-sectional thickness, the flatness ratio of the mold, and the formula (1) defined in the present invention. This is a test for an example of the present invention that satisfies all the relationships. In both cases, the quality was good on both the inner and outer surfaces after pipe production.

  Moreover, since test numbers 1 and 13 have a mold thickness of less than 240 mm, it is difficult for porosity to occur in the slab, and the quality of the inner surface and the outer surface after pipe making is not required without using the method of the present invention. It was good. In Test No. 25, since the solidification shrinkage rate of steel is less than 4%, porosity is hardly generated in the slab, and therefore the quality of the tube inner surface and outer surface is good without using the method of the present invention. there were.

  In Test Nos. 2 and 14, which are comparative examples in which the flatness ratio of the mold was too large, a bowl-shaped billet wrinkle occurred, resulting in poor tube outer surface quality. In Test Nos. 12 and 24, which are comparative examples in which the flatness ratio of the mold was too small, the quality of the inner surface of the pipe was poor, even though the conditions under the pressing pressure were within the appropriate range.

  Test numbers 9 and 21 of comparative examples in which the value of the ratio (L / Lo) at the time of press reduction is less than (0.7-0.03 × Δδ) and does not satisfy the relationship of the above expression (1) are Later, when roll-rolling was performed, the steel slab posture was unstable, so that the rolling stability was lowered, the billet shape was poor, and the tube outer surface flaws occurred due to the folding of the corner portion. Moreover, the test numbers 10 and 22 of the comparative examples in which the value of the ratio (L / Lo) exceeds (1.0−0.03 × Δδ) and does not satisfy the relationship of the expression (1) are sufficiently porous. Without crimping, flaws on the inside of the tube occurred.

  According to the method of the present invention, even in a steel type in which porosity is likely to be generated, by adjusting the press reduction amount according to the flatness ratio of the slab cross section and the solidification shrinkage rate specific to the steel type to an appropriate range, the internal quality Can be manufactured at low cost without impairing productivity. Therefore, the present invention is a method for manufacturing billet steel pieces that can be widely applied in the field of manufacturing round billets for seamless pipe manufacturing, and can be widely used as a technique for improving the inner and outer surface quality of steel pipes.

It is a figure which shows the relationship between a solidification shrinkage rate and a pipe inner surface defect generation condition. It is a figure which shows the relationship between a mold flat ratio and a pipe outer surface wrinkle generation | occurrence | production index. It is a figure which shows the relationship between a solidification shrinkage rate and a press reduction ratio (L / Lo), and a pipe inner surface flaw generation | occurrence | production condition.

Claims (2)

Using a molten steel having a solidification shrinkage rate of 4% or more when solidifying from a liquid phase to a solid phase, a mold having a cross-sectional thickness of 240 mm or more and a width of 1.8 to 4.0 times the thickness is used. continuous casting Te, the obtained Bloom cast piece, the width direction, meets the following relationship (1), and a pressing pressure over the entire length of the rolling reduction under conditions in the range of 100~300mm A method for producing a billet billet characterized by performing roll rolling after application.
0.7 ≦ L / Lo + 0.03 × Δδ ≦ 1.0 (1)
Here, Lo is the distance (mm) between the pressed surfaces before pressing at the time of the maximum reduction by the press, L is the distance (mm) between pressing surfaces after the pressing at the maximum reduction by the press, and Δδ is the solidification shrinkage rate (%). ) Respectively. 2. The method for producing a billet steel slab according to claim 1, wherein, when the roll rolling is performed after the press reduction, the roll slab is continuously rolled in a hot state without heating the bloom cast slab after the press reduction.

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