JP2006212671A - METHOD FOR PREVENTING SURFACE FLAW AT THE TIME OF ROLLING IN Ni-CONTAINING STEEL - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preventing the generation of surface flaws generated at the time of rolling in an Ni-containing steel. <P>SOLUTION: In the method, the surface of an Ni-containing steel comprising 1 to 10% Ni is mechanical ground, so as to remove an intergranular oxidized part and a cracked part in the surface, thereafter, the depth d<SB>0</SB>and the steepness parameter θ of the recessed part in the surface of the slab after the surface grinding are allowed to satisfy formula (1), and then, the slab is coated with an oxidation preventive, is heated at 1,000 to 1,180°C, and is rolled. The formula (1) is d<SB>0</SB>×a×cos<SP>2</SP>θ/R<SB>1</SB>×10<SP>3</SP>≤0.004×(T-700)×R<SB>1</SB>+40; wherein, a is the depth ratio after the first pass of the rolling in the depth of the recessed part and a=0.3; d<SB>0</SB>is the depth (mm) of the recessed part in the surface of the slab; θ is the steepness parameter (° ) of the recessed part in the surface of the slab expressed by formula (2); R<SB>1</SB>is the draft ratio on and after the first pass of the rolling and satisfies the sheet thickness of the steel sheet after the first pass of the rolling/the sheet thickness of the steel sheet after the completion of the rolling; and T is the heating temperature (°C). The formula (2) is tanθ=L<SB>0</SB>/d<SB>0</SB>; wherein, L<SB>0</SB>is the representative length (mm) in the steepness of the recessed part in the surface of the slab. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for preventing rolling surface flaws in heating and rolling of Ni-containing steel containing 1 to 10% of Ni.

  Conventionally, Ni-containing steel has been widely used as a low-temperature welded structural steel in liquid tanks and the like. It has been well known that Ni-containing steels easily generate (1) slab surface cracks, (2) scale indentations during heating and rolling, and become steel sheet surface after rolling. Ni-containing steels are often used for applications that require strict safety, such as low-temperature liquid tanks. Therefore, strict steel plate surface inspection and surface care after rolling are required, and this steel plate surface care process is a production hindrance. It has become.

The main thing of the prior art currently disclosed with respect to the steel plate surface flaw of such Ni-containing steel is a technology for regulating steel components and casting conditions as a technology relating to prevention of slab surface cracking.
In Patent Document 1, in continuous casting of low temperature steel containing Ni: 5.5 to 10%, S: 0.0020% or less, N: 0.0045% or less, Ca: 0.0020 to 0.0070 A method for preventing surface flaws in continuous casting of Ni-containing low-temperature steel is disclosed, which is characterized by performing continuous casting with a% restriction.

  In Patent Document 2, in continuous casting of low-temperature steel containing 5 to 10% of Ni, the cooling rate of the slab surface in the region where the slab surface temperature is 1150 ° C. to 950 ° C. is 20 ° C./min or less. A method for preventing surface cracking in continuous casting of Ni-containing steel is disclosed.

  In Patent Document 3, in the continuous casting of steel containing 5 to 10% of Ni, in addition to the restriction of P ≦ 0.01% and S ≦ 0.005%, the internal solidification interface strain at the time of continuous casting A technique characterized by defining a rate and regulating it is disclosed.

  In Patent Document 4, in continuous casting of low-temperature steel containing 5.5 to 10% of Ni, the steel components are P ≦ 0.002%, S ≦ 0.002%, Al ≦ 0.02%, and N : In combination with the restriction of 0.001 to 0.004%, the secondary cooling condition of the slab during continuous casting is regulated so that the thickness of the columnar γ grain layer of the slab surface layer is 25 mm or less. A method for producing a Ni-containing steel slab is disclosed.

  In Patent Document 5, in continuous casting of steel containing 5 to 10% of Ni, Al: 0.005 to 0.030%, N: 0.0030% or less, and regulation of the product of the concentration of Al and N In addition, a method for producing a low-temperature Ni-containing steel is disclosed, in which cooling is performed so that the surface temperature of the slab is 750 ° C. or higher in a region subjected to straightening stress after continuous casting.

  In Patent Document 6, P ≦ 0.0010%, S ≦ 0.0010%, Al 0.002 to 0.030%, N ≦ 0 in continuous casting of steel containing 5.5 to 10% Ni. .0040% restriction, further restricting the concentration product of Al and N, and in the secondary cooling of the slab during continuous casting, the ratio of the width and thickness of the slab, the long side surface and the short side surface A continuous casting method for low temperature Ni-containing steel is disclosed, which is characterized by regulating the relationship with the cooling water amount ratio.

  In Patent Document 7, in steel containing 8.5 to 10% of Ni, Si ≦ 0.10%, P ≦ 0.0020%, S ≦ 0.0020%, Al is 0.007 to 0.015%. N-containing steel that can reduce surface defects by reducing grain boundary oxidation and inclusions by limiting N ≦ 0.0040% is disclosed.

Patent Document 8 discloses an antioxidant capable of preventing scale indentation flaws, and states that there is no surface flaw of 9% Ni steel.
Further, although not Ni-containing steel, Non-Patent Document 1 discloses that the concave portion of the steel slab surface of SS41 steel has an initial depth of 5 or 15 mm on the surface of the steel slab after hot rolling. It is disclosed that it becomes a surface defect.
JP 57-261141 A Japanese Patent Laid-Open No. 1-2228644 JP-A-8-197193 JP-A-9-285855 JP-A-10-156696 JP-A-10-166126 JP 2000-256798 A JP-A-53-2311 Iron and Steel 1975, Vol. 61, no. 12, P.I. 205, S555, Japan Steel Association

  The problem to be solved by the present invention is to prevent the occurrence of surface flaws generated during the rolling of Ni-containing steel in order to reduce the load of the steel sheet surface care process, which is a problem at the manufacturing site as a production obstacle.

  When preventing surface flaws, (1) For slab surface cracks, the present inventors have prevented the occurrence of slab surface cracks by using a vertical continuous casting machine. However, it has been difficult to stably prevent the grain boundary oxidation portion that is formed from the surface of the slab that opens and becomes a ridge during the subsequent rolling toward the inside of the slab. Therefore, the present inventors decided to remove the necessary depth (for example, a depth of 0.5 to 10 mm) from the grain boundary oxidized portion on the surface of the slab by mechanical grinding. In some cases, the slab is subjected to ingot rolling prior to the main rolling, but in the case of ingot rolling as well, the grain boundary oxidized portion and the cracked portion where the grain boundary oxidized portion of the slab has opened during the ingot rolling are required to have the required depth. (For example, a depth of 0.5 to 10 mm) was removed by mechanical grinding. These as-cast or slabs after partial rolling are subjected to surface grinding treatment are hereinafter referred to as “steel pieces after surface grinding” and slabs.

(2) For scale indentation during heating and rolling, the present inventors apply an antioxidant to the surface of the steel slab after surface grinding before inserting the steel slab after surface grinding into the furnace. In order to prevent scale formation during heating. Furthermore, with respect to the scale generated during rolling after the antioxidant is peeled off by high-pressure water in the descaler after extraction in the heating furnace, the rolling temperature is also set to 1180 ° C. or lower by regulating the heating temperature to 1180 ° C. or lower. I made it. Due to this restriction, the Ni-containing steel does not grow thick during rolling, so that it is possible to prevent the occurrence of scale indentation due to the scale generated during rolling. These measures were very effective, and there was no occurrence of scale indentation in steel sheets.

  However, it has been found that even when the above-mentioned steel slab after surface grinding and the application of an antioxidant are taken, surface defects of unknown cause occur in the steel sheet after rolling. Therefore, the present inventors considered that there may be a cause of surface flaws in addition to the two causes of the grain boundary oxidation part of the slab and the scale indentation flaw.

In the prior arts of Patent Documents 1 to 7 described above, only a technique for reducing the generation of surface cracks and grain boundary oxidation parts on the surface of a Ni-containing steel slab is disclosed, and it is generated for other reasons. There is no description regarding the surface defects of the steel sheet after rolling of the Ni-containing steel. Therefore, it has been difficult to suppress the formation of steel sheet surface flaws after rolling of the Ni-containing steel only with these conventional techniques.
The specific technical problem to be solved by the present invention is to establish a method capable of preventing the occurrence of wrinkles on the rolling surface flaws of Ni-containing steel generated due to causes other than the grain boundary oxidation part of the slab and the scale indentation flaws. It is to be.

  The present inventors mechanically grind and remove the grain boundary oxidation portion generated from the surface of the cast slab or the steel slab after ingot rolling toward the inside, and further, the steel after the surface grinding. After regulating the form of the concave portion on one surface and then applying an antioxidant, heating and rolling, it was newly discovered that the occurrence of rolling surface defects could be completely eliminated, and the present invention was made.

The gist of the present invention is as follows.
(1) In heating and rolling of Ni-containing steel containing 1 to 10% of Ni, a method for preventing rolling surface flaws, in mass%,
0.030 ≦ C ≦ 0.200
0.02 ≦ Si ≦ 0.50
0.30 ≦ Mn ≦ 2.0
0.001 ≦ P ≦ 0.020
0.0001 ≦ S ≦ 0.0060
1.0 ≦ Ni ≦ 10.0
0.010 ≦ Al ≦ 0.080
0.0020 ≦ N ≦ 0.0060
0.0005 ≦ O ≦ 0.0040
The steel after surface grinding after mechanically grinding the surface of the cast slab or the steel slab of which the slab has been rolled into pieces before heating and rolling to remove the grain boundary oxidation parts and cracks on the surface After the depth d ₀ and the steepness parameter θ of the concave portion on one surface satisfy the formula (1), an antioxidant is applied, heated at 1000 to 1180 ° C., and rolled. The rolling surface wrinkle prevention method of Ni containing steel.
d ₀ × a × cos ² θ / R ₁ × 10 ³ ≦ 0.004 × (T−700) × R ₁ +40 Formula (1)
a: Depth ratio after 1 pass of rolling of the recess depth, a = 0.3.
d ₀ : Depth of recess on steel piece surface (mm)
θ: Steepness parameter (°) of the recess on the surface of the steel slab represented by formula (2)
R ₁ : Reduction ratio after one pass of rolling,
Steel plate thickness / rolling finish thickness after 1 pass of rolling.
T: Heating temperature (° C)
tan θ = L ₀ / d ₀ formula (2)
L ₀ : Steepness representative length of recess on the surface of a steel piece (mm)
(The measurement positions of d ₀ and L ₀ are as shown in FIG. 1.)
(2) Furthermore, the strength and toughness-improving element groups of the base material and joint are mass%,
0.030 ≦ Cu ≦ 1.50
0.030 ≦ Cr ≦ 1.00
0.02 ≦ Mo ≦ 1.00
0.003 ≦ Nb ≦ 0.100
0.003 ≦ V ≦ 0.100
0.005 ≦ Ti ≦ 0.020
0.0005 ≦ B ≦ 0.0020
0.0005 ≦ Ca ≦ 0.0050
0.0005 ≦ Mg ≦ 0.0050
0.0010 ≦ REM ≦ 0.0100
The method for preventing rolling surface flaws of Ni-containing steel according to claim 1, wherein one or more of these are contained.

  The rolling surface flaw prevention method of the present invention can prevent the occurrence of surface flaws generated during the rolling of Ni-containing steel, and can reduce and omit the steel sheet surface care process after rolling, which is a production hindrance, improving productivity. Manufacturing costs can be reduced. Therefore, the present invention is extremely effective industrially.

Hereinafter, the reasons for limiting the chemical components and the production method of the present invention will be described.
C is an element essential for securing the strength of the base material. If it is less than 0.03%, the strength of the base material cannot be secured, so 0.03% was made the lower limit. On the other hand, when a large amount of C is contained, cementite and island martensite, which are the starting points of brittle fracture, are increased, so that suitable toughness cannot be obtained. When the content exceeds 0.20%, the toughness is significantly reduced. From the viewpoint of securing toughness, the more preferable upper limit value is 0.15%.

  Si is an element effective for increasing the strength of the base material. If less than 0.02%, this effect cannot be obtained, so the lower limit was made 0.02%. On the other hand, if the content exceeds 0.50%, island martensite is generated in the structure of the weld heat affected zone (HAZ), and suitable HAZ toughness cannot be obtained. Therefore, the upper limit was made 0.50%. From the viewpoint of securing HAZ toughness, the more preferable upper limit value is 0.30%.

  Mn is an effective element for increasing the strength of the base material. Since this effect cannot be obtained if the content is less than 0.30%, the lower limit is set to 0.30%. On the other hand, if the content exceeds 2.0%, temper embrittlement is promoted, so that a suitable base material and HAZ toughness cannot be obtained. Therefore, the upper limit was made 2.0%.

  P is an element that causes grain boundary embrittlement and is harmful to toughness. If the content exceeds 0.020%, the toughness is significantly lowered, so 0.020% is made the upper limit. From the viewpoint of securing toughness, the more preferable upper limit value is 0.010%. Reduction of the P content to less than 0.001% leads to production failure and cost increase, so the lower limit was set to 0.001%.

  S is an element harmful to toughness as an inclusion such as MnS, and is preferably as low as possible. When the content exceeds 0.0060%, the toughness is significantly lowered, so 0.0060% is made the upper limit. From the viewpoint of securing toughness, a more preferable upper limit value is 0.0040%. Reduction of the S content to less than 0.0001% leads to production failure and cost increase, so the lower limit was set to 0.0001%.

  Ni is an essential element for ensuring low temperature toughness, and if the content is less than 1.0%, these effects cannot be obtained, so the lower limit was set to 1.0%. Ni is an expensive element, and if it is contained in excess of 10.0%, the economic efficiency is impaired, so the upper limit was made 10.0%.

  Al is an element necessary for deoxidation of steel and grain refinement of the base material by AlN, and since these effects cannot be obtained at a content of less than 0.010%, the lower limit is set to 0.010%. . On the other hand, if the content exceeds 0.080%, a coarse base material and HAZ toughness cannot be obtained due to coarse AlN, alumina clusters, and the like. Therefore, the upper limit is set to 0.080%. From the viewpoint of securing toughness, the more preferable upper limit value is 0.050%.

  N is an element necessary for crystal grain refinement of the base material with AlN, and if the content is less than 0.0020%, these effects cannot be obtained, so the lower limit was made 0.0020%. On the other hand, when the content exceeds 0.0060%, a suitable base material and HAZ toughness cannot be obtained due to coarse AlN. Therefore, the upper limit is set to 0.0060%. From the viewpoint of securing toughness, the more preferable upper limit value is 0.0050%.

  O is an element harmful to toughness as an inclusion, and is preferably as low as possible. When the content exceeds 0.0040%, the toughness is significantly lowered, so 0.0040% is made the upper limit. From the viewpoint of securing toughness, the more preferable upper limit value is 0.0030%. Reduction of the O content to less than 0.0005% leads to production failure and cost increase, so the lower limit was set to 0.0005%.

Furthermore, the limited range of the selective elements effective for improving the strength and toughness of the base material and the joint was determined for the following reason.
Cu is effective in increasing the strength of the base material. If less than 0.03%, this effect cannot be obtained, so the lower limit was made 0.03%. On the other hand, if the content exceeds 1.5%, HAZ is cured and suitable HAZ toughness cannot be obtained. Therefore, the upper limit is set to 1.50%.

  Cr is effective in increasing the strength of the base material. If less than 0.03%, this effect cannot be obtained, so the lower limit was made 0.03%. On the other hand, when the content exceeds 1.0%, a hardened structure is generated in the HAZ, and a suitable HAZ toughness cannot be obtained. Therefore, the upper limit is set to 1.0%.

  Mo is effective in increasing the strength of the base material. If less than 0.02%, this effect cannot be obtained, so the lower limit was made 0.02%. On the other hand, when the content exceeds 1.0%, a hardened structure is generated in the HAZ, and a suitable HAZ toughness cannot be obtained. Therefore, the upper limit is set to 1.0%.

  Nb is an element effective for increasing the strength and refining of the base material. If less than 0.003%, these effects cannot be obtained, so the lower limit was made 0.003%. On the other hand, if the content exceeds 0.100%, precipitation of Nb carbonitrides in the HAZ becomes remarkable, and suitable HAZ toughness cannot be obtained. Therefore, the upper limit is set to 0.100%.

  V is an element effective for increasing the strength and refining of the base material. If less than 0.003%, these effects cannot be obtained, so the lower limit was made 0.003%. On the other hand, when the content exceeds 0.10%, precipitation of carbonitrides in the HAZ becomes remarkable, and suitable HAZ toughness cannot be obtained. Therefore, the upper limit is set to 0.100%.

  Ti is an element effective for increasing the strength and refining of the base material. If less than 0.005%, these effects cannot be obtained, so the lower limit was made 0.005%. On the other hand, if the content exceeds 0.020%, coarse TiN is generated and this becomes a starting point of fracture, so that suitable HAZ toughness cannot be obtained. Therefore, the upper limit is set to 0.020%.

  B exhibits a remarkable increase in strength particularly when controlled cooling and quenching heat treatment are performed. If the content is less than 0.0005%, the effect of increasing the strength cannot be obtained, so the lower limit was set to 0.0005%. On the other hand, if the content exceeds 0.0020%, coarse B nitrides and carbon borides are precipitated and serve as starting points for fracture, so that suitable HAZ toughness cannot be obtained. Therefore, the upper limit is set to 0.0020%.

Ca is an element effective in improving toughness by controlling the form of inclusions such as Al ₂ O ₃ and MnS. If the content is less than 0.0005%, the effect of improving toughness cannot be obtained, so the lower limit was made 0.0005%. On the other hand, if the content exceeds 0.0050%, coarse Ca-containing inclusions are generated and this becomes the starting point of fracture, so that a suitable base material and HAZ toughness cannot be obtained. Therefore, the upper limit is set to 0.0050%.

Mg is an element effective for improving toughness by controlling the form of inclusions such as Al ₂ O ₃ and MnS. If the content is less than 0.0005%, the effect of improving toughness cannot be obtained, so the lower limit was made 0.0005%. On the other hand, if the content exceeds 0.0050%, coarse Mg-containing inclusions are generated and this becomes the starting point of fracture, so that a suitable base material and HAZ toughness cannot be obtained. Therefore, the upper limit is set to 0.0050%.

REM is an element effective for improving toughness by controlling the form of inclusions such as Al ₂ O ₃ and MnS. If the content is less than 0.0010%, the effect of improving toughness cannot be obtained, so the lower limit was made 0.0010%. On the other hand, if the content exceeds 0.0100%, coarse REM-containing inclusions are generated and this becomes the starting point of fracture, so that a suitable base material and HAZ toughness cannot be obtained. Therefore, the upper limit is set to 0.0100%.

Next, the reasons for limiting the production method of the present invention will be described.
In the present invention, it is essential to mechanically grind the surface of a slab or a steel slab obtained by subjecting a slab to ingot rolling before heating and rolling. This is because the cast slab or the steel slab obtained by rolling the slab has a grain boundary oxidation part generated from the surface to the inside, and a crack part in which the grain boundary oxidation part of the slab opens during the batch rolling. This is because these become surface defects during product rolling. The grinding depth needs to be a depth (for example, a depth of 0.5 to 10 mm) necessary for completely removing the grain boundary oxidized portion and the surface cracked portion.

  Subsequently, the importance of strictly limiting the concave shape of the steel piece after surface grinding, which is the greatest feature of the present invention, will be described. Most steel types of thick steel plates are heated and rolled without using antioxidants. Even in these thick steel plates that do not use an antioxidant, when a surface recess having a depth of several millimeters (for example, a depth of 5 mm or more) occurs in the slab before heating due to casting abnormality or the like, this is applied to the steel plate after rolling. For example, Non-Patent Document 1 discloses that a recess having an initial depth of 5 mm and 15 mm on the surface of SS41 steel has a surface defect after hot rolling. Is disclosed. However, even if the slab before heating has minute recesses with a depth of about 1.0 mm, in most steel types that do not use an antioxidant, these minute recesses are scaled off during heating, and the steel sheet after rolling There was no surface defect. For this reason, in the heating and rolling of thick steel plates, it has not been conventionally considered that fine slab surface irregularities with a depth of about 1.0 mm cause the steel plate surface flaws. Because the cause of slab surface cracks and grain boundary oxidation parts are considered, and countermeasures for this have been studied exclusively, a report that found the cause of steel sheet surface flaws in Ni-containing steel in the minute recesses on the slab surface Is not found.

  As a result of repeated research, the inventors have applied an antioxidant for the purpose of preventing scale flaws in Ni-containing steel. In the case of heating and rolling, a depth of about 1 mm, which is about 1/5 of the depth that has been considered harmful in the production of thick steel sheets, remains on the surface of the steel sheet even if it is a slight recess. I found out.

Most of the steel slab surface ground using a commercially available slab grinder or a slab cutting machine, for example, has a gentle wave shape with a height of 0.1 to 0.2 mm generated at a pitch of 1 to 3 mm. Such a gentle wave shape does not cause surface defects during rolling. However, a part of the surface of the steel slab after grinding may be considerably larger than the gently corrugated irregularities on the surface of the steel slab, for example, a depth of 1 A recess of about 3 mm may be generated. As described above, when the present inventors apply an antioxidant with Ni-containing steel and heat and roll it, even a slight recess of about 1 mm in depth remains on the surface of the steel sheet and becomes wrinkles. In order to prevent the partial recesses on the surface of the steel slab from becoming surface defects during rolling, many studies were made on the recess shape, heating, and rolling conditions. As a result, it was found that by limiting the shape of the concave portion on the surface of the steel slab so as to satisfy the formula (1), it was possible to prevent the formation of a steel sheet surface flaw, and the present invention was achieved.
d ₀ × a × cos ² θ / R ₁ × 10 ³ ≦ 0.004 × (T−700) × R ₁ +40 Formula (1)

  The equation (1) will be described. The left side of equation (1) is a parameter of the depth of the steel plate surface after rolling, and the right side of equation (1) is a parameter of the scale-off depth of the steel plate surface during rolling. Even when the concave part on the surface of the steel slab collapses at an acute angle in the first pass of rolling to form a surface defect, the surface defect is reduced by reducing the depth of the surface defect on the steel plate to a depth that is scaled off during rolling. Does not remain on the steel sheet.

The parameter of the depth of the steel sheet surface flaw after rolling on the left side of the formula (1) will be described. The sharp recesses of the steel slab are approximately 0.3 times the original depth after one pass of rolling. Therefore, the depth ratio a after one pass of rolling of the recess depth is set to 0.3. Compared with sharp recesses, the depth of a slab is shallower and may disappear after one pass of rolling, so the sharpness (steepness) of the slab surface has an effect on the occurrence of rolling surface flaws. To do. Accordingly, as a result of measuring the steepness parameter θ of the recess shown in FIG. 1 and analyzing the influence of the recess depth after one pass of rolling, the recess depth after one pass of rolling is d ₀ × a × cos ² θ. The correlation was found to be good. After one pass of rolling, the depth of surface defects is inversely proportional to the reduction ratio, so the reduction ratio after one pass of rolling is R ₁ (R ₁ = steel plate thickness after one pass of rolling / steel plate thickness after the end of rolling) ), By multiplying d ₀ × a × cos ² θ, which is the depth of the recess after one pass of rolling, by 1 / R ₁ , it becomes a parameter of the depth of the steel sheet surface flaw after rolling.

The steepness parameter θ of the concave portion shown in FIG. 1 was obtained by tan θ = L ₀ / d ₀ . Here, L ₀ is the representative length (mm) of the steepness of the recess on the surface of the steel slab, and is the horizontal distance between the recess depth measurement position and the end of the recess closer to this position. Measure parallel to the rolling direction of the eye.

  The parameter of the surface scale off depth on the right side of Equation (1) will be described. When the billet is heated, the scale-off of the billet surface hardly occurs due to the effect of the antioxidant. The antioxidant is peeled off by ejecting high pressure water with a descaler after extracting the steel piece from the heating furnace. For this reason, the steel plate surface is oxidized during the subsequent rolling, and the steel plate surface is scaled off by descaling during rolling. It is accurate to calculate the scale-off depth during rolling from the rolling temperature and time of each pass, but it is useful as the equation (1) even if the heating temperature is simply compared relative to the rolling temperature. It was found that the properties were high, and since it was simple, it was relatively represented by the heating temperature. The scale-off depth is proportional to the reduction ratio. From the analysis of the experimental results described below, the coefficient on the right side of Equation (1) was determined to be 0.004 and 40.

In the experiment, the steel having the components shown in Table 1 was used, and the surface of the rolled steel slab was ground using a slab grinder to remove the grain boundary oxidized portion and cracked portion on the surface, and then the steel after surface grinding. An artificial defect was imparted with a drill to one surface (a gentle wave shape with a height of 0.1 to 0.2 mm, a steel piece thickness of 150 mm), an antioxidant was applied, heated at 1140 ° C., and rolled. The artificial defect has a recess depth d ₀ of 0.5, _1.0 , 2.0, and _3.0 mm, all of which have four types of sharp recesses with a recess steepness parameter θ of 0 °, and a depth d. _{For 0} = 2.0 mm, two types of gradual recesses with a steepness parameter θ of 45 ° and 63 ° were added, for a total of six types. Three pieces of steel with the above artificial defects were prepared, and the plate thickness was changed so that the reduction ratio R ₁ after one pass of rolling was 5.0, 8.0, 12.0, respectively (27 , 17, 11 mm). The surface of the rolled steel sheet was shot blasted and then visually inspected for presence of scale indentation flaws and magnetic particle inspection.

FIG. 2 plots the experimental results of changing the recess depth d ₀ and the rolling reduction R ₁ for the recess of θ = 0 ° on the steel sheet surface depth parameter after rolling on the left side of Equation (1). The scale-off depth parameter of the steel sheet surface during rolling on the right side of Formula (1) is also shown. In FIG. 2, the points marked with ◯ are steel plates where the steel plate surface flaw was not detected by the magnetic particle inspection, and the points marked with × were steel plates where the steel plate surface flaw was detected by the magnetic particle flaw inspection. No scale indentation occurred in any of the steel plates. The boundary between the ○ mark and the X mark in the experimental result is in good agreement with the intersection of the right side and the left side of Equation (1). FIG. 3 shows the scope of the present invention in FIG.

FIG. 4 plots the experimental results of changing the steepness parameter θ and the rolling reduction R ₁ on a concave portion having a depth d ₀ = 2 mm on the steel sheet surface depth parameter after rolling on the left side of the equation (1). The scale-off depth parameter of the steel sheet surface during rolling on the right side of Formula (1) is also shown. The meanings of the circles and the crosses in FIG. 4 are the same as those in FIG. No scale indentation occurred in any of the steel plates. The boundary between the ○ mark and the X mark in the experimental result is in good agreement with the intersection of the right side and the left side of Equation (1).

  The same artificial defect test was performed with heating temperatures of 1000 ° C. and 1180 ° C. The experimental results are shown in FIGS. The meanings of the circles and the crosses in FIGS. 5 and 6 are the same as those in FIG. No scale indentation occurred in any steel plate. There is a difference in the presence or absence of surface defects on the steel sheet due to the difference in heating temperature. Including the above experimental result and the present experimental result, the coefficient on the right side of the equation (1) was optimized so that the boundary between the circles and the crosses matched.

  As described above, if the wave shape is a gentle wave having a height of 0.1 to 0.2 mm, it does not become a surface defect during rolling. Therefore, after grinding, it is effective for preventing surface flaws of Ni-containing steel to prevent the formation of recesses on the surface of the steel slab due to inversion of the steel slab, pushing of foreign matter during movement, foreign matter jumping during grinding, etc. . If a recess that does not satisfy equation (1) is still produced on the surface of the steel slab, only the periphery of the recess is partially ground with a hand grinder or the like, and the steepness of the recess is satisfied until equation (1) is satisfied. Generation of surface defects can be prevented by reducing the parameter θ.

  In addition to the restriction of the shape of the concave portion on the steel slab surface described above, in the present invention, it is essential to apply an antioxidant to the steel slab surface. This is because when the steel slab surface is heated and rolled without applying an antioxidant, a steel plate care process is required due to generation of scale indentation flaws. The antioxidant prevents oxidation of the surface of the steel slab in heating at 1000 ° C. to 1180 ° C., which is the heating temperature limited in the present invention, and easily peels off by high-pressure water in the descaler after extraction in the heating furnace. Any antioxidant may be used as long as it is commercially available.

  In the present invention, the heating temperature is limited to 1000 ° C. or higher and 1180 ° C. or lower. When the heating temperature is lower than 1000 ° C., the rolling productivity is lowered, so the lower limit is set to 1000 ° C. When the heating temperature is higher than 1180 ° C., the scale generated during high-temperature rolling is pushed into the steel sheet and becomes surface flaws, so the upper limit is set to 1180 ° C.

Examples of the present invention are shown below. Steel was melted by a converter, and a slab having a thickness of 245 to 400 mm was produced by a vertical continuous casting machine. Table 1 shows the chemical composition of the steel material. Steels A to G in Table 2 are all steels having chemical components within the scope of the present invention. In Tables 3 and 4, steel plates heated and rolled using these steels, slab thickness, presence / absence of ingot rolling, presence / absence of surface grinding at the required depth of the ingot or slab after ingot rolling, The thickness of the steel slab after surface grinding, the maximum value of d ₀ × a × cos ² θ / R ₁ × 10 ^{3 on} the left side of equation (1) and the surface recess on the surface of the steel slab after surface grinding, and the right side of equation (1) 0.004 × (T−700) × R ₁ +40 value, satisfaction / dissatisfaction of formula (1), presence / absence of application of antioxidant, heating temperature, steel sheet thickness and rolling ratio R ₁ after one pass of rolling, And the presence or absence of a steel sheet surface flaw after rolling is shown.

Surface grinding at the required depth of the slab or slab after rolling is performed using a commercially available slab grinder or slab cutting machine, and the surface of the slab after grinding is gently 0.1 to 0.2 mm in height. A simple wave shape was generated at a pitch of 1 to 3 mm.
The measurement of the surface recess shape on the surface of the steel slab after surface grinding was performed by measuring 2 to 10 large dents on the front and back surfaces of one steel slab, and the value of d ₀ × cos ² θ was the maximum. The value of Per front and back surfaces of a sheet of steel strips, using the value of recesses value of d ₀ × cos ² θ was maximum, was determined value of left side of the equation (1). On the surface after measuring the concave shape, consideration was given so that no new indentation flaws would be generated thereafter.

  In the steel sheet surface flaw inspection after rolling, the front and back surfaces of the steel sheet were subjected to a visual inspection for the presence of a scale indentation flaw and a magnetic particle inspection inspection after shot blasting. It is possible to detect fine surface flaws that cannot be visually detected by magnetic particle flaw detection. Both the front and back surfaces were judged to have no steel plate surface flaws when there was no scale indentation flaws by visual inspection and no fine surface flaws were detected by magnetic particle inspection. On either one of the front and back surfaces, when a scale indentation flaw was confirmed by visual inspection or when one or more minute surface flaws were detected by magnetic particle inspection, it was determined that a steel plate surface flaw was present. In addition, Ni-containing steel becomes a product after it is directly water-cooled after rolling or after a heat treatment process such as tempering, quenching and tempering offline, but the steel sheet surface inspection result before shipment is the steel sheet surface inspection result after rolling. Was the same.

  As is apparent from Tables 3 and 4, numbers 1, 2, 6, 7, 10, 11, 15, 16, 19, 20, 23, 24, 25, and 27 according to the production method of the present invention are steel sheet surface defects. There was no outbreak. On the other hand, in Nos. 3, 4, 8, 9, 12, 13, 21, 22, and 28, the concave shape of the steel slab surface after surface grinding did not satisfy the formula (1), and the steel plate surface flaw occurred. . In No. 5, since the grinding amount on the surface of the steel slab after the partial rolling was insufficient, the steel plate surface flaw occurred. Nos. 14, 18, and 26 were heated and rolled without applying an antioxidant to the surface of the steel slab after surface grinding, and steel plate surface flaws (scale indentation flaws) were generated. No. 17 was heated at a heating temperature of 1200 ° C. and exceeding the limit range of the present invention, so that a steel sheet surface flaw (scale indentation flaw) occurred.

Of the recess of the steel strip surface, a diagram illustrating depth d _0, and the measurement position of the steepness representative length L _0, the relationship between the steepness parameter theta. For the recess of θ = 0 °, the experimental results of changing the recess depth d ₀ and the rolling reduction R ₁ , the steel sheet surface depth parameter after rolling on the left side of equation (1), and the right side of equation (1) It is a figure which shows the relationship with the scale-off depth parameter of the steel plate surface under rolling. In FIG. 2, it is a figure which shows the scope of the present invention. Experimental results in which the steepness parameter θ and the rolling reduction R ₁ were changed for a recess having a depth d ₀ = 2 mm, the steel sheet surface depth parameter after rolling on the left side of equation (1), and the right side of equation (1) It is a figure which shows the relationship with the scale-off depth parameter of the steel plate surface under rolling. Experimental results at a heating temperature of 1000 ° C. in which the recess depth d ₀ and the rolling reduction R ₁ are changed for the recess of θ = 0 °, the steel sheet surface depth parameter after rolling on the left side of the equation (1), and It is a figure which shows the relationship with the scale-off depth parameter of the steel plate surface under rolling of the formula (1) right side. Experimental results at a heating temperature of 1180 ° C. in which the recess depth d ₀ and the rolling reduction R ₁ are changed for the recess of θ = 0 °, and the steel sheet surface depth parameter after rolling on the left side of Equation (1), and It is a figure which shows the relationship with the scale-off depth parameter of the steel plate surface under rolling of the formula (1) right side.

Claims (2)

In heating and rolling of Ni-containing steel containing 1 to 10% of Ni, a method for preventing rolling surface flaws, in mass%,
0.030 ≦ C ≦ 0.200
0.02 ≦ Si ≦ 0.50
0.30 ≦ Mn ≦ 2.0
0.001 ≦ P ≦ 0.020
0.0001 ≦ S ≦ 0.0060
1.0 ≦ Ni ≦ 10.0
0.010 ≦ Al ≦ 0.080
0.0020 ≦ N ≦ 0.0060
0.0005 ≦ O ≦ 0.0040
The steel after surface grinding after mechanically grinding the surface of the cast slab or the steel slab of which the slab has been rolled into pieces before heating and rolling to remove the grain boundary oxidation parts and cracks on the surface After the depth d ₀ and the steepness parameter θ of the concave portion on one surface satisfy the formula (1), an antioxidant is applied, heated at 1000 to 1180 ° C., and rolled. The rolling surface wrinkle prevention method of Ni containing steel.
d ₀ × a × cos ² θ / R ₁ × 10 ³ ≦ 0.004 × (T−700) × R ₁ +40 Formula (1)
a: Depth ratio after 1 pass of rolling of the recess depth, a = 0.3
d ₀ : Depth of recess on steel piece surface (mm)
θ: Steepness parameter (°) of the recess on the surface of the steel slab represented by formula (2)
R ₁ : Reduction ratio after one pass of rolling,
Steel plate thickness after one pass of rolling / Steel plate thickness after rolling T: Heating temperature (° C)
tan θ = L ₀ / d ₀ formula (2)
L ₀ : Steepness representative length of recess on the surface of a steel piece (mm) In addition, the weight percentage of the base material
0.030 ≦ Cu ≦ 1.50
0.030 ≦ Cr ≦ 1.00
0.02 ≦ Mo ≦ 1.00
0.003 ≦ Nb ≦ 0.100
0.003 ≦ V ≦ 0.100
0.005 ≦ Ti ≦ 0.020
0.0005 ≦ B ≦ 0.0020
0.0005 ≦ Ca ≦ 0.0050
0.0005 ≦ Mg ≦ 0.0050
0.0010 ≦ REM ≦ 0.0100
The method for preventing rolling surface flaws of Ni-containing steel according to claim 1, wherein one or more of these are contained.

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