JP2007090429A - Hot rolling method of bar material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot rolling method of bar material for manufacturing a bar product which suffices rigid requirements in recent years for guarantee against surface flaws by preventing the occurrence of minute surface flaws caused by rolling deformation in the hot rolling process for bar materials such as wire rods, steel bars and square bars. <P>SOLUTION: In the hot rolling process of bar material, a billet as a raw material is sequentially reduced in its cross-section area by respective rolling process of passing through calibers of rollers disposed plurally and is finished to a prescribed size. The cross-section area of a rolled product is decreased so that compressive strain in the circumferential direction of the rolled product at the pass in the respective rolling process becomes not less than -0.5, and thereby a troublesome surface flaw, which may cause a processing defect in the succeeding process, is not produced by the rolling deformation, and it becomes possible to provide the bar product having excellent surface quality which can meet rigid requirements in recent years for guarantee against surface flaws. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a hot rolling method for strips that reduces the surface flaws of strips such as wire rods, steel bars and square bars.

  It is necessary to ensure that the surface defects are within an acceptable range for strip products such as wire rods, steel bars, square bars and the like manufactured by hot rolling. This is because, if surface flaws exceeding the permissible level remain in the strip product, for example, processing defects such as cracks may occur starting from the flange portion during forging in the subsequent so-called secondary processing step. is there. For example, in the pass (hole type) schedule for each product dimension such as wire rod and bar steel, in each pass, if the hole type is designed so that "rolling out" does not occur, the rolled material protrudes from the roll flange part. Combinations of hole shapes are possible. Usually, from the viewpoint of productivity (rolling efficiency), by using a common hole shape, a common hole shape is used as much as possible for each product dimension. With this common hole shape, a high reduction in area is achieved only by adjusting the roll gap. Therefore, rolling is performed with a low reduction in area.

As an example of the hole design, when the “square-oval” rolling method is used as part of the pass schedule, the oval hole is designed with the following dimensional range (eg, non- Patent Document 1).
B / H = 2.4 to 4.6 (B: width of rolled material on the punched side, H: height of rolled material on the punched side)
C / H = 1.1 to 2.4 (C: opposite side dimension of rolled material (corner) on the inlet side of the hole mold, H: height of rolled material on the outlet side of the hole mold)
r = (1/4 to 1/6) C (r: corner of rolled material (corner) on the entrance side of the hole mold, C: opposite side dimension of rolled material (corner) on the entrance side of the hole mold)
As described above, the range is provided for each dimension ratio and the corner r of the entry side material in order to enable a hole-type design that can secure a required area reduction rate by changing each dimension ratio within the above range. It is.
Metal, Vol. 25 (1955), No. 3, p. 233

  However, even if the hole mold is designed by selecting the dimension ratio and the corner r within the above range, the circumferential compressive strain of the surface layer of the rolled material filled in the oval hole mold exceeds the surface flaw generation limit value. In that case, naturally surface flaws occur. FIGS. 1A and 1B show an example of the “square-oval” rolling method, and Table 1 lists required data such as the hole size and the size ratio. In this case, the minimum value of the compressive strain in the circumferential direction of the rolled material filled in the oval (elliptical) hole mold 1 obtained by deformation analysis using FEM is −0.88. The definition of the compressive strain in the circumferential direction of the rolled material will be described later. A square steel material 2 shown in FIG. 1 (b) was heated to a normal hot rolling temperature range using a pair of rolls obtained by processing the oval hole mold 1 of FIG. 1 (a), and a hot rolling experiment was carried out. However, generation of surface flaws was observed in the vicinity of the minimum compression strain. The oval hole mold 1 having the dimensions shown in Table 1 shown in FIG. 1 (a) is one of the hole molds used in actual rolling, and the occurrence of surface defects in the hot rolling experiment is usually 10 stands. This shows that surface flaws were generated in at least one stand (1 pass) or more in the actual rolling process in which rolling was performed at (10 passes) or more and finished into a wire rod or steel bar product.

As means for improving the surface properties such as surface wrinkles of the strip material, for example, in Patent Document 1, a “square-oval” rolling method is used, and the shape of the recesses formed on the free surface of the rolled oval material is used as a sizing roller. And a method for improving the state of occurrence of surface defects is disclosed.
JP-A 63-16801

  However, the occurrence of recesses on the free surface of the rolling material is caused by the fact that the reduction in the area of the rolling mill must be reduced because the motor power or rigidity of the rolling mill is insufficient. The latest mill with sufficient motor power does not use a light pressure path that creates a recess on the free surface. Even if a concave portion is formed on the free surface of the rolled material, there is a possibility that a new concave portion is generated on the free surface during shaping by shaping using a sizing roller under light pressure. Furthermore, when a new concave portion is generated, a circumferential maximum compressive strain (minimum value of the compressive strain) is generated at the deepest portion of the concave portion, and a fine surface flaw having a scale taken therein is generated. Therefore, even if shaping with a sizing roller, fine surface wrinkles cannot be improved, and in recent years, for example, a very severe surface wrinkle guarantee that the product wrinkle depth is 0.02 mm or less cannot be performed.

  Therefore, an object of the present invention is to reduce the occurrence of fine surface defects due to rolling deformation in the hot rolling process of strips such as wire rods, bar steel, square bars, etc. It is to provide a method of hot rolling a strip for manufacturing.

  In order to solve the above problems, the present invention employs the following configuration.

  That is, the method of hot rolling strip material according to claim 1 is to reduce the cross-sectional area sequentially by the respective rolling methods by the hole mold provided in the roll of a plurality of rolling mills arranged from the raw billet, to the required product dimensions. A method of hot rolling a strip material to be finished, characterized in that the cross-sectional area of the rolled material is reduced so that the compressive strain in the circumferential direction of the rolled material in the hole molds of the respective rolling methods is −0.5 or more. And

As a result of conducting model experiments and deformation analysis in order to investigate the cause of surface flaws in the rolled material due to rolling deformation in the hole mold, the present inventors found that the compression strain in the circumferential direction of the rolled material caused by rolling deformation and the post-rolling It was found that there was a correlation between the surface depth in the cross section of the rolled material. Here, as shown in FIG. 2, the circumferential compressive strain is the length S ₀ of the curved side of the rolled material surface side of the element e before and after the rolling deformation by the hole mold 4 of the entry-side rolled material 3. , S ₁ is a conventional strain calculated by the following equation.
ε = (S ₁ −S ₀ ) / S ₀

The compressive strain ε is generally indicated by a negative value because the length S ₁ after the rolling deformation is smaller than the length S ₀ before the rolling deformation, but the amount of change that changes from S ₀ to S _1. The larger the value, the smaller the value. Therefore, since the value obtained as the compression strain ε is usually represented by a negative value, in this specification, for example, the maximum value of the change amount itself is expressed as “minimum value of the compression strain”. .

  FIG. 3 shows the result of the one-pass hot experimental rolling of the “square-oval” rolling method shown in FIGS. 1 (a) and 1 (b). In this experimental rolling, an oval hole type (radius R = 23 .2) obtained by processing a square steel of different opposite side dimensions and corner R (17 mm to 21 mm square, JIS SWRCH45K) finished so that no wrinkles exist on the surface into a roll having a diameter of 230 mm. 6 mm, height H = 5 mm), and the compression strain ε in the circumferential direction due to rolling deformation was changed. The rolling was performed at a roll peripheral speed of 1.5 m / min by heating the square steel of the entry side material to 1000 ° C. For each square steel dimension of the entry side material, calculate the circumferential compressive strain ε using the FEM (Rigid Plastic Generalized Plane Strain Model) deformation analysis result, and minimize the lateral compressive strain ε in the circumferential direction of the rolled material. The value was determined, and the surface depth of the cross section of the rolled material after one-pass rolling was observed with an optical microscope to determine the depth of the cross section on the vertical axis. The ● shown in FIG. 3 is the result obtained by this deformation analysis. From FIG. 3, even when the surface does not have any surface defects, the surface defects occur as the compressive strain ε in the circumferential direction becomes smaller. I understand. Further, as the circumferential compressive strain ε increases, the surface wrinkle depth becomes shallower. When the circumferential compressive strain ε is −0.5 or more, the surface wrinkle depth becomes 0.02 mm or less. It can be seen that surface flaws that would cause processing defects do not occur, and that surface flaws do not occur due to rolling deformation by setting the circumferential compressive strain ε to −0.35 or more.

  In addition, there is a concern that an analysis error may occur in the deformation analysis, so a model is obtained by measuring the interval before and after rolling with hot rolling by putting a marking line parallel to the surface of the actual rolled material at 1 mm intervals in the circumferential direction. Experiments were conducted to investigate the relationship between compressive strain and surface depth. The circles in FIG. 3 are the results obtained in this investigation, which almost correspond to the results obtained in the deformation analysis described above, and the surface defects that cause problems when the circumferential compressive strain ε is −0.5 or more are also shown. It turns out that it does not occur. It can also be seen that surface flaws do not occur due to rolling deformation by setting the circumferential compressive strain ε to −0.35 or more.

  In addition, a model experiment was conducted under the same conditions as described above by putting a marking line on the cold lead, and the relationship between the compressive strain and the surface wrinkle depth was investigated. The ▲ in FIG. 3 is the result obtained by this investigation, which almost corresponds to the result obtained by the deformation analysis described above, and the surface flaw that becomes a problem when the circumferential compressive strain ε is −0.5 or more is also shown. It turns out that it does not occur. It can also be seen that surface flaws do not occur due to rolling deformation by setting the circumferential compressive strain ε to −0.35 or more.

  In other words, even when the hot material or lead (cold) was actually rolled, a result almost the same as the result of the deformation analysis could be obtained. Therefore, it was confirmed by actual measurement that the result obtained by deformation analysis was correct.

  The method for hot rolling strip according to claim 2 is characterized in that the rolling method includes a square-oval rolling method.

  In the hot rolling of the strip material, usually, it is an “oval-oval” rolling type oval hole type, and there is a tendency that the largest circumferential compressive strain is generated in the absolute value on or near the free surface of the rolled material. In this "square-oval" rolling method, rolling deformation is performed so that the compressive strain in the circumferential direction becomes -0.5 or more. Is important to prevent.

  The method of hot rolling a strip according to claim 3 is a hot rolling method in which the strip is a wire or a steel bar.

  In the case of wire rods and bar steels, cold processing with a high degree of processing is often performed in subsequent processing steps, and in particular, the quality requirements for surface defects remaining on the product surface have become more severe in recent years. It is extremely important to prevent the occurrence of surface flaws.

  In the present invention, in the hot hole rolling process for strips such as wire rods, bar steel, square bars (square bars), etc., the compression strain in the circumferential direction of the rolled material due to rolling deformation in the hole molds of each rolling method is −0.5 or more. Since the hot rolling is performed in the hole type schedule, the surface defects that cause processing defects in subsequent processing steps due to rolling deformation do not occur, and the recent severe surface defects guarantee is also supported. It is possible to provide a strip product with excellent surface quality.

  Hereinafter, embodiments of the present invention will be described in detail based on examples.

  Table 2 shows the hole types provided in each roll stand (hereinafter referred to as a stand) of an actual rolling mill row for rolling a wire rod having a product size of φ17 mm in 15 passes (hole type) from a 115 mm square material billet (steel grade SCM435). Is shown. In the No. 1 to No. 7 stand of the rough rolling mill, each rolling of “box-oval (elliptical)”, “oval-round” and “round-oval”, “oval-angle” and “square-oval” For the No. 9 to No. 11 stand of the intermediate rolling mill row, the “Oval-Square” and “Square-Oval” rolling methods are used for the No. 12 to No. 15 stand of the finish rolling mill row. The “round” and “round-oval” rolling systems are used respectively. When the minimum value of the compressive strain in the circumferential direction of the rolled material due to rolling deformation in the hole mold of each rolling method was determined using FEM, the rolled material was obtained only in the rolling deformation of the No. 11 stand corner (23 mm square) hole mold. It was found that the minimum value of the circumferential compressive strain was not more than -0.5. For this reason, the hole type correction which enlarged the corner | angular (23 mm square) hole type groove bottom part R of No. 11 stand was performed so that the said compression strain might be more than -0.5. Thus, not only the “square-oval” rolling method, but also the “oval-square” rolling method, the compressive strain in the circumferential direction of the rolled material may not be −0.5 or more. In general, in the “round-oval” and “oval-round” rolling methods, there is almost no problem with the compressive strain in the circumferential direction of the rolled material.

  The steel billet is cleaned so that no surface flaws remain on the 115 mm square billet, heated to a hot rolling temperature range of about 1050 ° C. in a heating furnace, and then finished into a φ17 mm coil wire by the rolling method shown in Table 2. It was. After cutting off the defective portion at the front and rear ends of the coil wire, the surface flaw inspection was performed by sampling again from the front end, the rear end and the center of the coil wire, and the surface flaw exceeding a depth of 0.02 mm was obtained. Was not recognized.

  Table 3 shows the minimum value of the compressive strain in the circumferential direction due to rolling deformation at the corner (23 mm square) of No. 11 stand shown in Table 2 when rolling from the 115 mm square billet to a φ17 mm coil wire rod. Is the result of determining the occurrence state of product surface wrinkles when the square hole type groove bottom portion R is changed, with a wrinkle depth of 0.02 mm as a boundary. X in the table indicates that surface defects exceeding a depth of 0.02 mm are observed, ○ indicates that surface defects of a depth of 0.02 mm or less are observed, and ◎ indicates that generation of surface defects is not observed. Each is shown. From Table 3, when the circumferential compressive strain due to rolling deformation is less than -0.5, the occurrence of surface defects with a depth exceeding 0.02 mm is observed, whereas the compressive strain is -0.5 or more. It can be seen that generation of surface flaws with a depth exceeding 0.02 mm is not observed, and generation of surface flaws due to rolling deformation is not observed when this compressive strain is -0.35 or more.

  Thus, for example, the compressive strain in the circumferential direction due to the rolling deformation in each rolling method shown in Table 2 is calculated using a deformation analysis means such as FEM, and this compressive strain is less than −0.5. In this case, as described above, the shape of the hole shape such as increasing the groove bottom R is changed so that the compressive strain in the circumferential direction of the rolled material in all the hole shapes used for rolling becomes −0.5 or more. Thereby, it becomes possible to suppress the surface wrinkle depth accompanying rolling deformation to 0.02 mm or less. In the present embodiment, each stand is fixed to each stand by correcting the shape of the mold so that all the compressive strains in the circumferential direction are −0.5 or more, preferably −0.35 or more. This is to optimize the distribution of the area reduction rate, and in principle, does not require an increase in the number of passes.

  As shown in Table 2, the rolling schedule of strip material has rolling methods (hole series) such as box → box, oval (oval) → circle → oval, diamond → square → diamond, etc. Even in the rolling method, since the hole shape is defined, there can be an infinite number of pass schedules. However, in the present invention, the compressive strain in the rolling circumferential direction is −0.5 or more, preferably −0.35 or more. Thus, any pass schedule can be accommodated by designing the hole shape in each rolling method.

(A) It is explanatory drawing which shows an oval (ellipse) hole type | mold. (B) It is explanatory drawing which shows a "square-oval" rolling system typically. It is explanatory drawing which shows typically the compressive strain of a rolling material circumferential direction. It is explanatory drawing which shows the relationship between the compressive strain of the rolling material circumferential direction by rolling deformation | transformation, and surface wrinkle depth.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Oval (ellipse) hole type | mold 1a ... Oval rolling material 2 ... Square steel material 3 ... Entrance side rolling material 4 ... Hole type 5 ... Outlet rolling material

Claims (3)

  It is a hot rolling method of strip material in which the cross-sectional area is sequentially reduced by the respective rolling methods by the hole mold provided in the rolls of a plurality of rolling mills arranged from the material billet, and finished to the required product dimensions, each of the rolling A method for hot rolling strip material, characterized in that the cross-sectional area of the rolled material is reduced so that the compression strain in the circumferential direction of the rolled material in a hole type of the method is -0.5 or more.   The hot rolling method for a strip according to claim 1, wherein the rolling method includes a square-oval rolling method.   The method of hot rolling a strip according to claim 1 or 2, wherein the strip is a wire rod or a steel bar.

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