JP2008221234A - Hot rolling method for bar steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot rolling method for a bar steel where surface flaws generated when a bar steel is hot-rolled can be suppressed without performing large equipment investment, and a bar steel capable of producing a bar steel product satisfying severe surface flaw warranty in recent years. <P>SOLUTION: A stock billet 4 (the material to be rolled) composed of a steel comprising, by mass, ≥0.05% Si and at least one or more kinds selected from Cr and Ni by ≥0.1% is heated at ≤1,200°C. The material 4a to be rolled is thereafter exposed to a wet atmosphere with a dew point of 30 to 60°C for ≤2 s and is subjected to water vapor oxidation treatment at least once, and is subsequently subjected to mechanical descaling, and is hot-rolled, so as to be finished to a bar steel with required shape and dimensions. The properties of scale including subscale whose peeling is difficult is modified into the properties of scale easy to be peeled by the action of the water vapor, thus the scale can be effectively removed without requiring large-scale equipment. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a hot rolling method for a bar material that can suppress the generation of surface defects due to scale and surface fine cracks due to a temperature drop, and that can produce a bar material having excellent surface properties.

  The strip material heated in the heating furnace is descaled with high-pressure water, and then divided into multiple passes sequentially by various hole molds provided in a pair of roll stands arranged at predetermined intervals. It has been conventionally performed by rolling to finish a strip material having a predetermined product shape and size by sequentially reducing the cross-sectional area.

  However, when a steel bar product such as a wire rod, bar steel, square bar or the like is manufactured by this hot rolling method, a large number of surface defects may be formed on the surface of the finished steel bar, which has been a problem for some time. If secondary processing such as forging is performed with such surface defects remaining, processing defects such as cracks may occur starting from the surface defects.

  One of the main causes of surface flaws is the subscale with high adhesion to the steel material generated on the material surface during the heating furnace and rolling process, that is, scale (iron oxide) and ground iron (steel). An iron oxide that contains an easily oxidizable alloy element, such as Cr or Si, generated at the interface, and encloses fine pores, can be considered. For example, Patent Document 1 and Patent Document 2 disclose inventions for manufacturing a steel bar material that can easily remove the subscale generated on the steel material surface in a heating furnace or the like. These methods described in Patent Documents 1 and 2 change the subscale structure into a structure that can be easily peeled off, and are very excellent inventions that can reliably produce a strip material having excellent surface properties. However, there was room for improvement due to problems with scale loss and furnace operation.

  Further, as a more general method, there is a method in which the collision pressure is increased by increasing the discharge pressure of the high-pressure water descaler without changing the scale structure, and the scale that is difficult to peel off is removed. However, in order to completely remove the subscale with the high-pressure water descaler, for example, a discharge pressure of 500 kgf / cm2 (49.0 MPa) or more is required, and the load on the rolling mill increases due to the temperature drop of the steel material due to descaling. There was a problem that a large temperature drop on the surface caused cracks in the steel material surface, which caused new surface defects.

As a means for improving such a problem, for example, as described in Patent Document 3, there is a mechanical descaling method using a brush roll. This descaling method is a useful means for completely exfoliating the subscale because the scale is mechanically scraped without lowering the temperature of the steel material. However, in order to completely exfoliate the subscale, it is necessary to contact the brush several times at the same location of the steel material. To achieve this contact with the moving steel material, the brush rotation is very high. It is necessary to install a plurality of descaling equipment that incorporates brush rolls (number of brush roll peripheral speeds). Therefore, when equipment is determined from the viewpoint of realistic equipment costs, this equipment has a problem that subscale removal can be expected to some extent, but it cannot be completely removed.
JP 2003-119517 A Japanese Patent Laid-Open No. 2002-316207 JP-A-4-115863

  The present invention has been made for the purpose of solving the above-mentioned conventional problems, and the problem is to suppress surface flaws that occur when hot-rolling steel strip without making a large capital investment. It is an object of the present invention to provide a method for hot rolling a bar material that can manufacture a bar product that satisfies the recent severe surface defect guarantee.

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

  That is, in the method of hot rolling steel strip according to claim 1, after heating the material to be rolled in the material billet state, the cross-sectional area of the material to be rolled is successively reduced by the respective roll hole molds of a plurality of rolling mills arranged. A hot strip rolling method for finishing a strip having a required shape and size, wherein the material billet is 0.05% by mass or more of Si, and at least one of Cr and Ni is 0.0. A heating step comprising a steel containing 1% by mass or more, wherein the heating is performed at 1200 ° C. or less, and after the heating step, the material to be rolled is at least once in a humid atmosphere with a dew point of 30-60 ° C. for 2 seconds or less. A steam oxidation treatment step to be exposed, and a step of descaling the material to be rolled by a mechanical descaler using a brush roll after the steam oxidation treatment step are provided.

  In the hot rolling method of the above-mentioned strip steel material, the strip steel material is 0.05 mass% or more of Si, and a steel type having a composition containing 0.1 mass% or more of at least one of Cr and Ni, and the heating temperature is The precondition that the temperature is 1200 ° C. or less is that the subscale is easily generated in the case of such a steel type and the heating temperature, and the generation of the subscale causes surface flaws due to scale indentation during the hot rolling process. This is because there is a problem that it is easy. As described above, by subjecting the material billet and the material to be rolled in the rolling process to a steam oxidation treatment that is exposed to a wet atmosphere containing water vapor, the water vapor reaches the interface between the subscale and the steel. As a result, the surface of the ground iron (steel) is directly oxidized to form a thin oxide layer (wustite (FeO)) at the interface between the subscale and the ground iron. Since this wustite (FeO) has low strength, it cracks and propagates due to thermal stress at the time of its generation and destroys the upper subscale. In addition, the effect of water vapor uniformly oxidizes the interface between the iron and iron, making the interface smooth and making it easier to remove subscales from the iron and iron. Greatly improved. In addition, since the descaling is performed by a mechanical descaler using a brush roll, the temperature drop of the material billet and material to be rolled is extremely small compared to the conventional descaling using high-pressure water, and the load increase of the rolling mill is suppressed. Therefore, descaling can be performed effectively with simple equipment.

  In order to evaluate the subscale peelability in a humid atmosphere supplied with water vapor, the present inventors performed the following subscale peelability evaluation experiment. Table 1 shows the experimental conditions.

FIG. 1 (a) schematically shows the experimental situation. After the test piece 9 was heated to 1200 ° C., the moisture supply amount was adjusted such that the dew point was 30 ° C. at this temperature. The test piece (steel material) in which the scale layer including the subscale is uniformly formed is fixed to the rectangular surface (plane) and fixed to the rectangular plane (surface) of the test piece. The test brush roll 10 is pressed and rotated, and the relationship between the number of times the test brush roll 10 contacts the surface of the test piece 9 and the peeling length Ld of the scale layer shown in FIG. Determined by For comparison, after heating the test piece 9 to 1200 ° C., the test brush roll 10 was pressed and rotated as it was without being exposed to a humid atmosphere, and the contact number C of the brush roll 10 and the peeling length of the scale layer were measured. The relationship with Ld was also obtained. The test brush roll 10 has a metal shaft portion 10a radially provided with a metal brush portion (needle-like hair portion) 10b having a length of 30 mm, and has a diameter (outermost peripheral diameter) of 200 mm. . The number of rotations of the brush roll 10 was increased in a stepwise manner as the number of contact times C increased, from the smaller number of contact times C to 100 rpm (C = 5236), 140 rpm, 300 rpm, and 500 rpm (C = 26180). Table 1 shows the experimental conditions, and FIG. 2 shows the experimental results. In addition, the peeling length Ld of the scale layer is schematically shown in FIG. 1B, and the peeling length Ld of the scale layer shown in FIG. 2 is an average value in the width (W) direction of the test piece. is there. The theoretical contact length Lt shown in FIG. 2 is a geometry determined from the diameter (outermost circumference diameter) of the test brush roll 10 and the pushing amount d, as shown in FIG. Contact length. Further, the number of contact C of the brush roll was determined by the following formula (1).
Number of contacts C = π × Db × Nb / 60 × S / B -------------- (1)
Here, Db: brush roll diameter, Nb: brush roll rotation speed (rpm), S: brush roll contact time (s), and B: interval between brushes (needle hairs) (= 1 mm).

  From FIG. 2, the peeling length Ld increases as the number of contact times C increases. However, when not exposed to a wet atmosphere with water vapor, the number of contact times of about 26000 times is required until the theoretical contact length Lt is reached. On the other hand, when exposed to a humid atmosphere for 2 seconds, it can be seen that the theoretical contact length Lt is reached in about 16000 times. This means that the produced subscale can be completely peeled off with the scale layer with fewer contact times due to the effect of exposing the heated test piece, that is, the steel material, to a moist atmosphere by supplying water vapor. Oxidation treatment effect makes it easy to easily remove subscales that have been difficult to peel off without greatly increasing the number of rotations of the brush roll or installing multiple brush rolls, i.e., multiple mechanical descaling devices. It means that it can peel completely.

The reason why the dew point of the wet atmosphere is in the range of 30 to 60 ° C. is as follows. That is, when the dew point is less than 30 ° C., the water vapor concentration in the wet atmosphere is low, so that the subscale is not sufficiently destroyed. When the dew point is higher than 60 ° C., the water vapor concentration in the wet atmosphere is high. This is because the scale grows too much and its peelability becomes unclear. From the viewpoint of suppressing the reduction in scale peelability due to such scale growth, it is more desirable that the dew point in the wet atmosphere in the steam oxidation treatment step is 55 ° C. or less, and therefore the range of this dew point. Is more preferably 30 to 55 ° C. When the wet atmospheres with dew points of 30 to 60 ° C. and 30 to 55 ° C. are expressed using the absolute water vapor amount, the absolute water vapor amount is 0.3 to 122 g / the wet atmosphere 1 m ³ , and the absolute water vapor amount is 30.3 to 99.1 g, respectively. / Wet atmosphere 1 m ³ .

The reason for exposing the material billet and the material to be rolled to the wet atmosphere in the steam oxidation treatment step to 2 seconds or less is as follows. In other words, the effect of forming wustite (FeO) at the interface between the iron and steel by steam oxidation is more likely to occur as the steam treatment time for which the thickness remains thin is shorter. Therefore, when the steam oxidation time exceeds 2 seconds, surface oxidation of the material billet and the material to be rolled proceeds, and wustite (FeO) formed at the interface between the iron and steel is magnetite (Fe ₃ O ₄ ), the subscale becomes difficult to peel off, which will promote the generation of surface flaws due to the scale indentation in hot rolling, and the effect of smoothing the iron-iron interface is lost. This is because the effect of improving the mechanical descaling property is lost. If the steam oxidation treatment time is too short, the steam cannot reach the lower layer of the subscale, and the ground iron is not oxidized. Therefore, the effect of destroying the subscale is lost. Therefore, it is preferable that the exposure time in a humid atmosphere be 0.1 seconds or longer.

  The hot rolling method of the strip according to claim 2 is characterized in that the steam oxidation treatment step is provided immediately after the heating step and immediately after the rough rolling step.

  Thus, when the steam oxidation treatment is performed twice, the so-called primary subscale generated in the heating process, and the rolling speed is slow and the rolling time is relatively long, that is, in the rough rolling process where the oxidation time is relatively long. Both of the generated so-called secondary subscales can be effectively removed by mechanical descaling, and the generation of surface flaws due to scale pressing can be further suppressed.

  In this invention, after heating a strip steel material containing 0.05 mass% or more of Si and 0.1 mass% or more of at least one of Cr and Ni at 1200 ° C. or less, the dew point is 30 at least once. Since steam oxidation treatment is performed for 2 seconds or less in a humid atmosphere at -60 ° C, scales containing subscales that are difficult to peel off are modified to scale properties that are easy to peel off and removed by mechanical descaling. Therefore, it is possible to effectively perform descaling without requiring large-scale equipment, and it is possible to manufacture a strip product that satisfies the recent severe surface defect guarantee.

  Embodiments of the present invention will be described below with reference to FIGS. 3 and 4 attached with examples.

  FIG. 3 schematically shows a part of the arrangement of the heating furnace 1 and rolling mill in a mill layout (rolling line) of a wire rod, which is a kind of strip steel, that is, a rough rolling mill row 2 and an intermediate rolling mill row 3. Is. In the rough rolling mill row 2 and the intermediate rolling mill row 3, a plurality of horizontal rolling mills 2a and 3a and vertical rolling mills 2b and 3b each incorporating a pair of rolls provided with a hole shape are arranged at a required interval. . On the exit side of the heating furnace 1, a steam oxidation treatment device 5 that performs steam oxidation treatment of the material billet 4 that is the material to be rolled 4 a is disposed. On the exit side of the steam oxidation treatment device 5, a mechanical descaling device 6 is provided. Is installed.

  The steam oxidation treatment apparatus 5 is supplied with steam from the steam supply pipes 7 and 7a to form a wet atmosphere, and the supply amount of the steam is adjusted so that the dew point in the wet atmosphere is 30 to 60 ° C. Is done. The dew point in the wet atmosphere can be measured by collecting an atmosphere in the vicinity of the surface within 50 cm in height from the surface of the material to be rolled (including the material billet 4) 4a and using a dew point meter, for example, a specular dew point meter. .

  In the mechanical descaling device 6 installed on the outlet side of the steam oxidation processing device 5, a brush roll is used as descaling means. In this mechanical descaling device 6, for example, as shown in FIG. 4A, the brush rolls 8 a to 8 d are arranged so that each surface (four surfaces) of the material billet 4 has a center A <b> 1 that faces each other. It arrange | positions so that the center part of the roll width direction (roll axial direction) of brush roll 8a, 8b and 8c, 8d may be located in A2. The brush rolls 8a to 8d are designed to alleviate local wear and the like of the brush when the brush rolls 8a to 8d are used. Inclined by an angle θ (for example, 30 °) and pressed with a required force (for example, 100 to 200 kgf). In addition, a steam oxidation treatment device 5a is also installed on the exit side of the rough rolling mill row 2 so that steam oxidation treatment and mechanical descaling can be performed as required, such as the steel type and the required surface quality level. A mechanical descaling device 6a can be arranged on the side. From the final rolling mill (2b) of the rough rolling mill row 2, since the material to be rolled whose cross-sectional shape is “square” is usually discharged, the mechanical descaling apparatus installed on the outlet side of the steam oxidation processing apparatus 5a Also in 6a, it can be set as the arrangement | positioning of the brush roll similar to what was shown to Fig.4 (a) and (b).

  After being heated at 1200 ° C. or less in the heating furnace 4, a material made of steel containing 0.05 mass% or more of Si and 0.1 mass% or more of at least one of Cr and Ni extracted. The billet 4 has a scale layer containing a subscale that is difficult to peel off on the surface of the billet 4 in a wet atmosphere in the steam oxidation treatment apparatus 5 in which the dew point is adjusted to a range of 30 to 60 ° C. by supplying steam for 2 seconds or less. As described above, after being modified to a scale property that is easily peeled off by the action of water vapor, the generated scale is destroyed by the rotation of the brush rolls 8a to 8d in the mechanical descaling device 6. , Peeled off from the interface. Then, the scaled material billet 4 is sequentially rolled by the horizontal rolling mill 2a and the vertical rolling mill 2b of the rough rolling mill row 2, and the cross-sectional area thereof is sequentially reduced, so that the final rolling mill 2bf of the rough rolling mill row 2 is reduced. From this, the material to be rolled 4a having a cross section of “square” is released, and further rolled in the intermediate rolling mill row 3 and the finish rolling mill row (not shown) to obtain a wire rod (steel bar material) having a required dimension (shape / dimension). ) Finished product.

  As described above, the material to be rolled whose cross-sectional shape discharged from the final rolling mill (2b) of the rough rolling mill row 2 is “corner” in the steam oxidation treatment apparatus 5a according to the steel type and the required surface quality level. As in the case of the material billet 4, 4a is exposed to a humid atmosphere whose dew point is adjusted in the range of 30 to 60 ° C. for a short time of 2 seconds or less, and the rolling speed is slow. (2) A secondary scale layer including a subscale formed in a rough rolling process having a relatively long oxidation time is modified to have a scale property that is easily peeled off by the action of water vapor, and then a scale generated by the mechanical descaling device 6a. Is destroyed and peeled off from the interface. In this way, the scale layer including the subscale that is difficult to peel off is formed on the rolled material 4a in the heating process of the material billet 4 and, if necessary, in the rough rolling stage having a relatively long oxidation time. By removing from the surface, generation of surface flaws due to scale pressing during the hot rolling process can be prevented.

  In order to confirm the effect of reducing surface flaws by the combination of “steam oxidation treatment + mechanical descaler” of the present invention, each 150 mm square billet of steel types A to E whose composition is shown in Table 2 is 1000 in the combustion gas atmosphere of the heating furnace. Heating to a range of ˜1350 ° C., and immediately after extraction from the heating furnace, by performing the four descaling processes of Step 1 to Step 4 shown in Table 3, respectively, an actual machine experiment for manufacturing a wire rod having a diameter of 8.0 mm was performed. Carried out. With respect to the manufactured wire having a diameter of 8.0 mm, the state of occurrence of surface defects was evaluated by the following method. That is, 10 or more cross-sections perpendicular to the longitudinal direction (rolling direction) were sampled from the front end side to the rear end side of a 8.0-mm diameter wire wound in a coil shape, and each cross-section was taken as an observation cross-section to obtain an optical microscope. Surface magnification was observed by (magnification: 100 to 200). The wrinkles having a depth of 0.01 mm or more were counted as surface wrinkles, and the average value (total number of wrinkles / total number of observation cross sections = number of wrinkles per observation cross section) was calculated. And, the average value of the number of wrinkles is 0 (no wrinkles) rank 0, those with more than 0 to 10 or less rank 1, those with more than 10 to 20 or less, rank 2, those with more than 20 to 30 or less Was defined as rank 4 and the occurrence of surface flaws on the wire rod after each step was evaluated. There is no problem as long as the surface flaws of rank 1 or less are produced as a product. Thus, the result of having evaluated the generation | occurrence | production state of the surface flaw about the process 1-the process 4 is shown in Tables 4-7.

  Table 4 shows the experimental results for Step 1 which is a standard (comparison) condition, but the surface defect rank is 3 to 4, and the surface defect rank 1 or less, which is no problem as a wire product, has not been achieved. . Although there are variations depending on the steel type, the surface defect rank is lower in the condition of 1050 ° C. where the heating temperature is low than in the condition of 1200 ° C. where the heating temperature is high. This is due to the fact that surface cracks have occurred due to the temperature drop on the surface of the billet (rolled material) due to high-pressure water descaling, and that the descaling capability increases as the surface temperature increases.

  Table 5 shows the experimental results for step 2. Although the surface defect rank is improved as compared with Step 1, it does not reach the surface defect rank 1 or lower, which is no problem as a wire product. The improvement compared to Step 1 is that the temperature drop on the surface of the billet (rolled material) is mitigated by changing the descaling means from the high-pressure water descaler to the mechanical descaler, and the fluid descaler It is inferred that mechanical descaling has a higher descaling capability than scaling. Also in the case of step 2, the rolling condition with a higher heating temperature has a better surface defect rank, as in the case of step 1, the higher the billet (rolled material) surface, the better the descaling capability. By improving.

  Table 6 shows the experimental results for step 3. For Experiment No. (21, 22, 26 to 28, 31 to 33, 36, 38, 39) in which the conditions (dew point and treatment time) of the steam oxidation treatment are within the scope of the present invention, the scale peelability by the steam oxidation treatment Therefore, the surface defect rank is in the range of 2 to 3 for any heating temperature. In addition, when the steam oxidation treatment conditions are compared between the experiment Nos. In the present invention, in the case of Step 3, the experiment Nos. (21, 27, 31, 36, 38) having a heating temperature of 1050 ° C. or less are Compared with the experiment No. where other heating temperatures are high, the surface defect rank is worse. As in the case of step 1, this is because, when the heating temperature is low, surface cracks are generated due to a temperature drop on the billet (rolled material) surface due to high-pressure water descaling. However, even when the heating temperature is low, the surface flaw rank is improved as compared with the case of Step 1 (only high-pressure water descaling), and the effect of improving the scale peelability by the steam oxidation treatment is recognized. With respect to the rolling conditions in step 3, for the experiment No. in which the steam oxidation treatment conditions are within the technical scope of the present invention and the heating temperature is higher than 1050 ° C., the surface defect rank is 2, and in the case of step 1 However, it has not yet reached the surface defect rank 1 or lower, which is no problem as a wire product.

  Table 7 shows the experimental results for Step 4. By setting the steam oxidation treatment conditions (dew point and treatment time) within the technical scope of the present invention and changing the descaling means from the high pressure water descaler to the mechanical descaler, the surface defect rank is 1 in any experiment No. The following have been improved. This improvement effect is obtained by synergistically obtaining the effect of improving the scale peelability by the steam oxidation treatment and the effect of preventing the surface temperature of the billet (rolled material) from being lowered by using the mechanical descaler. With the steam oxidation treatment conditions and rolling (heating) conditions within the technical range, a wire product with a good surface property having a surface defect rank of 1 or less can be produced.

It should be noted that, for example, a low pressure of about 40 kgf / cm @ 2 (3.9 MPa), for example, so as to hardly cause a drop in the surface temperature of the material billet and the material to be rolled on the rear side of the mechanical descalers 6, 6a (see FIG. 3) A water descaler may be disposed to remove scale waste remaining on the billet and the surface of the material to be rolled after descaling by the mechanical descalers 6 and 6a. In addition, a high-pressure water descaler having a pressure of about 150 kgf / cm 2 (14.7 MPa) is arranged on the inlet side of the steam oxidation treatment apparatus 5 (see FIG. 3), and a normal thick scale (FeO, Fe ₃ O produced in a heating furnace) is used. ₄ , Fe ₂ O ₃ ) may be removed, and after steam oxidation with the steam oxidation treatment apparatus 5, the subscale that could not be peeled off with the high-pressure water descaler may be removed with the mechanical descaler 6.

(A), (b) It is explanatory drawing which shows typically a subscale peelability evaluation experiment outline. It is explanatory drawing which shows the relationship between the frequency | count of contact of a brush roll, and scale peeling length. It is explanatory drawing which shows typically an example of the layout of the rolling line of a wire. (A) It is explanatory drawing which shows an example of arrangement | positioning of a brush roll. (B) It is explanatory drawing which shows an example of the inclination-angle of the brush roll with respect to the raw material billet surface.

Explanation of symbols

1: Heating furnace 2: Rough rolling mill row 3: Intermediate rolling mill row 2a, 3a: Horizontal rolling mill 2b, 3b: Vertical rolling mill 4: Material billet 4a: Rolled material 5, 5a: Steam oxidation treatment apparatus 6, 6a : Mechanical descaling device 7, 7a: Water vapor supply pipes 8a to 8d: Brush roll 9: Test piece 10: Test brush roll 10a: Brush roll shaft portion 10b: Brush (needle hair) portion

Claims (2)

  After heating the material to be rolled in the billet state, the cross-sectional area of the material to be rolled is sequentially reduced by the roll hole molds of a plurality of rolling mills arranged to finish the steel material having the required shape and dimensions. The material billet is made of steel containing 0.05% by mass or more of Si and 0.1% by mass or more of Cr and Ni, and the heating is performed at 1200 ° C. or less. A heating step performed in the step, a steam oxidation treatment step in which the material to be rolled is exposed at least once in a humid atmosphere having a dew point of 30 to 60 ° C. for 2 seconds or less, and a brush roll after the steam oxidation treatment step. A hot rolling method for a strip steel material, comprising a step of descaling the material to be rolled by a mechanical descaler using a steel plate.   The method of hot rolling steel bars according to claim 1, wherein the steam oxidation treatment step is provided immediately after the heating step and immediately after the rough rolling step.

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