JP2007118028A - Method for mechanically descaling steel material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a descaling method by which both ordinary oxidized scale which is generated when heating the steel material containing Si, Cr or Ni up to the hot-rolling temperature range and composed of three layers and a sub-scale which is generated on the interface of the oxidized scale and a base steel and difficult to be peeled can be efficiently removed in both aspects of equipment and energy consumption. <P>SOLUTION: In this descaling of the steel material for hot rolling containing at least one kind of Si, Cr and Ni of ≥0.1%, mechanical descaling is performed at the surface temperature of ≥900°C after heating the steel material at the atmospheric temperature of ≥1,200°C in a heating furnace. Under such mechanical descaling conditions, the porosity of the sub-scale is increased, the sub-scale is made into more porous structure or the rate of decrease of the hardness of the scale is increased. Then, the sub-scale is easily destroyed and peeled with a brush roll. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a mechanical descaling method in a hot rolling process of steel material for removing scale generated on the surface of the steel material before starting rolling.

  When manufacturing rolled products such as steel plates, wire rods, and steel bars by hot rolling, the scale (oxide layer) generated during the heating process adheres to the surface of steel materials such as slabs and billets extracted from the heating furnace. When rolling is started without removing the scale, the scale is pushed into the steel material, surface flaws are generated, and surface defects occur in the product. For this reason, the descaling method which removes a scale by spraying high pressure water on the said steel material surface before the rolling start extracted from the heating furnace is generally used. However, depending on the steel material, it may be difficult to completely remove the scale by the descaling method using high-pressure water.Therefore, the impact pressure on the steel surface is increased by increasing the pressure of high-pressure water or increasing the spraying flow rate. Measures have been taken to raise For this measure, large equipment is required to greatly increase the pressure of high-pressure water, and it is necessary to increase the heating temperature because the temperature drop of the steel material increases due to an increase in the spray flow rate. With the problem of.

  As a descaling method for solving such problems, for example, in Patent Document 1, a brush roll and a spray nozzle are provided on the entrance side of the rolling mill, and the temperature of the steel material is measured on the front side of the brush roll. Descaling using a mechanical method (mechanical descaling) using a brush roll and a method using fluid jet using high-pressure water to control the peripheral speed and / or fluid spray pressure of the brush roll based on the measured temperature A method is disclosed. In this descaling method, it is found that there is a strong correlation between the steel material temperature and the descaling effect, and in the steel material temperature range where the scale defect is likely to occur (in the embodiment, 1200 ° C. or more or 950 ° C. or less), the peripheral speed and spray pressure of the brush roll. In the steel temperature range (in the embodiment, more than 950 ° C. and less than 1200 ° C.) in which one or both of these are increased, the reverse operation is performed, and the descaling is insufficient or excessively descaled over the entire length of the steel material. Therefore, it is possible to perform an appropriate descaling process that does not cause the problem.

Moreover, in patent document 2, after reaching the mechanical brush, the steel material is rapidly cooled with pressure water to promote the generation of brittle Fe ₃ O ₄ and improve the peelability of the scale. Discloses a descaling method that performs the steps of breaking and peeling the scale, and scattering the scale in a floating state with pressurized water, improving the scale removal capability, reducing the power cost, and preventing overcooling Is planned.
JP 59-21426 A JP 59-82110 A

However, according to an actual machine experiment conducted by the present inventors, in the case of a steel material containing 0.1% or more of at least one of the alloy elements Si, Cr, Ni, which is an object of descalability improvement, When the temperature becomes a low temperature of about 850 ° C. or less, the peelability of the scale sharply decreases, and as disclosed in Patent Document 1, the rotational speed of the brush roll for mechanical descaling or the spray pressure for fluid jet descaling is reduced. It has been found that increasing the scale does not improve the scale peelability. Further, as disclosed in Patent Document 2, by rapidly cooling the steel material surface being transferred along the rolling line, the generation of Fe ₃ O _{4 having} brittle properties is promoted, but Si, Cr, In steel materials containing alloy elements such as Ni, scales that are difficult to peel, such as firelite (Fe ₂ SiO ₂ ), which is a compound of these alloy elements and Fe, O ₂ , remain as they are, and are simply mechanical. It has also been found that the scale cannot be peeled off simply by combining descaling and fluid ejection descaling.

  Then, the subject of this invention is the normal oxidation scale (iron-oxygen compound) which consists of three layers produced | generated on the steel material surface, when the steel material containing Si, Cr, or Ni is heated to a hot rolling temperature range, An object of the present invention is to provide a descaling method that can efficiently remove both the oxide scale and the scale that is difficult to peel (subscale) generated at the interface between the iron and steel (steel material) both in terms of equipment and energy consumption.

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

  That is, the mechanical descaling method of a steel material according to claim 1 is a mechanical descaling method of a steel material for hot rolling containing 0.1% or more of at least one of Si, Cr, and Ni. After heating at an atmospheric temperature of 1200 ° C. or higher in a heating furnace, mechanical descaling is performed at a surface temperature of the steel material of 900 ° C. or higher.

  When the heating temperature of the steel material is increased, bubbles are generated in a scale that is difficult to peel (subscale) generated at the interface between the oxide scale and the steel and the scale adhesion to the steel is reduced. Focusing on Cr as the alloy element, each cylindrical specimen cut from a steel material having a Cr content of about 1.5 mass% is heated to a different temperature range of 1100 to 1300 ° C., and then the test temperature (1000 ° C.). The area ratio of the scale remaining on the surface of the test piece after the compression test (cooling strain 50%, strain rate 10 mm / s) after cooling to room temperature was measured. From the measurement results, steel materials containing a Cr content of about 1.5% by mass are steel materials that have a large Cr content and a large amount of subscale. Therefore, when the heating temperature rises, the porosity of the subscale increases. As a result, it was found that the scale scale adhesion to the ground iron decreased, the area ratio of the remaining scale decreased, and the mechanical descaling property improved. In particular, when the heating temperature is 1200 ° C. or more, even after the uniaxial compression processing test as described above, the remaining scale area ratio becomes less than 50%, and the generated scale itself becomes higher, that is, At 900 ° C or higher, both the scale and subscale have a reduced hardness reduction ratio, and the subscale fracture strength itself also decreases, so by keeping the heating atmosphere temperature and the steel material temperature during mechanical descaling within the above temperature range, The subscale mechanical descaling is improved. The ambient temperature is desirably 1400 ° C. or lower in order to prevent local melting due to overheating.

  A steel mechanical descaling method according to claim 2 is a mechanical descaling method of a steel material for hot rolling containing 0.1% or more of at least one of Si, Cr, and Ni, when mechanical descaling is performed. The steel material surface temperature is 900 ° C. or higher, and the steel material surface temperature is rapidly increased immediately before mechanical descaling.

  Even if it is a low-temperature heating material of less than 900 ° C for the purpose of material control, if the in-furnace time becomes long due to troubles in the rolling line, etc., the scale that becomes a problem that the subscale grows and is difficult to peel off is the interface To generate. As described above, if only the steel surface temperature is rapidly heated to 900 ° C. or higher, the descaleability of the subscale can be improved while controlling the material. This rapid heating can be performed by installing a rapid heating device on the entry side of the mechanical descaling device or by setting the atmospheric temperature in the final heating zone of the heating furnace to be high.

  In the present invention, a steel material for hot rolling containing at least one of Si, Cr, and Ni as an alloy element is extracted by heating at an atmospheric temperature of 1200 ° C. or higher, and then the steel surface temperature is 900 ° C. or higher. Since the mechanical descaling is performed by using, it is possible to efficiently remove the scale including the normal oxide scale and the scale (subscale) which is difficult to be peeled off generated at the interface of the ground iron. As a result, the occurrence of surface flaws due to scale is suppressed, the surface defects in the product can be greatly reduced, and the product surface quality is improved.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to FIGS.

  1 shows a heating furnace 1 and a rough rolling machine row 2 of a wire rod rolling line. As shown in FIG. 2 (a), on the entry side of one rolling mill, the brush rolls 3a, 3b, and 3c are placed at the centers A1 and A2 of the opposing surfaces with respect to each surface (four surfaces) of the billet 6. The mechanical descaling device 3 is disposed so that the central portion of the roll width of 3d is located, and the fluid ejecting descaling device 4 that ejects high-pressure water is disposed following the mechanical descaling device 3. Moreover, between the heating furnace 1 and the mechanical descaling apparatus 3, the rapid heating apparatus 5 provided with the high frequency heating coil, for example is arrange | positioned. At least one of Si, Cr, and Ni extracted from the heating furnace 1 so as to satisfy the required hot rolling temperature by being heated at an atmospheric temperature of 1200 ° C. or higher for a required time, for example, about 20 minutes. As shown in FIG. 2 (b), the brush rolls 3a to 3d on the four surfaces of the steel billet 6 containing one kind as an alloy element alleviate local wear of the brush when it is continuously used. The axis of the billet 6 is inclined with respect to the vertical direction by a certain angle, for example, 30 ° and pressed with a required force (for example, 100 to 200 kgf). The scale generated on the surface is broken and peeled off before rolling starts. In general, the fluid jet descaling device 4 jets high-pressure water onto the four surfaces of the billet 6 and blows away the scale peeling pieces remaining on the surface of the billet 6. The billet 6 thus descaled, that is, the rolled material 6a, is NO. The first rolling reduction is received by one rolling mill (NO. 1st), and the cross-sectional area sequentially decreases with the progress of rolling in the succeeding rolling mill to finish the wire with a predetermined size. In addition, examples of the hot-rolling steel material that contains at least one of Si, Cr, and Ni as an alloy element and can easily generate the subscale include bearing steel, spring steel, and heat-resistant steel.

  In order to investigate the influence of the descaling temperature, that is, the billet (rolled material) temperature, on the descaling property (scaling removal capability) when the mechanical descaling device 3 is used, Φ5. When rolling a 5 mm wire, an actual mechanical descaling experiment was performed in combination with fluid jet descaling. After carrying out mechanical descaling and fluid jet descaling on a billet extracted from a heating furnace at a heating atmosphere temperature of 1200 ° C. in a temperature range of 800 to 1200 ° C., these billets (rolled material) are NO. 1-NO. After roughly rolling to a 5.5 mm diameter wire rod with a rolling mill line consisting of 8 rolling mills, the sample material is sampled from this wire product, pickled and visually observed for surface defects. Observed. Then, the surface wrinkle generating portion was shaved until the wrinkle disappeared, and the surface wrinkle depth was measured to obtain the maximum surface wrinkle depth in each descaling temperature. FIG. 3 is a plot of this maximum surface depth on the billet (steel) temperature during descaling. It can be seen that at a descaling temperature of 900 ° C. or higher, the maximum product surface wrinkle depth is significantly reduced and is within the product surface wrinkle tolerance of 0.02 mm.

In order to investigate the influence of the descaling temperature, that is, the billet (rolled material) temperature on the descaling property (scaling removal capability) when the mechanical descaling device is used, as in the case shown in FIG. As shown in Table 1, the billet temperature (descaling temperature) for mechanical descaling was changed for four cases with a heating temperature of 1000 ° C to 1300 ° C as shown in Table 1 when rolling a Φ5.5 mm wire from a billet (bearing steel). An actual mechanical descaling experiment was also carried out with fluid jet descaling. At that time, the pressing force of the brush roll was changed by two levels. The billet heated to each temperature in the temperature range is subjected to mechanical descaling, the peeled scale is removed by fluid jet descaling, and then the billet (rolled material) is removed from the rough rolling mill row to the finish rolling mill. Then, the cross-sectional area was sequentially reduced to finish rolling to a wire having a diameter of 5.5 mm, and then a sample material was collected from the wire product, pickled, and visually observed for occurrence of surface defects. Then, the surface wrinkle generating portion was shaved until the wrinkle disappeared, and the surface wrinkle depth was measured to obtain the maximum surface wrinkle depth in each descaling temperature. With the product surface wrinkle depth of 0.02 mm as a criterion, the case where the maximum surface wrinkle depth is 0.02 mm or less is evaluated as “good”, the case of exceeding 0.02 mm is determined as “failed”, and the mark “No. 1-No. The results are listed in Table 1 for each of the 24 descaling conditions.
From Table 1, when the heating temperature is 1200 ° C. or higher and the descaling temperature is 900 ° C. or higher, the product maximum surface wrinkle depth is 0.02 mm or less and satisfies the wrinkle determination standard for any brush roll pressing force. . On the other hand, when the heating temperature and the descaling temperature do not reach these temperature ranges, the product maximum surface wrinkle depth is 0.02 mm or more and does not satisfy the wrinkle criterion.

  Thus, the descaling property was good when the heating temperature was 1200 ° C. or higher and the descaling temperature was 900 ° C. or higher. The subscale structure generated in a high temperature heating atmosphere was porous, and the high temperature was high. This is because the reduction ratio of the subscale hardness increases, and the fracture strength of the subscale itself decreases, which facilitates breakage and peeling by the brush roll. In addition, the conventional fluid injection descaling that injects high-pressure water alone does not provide sufficient force to peel off the subscale from the iron-iron interface even when the above-mentioned advantages are achieved when descaling is performed at high temperatures. Therefore, it can be considered that the scale generation amount (generation thickness) itself has failed to increase at high temperatures.

  For the purpose of material control, low temperature heating is performed in a heating furnace, and for low temperature heating materials extracted at a temperature lower than 900 ° C., if the in-furnace time becomes long due to troubles in the rolling line, the subscale is Since a scale that grows and becomes a problem that is difficult to peel off is generated at the iron-iron interface, if only the surface temperature of the billet is rapidly heated to 900 ° C. or higher by the rapid heating device 5 shown in FIG. While performing, it is possible to improve the subscale descaleability. This rapid heating is performed not only immediately after extraction in the heating furnace but also in the middle of the hot rolling process, for example, at the time of low temperature rolling, the steel material surface may be raised to 900 ° C. or higher to perform mechanical descaling. it can.

  FIG. 4 shows a sample of the rolled material on the outlet side of the rough rolling mill row 2 shown in FIG. 1 during the above-mentioned actual mechanical descale experiment, and the number of surface defects generated in the sample rolled material and the billet during descaling. The relationship with temperature is shown typically. Compared to the case of fluid jet descaling with only high-pressure water spraying, if the scale that has been broken or peeled off is blown away by fluid jet descaling after mechanical descaling, the number of surface defects will be reduced, and descaling will occur. And the depth of surface defects remaining in the product (finished wire) is reduced. This improvement in descaling property (decrease in the number of surface defects) becomes more significant as the mechanical descaling temperature becomes higher. In addition, the mechanical descaling method of this invention can be used also for the scale removal at the time of hot rolling of other steel materials, such as not only a wire rod and a bar steel but a steel plate.

It is explanatory drawing which shows an example of arrangement | positioning of the rough rolling machine row | line of a wire rod rolling line, and a mechanical descaling apparatus. (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 billet surface. It is explanatory drawing which shows the influence which descaling temperature exerts on the product surface maximum depth. It is explanatory drawing which shows typically descaling temperature and the number of surface flaw generation | occurrence | production.

Explanation of symbols

1: Heating furnace 2: Rough rolling mill row 3: Mechanical descaling devices 3a to 3d: Brush roll 4: Fluid jet descaling device 5: Rapid heating device 6: Billet 6a: Rolled material

Claims (2)

  A mechanical descaling method of a steel material for hot rolling containing 0.1% or more of at least one of Si, Cr, and Ni, after heating the steel material at an atmospheric temperature of 1200 ° C. or higher in a heating furnace, A mechanical descaling method for a steel material, wherein the surface temperature of the steel material is 900 ° C. or higher and mechanical descaling is performed. A mechanical descaling method of a steel material for hot rolling containing 0.1% or more of at least one of Si, Cr and Ni, the steel material surface temperature when performing mechanical descaling is 900 ° C. or more, and A mechanical descaling method for steel, characterized by rapidly increasing the steel surface temperature immediately before mechanical descaling.

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