JP5064934B2 - Method for producing Si and Cr-containing strips with excellent scale peelability - Google Patents

Description

  The present invention relates to a method for producing a hot-rolled steel bar containing Si and Cr, and the occurrence of surface flaws due to the scale inevitably generated during hot rolling, particularly the subscale derived from Si and Cr contained in the steel. The present invention relates to a hot rolling method for producing a strip material that can be thoroughly suppressed and has excellent surface properties.

  Under the situation where the surface quality required for steel bar products such as steel bars and steel wire rods or section steels is becoming more severe, in steel types that contain a large amount of easily oxidizable alloy elements such as Si and Cr required for these applications, Subscales containing these alloy elements are inevitable during the heating and rolling process. This scale loss can be reduced somewhat by heating the billet at a lower temperature, for example, 1150 ° C. or lower. However, the subscale having a composition and structure derived from Si · Cr generated at a low temperature up to 1150 ° C. is dense and thin, and is difficult to remove even by performing high-pressure water descaling. When the subscale is rolled while remaining on the surface of the billet, it is pushed into the steel and becomes scale wrinkles, which deteriorates the surface quality of the product. In particular, in the case of Ni-containing steel, it is a problem to further improve the adhesion between the steel and the subscale.

  As a countermeasure against such subscale, a method of manufacturing a steel bar having excellent surface properties by weakening the subscale by adding water and improving descalability has already been known.

  The following Patent Document 1 or 2 discloses a typical example, and the former method 1 is that the billet is heated at 1100 ° C. or higher for 10 minutes or more or 1200 ° C. or higher while supplying moisture in a heating furnace. By heating for more than a minute, pores are generated in the subscale and weakened.

  Patent Document 2 proposes a method of performing two-stage heating in an atmosphere to which water vapor is added. The first stage is high-temperature heating at a temperature of 1200 to 1350 ° C. and a water vapor concentration of 20 to 30%, and the second stage is low-temperature heating at a temperature of 1000 to 1200 ° C. and a water vapor concentration of 20% or less. This is a method of improving the subscale peelability during high-pressure water descaling by steam heating over two stages.

The above two types of inventions are basically good methods for improving the subscale peelability, but have the following problems in production management. First, high-temperature heating in the heating furnace may cause a large amount of scale loss when an on-site situation occurs in which the billet stays in the heating furnace for a long time while maintaining its temperature level. Next, the steam addition to the heating furnace is because it is not easy to immediately stabilize the furnace atmosphere when it is necessary to switch to another steel type that does not require steam addition for the convenience of the line. There are concerns about operational difficulties to ensure the scale stability of the following steel grades.
JP 2002-316207 A JP 2003-119517 A

  The present invention is based on a technique for improving the subscale peelability using moisture, and the practicality is sufficiently taken into consideration in terms of both equipment and operation, and the Si / Cr-containing strip having excellent surface properties. This method is intended for the production of steel. Therefore, the present invention combines the low temperature heating and rapid heating of the billet inside the heating furnace, and controls the wet atmosphere by adding water vapor outside the heating furnace, so that the scale generated in the heating furnace has good peelability. It is an object of the present invention to provide a method that can be easily carried out in terms of actual operation to improve the properties. That is, the present invention can produce a steel strip product that can suppress the occurrence of surface defects formed when hot rolling the steel strip without making a large capital investment, and satisfies the recent severe surface defects guarantee. It is a specific object to provide a hot rolling method for a strip steel material that can be performed.

In order to solve the above problems, the present invention is characterized by the following means.
(1) A steel billet containing 0.05 to 3% by mass (abbreviated as%; the same applies hereinafter) and 0.1 to 5% Cr in a heating furnace at a temperature of 1000 ° C. to 1150 ° C. for 60 minutes. Within 5 ° C./min. To a temperature of 1200 to 1350 ° C. before the extraction. Perform rapid heating at the above heating rate, hold within 10 minutes, extract from the heating furnace, then perform steam oxidation treatment once or twice or more in a humid atmosphere with a dew point of 30-60 ° C for 2 seconds or less A method for producing a strip material containing Si and Cr excellent in high-pressure water descaling and rolling scale peeling.
(2) Immediately after extraction from the heating furnace, dew point: scale after high pressure water descaling and rolling after performing steam oxidation treatment for 2 seconds or less in a humid atmosphere of 30-60 ° C. A method for producing a strip material containing Si and Cr having excellent releasability.
(3) Between the rolling mills before and after the rolling process, after performing steam oxidation treatment for 2 seconds or less in a moist atmosphere with a dew point of 30 to 60 ° C., the above-mentioned (1) for high-pressure water descaling The manufacturing method of Si and Cr containing strip steel material excellent in scale peelability of description.
(4) After the extraction from the heating furnace, the scale described in (2) above is subjected to high-pressure water descaling and rolling after performing steam oxidation treatment with a dew point of 30 to 60 ° C. in a humid atmosphere for a short period of 2 seconds or less. A method for producing a strip material containing Si and Cr having excellent releasability.
(5) A method for producing a Si or Cr-containing strip steel material excellent in scale peelability as described in (1), (2), (3) or (4) above, wherein a steel billet containing 0.1% or more of Ni is used. .

  One advantage of the present invention is that scale loss can be reduced as much as possible by first heating the steel billet at a low temperature and controlling the high-temperature heating before extraction rapidly and in a short time. For inevitable subscales, it is possible to weaken the subscale itself and make descaling with high-pressure water more effective by performing one or more exposures in a humid atmosphere under certain conditions. it can. As a result, the surface flaw caused by the residual subscale is reduced to the utmost limit, making it possible to produce a strip with excellent surface properties.

  Moreover, since this method has operational simplicity that can easily cope with hot rolling of a wide variety of materials, it does not require a large amount of capital investment, and a steel product that satisfies the recent severe surface defect guarantee can be obtained. It is possible to manufacture.

  In the present invention, as the most practical type of strip material required from various applications, a strip material containing 0.05% or more of Si and 0.1% or more of Cr is used as a basic steel composition. The manufacturing target. In addition, the upper limit of the Si amount that exhibits the effect of the present invention is 3%, and the upper limit of the Cr amount is about 5%. In addition, there may be a case where 0.1% or more of Ni is additionally contained as an alloying element due to the demands of applications.

  The present invention is a combination of heating the steel billet at a low temperature and then quickly heating it before extraction to perform a high temperature heating for a short time, and further performing a wet atmosphere treatment by adding steam after extracting the steel billet from the heating furnace, The basic idea is to weaken the dense subscale generated during low-temperature heating so that it can be easily peeled off.

Under normal atmosphere, the occurrence of scale loss during hot rolling of the steel billet is inevitable, but the lower the heating temperature, the more the scale loss is reduced. For example, low temperature heating at 1150 ° C. or lower is desired. However, in a steel type that contains a large amount of easily oxidizable alloy elements such as Si and Cr, a dense subscale composed of FeCr ₂ O ₄ and Fe ₂ SiO ₄ as shown in FIG. It occurs at the interface of and induces descaling failure. It has been found that these fine subscales are likely to occur when the steel contains 0.05% or more of Si and 0.1% or more of Cr. In addition, when Ni is added as an alloying element, if the added amount exceeds 0.1%, Ni concentrates unevenly at the interface between the scale and the steel material. Gets worse.

  As described above, these subscales left without being descaled are pushed into the surface of the iron core during rolling, and cause surface defects.

  Therefore, in the present invention, the steel billet is first heated in a heating furnace at a temperature of 1000 ° C. or higher and 1150 ° C. for 60 minutes or less, and then extracted at 5 ° C./min. Under the above heating rate, it is rapidly heated to a temperature of 1200 to 1350 ° C., held within 10 minutes, and extracted from a heating furnace.

  In this way, combining low temperature heating with rapid heating at a high temperature suppresses the generation of subscales due to low temperature heating, and by holding the subscale for a short time at a high temperature prior to extraction. Growth stress due to rapid growth occurs, and the subscale of the interface is destroyed.

  When the low temperature heating is less than 1000 ° C., the difference from the extraction temperature from the heating furnace (rolling temperature + α) is large, and in order to secure the time for rapid heating and the temperature necessary for rolling, The holding time must be extended, resulting in a longer staying time in the high temperature range and an increased scale loss. In addition, what is necessary is just to determine the minimum temperature of the low temperature heating made into 1000 degreeC or more so that the temperature required for rolling may be ensured.

When this low-temperature heating is performed at a temperature higher than 1150 ° C., combined with the subsequent high-temperature heating, not only the subscale (FeCr ₂ O ₄ , Fe ₂ SiO ₄ ) but also the scale (FeO, Fe ₃ O) of the upper layer. ₄ and Fe ₂ O ₃ ) grow too much, increasing the scale loss.

  And the upper limit of the holding time of the said low temperature heating shall be 60 minutes. If it exceeds 60 minutes, the scale loss increases due to the rapid heating of the subsequent process and the effect of water vapor addition. The lower limit of the holding time is determined in consideration of the thickness of the subscale to be generated. However, if the holding time is insufficient, the subscale becomes too thin and the peelability becomes difficult. Therefore, 15 minutes is preferable as the lower limit of the low-temperature heating holding time at which the subscale is appropriately generated.

  Next, in the present invention, the steel billet that has been subjected to low-temperature heating is controlled to a temperature range of 1200 to 1350 ° C. while being controlled at a heating rate of 5 ° C. or more per minute, and rapid heating that maintains this within 10 minutes is performed. It is characterized by performing.

By this high temperature rapid heating, the scale generated during the low temperature heating grows at a stretch, and the subscales (FeCr ₂ O ₄ , Fe ₂ SiO ₄ ) are destroyed by the action. For this purpose, a heating rate of 5 ° C./min or more is necessary, and below this, scale growth is insufficient to destroy the subscale.

In addition, when the final temperature after rapid heating is less than 1200 ° C., the subscale (FeCr ₂ O ₄ , Fe ₂ SiO ₄ ) has little destruction effect, and when it exceeds 1350 ° C., scale growth becomes excessive and scale loss increases. On the contrary, it makes the descaling difficult. Further, heating at a high temperature exceeding 10 minutes is not preferable from the viewpoint of scale loss and peelability because the scale growth is promoted too much.

  Next, the present invention provides a steam oxidation treatment in which a steel billet that has been heat-treated under the above conditions and extracted from a heating furnace is exposed to a short time of 2 seconds or less in a humid atmosphere with a dew point of 30 to 60 ° C. once. Or, it is performed twice or more, and is characterized by high-pressure water descaling and rolling.

  When a steel billet is subjected to steam oxidation in such a humid atmosphere for a very short time, the steam acts on the interface between the subscale and the steel through the subscale, forming brittle wustite (FeO) at this interface. To do. And this weak wustite makes it fine as if the subscale in the upper layer is pulverized, and the peelability of the scale in FIG. 2a is easily improved. This effect is the same even when Ni is added, and the effect of reducing the interface concentration of Ni is also overlapped to improve the peelability of the scale.

  Moreover, the presence of water vapor has the effect of reducing irregularities at the interface between the scale and the steel in addition to the above effects, and is extremely effective for suppressing wrinkles during rolling.

  The reason why the wet atmosphere treatment is performed in an atmosphere having a dew point of 30 to 60 ° C. is that the dew point necessary for the destruction of the subscale is 30 ° C. or more. In a humid atmosphere with a dew point of 60 ° C. or higher, the scale grows too much and the scale peeling becomes difficult.

  The reason why the exposure time to the humid atmosphere is set to 2 seconds or less is because it has been found that the shorter the treatment time, the easier the effect of forming the wustite at the interface by the steam oxidation in the wet atmosphere. In fact, when the treatment time in the humid atmosphere exceeds 2 seconds, the oxidation proceeds and wustite (FeO) formed at the interface changes to magnetite (Fe3O4). Since this magnetite is highly compatible with the ground iron, it makes it difficult to peel off the subscale and promotes the generation of surface defects due to the indentation of the scale. At the same time, the effect of smoothing the interface between the ground iron and the scale is lost.

  The wet atmosphere treatment is performed before descaling with high-pressure water on the steel billet immediately after extraction from the heating furnace, as shown in FIG. 3a, or, as shown in FIG. After the transition to, it is carried out toward the rolled material before the high-pressure water descaling between the two front and rear machines in the rolling mill row. Alternatively, both of the above can be used in combination. Which one is to be adopted may be selected in consideration of the type of the product strip material, its steel type, and the like.

The wet atmosphere is adjusted to a predetermined dew point by installing a wet atmosphere booth FIG. 2 and introducing water vapor from the water vapor supply pipe. The dew point in the wet atmosphere is near the surface of the steel billet or strip and can be easily measured using a specular dew point meter by collecting atmospheric gas within a height of 50 cm from the steel surface. .
(Example 1)
Seven types of steel billets (150 mm square) melted in the alloy composition shown in Table 1 were prepared, and as shown in Table 2, first, most of them were in the temperature range of 1000 to 1150 ° C. regulated by the present invention. Is heated at a low temperature. Only 9 was heated in a higher temperature range than the same temperature range as a comparative example.

  Next, after each heated steel billet is extracted from the heating furnace, as shown in FIG. 3a, steam oxidation treatment is immediately performed in a humid atmosphere in a humid atmosphere booth installed close to the outlet side of the furnace. High pressure water descaling was performed. Subsequently, each was hot-rolled to produce a steel wire having a diameter of 8.0 mm.

LNG gas was used as the combustion gas for the heating furnace, and the atmosphere in the furnace was adjusted to 72% N ₂ -18% H ₂ O-9% CO ₂ -1% O ₂ .

  In order to evaluate the surface defects due to the scale of each manufactured steel wire, the presence and number of surface defects were counted by a method of observing the cross section of the steel wire at an optical microscope at a magnification of 100 times.

Also, the following method was used to confirm whether or not these were surface defects caused by scale. That is, when the cross section of the entire surface flaw is analyzed by EPMA mapping at a magnification of 500 times, if there is a region in which Si or Cr or both are more than twice the composition of each element in the steel wire, It was determined that the surface defect was caused by the scale in which the subscale (FeCr ₂ O ₄ , Fe ₂ SiO ₄ ) was pressed.

  On the other hand, the surface defects observed at 10 or more points in the cross section perpendicular to the longitudinal direction of the steel wire (steel rolling direction) were measured, and the average value was calculated by the following equation.

・ Total number of wrinkles / total number of cross sections to be measured = number of wrinkles per measurement cross section Here, wrinkles refers to surface wrinkles caused by scales that reach a depth of 10 μm or more. Classification and evaluation were performed. It should be noted that a wrinkle having a depth of less than 10 μm is not recognized as a surface flaw and does not substantially cause a problem such as a crack during processing. Therefore, the determination is made only for a surface flaw reaching 10 μm or more. .

・ Rank 0: The average number of hooks is 0 (no hooks) ・ Rank 1: The average number of hooks is more than 0 and less than 10 ・ Rank 2: More than 10 and less than 20 ・ Rank 3: 20 or more and less than 30 ・ Rank 4: More than 30 If rank 1 or less, there is no problem as a product with respect to surface defects due to scale.

  As is apparent from Table 2, the low temperature heating temperature / time of the steel billet, the rapid heating rate / attainment temperature / time before extraction, and the dew point / time of the steam oxidation treatment in the humid atmosphere on the heating furnace exit side are all present. The product wire rods of the following examples of the present invention that satisfy the conditions regulated by the invention were all satisfactory when the surface flaw level was 0 or 1.

-Example No. 3, 4, 5, 8, 11, 12, 13, 15, 18 and 19
On the other hand, in the comparative examples that deviate from one or more of the conditions regulated by the present invention, as described below, the surface flaw level is evaluated as 3 or more, and it is confirmed that the quality cannot be satisfied. It was done.

  Comparative Example No. Since 1, 10 and 14 were extracted only by heating the steel billet at a low temperature of 1150 ° C. or lower, a dense subscale was generated, which could hardly be removed even by high-pressure water descaling. Residual subscale is pushed in and a large amount of surface flaws are generated.

  Comparative Example No. 2, 16 and 21 are examples in which the scale grows excessively because the holding time after rapid heating is too long and cannot be completely removed even by high-pressure water descaling, resulting in a large amount of surface defects.

  Comparative Example No. No. 17 is a surface formed by pressing the scale, making it difficult to peel off the subscale because magnetite (Fe 3 O 4) having high fusion with the ground iron was formed at the interface because the steam oxidation time in the humid atmosphere was too long. This is an example of a large amount of soot.

  Comparative Example No. In Nos. 6 and 20, the holding temperature after rapid heating is too high, the scale grows excessively and cannot be completely removed even by descaling, and a large amount of surface defects are generated.

  Comparative Example No. No. 7 is an example in which the dew point of the humid atmosphere is too high, the scale grows excessively, cannot be completely removed even by descaling, and a large amount of surface defects occur.

  Comparative Example No. No. 9 is an example in which the initial heating temperature to be heated at a low temperature is too high, the scale grows excessively, cannot be completely removed even by descaling, and a large amount of surface flaws are generated.

(Example 2)
Seven types of steel billets (150 mm square) melted in the alloy composition shown in Table 3 were prepared. As shown in Table 4, first, most of the steel billet was in the temperature range of 1000 to 1150 ° C. regulated by the present invention. Is heated at a low temperature. Only No. 23 was heated in a higher temperature range than the same temperature range as a comparative example.

  Next, after extracting each heated steel billet from a heating furnace, as shown in FIG. 3b, after moving to the rolling process, steam oxidation treatment is performed between the two machines before and after the rolling mill row, and high pressure water descaling is performed. Was done. Further, hot rolling was performed to manufacture a steel wire having a diameter of 8.0 mm.

LNG gas was used as the combustion gas for the heating furnace, and the atmosphere in the furnace was adjusted to 72% N ₂ -18% H ₂ O-9% CO ₂ -1% O ₂ .

  The method for evaluating surface defects is the same as in Example 1.

  As is apparent from Table 4, the low temperature heating temperature / time of the steel billet, the rapid heating rate / attainment temperature / time before extraction, and the dew point / time of the steam oxidation treatment in a humid atmosphere between the rolling mills are all included in the present invention. The product wire rods of the following examples of the present invention satisfying the conditions regulated by the above were all satisfactory when the surface flaw level was 0 or 1.

-Example No. 22, 24, 27, 31, 32, 34, 38
On the other hand, in the comparative examples that deviate from one or more of the conditions regulated by the present invention, as described below, the surface flaw level is evaluated as 3 or more, and it is confirmed that the quality cannot be satisfied. It was done.

  Comparative Example No. 25, 29, 33 and 36 were extracted only by heating the steel billet at a low temperature of 1150 ° C. or lower, so that a dense subscale was generated, which could hardly be removed even by high-pressure water descaling. Then, the residual subscale is pushed in and a large amount of surface flaws are generated.

  Comparative Example No. Nos. 28 and 35 are examples in which the scale grows excessively because the holding time after rapid heating is too long and cannot be completely removed even by high-pressure water descaling, and a large amount of surface flaws are generated.

  Comparative Example No. No. 37 was formed because magnetite (Fe 3 O 4) having high fusion with the ground iron was formed at the interface because the steam oxidation time in a humid atmosphere was too long. This is an example of a large amount of occurrence.

  Comparative Example No. In No. 30, the holding temperature after rapid heating is too high, the scale grows excessively and cannot be completely removed even by descaling, and a large amount of surface defects are generated.

  Comparative Example No. No. 26 is an example in which the dew point of the humid atmosphere is too high, the scale grows excessively, cannot be completely removed even by descaling, and a large amount of surface flaws occur.

  Comparative Example No. No. 23 is an example in which the initial heating temperature to be heated at a low temperature is too high, the scale grows excessively, cannot be completely removed even by descaling, and a large amount of surface defects occur.

The schematic diagram of the cross-sectional structure of the scale produced | generated when a steel billet is heated at low temperature. The schematic diagram of the cross-sectional structure of the scale produced | generated when the process by the rapid heating and water vapor addition by this invention is given. The schematic of the rolling equipment used for implementation of the hot rolling method of this invention.

Claims (5)

A steel billet containing 0.05 to 3% by mass (abbreviated as%; the same applies hereinafter) and 0.1 to 5% Cr is held in a heating furnace at a temperature of 1000 ° C. to 1150 ° C. within 60 minutes. After performing the low temperature heating, before the extraction, the temperature reaches 1200 to 1350 ° C. at 5 ° C./min. Perform rapid heating at the above heating rate, hold for 10 minutes or less, extract from the heating furnace, and then perform steam oxidation treatment once exposed to a short time of 2 seconds or less in a humid atmosphere with a dew point of 30 to 60 ° C. A method for producing a strip material containing Si and Cr excellent in high-pressure water descaling and rolling scale peeling, which is performed twice or more times.   The scale exfoliation property according to claim 1, which is subjected to high-pressure water descaling and rolling immediately after extraction from a heating furnace, after performing steam oxidation treatment for 2 seconds or less in a humid atmosphere with a dew point of 30 to 60 ° C. Si, Cr-containing strip steel material manufacturing method.   The scale peeling according to claim 1, wherein high-pressure water descaling is performed after performing steam oxidation treatment in a humid atmosphere at a dew point of 30 to 60 ° C. for a short time of 2 seconds or less between the rolling mills before and after the rolling process. A method for producing Si- and Cr-containing strips with excellent properties.   3. Excellent exfoliation of scale according to claim 2, wherein after extraction from a heating furnace, steam oxidation is performed in a humid atmosphere with a dew point of 30 to 60 ° C. for a short time of 2 seconds or less, followed by high-pressure water descaling and rolling. Si, Cr-containing strip steel material manufacturing method.   The method for producing a Si or Cr-containing strip steel material having excellent scale peelability according to claim 1, 2, 3 or 4, wherein a steel billet containing 0.1% or more of Ni is used.

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