JP2007229747A - Method for producing bar steel having excellent surface property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a bar steel having excellent surface properties in which surface flaws (such as scale penetration flaws) caused by scale in a hot rolling-finished state are reduced. <P>SOLUTION: The method for producing a bar steel having excellent surface properties is characterized in that, when a bar steel comprising Si of ≥0.05 mass% and one or more kinds selected from Cr and Ni of ≥0.1 mass% is heated at ≤1,200°C (e.g., at 1,150°C) and is thereafter subjected to hot rolling, the bar steel is exposed to a humid atmosphere having a dew point of 30 to 60°C for ≤2 s for at least one time till the completion of the hot rolling (e.g., directly after its extraction from a heating furnace and/or between hot rolling mills). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention belongs to a technical field relating to a method for producing a strip material having excellent surface properties.

  The surface quality required for steel products is becoming stricter year by year, but surface defects such as fine surface wrinkles and rough skin often occur in the hot rolled finish. In particular, in steel types containing a large amount of easily oxidizable alloy elements such as Cr and Si, iron oxides containing alloy elements (hereinafter also referred to as subscales) are formed during the heating and rolling processes. The subscale is dense and thin at low temperatures up to around 1200 ° C, and when Ni is included, it has the effect of further improving the adhesion to steel, so it is descaling with high-pressure water (hereinafter also referred to as high-pressure water descaling). ) Is difficult to remove. If it is rolled while remaining on the steel surface, the subscale is pushed into the steel and becomes scale flaws, deteriorating the surface quality of the product.

In order to improve the descaling property of the subscale, a method of weakening the subscale by high-temperature heating in the heating furnace and supply of steam into the heating furnace is disclosed (Japanese Patent Laid-Open Nos. 2003-119517 and 2002). -316207). However, this method has problems in terms of scale loss and furnace operation. Furthermore, a secondary scale having a subscale with high adhesion is formed even during rolling. For this reason, a method for facilitating peeling of the subscale is demanded.
Japanese Patent Laid-Open No. 2003-119517 JP 2002-316207 A

  Since the subscale produced by heating (low temperature heating) at 1200 ° C. or less in the heating furnace is dense and strong, it is difficult to remove by normal high-pressure water descaling. Further, a secondary scale having a subscale with high adhesion is formed between the rolling mills (between the rolling mill and the rolling mill). For this reason, it is necessary to modify these hardly peelable subscales to easily peelable subscales before descaling. The time from the furnace extraction to the first descaling and the passing time between rolling mills are within a few seconds, and it is necessary to modify the scale within an extremely short time.

  The present invention has been made paying attention to such circumstances, and an object thereof is to provide a method for producing a steel bar having a small surface flaw due to scale and excellent surface properties.

  In order to achieve the above-mentioned object, the present inventors have conducted intensive studies and have completed the present invention. According to the present invention, the above object can be achieved.

  The manufacturing method of the steel bar material according to the present invention which has been completed in this way and has achieved the above object is a steel bar material containing Si: 0.05% by mass or more and one or more of Cr and Ni: 0.1% by mass or more. After heating at 1200 ° C. or lower, when hot rolling, the surface texture is characterized by exposing it to a humid atmosphere at a dew point of 30 to 60 ° C. for at least once before the hot rolling is completed. It is a manufacturing method of the strip steel material excellent in.

  According to the method for manufacturing a steel bar according to the present invention, it is possible to manufacture a steel bar having less surface wrinkles due to scale and having excellent surface properties.

  In order to achieve the above-mentioned object, the present inventors have studied a method for facilitating peeling of a hardly peelable subscale generated at 1200 ° C. or less in a heating furnace or between rolling mills. As a result, the following was found.

When the surface of a steel material with a dense subscale is oxidized (steam oxidation) in wet air containing water vapor (humid atmosphere), the water vapor passes through the subscale in a very short time within a few seconds. The steel surface is directly oxidized by oxygen moving inward. As a result, the subscale composed of Fe ₂ SiO ₄ and FeCr ₂ O ₄ is thinly formed with wustite (FeO) at the interface where it contacts the ground iron. Destroy. In addition, since it is uniformly oxidized by the action of water vapor, the interface becomes smooth and the subscale is easily removed from the ground iron, and the high-pressure water descaling property is greatly improved. Therefore, it is possible to suppress the occurrence of scale indentation flaws due to the scale being pushed into the steel surface during rolling. At this time, the dew point of the wet atmosphere is 30 to 60 ° C., and the time of exposure to the wet atmosphere (steam oxidation time) is preferably 2 seconds or less. In this case, the above-described effects are sufficiently exhibited. .

  The present invention has been completed based on such knowledge, and relates to a method for manufacturing a strip steel material. The method for producing the steel strip according to the present invention thus completed is as follows. After heating the steel strip containing Si: 0.05% by mass or more and one or more of Cr and Ni: 0.1% by mass or more at 1200 ° C. or less. The method for producing a strip with excellent surface properties, characterized in that, at the time of hot rolling, at least once before the hot rolling is completed, it is exposed to a humid atmosphere with a dew point of 30 to 60 ° C. for 2 seconds or less. It is.

  According to the method for manufacturing a steel bar according to the present invention, as can be seen from the above knowledge, it is possible to manufacture a steel bar having less surface wrinkles due to scale and excellent surface properties.

  More specifically, the above dew point: the treatment of exposing the steel strip in a humid atmosphere of 30 to 60 ° C. for 2 seconds or less (hereinafter also referred to as wet atmosphere treatment (A)), heating the steel strip at 1200 ° C. or less. After that, when it is performed before hot rolling, the dense subscale generated by the heating is broken by the wet atmosphere treatment (A) and modified to a scale property that is easy to peel off. Therefore, it is possible to remove the scale by high-pressure water descaling after the wet atmosphere treatment (A), and therefore, it is possible to obtain a strip having excellent surface properties with less surface wrinkles due to scale indentation in hot rolling. it can.

  After heating the strip steel at 1200 ° C or lower, descaling, and then hot rolling, the wet atmosphere treatment (A) is between the hot rolling mills (between the rolling mill and the rolling mill). In some cases, the above-described descaling cannot be removed, and the remaining subscale and the subscale generated during hot rolling are destroyed by the wet atmosphere treatment (A) and modified to a scale property that is easy to peel off. Since it is uniformly oxidized by the action of water vapor, the interface becomes smooth and the subscale is easily removed from the ground iron. Therefore, it is possible to remove the scale by high-pressure water descaling after the wet atmosphere treatment (A), and therefore, it is possible to produce a steel bar having excellent surface properties with less surface wrinkles due to scale pressing in subsequent hot rolling. Obtainable.

  In this case, high-pressure water descaling is performed after the wet atmosphere treatment (A) between the hot rolling mills, but it cannot be removed even by descaling after the above-mentioned bar material heating, and the remaining subscale is slight. In addition, when the subscale generated during the hot rolling is slight, it may not be necessary to perform descaling after the wet atmosphere treatment (A). This is because such a minor subscale may be removed during hot rolling after the wet atmosphere treatment (A). Therefore, in such a case, it is possible to obtain a strip having excellent surface properties with less surface flaws due to scale indentation in hot rolling without performing descaling after the wet atmosphere treatment (A).

  The manufacturing method of the steel strip according to the present invention is, in terms of process, the above process, that is, the heating of the steel strip at 1200 ° C. or lower → the wet atmosphere treatment (A) → descaling → hot rolling, , Heating of strip steel at 1200 ° C or lower → Descaling → Hot rolling → Wet atmosphere treatment between hot rolling mills (A) → Descaling → Hot rolling process, heating of strip steel at 1200 ° C or lower → Besides the descaling → hot rolling → hot rolling mill wet atmosphere treatment (A) → hot rolling process, there are the following processes. That is, heating the strip steel at 1200 ° C. or lower → wet atmosphere treatment (A) → descaling → hot rolling → wet atmosphere treatment between hot rolling mills (A) → hot rolling or 1200 There is a process of heating at a temperature of ℃ or lower → wet atmosphere treatment (A) → descaling → hot rolling → hot rolling mill, wet atmosphere treatment (A) → descaling → hot rolling, and these also relate to the present invention. It is included in the manufacturing method of strip steel. Which process is to be adopted may be determined according to the time and degree of subscale generation. The time point and degree of generation of this subscale are determined by the composition of the strip steel, heating temperature, hot rolling conditions, and the like.

  In the manufacturing method of the steel bar according to the present invention, the steel types of the steel bars are componentally Si: 0.05% by mass (wt%) or more, one or more of Cr and Ni: 0.1% by mass or more (two types of Cr and Ni In this case, the total amount of Cr and Ni is 0.1 mass% or more), and the heating temperature before hot rolling is 1200 ° C. or less. The reason is as follows. In the case of such steel types and heating temperatures, subscales are likely to be generated, and conventionally, there has been a problem that surface flaws are likely to occur due to scale pressing during hot rolling. Since the present invention intends to solve such problems, it is premised and necessary to have such a steel type and heating temperature. The present invention is effective in the case of such steel types and heating temperatures.

The wet atmosphere in the treatment of exposure in a wet atmosphere (hereinafter also referred to as wet atmosphere treatment) is dew point: 30 to 60 ° C. The reason is as follows. The dew point of the moist atmosphere required for subscale destruction is 30 ° C. or higher. In the case of a moist atmosphere with a dew point of less than 30 ° C., subscale destruction is insufficient. Dew point: In a humid atmosphere of 60 ° C. or higher, the scale grows too much and the scale peeling becomes worse. From this point, the wet atmosphere is dew point: 30-60 ° C. In addition, from the viewpoint of further suppressing a decrease in scale peelability due to such scale growth, the wet atmosphere in the wet atmosphere treatment is desirably a dew point of 55 ° C. or less, and thus a dew point of 30 to 55 ° C. . Dew point: When a humid atmosphere of 30 to 60 ° C. is expressed using the absolute water vapor amount, the absolute water vapor amount is 30.3 to 122 g / m ³ . When a moist atmosphere with a dew point of 30 to 55 ° C. is expressed using an absolute water vapor amount, the absolute water vapor amount is 30.3 to 99.1 g / m ³ .

  The exposure time in the humid atmosphere in the humid atmosphere treatment (humid atmosphere treatment time) is 2 seconds or less. The reason is as follows. The effect of forming wustite at the interface due to steam oxidation in the wet atmosphere treatment is more likely to occur as the wet atmosphere treatment time (steam oxidation time) is shorter. When the wet atmosphere treatment time exceeds 2 seconds, surface oxidation proceeds, and wustite formed at the interface changes to magnetite. Magnetite has high consistency with the ground iron, and the subscale is difficult to peel off, which promotes generation of surface flaws due to scale pressing during hot rolling. In addition, the effect of smoothing the interface between the ground iron and the scale is lost. From this point, the wet atmosphere treatment time: 2 seconds or less. Note that 0 seconds are not included in the above 2 seconds or less.

  The wet atmosphere treatment (A) is performed at least once. That is, the wet atmosphere treatment (A) is not limited to once, but is performed once or twice or more as necessary.

  In the present invention, the steel bar is a general term for rod-shaped or linear steel materials. There are shape steel, steel sheet pile, rail, steel bar, steel wire rod, and the like.

  The wet atmosphere is a wet atmosphere containing water vapor. Usually, moist air containing water vapor is used as the moist atmosphere. The dew point in a humid atmosphere is measured using a specular dew point meter by collecting atmospheric gas near the surface of the steel bar such as steel wire (within a height within 50 cm from the steel surface). The wet atmosphere is adjusted to a predetermined dew point by installing a wet atmosphere booth and introducing water vapor from the water vapor supply pipe.

  Examples of the present invention and comparative examples will be described below. The present invention is not limited to this embodiment, and can be implemented with appropriate modifications within a range that can be adapted to the gist of the present invention, all of which are within the technical scope of the present invention. include.

[Example 1]
A steel billet (150 mm square) having the composition shown in Table 1 was heated in a heating furnace in a combustion gas atmosphere, and immediately after extraction from the heating furnace, was subjected to a humid atmosphere treatment (steam oxidation treatment) in a humid atmosphere booth. After scaling, it was hot-rolled to produce a steel wire (diameter: 13.5 mm). At this time, the heating temperature in the heating furnace, the dew point of the wet atmosphere in the wet atmosphere booth, and the wet atmosphere treatment time (steam oxidation treatment time) in the wet atmosphere booth were changed as shown in Table 2.

  The number of scale wrinkles (surface wrinkles resulting from the scale) was measured for the steel wire thus manufactured. The results are shown in Table 2.

  The presence / absence and number of scale wrinkles can be evaluated by observing a cross section of the steel material with an optical microscope at a magnification of about 100 to 200 times. Analyzing the cross section of the scale soot by EPMA mapping, it is possible to confirm the enrichment of Si and Cr in addition to Fe and O, and it can be determined that the scale is a scale so that the sub-scale is pushed in.

  Measurement of the number of scale ridges for the steel wire was performed by such a method. That is, by such a method, the number of scale ridges observed at 10 or more cross-sections perpendicular to the longitudinal direction (steel rolling direction) of the steel wire was measured, and the average value (the number of scale ridges) Total number / total number of measured cross sections = number of scales per measured cross section) was calculated. And, the average value of scale 疵 number is 0 (no scale 疵) is rank 0, the average value of scale 疵 number is more than 0 and less than or equal to rank 1, rank is more than 10 and less than 20 rank. 2, 20 or more and less than 30 were ranked 3 and more than 30 were ranked 4. If it is rank 1 or less, there is no problem as a product.

  As can be seen from Table 2, in the case of Nos. 5, 10, 14, and 17, the surface texture level is ranks 3 to 4, and there are many scale wrinkles. This is because the steam oxidation treatment time (wet atmosphere treatment time) in the wet atmosphere booth was too long, over 2 seconds, and magnetite was formed at the interface between the subscale and the steel, resulting in poor scale peelability.

  In the case of No. 3, the surface texture level is rank 3, and there are many scale defects. This is because the dew point of the moist atmosphere in the moist atmosphere booth is 10 ° C. and the amount of water in the moist atmosphere is small, so that the water vapor effect is insufficient and the subscale is not sufficiently destroyed.

  In the case of No. 9, the surface texture level is rank 2, and in the case of No. 21, the surface texture level is rank 4, and in either case there are many scale wrinkles. This is because the dew point of the moist atmosphere in the moist atmosphere booth is 76 ° C, and the moisture effect in the moist atmosphere is too much and the water vapor effect is too strong. This is because the growth of wustite accelerates and the scale grows too thick and the scale peeling becomes worse.

  In the case of No. 4, the surface texture level is rank 2, and in the case of No. 15 and 20, the surface texture level is rank 3, and in either case there are many scale defects. This is because the heating temperature in the heating furnace is high, so a porous and thick scale is generated, and a thicker scale is generated by steam oxidation. Because it became.

  In the case of No.1, 2, 6, 7, 8, 11, 12, 13, 16, 18, 19, the surface texture level is rank 0 or rank 1, and there is no scale wrinkle or there is scale flaw Few. This is because the heating temperature in the heating furnace, the dew point of the wet atmosphere in the wet atmosphere booth, and the wet atmosphere treatment time (steam oxidation time) in the wet atmosphere booth are the conditions according to the present invention (heating temperature: 1200 ° C. or less). This is because the subscale was destroyed by the water vapor effect and completely or almost completely removed by descaling in order to satisfy the dew point of the wet atmosphere: 30 to 60 ° C. and the wet atmosphere treatment time: 2 seconds or less.

  As described above, after heating the steel billet in the heating furnace under the conditions according to the present invention (heating temperature: 1200 ° C. or less), immediately after extraction from the heating furnace, the conditions according to the present invention (wet atmosphere dew point: When wet atmosphere treatment (steam oxidation treatment) is performed at 30 to 60 ° C and humid atmosphere treatment time: 2 seconds or less), the steam reaches the steel interface through the scale generated in the heating furnace, and wustite is formed. As a result, the interface becomes brittle, and immediately after descaling with a high-pressure water jet or the like, the scale layer is completely or almost completely removed, and the scale residue created in the heating furnace is pushed into the steel during rolling. As a result, scale wrinkles are less likely to occur.

[Example 2]
Using a steel billet having the composition shown in Table 1, a steel wire was manufactured by the following process. That is, a steel billet (150 mm square) having the composition shown in Table 1 is heated in a furnace in a combustion gas atmosphere → high pressure water descaling → hot rolling → wet atmosphere treatment (water vapor in a wet atmosphere booth between hot rolling mills) A steel wire (diameter: 13.5 mm) was manufactured by the process of oxidation treatment) → high pressure water descaling → hot rolling. At this time, the hot rolling start temperature, the dew point of the wet atmosphere in the wet atmosphere booth, and the steam oxidation treatment time in the wet atmosphere booth were changed as shown in Table 3. In addition, although the said hot rolling start temperature is slightly lower than the heating temperature in a heating furnace, it is substantially equal to the heating temperature in a heating furnace. Therefore, although the heating temperature in a heating furnace is slightly higher than the hot rolling start temperature shown in Table 3, it is substantially equal to this hot rolling start temperature, and all are 1200 degrees C or less.

  With respect to the steel wire thus produced, the number of scale ridges (surface ridges resulting from the scale) was measured by the same method as in Example 1, and the average value of the number of scale ridges was determined. And as in the case of Example 1, the average value of scale 疵 number is 0 (no scale 疵) is rank 0, the average value of scale 疵 number is more than 0 and less than 10 rank 1, and more than 10 20 Those with less than 30 were ranked 2, those with 20 or more but less than 30 were ranked 3, and those with more than 30 were ranked 4. The results are shown in Table 3.

  As can be seen from Table 3, the dew point of the moist atmosphere in the moist atmosphere booth is in the range of 30 to 60 ° C, and the moist atmosphere treatment time satisfies 2 seconds or less (No. 1, 5, 6, 8, 10 , 11, 13, 14), the surface texture level is rank 0 or rank 1 and there is no scale wrinkle or there is little scale wrinkle. This is because surface defects due to the irregularities on the steel surface were reduced by steam oxidation, and the surface quality was improved. That is, the subscale generated during hot rolling is destroyed by the wet atmosphere treatment, modified to a scale property that easily peels off, and is uniformly oxidized by the action of water vapor so that the interface is smooth and clean. Since the scale is easily removed from the ground iron, it can be removed by high-pressure water descaling after wet atmosphere treatment, which reduces the occurrence of surface flaws due to scale pressing in subsequent hot rolling. It is.

  When the wet atmosphere treatment time exceeds 2 seconds (No. 3, 7, 9, 12, 15), the surface texture level is rank 3 to 4, and there are many scale wrinkles. This is because magnetite formed at the interface between the subscale and steel, resulting in poor scale peelability.

  When the dew point of the humid atmosphere in the humid atmosphere booth is less than 30 ° C (No. 2), the surface texture level is rank 4 and there are many scale defects. This is because the moisture content in the humid atmosphere is small, so that the water vapor effect is insufficient and the subscale is not sufficiently broken.

  As described above, the steel billet was heated in the heating furnace under the conditions according to the present invention (heating temperature: 1200 ° C. or less), and then descaled, and then the conditions according to the present invention in the wet atmosphere booth between the hot rolling mills ( When wet atmosphere treatment is performed at a dew point of wet atmosphere: 30 to 60 ° C., wet atmosphere treatment time: 2 seconds or less), water vapor reaches the interface through the surface scale, and the surface of the iron base is uniformly oxidized. In order to scale (form wustite), there is an effect of smoothing the interface between the scale and the steel, and the surface wrinkle depth due to the irregularities generated in the previous process is reduced. Accordingly, it is difficult for the surface wrinkle depth to remain until the product, and a product having excellent surface properties can be obtained.

  According to the method for manufacturing a bar material according to the present invention, it is possible to manufacture a bar steel material having less surface wrinkles due to scale and having excellent surface properties. It can be suitably used as a method for producing an excellent steel bar material, and is useful.

Claims (1)

When a steel strip containing Si: 0.05% by mass or more and one or more of Cr and Ni: 0.1% by mass or more is heated at 1200 ° C. or less and then hot-rolled, at least 1 by the end of the hot rolling. Time, dew point: A method for producing a strip material having excellent surface properties, characterized by being exposed to a humid atmosphere of 30 to 60 ° C. for 2 seconds or less.