KR19990053146A - Manufacturing method of hot rolled steel sheet for high-strength steel pipe by thin slab direct rolling - Google Patents

Abstract

In the present invention, in the thin slab direct rolling process having a rough rolling facility, C is less than 0.04% by weight (less than%) for the purpose of easily manufacturing SPHT1, which is the softest of the hot rolled steel sheets for steel pipes, and preventing edge cracking of hot rolled steel strips. , Mn content is 0.3% or more and 0.5% or less, S is 0.012% or less, Mn / S ratio is maintained at 35 or more, P is 0.012% or less, Si is 0.02% or less, Cu 0.1% or less, Sn 0.01% In the case of induction heating of steel composed of Al content of 0.02% or more and 0.04% or less and other unavoidable residual elements after continuous casting-rolling direct rolling, the induction heating temperature is set to 1050 ° C or less, and this is a conventional thin slab direct rolling method. Hot-rolled, but the finish rolling temperature is 850 ℃ or more and 880 ℃ or less, hot rolling for hot-rolled steel tube by the thin slab direct rolling method exceeding 41% elongation based on tensile strength 41kgf / mm ² or less, 3mm thickness The steel plate manufacturing method is made into the summary.

Description

Manufacturing method of hot rolled steel sheet for high-strength steel pipe by thin slab direct rolling

The present invention relates to a method for manufacturing a hot rolled steel sheet for high-strength steel pipe by the thin slab direct rolling method, in particular, while adjusting the C, P, Si, etc. among the alloy elements that can be easily regulated and adjustable in the thin slab direct rolling method, hot finish As a method of easily manufacturing the softest SPHT1 material among hot rolled steel sheets for steel pipes by controlling process factors such as rolling temperature, it has more ductility and secures similar tensile strength and surface quality as compared to the product manufactured by the blast furnace method. Of course, the present invention relates to a method for manufacturing hot rolled steel sheet for high-strength steel pipe by the thin slab direct rolling method, which has an advantage of significantly improving product error rate by preventing hot edge cracks.

A thin slab continuous casting direct hot rolling method called a mini mill has recently emerged as a new steelmaking process (hereinafter referred to as a thin slab direct rolling method). This method is effective in energy saving because of low equipment cost, production of hot rolled steel sheet, and no reheating of hot slab. However, because the scrap iron is melted again in an electric furnace, the content of residual elements such as Cu, Sn, Cr, and Ni is high, and since nitrogen in the air is contained in molten steel, the strength of the steel is higher and the ductility is higher than that of the conventional blast furnace method. There is a problem of deterioration, and it is not easy to manufacture SPHT1 material for steel pipe (JIS, G3132), which requires appropriate strength and ductility. The SPHT1 material proposed in the present invention is the softest of the hot rolled steel sheets for steel pipes, and is mainly used for home and industrial piping materials. Since most of them are used by galvanizing after pipes, the surface and plating quality must be excellent, and the strength and elongation must be provided at the same time. do. However, unlike the existing blast furnace method, in the thin slab direct rolling method, the solid slab solidified by continuous casting is directly rolled, so that the steel is strengthened by supercooling and plastic organic precipitation. It is difficult to secure the target material. However, if the adjustable elements such as carbon, Mn, Al, etc. are adjusted in terms of securing the material, surface defects such as edge cracks are likely to occur, which is also undesirable in terms of product yield. Edge crack is a kind of high temperature brittleness of steel, and when processing is performed when acementite grains contain liquid metal or cornerstone ferrite, the crack is easily formed and propagated by using the grain boundary precipitate or grain triple point as nucleus. As a losing phenomenon, it is necessary to reduce the number of precipitates as much as possible, and to precipitate low melting compounds such as FeS remaining in the liquid phase in the liquid phase as coarse precipitates at higher temperatures.

The present invention facilitates the softest SPHT1 material among the hot rolled steel sheets for steel pipes by controlling C, P, Si, etc. among the alloy elements that can be easily regulated and adjusted in the thin slab direct rolling method, and controlling process factors such as hot finish rolling temperature. As a manufacturing method, the thin slab direct rolling method has the advantage of significantly improving the product realization rate by preventing hot edge cracking as well as securing ductility and similar tensile strength and surface quality compared to the product manufactured by the blast furnace method. It is an object of the present invention to provide a method for manufacturing a hot rolled steel sheet for high forming steel pipe.

The present invention for achieving the above object is a chemical composition of C: 0.04% by weight (% or less), Mn: 0.3% or more and 0.5% or less, S: 0.012% or less, Mn / S ratio is maintained at 35 or more Continuous casting-cast rolling of steel consisting of P: 0.012% or less, Si: 0.02% or less, Cu: 0.1% or less, Sn: 0.01% or less, Al: 0.02% or more and 0.04% or less, and other Fe and unavoidable residual elements In direct heating after direct rolling, induction heating temperature is 1050 ℃ or less and hot rolling is performed by conventional thin slab direct rolling method, but thin slab direct rolling method which finish rolling temperature is 850 ℃ or more and 880 ℃ or less. It is achieved by a method for producing a hot rolled steel sheet for high-molded steel pipe by.

1 is a diagram showing a relationship between an Mn / S ratio and an edge crack average depth.

2 is a view showing a change in tensile strength according to the carbon content.

Hereinafter, the present invention will be described in detail.

In the present invention, in the thin slab direct rolling process having a rough rolling facility, C is less than 0.04% by weight (less than%) for the purpose of easily manufacturing SPHT1, which is the softest of the hot rolled steel sheets for steel pipes, and preventing edge cracking of hot rolled steel strips. , Mn content is 0.3% or more and 0.5% or less, S is 0.012% or less, Mn / S ratio is maintained at 35 or more, P is 0.012% or less, Si is 0.02% or less, Cu 0.1% or less, Sn 0.01% or less , Induction heating temperature after continuous casting-co-rolling and direct-rolling of steel composed of more than 0.02% and less than 0.04% of Al content and other unavoidable residual elements is 1050 ° C or less, and the conventional slab direct rolling method but the hot rolling finish rolling temperature is the tensile strength of 41kgf / mm ² or less, goyeon hot-rolled steel sheet for forming steel pipes according to the foil slab directly rolling in excess of the elongation 41%, based on the thickness 3mm recipe to ensure that the less than 850 ℃ 880 ℃ .

The reason for limiting these as mentioned above is demonstrated below.

Hot rolled steel sheets for steel pipes are classified into material grades ranging from SPHT1 to T4 according to their tensile strength.T1 materials used for home and industrial piping materials have a small diameter, so those with a tensile strength of 30 to 45kgf / mm ² and elongation of 35 to 42% It is used. The T1 material produced by the conventional blast furnace method is mainly 5mm or less in thickness, the tensile strength is 40kgf / mm ² , the level of elongation 40%. Recently, with the introduction of the thin slab direct rolling method, the development of technology to replace the material corresponding to the blast furnace material is progressing.However, due to the characteristics of the thin slab direct rolling method, the cooling rate of the cast slab is high and the amount of plastic organic precipitation during rolling increases the strength. Not easy at In addition, in order to reduce the strength to the blast furnace material level, the adjustment of the elements, which is helpful for reducing the strength, that is, lowering Mn, and increasing the S and Al content, various surface cracks and edge cracks occur, making it difficult to secure the surface quality. Therefore, the best way is to lower the carbon content. In the present invention, the carbon content is limited to 0.04% or less in order to obtain a tensile strength of 42 kgf / mm ² or less. Although the lower limit of carbon is not specified in the present invention, considering the recent capacity and economic feasibility, the lower limit is determined to be 0.02%. Mn content and S content are very important in the present invention because they directly relate to surface quality such as strength and edge cracks of the product. Although Mn is preferably contained less as an element to increase the strength, FeS existing in a liquid state at high temperature is precipitated as a solid as MnS, which has an effect of suppressing edge cracking. Edge cracks occur for two reasons. Firstly, the liquid S segregated in the crab at high temperature causes high temperature intergranular brittleness, and secondly, the higher the S content, the more precipitated amounts of FeS, CuS, and MnS precipitated in the ferrite region. When the edge temperature drops, the crack generation site and the propagation-integration of the ferrite become easy, so that the occurrence of edge cracks is easy. In addition, in order to prevent this, by inhibiting FeS and CuS precipitation by adding 0.3% or more of Mn as suggested in the present invention, and growing coarse MnS to lower the deposition density at the liquid phase S and low temperature. Occurrence is suppressed. However, Mn has an upper limit of 0.5% due to the standard system, and further addition results in an increase in strength, so the upper limit is limited to 0.5%. The smaller the amount of S, the more effective the edge crack suppression, and the upper limit thereof was 0.012% in the present invention. However, experimentally, even though the range of Mn and S satisfies the scope of the present invention, edge cracks may occur when the Mn / S ratio is not more than 35. Therefore, Mn and S satisfy the scope of the present invention and Mn. It is preferable that / S ratio satisfy | fills 35 or more.

P was found to have a significant effect on strength contribution as a result of the investigation in the present invention, unlike the conventional steel grades, it was limited to less than 0.012%, which is a rather strict range. Since Si is an undesirable element in terms of strength increase and plating property of the hot rolled steel sheet for steel pipes, Si is limited to 0.02% or less, which is a relatively low range.

Unlike the conventional technology, the Al content was limited to 0.02 to 0.04% in the present invention. As the Al is added, the AlN precipitation increases, and it is found to be effective in reducing the strength by removing the dissolved nitrogen in the steel. On the contrary, it was found to be disadvantageous in terms of edge cracks and surface cracks. As described above, since the deposition density of AlN is higher, the crack generation point promotes edge crack generation and surface crack generation. Therefore, in the present invention, the Al content is limited to 0.04% or less, and if the amount is less than 0.02%, the strength increase due to solid solution nitrogen and the strong oxygen are not sufficiently removed, but rather the problem of generating surface cracks is lower than 0.02%. It was set as. Among the remaining elements, Cu, which promotes edge crack generation, was loaded at 0.1% or less and Sn at 0.01% or less. According to the present invention, methods for minimizing strength and effective methods for securing surface quality and suppressing edge cracks have been described. As described above, Cu and Sn, which cause surface cracks and edge cracks, were strongly inhibited without significant influence on strength and ductility. Cu tends to exist in a molten state to 1100 ℃, the lower temperature to precipitate fine Cu ₂ S. Unlike MnS, it precipitates at low temperatures and has a very fine size, thus providing a place for cracking. Since Sn has a low melting point, a very low solubility in iron, and is not preferable as an element for lowering the solubility of Cu, Cu is limited to 0.1% or less and Sn to 0.01% or less. Such a component range can be produced more easily in the thin slab direct rolling method.

N is not otherwise regulated in the present patent. If it is not denitrified, nitrogen is generally contained in 0.006% to 0.01% of the bar. In the present invention, since the nitrogen content is investigated at the level of 0.01%, N is usually lower than this. And other unavoidable residual elements. The induction heating temperature was set to 1050 ° C. or less in continuous induction heating after continuous casting-cast rolling direct rolling. The reason for this is that by performing heat treatment at a low temperature, a greater amount of masonry is made and a coarse lake is formed, which is effective in reducing strength and suppressing microcracks. Due to the structural configuration of the thin slab direct rolling, since the hot rolling temperature is generally affected by the temperature of the bat after induction heating, the present invention limits the hot finish rolling degree and does not present a lower limit of the induction heating temperature. The minimum of hot finishing rolling temperature was 850 degreeC. This is because dynamic recrystallization is more delayed at low temperatures, resulting in greater material differences in the longitudinal direction and the width direction of the product due to the larger structural differences between the edges and the centers where cooling is more severe. However, the higher the finishing hot rolling temperature, the higher the amount of dissolved solids and the smaller the precipitate was. Therefore, the upper limit was 880 ° C. The hot rolled steel sheet manufactured as described above may be wound in the usual manner, but the present invention does not include the hot rolled coil winding temperature on the material, which is not included in the claims, considering the iron oxide layer formation thickness and material level. The 650 ° C. range is preferred. The production of hot-rolled steel sheets for steel pipes according to the present invention provides superior elongation than conventional blast furnace products, and can significantly improve surface cracks and quality compared to other products manufactured by thin slab direct rolling. Have

Hereinafter, embodiments of the present invention will be described.

In the well-known thin slab direct rolling method, after rough rolling, a bar is wound in a coil box to maintain a predetermined time and finish rolling to a predetermined thickness (aka ISP method) and to keep the slab without rough rolling. Then, it is wound into a coil box, maintained for a certain time, and then roughly rolled into finishing method (aka CSP method). The former has the advantage that it is possible to manufacture the product of the thin product after finishing rolling after rough rolling, but has a problem that the generation of edge cracks is easy. The present invention was tested in the thin slab direct rolling method with the former rough rolling.

The test specimens were cast in a vacuum induction furnace with a thickness of 80 mm and a width of 170 mm, and then the high-temperature oxide film was removed with a high-pressure gas and rough-rolled immediately before the rough rolling, referring to the thermal history calculated by computer by heat transfer theory. . The bar thickness after rough rolling is 25 mm and the rough rolling reduction rate is 58%. Some of the bars prepared for evaluating edge cracks and surface microcracks were ground to the center in the thickness direction and then machined with sandpaper up to 1000 times, and then exposed to fine cracks through etching in a weak acid. The crack depth was measured with vernier calipers of about 1 / 100mm and the average crack depth was measured. The remaining samples were sheared after rough rolling at 1040 to 1050 ° C. and finally the finish rolling temperature was set as desired. After the annual rolling was completed, the furnace was maintained for 1 hour in a furnace preheated to 600 ° C., and then cooled by heat, followed by heat treatment of hot rolled winding equivalent, and then the tensile test specimen of JIS No. 5 was taken in the rolling direction to evaluate mechanical properties.

Residual components in the steel produced in the present invention was set to the same level as used in the conventional thin slab direct rolling method, and unless otherwise specified, Cr is 0.04%, Ni is 0.03%, and N is 0.09 to 0.011%. It was.

Figure 1 shows the relationship between the Mn / S ratio and the average depth of the edge cracks by measuring the average depth of the evi crack with respect to the Mn / S ratio for the bar of the sample. It can be seen that the Mn / S ratio for preventing the edge crack is 35 or more in the composition range of the present invention.

Figure 2 shows the change in tensile strength according to the carbon content, showing the tensile strength in the carbon content among the other components among those shown in Table 1 below the scope of the present invention. Tensile Strength 41kgf / mm ² Hereinafter, the present invention will be described in more detail with reference to Examples.

Table 1 below shows an example of manufacturing a hot rolled steel sheet for high-molded steel pipe by the thin slab direct rolling method. These are manufactured as described above. The invention methods 'a' to 'da' are those whose composition and manufacturing process variables fall within the scope of the present invention, which satisfies a tensile strength of 41 kgf / mm ² or less and no edge cracks or surface cracks occur. In particular, the elongation is more than 41%, showing superior ductility compared to the blast furnace method. This is considered to be a characteristic of the thin slab direct rolling method because the uniform ferrite of the polygon is formed systematically.

The steel grade 'la', which is a comparative method, was satisfied, but the tensile strength was high because the temperature of the bar was high as 1100 ℃ after induction heating. The hemp is 0.05% carbon, the bars have a high Mn content, and the yarns have a high P content, exceeding all of the target tensile strengths. Steel grade 'Ah' has high strength because of high Si content, but many scales occurred on the surface. It does not have good pickling properties, which hinders galvanization. Steel grades 'za' to 'ka' could secure the target tensile strength, but the high Al content and low Mn / S ratio resulted in edge cracks with an average depth of 1 to 3.5mm. Steel grade 'Ka' can secure the target tensile strength and elongation, but a small amount of edge cracks and surface cracks occurred due to the high content of Cu and Sn. Therefore, as can be seen in the embodiment of Table 1, according to the present invention, surface cracks are suppressed, surface and plating properties are excellent, and for high-ductility steel pipes having an elongation of 41 kgf / mm ² or less and an elongation of more than 41% based on a thickness of 3 mm. Hot rolled steel sheet can be easily produced by the thin slab direct rolling method.

C Mn P S Mn / S Si Cu Sn Al IHDT FDT TS El Surface condition Invention end 0.03 0.4 0.011 0.011 36.36 0.01 0.08 0.008 0.04 1050 860 39 42.77 o I 0.02 0.35 0.006 0.008 43.75 0.01 0.05 0.004 0.03 1045 880 39.33 43.2 o All 0.04 0.5 0.01 0.012 41.67 0.02 0.08 0.006 0.02 1050 864 40.81 42.99 o Comparative method la 0.04 0.5 0.01 0.005 100 0.02 0.08 0.006 0.02 1100 890 42.33 39.98 o hemp 0.05 0.45 0.01 0.008 56.25 0.02 0.08 0.006 0.02 1050 862 42.56 40.31 o bar 0.04 0.6 0.009 0.005 120 0.02 0.07 0.007 0.03 1050 870 41.81 41.52 o four 0.04 0.45 0.015 0.008 56.25 0.02 0.08 0.007 0.03 1050 873 41.14 40.96 o Ah 0.04 0.5 0.011 0.005 100 0.035 0.08 0.008 0.025 1050 878 41.94 41.12 Surface scale character 0.04 0.3 0.009 0.008 37.5 0.02 0.08 0.007 0.05 1050 880 39.23 40.88 Edge crack car 0.04 0.25 0.011 0.02 12.5 0.02 0.08 0.002 0.04 1043 880 38.45 40.84 Edge crack Ka 0.04 0.31 0.009 0.007 44.29 0.02 0.12 0.011 0.04 1040 909 40.24 41.72 Surface crack

Ingredients: weight percent

-IHDT: Bar temperature after induction heating (℃)-FDT: Hot finishing rolling temperature (℃)

-TS: Tensile Strength (kgf / mm2)-El: Elongation (%)

In the present invention, by controlling the C, P, Si, etc. among the alloy elements that can be easily regulated and adjusted in the thin slab direct rolling method, while controlling the process factors such as hot finish rolling temperature, the softest SPHT1 material among the hot rolled steel sheet for steel pipes As a method of manufacturing the same, the ductility is superior to the product manufactured by the blast furnace method, and the similar tensile strength and surface quality is obtained, as well as an excellent effect of remarkably improving the product error rate by preventing hot edge cracking.

Claims (1)

By chemical composition, C: 0.04% by weight (% or less), Mn: 0.3% or more and 0.5% or less, S: 0.012% or less, Mn / S ratio is maintained at 35 or more, P: 0.012% or less, Si: 0.02% or less, Cu: 0.1% or less, Sn: 0.01% or less, Al: 0.02% or more and 0.04% or less, in continuous casting-cast rolling, induction heating after direct rolling, Induction heating temperature is 1050 ℃ or less and hot rolled by the conventional thin slab direct rolling method, the finish rolling temperature for high-molded steel pipe by the thin slab direct rolling method characterized in that the 850 ℃ or more. Hot rolled steel sheet manufacturing method.