KR100327792B1 - Method for manufacturing hot rolled steel sheet for pipe by thin slab direct rolling process - Google Patents

Abstract

PURPOSE: A method for manufacturing a hot rolled steel sheet for pipes by thin slab direct rolling process is provided to produce hot rolled steel sheets with a tensile strength of less than 41 kgf/mm¬2 and an elongation ratio of higher than 41 %, based on a hot rolled steel sheet with a thickness of less than 3 mm. CONSTITUTION: The method for manufacturing a hot rolled steel sheet for pipes by thin slab direct rolling process includes step of continuous casting a steel comprising 0.04 wt.% or less of C, Mn 0.3 wt.% to 0.5 wt.%, 0.012 wt.% or less of S, 0.012 wt.% or less of P, 0.02 wt.% or less of Si, 0.1 wt.% or less of Cu, 0.01 wt.% or less of Sn, Al 0.02 wt.% to 0.04 wt.%, a balance of Fe and incidental impurities, wherein Mn/S is between 35 and 55; rough rolling the continuous cast steel; direct rolling the rough rolled steel strip; induction heating the direct rolled steel strip at less than 1050°C; hot rolling the induction heat treated steel strip by thin slab direct rolling process in such a condition that the temperature of finish rolling is controlled to be in the range of 850°C to 880°C.

Description

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

The present invention relates to a method for manufacturing a hot rolled steel sheet for steel pipe by the thin slab direct rolling method, in particular to adjust the C, P, Si, etc. among the alloy elements that can be easily regulated and adjustable in the thin slab direct rolling method while hot finishing rolling temperature It is possible to easily manufacture the softest SPHT1 material among the hot rolled steel sheets for steel pipes by controlling the process factors, and as compared with the SPHT1 products manufactured by the blast furnace method, it has improved ductility and similar tensile strength and surface quality. Of course, the present invention relates to a method for manufacturing a hot rolled steel sheet for steel pipe by a thin slab direct rolling method, which can significantly improve a 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 process is effective in energy savings because of low equipment costs, the manufacture of hot rolled steel sheets of thin metal, and no reheating to hot roll the continuously cast 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 ductility than the conventional blast furnace manufacturing method. It is not easy to manufacture SPHT1 material for steel pipes (JIS, G3132) that requires proper 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 error rate. The above-mentioned edge crack is a kind of high temperature brittleness of steel. When processing is performed when liquid metal or cornerstone ferrite is present in the austenite grain boundary, the cracks are easily formed and propagated using the grain boundary precipitate or grain triple point as nuclei. This is a tearing phenomenon. Therefore, it is necessary to reduce the number of precipitates as much as possible, and to precipitate the low-melting point compounds such as FeS remaining in the liquid phase as coarse precipitates at a higher temperature.

The present invention devised the above solution, hot-rolled steel pipe by controlling the process factors such as hot finish rolling temperature while adjusting C, P, Si, etc. among the alloy elements that can be easily regulated and adjustable in the thin slab direct rolling method As a method of easily manufacturing the softest SPHT1 material among steel sheets, the ductility is improved and the tensile strength and surface quality are similar to those of the blast furnace method. It is an object of the present invention to provide a method for manufacturing a hot rolled steel sheet for steel pipe by the thin slab direct rolling method which can significantly improve the efficiency of the steel sheet.

1 is a view showing a change in the average depth of the edge cracks according to the Mn / S ratio.

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

In order to achieve the above object, the present invention provides a weight% of C: 0.04 or less, Mn: 0.3% to 0.5%, S: 0.012% or less, and maintains Mn / S ratio of 35 to 55, P: 0.012% Below, Si: 0.02% or less, Cu: 0.1% or less, Sn: 0.01% or less, Al: 0.02% to 0.04%, and other unavoidable residual steels after continuous casting-rolling direct rolling to guide the steel back to 1050 ° C or less. It is heated and hot-rolled by a conventional thin slab direct rolling method, but provides a method of manufacturing a hot rolled steel sheet for steel pipes by the thin slab direct rolling method characterized in that the finishing rolling temperature is controlled to be in the range of 850 ℃ ~ 880 ℃. .

Hereinafter, the present invention will be described in detail.

In the present invention, in the thin slab direct rolling process equipped with a rough rolling facility, the softest SPHT1 material among the hot rolled steel sheets for steel pipes is easily manufactured, and C is less than 0.04 in weight% for the purpose of preventing edge cracking of the hot rolled steel sheets. , Mn is 0.3% to 0.5%, S is 0.012% or less, Mn / S ratio is maintained at 35 to 55, P is 0.012% or less, Si is 0.02% or less, Cu is 0.1% or less, Sn 0.01 % Or less, Al is 0.02% to 0.04%, induction heating temperature is controlled to 1050 ℃ or less in continuous induction heating after continuous casting-corolling direct-rolling steel composed of other unavoidable residual elements. Hot rolling by direct rolling method, but the finish rolling temperature is controlled to be in the range of 850 ℃ to 880 ℃, and the hot rolling for steel pipe by thin slab direct rolling method with tensile strength less than 41kgf / mm ² and elongation exceeding 41% based on thickness 3mm Steel plate manufacturing method.

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 tensile strength. SPHT1 materials used for home and industrial piping materials have small pipe diameters, so those with tensile strength of 30 to 45 kgf / mm ² and elongation of 35 to 42%. It is used. The SPHT1 material produced by the current blast furnace method is mainly 5mm or less in thickness and has a tensile strength of 40kgf / mm ² and an elongation of 40%. Recently, with the introduction of the thin slab direct rolling method, the development of technology to replace the material corresponding to the SPHT1 material produced by the blast furnace method is in progress, but due to the characteristics of the thin slab direct rolling method, the cooling rate of the cast steel is high, and the amount of plastic organic precipitation during rolling Because of this many, it is not easy in terms of securing strength. In addition, in order to lower 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 contents, various surface cracks and edge cracks are generated, making it difficult to secure good 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 41 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 MnS, thereby 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 introducing 0.3% or more of Mn as suggested in the present invention, and by growing the coarse MnS by lowering the density of precipitation in the liquid phase and low temperature, the edge cracks. The occurrence of 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%.

Since S is more effective in suppressing edge cracks as the amount is reduced to a small amount, the upper limit thereof is set to 0.012% in the present invention. However, experimentally, as shown in FIG. 1, even if the range of Mn and S satisfies the scope of the present invention, an edge crack may occur when the Mn / S ratio is not more than 35. Since there is a problem that the required high ductility is not secured, it is preferable that Mn and S satisfy the scope of the present invention and that the Mn / S ratio satisfies 35 to 55.

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 grade 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 appeared 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 upper limit of the Al content is limited to 0.04%, and if the content is less than 0.02%, there is a problem of generating surface cracks due to insufficient strength increase and strong oxygen due to solid solution nitrogen. The rate was made 0.02%.

Among the remaining elements, Cu, which promotes edge crack generation, was loaded at 0.1% or less and Sn at 0.01% or less. In the present invention, methods for minimizing strength and effective methods for securing surface quality and edge crack suppression are As mentioned 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. Sn is not preferred as an element having low melting point, very low solubility in iron, and lowering the solubility of Cu. Therefore, Cu was limited to 0.1% or less and Sn to 0.01% or less.

N is not regulated separately in the present patent. If it is not denitrified, 0.006 to 0.01% of nitrogen is usually contained in steel. In the present invention, since the nitrogen content is examined at a level of 0.01%, since it 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 a heat treatment at low temperature, a greater amount of weights and coarsening are made, 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 bar after the induction heating, the present invention limits the hot finish rolling degree and does not present the 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. However, since the influence of the hot rolled winding temperature on the material is not large in the present invention, it is not included in the claims, but considering the iron oxide layer formation thickness and the material level, 550 The range of 650 ° C. is preferred. The production of hot-rolled steel sheet for steel pipes according to the present invention is superior in elongation than conventional blast furnace products, and can significantly improve surface cracks and edge cracks compared to products manufactured by other thin slab direct rolling methods. Has

Hereinafter, embodiments of the present invention will be described.

In the well-known thin slab direct rolling method, after performing rough rolling, the bar is wound in a coil box and maintained for a predetermined time to 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 tested specimens were cast from the vacuum induction furnace with the composition ranges in Table 1 to 80 mm thick and 170 mm wide, and then the high-temperature oxide film immediately before rough rolling by referring to the thermal history calculated by computer by heat transfer theory. It was removed with a high pressure gas and rough rolled. The bar thickness after rough rolling was 25 mm, and the rough rolling reduction rate was 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 a vernier caliper of about 1/100 mm and the average crack depth was measured. The remaining sample was sheared after rough rolling, charged into a furnace heated to 1040 to 1050 ° C., preserved for about 3 minutes, and then subjected to repeated rolling three times to produce a hot rolled steel sheet having a final thickness of 3 mm. At this time, the reduction ratio was about 88%. Rolling start temperature was carried out at 1000 ~ 1050 ℃ and finally the finish rolling temperature was set to 850 ~ 880 ℃ as desired. After hot rolling was completed, the furnace was maintained for 1 hour in a furnace preheated to 600 ° C., and then cooled to a heat treatment at a temperature corresponding to the hot rolling temperature, 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 average depth of the edge crack according to the Mn / S ratio to show the relationship between the Mn / S ratio and the edge crack average depth of the specimen. As shown in this figure, it can be seen that the Mn / S ratio of the present invention for preventing edge crack is 35 to 55.

Figure 2 shows the change in tensile strength according to the carbon content, as shown in Table 1, showing the tensile strength according to the carbon content of the other components within the scope of the present invention. In this figure, it can be seen that carbon content of 0.04% or less is required to obtain a tensile strength of 41 kgf / mm ² or less.

The following describes the embodiment of the present invention in more detail with reference to Table 1.

The method of the present invention 'ga' ~ 'da' steel composition of the composition and manufacturing process variables are those within the scope of the present invention, the tensile strength is less than 41kgf / mm ² and did not occur edge cracks and surface cracks. In particular, the elongation is more than 41%, showing superior ductility compared to the comparative 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. 'Ma' has a high carbon content of 0.05%, 'Bar' has a high Mn content, and 'Sa' has a high P content, exceeding the target tensile strength. Steel grade 'Ah' has high strength because of high Si content, but a lot of red scale occurred on the surface. This leads to poor pickling and impairs galvanization. Steel grades 'za' to 'ka' can secure the target tensile strength, but high Al content or low Mn / S ratio resulted in edge cracks of 1 ~ 3.5mm. In addition, steel grade 'Ka' can secure the target tensile strength and elongation, but a small amount of edge crack and surface crack occurred due to 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, and the surface and plating properties are excellent, and for high-strength 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.

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 Compared to the product manufactured by the blast furnace method, it was more ductile and secured similar tensile strength and surface quality as well as preventing hot edge cracking.

Claims (1)

C: 0.04 or less, Mn: 0.3% to 0.5%, S: 0.012% or less, Mn / S ratio of 35 to 55, P: 0.012% or less, Si: 0.02% or less, Cu: 0.1% or less, Sn: 0.01% or less, Al: 0.02% to 0.04%, other steels composed of Fe and unavoidable residual elements are subjected to continuous casting-cast rolling, direct rolling, and then induction heating up to 1050 ° C or lower, Hot-rolled by the slab direct rolling method, the finish rolling temperature is controlled to be in the range of 850 ℃ ~ 880 ℃ hot slab steel sheet manufacturing method for the steel pipe by the thin slab direct rolling method.

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