KR20110010898A - Hot rolled steel sheet with low yield ratio and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A hot rolled steel sheet with a low yield ratio and a method for manufacturing the same are provided to simultaneously improve strength and yield ratio without separate addition elements. CONSTITUTION: A hot rolled steel sheet with a low yield ratio comprises aluminum 0.01~0.06 weight%, sulfur less than 0.005 weight%, carbon 0.03~0.08 weight%, manganese 1.0~1.3 weight%, silicon 0.1~0.2 weight%, niobium 0.01~0.03 weight%, vanadium 0.03~0.06 weight%, P less than 0.02 weight%, and the inevitably contained impurity.

Description

HOT ROLLED STEEL SHEET WITH LOW YIELD RATIO AND METHOD FOR MANUFACTURING THE SAME

The present invention relates to a hot rolled steel sheet having a resistance ratio and a method of manufacturing the same. More specifically, the present invention relates to a hot-rolled steel sheet and a method of manufacturing the same, which satisfy yield yield and strength simultaneously without additional elements by optimizing grain size by applying two-stage cooling on a rolled ROT.

Line pipe steel requires high strength as its main property for high crude oil and natural gas transportation efficiency. Also, it is suitable for use in cold areas, low temperature toughness for low temperature transportation, and corrosion resistance such as beaches. Weldability is needed to improve manufacturing and field workability. These characteristics are essential requirements for the development of crude oil or economic transport of energy in Alaska, Siberia and the cold regions around the Antarctic and Arctic.

Along with these characteristics, another important characteristic required for the stable use of line pipe steel is the yield ratio. Yield ratio is the value obtained by dividing the yield strength by the tensile strength, which indicates the resistance to plastic deformation after failure by external stress, which is closely related to the safety of structural materials. In particular, a pressure difference of more than 100 atmospheres occurs inside the line pipe, which causes a high rate of ductile breakdown similar to brittle fracture rate in the pipe when the yield ratio is high. Therefore, it is important to have a low yield ratio in line pipe material while maintaining the strength as a strictly regulated property.

Yield ratio is a property that depends on yield strength and work hardening rate, and depends on matrix structure, type and fraction of secondary phase, and carbon content. As a method for obtaining a resistance ratio, conventionally, a method of forming a low temperature transformation phase by adding a hardening enhancing element such as Mo or Cr or rapidly cooling during winding or by increasing the carbon content by interacting with a large amount of solid solution carbon and dislocation The method of lowering the yield ratio was used.

However, this conventional method has a high possibility of winding faults when applying low temperature winding to form low temperature transformation as well as a cost burden of using expensive hardened elements and equipment limitations for rapid cooling. In addition, in recent years, the low carbon (less than 0.1wt.%) Is designed to secure low-temperature toughness, so it is difficult to increase the carbon content only for securing the resistance ratio.

Meanwhile, various methods are being studied according to the demand for high strength and low temperature toughening of the material, and the most influential means is grain refinement. In order to refine the grains, a small amount of alloying elements such as Nb and V may be appropriately added or the strength may be increased through controlled rolling. However, strength and yield ratios have opposite correlations to the microstructural variables that are common to each other, and therefore, there is a need for a technique that appropriately adjusts grain size to a desired characteristic. In particular, as the thickness of the steel sheet is thinner, the total rolling reduction increases, so that the grains are more easily refined. As a result, the yield strength increases but the yield ratio also increases, so it is not easy to control the optimum conditions in the actual rolling process. Therefore, in order to develop a steel having low yield ratio and excellent low temperature toughness and at the same time high strength, research on the independent influence of the microstructure on each property and the actual manufacturing conditions for implementing the same must be preceded.

In general, hot rolled steel sheet is produced by reheating a slab manufactured by continuous casting in a heating furnace and winding through rough rolling and finishing rolling processes. At this time, after the end of rolling, the cooling zone (ROT) is sprayed with water to the target winding temperature (CT) to go through the cooling process and wound up. Conventional cooling patterns include a front-end cooling method that injects water from the front of the Run Out Table (ROT) or a rear-end cooling method that injects water from the rear end, depending on the target material and microstructure. Will be designed. In particular, in the case of thin thin plates, the yield ratio is increased to about 0.92 when the existing cooling pattern is applied.

The technical structure described above is a background technique for assisting the understanding of the present invention, and does not mean the prior art widely known in the technical field to which the present invention belongs.

The present invention is to improve the above problems, to provide a hot-rolled steel sheet and a method of manufacturing the same that can be applied to a 6.3mm API-X56 grade line pipe thickness by satisfying the yield ratio and strength at the same time without additional elements. There is this.

One aspect of the present invention relates to a hot rolled steel sheet having a resistance ratio. The hot rolled steel sheet is 0.03 to 0.08% by weight of carbon (C), 1.0 to 1.3% by weight of manganese (Mn), 0.1 to 0.2% by weight of silicon (Si), 0.01 to 0.03% by weight of niobium (Nb), and 0.03 to 0.03% of vanadium (V). It is contained in 0.06% by weight, the remaining content is made of iron (Fe) and impurities inevitably contained in the iron, characterized in that the yield ratio is 0.84-0.88%.

In a specific embodiment, the hot rolled steel sheet may further include components selected from 0.02 wt% or less of phosphorus (P), 0.005 wt% or less of sulfur (S), and 0.01 to 0.06 wt% of aluminum (Al).

In a specific embodiment, the hot rolled steel sheet has a tensile strength of 490 to 758 Mpa measured at a rate of 20 mm / min using a 25 ton Zubi tensile tester for a tensile test piece of API 5L (2 "Gage Length, W 38.1 mm), The yield strength is in the range of 386-544 Mpa.

The hot rolled steel sheet has a metal structure of ferrite phase and a small amount of bainite phase, and the fraction of bainite is 0.001 to 5%.

Another aspect of the present invention relates to a method for manufacturing a hot rolled steel sheet having the above resistance ratio. The method comprises 0.03% to 0.08% by weight of carbon (C), 1.0% to 1.3% by weight of manganese (Mn), 0.1% to 0.2% by weight of silicon (Si), 0.01% to 0.03% by weight of niobium (Nb) and 0.03% to vanadium (V). 0.06% by weight, and the remaining content is hot-rolled steel slab composed of iron (Fe) and impurities inevitably contained in the steel sheet; First cooling the hot rolled steel sheet to a ferrite transformation temperature or less; Cooling the primary cooled steel sheet to induce transformation and grain growth; Second cooling the air-cooled steel sheet to a coiling temperature; And winding the cooled steel sheet.

In an embodiment said primary cooling is shear cooling on a runout table (ROT) and cooled to 700-830 ° C.

In an embodiment, the air cooling time is 5 seconds to 20 seconds.

In a specific embodiment said secondary cooling is post-stage cooling on a runout table (ROT) and cooled to 500-600 ° C.

Another aspect of the invention relates to the use of the hot rolled steel sheet. Since the hot rolled steel sheet satisfies the yield ratio and strength at the same time, it can be suitably applied to the line pipe, and particularly preferably applied to the line pipe 6.3mm API-X56 Grade.

Hot rolled steel sheet having a resistance ratio according to the present invention by applying the stepwise cooling on the rolled ROT to optimize the grain size to satisfy the yield ratio and strength at the same time without any additional elements, and to reduce the cost, excellent in terms of manufacturing and use stability There is an effect that can be applied to API-X56 Grade line pipe.

Hereinafter, a hot rolled steel sheet having a resistance ratio of the present invention and a manufacturing method thereof will be described in detail.

Hot rolled steel

In a specific embodiment, the hot rolled steel sheet of the present invention includes 0.03 to 0.08 wt% of carbon (C), 1.0 to 1.3 wt% of manganese (Mn), 0.1 to 0.2 wt% of silicon (Si), 0.01 to 0.03 wt% of niobium (Nb), and vanadium. (V) 0.03 to 0.06% by weight, and the remaining content is composed of iron (Fe) and impurities inevitably contained in the iron.

In another embodiment, the hot rolled steel sheet may further include components selected from 0.02 wt% or less of phosphorus (P), 0.005 wt% or less of sulfur (S), and 0.01 to 0.06 wt% of aluminum (Al).

Hereinafter, the content and properties of each component will be described.

Carbon (C): 0.03-0.08 weight%

Carbon element is added to secure the strength of the steel sheet. In the present invention, the content of carbon is limited to 0.03 to 0.08% by weight, and when produced in low carbon, it is possible to secure low temperature toughness and have excellent weldability and workability. If the carbon content exceeds 0.08% by weight, pearlite may increase, resulting in a crack source, and workability and weldability may be deteriorated.

manganese( Mn ): 1.0-1.3 wt%

Manganese is an austenite stabilizing element that is effective for solid solution strengthening. When the manganese content is more than 1.3% by weight, weldability may be lowered, and when it is less than 1.0% by weight, tensile strength may be lowered.

silicon( Si 0.1 to 0.2 wt%

Silicon acts as a deoxidation or solid solution strengthening element of molten steel and is added at 0.1 to 0.2% by weight. If the silicon is less than 0.1% by weight, it is difficult to obtain clean steel, and if it exceeds 0.2% by weight, ERW weldability may be impaired.

Niobium [ Nb ]: 0.01 to 0.03 wt%

Niobium is finely precipitated during hot rolling to improve strength, and niobium as a solid solution suppresses the transformation of austenite into pearlite. In the present invention, the content of niobium is 0.01 to 0.03% by weight. If the niobium content exceeds 0.03% by weight, the yield strength rises, making it difficult to obtain a resistance ratio. In addition, when the niobium content is less than 0.01% by weight, excellent strength cannot be obtained.

Vanadium [V]: 0.03-0.06 wt%

Vanadium is precipitated during winding after rolling, and has an effect of improving strength. In the present invention, the vanadium content is limited to 0.03 to 0.06% by weight, but if less than 0.03% by weight, the strength may be lowered. When the amount of vanadium is more than 0.06% by weight, excessive vanadium carbide may be precipitated to reduce toughness.

Phosphorus (P): 0.02 wt% or less

Phosphorus is a useful element for securing the strength of materials. However, when the content of phosphorus exceeds 0.02% by weight, not only workability but also weldability may decrease. Preferably the content of phosphorus is at most 0.015% by weight, more preferably at most 0.01% by weight and most preferably at most 0.007% by weight.

Sulfur (S): 0.005 wt% or less

Increasing the content of sulfur to form emulsion-based inclusions (MnS, etc.), may cause cracks and the like. In the present invention, since a large amount of manganese is added, the sulfur content is preferably lower. In specific embodiments, the sulfur content is 0.005% by weight or less, more preferably 0.003% by weight or less, most preferably 0.002% by weight or less.

aluminum( Al ): 0.01 to 0.06 wt%

Aluminum is a component that serves as a deoxidizer, and keeps the amount of dissolved oxygen in the steel sufficiently low. When the content of aluminum exceeds 0.06% by weight, the workability may be inhibited, so it is preferable to limit the amount to 0.01 to 0.06% by weight.

The steel sheet of this invention contains the said component, and remainder is substantially iron (Fe) and an unavoidable impurity, and oxygen (O) etc. are contained as an element contained according to conditions, such as a raw material, a material, and a manufacturing facility.

Hot rolled steel sheet manufacturing method

The hot rolled steel sheet of the present invention is continuously cast molten steel 0.03 to 0.08% by weight of carbon (C), 1.0 to 1.3% by weight of manganese (Mn), 0.1 to 0.2% by weight of silicon (Si), 0.01 to 0.03% by weight of niobium (Nb) And vanadium (V) in an amount of 0.03 to 0.06% by weight, and the remaining content is iron (Fe) and a steel slab having a composition consisting of impurities inevitably contained in the iron, and then the steel slab is hot rolled in the form of a steel sheet. and; First cooling the hot rolled steel sheet to a ferrite transformation temperature or less; Cooling the primary cooled steel sheet to induce transformation and grain growth; Second cooling the air-cooled steel sheet to a coiling temperature; The steel sheet may be manufactured by winding the cooled steel sheet.

In the specific example, the extraction temperature in the hot rolling step may be 1150 to 1250 ° C and the rolling finish temperature may be 800 to 900 ° C.

The hot rolled steel sheet is subjected to two-stage cooling. Thus, by optimizing the grain size through two-stage cooling, the yield ratio and strength can be simultaneously satisfied without additional elements.

When the shear cooling method or the post-stage cooling method is applied, the final microstructure is composed of fine ferrite and a small amount of pearlite, whereas when two-stage cooling is applied as in the present invention, the microstructure is composed of ferrite and some bainite grown during air cooling. . Thus, in order to obtain the target microstructure during cooling, it is necessary to consider the transformation behavior according to the component system, and it is necessary to set the optimum air cooling temperature, air cooling time and winding temperature.

In an embodiment the primary cooling is achieved by spraying water at the ROT front end (Bank # 1). The hot rolled steel sheet is cooled below the ferrite transformation temperature through the shear cooling. In a specific example, it cools to 700-830 degreeC. After reaching the target temperature at Bank # 3 ~ # 4, go through air cooling. Transformation and grain growth occur during air cooling, which can lower grain yield by coarsening grains. In a specific example, air cooling time is 5 second-20 second, Preferably it is 6 second-15 second, More preferably, it is 7 second-10 second.

The steel sheet subjected to the air cooling is subjected to secondary cooling up to the coiling temperature. In the exemplary embodiment, the secondary cooling is performed by spraying water again at a rear end (Bank # 9) to form a low temperature transformation phase in the secondary phase through such rear end cooling. In an embodiment the secondary cooling is cooled to 500-600 ° C.

The hot-rolled steel sheet of the present invention manufactured by the above method was measured for a tensile test piece of API 5L (2 "Gage Length, W 38.1mm) at a rate of 20 mm / min using a 25 ton TZBIC tensile tester at 490 It is -758 Mpa, and yield strength is the range of 386-544 Mpa.

Hot rolled steel sheet of the present invention satisfies the yield ratio and strength at the same time can be used suitably for the line pipe. In particular, it is preferably applied to a 6.3 mm thick API-X56 Grade line pipe.

The invention can be better understood by the following examples, which are intended for the purpose of illustration of the invention and are not intended to limit the scope of protection defined by the appended claims.

Example

Example  One

After producing the slab continuously cast according to the composition and content as shown in Table 1, the extraction temperature was set to 1200 ℃, the rolling finish temperature was set to 880 ℃. After shear cooling in ROT Bank # 1 it was cooled to 780 ℃. After 8 seconds of air cooling in Banks # 2 to # 8, it was post-cooled in Banks # 9 to # 11. The specimen was prepared by winding at 550 ° C. A schematic of the cooling on the ROT is shown in FIG. 1. The yield ratio of the prepared specimens was measured and shown in Table 2, and the tissue photograph taken at 200 times magnification with an optical microscope for the specimen is shown in FIG.

Comparative example  One

After producing the slab continuously cast according to the composition and content as shown in Table 1, the extraction temperature was set to 1200 ℃, rolling finish temperature was 840 ℃. Thereafter, only shear cooling was performed in Bank # 1, followed by winding at 550 ° C. to prepare a specimen. The yield ratio of the prepared specimens was measured and shown in Table 2, and the tissue photograph taken at 200 times magnification with an optical microscope for the specimen is shown in FIG. 3.

Comparative example  2

After producing the slab continuously cast according to the composition and content as shown in Table 1, the extraction temperature was set to 1200 ℃, rolling finish temperature was 850 ℃. After 8 seconds of air cooling at 830 ° C., the latter stages were cooled in Banks # 9 to # 11. The specimen was prepared by winding at 610 ° C. The yield ratio of the prepared specimens was measured and shown in Table 2, and the tissue photograph taken at 200 times magnification with an optical microscope for the specimen is shown in FIG. 4.

Comparative example  3

After producing the slab continuously cast according to the composition and content as shown in Table 1, the extraction temperature was set to 1200 ℃, the rolling finish temperature was set to 880 ℃. Thereafter, shear cooling was performed in Bank # 1 and cooled to 780 ° C. After 8 seconds of air cooling in Banks # 2 to # 8, it was post-cooled in Banks # 9 to # 11. The specimen was prepared by winding at 660 ° C. The yield ratio of the prepared specimens was measured and shown in Table 2, and the tissue photograph taken at 200 times magnification with an optical microscope for the specimen is shown in FIG. 5.

Comparative example  4

After producing the slab continuously cast according to the composition and content as shown in Table 1, the extraction temperature was set to 1200 ℃, the rolling finish temperature was set to 880 ℃. Thereafter, shear cooling was performed in Bank # 1 and cooled to 680 ° C. After 8 seconds of air cooling in Banks # 2 to # 8, it was post-cooled in Banks # 9 to # 11. The specimen was prepared by winding at 570 ° C. The yield ratio of the prepared specimens was measured and shown in Table 2, and the tissue photograph taken at 200 times magnification with an optical microscope for the specimen is shown in FIG. 6.

C Mn Si Al P S Nb V 0.065 ↓ 1.10 0.13 0.025 0.015 ↓ 0.005 ↓ 0.020 0.045

Extraction temperature
(℃) FDT (℃) Air cooling temperature (℃) Air cooling time Winding temperature (℃) Yield fee Example 1200 880 780 8 sec 550 0.86 Comparative Example 1 1200 840  -  - 580 0.90 Comparative Example 2 1200 850 830 8 sec 610 0.88 Comparative Example 3 1200 880 780 8 sec 660 0.87 Comparative Example 4 1200 880 680 8 sec 570 0.89

As shown in Table 2, in Example 1, the water is injected at the front end to cool down to 780 ° C., and then ferrite transformation and growth occurs during air cooling from Bank # 3 to the rear end to coarse the ferrite grains. At the rear end, it is cooled to 550 ° C and wound up. The pearlite transformation does not occur below the winding temperature of 550 ° C., and some bainite is produced. Thus, the final tissue of Example 1 consists of coarse ferrite and a small amount of bainite. In this case, the yield ratio is 0.86, which is lower than that of the existing cooling method.

In the case of Comparative Example 1 to which shear cooling is applied, ferrite, which is a metamorphic structure, is also generated finely because the time for austenite recrystallization or growth is very short after finishing rolling, and some perlite is also generated, resulting in a high yield ratio of 0.90. In the case of Comparative Example 2 to which post stage cooling was applied, air cooling was maintained up to the rear stage of ROT, and in the bank # 3 ~ 4 during air cooling, the temperature was about 830 ° C in the austenite region. Perlite is produced. In this case, the yield ratio of grain coarsening is lowered to 0.88.

Even in the case of applying two-stage cooling as in Comparative Example 3, when the coiling temperature is 660 ° C., some pearlite is generated. In addition, even when applying two-stage cooling as in Comparative Example 4, even when the air-cooling temperature is lowered to 680 ℃ it can be seen that the effect of lowering the yield ratio. This is because the air cooling temperature is low, the pearlite generation is activated rather than the ferrite growth during air cooling, the pearlite fraction increases.

Graphs comparing yield ratios according to cooling patterns of Comparative Example 1 and Example 1 are shown in FIGS. 7 (A) and (B), respectively, and the comparison of impact toughness by temperature was performed by Charpy V-Notch test. ) And (B), respectively. Is 5.0 t sub-size and ○ is converted to full-size.

Simple modifications and variations of the present invention can be easily made by those skilled in the art, and all such modifications or changes can be seen to be included in the scope of the present invention.

1 is a schematic diagram of cooling of the ROT phase of Example 1 and Comparative Examples 1-2.

Figure 2 is a tissue photograph taken at 200 times magnification with an optical microscope for the specimen prepared in Example 1.

Figure 3 is a tissue photograph taken at 200 times magnification with an optical microscope for the specimen prepared in Comparative Example 1.

Figure 4 is a tissue photograph taken at 200 times magnification with an optical microscope for the specimen prepared in Comparative Example 2.

5 is a tissue photograph taken at 200 times magnification with an optical microscope for the specimen prepared in Comparative Example 3.

6 is a tissue photograph taken at 200 times magnification with an optical microscope for the specimen prepared in Comparative Example 4.

7 (A) and (B) show graphs comparing yield ratios according to cooling patterns of Comparative Example 1 and Example 1, respectively.

8 (A) and (B) shows a comparison of impact toughness for each temperature by Charpy V-Notch test for Comparative Example 1 and Example 1.

Claims (9)

0.03% to 0.08% by weight of carbon (C), 1.0% to 1.3% by weight of manganese (Mn), 0.1% to 0.2% by weight of silicon (Si), 0.01% to 0.03% by weight of niobium (Nb) and 0.03% to 0.06% by weight of vanadium (V) It contains, the remaining content is made of iron (Fe) and impurity inevitable contained in the iron and the yield ratio is a hot-rolled steel sheet having a resistance yield ratio, characterized in that 0.84 ~ 0.98%. The method of claim 1, The hot rolled steel sheet further comprises a component selected from phosphorus (P) 0.02% by weight or less, sulfur (S) 0.005% by weight or less and aluminum (Al) 0.01 to 0.06% by weight. The method according to claim 1 or 2, The hot rolled steel sheet has a tensile strength of 490 to 758 Mpa measured at a rate of 20 mm / min using a 25 ton TZBIC tensile tester for a tensile test piece of API 5L (2 "Gage Length, W 38.1mm), and yield strength Hot rolled steel sheet, characterized in that 386 ~ 544Mpa. The method according to claim 1 or 2, The hot rolled steel sheet has a ferrite phase and bainite metal structure, the fraction of the bainite is a hot rolled steel sheet, characterized in that 0.001 to 5%. 0.03% to 0.08% by weight of carbon (C), 1.0% to 1.3% by weight of manganese (Mn), 0.1% to 0.2% by weight of silicon (Si), 0.01% to 0.03% by weight of niobium (Nb) and 0.03% to 0.06% by weight of vanadium (V) A steel slab comprising iron (Fe) and an impurity inevitably contained in the iron in a steel sheet form; First cooling the hot rolled steel sheet to a ferrite transformation temperature or less; Cooling the primary cooled steel sheet to induce transformation and grain growth; Second cooling the air-cooled steel sheet to a coiling temperature; And Winding the cooled steel sheet; Method for producing a hot rolled steel sheet having a resistive ratio comprising the step. The method of claim 5, The primary cooling is a shear cooling on a runout table (ROT), the method of manufacturing a hot rolled steel sheet, characterized in that the cooling to 700 ~ 830 ℃. The method of claim 5, The air cooling time is a method of producing a hot rolled steel sheet, characterized in that 5 seconds to 20 seconds. The method of claim 5, The secondary cooling is a post-stage cooling on a runout table (ROT), the method of manufacturing a hot rolled steel sheet, characterized in that the cooling to 500 ~ 600 ℃. A line pipe made of the hot rolled steel sheet of claim 1.

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