CN101161847B - High-toughness steel for hot-bending pipe and production method of hot-rolled flat plate thereof - Google Patents

Abstract

The invention provides a high-toughness steel for hot-simmering pipe bending and a production method of the hot-rolled flat plate. The chemical composition of the steel is (weight percent) C: 0.04% to 0.12%, Si: 0.10% to 0.30%, Mn: 1.50% to 1.65%, Nb: 0.04% to 0.08%, Ti: 0.008% to 0.025%, Ni: 0.15% to 0.36%, Mo: 0.15% to 0.45%, Cu: 0.15% to 0.30%, and the balance is iron and unavoidable impurities. The production method of its hot-rolled flat plate includes smelting, refining outside the furnace, and continuous casting. The steel rolling adopts the controlled rolling and controlled cooling process. The heating temperature of the slab is 1160-1220°C, the rough rolling temperature is 1010-1150°C, and the finish rolling temperature is 800°C. ~950°C, the controlled cooling rate after rolling is 15~30°C/s, and the final cooling temperature is 450~600°C. The composition design of the invention is reasonable, and compared with the prior art, the production cost can be reduced by about 15%; the hot-rolled steel plate of the invention can obtain a composite structure mainly composed of bainite, has good low-temperature toughness, and its -20°C impact energy Greater than 220J, the shear area of -15°C DWTT is over 90%, the yield strength of the product is over 520MPa, and the tensile strength is over 570MPa. The toughness of the steel plate of the invention does not decrease after hot bending treatment, and has better strength and toughness.

Description

High-toughness steel for hot-simmering pipe bending and its production method for hot-rolled flat plate

technical field

The invention belongs to the field of low-carbon micro-alloy steel, and in particular relates to a production method for manufacturing high-pressure, large-diameter oil and gas pipeline hot-simmered bent pipe steel and its hot-rolled flat plate.

Background technique

Elbow is one of the important pipe fitting products on the pipeline. It mainly plays two roles on the pipeline. One is to change the direction of the pipeline according to the needs; The tensile and compressive stress and torque attached to the straight pipe due to the change of external environment temperature. The bent pipe is one of the most demanding structural parts in the pipeline. In the pipelines that have been built and operated, the pipeline accidents related to the bent pipe account for a relatively high proportion of the entire pipeline accidents. It can be seen that the comprehensive quality of the bent pipe is high or low. It is one of the bottlenecks restricting the safe and smooth operation of the pipeline.

my country's elbow manufacturing adopts GB/T12459-1990 and SY/T5257-91. The X70-grade bent pipe currently produced is usually made of X70-grade pipeline steel pipe as the parent pipe, which is made after hot-simmering processing. Since the parent pipe is a controlled-rolled steel design, its strength and toughness are greatly reduced after heat treatment. In addition, due to the lower carbon content and more other alloying elements, its production cost is relatively high.

Japanese patent JP2002129288 discloses a high-strength bent pipe and its production method. APIX80-100 steel contains: C≤0.03%, Si≤0.3%, Mn: 0.8%-2.5%, P≤0.015%, S≤0.005%, Nb: 0.01-0.05%, Ti: 0.005-0.030%, Al≤0.05%, N: 0.001-0.06%, and other arbitrary metals Ni: 0.01-1.0%, Cu: 0.1-1.2%, Cr: 0.1-1.0% , V: 0.01-0.10%, Ca: 0.001-0.005%, Mg: 0.0001-0.002%, and the rest is iron. The raw steel pipe is hot-rolled at 800-1000°C, bent and then quenched to obtain the final bent pipe product. Due to the lower carbon content and more other alloying elements, the patented technology has higher production costs, and does not include X70 grade elbows.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies in the prior art, to provide a kind of X70 and X80 high-toughness steel for hot-simmering pipe bending and a method for producing the hot-rolled flat plate. By adjusting the chemical composition of the parent tube to make it suitable for heat treatment, while reducing costs, better strengthening and toughening effects can be achieved.

The chemical composition of the steel for high toughness hot-simmering pipe bending of the present invention is (weight percentage): C: 0.04% to 0.12%, Si: 0.10% to 0.30%, Mn: 1.50% to 1.65%, Nb: 0.04% to 0.08%, Ti: 0.008% to 0.025%, Ni: 0.15% to 0.36%, Mo: 0.15% to 0.45%, Cu: 0.15% to 0.30%, and the balance is iron and unavoidable impurities.

The hot-rolled flat plate production method of high-toughness hot-simmered pipe-bending steel of the present invention includes smelting, refining outside the furnace, and continuous casting. The temperature range is 1010-1150°C, the finish rolling temperature range is 800-950°C, the controlled cooling rate after rolling is 15-30°C/s, the final cooling temperature range is 450-600°C, and then air-cooled.

The reason for the composition design of the high toughness hot-simmering steel for bending pipes of the present invention:

Carbon: As the carbon content increases, the strength of the steel increases, while the toughness and weldability decrease. However, due to the maturation of the controlled rolling and controlled cooling process and microalloying technology, and simultaneously to improve the performance of the welding heat-affected zone (HAZ), the carbon content in the steel gradually decreases. The carbon content should be 0.04% to 0.12%.

Manganese: It has a solid solution strengthening effect, and can also reduce the γ-α phase transition temperature, thereby refining the ferrite grains, which can improve the strength and toughness of the steel at the same time, increasing the Mn content can refine the grains, but if it exceeds 1.65% , center segregation will appear, which will adversely affect casting performance and impact toughness; if it is less than 1.50%, the purpose of improving strength will not be achieved.

Silicon: It is an element that stabilizes ferrite structure and can also inhibit the formation of carbides. This element plays an important role in TRIP steel and DP steel, but not in API steel. In API steel, if the Si content is increased, it will have an adverse effect on the transformation properties and toughness due to the formation of pearlite. Therefore, the present invention limits the content of Si to 0.10%-0.30%.

Niobium: It can delay austenite recrystallization, reduce the phase transition temperature, and obtain the required properties through solid solution strengthening, phase transformation strengthening, precipitation strengthening and other mechanisms. 0.04% ~ 0.08% Nb steel, combined with a reasonable rolling process, can obtain a uniform composite phase mainly composed of bainite and good toughness.

Titanium: After adding a small amount of Ti, the embrittlement temperature zone disappears. This is because TiN is easier to form than Nb(N,C) in the austenite high temperature zone, so N is fixed by TiN in the austenite high temperature zone, and Nb precipitates change from Nb(N,C) to austenite NbC that is difficult to precipitate in the low temperature region and the γ+α dual phase region.

Molybdenum: It can reduce the phase transition temperature, inhibit the formation of massive ferrite, promote the transformation of acicular ferrite, and improve the precipitation strengthening effect of Nb (C, N). The pipeline steel of this alloy system has high density Dislocation fine bainite structure, high strength (up to 520MPa), good impact toughness. In the X70 pipeline steel for hot-simmering and bending, the Mo content is ideal at 0.15% to 0.45%.

Copper and nickel: the strength of steel can be improved through solid solution strengthening, and Cu can also improve the corrosion resistance of steel. The addition of Ni is mainly to improve the hot brittleness easily caused by Cu in steel, and it is beneficial to toughness. In the steel for thick gauge bent pipes, the strength decrease caused by the increase of thickness can also be compensated. The content of copper in the present invention is selected as 0.15%-0.30%, and the content of nickel is selected as 0.15%-0.36%.

The present invention controls the impurity elements in the steel to be P≤0.020%, S≤0.003%, the lower the better if the production is possible.

The hot-rolled plate production method of high-toughness hot-simmering steel for pipe bending of the present invention is made up of following steps:

Raw material preparation according to the technical plan...Molten iron pre-desulfurization...LD converter smelting...External refining...Continuous casting...Slab reheating...Controlled rolling...Controlled cooling...Straightening...Sizing...Flaw detection...Finishing...Inspection...Storage.

Hot rolling process design of the present invention:

Slab heating temperature: The main purpose of slab heating is to prepare for the solid solution of alloy elements and uniform austenite for subsequent rolling deformation. If the heating temperature is too high, it will easily cause the growth of austenite grains, which will affect the quality of the steel. Toughness; if the heating temperature is too low, the alloying elements cannot be fully dissolved, and the effect of alloy strengthening cannot be achieved. Therefore, the selected heating temperature is 1160-1220°C.

Rough rolling temperature: The rough rolling stage is the recrystallization rolling stage, which is above the recrystallization temperature to make the austenite fully refined and recrystallized. If the temperature is too low, it is easy to cause mixed crystals. The present invention sets the rough rolling temperature range as It is 1010～1150℃.

Finish rolling temperature: The finish rolling stage is the rolling stage in the non-recrystallized area. Austenite is elongated and flattened along the rolling direction, and deformation bands are formed in the grain. This work-hardened austenite is easy to promote the formation of ferrite. nuclear. When the temperature is high, mixed crystals are prone to occur, and when the temperature is low, it enters the two-phase region and pro-eutectoid ferrite appears, which is not good for the strength and toughness. Therefore, the finishing temperature range is set at 800-950°C. This finishing temperature range as wide as 150°C creates the possibility for the single-rolled pipeline steel hot-rolled flat plate to complete the dislocation accumulation in the finishing rolling zone and the subsequent grain refinement task, and is also suitable for the pipeline steel hot-rolled flat plate It is characterized by a large temperature drop during finish rolling.

Controlled cooling rate after rolling: Set the controlled cooling rate after rolling to 15-30°C/s, and a composite structure mainly composed of bainite can be obtained, which has good low-temperature toughness.

Final cooling temperature: The final cooling temperature is the interval for controlling the transformation products of the tissue. If the temperature is high, pearlite is prone to appear, resulting in insufficient strength. At low temperature, the number of MA phases increases, resulting in a decrease in toughness. Therefore, the final cooling temperature range is selected as 450-600 ℃.

The composition design of the high-toughness steel for hot-simmering pipe bending of the present invention is reasonable, and its production cost can be reduced by about 15% compared with the prior art; Structure, with good low temperature toughness, its -20°C impact energy is greater than 220J, -15°C DWTT shear area reaches more than 90%, the product yield strength can reach the design requirements of 520MPa or more, and the tensile strength can reach the design requirements of 570MPa or more The toughness of the steel plate with composite structure of the present invention does not decrease after hot bending treatment (heating at 900-1000° C., tempering at 500-560° C.), and has good strength and toughness.

Description of drawings

Accompanying drawing 1 and accompanying drawing 2 are respectively the metallographic structure of the steel X70 and X80 used for high-toughness hot-bending pipe of the present invention.

Detailed ways

See Table 1 for the chemical composition of six examples of high-toughness steel for hot-simmering pipe bending according to the present invention, see Table 2 for the rolling process of the corresponding examples, see Table 3 for the performance test results, and see Table 4 for the performance after hot-simmering.

Table 1 The chemical composition of the steel for high toughness hot-simmering bent pipes of the present invention

Table 2 The rolling process of steel for high toughness hot-simmering pipe bending of the present invention

serial number heating temperature, ℃ Rough rolling temperature range, ℃ Finishing temperature range, ℃ Cooling rate, ℃/s Final cooling temperature, ℃ 1 1194 1100-1010 950-802 twenty one 580

2 1220 1148-1030 950-805 18 594 3 1220 1150-1050 930-800 15 600 4 1160 1130-1010 950-820 26 450 5 1163 1109-1012 950-813 30 467 6 1200 1130-1010 950-820 20 500

Table 3 The performance of steel for high toughness hot-simmering bent pipes of the present invention

Table 4 Properties of the steel for high toughness hot-simmering bent pipes of the present invention after hot-simmering

Claims (2)

1. high-ductility steel for hot-bending bends, the chemical ingredients that it is characterized in that this steel is counted C:0.04%～0.12%, Si:0.10%～0.30%, Mn:1.50%～1.65%, Nb:0.04%～0.08%, Ti:0.008%～0.025%, Ni:0.15%～0.36%, Mo:0.15%～0.45%, Cu:0.15%～0.30% according to weight percent, and surplus is iron and unavoidable impurities.

2. the hot rolled slab production method of the described high-ductility steel for hot-bending bends of claim 1, comprise smelting, external refining, continuous casting, cooling controlling and rolling controlling process is adopted in steel rolling, the Heating temperature that it is characterized in that slab is 1160～1220 ℃, the roughing temperature range is 1010～1150 ℃, and the final rolling temperature interval is 800～950 ℃, and rolling back controlled chilling speed is 15～30 ℃/s, the final cooling temperature interval is 450～600 ℃, afterwards air cooling.

Images