CN104561825A - Low-cost X80 pipeline steel and manufacturing method thereof - Google Patents

Abstract

The invention discloses a low-cost X80 pipeline steel and a manufacturing method thereof. The weight percentages of the components are: C0.065%-0.085%, Mn1.0%-2.0%, Si0.25%-0.35%, Cu0. 10%-0.25%, Ni0.10%-0.30%, Cr0.10%-0.50%, Nb0.02%-0.04%, Ti0.005%-0.03%, V0.02%%-0.04%, Alt0.02 -0.06%, Ca≤0.006%P≤0.015%, S≤0.003%, N≤0.012%, the balance is Fe and trace unavoidable impurities. The hot rolling process is controlled as follows: slab heating temperature: 1100-1200°C; rolling temperature in the recrystallization zone: 900-1150°C; rolling temperature in the non-recrystallization zone: 700-950°C; termination rolling temperature: 700-800°C °C; stop cooling temperature: 350-450 °C, cooling rate: 10-25 °C/s, the advantages and effects of the present invention are: reducing the addition of alloy elements, low cost, production efficiency, and good overall performance of the product.

Description

A low-cost X80 pipeline steel and its manufacturing method

technical field

The invention belongs to the technical field of pipeline steel production, and in particular relates to a low-cost X80 pipeline hot-rolled steel plate and a manufacturing method thereof.

Background technique

In recent years, the changes in energy structure and the increase in energy demand have greatly promoted the development of long-distance oil and gas transmission pipelines. In order to improve transmission efficiency and reduce engineering investment, it has become an inevitable trend to develop steel for long-distance oil and gas transmission pipelines to high-grade steel.

Under the same transportation conditions, the application of high-grade pipeline steel products can reduce the wall thickness of steel pipes, save steel consumption, reduce engineering investment and improve construction efficiency; or increase the transportation pressure under the condition of constant pipe diameter and wall thickness , to achieve the purpose of increasing the conveying capacity. The continuous improvement of pipeline steel grade has become the development trend of pipeline steel. At present, the highest grade of pipeline steel widely used in the world has reached X80 (yield strength Rt0.5≥555MPa), and a small number of test sections have adopted X100 and X120 steel pipes.

As the current mainstream steel grade, X80 has disadvantages such as high addition of alloying elements and heat treatment, resulting in high manufacturing costs. Mo is an alloying element often added to pipeline steel, and its price is expensive. For example, patents such as US5545270A, US5531842, WO2009119570, and WO2009119579 all require the addition of considerable Mo elements, which increases product manufacturing costs. In addition, the patents US5545270A and US5531842 require three-stage rolling of the billet, which increases the complexity of the rolling process, and is rolled in the dual-phase region, with low rolling temperature and heavy equipment load.

Patent CN101456034A provides a kind of X70, X80 pipeline steel and its preparation method based on strain design requirements, and its C content is (0.02-0.05)wt%. C is a cheap strengthening element, increasing its content in an appropriate amount can save the addition of other alloying elements, which is beneficial to reduce costs.

Patent WO2009125863 provides a method for preparing X80 and above grade pipeline steel, in which B, W, Zr, Ta, Mg and other elements are added to the steel, which increases the difficulty of molten steel smelting and slab continuous casting.

Patents JP2009161824, JP2009174020, paper Development of high-deformability linepipe with resistance to strain-aged hardening by HOP (heat-treatment on-line process), JFE Technical Report No.12 (Oct.2008), disclosed X80 pipeline steel preparation process The online heat treatment process is added in the process, which makes the process complicated and the cost of heat treatment is high.

Contents of the invention

Aiming at the problems of low toughness and high cost of the above-mentioned steel for pipelines, the present invention provides a low-cost X80 hot-rolled steel plate for pipelines and its production method, so as to reduce the investment in alloy cost and manufacturing process of X80 pipeline steel, and improve the quality of the product. Comprehensive performance for cost-effective production.

The low-cost high-performance steel plate for pipelines of the present invention has the following components by weight percentage: C0.065%-0.085%, Mn1.0%-2.0%, Si0.25%-0.35%, Cu0.10%-0.25% , Ni0.10%-0.30%, Cr0.10%-0.50%, Nb0.02%-0.04%, Ti0.005%-0.03%, V0.02%%-0.04%, Alt0.02-0.06%, Ca ≤0.006%P≤0.015%, S≤0.003%, N≤0.012%, the balance is Fe and trace unavoidable impurities.

In the present invention, the 230mm thick continuous casting slab is preferably used to produce hot-rolled steel plates with a thickness specification not exceeding 25mm.

Among the present invention, the effect of main components is as follows:

C: The most economical and basic strengthening element in steel, it has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening, but increasing the C content has a negative impact on the ductility, toughness and weldability of steel. Therefore, this The invention sets the upper limit of C content at 0.085%. The development process of modern pipeline steel is a process of continuously reducing the C content. In order to give full play to the precipitation strengthening effect of Nb and other elements and improve the strength of welded joints, the lower limit of C content in the present invention is set at 0.065%.

Si: The purpose of adding Si is to deoxidize and improve the strength of the matrix during the steelmaking process. In addition, adding Si can reduce the precipitation of pearlite, which is beneficial to the improvement of matrix strength and toughness. However, if excessive Si is added, the toughness of the welding heat-affected zone of the base metal will be significantly reduced, and the field welding workability will also be deteriorated. Therefore, in the present invention, the Si content is 0.25%-0.35%.

Mn: Improve the strength of steel through solid solution strengthening. It is the most important and economical strengthening element in pipeline steel to compensate for the loss of strength caused by the reduction of C content. Mn is also an element that expands the γ phase region, which can reduce the γ→α phase transition temperature of steel, help to obtain fine phase transition products, improve the toughness of steel, and reduce the ductile-brittle transition temperature. In order to ensure the balance between strength and low temperature toughness, the minimum content of Mn is set at 1.0%. Increasing the content of Mn increases the hardenability of the steel. When the content increases to a certain extent, it will lead to a decrease in welding performance, especially a serious deterioration of the toughness of the welding heat-affected zone. In addition, too high Mn content will also increase the center segregation of the continuous casting slab, which will increase the anisotropy of the properties of the steel plate. Therefore, the upper limit of Mn content in the present invention is designed to be 2.0%.

Nb, V: It is one of the most important elements in modern microalloyed pipeline steel, and it has a very obvious effect on grain refinement. During the hot rolling process, the strain-induced precipitation of the two-phase particles containing Nb\V hinders the recovery and recrystallization of the deformed austenite. It is transformed into fine phase transformation products, so that the steel has high strength and high toughness. In the present invention, appropriate amounts of Nb and V are added in accordance with the C content to play the role of precipitates, and the range of Nb and V content selected in the present invention is 0.02%-0.04%.

Ti: It is a strong N-fixing element. The stoichiometric ratio of Ti/N is 3.42. About 30ppm of N in steel can be fixed by using about 0.01% Ti, and fine high-temperature stable TiN precipitation can be formed during slab continuous casting Mutually. Such fine TiN particles can effectively inhibit the growth of austenite grains when the slab is reheated, help to increase the solid solubility of Nb in austenite, and improve the impact toughness of the welded heat-affected zone. When the content of Als is too low (for example, less than 0.005%), Ti will form oxides, and these endogenous particles can act as nucleation cores of intragranular ferrite, refining the structure of the welded heat-affected zone. In order to obtain this effect, at least 0.005% Ti should be added. When the amount of Ti added exceeds a certain value, the TiN particles will be coarsened, and the precipitation strengthening effect of TiC will appear, resulting in deterioration of low temperature toughness. Therefore, the present invention selects the Ti content in the range of 0.005%-0.03%.

Cr: It is the main element that expands the γ phase region, delays the formation of ferrite during the γ→α phase transformation, and promotes the formation of acicular ferrite. It plays an important role in controlling the phase transformation structure. Under certain cooling conditions and final rolling When added to ultra-low carbon pipeline steel at low temperature, obvious acicular ferrite and bainite structures can be obtained. At the same time, due to the phase transformation to low temperature, the structure can be further refined, which is conducive to the improvement of low temperature toughness. improve. In order to obtain a reasonable combination of strength and toughness, the present invention selects the Cr content range of Cr0.10%-0.50%.

Cu, Ni: 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 low temperature toughness. In thick-gauge pipeline steel, it can also compensate for the strength drop caused by the lack of water-cooling strength caused by the increase in thickness. The present invention selects Cu0.10%-0.25%, Ni0.10%-0.30%.

P, S: These are inevitable impurity elements in steel, the lower the better. Due to the consideration of smelting cost, it cannot be unlimitedly low. Therefore, the present invention sets the upper limit of P and S content as 0.015% and 0.003%. Controlling the inclusion morphology of sulfide through ultra-low S (less than 30ppm) and Ca treatment can make pipeline steel have high impact toughness.

The invention aims at micro-alloy low-carbon bainite structure with high strength and high toughness, based on material strengthening theories such as grain refinement, phase transformation strengthening, precipitation strengthening and dislocation strengthening, etc., for the composition of pipeline steel with bainite structure The design adopts low carbon, ultra-low sulfur, Nb, V, Ti composite micro-alloying, Cr alloying with controlled structure and appropriate addition of Cu and Ni. Controlled rolling and cooling process to obtain a structure dominated by bainite + acicular ferrite to ensure that the pipeline steel has high strength, toughness and low cost.

The process route of the manufacturing method of the low-cost high-performance pipeline steel of the present invention is as follows: raw material preparation → converter or electric furnace smelting → out-of-furnace refining → casting → slab reheating → controlled rolling → controlled cooling.

Its characteristic is that the hot rolling process is controlled as follows:

(1) Slab heating temperature: 1100-1200°C;

(2) Temperature control range of controlled rolling in the recrystallization zone: 900-1150°C;

(3) Temperature control range of controlled rolling in non-recrystallization zone: 700-950°C;

(4) End rolling temperature: 700-800°C;

(5) Stop cooling temperature: 350-450°C

(6) Cooling rate: 10-25°C/s

In order to ensure a good matching of strength and toughness, two-stage controlled rolling technology is adopted in the steel plate production process, in which the first stage rolling is rolling in the recrystallization zone. The amount of deformation is not less than 25%. The rolling in the second stage is rolling in the non-recrystallization zone. The cumulative deformation in this stage is not less than 60%, and at least two passes are completed below 780°C. After the steel plate is rolled, in the water cooling process, the specified temperature range is used to match the corresponding cooling rate.

The hot rolling process of the present invention adopts the thermomechanical treatment technology of controlled rolling and controlled cooling, and controls the structure of the final product through reasonable components and processes to obtain a low-carbon bainite structure with high strength and high toughness, so that the steel plate produced can reach X80 Pipeline Steel Requirements.

Compared with the existing large-scale application of X80, the performance of the pipeline steel produced according to the above technical scheme meets the following requirements:

(1) Tensile properties:

Target: Rt0.5≥555MPa, Rm≥625MPa, yield strength ratio Rt0.5/Rm≤0.90.

(2) V-notch impact performance:

Target: test temperature -20°C, average impact energy of 10mm×10mm×55mm sample ≥230J, single value of shear area ≥80%, average ≥90%.

(3) DWTT performance

Target: test temperature -15°C, average shear area SA%≥85%, individual SA%≥70%.

(4) Transverse cold bending performance:

Target: d=2a, 180°, intact.

The characteristics of the present invention are: (1) Compared with the previous pipeline steel components, the reasonable formulation of the present invention considers that Mo and B are not added, and the addition of Mn, Cu, Ni, Cr, Nb, V and Ti is strictly controlled to reduce the cost ; (2) It is characterized by low-carbon and Mn-limited alloying elements. Under the premise of ensuring strength, it increases impact toughness and good welding performance, so that pipeline steel has good crack arrest ability; (3) makes full use of C and The relationship between Nb elements, under a certain coordination, make it generate NbC, produce fine-grain strengthening effect, and use the effect of Nb to increase the recrystallization temperature to cooperate with the controlled rolling process, which not only improves the comprehensive performance of the product, but also can adopt flexible heat treatment. The rolling production process improves production efficiency and reduces the load of the rolling mill. The products produced have high impact toughness, can fully guarantee high strength and toughness, and have good crack arrest ability; (4) To ensure good strength and toughness matching , a two-stage controlled rolling process is adopted and there is a quantitative lower limit for the rolling pass, pass reduction and cumulative reduction of each stage.

The advantages and effects of the present invention are: 1) reducing the amount of alloying elements added, reducing alloy cost investment, and increasing profit margins; Small mill load. The invention has the advantages of low cost, production efficiency, good comprehensive performance of products and the like.

Detailed ways

The present invention is described in detail below in conjunction with specific embodiment

Tables 1-3 are the chemical composition, process parameters and performance results of Examples 1-5 and Comparative Examples 6-8 composed according to the composition of the present invention. Among them, comparative example 6 is from patent CN201110179963.9, comparative example 7 is from CN201210327206.6, and comparative example 8 is from CN201210003018.8.

1. Chemical composition

The chemical compositions (wt%) of Examples 1-5 and Comparative Examples 6-8 are shown in Table 1.

Table 1 Chemical composition (wt%)

2. Hot rolling process

The process route is as follows: material preparation → converter or electric furnace smelting → out-of-furnace refining → casting → slab reheating → controlled rolling → controlled cooling. The processing parameters of embodiment 1-5 and comparative example 6-8 are shown in table 2.

Table 2 process parameters

Note: * means that the original patent did not provide specific values of relevant parameters.

3. Performance Results

Mechanics, Charpy impact and DWTT tests were carried out respectively, and the performance test results of the steel plates of Examples 1-5 and Comparative Examples 6-8 are shown in Table 3. The sample used for tensile measurement in the table is a round bar sample with a diameter of 12.7 mm.

Table 3 mechanical properties results

Note: * means that the original patent did not provide specific values of relevant parameters.

Claims (3)

1. A low-cost X80 pipeline steel, characterized in that the weight percent of the chemical composition is: C0.065%-0.085%, Mn1.0%-2.0%, Si0.25%-0.35%, Cu0.10 %-0.25%, Ni0.10%-0.30%, Cr0.10%-0.50%, Nb0.02%-0.04%, Ti0.005%-0.03%, V0.02%%-0.04%, Alt0.02- 0.06%, Ca≤0.006%P≤0.015%, S≤0.003%, N≤0.012%, the balance is Fe and trace unavoidable impurities. 2. A method for manufacturing a low-cost X80 pipeline steel as claimed in claim 1, the process route is as follows: material preparation → converter or electric furnace smelting → out-of-furnace refining → casting → slab reheating → controlled rolling → control cooling; it is characterized in that the specific hot rolling process is controlled as follows: 1) Slab heating temperature: 1100-1200°C; 2) Two-stage controlled rolling technology is adopted in the steel plate production process, in which the first stage rolling is rolling in the recrystallization zone, and the temperature control range of controlled rolling in the recrystallization zone: 900-1150°C; controlled rolling in the non-recrystallization zone Temperature control range: 700-950°C; 3) Stop rolling temperature: 700-800°C; 4) Stop cooling temperature: 350-450°C; 5) Cooling rate: 10-25°C/s. 3. A method for manufacturing low-cost X80 pipeline steel according to claim 2, characterized in that two-stage controlled rolling is adopted, the first stage is completed at least three passes above 1000°C, and a single pass The amount of deformation is not less than 25%. In the second stage, at least two passes are completed below 780°C. The total cumulative deformation in this stage is not less than 60%.