CN107988562A - X65-grade low-cost submarine pipeline steel and manufacturing method thereof - Google Patents

Abstract

The invention provides an X65 grade low-cost submarine pipeline steel and a manufacturing method thereof. The composition of the pipeline steel is as follows by weight percentage: C: 0.05%-0.09%, Si: 0.10%-0.30%, Mn: 1.40%- 1.60%, P: ≤0.018%, S: ≤0.008%, Ti: 0.008%‑0.020%, Als: 0.02%‑0.06%, Nb: 0.02%‑0.04%, Cr: 0.10%‑0.15%, N: ≤ 0.008%, Pcm≤0.20, the rest is Fe and unavoidable elements; manufacturing method: smelting, continuous casting, continuous casting slab reheating, rolling, cooling, coiling; the pipeline steel produced by the invention has higher strength and good Excellent weldability; the load of rolling and coiling equipment is small, and rolling and coiling are easy to implement.

Description

A kind of X65 low-cost submarine pipeline steel and its manufacturing method

technical field

The invention belongs to the field of metal materials, and in particular relates to a low-cost spiral submerged arc welding pipeline steel hot-rolled coil and a manufacturing method thereof.

Background technique

According to the transmission medium and the marine environment, the requirements for steel for submarine pipelines are different. Used to transport oil and natural gas, the steel for submarine pipelines working in low temperature, high pressure, strong corrosion and harsh marine environment not only bears the combined effect of internal and external pressure, axial force, bending moment and other static loads and temperature loads, but also Bearing the dynamic load of alternating external pressure, waves and ocean currents, the pipeline is subjected to the combined action of various loads and causes various forms of damage. Therefore, there are strict requirements for strength and toughness, compressive performance, corrosion resistance, dimensional accuracy and other indicators. In order to meet the corresponding requirements, this type of pipeline steel is usually designed with a large amount of precious alloy elements added, and harsh rolling and cooling processes are adopted, making it difficult to manufacture. Simplify

However, for medium and low-end submarine pipelines used to transport non-oil and natural gas, such as up-to-standard sewage discharge, coal-water slurry and other media, working in shallow sea areas with general marine environments, low-cost designs can be adopted, and the rolling and cooling process can be adopted at the same time. Easy to implement. This type of low-end submarine pipeline steel is designed with moderate C and Mn content, low alloy cost, high strength and good weldability, which can fully meet the standard requirements. Under the market conditions of excess steel production capacity, it has a greater potential Market competitiveness and market prospects.

At present, the use of conventional hot rolling mills to produce X65 pipeline steel hot-rolled coils for submarine pipelines has been initially applied in engineering, but it is generally designed to meet the high-end market demand for oil and natural gas transportation, and a large amount of precious alloys (Ni , Cu, V, etc.), the alloy cost is high. However, there is no relevant research on the steel for ordinary submarine pipelines used for transporting non-oil and natural gas. In particular, there is no report on the production of low-cost X65 pipeline steel hot-rolled coils using low niobium and adding a small amount of alloy chromium. The following brief introduction is closer to the present invention Patents and documents:

Chinese invention CN103834874A, thick wall and high DWTT performance X65-X70 submarine pipeline steel and its manufacturing method. The composition contains C: 0.03%-0.05%, Mn: 1.47%-1.70%, Cr: 0.10%-0.20%, Cu: 0.12%-0.20%, Ni: 0.36%-0.45%, Nb: 0.04%-0.05% . The starting temperature of finish rolling is 80±20°C, and the finishing temperature of finish rolling is 790±15°C. In this patent, the C content is low, and more alloy elements Ni and Cu need to be added to ensure the strength, and the cost of the alloy is high. The starting temperature of finish rolling is low, and the load of rolling mill is heavy.

Chinese invention CN103451536A, a low-cost thick gauge submarine pipeline steel plate and its manufacturing method. The composition contains C: 0.04%-0.06%, Mn: 1.43%-1.47%, Ni: 0.10%-0.15%, Nb: 0.03%-0.04%. The final rolling temperature is 750-810°C, the steel plate is relaxed and cooled after rolling, and the final cooling temperature is 350-450°C. In this patent, the C content is low, and more alloying element Ni needs to be added to ensure the strength, and the cost of the alloy is high. The relaxation and slow cooling of the steel plate after rolling is not suitable for hot rolling, the final cooling temperature is low, and coiling is difficult.

Chinese invention CN105132833A, an economical high-strength submarine pipeline steel and its production method. The composition contains C: 0.05%-0.08%, Mn: 0.8%-1.3%, Cr: 0.10%-0.30%, V: 0.020%-0.045%, Nb: 0.015%-0.040%. The cooling rate is not lower than 70°C/s, and the coiling temperature is 200-350°C. In this patent, the C content is low, and more alloy element Ni needs to be added to ensure the strength, and the cost of the alloy is high; the cooling speed is fast, difficult to control, the coiling temperature is low, and the requirements for the coiler are high.

Thesis "Development of Thick Gauge Submarine Pipeline Steel X65 Production Process" (Journal of Liaoning University of Science and Technology, 2012, 35(4): 402-406), in which C: 0.10%, Mn: 1.65%, Ni: 0.30%, Cr: 0.30%, Mo: 0.50%, Nb: 0.06%, V: 0.06%, Ti: 0.04%; produced by low final rolling temperature + low redness temperature. In this patent, the C content is slightly high, the Mn content is high, and a large amount of alloying elements Ni, Mo, Nb, V, etc. are contained at the same time, and the cost of the alloy is high.

In the above disclosed patents or documents, a large amount of precious alloys (Ni, Cu, V, etc.) are added, and the cost of the alloy is high; the rolling and coiling temperature is low in the process, and the requirements for the capacity of the rolling mill and the capacity of the coiler are high. Both are obviously different from the low-cost design of the present invention using C-Mn-Nb-Cr.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned problems and disadvantages and provide a hot-rolled coil of pipeline steel for X65 steel grade submarine pipelines and its manufacturing method, which has high strength and good weldability, saves alloy costs, and can be rolled and coiled. The load of the taking equipment is small, and rolling and coiling are easy to implement.

The purpose of the present invention is achieved like this:

An X65 grade low-cost submarine pipeline steel, the composition of the steel plate is as follows by weight percentage: C: 0.05%-0.09%, Si: 0.10%-0.30%, Mn: 1.40%-1.60%, P: ≤0.018%, S: ≤0.008%, Ti: 0.008%-0.020%, Als: 0.02%-0.06%, Nb: 0.02%-0.04%, Cr: 0.10%-0.15%, N: ≤0.008%, Pcm≤0.20, where Pcm =C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B, and the rest are Fe and unavoidable elements.

The structure of the steel plate is a mixed structure of acicular ferrite and a small amount of ferrite, wherein the volume percentage of ferrite is 4%-8%.

Compared with the prior art, the content of C and Mn in the present invention is moderate, the content of Nb is low, and a small amount of Cr replaces precious elements such as Ni, Mo, V, etc., saving alloy cost.

The composition design reason of the present invention is as follows:

C: Carbon is a solid solution element, which mainly plays a solid solution strengthening role. It is the most effective element to ensure strength and can improve hardenability. It forms carbides and precipitates with strong carbide-forming elements to play a role in precipitation strengthening. Therefore, the carbon content should not be too low; however, the increase of the carbon content is unfavorable to the toughness and weldability of the material, so the carbon content should not be too high, and the carbon content of the present invention is preferably controlled at 0.05% to 0.09%.

Si: Silicon can play a role of solid solution strengthening, but if its content is too high, the plasticity and toughness of steel will be reduced, and the optimum range is 0.10%-0.30%.

Mn: Manganese mainly plays the role of solid solution strengthening and phase transformation strengthening, which can make up for the loss of yield strength of pipeline steel due to the reduction of carbon content, and can also increase the stability of austenite, which is also beneficial to low temperature toughness and hardenability. Manganese also acts to lower the phase transition temperature, contributes to grain refinement, and improves strength and toughness. However, too high manganese content can easily induce segregation, resulting in uneven composition and structure. It is more appropriate to control the Mn content at 1.40% to 1.60%.

P: Phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates welding performance, reduces plasticity, and deteriorates cold bending performance. Its content should be controlled to ≤0.018%.

S: Sulfur is a harmful element in steel, which causes hot brittleness of steel, reduces the ductility and toughness of steel, and is also harmful to welding performance. Its content should be controlled to ≤0.008%.

Ti: Titanium is a strong carbonitride forming element, which significantly refines the austenite grains and can compensate for the decrease in strength caused by the reduction of carbon. If the content is too high, it is easy to form coarse TiN and reduce the performance of the material. The suitable range is 0.008%-0.020%.

Als: Aluminum is a commonly used deoxidizer. Adding a small amount of aluminum to steel can refine grains and improve impact toughness. The suitable range is 0.02%-0.06%.

Nb: Niobium is a fine-grained and precipitation-strengthening element that improves weldability. It has a strong grain refinement effect in pipeline steel. This effect is mainly due to the delay or prevention of austenite recrystallization in the rack behind hot rolling. Nb reduces the transformation temperature from austenite to ferrite, and in Increase the rate of ferrite nucleation while reducing the rate of grain growth. At the same time, Nb can promote the formation of acicular ferrite in the rapid cooling stage after rolling, and has a precipitation strengthening effect in the slow cooling stage after coiling, which improves the strength without reducing the low temperature toughness. But too high will increase the alloy cost, and the suitable range is 0.02%-0.04%.

Cr: Chromium has a strong solid solution strengthening effect, and can also effectively improve the stability of the structure. In terms of improving strength and refining grains, Cr, like Mo, can inhibit the formation of massive ferrite, refine grains and promote the transformation of acicular ferrite, and obtain multiple and fine acicular ferrite structures. Ensure the grain size is above 10 grades, thereby greatly improving the strength and ensuring the weldability of the pipe, especially for thick gauge pipeline steel, it is necessary to add a certain amount of Cr to improve the hardenability to compensate for the strength loss caused by the thickness and improve the thickness direction. Uniformity of structure and performance. At the same time, the combined use of Cr and Nb can promote the precipitation of Nb and improve the precipitation strengthening effect of Nb. Adding more than 0.10% Cr can improve the corrosion resistance of steel. With the increase of Cr content, the strength increases greatly, but if the addition amount is too large, the toughness and weldability of the welding heat-affected zone will be significantly reduced. Therefore, the suitable range of Cr in the present invention is 0.10%-0.15%.

N: solid solution nitrogen has a strong effect of pinning dislocations and has a bad effect on toughness, and its content should be controlled to ≤0.008%.

A method for manufacturing X65 grade low-cost submarine pipeline steel, including smelting, continuous casting, continuous casting slab reheating, rolling, cooling, and coiling;

(1) Smelting and continuous casting: pretreatment of molten iron, converter smelting - through top blowing or top-bottom compound blowing, refining outside the furnace, light desulfurization treatment in LF furnace and calcium treatment to control the shape of inclusions and improve the ductility of steel, Toughness and cold bending performance, slab continuous casting to make continuous casting slab, continuous casting adopts electromagnetic stirring or dynamic light reduction to improve the quality of continuous casting slab;

(2) Rolling: The continuous casting slab is heated to 1130-1160°C in a heating furnace and held for 140-300 minutes. This heating temperature range can ensure sufficient solid solution of alloying elements and prevent excessive growth of austenite grains; Then, two-stage controlled rolling is carried out in the hot continuous rolling mill. The first-stage rolling temperature is 1120-1160°C, the final rolling temperature is greater than 970°C, and the cumulative deformation is greater than 60%, so that the grains of austenite after recrystallization are fine. In the second stage, the starting rolling temperature is less than 960°C, and the final rolling temperature is 790-840°C. Under this rolling process, deformation and phase transformation proceed simultaneously, austenite grains are elongated, and austenite grain boundaries increase. The emergence of slip bands and slip bands provides favorable conditions for ferrite nucleation, and fine-grained ferrite is obtained;

(3) Cooling: After rolling, the coil is cooled at a cooling rate of 10-25°C/s, and the coil is coiled at a temperature of 480-600°C to obtain a mixture of fine and uniform acicular ferrite and a small amount of ferrite Organization, in which the ferrite content is 4%-8%.

The beneficial effects of the present invention are:

(1) The content of C and Mn in the present invention is moderate, Pcm≤0.20%, has higher strength and good weldability;

(2) The present invention adopts the low-cost design of C-Mn-Nb-Cr instead of adding a large amount of precious alloying elements Ni, Cu, V, etc., saving the alloy cost.

(3) The finishing temperature and coiling temperature of the present invention are moderate, the load of rolling and coiling equipment is small, and rolling and coiling are easy to implement.

Description of drawings

Fig. 1 is the metallographic diagram of the microstructure of Example 1 of the present invention.

Detailed ways

Below by embodiment the present invention will be further described.

In the embodiments of the present invention, smelting, continuous casting, reheating, rolling, cooling and coiling of continuous casting slabs are carried out according to the composition ratio of the technical solution. The composition of the steel of the embodiment of the present invention is shown in Table 1. The main process parameters of the steel of the embodiment of the present invention are shown in Table 2. The tensile properties and ferrite content of the steels in the examples of the present invention are shown in Table 3. The toughness index of the steel of the embodiment of the present invention is shown in Table 4.

Table 1 Composition (wt, %) of the steel of the embodiment of the present invention

Table 2 Main process parameters of the steel of the embodiment of the present invention

Table 3 Tensile properties and ferrite content of the steels of the examples of the present invention

Table 4 Toughness index of the steel of the embodiment of the present invention

It can be seen from Tables 3 and 4 that the low-cost X65 hot-rolled coils for submarine pipelines produced by adopting the composition, smelting, continuous casting, heating, rolling, cooling and coiling processes of the present invention meet the API SPEC 5L standard in mechanical properties The requirements for X65 grade steel pipes can be used to manufacture X65 grade steel pipes for submarine pipelines that meet the requirements of the API SPEC 5L standard.

Claims (3)

1. A X65 grade low-cost submarine pipeline steel, characterized in that the composition of the pipeline steel is as follows by weight percentage: C: 0.05%-0.09%, Si: 0.10%-0.30%, Mn: 1.40%-1.60% , P: ≤0.018%, S: ≤0.008%, Ti: 0.008%-0.020%, Als: 0.02%-0.06%, Nb: 0.02%-0.04%, Cr: 0.10%-0.15%, N: ≤0.008% , Pcm≤0.20, the rest are Fe and unavoidable elements; Wherein Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B. 2. X65 grade low-cost submarine pipeline steel according to claim 1, characterized in that the structure of the steel plate is a mixed structure of acicular ferrite and a small amount of ferrite, wherein the volume percentage of ferrite is 4 %-8%. 3. A method for manufacturing X65 grade low-cost submarine pipeline steel according to claim 1 or 2, including smelting, continuous casting, continuous casting slab reheating, rolling, cooling, coiling, characterized in that: (1) Rolling: The continuous casting slab is heated to 1130-1160°C by a heating furnace, and held for 140-300 minutes; then two-stage controlled rolling is carried out in the hot continuous rolling mill, and the rolling temperature of the first stage is 1120-1160°C , the final rolling temperature is greater than 970°C, and the cumulative deformation is greater than 60%; the second stage rolling temperature is less than 960°C, and the final rolling temperature is 790-840°C; (2) Cooling: After rolling, the coil is cooled at a cooling rate of 10-25°C/s, and the coil is coiled at a temperature of 480-600°C.