Low-yield-ratio low-temperature high-toughness steel plate for seabed X80MO pipeline and manufacturing method of low-yield-ratio low-temperature high-toughness steel plate

Abstract

The invention relates to a low-yield-ratio, low-temperature and high-toughness steel plate for a seabed X80MO pipeline and a manufacturing method of the steel plate, and belongs to the field of metallurgy. The production thickness of the steel plate is 25.4 mm-36 mm, the surface and the near surface of the steel plate are upper bainite structures, and the core of the steel plate is acicular ferrite. The transverse yield strength is larger than or equal to 555 MPa, the transverse tensile strength is larger than or equal to 680 MPa, and the transverse yield ratio is smaller than or equal to 0.82; the longitudinal yield strength is larger than or equal to 555 MPa, the longitudinal tensile strength is larger than or equal to 670 MPa, and the longitudinal yield ratio is smaller than or equal to 0.83; the ductility of a steel plate round bar sample is not smaller than 24%, the impact energy at the temperature of-30 DEG C is larger than or equal to 400 J, the Vickers hardness of 1 / 4 thickness and 1 / 2 thickness in the thickness direction of the steel plate and near the surface is 215-250 Hv10, the low-temperature drop hammer performance is not lower than 85% of shearing area / whole wall thickness at the temperature of-25 DEG C, and CTOD at the temperature of-25 DEG C is only not lower than 0.30 mm. The surface of the steel plate has high hardness, the core part has high toughness, and the comprehensive performance meets the deep sea use requirements of pipeline steel.

Description

Technical Field

[0001] The present invention relates to the technical field of iron-based alloys, and in particular to submarine pipeline steel and a manufacturing method thereof. Background Art

[0002] Fossil fuels still dominate the global energy mix. The rapid growth of the global economy in recent years has significantly increased demand for fossil fuels. To meet this immense demand, land-based resources have been depleted over the years. Energy extraction has shifted to the oceans. Subsea pipelines have stricter material performance requirements than land-based pipelines. In addition to mechanical properties, they also require longitudinal performance, crush resistance, and, even more importantly, high toughness.

[0003] Currently, the highest steel grade used for deep-sea pipelines is X70 (L485). There are few applications or related reports on X80 steel grade. Patent document CN110331343A discloses a low-yield ratio X80MO submarine pipeline steel plate and its manufacturing method. However, due to the high heating temperature, this method affects grain refinement, thereby limiting its production thickness.

[0004] Due to the special service environment of submarine pipelines, the steel plates must have high strength and high toughness at low temperatures. At the same time, the steel plates must have uniform strength in the longitudinal and transverse directions and be resistant to crushing. Summary of the Invention

[0005] Based on the service requirements of existing deep-sea risers, the present invention designs and tests a steel plate and its manufacturing method from the perspective of elemental composition and microstructure, which meets the requirements of high toughness at low temperatures, high strength uniformity in the longitudinal and transverse directions, and hardness.

[0006] The technical solution adopted by the present invention to solve the above problems is: a low-yield ratio, low-temperature, high-toughness submarine X80MO pipeline steel plate, whose elemental composition is as follows: C: 0.02-0.039%, Mn: 1.5-1.6%, Si: 0.31-0.4%, S: ≤0.001%, P: ≤0.010%, Nb: 0.041-0.08%, Ti: 0.008-0.03%, V: ≤0.030%, Alt: ≤0.06%, N: ≤0.010%, O: ≤0.006%, Mo: 0.1-0.30%, Cu: ≤0.30%, Ni: 0.15-1.5%, Cr: 0.15-0.45%, Ca: ≤0.01%, and the rest is Fe and unavoidable impurities.

[0007] The steel plate manufacturing process specifically involves molten steelmaking (KR hot metal pretreatment, converter smelting, LF refining, and RH vacuum degassing), continuous casting, heating, TMCP controlled rolling, and post-rolling cooling, achieving excellent comprehensive properties. Before rolling, the slab is reheated to 1150-1179°C for 3.5 hours to ensure solid solution of Nb carbonitrides and prevent excessive austenite grain growth. Rolling is divided into roughing and finishing stages. The roughing stage is typically the recrystallization stage. Rolling in the recrystallization zone requires a final rolling temperature of 1080-1120°C, an aspect ratio of no less than 1.6, and a reduction ratio of ≥22% per pass over two to three consecutive passes. Following the completion of the roughing and recrystallization zone rolling, finishing rolling immediately begins. This process also utilizes recrystallization rolling for this product, resulting in relatively equiaxed austenite grains. The temperature at the end of the finishing rolling phase is controlled at no less than 910°C. Finishing rolling is carried out with gradually increasing reduction ratios with each pass to refine the grains. Immediately after finishing rolling, the steel plate is cooled using DQ ultra-rapid cooling until the surface temperature reaches 450-500°C. It is then cooled using ACC to 350-400°C before finally being air-cooled to room temperature. The cooling rate for DQ is 25-35°C / s, while the cooling rate for ACC is 12-20°C / s.

[0008] The main mechanical properties of the steel plate obtained are: transverse yield strength of not less than 555 MPa; tensile strength of not less than 680 MPa, with a yield strength ratio of not more than 0.82; longitudinal yield strength of not less than 555 MPa; tensile strength of not less than 670 MPa, with a yield strength ratio of not more than 0.83; and elongation of not less than 24% (round bar sample). The steel plate has uniform strength in both longitudinal and transverse directions with minimal variability. The impact energy at -30°C is not less than 400 J; and the Vickers hardness along the thickness of the steel plate is 215-250 Hv10 near the surface, 1 / 4 thickness, and 1 / 2 thickness (core) at -25°C. Low-temperature drop hammer performance at -25°C is not less than 85% shear area (full wall thickness), and CTOD (crack tip opening displacement) at -25°C is not less than 0.3 mm.

[0009] The composition design concept is compared with the conventional X80 steel plate composition design and process. The conventional elements are somewhat the same, but it has its own characteristics, especially in terms of process. The process needs to be combined with the role of alloying elements in steel, as follows: C: The most economical and basic strengthening element in steel. It can significantly improve the strength of steel through solid solution strengthening and precipitation strengthening. Combined with the dual hardness requirements of steel plates and the requirements of high toughness at low temperatures, it is necessary to fully consider the relationship between organizational transformation and second-phase particle precipitation conditions, and also consider welding performance. Therefore, the C content in steel is controlled at 0.02-0.039%.

[0010] Si: is a solid solution element and is beneficial to improving strength. However, this patent application controls Si to 0.31-0.4 to inhibit carbide precipitation, thereby facilitating the acquisition of a hard phase structure and low yield strength ratio performance.

[0011] Mn: It increases steel strength through solid solution strengthening and is the most important element in pipeline steel to compensate for the strength loss caused by reduced carbon content. Mn also expands the γ phase, lowering the γ→α phase transition temperature, helping to produce finer phase transformation products. This increases steel toughness and lowers the ductile-brittle transition temperature. Mn also improves the hardenability of steel. In this invention, the Mn content is designed to be within the range of 1.50-1.6%.

[0012] Nb: It is one of the most important microalloying elements in modern microalloyed steels, especially pipeline steels, and has a significant effect on grain refinement. The solid solution drag of Nb and the strain-induced precipitation of Nb (C, N) during hot rolling can hinder the recovery and recrystallization of deformed austenite. Therefore, excessive Nb has a strong inhibitory effect on austenite recrystallization, which is not conducive to the subsequent acquisition of relatively uniform austenite in the longitudinal and transverse directions. Considering the relationship between C and Nb content, the Nb content is controlled within the range of 0.041-0.08%. V: It has a higher precipitation strengthening and weaker grain refining effect. When used in combination with the three microalloying elements Nb, V and Ti, V mainly exerts its precipitation strengthening effect, but the precipitation of V is inhibited under rapid cooling and a certain final cooling temperature.

[0013] Ti: It is a strong nitrogen-fixing element with a Ti / N stoichiometric ratio of 3.42. About 0.02% Ti can fix nitrogen below 60ppm in steel. TiN precipitation phase can be formed during the slab continuous casting process. This fine precipitation phase can effectively prevent the growth of austenite grains during the heating process of the slab, help to increase the solid solubility of Nb in austenite, and improve the impact toughness of the weld heat-affected zone. It is an indispensable element in pipeline steel.

[0014] Mo: This element delays the formation of the ferrite phase during the γ→α phase transformation, promoting the formation of acicular ferrite. It plays an important role in controlling phase transformation and also improves the hardenability of steel. By adding a certain amount of Mo at a certain cooling rate and final cooling temperature, a distinct acicular ferrite or bainite structure can be obtained.

[0015] S and P: are inevitable impurity elements in pipeline steel. The lower the better. By changing the sulfide form through ultra-low sulfur and Ca treatment, the pipeline steel can have high impact toughness.

[0016] Cu and Ni: can improve the strength of steel through solid solution strengthening. The addition of Ni can improve the toughness of steel and improve the hot brittleness easily caused by Cu in steel.

[0017] Cr: The addition of Cr can improve the hardenability of steel and is beneficial to obtaining moderate hardness, so the Cr content is controlled at 0.15-0.45%.

[0018] The present invention has the following characteristics: 1) In order to ensure the longitudinal and transverse performance and ensure high toughness, a certain width-to-width ratio must be maintained. 2) Taking into account the dual hardness requirements and lower yield strength ratio requirements, the role of micro-alloying elements such as Nb and V is fully considered in the composition design.

[0019] 3) Considering the dual hardness requirements, especially the high surface hardness and low-temperature toughness requirements, and their conflicting relationship, a staged cooling process is adopted, with ultra-fast cooling in the first stage and slow cooling in the second stage. This results in a microstructure with upper bainite on the surface and acicular ferrite in the core, resulting in a microstructure with high surface hardness and high core toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a microstructure metallographic diagram of the steel plate of the present invention near the surface along the thickness direction; Figure 2 This is a metallographic diagram of the core structure of the steel plate of the present invention along the thickness direction. DETAILED DESCRIPTION

[0021] The present invention is further described in detail below with reference to the examples. The examples are illustrative and intended to explain the present invention, but are not to be construed as limiting the present invention.

[0022] A low-yield-to-strength ratio, low-temperature, high-toughness submarine X80MO pipeline steel plate. The process route is as follows: prepare materials according to the technical plan → smelt in a converter or electric furnace → refining outside the furnace → continuous casting → slab reheating → specific TMCP process + air cooling to room temperature. Specifically: in the hot-rolled plate manufacturing process, the slab reheating temperature is: 1150~1179℃; recrystallization rolling is adopted in the rough rolling zone, and the rolling termination temperature is controlled to be: 1080~1120℃, and the single-pass reduction rate for 2-3 consecutive passes is ≥22%, and the width expansion ratio is not less than 1.6; the finishing rolling zone still adopts recrystallization zone rolling as much as possible, and the final rolling temperature is not less than 910℃, while obtaining the required finished product thickness; after the finishing rolling is completed, the steel plate is immediately cooled rapidly by DQ ultra-fast cooling, and the surface cooling temperature of the steel plate is controlled at 450-500℃; the cooling rate is controlled at 25-35℃ / s. After the DQ ultra-fast cooling is completed, the steel plate is cooled by ACC later, and the cooling rate is controlled at 15-20℃ / s. The final cooling temperature is 350-400℃, and then air-cooled to room temperature.

[0023] The chemical composition of the embodiment of the present invention is shown in Table 1 Table 1 shows the chemical composition (wt%) of an X80MO extra thick steel plate according to the present invention. Example C Mn Si S P Nb Ti V Alt Mo+Cu+Ni+Cr B Ca 1 0.03 1.60 0.31 0.0005 0.008 0.05 0.015 ≤0.02 0.028 ≤1.5 - ≤0.01 2 0.039 1.54 0.35 0.0005 0.009 0.06 0.018 ≤0.02 0.030 ≤1.5 - ≤0.01 3 0.02 1.50 0.40 0.0015 0.007 0.08 0.021 ≤0..02 0.035 ≤1.5 - ≤0.01 Table 2 shows the rolling process parameters of the embodiment:

[0024] Table 3-6 shows the mechanical properties of the steel plates of Examples 1-3 Table 3

[0025] Table 4

[0026] Table 5

[0027] Table 6

[0028] In addition to the above embodiments, the present invention also includes other implementation methods. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the scope of protection of the claims of the present invention.

Claims

1. A low yield ratio, low temperature, high toughness submarine X80MO pipeline steel plate, characterized by: The weight percentage of the elemental composition is C: 0.02-0.039%, Mn: 1.5-1.6%, Si: 0.31-0.4%, S: ≤0.001%, P: ≤0.010%, Nb: 0.041-0.08%, Ti: 0.008-0.03%, V: ≤0.030%, Alt: ≤0.06%, N: ≤0.010%, O: ≤0.006%, Mo: 0.1-0.30%, Cu: ≤0.30%, Ni: 0.15-1.5%, Cr: 0.15-0.45%, Ca: ≤0.01%, and the rest is Fe and unavoidable impurities.

2. The steel plate according to claim 1, wherein: The production thickness is 25.4mm-36mm. The surface and near-surface of the steel plate are upper bainite structure, and the core of the steel plate is acicular ferrite.

3. The steel plate according to claim 2, characterized in that: Transverse yield strength ≥555MPa, transverse tensile strength ≥680MPa, transverse yield ratio ≤0.82; longitudinal yield strength ≥555MPa, longitudinal tensile strength ≥670MPa, longitudinal yield ratio ≤0.83; elongation of steel plate round bar sample not less than 24%, -30℃ impact energy ≥400J, Vickers hardness along the thickness direction of the steel plate, near the surface, 1 / 4 thickness and 1 / 2 thickness is 215-250Hv10, low-temperature drop hammer performance at -25℃ is not less than 85% shear area / full wall thickness, and CTOD at -25℃ is not less than 0.30mm.

4. A method for manufacturing the steel plate according to claim 1, characterized in that: It includes molten steel smelting, casting, TMCP rolling and cooling. Specifically, molten steel is smelted according to the elemental composition of the steel plate, the molten steel is cast into slabs, and the slabs are rolled by TMCP. The reheating temperature of the rolling is: 1150 ~ 1179 ° C, ensuring that Nb carbonitrides are dissolved while the austenite grains do not grow excessively. The rolling is divided into rough rolling stage and finishing rolling stage. Rough rolling is rolling in the recrystallization zone. The final rolling temperature of rough rolling is: 1080 ~ 1120 ° C, and the width-expansion ratio is not less than 1.6, and it is repeated for 2-3 consecutive passes. The single-pass reduction rate is ≥22%; finishing rolling is performed immediately after the rough rolling is completed, and the finishing rolling adopts recrystallization zone rolling. The final rolling temperature of finishing rolling is not lower than 910℃, so as to obtain equiaxed austenite grains; the steel plate is subjected to controlled cooling after rolling: DQ rapid cooling is performed immediately after rolling, with a cooling rate of 25-35℃ / s, until the surface temperature of the steel plate reaches 450-500℃; then the steel plate is cooled by ACC laminar flow cooling, with a cooling rate of 12-20℃ / s, and the final cooling temperature is 350-400℃, and then air-cooled to room temperature.

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