Low-Cr pipeline steel plate, heat treatment process and low-Cr pipeline steel pipe

Abstract

The invention relates to the technical field of heat treatment of pipeline steel, in particular to a low-Cr pipeline steel plate and a heat treatment process, and the low-Cr pipeline steel plate is prepared from the following chemical elements in percentage by weight: 0.04 to 0.12 percent of C, 0.10 to 0.30 percent of Si, 1.20 to 2.00 percent of Mn, 0.50 to 1.50 percent of Cr, 0.10 to 0.35 percent of Mo, 0.10 to 0.30 percent of Ni, 0.02 to 0.08 percent of Nb, 0.10 to 0.25 percent of Cu, 0.03 to 0.10 percent of V, 0.01 to 0.04 percent of Ti, 0.015 to 0.050 percent of Al, less than or equal to 0.015 percent of P, less than or equal to 0.005 percent of S, less than or equal to 0.008 percent of N and the balance of Fe and inevitable impurity elements. The invention further discloses a low-Cr pipeline steel pipe manufactured by the low-Cr pipeline steel plate and a heat treatment process. The manufactured steel plate is low in manufacturing cost, excellent in yield strength, tensile strength, corrosion resistance, yield ratio, ductility, impact toughness and other mechanical properties and capable of meeting the strict standard of modern oil and gas conveying pipelines for material performance; the method is suitable for X60-X80-grade oil and gas conveying pipelines with high requirements on high strength, toughness and corrosion resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of pipeline steel heat treatment, and in particular to a low-Cr pipeline steel plate, a heat treatment process and a low-Cr pipeline steel pipe. Background Art

[0002] In recent years, with the rapid development of the global oil and gas industry, the oil and gas transportation environment has become increasingly harsh. As the core carrier of energy transportation, the safety and reliability of low-Cr pipeline steel transportation systems have attracted much attention. Although modern oil and gas transportation steel pipes have advantages such as high strength, excellent plasticity and good welding performance, they are very prone to corrosion failure in actual service due to long-term exposure to complex working conditions such as high humidity, high temperature and high pressure. This will not only cause oil and gas leakage accidents and cause significant economic losses, but may also lead to serious ecological damage. In response to this serious problem, in-depth research on the corrosion failure mechanism of pipeline steel and the development of effective protection technologies are of great practical significance. Existing research often improves the material by adding trace alloying elements. However, this is economically costly for finished commercial low-Cr pipeline steel and directly affects the mechanical properties of the material. Therefore, this traditional alloying modification method has great limitations on the application of low-Cr pipeline steel for oil and gas transportation. Summary of the Invention

[0003] The purpose of the present invention is to provide a low-Cr pipeline steel plate, a heat treatment process, and a low-Cr pipeline steel pipe. The steel plate has a low manufacturing cost and excels in mechanical properties such as yield strength, tensile strength, corrosion resistance, yield strength ratio, elongation, and impact toughness. It can meet the stringent material performance standards of modern oil and gas pipelines and is suitable for X60-X80 grade oil and gas pipelines with high requirements for high strength, toughness, and corrosion resistance.

[0004] A first aspect of the present invention provides a low-Cr pipeline steel plate, wherein the contents of the chemical elements are as follows: C 0.04-0.12wt.%, Si 0.10-0.30wt.%, Mn 1.20-2.00wt.%, Cr 0.50-1.50wt.%, Mo 0.10-0.35wt.%, Ni 0.10-0.30wt.%, Nb 0.02-0.08wt.%, Cu 0.10-0.25wt.%, V 0.03-0.10wt.%, Ti 0.01-0.04wt.%, Al 0.015-0.050wt.%, P≤0.015wt.%, S≤0.005wt.%, N≤0.008wt.%, and the remainder is Fe and unavoidable impurity elements.

[0005] A second aspect of the present invention provides a heat treatment process for a low-Cr pipeline steel plate, wherein a steel billet after material preparation, smelting, and casting is subjected to heat treatment to produce the low-Cr pipeline steel plate according to claim 1. The heat treatment process is a composite heat treatment process, specifically comprising: S1, controlled rolling and controlled cooling (TMCP) stage: the steel billet after casting is cooled down at room temperature until it is completely austenitized and then kept at this temperature for 30-60 minutes, followed by multiple hot rolling, with the final rolling temperature being 850-900°C. Finally, the steel billet after multiple hot rolling is accelerated cooled to 500-600°C at a cooling rate of 15-25°C / s, wherein the temperature of the steel billet when it is completely austenitized is 1050-1100°C; S2, two-phase zone quenching stage: the steel billet after controlled rolling and controlled cooling treatment is heated again to 750-800℃ and kept at this temperature for 20-30min, first water-cooled to 300℃, and then air-cooled to room temperature; S3, gradient tempering stage: the steel billet after two-phase zone quenching treatment is subjected to three-stage tempering treatment, then furnace cooled to 400℃, and then air cooled to room temperature, wherein the first stage tempering temperature is 300-450℃, and the heat preservation is 1-2h; the second stage tempering temperature is 450-550℃, and the heat preservation is 1-2h; the third stage tempering temperature is 550-650℃, and the heat preservation is 0.5-1.5h.

[0006] Preferably, the cumulative deformation of the billet thickness after multiple hot rolling passes in the controlled rolling and controlled cooling TMCP stage is 70-80%, the final cooling structure is a mixed structure of fine bainite / acicular ferrite, and the grain size reaches ASTM grade 8-10.

[0007] Preferably, the Cr element is enriched in the ferrite phase during the two-phase quenching stage, and the surface Cr content increases by 40-60% relative to the Cr content inside the matrix, forming a continuous Cr2O3 passivation film.

[0008] Preferably, the microstructure after quenching in the two-phase region is a Cr-rich ferrite + low-carbon martensite / bainite complex structure, with a retained austenite content of 5-10%.

[0009] Preferably, the carbon equivalent Ceq of the steel plate produced by the heat treatment process of the low-Cr pipeline steel plate is ≤0.40%, and the welding cold crack sensitivity index Pcm is ≤0.18%.

[0010] Preferably, the steel plate made by the heat treatment process of the low Cr pipeline steel plate has a yield strength ≥500 MPa, a tensile strength ≥550 MPa, an elongation ≥15%, a corrosion rate in a 5% NaCl+0.5% H2S solution ≤0.30 mm / y, and an impact energy at -20°C ≥100 J.

[0011] A third aspect of the present invention provides a low-Cr pipeline steel pipe, which is manufactured from a steel plate produced according to a heat treatment process for a low-Cr pipeline steel plate.

[0012] Preferably, the low Cr pipeline steel pipe is suitable for X60-X80 grade oil and gas pipelines.

[0013] Preferably, when the low Cr pipeline steel pipe is suitable for X60 grade oil and gas transmission pipeline, the content of each chemical element in the low Cr pipeline steel plate is as follows: C 0.06-0.10wt.%, Si 0.15-0.25wt.%, Mn 1.30-1.60wt.%, Cr 0.80-1.20wt.%, Mo 0.10-0.20wt.%, Ni 0.10-0.30wt.%, Nb 0.03-0.05wt.%, Cu 0.10-0.25wt.%, V 0.03-0.06wt.%, Ti 0.01-0.02wt.%, Al 0.015-0.050wt.%, P≤0.015wt.%, S≤0.005wt.%, N≤0.008wt.%, and the rest are Fe and unavoidable impurity elements; When applicable to X70 grade oil and gas transmission pipelines, the contents of various chemical elements in low Cr pipeline steel plates are as follows: C 0.05-0.12wt.%, Si 0.10-0.30wt.%, Mn 1.20-1.80wt.%, Cr 0.50-1.50wt.%, Mo 0.10-0.30wt.%, Ni 0.10-0.30wt.%, Nb 0.02-0.06wt.%, Cu 0.10-0.25wt.%, V 0.03-0.08wt.%, Ti 0.01-0.03wt.%, Al 0.015-0.050wt.%, P≤0.015wt.%, S≤0.005wt.%, N≤0.008wt.%, and the rest are Fe and unavoidable impurity elements; When suitable for X80 grade oil and gas pipelines, the contents of various chemical elements in low Cr pipeline steel plates are as follows: C 0.04~0.08wt.%, Si 0.10~0.25wt.%, Mn 1.70~2.00wt.%, Cr 0.80~1.20wt.%, Mo 0.20~0.35wt.%, Ni 0.10~0.30wt.%, Nb 0.05~0.08wt.%, Cu 0.10~0.25wt.%, V 0.05~0.10wt.%, Ti 0.02~0.04wt.%, Al 0.015~0.050wt.%, P≤0.012wt.%, S≤0.003wt.%, N≤0.008wt.%, and the rest are Fe and unavoidable impurity elements.

[0014] Compared with the prior art, the present invention has the following beneficial effects: 1. The heat treatment process of the present invention significantly improves the corrosion resistance of low-Cr pipeline steel under extreme working conditions through the synergistic effect of multiple mechanisms. This process creatively utilizes two-phase quenching to achieve selective enrichment of Cr in the ferrite phase, increasing the surface Cr content by 40-60% relative to the Cr content in the matrix, prompting the formation of a continuous, dense Cr2O3 passivation film with self-repairing capabilities on the material surface. Combined with the ultrafine-grained structure (ASTM grades 8-10) obtained in the controlled rolling and controlled cooling (TMCP) stage and the uniformly distributed nanoscale NbC / TiN precipitates, the material exhibits excellent performance in a variety of corrosive media: in an acidic environment containing H2S / CO2, the sulfide stress corrosion cracking (SSCC) threshold stress reaches 2.1 times that of conventional X65 steel; in a 3.5wt.% NaCl aqueous solution, the corrosion current density is reduced by 50-60%, and the pitting initiation potential is significantly improved (positive shift of 100mVSCE to +350mVSCE); in a Cl-containing aqueous solution, the corrosion current density is reduced by 50-60%, and the pitting initiation potential is significantly improved (positive shift of 100mVSCE to +350mVSCE). － The service life in formation water is extended by more than 40%. This multiple protection mechanism enables pipeline steel to resist sulfide corrosion in sour gas fields and Cl corrosion in marine environments. － Erosion, providing reliable material solutions for the development of complex oil and gas fields.

[0015] 2. The present invention achieves an optimal strength-toughness match by combining strain-induced precipitation (Nb / Ti carbonitrides) and grain refinement (grain size 8-12 μm) in the TMCP stage through controlled rolling and controlled cooling, and synergistically achieving a ferrite + lath bainite duplex structure obtained by two-phase quenching. This ensures that the heat-treated low-Cr pipeline steel not only has excellent strength properties but also exhibits outstanding low-temperature toughness.

[0016] 3. This invention successfully overcomes the technical bottlenecks of traditional heat treatment through breakthrough technological innovation, primarily through coordinated optimization in the following three aspects: First, in terms of process control, by precisely regulating the two-phase zone temperature and tempering parameters, an optimized distribution of Cr (over 95% in solid solution) and control of harmful phases (Cr2N precipitation <0.3%) are achieved. Second, in terms of alloy design, an optimized ratio of 0.50-1.50 wt.% Cr, combined with heat treatment strengthening, reduces material costs by 35% while achieving corrosion resistance comparable to that of 3 wt.% Cr alloy steel. Finally, in terms of engineering application, the innovative two-phase zone treatment significantly improves the HAZ performance of the weld heat-affected zone (HAZ), reducing the softening zone width by 50% and hardness fluctuations to within HV20. It also reduces the cold crack sensitivity index (Pcm) to 0.18%, enabling compliance with the API 5L standard without the need for post-weld heat treatment. This provides a technologically advanced and economically sound solution for oil and gas pipeline projects. DETAILED DESCRIPTION

[0017] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0018] A first aspect of the present invention provides a low-Cr pipeline steel plate, wherein the contents of the chemical elements are as follows: C 0.04-0.12wt.%, Si 0.10-0.30wt.%, Mn 1.20-2.00wt.%, Cr 0.50-1.50wt.%, Mo 0.10-0.35wt.%, Ni 0.10-0.30wt.%, Nb 0.02-0.08wt.%, Cu 0.10-0.25wt.%, V 0.03-0.10wt.%, Ti 0.01-0.04wt.%, Al 0.015-0.050wt.%, P≤0.015wt.%, S≤0.005wt.%, N≤0.008wt.%, and the remainder is Fe and unavoidable impurity elements.

[0019] The design principles of the chemical elements in the low Cr pipeline steel plate of the present invention are as follows: 1. The present invention uses chromium (Cr), copper (Cu), and nickel (Ni) as core corrosion-resistant elements. Cr, as a passive film-forming element, selectively enriches in the ferrite phase to promote the formation of a dense Cr2O3 film. Cu+Ni and Cr form a "Cr-Cu-Ni" synergistic system, with Cu enhancing the adhesion of the passive film and Ni improving the partition coefficient of Cr in the γ / α phase. Therefore, the Cr content in the present invention is controlled to 0.50-1.50 wt.%, the Cu content to 0.10-0.25 wt.%, and the Ni content to 0.10-0.30 wt.%.

[0020] 2. This invention utilizes the synergistic effect of niobium (Nb) with titanium (Ti) and vanadium (V) microalloying elements to control microstructure refinement. Nb and Ti form complex carbonitrides, which inhibit recrystallization through strain-induced precipitation during controlled rolling and controlled cooling, achieving an ASTM grain size of 8 or greater. V primarily precipitates nanoscale VC during tempering, providing a secondary hardening effect. Therefore, the Nb content in this invention is controlled to 0.02-0.08wt%, the Ti content to 0.01-0.04wt%, and the V content to 0.03-0.10wt%.

[0021] 3. This invention utilizes carbon (C), manganese (Mn), and silicon (Si) as a mechanical performance guarantee system. C is controlled to a lower limit, combined with Nb-V-Ti microalloying to achieve a "low-carbon, high-strength" design. A carbon equivalent (Ceq) of ≤ 0.40% ensures weldability. Mn, through solid solution strengthening and austenite stabilization, reduces cold crack susceptibility while improving strength. Si, a ferrite-forming element, suppresses the expansion of the γ phase during heating in the austenite region, refining the final microstructure. Therefore, the C content in this invention is controlled to 0.04-0.12 wt.%, the Mn content to 1.20-2.00 wt.%, and the Si content to 0.10-0.30 wt.%.

[0022] It should be noted that the "low carbon" in this application is to avoid the disadvantages of traditional high carbon steel, and the "high strength" is achieved by relying on micro-alloying Nb-V-Ti and controlled rolling and controlled cooling processes. The carbonitrides of Nb, V, and Ti precipitate during controlled rolling, controlled cooling, and tempering, providing precipitation strengthening and fine grain strengthening to compensate for the strength loss caused by low carbon.

[0023] 4. The present invention uses sulfur (S) and phosphorus (P) to limit impurity elements. S is used to control the sulfide morphology and increase the MnS inclusion spheroidization rate to over 90%, while P is used to suppress grain boundary segregation. Therefore, the S content in the present invention is controlled to ≤ 0.005 wt.%, and the P content is controlled to ≤ 0.015 wt.%.

[0024] 5. This invention utilizes molybdenum (Mo) and aluminum (Al) plus nitrogen (N) to adapt to specific environments. Mo improves the stability of the passive film, increasing the pitting corrosion equivalent (PREN) by 15-20%. Al forms AlN with N, inhibiting the precipitation of the harmful Cr2N phase while also avoiding Cr consumption and the risk of embrittlement. Therefore, the Mo content in this invention is controlled to 0.10-0.35 wt.%, the Al content to 0.015-0.050 wt.%, and the N content to ≤0.008 wt.%.

[0025] A second aspect of the present invention provides a heat treatment process for a low-Cr pipeline steel plate, wherein a low-Cr pipeline steel plate is produced by heat treating a steel billet after material preparation, smelting, and casting. The heat treatment process is a composite heat treatment process, specifically comprising: S1. Controlled rolling and controlled cooling (TMCP) stage: The cast steel billet is cooled down at room temperature until it is fully austenitized and then kept warm for 30 to 60 minutes. It is then hot rolled in multiple passes with a final rolling temperature of 850 to 900°C. Finally, the steel billet after multiple hot rolling processes is accelerated to 500 to 600°C at a cooling rate of 15 to 25°C / s, where the temperature of the steel billet when it is fully austenitized is 1050 to 1100°C.

[0026] In the heat treatment process of low-Cr pipeline steel, the temperature at which the steel billet is fully austenitized is set to 1050-1100°C in this application, rather than the theoretically required 890°C for complete austenitization, mainly based on various engineering practice considerations. First, higher temperatures can ensure that key alloying elements such as Cr, Nb, and V are fully dissolved. Experimental data show that when the temperature is lower than 950°C, about 15-20% of Cr still exists in the form of carbides, which significantly reduces the effective Cr content in the steel and affects the subsequent corrosion resistance. Secondly, high-temperature treatment can effectively eliminate the banded structure defects of hot-rolled steel. When treated at 1100°C, the banded structure elimination rate can reach 98%, which is much better than the effect of 900°C treatment.

[0027] In addition, from a kinetic perspective, high temperature can significantly accelerate the diffusion process of alloying elements. Taking Cr as an example, its diffusion activation energy is 240kJ / mol, and its diffusion rate at 1100°C is more than 60% faster than that at 900°C, which is particularly important for improving dendritic segregation in ingots. At the same time, the higher austenitizing temperature provides a wider operating window for the subsequent controlled rolling and controlled cooling TMCP stage, which can better cope with the temperature fluctuation of ±30°C in actual production. It is worth noting that a short-term holding temperature of 1050-1100°C for 30-60 minutes can actually obtain finer austenite grains. This is because high temperature promotes the secondary recrystallization process, and a more uniform structure can be obtained compared to the 890°C treatment.

[0028] From an industrial production perspective, this temperature setting is well matched to the continuous casting billet discharge temperature (1150-1250°C), directly utilizing the billet's waste heat and saving 20-30% of heating energy. Actual production verification has shown that austenitizing X70-grade Cr-containing pipeline steel at 1050-1100°C can reduce the corrosion rate by 40% while increasing its yield strength by approximately 15%. To ensure process stability, a precise temperature control system with a tolerance of ±10°C is required, and trace amounts of Ti are added to suppress abnormal grain growth. This high-temperature austenitization design ensures microstructure uniformity while creating more ideal conditions for subsequent two-phase processing.

[0029] In this application, the steel billet after casting is cooled at room temperature to make the structure completely austenitized, and then subjected to heat preservation treatment to make the Cr in the steel billet uniformly distributed; at the same time, the pearlite is suppressed at a cooling rate of 15 to 25°C / s to promote bainite transformation.

[0030] Preferably, the cumulative deformation of the billet thickness after multiple hot rolling passes in the controlled rolling and controlled cooling TMCP stage is 70-80%, the final cooling structure is a mixed structure of fine bainite / acicular ferrite, and the grain size reaches ASTM grade 8-10.

[0031] It should be noted that the cumulative deformation of the steel billet thickness after multiple hot rolling passes in the controlled rolling and controlled cooling TMCP stage is 70-80%, which means that the thickness of the steel billet is reduced by 70-80% after multiple hot rolling passes in the controlled rolling and controlled cooling TMCP stage.

[0032] S2. Two-phase zone quenching stage: The steel billet after controlled rolling and controlled cooling treatment is heated again to 750-800℃ and kept warm for 20-30min. It is first water-cooled to 300℃ and then air-cooled to room temperature.

[0033] Preferably, the Cr element is enriched in the ferrite phase during the two-phase quenching stage, and the surface Cr content increases by 40-60% relative to the Cr content inside the matrix, forming a continuous Cr2O3 passivation film.

[0034] It should be noted that the matrix refers to the inside of the steel billet material.

[0035] Preferably, the microstructure after quenching in the two-phase region is a Cr-rich ferrite + low-carbon martensite / bainite complex structure, with a retained austenite content of 5-10%.

[0036] In the present application, the steel billet after controlled rolling and controlled cooling treatment is heated again to obtain an α+γ two-phase region and enrich Cr in ferrite, and then a heat preservation treatment is performed to ensure the Cr diffusion balance.

[0037] S3, gradient tempering stage: the steel billet after two-phase zone quenching treatment is subjected to three-stage tempering treatment, then furnace cooled to 400℃, and then air cooled to room temperature, wherein the first stage tempering temperature is 300-450℃, and the heat preservation is 1-2h; the second stage tempering temperature is 450-550℃, and the heat preservation is 1-2h; the third stage tempering temperature is 550-650℃, and the heat preservation is 0.5-1.5h.

[0038] In this application, a gradient tempering design is adopted to avoid high Cr cost and ensure effective passivation. The first stage tempering treatment avoids the precipitation of Cr carbides and maintains Cr solid solution; the second stage tempering treatment promotes the precipitation of nano-scale carbides VC / NbC; the third stage tempering treatment stabilizes the structure and avoids Cr. 23 C6 coarsening. Subsequently, the steel billet after the three-stage tempering treatment is furnace cooled and air cooled to prevent temper brittleness.

[0039] In terms of phase transformation control, rapid quenching after two-phase treatment causes the untransformed austenite to form a low-carbon martensite / bainite structure, which decomposes into a mixed structure of tempered martensite and fine carbides after tempering. This multiphase structure not only provides excellent strength and toughness matching, but also forms a microgalvanic effect with the Cr-enriched ferrite, which effectively slows the overall corrosion rate.

[0040] It is particularly noteworthy that a small amount of retained austenite (5-10%) is retained during the process. In a corrosive environment, this can induce a phase transformation-induced plasticity effect, which not only improves the toughness of the material but also releases Cr through phase transformation in the localized corrosion zone, enabling the self-repair function of the passivation film. The small amount of retained austenite is relative to the overall microstructure of the steel plate after quenching in the two-phase region.

[0041] Actual industrial applications have shown that the salt spray test corrosion rate of X70 grade low-Cr pipeline steel treated by this process can be reduced by 40% compared with the traditional process, while maintaining a yield strength of more than 500 MPa and excellent low-temperature impact toughness, achieving synergistic optimization of corrosion resistance and mechanical properties.

[0042] In this application, material preparation refers to preparing raw materials according to the designed composition ratio to ensure that the chemical composition accurately meets the range in the claims; smelting refers to melting the raw materials in an electric furnace to obtain pure molten steel; casting refers to continuously casting the molten steel into steel billets, controlling the cooling rate to avoid segregation, and obtaining uniform and defect-free steel billets, which provide a basis for the subsequent controlled rolling and controlled cooling (TMCP) stage heat treatment.

[0043] It should be noted that the low-Cr pipeline steel plate provided in this application is produced after material preparation, smelting, casting and heat treatment, among which material preparation, smelting and casting are conventional technical means in this field. They are not improved in this application, only the heat treatment process is improved. Therefore, the material preparation, smelting and casting processes are not elaborated in detail in this application.

[0044] The present invention uses a composite heat treatment process of controlled rolling and controlled cooling (TMCP) stage + two-phase quenching + gradient tempering to selectively precipitate multiple key phases on the surface of the low-Cr pipeline steel billet, thereby jointly improving the corrosion resistance and mechanical properties of the material. Specifically, the original austenite grains are refined through controlled rolling and controlled cooling in the TMCP stage; secondly, the Cr element is significantly enriched in the ferrite phase during two-phase quenching. This Cr-enriched ferrite, namely α-Fe(Cr), can serve as the core formation site of the passivation film and preferentially generate a dense Cr2O3 protective film in a corrosive environment; finally, in the gradient tempering stage, microalloying elements Nb, Ti, and V are used to precipitate nano-scale carbonitrides. These fine precipitated phases can pin grain boundaries to inhibit intergranular corrosion and improve material strength through precipitation strengthening.

[0045] Preferably, the carbon equivalent Ceq of the steel plate produced by the heat treatment process of the low-Cr pipeline steel plate is ≤0.40%, and the welding cold crack sensitivity index Pcm is ≤0.18%.

[0046] Preferably, the steel plate made by the heat treatment process of the low Cr pipeline steel plate has a yield strength ≥500 MPa, a tensile strength ≥550 MPa, an elongation ≥15%, a corrosion rate in a 5% NaCl+0.5% H2S solution ≤0.30 mm / y, and an impact energy at -20°C ≥100 J.

[0047] It should be noted that the 5% NaCl + 0.5% H2S solution in this application is obtained by passing H2S gas into a 5 wt.% NaCl aqueous solution until the concentration of dissolved H2S reaches 0.5% of the total mass of the NaCl aqueous solution.

[0048] A third aspect of the present invention provides a low-Cr pipeline steel pipe, which is manufactured using a steel plate manufactured using a heat treatment process for a low-Cr pipeline steel plate.

[0049] Preferably, the low Cr pipeline steel pipe is suitable for X60-X80 grade oil and gas pipelines.

[0050] In the embodiment of the present application, when the low Cr pipeline steel pipe is suitable for X60 grade oil and gas transmission pipeline, the content of each chemical element in the low Cr pipeline steel plate is as follows: C 0.06-0.10wt.%, Si 0.15-0.25wt.%, Mn 1.30-1.60wt.%, Cr 0.80-1.20wt.%, Mo 0.10-0.20wt.%, Ni 0.10-0.30wt.%, Nb 0.03-0.05wt.%, Cu 0.10-0.25wt.%, V 0.03-0.06wt.%, Ti 0.01-0.02wt.%, Al 0.015-0.050wt.%, P≤0.015wt.%, S≤0.005wt.%, N≤0.008wt.%, and the rest are Fe and unavoidable impurity elements.

[0051] In the embodiment of the present application, when the low Cr pipeline steel pipe is suitable for X70 grade oil and gas transmission pipeline, the content of each chemical element in the low Cr pipeline steel plate is as follows: C 0.05-0.12wt.%, Si 0.10-0.30wt.%, Mn 1.20-1.80wt.%, Cr 0.50-1.50wt.%, Mo 0.10-0.30wt.%, Ni 0.10-0.30wt.%, Nb 0.02-0.06wt.%, Cu 0.10-0.25wt.%, V 0.03-0.08wt.%, Ti 0.01-0.03wt.%, Al 0.015-0.050wt.%, P≤0.015wt.%, S≤0.005wt.%, N≤0.008wt.%, and the rest are Fe and unavoidable impurity elements.

[0052] In the embodiment of the present application, when the low Cr pipeline steel pipe is suitable for X80 grade oil and gas transmission pipeline, the content of each chemical element in the low Cr pipeline steel plate is as follows: C 0.04-0.08wt.%, Si 0.10-0.25wt.%, Mn 1.70-2.00wt.%, Cr 0.80-1.20wt.%, Mo 0.20-0.35wt.%, Ni 0.10-0.30wt.%, Nb 0.05-0.08wt.%, Cu 0.10-0.25wt.%, V 0.05-0.10wt.%, Ti 0.02-0.04wt.%, Al 0.015-0.050wt.%, P≤0.012wt.%, S≤0.003wt.%, N≤0.008wt.%, and the rest are Fe and unavoidable impurity elements.

[0053] Based on the aforementioned low-Cr pipeline steel plate, the present invention also conducted the following multiple sets of experiments. The chemical element contents of the low-Cr pipeline steel plate suitable for X60-grade oil and gas pipelines are shown in Table 1, the chemical element contents of the low-Cr pipeline steel plate suitable for X70-grade oil and gas pipelines are shown in Table 2, and the chemical element contents of the low-Cr pipeline steel plate suitable for X80-grade oil and gas pipelines are shown in Table 3. It should be noted that the raw materials used in the following experiments were all commercially available raw materials.

[0054]

[0055]

[0056]

[0057] Based on the aforementioned heat treatment process for low-Cr pipeline steel plates, and according to the chemical element contents in low-Cr pipeline steel plates suitable for X70-grade oil and gas pipelines shown in Table 2, the present invention further conducted the following multiple groups of experiments.

[0058] Example 1

[0059] A heat treatment process for a low-Cr pipeline steel plate is provided, wherein a steel billet after material preparation, smelting, and casting is subjected to heat treatment to produce a low-Cr pipeline steel plate. The heat treatment process is a composite heat treatment process, specifically comprising: S1. Controlled rolling and controlled cooling (TMCP) stage: The cast steel billet is cooled at room temperature until it is fully austenitized and then kept warm for 60 minutes. It is then hot rolled in multiple passes with a final rolling temperature of 850°C. Finally, the steel billet after multiple hot rolling is accelerated to 500°C at a cooling rate of 20°C / s. The temperature of the steel billet when it is fully austenitized is 1050°C, and the cumulative deformation of the steel billet thickness after multiple hot rolling is 75%.

[0060] S2, two-phase zone quenching stage: the steel billet after controlled rolling and controlled cooling treatment is heated again to 750℃ and kept warm for 30min, first water-cooled to 300℃, and then air-cooled to room temperature.

[0061] S3, gradient tempering stage: the steel billet after two-phase zone quenching treatment is subjected to three-stage tempering treatment, then furnace cooled to 400℃, and then air cooled to room temperature, wherein the first stage tempering temperature is 350℃, and the heat preservation is 1.5h; the second stage tempering temperature is 500℃, and the heat preservation is 1h; the third stage tempering temperature is 580℃, and the heat preservation is 0.5h.

[0062] Example 2

[0063] A heat treatment process for a low-Cr pipeline steel plate is provided, wherein a steel billet after material preparation, smelting, and casting is subjected to heat treatment to produce a low-Cr pipeline steel plate. The heat treatment process is a composite heat treatment process, specifically comprising: S1. Controlled rolling and controlled cooling (TMCP) stage: The cast steel billet is cooled down to fully austenitized at room temperature and then kept at this temperature for 45 minutes. It is then hot rolled in multiple passes with a final rolling temperature of 870°C. Finally, the steel billet after multiple hot rolling is accelerated to 550°C at a cooling rate of 18°C / s. The temperature of the steel billet when it is fully austenitized is 1080°C, and the cumulative deformation of the steel billet thickness after multiple hot rolling is 80%.

[0064] S2, two-phase zone quenching stage: the steel billet after controlled rolling and controlled cooling treatment is heated again to 780℃ and kept warm for 25min, first water-cooled to 300℃, and then air-cooled to room temperature.

[0065] S3, gradient tempering stage: the steel billet after two-phase zone quenching treatment is subjected to three-stage tempering treatment, then furnace cooled to 400℃, and then air cooled to room temperature, wherein the first stage tempering temperature is 400℃, and the heat preservation is 1.2h; the second stage tempering temperature is 520℃, and the heat preservation is 1.2h; the third stage tempering temperature is 600℃, and the heat preservation is 1h.

[0066] Example 3

[0067] A heat treatment process for a low-Cr pipeline steel plate is provided, wherein a steel billet after material preparation, smelting, and casting is subjected to heat treatment to produce a low-Cr pipeline steel plate. The heat treatment process is a composite heat treatment process, specifically comprising: S1. Controlled rolling and controlled cooling (TMCP) stage: The cast steel billet is cooled at room temperature until it is fully austenitized and then kept warm for 30 minutes. It is then hot rolled in multiple passes with a final rolling temperature of 890°C. Finally, the steel billet after multiple hot rolling is accelerated to 520°C at a cooling rate of 25°C / s. The temperature of the steel billet when it is fully austenitized is 1100°C, and the cumulative deformation of the steel billet thickness after multiple hot rolling is 70%.

[0068] S2, two-phase region quenching stage: the steel billet after controlled rolling and controlled cooling treatment is heated to 790 DEG C again for 20 min, first water cooled to 300 DEG C, and then air cooled to room temperature.

[0069] S3, gradient tempering stage: the steel billet after two-phase region quenching treatment is subjected to three-stage tempering treatment, then furnace cooled to 400 DEG C, and then air cooled to room temperature, wherein the first stage tempering temperature is 380 DEG C, the holding time is 1.8 h; the second stage tempering temperature is 480 DEG C, the holding time is 1.5 h; the third stage tempering temperature is 620 DEG C, the holding time is 0.8 h.

[0070] Example 4

[0071] A heat treatment process of a low-Cr pipeline steel plate, a low-Cr pipeline steel plate is prepared after a steel billet after preparation, smelting and casting is subjected to heat treatment, the heat treatment process is a composite heat treatment process, specifically comprising: S1, TMCP stage of controlled rolling and controlled cooling: the steel billet after casting is cooled to complete austenitization at room temperature and then held for 50 min, then multi-pass hot rolling, the final rolling temperature is 880 DEG C, finally the steel billet after multi-pass hot rolling is accelerated cooled to 580 DEG C at a cooling rate of 22 DEG C / s, wherein the temperature of the steel billet when complete austenitization is 1070 DEG C, and the cumulative deformation of the steel billet after multi-pass hot rolling is 80%.

[0072] S2, two-phase region quenching stage: the steel billet after controlled rolling and controlled cooling treatment is heated to 770 DEG C again for 30 min, first water cooled to 300 DEG C, and then air cooled to room temperature.

[0073] S3, gradient tempering stage: the steel billet after two-phase region quenching treatment is subjected to three-stage tempering treatment, then furnace cooled to 400 DEG C, and then air cooled to room temperature, wherein the first stage tempering temperature is 450 DEG C, the holding time is 1 h; the second stage tempering temperature is 550 DEG C, the holding time is 2 h; the third stage tempering temperature is 650 DEG C, the holding time is 0.5 h.

[0074] Example 5

[0075] A heat treatment process of a low-Cr pipeline steel plate, a low-Cr pipeline steel plate is prepared after a steel billet after preparation, smelting and casting is subjected to heat treatment, the heat treatment process is a composite heat treatment process, specifically comprising: S1. Controlled rolling and controlled cooling (TMCP) stage: The cast steel billet is cooled down at room temperature until it is fully austenitized and then kept warm for 40 minutes. It is then hot rolled in multiple passes with a final rolling temperature of 900°C. Finally, the steel billet after multiple hot rolling is accelerated to 600°C at a cooling rate of 15°C / s. The temperature of the steel billet when it is fully austenitized is 1060°C, and the cumulative deformation of the steel billet thickness after multiple hot rolling is 75%.

[0076] S2, two-phase zone quenching stage: the steel billet after controlled rolling and controlled cooling treatment is heated again to 800℃ and kept warm for 20min, first water-cooled to 300℃, and then air-cooled to room temperature.

[0077] S3, gradient tempering stage: the steel billet after two-phase zone quenching treatment is subjected to three-stage tempering treatment, then furnace cooled to 400℃, and then air cooled to room temperature, wherein the first stage tempering temperature is 300℃, and the heat preservation is 2h; the second stage tempering temperature is 450℃, and the heat preservation is 1.5h; the third stage tempering temperature is 550℃, and the heat preservation is 1.5h.

[0078] Based on the contents of various chemical elements in the low-Cr pipeline steel plate suitable for X70-grade oil and gas pipelines shown in Scheme 1 in Table 2, the present invention conducted the following multiple groups of control experiments.

[0079] Comparative Example 1

[0080] A heat treatment process for a low-Cr pipeline steel plate is provided, wherein a steel billet after material preparation, smelting, and casting is subjected to heat treatment to produce a low-Cr pipeline steel plate. The heat treatment process is a composite heat treatment process, specifically comprising: S1. Controlled rolling and controlled cooling (TMCP) stage: The cast steel billet is cooled at room temperature until it is fully austenitized and then kept warm for 60 minutes. It is then hot rolled in multiple passes with a final rolling temperature of 950°C. Finally, the steel billet after multiple hot rolling is accelerated to 700°C at a cooling rate of 10°C / s. The temperature of the steel billet when it is fully austenitized is 1200°C, and the cumulative deformation of the steel billet thickness after multiple hot rolling is 60%.

[0081] S2, two-phase zone quenching stage: the steel billet after controlled rolling and controlled cooling treatment is heated again to 700℃ and kept warm for 30min, first water-cooled to 300℃, and then air-cooled to room temperature.

[0082] S3, tempering stage: heat the steel billet after two-phase zone quenching treatment to 700℃, keep it warm for 2h, and then air cool it to room temperature.

[0083] Comparative Example 2

[0084] The application discloses a heat treatment process of a low-Cr pipeline steel plate. S1, TMCP stage of controlled rolling and controlled cooling: after the steel billet is cast, the steel billet is cooled at room temperature to complete austenitization and then is kept for 20 min, and then is subjected to multi-pass hot rolling, and the final rolling temperature is 800 DEG C; finally, the steel billet after the multi-pass hot rolling is accelerated to 400 DEG C at a cooling speed of 30 DEG C / s, wherein the temperature of the steel billet when complete austenitization is 1000 DEG C, and the cumulative deformation of the steel billet after the multi-pass hot rolling is 85%.

[0085] S2, two-phase region quenching stage: the steel billet after the controlled rolling and controlled cooling is heated to 850 DEG C again and kept for 10 min, is cooled to 300 DEG C by water cooling, and then is cooled to room temperature by air cooling.

[0086] S3, tempering stage: the steel billet after the two-phase region quenching is heated to 500 DEG C, is kept for 0.5 h, and then is cooled to room temperature by air cooling.

[0087] Comparative example 3

[0088] The application discloses a heat treatment process of a low-Cr pipeline steel plate. S1, TMCP stage of controlled rolling and controlled cooling: after the steel billet is cast, the steel billet is cooled at room temperature to complete austenitization and then is kept for 20 min, and then is subjected to multi-pass hot rolling, and the final rolling temperature is 800 DEG C; finally, the steel billet after the multi-pass hot rolling is accelerated to 400 DEG C at a cooling speed of 30 DEG C / s, wherein the temperature of the steel billet when complete austenitization is 1000 DEG C, and the cumulative deformation of the steel billet after the multi-pass hot rolling is 85%.

[0089] S2, two-phase region quenching stage: the steel billet after the controlled rolling and controlled cooling is heated to 850 DEG C again and kept for 10 min, is cooled to 300 DEG C by water cooling, and then is cooled to room temperature by air cooling.

[0090] S3, tempering stage: the steel billet after the two-phase region quenching is heated to 500 DEG C, is kept for 0.5 h, and then is cooled to room temperature by air cooling.

[0091] Comparative example 4

[0092] The application discloses a heat treatment process of a low-Cr pipeline steel plate. S1, TMCP stage: the billet after casting is cooled to room temperature, and then heated to complete austenitization and kept for 30 min, and then hot rolled in multiple passes, with a final rolling temperature of 900 ℃, and finally accelerated cooling to 500 ℃ at a cooling rate of 25 ℃ / s, wherein the temperature of the billet when completely austenitized is 1100 ℃, and the cumulative deformation of the billet after multiple hot rolling is 80%.

[0093] S2, two-phase region quenching stage: the billet after TMCP treatment is heated to 750 ℃ again and kept for 5 min, and then water-cooled to 300 ℃, and then air-cooled to room temperature.

[0094] Comparative Example 5

[0095] A heat treatment process for a low-Cr pipeline steel plate, wherein a billet after preparation, smelting and casting is subjected to heat treatment to obtain a low-Cr pipeline steel plate, and the heat treatment process specifically comprises: S1, TMCP stage: the billet after casting is cooled to room temperature, and then heated to complete austenitization and kept for 60 min, and then hot rolled in multiple passes, with a final rolling temperature of 850 ℃, and finally accelerated cooling to 500 ℃ at a cooling rate of 20 ℃ / s, wherein the temperature of the billet when completely austenitized is 1050 ℃, and the cumulative deformation of the billet after multiple hot rolling is 75%.

[0096] S2, tempering stage: the billet after two-phase region quenching treatment is heated to 600 ℃ and kept for 2 h, and then air-cooled to room temperature.

[0097] Performance test: including yield strength test, tensile strength test, elongation test and corrosion rate test.

[0098] In this experiment, the elongation test procedure is as follows: (1) Take a plate-shaped sample along the rolling direction, and polish each surface of the sample with SiC sandpaper of 120#, 400#, 800#, 1000# and 2000# in sequence to mirror smoothness, and ensure that the surface is free of oxide scale and scratches.

[0099] (2) Use a universal testing machine to perform tensile test at a speed of 1 mm / min, and set 3 parallel samples for each group.

[0100] (3) Continuously load until the sample breaks, and tightly splice the broken sample to measure the final gauge length .

[0101] (4) Calculate the elongation of the sample by the following formula: , In the formula, is the initial length of the specimen.

[0102] It should be noted that the final gauge length in this experiment is and the initial length of the specimen Accurate to 0.1mm.

[0103] In this experiment, the corrosion rate test steps are as follows: (1) Use 120#, 400#, 800#, 1000#, and 2000# SiC sandpaper to polish each surface of the sample until it is mirror smooth, ensuring that there is no oxide scale and scratches on the surface. Then use alcohol or acetone to ultrasonically clean it. After drying, weigh it and record the initial mass W1.

[0104] (2) Prepare a corrosive solution (5% NaCl + 0.5% H2S) that simulates the oil and gas environment according to the test requirements. Place the solution in a glass container with a condensation reflux device and control the temperature to 80°C (oil and gas transportation environment conditions). Use a polytetrafluoroethylene clamp to hang the sample to ensure that it is completely immersed and avoid contact with the container. Set up three parallel samples for each group.

[0105] (3) After the sample is removed, it is immediately rinsed with deionized water, and then ultrasonically cleaned in an alkaline cyanide solution for 5 minutes to remove corrosion products. After drying, it is weighed W2, where the alkaline cyanide solution is an aqueous solution of 10wt.% KCN+1wt.% KMnO4.

[0106] (4) Calculation of the corrosion rate of low Cr pipeline steel plates in 5% NaCl + 0.5% H2S solution v , as shown below: , , Where, Indicates the weight loss of the sample, in g; Indicates the material density in g / cm 3 , steel usually takes 7.85; Indicates the exposed area of ​​the sample, in cm 2 ; Indicates corrosion time, unit: h.

[0107] In this experiment, based on the chemical element contents of the four low-Cr pipeline steel plates suitable for X70-grade oil and gas pipelines listed in Table 2, low-Cr pipeline steel plates were prepared using the processes described in Examples 1 to 5, and their performance was tested. The test results are shown in Tables 4-7. Table 4 presents the comparative performance test results of the low-Cr pipeline steel plates produced using the processes of Examples 1 to 5 for Scheme 1 in Table 2, Table 5 presents the comparative performance test results of the low-Cr pipeline steel plates produced using the processes of Examples 1 to 5 for Scheme 2 in Table 2, Table 6 presents the comparative performance test results of the low-Cr pipeline steel plates produced using the processes of Examples 1 to 5 for Scheme 3 in Table 2, and Table 7 presents the comparative performance test results of the low-Cr pipeline steel plates produced using the processes of Examples 1 to 5 for Scheme 1 in Table 4.

[0108]

[0109]

[0110]

[0111]

[0112] It can be clearly seen from Tables 4-7 that the elongation obtained by using Example 3 according to Scheme 1 in Table 2 is the highest, but some strength and corrosion resistance are sacrificed; while the elongation obtained by using Example 2 according to Scheme 1 in Table 2 is lower, but the overall performance is the best, that is, high strength + low corrosion rate.

[0113] The reason why Example 2, which is applied according to Scheme 1 in Table 2, shows the most excellent comprehensive performance is mainly due to the perfect balance between its carefully designed component system and heat treatment process. The component adopts a Cr content of 1.20wt.%, combined with 0.20wt.% Mo and 0.18wt.% Cu, to form an efficient corrosion-resistant synergistic system. The Cr element is enriched in the ferrite phase through two-phase heat treatment, which increases the effective Cr concentration on the surface and significantly enhances the density and self-healing ability of the Cr2O3 passivation film. At the same time, the 0.08wt.% low-carbon design combined with the micro-alloying of 0.04wt.% Nb and 0.05wt.% V ensures the excellent mechanical properties of the material. In addition, this embodiment forms a complex phase structure with ultrafine acicular ferrite as the matrix and nano-scale martensite islands as the second phase through optimized controlled rolling and controlled cooling TMCP process and two-phase quenching treatment. The acicular ferrite grain size is controlled within the 3-5μm range, providing excellent plastic deformation capability and ensuring an 18% elongation. The evenly distributed nano-martensite islands (50-100nm in size) enhance the material's yield strength to 620MPa through strong second-phase strengthening. This "soft matrix + hard second phase" microstructure design avoids the inherent strength limitations of a single ferrite structure while overcoming the limitations of a fully martensite structure's poor plasticity. More importantly, this embodiment utilizes a precise gradient tempering process to form a 5-10nm thick Cr-rich transition zone (with a Cr content of 8-10%) around the nano-martensite islands. This microscopic compositional manipulation allows the material to maintain high strength while reducing its corrosion rate to an exceptionally low 0.07mm / y.

[0114] In contrast, although Example 2 according to Scheme 2 in Table 2 partially compensates for the deficiency of low Cr by adding a high Si content and Mo, its yield strength and corrosion resistance are still significantly inferior, indicating that the Cr element must be at a level of 1.20wt.% to ensure the formation of a sufficiently dense passivation film, and the compensatory effect of Si is limited; the corrosion resistance of Example 2 according to Scheme 3 in Table 2 is comparable, but its higher carbon content leads to a decrease in elongation; although Example 2 according to Scheme 4 in Table 2 maintains similar corrosion resistance to Example 2 according to Scheme 1 in Table 2, the strength advantage of Example 2 according to Scheme 1 in Table 2 is more obvious, which is mainly attributed to its reasonable Mn content design, which not only ensures sufficient solid solution strengthening effect, but also avoids the work hardening problem that may be caused by high Mn. Based on the above analysis, the multi-scale coordinated control of the organizational characteristics in Example 2 according to Scheme 1 in Table 2 is the fundamental reason for achieving the optimal balance of strength-plasticity-corrosion resistance.

[0115] In addition, in this experiment, based on the content of each chemical element in the low-Cr pipeline steel plate suitable for X70-grade oil and gas pipelines in Scheme 1 listed in Table 2, low-Cr pipeline steel plates were prepared using the processes described in Comparative Examples 1 to 5, and their performance was tested. At the same time, the performance was compared with that of Example 2 applied according to Scheme 1 in Table 2. The comparison results are shown in Table 8.

[0116]

[0117] It can be clearly seen from Table 8 that the controlled rolling and controlled cooling TMCP + two-phase zone quenching + gradient tempering process adopted in Example 2 according to Scheme 1 in Table 2 has successfully broken through the performance bottleneck of the traditional process through multi-scale organizational regulation. From the perspective of mechanical properties, this embodiment shows the most excellent strength. Its yield strength of 620MPa and tensile strength of 705MPa are significantly higher than all comparative examples, which is mainly due to its fine microstructure control and optimized heat treatment process. It is particularly noteworthy that although the elongation of comparative example 1 reaches 30%, its strength index is significantly low, which shows that the process design that simply pursues plasticity while ignoring strength is difficult to meet the needs of engineering applications. In terms of corrosion resistance, this embodiment performs best with a corrosion rate of 0.07mm / y, which is 82.5% lower than that of comparative example 5. This excellent corrosion resistance is crucial for the long-term service of oil and gas pipelines. Furthermore, the data from Comparative Examples 2 and 3 show that when the yield strength falls below 450 MPa, the corrosion rate of the material increases dramatically (0.30-0.40 mm / y), demonstrating that ensuring a basic strength level is essential for achieving good corrosion resistance during material development. Comparative Example 4 exhibits a "high strength, high corrosion" characteristic. While its yield strength of 500 MPa is acceptable, its corrosion rate of 0.35 mm / y completely fails to meet engineering requirements. This demonstrates that a process design that simply pursues strength while ignoring corrosion resistance is a failure. Comparative Example 5 exhibits similar high plasticity as Comparative Example 1, but its strength is the lowest among the examples in Comparative Example 2, and its corrosion rate is even higher than that of Comparative Example 4, making it even more difficult to meet engineering requirements. This further demonstrates the difficulty of traditional processes in balancing strength and corrosion resistance. A comprehensive comparison shows that Example 2, which utilizes Scheme 1 in Table 2, achieves the best balance among the three key indicators of strength, plasticity, and corrosion resistance. Its excellent corrosion resistance, in particular, demonstrates the unique advantages of this process in suppressing localized corrosion and element segregation. The performance defects of each comparative example also confirm the scientific nature and advancement of the process design of this embodiment from the opposite side.

[0118] In summary, the present invention achieves dual optimization of material microstructure and surface properties under the synergistic effect of controlled rolling and controlled cooling TMCP + two-phase zone quenching + gradient tempering process: on the one hand, it significantly refines the steel grain structure and effectively reduces the active interface area of ​​electrochemical corrosion; on the other hand, it promotes the rapid formation of corrosion product film with excellent protective performance on the steel surface. This dense product film has outstanding Cl － The barrier capacity can significantly improve the long-term corrosion resistance of pipeline steel in harsh environments. At the same time, by precisely controlling the phase transformation process, the present invention ensures that the material achieves excellent mechanical properties such as excellent yield strength and tensile strength, an appropriate yield-to-strength ratio, good elongation, and outstanding impact toughness. This meets the stringent material performance requirements of modern oil and gas pipelines and is suitable for X60-X80 oil and gas pipelines, which have high requirements for high strength, toughness, and corrosion resistance.

[0119] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A low Cr pipeline steel plate, characterized in that: The contents of each chemical element are as follows: C 0.04~0.12wt.%, Si 0.10~0.30wt.%, Mn 1.20~2.00wt.%, Cr 0.50~1.50wt.%, Mo 0.10~0.35wt.%, Ni 0.10~0.30wt.%, Nb 0.02~0.08wt.%, Cu 0.10~0.25wt.%, V 0.03~0.10wt.%, Ti 0.01~0.04wt.%, Al 0.015~0.050wt.%, P≤0.015wt.%, S≤0.005wt.%, N≤0.008wt.%, and the rest are Fe and inevitable impurity elements.

2. A heat treatment process for low Cr pipeline steel plates, comprising heat treating a steel billet after material preparation, smelting, and casting to produce the low Cr pipeline steel plate according to claim 1, characterized in that: The heat treatment process is a composite heat treatment process, specifically including: S1, controlled rolling and controlled cooling (TMCP) stage: the steel billet after casting is cooled down at room temperature until it is completely austenitized and then kept at this temperature for 30-60 minutes, followed by multiple hot rolling, with the final rolling temperature being 850-900°C. Finally, the steel billet after multiple hot rolling is accelerated cooled to 500-600°C at a cooling rate of 15-25°C / s, wherein the temperature of the steel billet when it is completely austenitized is 1050-1100°C; S2, two-phase zone quenching stage: the steel billet after controlled rolling and controlled cooling treatment is heated again to 750-800℃ and kept at this temperature for 20-30min, first water-cooled to 300℃, and then air-cooled to room temperature; S3, gradient tempering stage: the steel billet after two-phase zone quenching treatment is subjected to three-stage tempering treatment, then furnace cooled to 400℃, and then air cooled to room temperature, wherein the first stage tempering temperature is 300-450℃, and the heat preservation is 1-2h; the second stage tempering temperature is 450-550℃, and the heat preservation is 1-2h; the third stage tempering temperature is 550-650℃, and the heat preservation is 0.5-1.5h.

3. The heat treatment process of a low Cr pipeline steel plate according to claim 2, characterized in that: After multiple hot rolling passes in the controlled rolling and controlled cooling (TMCP) stage, the cumulative deformation of the billet thickness is 70-80%, the final cooling structure is a mixed structure of fine bainite / acicular ferrite, and the grain size reaches ASTM grade 8-10.

4. The heat treatment process of a low Cr pipeline steel plate according to claim 2, characterized in that: During the two-phase quenching stage, the Cr element is enriched in the ferrite phase, and the surface Cr content increases by 40% to 60% relative to the Cr content inside the matrix, forming a continuous Cr2O3 passivation film.

5. The heat treatment process of a low Cr pipeline steel plate according to claim 2, characterized in that: After quenching in the two-phase region, the microstructure is a Cr-rich ferrite + low-carbon martensite / bainite complex structure with a retained austenite content of 5-10%.

6. The heat treatment process of a low Cr pipeline steel plate according to claim 2, characterized in that: The carbon equivalent Ceq of the steel plate produced by the heat treatment process of the low-Cr pipeline steel plate is less than or equal to 0.40%, and the welding cold crack sensitivity index Pcm is less than or equal to 0.18%.

7. The heat treatment process of a low Cr pipeline steel plate according to claim 2, characterized in that: The steel plate produced by the heat treatment process of the low-Cr pipeline steel plate has a yield strength of ≥500 MPa, a tensile strength of ≥550 MPa, an elongation of ≥15%, a corrosion rate in a 5% NaCl+0.5% H2S solution of ≤0.30 mm / y, and an impact energy of ≥100 J at -20°C.

8. A low-Cr pipeline steel pipe, manufactured from a steel plate produced by the heat treatment process for a low-Cr pipeline steel plate according to any one of claims 2 to 7.

9. The low Cr pipeline steel pipe according to claim 8, characterized in that: The low-Cr pipeline steel pipe is suitable for X60-X80 grade oil and gas transmission pipelines.

10. The low Cr pipeline steel pipe according to claim 9, characterized in that: When the low Cr pipeline steel pipe is suitable for X60 grade oil and gas transmission pipeline, the content of each chemical element in the low Cr pipeline steel plate is as follows: C 0.06-0.10wt.%, Si 0.15-0.25wt.%, Mn 1.30-1.60wt.%, Cr 0.80-1.20wt.%, Mo 0.10-0.20wt.%, Ni 0.10-0.30wt.%, Nb 0.03-0.05wt.%, Cu 0.10-0.25wt.%, V 0.03-0.06wt.%, Ti 0.01-0.02wt.%, Al 0.015-0.050wt.%, P≤0.015wt.%, S≤0.005wt.%, N≤0.008wt.%, and the rest are Fe and unavoidable impurity elements; When applicable to X70 grade oil and gas transmission pipelines, the contents of various chemical elements in low Cr pipeline steel plates are as follows: C 0.05-0.12wt.%, Si 0.10-0.30wt.%, Mn 1.20-1.80wt.%, Cr 0.50-1.50wt.%, Mo 0.10-0.30wt.%, Ni 0.10-0.30wt.%, Nb 0.02-0.06wt.%, Cu 0.10-0.25wt.%, V 0.03-0.08wt.%, Ti 0.01-0.03wt.%, Al 0.015-0.050wt.%, P≤0.015wt.%, S≤0.005wt.%, N≤0.008wt.%, and the rest are Fe and unavoidable impurity elements; When suitable for X80 grade oil and gas pipelines, the contents of various chemical elements in low Cr pipeline steel plates are as follows: C 0.04~0.08wt.%, Si 0.10~0.25wt.%, Mn 1.70~2.00wt.%, Cr 0.80~1.20wt.%, Mo 0.20~0.35wt.%, Ni 0.10~0.30wt.%, Nb 0.05~0.08wt.%, Cu 0.10~0.25wt.%, V 0.05~0.10wt.%, Ti 0.02~0.04wt.%, Al 0.015~0.050wt.%, P≤0.012wt.%, S≤0.003wt.%, N≤0.008wt.%, and the rest are Fe and unavoidable impurity elements.

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