A medium temperature pipeline steel and a method for manufacturing the same

Abstract

The application provides a medium-temperature pipeline steel and a preparation method thereof, and relates to the field of steel smelting, and comprises the following components: C: 0.12-0.16%, Si: 0.50-0.65%, Mn: 0.45-0.65%, Cr: 1.20-1.50%, Mo: 0.45-0.65%, Ti: 0.010-0.020%, Al: 0.03-0.05%, S: ≤0.002%, P: ≤0.012%, H: ≤0.00015%, and the rest is Fe and inevitable impurities; in the preparation, the target molten steel is smelted to obtain a casting blank, the casting blank is heated to a target temperature, and then two-stage controlled rolling and controlled cooling are adopted to reach room temperature, and then the obtained steel plate is sequentially subjected to quenching and tempering heat treatment to obtain a product steel plate; the formula and the preparation method of the medium-temperature pipeline steel are simultaneously optimized, the prepared steel plate has high strength, good machinability and excellent creep resistance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel smelting, in particular to a steel for medium-temperature pipeline and a preparation method thereof. BACKGROUND

[0002] The steel for medium-temperature pipeline is a low-carbon low-alloy heat-resistant steel, which is mainly applied to the manufacture of pressure pipelines in the oil refining and chemical industry, coal liquefaction and gasification industries; its service environment usually involves extreme working conditions such as medium temperature 300-550℃, high pressure 10-30MPa and hydrogen corrosion, so it puts forward very high requirements on the comprehensive mechanical properties of the material. Specifically, this kind of steel not only needs to have excellent high-temperature strength to resist creep and stress relaxation, but also needs to maintain good low-temperature toughness, fatigue resistance and hydrogen corrosion resistance during long-term service. In addition, since the steel plate needs to be processed into various complex-shaped pipeline components, the material not only needs to meet the basic mechanical property requirements, that is, "usable", but also needs to have good processability and weldability, that is, "good to use", which puts strict restrictions on the surface hardness of the steel plate. These application requirements make the composition design, microstructure control and heat treatment process of the steel face multiple challenges.

[0003] The prior art, such as patent CN106435396B, discloses a manufacturing method of a high-temperature-resistant and hydrogen sulfide corrosion-resistant pressure vessel steel plate, which adopts a composition design of C 0.03-0.12%, Al 0.40-1.50%, Cr 2.75-3.25%, Mo 0.30-0.70%, Nb 0.020-0.035%, realizes the synergy of 500℃ yield strength not less than 300MPa, hydrogen sulfide corrosion resistance (SSCC critical bending stress not less than 0.9σs) and-80℃ impact energy not less than 240J through off-line refining, controlled rolling, 920-1000℃ normalizing and 690-740℃ tempering process; the Al content in the formula is relatively high, the affinity between Al and oxygen is extremely strong, a large amount of Al2O3 inclusions will be generated in the high-Al molten steel, these hard oxides will deposit on the inner wall of the submerged entry nozzle to form nodules, which can easily cause continuous casting blockage; and the alloy cost of the formula is relatively high. Patent CN111004978B discloses a production method of a low-alloy high-temperature-resistant pressure vessel steel plate, which takes low-alloying combined with V / Nb micro-alloying as the core, realizes the target of 500℃ yield strength not less than 245MPa, tensile strength 480-620MPa and cost reduction through TMCP controlled rolling combined with laminar cooling and normalizing process; however, the production method adopts low Cr / Mo design, the product has insufficient 500℃ creep resistance, and the hydrogen corrosion resistance elements are insufficient, and the hydrogen embrittlement sensitivity is significant.

[0004] In view of the application needs of the current medium-temperature pipeline steel, a new medium-temperature pipeline steel and a preparation method thereof are urgently needed. The preparation method can not only improve the medium-temperature mechanical properties and solve the pipeline damage problem caused by creep, but also optimize the alloy ratio and process and solve the imbalance between strength and toughness. At the same time, the preparation method can balance the contradictory relationship among wear resistance, processability and weldability in the surface hardness control. SUMMARY

[0005] The application aims to provide a medium-temperature pipeline steel and a preparation method thereof. By optimizing the alloy ratio and preparation process of the medium-temperature pipeline steel, the steel organization is stable, and compared with the existing steel, the steel not only can maintain good strength in high-temperature service environment, but also has excellent processability and creep resistance.

[0006] To achieve the above-mentioned purpose, the application proposes the following technical solutions.

[0007] In a first aspect, a medium-temperature pipeline steel is provided, characterized in that the steel comprises the following chemical components in percentage by weight: C: 0.12-0.16%, Si: 0.50-0.65%, Mn: 0.45-0.65%, Cr: 1.20-1.50%, Mo: 0.45-0.65%, Ti: 0.010-0.020%, Al: 0.03-0.05%, S≤0.002%, P:≤0.012%, H≤0.00015%, and the rest is Fe and other unavoidable impurities.

[0008] Further, the microstructure of the steel plate of the medium-temperature pipeline steel is a complex structure of ferrite and tempered bainite, the content of ferrite is 15-25%, the content of tempered bainite is 75-85%, and the width of bainite lath after quenching is 0.2-0.5 μm, and the grain size of ferrite after tempering is 5-8 μm.

[0009] Further, the thickness specification of the product steel plate made of the medium-temperature pipeline steel is 8-40 mm; the yield strength of the product steel plate is 450-500 MPa, the tensile strength is 580-650 MPa, and the elongation is not less than 25%; the impact energy KV2 of the product steel plate at-20°C is not less than 280 J, the Brinell hardness value of the surface of the steel plate is 180-200 HBW; and the critical bending stress of the product steel plate against hydrogen sulfide stress corrosion is not less than 0.9σ S , and the yield strength R p0.2 of the steel plate in the medium-temperature range of 450-550°C is not less than 300 MPa.

[0010] In a second aspect, a preparation method of a medium-temperature pipeline steel is provided, comprising the following steps:

[0011] The molten iron is pre-desulfurized, smelted in a converter, refined in a ladle, and refined in RH vacuum, and then is continuously cast into a 220-320mm thick slab and is stacked and cooled to room temperature; the molten iron comprises the following chemical components in percentage by weight: C: 0.12-0.16%, Si: 0.50-0.65%, Mn: 0.45-0.65%, Cr: 1.20-1.50%, Mo: 0.45-0.65%, Ti: 0.010-0.020%, Al: 0.03-0.05%, S: ≤0.002%, P: ≤0.012%, H: ≤0.00015%, and the rest is Fe and other inevitable impurities;

[0012] The slab is subjected to a low-temperature heating system, which is that the slab is kept in a furnace for a time t and is raised from room temperature to a target temperature of 1140-1160℃, and the temperature difference between the upper and lower surfaces of the slab when discharged is not more than 20℃; wherein t = h + 80, min, and h represents the thickness of the slab;

[0013] The slab after low-temperature heating is subjected to two-stage controlled rolling, and the parameters of the two-stage controlled rolling are that the first-stage rough rolling temperature is 1030-1090℃, the first-stage finish rolling temperature is 960-990℃, and the slab is kept warm after rolling, and the warm thickness is 2.5-3.0H; the second-stage rough rolling temperature is (960-2H)±15℃, and the finish rolling temperature is (860-1.3H)±15℃; wherein H is the thickness of the product steel plate;

[0014] The steel plate after controlled rolling is first water-cooled to 600-660℃, and then is air-cooled to room temperature;

[0015] The steel plate air-cooled to room temperature is subjected to quenching heat treatment, and the parameters of the quenching heat treatment are that the quenching temperature is 910-930℃, the holding time is 2H+10min-2H+20min, the cooling rate after discharge is 25-35℃ / s, and then the steel plate is water-cooled to 320-380℃, and then is air-cooled to room temperature;

[0016] The steel plate quenched and cooled to room temperature is subjected to tempering heat treatment, and the parameters of the tempering heat treatment are that the tempering temperature is 670-730℃, and the steel plate is cooled to room temperature after a preset holding time, and a product steel plate is prepared.

[0017] Further, the specific heating process of the low-temperature heating system is that:

[0018] The casting blank is heated by a five-stage heating process including a heat recovery stage, a preheating stage, a first heating stage, a second heating stage and a soaking stage, wherein the temperature of the heat recovery stage is not more than 900 DEG C, the temperature of the preheating stage is 900-1050 DEG C, the temperature of the first heating stage is 1000-1100 DEG C, the temperature of the second heating stage is 1100-1160 DEG C, and the temperature of the soaking stage is 1130-1150 DEG C, and the residence time of the casting blank in the second heating stage is not less than 1 / 5t, and the residence time of the casting blank in the soaking stage is not less than 1 / 5t.

[0019] Further, the parameters for water cooling the controlled-rolled product steel plate to 600-660 DEG C are as follows: the water entry temperature is not less than 780 DEG C, and the cooling rate after water entry is 15-25 DEG C / s to 600-660 DEG C.

[0020] Further, the preset holding time in the tempering heat treatment process is as follows: when the thickness H of the product steel plate is not more than 20 mm, the holding time T is 4H+2 min-4H+8 min; and when the thickness H of the product steel plate is more than 20 mm, the holding time T is 3H+2 min-3H+8 min.

[0021] Further, the RH vacuum refining process of the molten steel includes controlling the degassing time to be not less than 15 minutes under the condition that the vacuum degree is not more than 2 mBar.

[0022] Further, the time for the casting blank to be subjected to the stack cooling is not less than 48 hours.

[0023] Further, the thickness specification of the product steel plate prepared is 8-40 mm, the microstructure of the product steel plate is a complex phase structure of ferrite and tempered bainite, the content of the ferrite is 15-25%, the content of the tempered bainite is 75-85%, and the bainite lath width after quenching is 0.2-0.5 μm, and the ferrite grain size after tempering is 5-8 μm.

[0024] The yield strength of the product steel plate is 450-500 MPa, the tensile strength is 580-650 MPa, and the elongation is not less than 25%; the product steel plate has an impact energy KV2 of not less than 280 J at -20 DEG C, and the Brinell hardness value of the surface of the product steel plate is 180-200 HBW; the critical bending stress of the product steel plate against hydrogen sulfide stress corrosion is not less than 0.9σ S , and the yield strength R p0.2 of the steel plate in the medium temperature range of 450-550 DEG C is not less than 300 MPa.

[0025] According to the above technical solution, the technical solution of the present application has the following beneficial effects:

[0026] 1) The medium temperature pipeline steel provided by the present application firstly optimizes the alloying ratio of Cr and Mo in the formula, combines controlled rolling and controlled cooling and a multi-stage heat treatment process, so that the steel plate maintains good strength in a high temperature service environment; at the same time, the Cr-Mo synergistic effect promotes the formation of stable complex carbides in the steel microstructure, effectively reduces the high temperature creep tendency of the steel through the double mechanisms of pinning dislocations and inhibiting grain boundary migration, and improves the creep resistance of the steel; the optimized microstructure type and structure size cooperate with each other to form a multi-level strengthening system, ensuring the performance stability of the steel under long-term high temperature and high pressure working conditions.

[0027] 2) The steel material is prepared by adopting the process route combining controlled rolling and controlled cooling and quenching and tempering, successfully constructing a complex phase structure of ferrite and tempered bainite, the content of ferrite being 15-25%, and the content of tempered bainite being 75-85%, the complex phase structure design fully giving play to the synergistic effect of the two phases; among them, the ferrite phase guarantees the good plasticity and toughness of the material, and the bainite phase provides high strength and creep resistance, breaking through the strength-plasticity balance limitation of traditional single-structure steel; at the same time, the optimized complex phase ratio significantly improves the material processing performance while maintaining high strength, realizing the synergistic improvement of wear resistance and weldability.

[0028] 3) The preparation method of the present application precisely controls the cooling parameters of the quenching process to obtain a fine-grained bainite matrix structure with high interface density, and the bainite lath width after quenching is 0.2-0.5 μm; this structure cooperates with the dispersed distribution of carbide network to effectively block the diffusion channel of hydrogen elements, significantly enhancing the hydrogen corrosion resistance of the material; and the method combines with the gradient tempering process design to realize the three-dimensional uniform precipitation of carbides, ensuring the stability of the structure while maintaining excellent consistency of high temperature strength.

[0029] 4) The present application further innovatively develops a thickness-adaptive dynamic rolling process, which realizes the optimization of rolling parameters of full-thickness specifications through an intelligent temperature control model; the dynamic rolling process breaks through the performance limitation of traditional rolling technology on plate thickness, significantly reduces the performance difference between the same plate and different plates, and ensures the high stability of the uniformity of the structure and the mechanical properties in batch production; the present application improves the service reliability of the material under complex working conditions through grain refinement and defect control.

[0030] It should be understood that all combinations of the aforementioned concepts and additional concepts described in greater detail below can be seen as part of the subject matter of the present disclosure as long as such concepts are not mutually inconsistent.

[0031] The foregoing and other aspects, embodiments and features of the present teachings can be better understood from the following description of the present teachings with reference to the drawings. Other aspects, embodiments and features of the present teachings will be apparent from the following description of the examples and appended claims, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0032] The drawings are not drawn to scale. In the drawings, each same or similar component will be denoted by the same reference numeral for each figures. For the sake of clarity, not every component can be labeled in every figure. Embodiments of various aspects of the present teachings will now be described, by way of example only, with reference to the accompanying drawings in which:

[0033] Figure 1 Microstructure photograph at 500 times of the longitudinal section 1 / 2 of the steel for a medium temperature pipeline produced in Example 1;

[0034] Figure 2 Microstructure photograph at 500 times of the longitudinal section 1 / 2 of the steel for a medium temperature pipeline produced in Comparative Example 1;

[0035] Figure 3 Microstructure photograph at 500 times of the longitudinal section 1 / 2 of the steel for a medium temperature pipeline produced in Comparative Example 2. DETAILED DESCRIPTION

[0036] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be described clearly and completely below with reference to the drawings of the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments of the present application. Based on the described embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort belong to the scope of protection of the present application. Unless otherwise defined, the technical terms or scientific terms used herein should be understood as the common meanings thereof by those of ordinary skill in the art.

[0037] The terms "first", "second", and similar terms used in the specification and claims of the present patent application do not necessarily connote an order of importance, but are used to distinguish one element from another. Also, the use of "a", "an" or "the" or similar referents in the context of describing the application (especially in the context of claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms "comprises", "comprising", "includes", "including" and the like can mean "including but not limited to".

[0038] The pipeline steel disclosed by the prior art can meet the basic production needs in performance, but there are problems such as high pipeline manufacturing cost, easy to produce oxidation inclusions to block the pipeline, frequent replacement, insufficient hydrogen corrosion resistance, and significant hydrogen embrittlement sensitivity, etc. due to the formula. Therefore, the present application aims to improve the steel formula and preparation process to propose a medium-temperature pipeline steel and its preparation method, which not only reduces the preparation and use cost of steel, but also further improves the performance.

[0039] Specifically, the medium-temperature pipeline steel disclosed by the present application comprises the following chemical components by weight percentage: C: 0.12-0.16%, Si: 0.50-0.65%, Mn: 0.45-0.65%, Cr: 1.20-1.50%, Mo: 0.45-0.65%, Ti: 0.010-0.020%, Al: 0.03-0.05%, S≤0.002%, P: ≤0.012%, H≤0.00015%, and the rest is Fe and other unavoidable impurities.

[0040] The role of each component and the selection of its amount in the above formula are analyzed as follows:

[0041] C: C element as a key solid solution strengthening element, the lattice distortion effect produced by interstitial solid solution can significantly improve the yield strength of the material, and its characteristic of reducing the critical cooling rate of austenite has a decisive influence on hardenability. However, excessive C element will promote the continuous precipitation of lamellar cementite, leading to a sharp deterioration of plasticity index and a significant increase in cold crack sensitivity, and it is easy to combine with diffused hydrogen to induce interface cracking risk in hydrogen-containing environment. Therefore, the content of carbon element is controlled at 0.12-0.16%.

[0042] Si: Si element and Mo element, Cr element, etc., has the effect of improving corrosion resistance and oxidation resistance, is an indispensable element for manufacturing heat-resistant steel; but because Si can deoxidize the metal in the weld pool, it has a slight strengthening effect on the weld, which is not conducive to welding; therefore, the Si content is controlled at 0.50~0.65%.

[0043] Mn: Mn element as a typical displacement solid solution strengthening element, the lattice distortion effect caused by the atomic radius difference can effectively improve the strength of ferrite matrix, and the C-Mn strengthening system formed by the synergistic effect of C and Mn is an important part of the basic strengthening mechanism of low carbon steel; however, when the Mn content exceeds the critical value, its high affinity with S element promotes the extension of strip-shaped MnS inclusions along the rolling direction, such sulfides not only become the preferred channel for hydrogen atom enrichment, but also evolve into the nucleation source of hydrogen-induced cracks under stress; therefore, the Mn content is controlled at 0.45~0.65%.

[0044] Cr: Cr element is one of the important alloying elements of steel for medium temperature environment, its strong carbide forming characteristics have a double strengthening mechanism in medium temperature environment: on the one hand, at 550℃ service temperature, the (M,Cr) 23 C6 type alloy carbide can effectively pin dislocations and inhibit grain boundary migration, hinder the high temperature diffusion trend of carbon by reducing carbon activity, and eliminate the brittleness tendency caused by grain boundary carbon segregation, so that the material still maintains excellent anti-creep stability under long-term high temperature stress; on the other hand, Cr element forms a continuous and dense Cr2O3 protective film by selective oxidation, and this oxidation layer has very low oxygen diffusion coefficient, which can build a dynamic oxidation barrier on the surface of the material, significantly improving the service life of the material in the oxidation corrosion environment below 600℃; however, when the Cr content exceeds 1.50%, the formation tendency of δ-ferrite phase increases, and carbide network is easily precipitated in the welding heat affected zone during rapid cooling, resulting in a decrease in material toughness reserve and an increase in welding cold crack sensitivity index; therefore, the Cr content is controlled at 1.20~1.50%.

[0045] Mo: Mo element is a key alloying element of steel for medium temperature environment, its strong carbide forming ability plays an important role in high temperature strengthening; Mo promotes the formation of bainite structure by inhibiting pearlite transformation, and this fine lath structure has higher interface density and dispersed carbides, which can effectively hinder the hydrogen diffusion path, thereby significantly improving the hydrogen corrosion resistance of the material; at the same time, the promotion of Mo to bainite transformation also improves the hardenability of the steel, ensuring the uniformity of the structure of thick section components; however, excessive Mo promotes the formation of brittle phase, causing instability of the structure in the welding heat affected zone and increasing the cold crack sensitivity; therefore, the Mo content is controlled at 0.45~0.65%.

[0046] Ti: Ti element is a strong carbonitride forming element, which preferentially combines with nitrogen in steel to form high-melting-point TiN nanoparticles; these fine precipitates can effectively pin austenite grain boundaries, significantly inhibit grain coarsening during hot working and welding, thereby improving the microstructure stability of the material; at the same time, the secondary precipitation of TiC phase can form a dispersion strengthening effect in the matrix, synergistically improving the low-temperature toughness and high-temperature strength of the steel; in addition, Ti can also delay hydrogen diffusion and reduce hydrogen embrittlement sensitivity through interaction with residual carbon; however, when the Ti content exceeds 0.020%, excessive TiN / TiC will aggregate to form coarse and brittle inclusions, which become stress concentration sources and promote crack initiation and propagation; therefore, the Ti content is controlled at 0.010~0.020%

[0047] Al: Al element as a strong deoxidizing element, can effectively eliminate the ferrous oxide inclusions in the molten steel, and the dispersed AlN particles generated can pin the austenite grain boundaries, significantly refine the grain structure and improve the low-temperature toughness of the material; but when the aluminum content exceeds 0.05%, AlN is easy to enrich at the grain boundaries during solidification to form coarse inclusions, which not only reduces the plasticity of the material, but also induces brittle fracture through hydrogen trapping effect; the high-hardness oxide inclusions generated by excessive aluminum combined with oxygen are easy to cause continuous casting nozzle clogging and surface crack defects of the billet; under the action of welding thermal cycle, the secondary precipitation of such oxides will worsen the ductility of the weld metal; therefore, the Al content is controlled at 0.03~0.05%.

[0048] P: P element is easy to enrich at the grain boundaries in steel, which weakens the grain boundary strength, significantly raises the brittle transition temperature of the material and induces temper embrittlement; during welding, the combined action of phosphorus and residual stress will exacerbate the risk of reheated cracking in the heat-affected zone; therefore, the P content is controlled at P≤0.012%.

[0049] S: S element is a harmful element in steel, sulfur exists in the form of FeS in steel, and its melting point is 985℃; and the billet heating temperature is generally above 1100℃, so when the billet is heated, the premature melting of FeS compounds leads to cracking of the billet during forging and rolling; sulfur also promotes the formation of welding hot cracks and reduces the corrosion resistance of the material; therefore, the S content is controlled at ≤0.002%.

[0050] H: Hydrogen is a "poisonous" element in steel, which can cause hydrogen embrittlement and white spot defects; the solubility in solid steel is very low, and when it is melted into the molten steel at high temperature and then rapidly cooled, hydrogen is retained to form high-pressure micro pores, which greatly reduces the plasticity, toughness and fracture toughness, and in severe cases, directly causes brittle fracture; therefore, the hydrogen content is controlled at ≤0.00015%.

[0051] The preparation method of the medium-temperature pipeline steel disclosed in the present application comprises the following steps:

[0052] Firstly, the molten iron is subjected to pre-desulphurization treatment, converter smelting, ladle refining, RH vacuum refining, then is continuously cast into a 220-320mm thick casting blank and is stacked and cooled to room temperature; wherein the molten iron comprises the following chemical components in percentage by weight: C: 0.12-0.16%, Si: 0.50-0.65%, Mn: 0.45-0.65%, Cr: 1.20-1.50%, Mo: 0.45-0.65%, Ti: 0.010-0.020%, Al: 0.03-0.05%, S≤0.002%, P: ≤0.012%, H≤0.00015%, and the rest is Fe and other inevitable impurities; in addition, the process of RH vacuum refining of the molten iron further comprises controlling the degassing time to be not less than 15 minutes under the condition that the vacuum degree is not more than 2mBar.

[0053] The casting blank is subjected to low-temperature heating system, the low-temperature heating system is that the casting blank is kept at the furnace for time t, and is raised from room temperature to target temperature 1140-1160℃, and the temperature difference between the upper and lower surfaces of the casting blank when discharged is not more than 20℃; wherein t=h+80, min, h represents the thickness of the casting blank; specifically, the casting blank is subjected to five-stage heating by using a heat recovery section, a preheating section, a first heating section, a second heating section and a soaking section, the temperature of the heat recovery section is not more than 900℃, the temperature of the preheating section is 900-1050℃, the temperature of the first heating section is 1000-1100℃, the temperature of the second heating section is 1100-1160℃, and the temperature of the soaking section is 1130-1150℃, and the residence time of the casting blank in the second heating section is not less than 1 / 5 t, and the residence time of the casting blank in the soaking section is not less than 1 / 5 t.

[0054] The casting blank after low-temperature heating is subjected to two-stage controlled rolling, and the parameters of the two-stage controlled rolling are that the first-stage rough rolling temperature is 1030-1090℃, the first-stage finish rolling temperature is 960-990℃, and the casting blank is kept warm after rolling, and the kept warm thickness is 2.5-3.0H; the second-stage rough rolling temperature is (960-2H)±15℃, and the finish rolling temperature is (860-1.3H)±15℃; wherein H is the thickness of the product steel plate.

[0055] The steel plate after controlled rolling is first water-cooled to 600-660℃, and then air-cooled to room temperature; wherein the parameters of the steel plate after controlled rolling being first water-cooled to 600-660℃ are that the water entry temperature is not less than 780℃, and the cooling rate after water entry is 15-25℃ / s to 600-660℃.

[0056] The steel plate air-cooled to room temperature is subjected to quenching heat treatment, and the parameters of the quenching heat treatment are that the quenching temperature is 910-930℃, the holding time is 2H+10min-2H+20min, the water cooling rate after discharge is 25-35℃ / s to 320-380℃, and then air-cooled to room temperature.

[0057] The quenched and cooled to room temperature steel plate is subjected to a tempering heat treatment with parameters of a tempering temperature of 670-730℃, and after a preset holding time, cooling to room temperature to obtain a product steel plate; wherein the holding time is 4H+2min-4H+8min when the thickness H of the product steel plate is ≤20mm, and is 3H+2min-3H+8min when the thickness H of the product steel plate is >20mm.

[0058] The product steel plate obtained after the tempering heat treatment has a thickness specification of 8-40mm, a microstructure of a complex phase structure of ferrite and tempered bainite, a content of ferrite of 15-25%, a content of tempered bainite of 75-85%, and a bainite lath width after quenching of 0.2-0.5μm and a ferrite grain size after tempering of 5-8μm; in terms of performance, the product steel plate has a yield strength of 450-500MPa, a tensile strength of 580-650MPa, and an elongation of not less than 25%; the product steel plate has an impact energy KV2 at -20℃ of not less than 280J, a Brinell hardness value on the surface of the steel plate of 180-200HBW, and a critical bending stress for hydrogen sulfide stress corrosion of not less than 0.9σ S , a yield strength R p0.2 of a steel plate in a medium temperature range of 450-550℃ of not less than 300MPa.

[0059] The medium temperature pipeline steel and the preparation method thereof disclosed in the present application will be further specifically introduced below in combination with specific embodiments.

[0060] Embodiment 1

[0061] A preparation method of a medium temperature pipeline steel is provided, comprising the following steps:

[0062] 1) The raw materials of each component are weighed according to the formula proportion, and then smelted, wherein the smelting step comprises molten iron desulfurization, converter smelting, ladle refining, and RH vacuum refining to obtain molten steel, and the chemical composition of the steel plate is as follows in terms of mass percentage: C: 0.13%, Si: 0.50%, Mn: 0.55%, Cr: 1.30%, Mo: 0.50%, Ti: 0.015%, Al: 0.04%, S: 0.002%, P: 0.010%, H: 0.00015%, and the rest is Fe and other unavoidable impurities; wherein the RH vacuum refining comprises degassing for 15 minutes under a vacuum degree of 2mBar, i.e. a soft stirring time of 15 minutes;

[0063] 2) The molten steel is continuously cast into a 320mm thick casting blank, and the blank is stack cooled for 48h;

[0064] 3) low-temperature heating system: the slab is heated by five stages, including heat recovery stage, preheating stage, first heating stage, second heating stage and soaking stage, the target temperature is 1140℃, the slab stays in the furnace for 400min, the temperature difference between the upper and lower surfaces of the slab is 20℃ when it is discharged, the temperature of the heat recovery stage is 900℃, the temperature of the preheating stage is 1000℃, the temperature of the first heating stage is 1050℃, the temperature of the second heating stage is 1140℃, the temperature of the soaking stage is 1140℃, and the slab stays in the second heating stage for 80min and stays in the soaking stage for 80min.

[0065] 4) rolling: two-stage controlled rolling is adopted, the first-stage rough rolling temperature is 1080℃, the first-stage finish rolling temperature is 990℃, the slab is cooled after the first-stage rolling, and the thickness of the slab after cooling is 120mm, the second-stage rough rolling temperature is 880℃, and the finish rolling temperature is 808℃.

[0066] 5) post-rolling cooling: the steel plate is quickly cooled in water after rolling, the water entry temperature is 785℃, the cooling rate is 15℃ / s after entering the water until the temperature is 600℃, and then the steel plate is air-cooled to room temperature.

[0067] 6) quenching heat treatment: the quenching temperature is 920℃, the holding time is 100min, the steel plate is cooled in water at a cooling rate of 25℃ / s until the temperature is 350℃, and then the steel plate is air-cooled to room temperature.

[0068] 7) tempering heat treatment: the tempering temperature is 700℃, the holding time is 125min, and the product steel plate is obtained.

[0069] The thickness of the product steel plate is H=40mm, and the microstructure of the product steel plate is observed by transmission electron microscopy, which is 20% ferrite + 80% tempered bainite, wherein the average bainite lath width after quenching is about 0.3μm, and the average ferrite grain size after tempering is about 7μm, as shown in Figure 1The steel plate is subjected to performance tests, including yield strength, tensile strength, elongation, -20℃ impact energy, surface Brinell hardness, critical bending stress for hydrogen sulfide stress corrosion resistance, and yield strength of steel plate in a medium temperature range of 550℃. The detection standards for yield strength, tensile strength, and elongation are the room temperature test methods disclosed in GB / T 228.1; the detection standard for -20℃ impact energy is the metal Charpy impact test method disclosed in GB / T 229; the detection standard for surface Brinell hardness is the Brinell hardness test of metal materials disclosed in GB / T 231.1; the detection standard for critical bending stress for hydrogen sulfide stress corrosion resistance is NACE TM0177 and NACE TM031; and the detection standard for yield strength of steel plate in a medium temperature range of 550℃ is the high temperature test method disclosed in GB / T 228.2. The results are as follows: the yield strength of the steel plate is 480 MPa, the tensile strength is 638 MPa, the elongation is 25%, the average -20℃ impact energy KV2 of the steel plate is 308 J, the surface Brinell hardness value of the steel plate is 190 HBW, the critical bending stress for hydrogen sulfide stress corrosion resistance of the steel plate is 442 MPa, and the yield strength R p0.2 of the steel plate in a medium temperature range of 550℃ is 320 MPa; that is, the product steel plate exhibits excellent hardness, impact performance, and the like.

[0070] Comparative Example 1

[0071] Comparative Example 1 and Example 1 use the same chemical composition of steel material formula and billet heating process, and the only difference is that the rolling and cooling processes are different;

[0072] Comparative Example 1 adopts one-stage rolling, and the one-stage opening rolling temperature is 1060℃, and the finish rolling temperature is 960℃. Then air cooling to room temperature; the heat treatment process adopted by Comparative Example 1 is: 1) quenching heat treatment: heating to 920℃, the holding time is 100 min, after discharging, water cooling to below 200℃ at a cooling rate of 25℃ / s, and then air cooling to room temperature; 2) tempering heat treatment: tempering temperature 700℃, holding time 125 min.

[0073] The produced steel plate has a thickness specification of 40 mm, and the structure is 10% ferrite and 90% tempered martensite, the average martensite lath width after quenching is about 1μm, and the average ferrite grain size after tempering is about 18μm; as Figure 2As shown, high martensite content results in poor steel hardness and machinability, and makes rolled pipes prone to surface cracking. Simultaneously, high martensite content also leads to reduced elongation; while the coarse original austenite structure results in poor impact performance. Performance tests were conducted on the above steel plates, including yield strength, tensile strength, elongation, impact energy at -20℃, surface Brinell hardness, critical bending stress against hydrogen sulfide stress corrosion, and yield strength in the 550℃ medium-temperature range. The results are as follows: the yield strength of the steel plate is 442 MPa, the tensile strength is 675 MPa, the elongation is 16%, the average impact energy (KV2) at -20℃ is 38 J, the surface Brinell hardness is 223 HBW, the critical bending stress against hydrogen sulfide stress corrosion is 392 MPa, and the yield strength R in the 550℃ medium-temperature range is... p0.2 The strength was 281 MPa; the steel performance test results were consistent with the microstructure.

[0074] Comparative Example 2

[0075] Comparative Example 2 and Example 1 use the same steel formula, chemical composition, billet heating process, and rolling and cooling processes; the only difference is the heat treatment process. The heat treatment process used in Comparative Example 2 is as follows: 1) Normalizing heat treatment: heating to 920°C, holding time is 100 min, and air cooling to room temperature after taking it out of the furnace; 2) Tempering heat treatment: tempering temperature is 700°C, holding time is 125 min.

[0076] The produced steel plate has a thickness of 40mm and a microstructure of 45% pearlite + 55% ferrite, with the ferrite grain size averaging 25μm. Figure 3 As shown, the microstructure is coarse, resulting in poor impact performance of the steel. Performance tests were conducted on the aforementioned steel plate, including yield strength, tensile strength, elongation, impact energy at -20℃, surface Brinell hardness, critical bending stress against hydrogen sulfide stress corrosion, and yield strength in the 550℃ medium-temperature range. The results were as follows: the yield strength of the steel plate was 459 MPa, the tensile strength was 566 MPa, the elongation was 23%, the average impact energy (KV2) at -20℃ was 56 J, and the surface Brinell hardness was 168 HBW. The critical bending stress against hydrogen sulfide stress corrosion was 397 MPa, and the yield strength R in the 550℃ medium-temperature range was... p0.2 The strength was 294 MPa; the steel performance test results were consistent with the microstructure.

[0077] The medium-temperature pipeline steel and its preparation method disclosed in this invention can significantly improve the medium-temperature mechanical properties of steel by designing the steel formula and optimizing the preparation process. At the same time, it solves the current problems of pipeline damage caused by steel creep and the imbalance between strength and toughness. Furthermore, the preparation method of this invention achieves control over the surface hardness of the steel while coordinating wear resistance, machinability and weldability, significantly improving the practicality of medium-temperature pipeline steel.

[0078] While the application has been described by way of example with reference to preferred embodiments, it is to be understood that this application is not limited to the embodiments disclosed, but is intended to cover modifications and variations within the spirit and scope of the application. Therefore, the scope of the application is defined not by the detailed description of the preferred embodiments but by the following claims.

Claims

1. A type of steel for medium-temperature pipelines, characterized in that, By weight percentage, it includes the following chemical composition: C: 0.12~0.16%, Si: 0.50~0.65%, Mn: 0.45~0.65%, Cr: 1.20~1.50%, Mo: 0.45~0.65%, Ti: 0.010~0.020%, Al: 0.03~0.05%, S≤0.002%, P:≤0.012%, H≤0.00015%, with the remainder being Fe and other unavoidable impurities; The thickness of the steel plates used in the medium-temperature pipeline products is 8~40mm; the yield strength of the steel plates is 450~500MPa, the tensile strength is 580~650MPa, and the elongation is not less than 25%; the impact energy (KV2) at -20℃ is not less than 280J, and the Brinell hardness value of the steel plate surface is 180~200HBW; the critical bending stress for hydrogen sulfide stress corrosion resistance of the steel plates is not less than 0.9σ. S Yield strength R of steel plates in the medium temperature range of 450~550℃ p0.2 Not less than 300MPa.

2. The steel for medium-temperature pipelines according to claim 1, characterized in that, The microstructure of the medium-temperature pipeline steel is a multiphase structure of ferrite and tempered bainite, with a ferrite content of 15-25% and a tempered bainite content of 75-85%. After quenching, the width of the bainite laths is 0.2-0.5 μm, and after tempering, the ferrite grain size is 5-8 μm.

3. A method for preparing medium-temperature pipeline steel as described in claim 1 or 2, characterized in that, Includes the following steps: After pre-desulfurization treatment, converter smelting, ladle refining, and RH vacuum refining, the molten iron is continuously cast into 220~320mm thick billets and stacked to cool to room temperature. The molten steel, by weight percentage, includes the following chemical composition: C: 0.12~0.16%, Si: 0.50~0.65%, Mn: 0.45~0.65%, Cr: 1.20~1.50%, Mo: 0.45~0.65%, Ti: 0.010~0.020%, Al: 0.03~0.05%, S≤0.002%, P:≤0.012%, H≤0.00015%, with the remainder being Fe and other unavoidable impurities. The billet is subjected to a low-temperature heating regime, which is as follows: the billet is kept in the furnace for a time t, and the temperature is raised from room temperature to the target temperature of 1140~1160℃. The temperature difference between the upper and lower surfaces of the billet does not exceed 20℃ when it is taken out of the furnace; where t=h+80,min, and h represents the thickness of the billet. The low-temperature heated billet is subjected to two-stage controlled rolling. The parameters of the two-stage controlled rolling are as follows: the first stage starting rolling temperature is 1030~1090℃, the first stage finishing rolling temperature is 960~990℃, and after rolling, it is allowed to heat up to a thickness of 2.5~3.0H; the second stage starting rolling temperature is (960-2H)±15℃, and the finishing rolling temperature is (860-1.3H)±15℃; where H is the thickness of the product steel plate. After controlled rolling, the steel plate is first water-cooled to 600~660℃, and then air-cooled to room temperature; The steel plate cooled to room temperature by air is subjected to quenching heat treatment. The parameters of the quenching heat treatment are: quenching temperature 910~930℃, holding time 2H+10min~2H+20min, water cooling to 320~380℃ at a cooling rate of 25~35℃ / s after taking it out of the furnace, and then air cooling to room temperature. The steel plate that has been quenched and cooled to room temperature is subjected to tempering heat treatment. The parameters of the tempering heat treatment are: tempering temperature 670~730℃, holding time for a preset time and then cooling to room temperature to obtain the product steel plate.

4. The method for preparing medium-temperature pipeline steel according to claim 3, characterized in that, The specific heating process of the aforementioned low-temperature heating regime is as follows: The billet is heated in five stages: a heat recovery stage, a preheating stage, a first heating stage, a second heating stage, and a soaking stage. The temperature of the heat recovery stage does not exceed 900℃, the temperature of the preheating stage is 900~1050℃, the temperature of the first heating stage is 1000~1100℃, the temperature of the second heating stage is 1100~1160℃, and the temperature of the soaking stage is 1130~1150℃. The residence time of the billet in the second heating stage is not less than 1 / 5 t, and the residence time of the billet in the soaking stage is not less than 1 / 5 t.

5. The method for preparing medium-temperature pipeline steel according to claim 3, characterized in that, The parameters for water cooling the steel plate after controlled rolling to 600~660℃ are as follows: the water temperature is not lower than 780℃, and after water immersion, it is cooled to 600~660℃ at a cooling rate of 15~25℃ / s.

6. The method for preparing medium-temperature pipeline steel according to claim 3, characterized in that, The preset holding time during the tempering heat treatment is as follows: when the thickness H of the product steel plate is ≤20mm, the holding time T is 4H+2min~4H+8min; when the thickness H of the product steel plate is >20mm, the holding time T is 3H+2min~3H+8min.

7. The method for preparing medium-temperature pipeline steel according to claim 3, characterized in that, The process of RH vacuum refining of molten steel includes controlling the degassing time to be no less than 15 minutes under a vacuum degree not exceeding 2 mBar.

8. The method for preparing medium-temperature pipeline steel according to claim 3, characterized in that, The stacking and cooling time for the cast billet shall not be less than 48 hours.

9. The method for preparing medium-temperature pipeline steel according to claim 3, characterized in that, The obtained product steel plate has a thickness of 8~40mm, and its microstructure is a multiphase structure of ferrite and tempered bainite, with a ferrite content of 15~25% and a tempered bainite content of 75~85%. The width of the bainite laths after quenching is 0.2~0.5μm, and the ferrite grain size after tempering is 5~8μm. The product steel plate has a yield strength of 450~500MPa, a tensile strength of 580~650MPa, and an elongation of not less than 25%; the product steel plate has an impact energy (KV2) of not less than 280J at -20℃, and a surface Brinell hardness of 180~200HBW; the product steel plate has a critical bending stress of not less than 0.9σ for resistance to hydrogen sulfide stress corrosion. S Yield strength R of steel plates in the medium temperature range of 450~550℃ p0.2 Not less than 300MPa.

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