X70 pipeline steel of extremely low-temperature high toughness for transport of co 2 and manufacturing method therefor

Abstract

The present invention relates to an X70 pipeline steel of extremely low-temperature high toughness for transport of CO2 and a manufacturing method therefor. In addition to Fe and unavoidable impurities, the X70 pipeline steel further comprises the following chemical components in percentage by weight: C: 0.03-0.08%, Si: 0.10-0.30%, Mn: 0.8-1.5%, P≤0.0070%, S≤0.0012%, Ni: 0.05-0.50%, Cr: 0.1-0.6%, Mo: 0.01-0.08%, Nb: 0.02-0.05%, V: 0-0.03%, Ti: 0.005-0.020%, Ca: 0.0010-0.0035%, all aluminum Alt: 0.010-0.040%, B≤0.0005%, O≤0.0030%, and Re: 0.0001-0.0030%, Re being Ce and La.

Description

X70 pipeline steel for extremely low temperature and high toughness CO2 transportation and manufacturing method thereof TECHNICAL FIELD

[0001] The present application belongs to the field of metal materials and its manufacturing, and particularly relates to a kind of X70 pipeline steel for extremely low temperature and high toughness CO2 transportation and manufacturing method thereof. BACKGROUND

[0002] Under the guidance of the "double carbon" target (i.e. carbon peak and carbon neutralization), CCUS (carbon capture, utilization and storage) technology has become the focus of attention, and the storage and transportation of CO2 is a key link in this technology chain. For long-distance and large-scale CO2 transportation, dense phase and supercritical state CO2 pipeline transportation is favored due to its efficiency and cost-effectiveness. However, this mode of transportation is also accompanied by a series of technical challenges.

[0003] Firstly, under low-temperature conditions such as leakage or planned / unintended depressurization, the high-pressure CO2 in the dense phase and supercritical state CO2 pipeline will cause a sharp drop in temperature due to the Joule-Thomson effect, reaching a low temperature condition of -40℃ or even below the triple point (-56℃), and long-term low-temperature conditions will make the pipeline material brittle. In addition, compared with conventional natural gas pipelines, dense phase and supercritical state CO2 transportation pipelines are more prone to fracture failure under high pressure. Due to the high saturation pressure of dense phase and supercritical state CO2 transportation pipelines and the phase transition easily occurring during depressurization, the pressure platform feature appears in the depressurization wave curve, and the depressurization wave velocity is significantly reduced, which increases the risk of rapid propagation of ductile fracture, making it very difficult to arrest the crack. Therefore, the low-temperature toughness of the pipe material is extremely strict.

[0004] Chinese patent 1 (application number 202111376646.6) discloses a method for regulating the dual-phase structure of X70 grade pipeline steel and increasing its low-temperature toughness. The obtained X70 grade pipeline steel contains ferrite and granular bainite, with the ferrite content being 40-60% and the granular bainite content being 40-60%. The mass percentage of each chemical component in the pipeline steel is as follows: C: 0.04-0.08%, Si: 0.15-0.25%, Mn: 1.50-1.80%, Cr: 0.15-0.25%, Mo: 0.10-0.20%, Nb: 0.04-0.06%, Ti: 0.01-0.03%, Al: 0.02-0.04%, Cu: 0.10-0.20%, P≤0.015%, S≤0.004%, Ceq: 0.35-0.48%, and the balance being Fe and other unavoidable impurities. The disadvantages of this patent are: 1) a large number of noble alloy elements are added, resulting in high cost; 2) the ferrite content is high, leading to relatively poor crack arrest performance, only meeting the DWTT requirement at -20°C, and the impact absorption energy at -60°C is less than 300 J. This is because when the polygonal ferrite content is high, the probability and tendency of crack propagation in the ferrite increase. When the polygonal ferrite content exceeds 40%, deformation slip mainly occurs in the tough phase, which needs to have high strength and plasticity to ensure good toughness. At this time, most of the polygonal ferrite loses the separation and surrounding of the hard phase of granular bainite, the constraint strengthening effect decreases or disappears, and the large-area continuous arrangement of polygonal ferrite cannot effectively limit crack propagation, thus excellent crack arrest toughness cannot be obtained.

[0005] Chinese patent 2 (application number 201910561444.5) discloses a production method for thick-gauge X70 grade pipeline steel with excellent low-temperature DWTT performance. The chemical elements of the pipeline steel are as follows in terms of weight percentage: C 0.043-0.07%; Si 0.22-0.40%; Mn 1.70-1.90%; P 0.004-0.009%; S 0.0009-0.0012%; Nb 0.08-0.09%; Ti 0.021-0.025%; Mo 0.06-0.15%; Ni 0.10-0.20%; Als 0.014-0.038%; N≤0.008%; and the balance being Fe and unavoidable impurities. The controlled rolling process adopts a "horizontal-vertical" type rough rolling + finishing rolling, and the controlled cooling process adopts single cooling rate rapid cooling. The disadvantages of this patent are: 1) a large number of alloy elements such as Mn, Nb, and Ti are added, resulting in high cost; 2) the inclusion elements O and B are not explicitly controlled, and the control means for key inclusions are not explicitly described; 3) the control requirements for the structure are not explicitly described; and 4) the toughness only meets the requirement of DWTT≥85% at -15°C, and the impact performance is not described.

[0006] The service characteristics of pipeline steel for carbon dioxide transportation put very strict requirements on the low-temperature toughness of the material, which cannot be met by conventional pipeline steel and the prior art. The prior art is relatively simple in controlling the low-temperature toughness of X70 pipeline steel, and therefore, under the premise of controlling the cost, how to obtain better low-temperature toughness under the premise of meeting the requirements in the strength level by optimizing the composition and process is a problem to be solved at present. SUMMARY

[0007] To solve the above problems, the purpose of the present application is to provide a kind of X70 pipeline steel for CO2 transportation with very low temperature high toughness and its manufacturing method. The pipeline steel of the present application can reach X70 strength level, and has excellent low-temperature toughness. Specifically, the yield strength R t0.5 of the X70 pipeline steel is ≥485MPa, the tensile strength R m is ≥570MPa, the yield strength ratio R t0.5 / R m is ≤0.93. The X70 pipeline steel has low-temperature impact and DWTT toughness, the Charpy impact energy at-60℃ is ≥300J, the impact ductile-brittle transition temperature is ≤-90℃, and the DWTT SA% at-40℃ is ≥85%. The X70 pipeline steel has low-temperature CTOD performance, the CTOD characteristic value δ m at-40℃ is ≥0.4mm.

[0008] To achieve the above purpose, the technical scheme of the present application is as follows:

[0009] The first aspect of the present application provides an X70 pipeline steel, which, in addition to Fe and unavoidable impurities, further comprises the following chemical components in the following weight percentage: C: 0.03-0.08%, Si: 0.10-0.30%, Mn: 0.8-1.5%, P≤0.0070%, S≤0.0012%, Ni: 0.05-0.50%, Cr: 0.1-0.6%, Mo: 0.01-0.08%, Nb: 0.02-0.05%, V: 0-0.03%, Ti: 0.005-0.020%, Ca: 0.0010-0.0035%, Alt: 0.010-0.040%, B≤0.0005%, O≤0.0030%, Re: 0.0001-0.0030%, Re is Ce and La, and simultaneously satisfies the following relationship:

[0010] (Ca+Re) / S≥2,

[0011] 4Mn+20Ni-14C≥3.5%,

[0012] Nb+V+Ti≤0.09%,

[0013] 0.3≤Nb / (Mo+75B)≤1.5,

[0014] wherein each element symbol is substituted for the weight percentage content of the corresponding element.

[0015] Preferably, the X70 pipeline steel contains the following chemical components with the weight percentage content as follows: C: 0.03-0.08%, Si: 0.10-0.30%, Mn: 0.8-1.5%, P≤0.0070%, S≤0.0012%, Ni: 0.05-0.50%, Cr: 0.1-0.6%, Mo: 0.01-0.08%, Nb: 0.02-0.05%, V: 0-0.03%, Ti: 0.005-0.020%, Ca: 0.0010-0.0035%, total aluminum Alt: 0.010-0.040%, B≤0.0005%, O≤0.0030%, Re: 0.0001-0.0030%, Re being Ce and La, and the balance being Fe and inevitable impurities.

[0016] Preferably, the chemical component content of the X70 pipeline steel also satisfies the following relationship:

[0017] 2≤(Ca+Re) / S≤41,

[0018] 3.5%≤4Mn+20Ni-14C≤12.30%,

[0019] 0.054%≤Nb+V+Ti≤0.09%.

[0020] Preferably, the X70 pipeline steel also satisfies: the non-recrystallization temperature T nr ≥920℃, preferably 920-1005℃, and the T nr ={ (887+464*C+890*Ti+363*Al-357*Si+(6445*Nb)-644*(SQRT(Nb))+[732*V-230*(SQRT(V))]}, wherein each element symbol is substituted for the weight percentage content of the corresponding element.

[0021] Preferably, the microstructure of the pipeline steel is: bainite + ferrite + martensite-austenite component M / A, wherein the bainite is granular, the ferrite is polygonal / quasi-polygonal, the average grain size of the ferrite is ≤ 7 μm, more preferably 5-7 μm, and the proportion of high-angle grain boundaries is 0.05-0.30 (i.e. 5%-30%), the high-angle grain boundary refers to a grain boundary with a difference in orientation of adjacent grains of more than 15°. Preferably, the volume fraction of bainite is 68%-96%, the volume fraction of ferrite is 2%-30%, and the rest is the martensite-austenite component M / A. The quasi-polygonal ferrite refers to an irregular polygonal ferrite type between equiaxed ferrite and acicular ferrite.

[0022] Preferably, the average grain size of the ferrite, the proportion of high-angle grain boundaries and the impact ductile-brittle transition temperature T c satisfy the following relationship:

[0023] In the formula:

[0024] T c is the impact ductile-brittle transition temperature, unit: ℃;

[0025] P f is the proportion of high-angle grain boundaries, dimensionless;

[0026] d is the average grain size of the ferrite, unit: μm.

[0027] Preferably, the yield strength R t0.5 of the X70 pipeline steel is ≥ 485 MPa, the tensile strength R m is ≥ 570 MPa, the yield strength ratio R t0.5 / R m is ≤ 0.93.

[0028] Preferably, the X70 pipeline steel has low-temperature impact and DWTT toughness, the Charpy impact energy at -60 ℃ is ≥ 300 J, the impact ductile-brittle transition temperature T c is ≤ -90 ℃, and the DWTT SA% at -40 ℃ is ≥ 85%.

[0029] Preferably, the X70 pipeline steel has low-temperature CTOD performance, the CTOD characteristic value δ m at -40 ℃ is ≥ 0.4 mm.

[0030] In the present application, the chemical composition design principle of the X70 pipeline steel with high toughness at extremely low temperature for CO2 transportation is as follows:

[0031] C: C is the most economical strengthening element in steel, which improves the strength of steel by interstitial solid solution strengthening. Increasing the carbon content can greatly improve the hardenability of steel, reduce the addition amount of other valuable alloying elements, and reduce the production cost. However, too high C content is not conducive to the ductility, toughness, weldability and corrosion resistance of the steel. Therefore, the C content in the present application is controlled between 0.03-0.08%.

[0032] Si: Si is a solid solution strengthening element, but too high Si content will adversely affect the surface quality and weldability. If the Si content exceeds 0.30%, the toughness may decrease. Therefore, the Si content in the present application is controlled between 0.10-0.30%.

[0033] Mn: Mn improves the strength of steel by solid solution strengthening, and is the most important and economical strengthening element in steel to compensate for the loss of strength caused by the reduction of C content. Manganese can be used to replace part of the carbon element in the production process, thereby reducing the carbon content of the steel and improving its toughness. Mn helps to obtain fine phase transformation products and also helps to control oxygen and sulfur during steelmaking. If the Mn content is low, it is difficult to achieve the target strength level, but Mn can also aggravate central segregation. Therefore, the Mn content in the present application is controlled between 0.8-1.5%.

[0034] Ni: Ni promotes the transformation of ferrite to austenite in steel. This transformation can reduce the plastic strain of steel under impact load, thereby further improving the toughness of the steel. Nickel can also refine the ferrite grains, and under the same strength conditions, it can improve the plasticity and toughness of the steel, especially the low temperature toughness. Therefore, the Ni content in the present application is controlled between 0.05-0.50%.

[0035] With the increase of carbon content in steel, the impact toughness brittle transition temperature shows an upward trend, and the maximum impact absorption energy decreases sharply. Mn and Ni are both austenite stabilizing elements that promote the formation of austenite, especially Ni can significantly improve the low temperature toughness. Research has found that when 4Mn+20Ni-14C is low, the low temperature mobile dislocation density is low, and cross slip is difficult, which deteriorates the toughness and plasticity of the material. Therefore, in the present application, 4Mn+20Ni-14C is controlled to be greater than or equal to 3.5%, so as to achieve more low temperature mobile dislocations, facilitate cross slip, and obtain a low toughness brittle transition temperature.

[0036] Cr: Cr has a certain solid solution strengthening effect, which can improve the hardenability of steel. When the Cr content is above 0.10%, a relatively dense protective layer is formed on the surface of the steel, which plays a protective role for the matrix and can effectively improve the corrosion resistance of the steel. However, too high Cr content in steel can reduce weldability. Therefore, the Cr content in the present application is controlled between 0.10-0.60%.

[0037] Mo: Mo can effectively promote the bainite transformation, play a role in strengthening the matrix, get more fine structure, but the plasticity of the steel decreases when the content of Mo is too much, and the cost of Mo is high. Therefore, the content of Mo in the application is controlled at a lower level of 0.01-0.08%. The addition of Cr and Mo alloying elements can also change the grain boundary structure and the interaction force between atoms of the steel, appropriately increase the elastic modulus, and thus improve the ability of the material to inhibit brittle fracture.

[0038] Ca: Calcium can play a role in modifying inclusions, and has a positive effect on improving the internal quality and anisotropy of the steel. However, when the content of Ca element in the steel is too much, it will have some adverse effects on the performance of the steel. Excessive calcium can cause segregation in the steel, that is, the uneven distribution of calcium element in the steel, which can affect the mechanical properties and processing performance of the steel, and can form larger inclusions, which may have poor distribution and stability in the steel, thereby adversely affecting the toughness of the steel. Therefore, the content of Ca in the application is controlled between 0.0010-0.0035%.

[0039] Re: Rare earth element Re has strong deoxidizing ability in steel, which can effectively purify the molten steel. After adding Re, the oxides or oxysulfides generated in the steel are usually spherical or nearly spherical, and these fine inclusions are more likely to aggregate and grow in the molten steel, which can achieve the effect of inclusion modification and coarsening. The generated sulfides and oxysulfides have high melting points and are difficult to dissolve, which can effectively reduce the solid solution amount of sulfur element during reheating and inhibit the precipitation of MnS during cooling. In terms of inclusion treatment, compared with Ca element, Re inclusions have better dispersibility and stability in the steel, which can hinder the growth of grains and refine the grain structure of the steel, thereby helping to improve the toughness and corrosion resistance of the steel. However, it should be noted that the addition amount of Re element needs to be strictly controlled to avoid adverse effects on other properties of the steel. Therefore, the content of Re in the application is controlled between 0.0001-0.0030%, and Re is Ce and La.

[0040] In order to obtain excellent toughness, the properties and morphology of inclusions in the steel must be strictly controlled, especially the MnS inclusions. The deformation degree of inclusions is affected by the ratio of Ca+Re to sulfur in the molten steel. When (Ca+Re) / S=1, the deformation amount of MnS is 50%, and when (Ca+Re) / S≥2, the deformation of MnS is basically completed. Therefore, the application controls (Ca+Re) / S≥2.

[0041] O, S: Oxygen exists in steel mainly in the form of inclusions such as FeO, MnO, SiO2, Al2O3, etc. These inclusions can reduce the strength and plasticity of the steel, especially having a serious impact on the impact toughness and the like, and with the increase of the S content, the MnS inclusions increase, significantly reducing the low-temperature toughness of the steel. In addition, the oxygen and sulfur contents in the steel have a significant impact on the solid solution amount of Re, and reducing the oxygen and sulfur contents can increase the solid solution amount of Re in the steel, the solid solution Re segregates at the grain boundary, thereby inhibiting the growth of austenite grains, reducing pearlite, improving banded structure, and increasing ferrite in the steel. However, O and S will react with Re, and in order to increase the solid solution amount of Re, the contents of O and S must be strictly controlled. Therefore, the O content is controlled to be ≤0.003%, and the S content is controlled to be ≤0.0012%.

[0042] P: Phosphorus is a major impurity element in steel, and phosphorus can easily cause cold brittleness of the steel, resulting in unstable performance of the steel, so the content of phosphorus in the steel should be reduced as much as possible. Therefore, the P content is controlled to be ≤0.007% in the present application.

[0043] Nb: Niobium is an important element of low-carbon micro-alloyed steel. The Nb strain-induced precipitation in the hot rolling process forms Nb carbonitride, which pins the grain boundary to inhibit the growth of deformed austenite. By controlled rolling and controlled cooling, the deformed austenite is transformed into fine products with high dislocation density. The solid solution Nb will be dispersedly precipitated in the matrix as a second phase particle NbC after the steel strip is coiled, which plays a role in precipitation strengthening. More Nb makes the slab prone to cracks, thereby affecting the surface quality, and also deteriorates the welding performance. Therefore, the Nb content is controlled to be between 0.02 and 0.05% in the present application.

[0044] V: Vanadium can form carbide particles to strengthen the grain boundary and intercrystalline of the steel, improve the hardness and strength of the steel, inhibit the occurrence of carbon precipitation at the grain boundary and intergranular corrosion, and improve the corrosion resistance of the alloy. Vanadium can refine the grain size of the steel, which helps to enhance the plastic deformation ability of the steel, inhibits crack propagation, and thus improves the toughness of the material. Therefore, the V content is controlled to be between 0 and 0.03% in the present application.

[0045] Ti: Titanium is a good deoxidizing and degassing agent and an effective element for fixing nitrogen and carbon. The unsolved carbonitride of Ti can prevent the growth of austenite grains when the steel is heated. The TiN and TiC precipitated during rough rolling in the high-temperature austenite region can effectively inhibit the growth of austenite grains, thereby refining the grains, and also can increase the solid solubility of Nb, reduce the micro-crack sensitivity of the Nb-containing steel. Generally, titanium is added in combination with Nb, and in addition, the precipitation during welding can also inhibit the growth of high-temperature grains, thereby improving the welding performance. Therefore, the Ti content is controlled to be between 0.005 and 0.020% in the present application.

[0046] Nb+V+Ti: These elements can form dispersed carbonitride with C and N, which can play a solid solution strengthening effect, and because the melting point of these carbonitride is very high, it plays the role of external crystal nucleus in the subsequent preparation process, which can effectively refine the grain structure and improve the performance of the structure, but excessive Nb, V and Ti addition can easily lead to the aggregation of carbonitride particles in the steel plate, reducing the low temperature toughness of the material, therefore, the present application controls Nb+V+Ti≤0.09%.

[0047] In addition, the present application controls the unrecrystallization temperature T nr ≥920℃ of the pipeline steel nr ={ (887+464*C+890*Ti+363*Al-357*Si+(6445*Nb)-644*(SQRT(Nb))+[732*V-230*(SQRT(V))]}, wherein SQRT is the square root. The formula calculates the value in front of the corresponding element weight percentage, by controlling the unrecrystallization temperature above 920℃, the recrystallization process can be slowed down, that is, hot rolling can be carried out at a higher temperature without the risk of deformed grain recrystallization and growth, and the result is that the austenite grain is small and uniform at the end of the hot rolling process, and the more fine and uniform microstructure is beneficial to obtain high toughness and reduce the impact toughness brittle transition temperature.

[0048] Total aluminum Alt: the role of aluminum is deoxidation, and appropriate Al is beneficial to refine the grain and improve the strength and toughness, but if Al is too much, it may form coarse precipitates to reduce the low temperature toughness of the steel. Therefore, the content of Alt in the present application is controlled between 0.010-0.040%.

[0049] B: the main role of boron in steel is to increase the hardenability and strength of the steel, thereby saving other more expensive metals, but adding B element can increase the brittleness of the steel, so its content needs to be strictly controlled. Therefore, the content of B in the present application is controlled≤0.0005%.

[0050] Overall, the design idea of the present application using the above components is as follows:

[0051] 1. Mainly based on the alloying design of good strength and low temperature toughness and other comprehensive performance, by adding Mn, Ni austenite stabilizing elements, promoting the formation of austenite, because the face-centered cubic structure of austenite is helpful to alleviate and inhibit stress concentration and crack propagation during plastic deformation and fracture, therefore, the impact toughness brittle transition temperature of the steel can be reduced and the impact toughness at low temperature can be improved.

[0052] 2. The addition of Cr and Mo alloying elements can change the grain boundary structure and the interaction force between atoms of the steel, improve the elastic modulus, improve the corrosion resistance and the ability to inhibit brittle fracture of the material.

[0053] 3. By adding Ca and Re (Ce+La) in combination, the internal quality is improved, and the (Ca+Re) / S ≥ 2 is controlled to achieve a stable inclusion modification effect. This results in better dispersion and stability of inclusions, which inhibit grain growth, refine the grain structure of the steel, and improve toughness. Strict control of impurities such as sulfur, oxygen, and phosphorus in the steel increases the amount of Re dissolved in the steel. The dissolved Re agglomerates at the grain boundaries, thereby inhibiting austenite grain growth, reducing unfavorable banded and pearlite structures, and preventing cold brittleness.

[0054] 4. Research has shown that controlling the proportion of large-angle grain boundaries (0.05–0.30) significantly impacts the toughness of steel. Large-angle grain boundaries are defined as those with an orientation difference greater than 15° between adjacent grains. Large-angle grain boundaries can hinder crack propagation by altering the direction of microcrack propagation. Therefore, if the proportion of large-angle grain boundaries is too low, the material's toughness deteriorates significantly. Conversely, an excessive number of large-angle grain boundaries increases material inhomogeneity, reduces strength and toughness, and fails to effectively prevent crack initiation and propagation, resulting in poor toughness. By adding Mo, B, Nb, and C elements, with the Nb / (Mo+75B) value ranging from 0.3 to 1.5, the solidification behavior and phase transformation process of the alloy are altered, thereby affecting the formation and proportion of large-angle grain boundaries. Both Mo and B elements promote the formation of large-angle grain boundaries, but excessive Mo and B content can strongly inhibit the formation of large-angle grain boundaries. In addition, with an appropriate increase in the precipitation of NbC in austenite, the stability and uniform distribution of large-angle grain boundaries can be promoted. Therefore, this invention adds a certain amount of Mo and B while controlling the Nb / (Mo+75B) value within the range of 0.3 to 1.5, thereby obtaining large-angle grain boundaries with stable content and uniform distribution.

[0055] 5. Obtain dispersed carbonitrides through Nb+V+Ti microalloying, and control the non-recrystallization temperature T. nr ≥920℃, suppresses the risk of grain coarsening, obtains fine and uniform microstructure, and achieves a low impact ductile-brittle transition temperature.

[0056] A second aspect of the present invention provides a method for manufacturing the above-mentioned X70 pipeline steel, the method comprising the following steps in sequence:

[0057] 1) Smelting and casting:

[0058] Based on the chemical composition of the above-mentioned X70 pipeline steel, it is smelted and refined, and then cast into slabs.

[0059] 2) The slab is rolled, and the rolling process includes heating, rough rolling, and finish rolling:

[0060] The heating temperature is 1100~1200℃, the heating time is 3.5~7h, and the uniform heat is ≤30℃;

[0061] the last pass reduction ratio of rough rolling is ≥ 16%, the finish rolling temperature of rough rolling is ≥ 950°C, and the finish rolling temperature of rough rolling is ≥ the unrecrystallization temperature T nr ;

[0062] T nr = {(887 + 464*C + 890*Ti + 363*Al - 357*Si + (6445*Nb) - 644*(SQRT(Nb)) + [732*V - 230*(SQRT(V))]}, in which each element symbol is substituted into the value in front of the weight percentage of each element;

[0063] the total reduction ratio of finish rolling is ≥ 75%, the start rolling temperature of finish rolling is ≤ 940°C, preferably 850-940°C, more preferably 889-940°C, and the start rolling temperature of finish rolling is ≤ the unrecrystallization temperature T nr , the finish rolling temperature of finish rolling is ≥ (A r3 + 20°C), wherein A r3 is the temperature at which austenite begins to precipitate free ferrite when cooled;

[0064] 3) cooling the rolled slab:

[0065] first air cooling to below A r3 temperature, controlling the open cooling temperature to be 700-790°C, and then rapid cooling to the stop cooling temperature: 350-550°C, the cooling speed of rapid cooling being: 8-28°C / s.

[0066] Preferably, step 1) refining uses LF and RH refining, in the LF refining process, the content of oxygen / sulfur and other impurities in the steel is strictly controlled, so that O≤0.003%, S≤0.0012%, Ca / Re treatment is used, and (Ca+Re) / S≥2 is controlled, A / B / C / D coarse and fine inclusions≤1.5 grade. Among them, A / B / C / D coarse and fine inclusions refers to A class (sulfide), B class (alumina), C class (silicate), D class (spherical oxide), A, B, C, D are divided into coarse and fine according to the ASTM E45-2018 standard according to the width.

[0067] In the manufacturing method of the X70 pipeline steel of the present application, the content of S in the molten steel is reduced by desulphurization with LF treatment, and O is removed by degassing with RH treatment process, and the content of oxygen / sulphur and other impurities in the steel is strictly controlled, so that the content of O is ≤0.003%, and the content of S is ≤0.0012%, the solid solution amount of Re in the steel is increased, the solid solution Re is segregated at the grain boundary, the austenite grain growth is inhibited, and the unfavorable banded and pearlite structure is reduced, and the cold brittleness is avoided. Ca / Re treatment is adopted, and (Ca+Re) / S≥2 is controlled, so that the optimal modification effect of inclusions is obtained, the inclusions are stably distributed in dispersion, the grain growth is hindered, and good internal quality and toughness are obtained. Through LF+RH double treatment, the internal quality of the molten steel is purified, the ultra-pure steel with low inclusions is obtained, and the A / B / C / D coarse and fine inclusions are ≤1.5 grade.

[0068] In the manufacturing method of the X70 pipeline steel of the present application, the slab heating temperature is 1100-1200℃, the heating time is 3.5-7h, and the soaking degree is ≤30℃, so that the sufficient heating temperature is ensured to make Nb, V and Ti fully solid-solute, and the heating soaking degree is reduced, so that the austenitization is fully uniform, and the uniform structure and plate shape are obtained.

[0069] In the manufacturing method of the X70 pipeline steel of the present application, the slab heating temperature is 1100-1200℃, the heating time is 3.5-7h, and the soaking degree is ≤30℃, so that the sufficient heating temperature is ensured to make Nb, V and Ti fully solid-solute, and the heating soaking degree is reduced, so that the austenitization is fully uniform, and the uniform structure and plate shape are obtained.

[0070] In the manufacturing method of the X70 pipeline steel of the present application, the final pass reduction rate of rough rolling is ≥16%, the rough rolling finishing temperature is ≥950℃, and the rough rolling finishing temperature is ≥ the non-recrystallization temperature T nr , so that the austenite grains can be fully refined by recrystallization after large reduction of rough rolling, and the structure toughness is improved. The recrystallization and deformation are alternately carried out in the rough rolling stage, the pass reduction distribution is optimized, the large reduction of ≥16% is implemented in the final pass, and the austenite grains are effectively refined.

[0071] In the manufacturing method of the X70 pipeline steel of the present application, the total reduction rate of finish rolling is ≥75%, the finish rolling starting temperature is ≤940℃, and the finish rolling starting temperature is ≤ the non-recrystallization temperature T nr , and the finish rolling finishing temperature is ≥A r3 +20℃. The non-recrystallization temperature rolling is adopted, so that the sufficient reduction deformation is ensured, the austenite grains are fully flattened, the nucleation ability of austenite transformation is increased, the deformation-induced ferrite phase transformation is promoted, and the fine and uniform ferrite grains are obtained in the subsequent cooling phase transformation. In addition, the finish rolling temperature is controlled to be ≥A r3+20℃, to avoid local entering into two-phase region and rolling, which produces more hard phase and reduces the low temperature toughness of the steel.

[0072] The traditional pipeline steel is cooled directly after rolling, which results in single bainite and insufficient toughness. The two-stage controlled cooling process is used to optimize the microstructure of the steel and obtain excellent performance. After the rolling process is completed, the steel is first cooled to A r3 temperature, which is below 700℃. The purpose of this step is to promote the formation of ferrite in the steel, including polygonal ferrite and quasi-polygonal ferrite, which has an important influence on the toughness and plasticity of the steel.

[0073] Then, in the manufacturing method of the X70 pipeline steel of the present application, the rapid cooling stage is entered, i.e. water accelerated cooling. The open cooling temperature after rolling is controlled at 700-790℃. The purpose of this stage is to convert the remaining unconverted supercooled austenite into granular bainite during the rapid cooling process. Granular bainite is a kind of microstructure with excellent mechanical properties, which can further improve the strength and toughness of the steel. The final cooling temperature is controlled at 350-550℃ to obtain fine and uniformly distributed M / A and improve the uniformity of the microstructure. The final cooling temperature of accelerated cooling affects the characteristics of bainite in the microstructure. When the final cooling temperature is greater than 550℃, coarse M / A particles are easily formed in the bainite and are mostly non-equiaxed, which leads to a decrease in toughness. When the final cooling temperature is too low, the plate shape of the steel plate cannot be guaranteed, the number of hard phases increases, and the toughness deteriorates.

[0074] Polygonal ferrite and quasi-polygonal ferrite are pro-eutectoid ferrite formed at a slower cooling speed. This means that after the steel plate is rolled, the cooling speed also needs to be controlled so that it has enough time to form, thus the cooling rate is controlled at 8-28℃ / s. During the crack propagation in the steel, the resistance in the small-angle grain boundary of the bainite is weak, and the propagation in the polygonal ferrite and quasi-polygonal ferrite is difficult, on the one hand, the small ferrite reduces the average size of the overall grain, so that the crack frequently changes the path and consumes its energy, on the other hand, the polygonal ferrite and quasi-polygonal ferrite are large-angle grain boundaries, the large-angle grain boundaries can change the propagation direction of the micro-crack, hinder the rapid propagation of the crack, improve the energy absorbed by the crack propagation of the material, improve the low-temperature impact toughness of the material, especially when the orientation difference of adjacent grains is more than 15°, such grain boundaries can make the crack blunt and inhibit the crack from continuing to expand, so that the low-temperature toughness of the material is greatly improved. The crack propagation needs to occur a large turning, so the toughness of the structure is improved, but when the proportion of the large-angle grain boundary is too high, the strength will decrease obviously, the uniformity of the structure increases, and the toughness will also be affected. By controlling the cooling rate in the range of 8-28℃ / s, a certain amount of polygonal ferrite and quasi-polygonal ferrite is formed, and the proportion of the large-angle grain boundary is controlled in the range of 0.05-0.30, so that better toughness is obtained.

[0075] Compared with the prior art, the present application has the following advantages:

[0076] The present application first proposes to control the low-temperature toughness of the steel plate by controlling the proportion of the large-angle grain boundary in the structure of the steel plate, and through research, the influence law of the grain size and the proportion of the large-angle grain boundary on the ductile-brittle transition performance of the material is found, and the expression of the impact ductile-brittle transition temperature T c is proposed, as shown below:

[0077] In the formula, P f represents the proportion of the large-angle grain boundary, and d represents the average grain size of the ferrite; by controlling T c ≤-90℃, the steel plate has excellent low-temperature toughness.

[0078] In the composition design, by adding Mn and Ni austenite stabilizing elements, the formation of austenite is promoted, and the impact toughness at low temperature is improved, the composite addition of Ca and Re combines the Nb+V+Ti micro-alloying effect, the grain structure is refined, the average grain size of the ferrite is controlled to be ≤7μm, and at the same time, through the Mo, B content and the Nb / (Mo+75B) proportion control, a stable content of uniformly distributed large-angle grain boundaries is obtained, so that the steel plate has high strength and good low-temperature toughness.

[0079] Based on the composition design, this invention promotes the transformation of the microstructure by controlling the roughing and finishing rolling stages. Combined with the subsequent two-stage controlled cooling process, it induces ferrite to obtain fine and uniform ferrite grains during the subsequent cooling phase transformation, controls the proportion of large-angle grain boundaries in the microstructure to be in the range of 0.05 to 0.30, and forms a certain amount of refined and uniformly distributed M / A, thereby improving the uniformity of the microstructure.

[0080] This invention, through composition and process control, produces a steel microstructure consisting of granular bainite + polygonal / quasi-polygonal ferrite + M / A, with an average ferrite grain size ≤7μm and a high-angle grain boundary ratio of 0.05–0.30. High-angle grain boundaries refer to grain boundaries where the orientation difference between adjacent grains is greater than 15°. Its yield strength R... t0.5 ≥485MPa, tensile strength R m ≥570MPa, yield strength ratio R t0.5 / R m ≤0.93; Low-temperature impact and DWTT toughness, Charpy impact energy at -60℃ ≥300J, impact ductile-brittle transition temperature T c ≤-90℃, -40℃ DWTT SA%≥85%; Low temperature CTOD performance, -40℃ CTOD characteristic value δ m ≥0.4mm. Traditional pipeline steel with a strength grade of X70 can usually meet the low-temperature toughness requirement of -20℃, but cannot meet the toughness requirement at even lower temperatures. Detailed Implementation

[0081] The present invention will be further described below with reference to specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0082] The performance parameters of the present invention were measured as follows.

[0083] Chemical composition of pipeline steel: determined using a spectrometer and a nitrogen and oxygen analyzer.

[0084] Microstructure of pipeline steel: photographed and observed using an optical microscope (manufacturer: ZEISS, model: Axio Imager.M2m).

[0085] In the microstructure of pipeline steel, the average grain size of ferrite and the volume fraction of each phase were determined / calculated using metallographic microscopy, and the proportion of large-angle grain boundaries was determined / calculated using electron backscatter diffraction.

[0086] The yield strength R of pipeline steel was measured using ASTM A370-2021 standard. t0.5 Tensile strength R m And through the obtained yield strength R t0.5 and tensile strength R mCalculate the yield strength ratio R t0.5 / R m .

[0087] The Charpy impact energy at -60°C and the impact ductile-brittle transition temperature T of pipeline steel were measured using ASTM A370-2021 standard. c -40℃ DWTT SA%.

[0088] The CTOD characteristic value δ at -40℃ for pipeline steel was measured using the ISO 12135-2021 standard or method. m .

[0089] Examples and Comparative Examples

[0090] The methods for manufacturing pipeline steel in the examples and comparative examples include the following steps in sequence:

[0091] 1) Smelting and casting: Based on the chemical element composition of pipeline steel, smelting and refining are carried out, and then casting is carried out into slabs;

[0092] 2) Rolling: Rolling the slab, wherein the rolling process includes heating, rough rolling and finish rolling;

[0093] 3) Cooling: Cool the rolled slab to obtain pipeline steel.

[0094] The chemical composition of the pipeline steel in the embodiments and comparative examples of the present invention is shown in Table 1, with the balance being Fe and unavoidable impurities.

[0095] The manufacturing process parameters of the pipeline steel in the embodiments and comparative examples of the present invention are shown in Table 3.

[0096] The mechanical properties and microstructure of the pipeline steel plates obtained in the embodiments and comparative examples of this invention are shown in Table 4. As can be seen from Table 4, the microstructure of the pipeline steel obtained by this invention is a refined and uniform microstructure consisting of granular bainite + polygonal / quasi-polygonal ferrite + M / A, with an average ferrite grain size ≤7μm. The volume fraction of bainite is 68%–96%, the volume fraction of ferrite is 2%–30%, and the remainder is martensite-austenite component M / A. The proportion of large-angle grain boundaries (or orientation differences between adjacent grains greater than 15°) in the microstructure is 0.05–0.30. The pipeline steel obtained by this invention can achieve the X70 strength level, with a yield strength R... t0.5 ≥485MPa, tensile strength R m ≥570MPa, yield strength ratio R t0.5 / R m ≤0.93; Low-temperature impact and DWTT toughness, Charpy impact energy at -60℃ ≥300J, impact ductile-brittle transition temperature T c ≤-90℃, -40℃ DWTT SA%≥85%; Low temperature CTOD performance, -40℃ CTOD characteristic value δm ≥ 0.4 mm.

[0097] In the pipeline steel of Comparative Example 1, the Mo content is high, the Ca content is low, the B content is high, the (Ca+Re) / S ratio is low, the Nb / (Mo+75B) ratio is low, and the proportion of large-angle grain boundaries in the structure is low (0.03). Although the strength can meet the requirements, the low-temperature toughness is poor. In the pipeline steel of Comparative Example 2, the V content is high, the Nb+V+Ti content is high, the 4Mn+20Ni-14C content is low, the proportion of large-angle grain boundaries in the structure is high (0.35), and the average ferrite grain size is large (9 μm). Although the strength can meet the requirements, the low-temperature toughness is poor. The ductile-brittle transition temperature T c of the pipeline steels in Comparative Example 1 and Comparative Example 2 is all ≥-77℃, the-60℃ Charpy impact energy is less than 300 J, the-40℃ DWTT SA% is less than 85%, and the-40℃ CTOD characteristic value δm is less than 0.4 mm.

Claims

1. An X70 pipeline steel, characterized in that, The X70 pipeline steel, in addition to Fe and unavoidable impurities, also contains the following chemical components by weight percentage: C: 0.03–0.08%, Si: 0.10–0.30%, Mn: 0.8–1.5%, P ≤ 0.0070%, S ≤ 0.0012%, Ni: 0.05–0.50%, Cr: 0.1–0.6%, Mo: 0.01–0. 0.8%, Nb: 0.02~0.05%, V: 0~0.03%, Ti: 0.005~0.020%, Ca: 0.0010~0.0035%, Al: 0.010~0.040%, B≤0.0005%, O≤0.0030%, Re: 0.0001~0.0030%, where Re is Ce and La, and simultaneously satisfies the following relationship: (Ca+Re) / S≥2, 4Mn + 20Ni - 14C ≥ 3.5%, Nb+V+Ti≤0.09%, 0.3≤Nb / (Mo+75B)≤1.5 Substitute the symbols of each element in the formula with the corresponding weight percentage content of each element.

2. The X70 pipeline steel according to claim 1, characterized in that, The X70 pipeline steel contains the following chemical composition by weight percentage: C: 0.03-0.08%, Si: 0.10-0.30%, Mn: 0.8-1.5%, P≤0.0070%, S≤0.0012%, Ni: 0.05-0.50%, Cr: 0.1-0.6%, Mo: 0.01-0.08%, Nb: 0.02-0.05%, V: 0-0.03%, Ti: 0.005-0.020%, Ca: 0.0010-0.0035%, Al: 0.010-0.040%, B≤0.0005%, O≤0.0030%, Re: 0.0001-0.0030%, where Re is Ce and La, and the balance is Fe and unavoidable impurities.

3. The X70 pipeline steel according to claim 1 or 2, characterized in that, The chemical composition of the X70 pipeline steel also satisfies the following relationship: 2≤(Ca+Re) / S≤41, 3.5%≤4Mn+20Ni-14C≤12.30%, 0.054%≤Nb+V+Ti≤0.09%.

4. The X70 pipeline steel according to any one of claims 1-3, characterized in that, The X70 pipeline steel meets the following requirement: non-recrystallization temperature T nr ≥920℃, preferably 920~1005℃, wherein T nr ={(887+464*C+890*Ti+363*Al-357*Si+(6445*Nb)-644*(SQRT(Nb))+[732*V-230*(SQRT(V))]}, where the symbols of each element are replaced with the values ​​preceding the weight percentage of the corresponding element.

5. The X70 pipeline steel according to any one of claims 1-4, characterized in that, The microstructure of the X70 pipeline steel is: bainite + ferrite + martensite-austenite component M / A, wherein the bainite is granular, the ferrite is polygonal / quasi-polygonal, preferably the average grain size of ferrite is ≤7μm, more preferably 5-7μm, and the proportion of large-angle grain boundaries is 0.05-0.30, wherein the large-angle grain boundary refers to the grain boundary where the orientation difference between adjacent grains is greater than 15°; preferably the volume fraction of bainite is 68%-96%, the volume fraction of ferrite is 2%-30%, and the remainder is martensite-austenite component M / A.

6. The X70 pipeline steel according to claim 5, characterized in that, The average grain size of the ferrite, the proportion of large-angle grain boundaries, and the impact ductile-brittle transition temperature T c The following relationship must be satisfied: In the formula: T c The impact ductile-brittle transition temperature, in °C; P f The proportion of large-angle grain boundaries is dimensionless. d represents the average grain size of ferrite, in μm.

7. The X70 pipeline steel according to any one of claims 1-6, characterized in that, The yield strength R of the X70 pipeline steel t0.5 ≥485MPa, tensile strength R m ≥570MPa, yield strength ratio R t0.5 / R m ≤0.93; The X70 pipeline steel has a Charpy impact energy ≥300J at -60℃ and an impact ductile-brittle transition temperature T0. c ≤-90℃,-40℃DWTT SA%≥85%; The -40℃ CTOD characteristic value δ of the X70 pipeline steel m ≥0.4mm.

8. A method for manufacturing X70 pipeline steel according to any one of claims 1 to 7, characterized in that, The method includes the following steps in sequence: 1) Smelting and casting: The X70 pipeline steel, based on the chemical composition of any one of claims 1-4, is smelted, refined, and then cast into slabs. 2) The slab is rolled, and the rolling process includes heating, rough rolling, and finish rolling: The heating temperature is 1100~1200℃, the heating time is 3.5~7h, and the uniform heat is ≤30℃; The final reduction rate of the roughing roll is ≥16%, the final rolling temperature of the roughing roll is ≥950℃, and the final rolling temperature of the roughing roll is ≥the non-recrystallization temperature T. nr ; T nr ={(887+464*C+890*Ti+363*Al-357*Si+(6445*Nb)-644*(SQRT(Nb))+[732*V-230*(SQRT(V))]}, where the symbols of each element are replaced with the values ​​preceding the weight percentage of the corresponding element; The total reduction rate of finishing rolling is ≥75%, the initial rolling temperature of finishing rolling is ≤940℃, preferably 850~940℃, and the initial rolling temperature of finishing rolling is ≤T of the non-recrystallization temperature. nr The finishing rolling temperature is ≥A r3 +20℃; 3) Cool the rolled slab: First air cool to A r3 Below the specified temperature, control the cooling temperature to 700-790℃, then rapidly cool to the cooling stop temperature of 350-550℃. The rapid cooling rate is 8-28℃ / s.

9. The method according to claim 8, characterized in that, In step 1): Refining is carried out using LF and RH refining. During the LF refining process, the oxygen / sulfur impurity content in the steel is strictly controlled so that O≤0.003% and S≤0.0012%. Ca / Re treatment is used to control (Ca+Re) / S≥2, and the coarse and fine inclusions of A / B / C / D are ≤1.5 grade.

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