X100-grade seamless steel pipe with low carbon equivalent and high toughness and preparation method thereof

Abstract

The invention discloses an X100-grade seamless steel tube with low carbon equivalent and high strength and toughness and a preparation method of the X100-grade seamless steel tube in the technical field of seamless steel tubes. The seamless steel pipe comprises the following chemical components in percentage by weight: 0.08%-0.12% of C, 0.25%-0.35% of Si, 1.10%-1.35% of Mn, less than or equal to 0.008% of P, less than or equal to 0.003% of S, 0.25%-0.40% of Mo, 0.035%-0.055% of Nb, 0.05%-0.08% of V, 0.010%-0.025% of Ti, 0.015%-0.040% of Al, 0.30%-0.45% of Ni, 0.25%-0.40% of Cr, less than or equal to 0.25% of Cu and the balance of Fe and inevitable impurities. Wherein CE (IW) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 is less than or equal to 0.43%, and Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 is less than or equal to 0.21%. The low-carbon component design is adopted, the carbon equivalent and the cold crack sensitive index are controlled, then strain induction is utilized to separate out a nanophase and refine grains, and the excellent low-temperature toughness and welding safety are achieved while the strength requirement of the steel pipe is met.

Description

Technical Field

[0001] This invention relates to the field of seamless steel pipe technology, and in particular to an X100 grade low carbon equivalent high strength and toughness seamless steel pipe and its preparation method. Background Technology

[0002] High-strength seamless steel pipes, especially those of X100 and above with a yield strength ≥690 MPa, are key materials for deep-sea oil and gas development, ultra-high-pressure pipelines, and marine engineering structures. Improving material strength helps achieve lightweight equipment, increased transportation efficiency, and reduced construction and operating costs; therefore, developing seamless steel pipes that combine ultra-high strength, excellent low-temperature toughness, and good weldability is of great significance.

[0003] However, the industrial production of this type of material has long faced a prominent technical contradiction: it is difficult to improve strength, weldability, and toughness in a coordinated manner.

[0004] Traditional techniques for achieving high strength typically involve increasing carbon content and the addition of alloying elements (such as Mn, Cr, and Mo). However, this leads to a significant increase in the welding carbon equivalent of the material. Excessive carbon equivalent exacerbates hardening and embrittlement of the weld heat-affected zone, and dramatically increases susceptibility to cold cracking, especially under harsh conditions such as offshore welding, seriously threatening structural safety. Furthermore, during the quenching and tempering heat treatment of thick-walled seamless steel pipes, insufficient hardenability in the core often results in uneven cross-sectional properties; the core strength and toughness are often lower than the surface layer, affecting overall service reliability.

[0005] To address these issues, existing technologies have explored various approaches. For instance, refining the microstructure through complex heat treatment processes such as "double quenching + tempering" can improve toughness to some extent, but these methods do not adequately address the limiting constraints on carbon equivalent in the composition system, thus failing to fundamentally solve the welding safety problem. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing an X100 grade low-carbon equivalent high-strength and high-toughness seamless steel pipe and its preparation method, which takes into account strength, toughness, and weldability.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution for the X100 grade low-carbon equivalent high-strength and high-toughness seamless steel pipe and its preparation method:

[0008] An X100 grade low-carbon equivalent high-strength and high-toughness seamless steel pipe, wherein the chemical composition of the seamless steel pipe, by weight percentage, includes the following chemical components:

[0009] C: 0.08%~0.12%, Si: 0.25%~0.35%, Mn: 1.10%~1.35%, P≤0.008%, S≤0.003%, Mo: 0.25%~0.40%, Nb: 0.035%~0.055%, V: 0.05%~0.08%, Ti: 0.010%~0.025%, Al: 0.015%~0.040%, Ni: 0.30%~0.45%, Cr: 0.25%~0.40%, Cu≤0.25%, with the balance being Fe and unavoidable impurities;

[0010] Among them, CE (IIW) = C + Mn / 6 + (Cr+Mo+V) / 5 + (Ni+Cu) / 15 ≤ 0.43%, Pcm= C +Si / 30 + (Mn+Cu+Cr) / 20 + Ni / 60 + Mo / 15 + V / 10≤ 0.21%.

[0011] The core design of this patent lies in adopting a low-carbon and dual-constraint principle, strictly limiting the carbon content to below 0.12%, thereby effectively reducing the carbon equivalent CE(IIW) ≤ 0.43% and the cold cracking sensitivity index Pcm ≤ 0.21%. This significantly improves weldability, ensures controllable hardness in the heat-affected zone, and minimizes the risk of cold cracking. Furthermore, it utilizes microalloying and composite alloying to synergistically guarantee the material's strength and toughness. In terms of microalloying, the precise proportions of Nb, V, and Ti elements are used to refine grains, induce precipitation, and fix nitrogen, strengthening the microstructure without excessively increasing the carbon equivalent. Simultaneously, under the strict constraints of CE and Pcm, the scientifically adjusted composite proportions of Mn, Mo, Ni, Cr, and Cu ensure that even thick-walled tube sections achieve sufficient and uniform martensite / bainite microstructure during quenching, thus laying a solid foundation for achieving high core strength.

[0012] A method for preparing X100 grade low carbon equivalent high strength and toughness seamless steel pipe includes the following steps:

[0013] S1. High-purity smelting and continuous casting: Smelting, refining and vacuum degassing are carried out according to the chemical composition described in claim 1, and the [H] in the molten steel is controlled to be ≤1.5ppm and [O] ≤20ppm, and then continuously cast into round billets;

[0014] S2. Billet heating and piercing: The round billet is heated to 1200-1220℃ and held at that temperature, and then pierced by skew rolling to obtain the tube.

[0015] S3. Controlled rolling: The tube is subjected to multiple hot rolling passes, including the following rolling stages performed in sequence:

[0016] S31. Recrystallization zone rolling stage: Rolling is carried out in the temperature range of finishing mill inlet temperature ≥980℃, with a cumulative deformation rate ≥50%;

[0017] S32. Rolling stage in the non-recrystallization zone: When the temperature drops below 950℃, continue rolling and control the final rolling temperature to 840℃~880℃, with a cumulative deformation rate ≥40%;

[0018] After rolling, the material is cooled at a rate of not less than 15℃ / s to induce strain-induced precipitation of microalloying elements.

[0019] S4. Quenching and tempering heat treatment: The rolled steel pipe is heated to 910-930℃ for austenitization, held at that temperature and then quenched; subsequently tempered at 520-580℃.

[0020] S5. Finishing and Inspection.

[0021] Preferably, in step S4, the tempering holding time is calculated based on the steel pipe wall thickness and is 1.5 to 2.5 min / mm.

[0022] Preferably, during the post-rolling cooling process in step S3, the strain-induced precipitates are (Nb,V)(C,N) type nanoscale precipitates with a size of 5–20 nm.

[0023] Preferably, both steps S31 and S32 involve rolling at least three passes.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] 1. This invention, through a dual-limit design of CE(IIW) ≤ 0.43% and Pcm ≤ 0.21%, minimizes the sensitivity to cold cracking in welding from a materials science perspective. This enables X100 grade ultra-high strength steel pipes to be safely applied to critical components such as offshore platforms and floating production storage and offloading (FPSO) units, which have extremely high welding requirements, thus solving the biggest obstacle to engineering applications.

[0026] 2. This invention creatively utilizes the hot rolling process as an active "strengthening phase pre-fabrication" stage. Through strain-induced precipitation induced by rolling in the non-recrystallization zone, nanoscale precipitation strengthening is introduced before tempering heat treatment, partially sharing the burden of final strengthening. This allows for the use of lower carbon and alloy content while maintaining the same strength requirements, achieving "reduced" green manufacturing, while simultaneously improving the material's toughness and weldability.

[0027] 3. Rolling in the recrystallization zone ensures a uniform and fine starting point for the microstructure; the combination of deformation and induced precipitation in the non-recrystallization zone makes the strengthening phase distribution more dispersed and its bond with the dislocation structure more tightly. The final "fine-grained tempered bainite / martensite + multi-scale precipitates" microstructure obtained after quenching and tempering has extremely high stability, ensuring uniform performance across the entire cross-section of the steel pipe, especially significantly improving the core toughness of thick-walled pipes. Detailed Implementation

[0028] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art will fall within the scope defined by the appended claims.

[0029] Example 1

[0030] An X100 grade low-carbon equivalent high-strength and high-toughness seamless steel pipe, comprising the following chemical composition by weight percentage:

[0031] C: 0.08%, Si: 0.25%, Mn: 1.15%, P: 0.005%, S: 0.002%, Mo: 0.25%, Nb: 0.045%, V: 0.05%, Ti: 0.018%, Al: 0.03%, Ni: 0.35%, Cr: 0.28%, Cu: 0.08%, with the balance being Fe and unavoidable impurities;

[0032] The welding carbon equivalent CE (IIW) = C + Mn / 6 + (Cr+Mo+V) / 5 + (Ni+Cu) / 15 = 0.416%, which satisfies CE (IIW) ≤ 0.43%.

[0033] The crack sensitivity index Pcm = C + Si / 30 + (Mn+Cu+Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 = 0.191%, which satisfies Pcm ≤ 0.21%.

[0034] A method for preparing X100 grade low carbon equivalent high strength and toughness seamless steel pipe includes the following steps:

[0035] S1. High-purity smelting and continuous casting: Smelting, refining and vacuum degassing are carried out according to the above chemical composition, and the [H] in the molten steel is controlled to be ≤1.5ppm and [O] ≤20ppm. Then, it is continuously cast into Φ450mm round billets.

[0036] S2. Billet heating and piercing: The round billet is heated to 1210℃ and held at that temperature, and then pierced by skew rolling to obtain the tube.

[0037] S3. Controlled rolling: The tube is rolled in 5 passes in the recrystallization zone at a finishing mill inlet temperature of 1050℃, with a cumulative deformation of 60%; the temperature is then reduced to approximately 940℃, followed by 3 passes in the non-recrystallization zone, with the final rolling temperature controlled at 860℃, and a cumulative deformation of 45%. Immediately after rolling, it is cooled at a rate of 20℃ / s.

[0038] S4. Quenching and tempering heat treatment: The rolled steel pipe is heated to 930℃ for austenitization, held for 40 minutes and then quenched; then tempered at 560℃ for 110 minutes (corresponding to a wall thickness of 55mm).

[0039] S5. Finishing and Inspection.

[0040] The final performance achieved is as follows:

[0041] Mechanical properties: Rp0.2=715 MPa, Rm=795 MPa, A=20%, average impact energy at -40℃ KV2: 135J.

[0042] Weldability evaluation: The surface crack rate and cross-sectional crack rate of the Y-groove weld were 0% and 0%, respectively. The highest hardness of the heat-affected zone was HV10=298.

[0043] Microstructure: Uniform tempered bainite, with the original austenite grain size reaching grade 10.5. Transmission electron microscopy reveals (Nb,V)(C,N) type precipitates of 12-30 nm.

[0044] Example 2

[0045] An X100 grade low-carbon equivalent high-strength and high-toughness seamless steel pipe, comprising the following chemical composition by weight percentage:

[0046] C: 0.1%, Si: 0.3%, Mn: 1.1%, P: 0.004%, S: 0.0015%, Mo: 0.25%, Nb: 0.045%, V: 0.05%, Ti: 0.018%, Al: 0.028%, Ni: 0.3%, Cr: 0.25%, Cu: 0.05%, with the balance being Fe and unavoidable impurities;

[0047] The welding carbon equivalent CE(IIW) = C + Mn / 6 + (Cr+Mo+V) / 5 + (Ni+Cu) / 15 = 0.417%, which satisfies CE(IIW)≤ 0.43%.

[0048] The crack sensitivity index Pcm = C + Si / 30 + (Mn+Cu+Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 = 0.207%, which satisfies Pcm ≤ 0.21%.

[0049] A method for preparing X100 grade low carbon equivalent high strength and toughness seamless steel pipe includes the following steps:

[0050] S1. High-purity smelting and continuous casting: Smelting, refining and vacuum degassing are carried out according to the above chemical composition, and the [H] in the molten steel is controlled to be ≤1.5ppm and [O] ≤20ppm. Then, it is continuously cast into Φ430mm round billets.

[0051] S2. Billet heating and piercing: The round billet is heated to 1215℃ and held at that temperature, and then pierced by skew rolling to obtain the tube.

[0052] S3. Controlled rolling: The tube is rolled in four passes in the recrystallization zone at a finishing mill inlet temperature of 1020℃, with a cumulative deformation of 55%. When the temperature drops to approximately 930℃, it is then rolled in three passes in the non-recrystallization zone, with the final rolling temperature controlled at 870℃, and a cumulative deformation of 42%. The tube is immediately cooled at a cooling rate of 18℃ / s after rolling.

[0053] S4. Quenching and tempering heat treatment: Heat the rolled steel pipe to 920℃ for austenitization, hold for 35 minutes and then quench; then temper at 550℃ for 100 minutes (corresponding to a wall thickness of 50mm).

[0054] S5. Finishing and Inspection.

[0055] The final performance obtained is as follows: Mechanical properties: Rp0.2=705 MPa, Rm=785 MPa, A=21%, -40℃ KV2

[0056] Average impact energy: 140J. Weldability evaluation: Crack test of oblique Y-groove weld, surface crack rate 0%, cross-section...

[0057] Crack rate 0%, maximum hardness of heat-affected zone HV10=290.

[0058] Microstructure: Uniform tempered bainite, with the original austenite grain size reaching grade 10.2. (Nb,V)(C,N) type precipitates of 8-25 nm are visible under transmission electron microscopy.

[0059] Example 3

[0060] An X100 grade low-carbon equivalent high-strength and high-toughness seamless steel pipe, comprising the following chemical composition by weight percentage:

[0061] C: 0.09%, Si: 0.28%, Mn: 1.12%, P: 0.005%, S: 0.002%, Mo: 0.28%, Nb: 0.045%, V: 0.05%, Ti: 0.018%, Al: 0.025%, Ni: 0.35%, Cr: 0.28%, Cu: 0.08%, with the balance being Fe and unavoidable impurities;

[0062] Among them, the welding carbon equivalent CE (IIW) = C + Mn / 6 + (Cr+Mo+V) / 5 + (Ni+Cu) / 15 = 0.427%, which satisfies CE (IIW) ≤ 0.43%;

[0063] The crack sensitivity index Pcm = C + Si / 30 + (Mn+Cu+Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 = 0.203%, which satisfies Pcm ≤ 0.21%.

[0064] A method for preparing X100 grade low carbon equivalent high strength and toughness seamless steel pipe includes the following steps:

[0065] S1. High-purity smelting and continuous casting: Smelting, refining and vacuum degassing are carried out according to the above chemical composition, and the [H] in the molten steel is controlled to be ≤1.5ppm and [O] ≤20ppm. Then, it is continuously cast into Φ410mm round billets.

[0066] S2. Billet heating and piercing: The round billet is heated to 1205℃ and held at that temperature, and then pierced by skew rolling to obtain the tube.

[0067] S3. Controlled rolling: The tube is rolled in 5 passes in the recrystallization zone at a finishing mill inlet temperature of 1005℃, with a cumulative deformation of 58%. When the temperature drops to about 935℃, it is then rolled in 4 passes in the non-recrystallization zone, with the final rolling temperature controlled at 850℃, and a cumulative deformation of 43%. After rolling, it is immediately cooled at a cooling rate of 15℃ / s.

[0068] S4. Quenching and tempering heat treatment: Heat the rolled steel pipe to 925℃ for austenitization, hold for 45 minutes and then quench; then temper at 570℃ for 90 minutes (corresponding to a wall thickness of 45mm).

[0069] S5. Finishing and Inspection.

[0070] The final performance achieved is as follows:

[0071] Mechanical properties: Rp0.2=710 MPa, Rm=790 MPa, A=20%, -40℃ KV

[0072] Average impact energy: 130J. Weldability evaluation: Crack test of oblique Y-groove weld, surface crack rate 0%, cross-section...

[0073] Crack rate 0%, maximum hardness of heat-affected zone HV10=295.

[0074] Microstructure: Uniform tempered bainite, with the original austenite grain size reaching grade 10.3. Transmission electron microscopy reveals (Nb,V)(C,N) type precipitates of 12-35 nm.

[0075] Comparative Example

[0076] Using existing steel pipes with the following chemical composition: C: 0.14%, Mn: 1.65%, Mo: 0.45%, CE(IIW) ≈ 0.52%, the manufactured steel pipes exhibit the following properties: slightly higher strength (Rp0.2 = 740 MPa), but only 65 J of impact energy at -40℃. The crack rate in the oblique Y-type test is as high as 40%, the highest hardness in the heat-affected zone reaches HV10 = 385, welding requires extremely high preheating temperatures, and there are significant risks in engineering applications.

[0077] Therefore, it can be seen that, under the premise of strictly controlling CE to ≤0.43% and Pcm ≤0.21%, the embodiments of the present invention successfully produced seamless steel pipes with yield strength not less than 700MPa and tensile strength not less than 780MPa, fully meeting the strength requirements of X100 steel grade. More importantly, all embodiments exhibit excellent low-temperature toughness and superior weldability.

Claims

1. An X100 grade low carbon equivalent high strength and toughness seamless steel pipe, characterized by, The seamless steel pipe has the following chemical components in percentage by weight: C: 0.08%~0.12%, Si: 0.25%~0.35%, Mn: 1.10%~1.35%, P≤0.008%, S≤0.003%, Mo: 0.25%~0.40%, Nb: 0.035%~0.055%, V: 0.05%~0.08%, Ti: 0.010%~0.025%, Al: 0.015%~0.040%, Ni: 0.30%~0.45%, Cr: 0.25%~0.40%, Cu≤0.25%, the balance being Fe and inevitable impurities; CE (IIW) = C + Mn / 6 + (Cr+Mo+V) / 5 + (Ni+Cu) / 15 ≤ 0.43%, Pcm= C + Si / 30 + (Mn+Cu+Cr) / 20 + Ni / 60 + Mo / 15 + V / 10≤ 0.21%.

2. The method of producing an X100 grade low carbon equivalent high strength and toughness seamless pipe according to claim 1, characterized by, The method comprises the following steps: S1, high-purity smelting and continuous casting: smelting, refining and vacuum degassing are performed according to the chemical components in claim 1, [H] in the molten steel is controlled to be ≤1.5ppm, [O] is controlled to be ≤20ppm, and then the round billet is continuously cast into a round billet; S2, pipe billet heating and piercing: the round billet is heated to 1200-1220℃ and kept, and then is subjected to inclined rolling piercing to obtain a rough pipe; S3, controlled rolling: the rough pipe is subjected to multi-pass hot rolling, which comprises the following sequentially executed rolling stages: S31, recrystallization zone rolling stage: rolling is performed in a temperature range with a finish rolling inlet temperature ≥980℃, and a cumulative deformation rate ≥50%; S32, non-recrystallization zone rolling stage: when the temperature drops below 950℃, rolling is continuously performed, and the finish rolling temperature is controlled to be 840℃~880℃, and the cumulative deformation rate is ≥40%; After rolling, cooling is performed at a cooling rate not lower than 15℃ / s to induce strain-induced precipitation of the micro-alloying elements; S4, quenching and tempering heat treatment: the rolled steel pipe is heated to 910~930℃ for austenitizing, and is quenched after holding; Then is tempered at 520~580℃; S5, finishing and detection.

3. The method of producing X100 grade low carbon equivalent high strength and toughness seamless pipe according to claim 2, characterized in that: In step S4, the holding time of the tempering is 1.5~2.5 min / mm calculated according to the wall thickness of the steel pipe.

4. The method of producing X100 grade low carbon equivalent and high strength and toughness seamless pipe according to claim 2, characterized in that: In the rolling and cooling process in step S3, the induced strain-induced precipitates are (Nb, V) (C, N) type nanoscale precipitates with a size of 5~20 nm.

5. The method of producing X100 grade low carbon equivalent and high strength and toughness seamless pipe according to claim 2, characterized in that: In steps S31 and S32, at least three passes are rolled.