CN108034885B - Steel plate for low-crack-sensitivity pipe fitting used under low-temperature condition and manufacturing method thereof - Google Patents
Steel plate for low-crack-sensitivity pipe fitting used under low-temperature condition and manufacturing method thereof Download PDFInfo
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Abstract
The invention relates to a low crack sensitivity steel plate for pipe fittings used under low temperature, the chemical composition (wt%) is C0.03-0.08, Si 0.2-0.35, Mn0.9-1.6, Al 0.02-0.04, Nb 0.01-0.07, V0.01-0.07, Ti0.01-0.02, Nb + V + Ti is less than or equal to 0.15%, Cr is less than or equal to 0.5, Ni is less than or equal to 0.5, Mo is less than or equal to 0.4, Cu is less than or equal to 0.3, Pcm is less than or equal to 0.18; the balance being Fe and unavoidable impurities. KR molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting blank continuous casting with the thickness of 450mm, covering and slow cooling of the continuous casting blank, inspection and cleaning of the continuous casting blank, heating of the casting blank, high-pressure water descaling, rolling, cooling, straightening and offline tempering. The steel plate has a single tempered bainite structure, the grain size is within the range of 3-6 mu m, and the steel plate has excellent low-temperature toughness and can meet the use requirement of X70 pipeline steel under the low-temperature condition of 60 ℃ below zero.
Description
Technical Field
The invention belongs to the technical field of metallurgy, and relates to a steel plate for a low-crack-sensitivity pipe fitting used under a low-temperature condition and a manufacturing method thereof.
Background
In the long-distance pipeline engineering project, in order to meet the long-distance transportation requirement, facilities such as a plurality of gas compression stations, gas transmission and distribution stations and the like need to be built, so that a large amount of steel for station pipe fittings is needed. In some severe cold regions, especially station pipe fittings used in low-temperature environments like those of the middle and Russian east, the limit temperature of the station is between-30 ℃ and-50 ℃, and better low-temperature performance is required on the basis of steel used for conventional pipe fittings. Meanwhile, in the whole pipe fitting manufacturing process, the weldability of the material has great influence on the low-temperature performance of the pipe fitting steel, so that the steel plate for the low-crack sensitive pipe fitting can better meet the engineering requirement under the low-temperature condition, and the engineering safety is improved.
In the production process of the pipe fitting, the pipe fitting needs to be subjected to the processes of bulging, drawing and other hot forming processes and then subjected to integral heat treatment, so that the required material can still meet the performance requirements of the pipe fitting steel after quenching and tempering. The traditional steel for pipe fittings is usually designed by adopting medium-high carbon and alloy components for improving hardenability so as to ensure the performance requirement after heat treatment, for example, the patent publication No. CN102011064 provides X80 grade low-temperature steel for pipe fittings, preparation and application thereof, the use of the steel for pipe fittings with the thickness of 30-70mm under the condition of-46 ℃ can be met, the range of C is 0.12-0.16%, the steel does not belong to steel with easy weldability, and the weldability of the material is obviously poor. In the manufacturing process, a tempering process is often adopted for production.
The invention provides a low-crack-sensitivity pipe fitting steel used under a low-temperature condition, which is produced by adopting low-crack-sensitivity component design and a hot rolling process plus a tempering process, and can meet the requirement that the pipe fitting steel subjected to integral heat treatment is used under the low-temperature condition of minus 60 ℃.
Disclosure of Invention
The invention aims to solve the technical problem of providing the low-crack-sensitivity pipe fitting steel used under the low-temperature condition aiming at the prior art, which is produced by adopting low-crack-sensitivity component design and hot rolling process plus tempering process, can meet the requirement that the pipe fitting steel subjected to integral heat treatment is used under the low-temperature condition of minus 60 ℃, the steel plate structure is a single tempered bainite structure, the grain size is within the range of 3-6 mu m, the steel plate has very excellent low-temperature flexibility, and can meet the use requirement of X70 pipeline steel under the low-temperature condition of minus 60 ℃.
The technical scheme adopted by the invention for solving the problems is as follows: the steel plate for the low-crack-sensitivity pipe fitting used under the low-temperature condition comprises the following chemical components, by mass, 0.03-0.08% of C, 0.2-0.35% of Si, 0.9-1.6% of Mn, 0.02-0.04% of Al, less than or equal to 0.15% of Nb + V + Ti, less than or equal to 0.5% of Cr, less than or equal to 0.5% of Ni, less than or equal to 0.4% of Mo, less than or equal to 0.3% of Cu, 0.01-0.02% of Ti and less than or equal to 0.18% of Pcm; the welding crack sensitivity coefficient Pcm is C + Si/30+ (Mn + Cu + Cr)/20+ Ni/60+ Mo/15+ V/10+ 5B; the balance being Fe and unavoidable impurities.
The thickness of the steel plate product is 40-60mm, the steel plate can be used at the low temperature of-60 ℃, and the Charpy impact energy at the thickness of the steel plate 1/4 at-60 ℃ is more than or equal to 200J.
The invention adopts low-carbon and micro-alloy elements such as Nb, V, Ti and the like, C is less than or equal to 0.08 percent, lower carbon equivalent and cold crack sensitivity coefficient can be obtained, the steel plate is ensured to be completely in the interval of easy-to-weld material, and the steel plate has good weldability; meanwhile, Pcm is set to be less than or equal to 0.18, and double guarantee is provided for good weldability of the steel plate.
The principle of the steel material of the present invention corresponding to the chemical components and the contents thereof is specifically explained as follows:
c: comprehensively measuring the influence of C on hardenability and weldability, the invention designs the selection range of C content to be 0.03-0.08% by comprehensive consideration.
Si: the deoxidizing element and the solid solution strengthening improve the strength of the steel, and the Si content selection range is 0.2-0.35% in comprehensive consideration.
Mn: the basic alloy elements of the low-alloy steel can enlarge the temperature range of an austenite phase, improve the strength of the steel mainly through solid solution strengthening, and can make up the strength loss caused by too low C; is one of the main alloy elements affecting strength, hardenability and weldability. Comprehensively considering, the selection range of the Mn content is 0.9-1.6%;
al: the deoxidizing element may serve to refine the grains by forming AlN. The selection range of the Al content is 0.02-0.04%.
Nb: in the controlled rolling process, the temperature range of an austenite recrystallization region can be increased, austenite grains are effectively refined, the effect of fine grain strengthening is achieved, and the improvement of strength and toughness is facilitated. In the process of quenching and tempering, the solid-dissolved Nb can form NbC to be precipitated in a dispersion strengthening mode, the strength can be improved without damaging the toughness, and the selection range of the Nb content is 0.01-0.07 percent in comprehensive consideration.
V: the carbonitride forming elements mainly adopt fine grain strengthening and precipitation strengthening, and V (C, N) is formed to refine grains in a dispersion strengthening and precipitation strengthening mode, so that the strength and the toughness of the steel are improved. The content of V in the invention is selected from the range of 0.01-0.07%.
Cr: the main element for improving the hardenability can well make up the defect of strength and hardenability reduction caused by lower C content. When the content is too high, the low-temperature impact toughness and weldability are adversely affected. Ni and Cu: the strength of steel can be improved through solid solution strengthening, the low-temperature impact toughness is improved, and the corrosion resistance of the material can be improved through Cu; the steel has a hot cracking tendency after Cu is added, and the hot cracking tendency of the Cu-containing steel can be improved by adding a proper amount of Ni. Mo: the addition of a proper amount of Mo can improve the hardenability of the material, delay the ferrite transformation and is beneficial to overcoming the tempering brittleness of steel during quenching and tempering.
Ti: the nitrogen element is fixed, and 0.02 percent of Ti can fix N in the steel below 60ppm, and the TiN is formed to play the roles of precipitation strengthening and fine grain strengthening, so that the crystal grains are effectively refined. However, if the Ti content is too high, a coarse precipitated phase is likely to occur, which is disadvantageous in toughness, and the Ti content is selected from the range of 0.01 to 0.02% in the present invention.
In the component design, low-carbon and microalloying elements are adopted, and the emphasis is that Pcm is less than or equal to 0.18, so that the material has good low crack sensitivity.
The present application further provides a method for manufacturing the steel sheet, comprising: KR molten iron pretreatment, converter, LF refining, RH vacuum degassing, plate blank continuous casting of 370-450mm, slow cooling, rolling, offline tempering, fine inspection, performance detection and warehousing.
(1) A steelmaking process, namely KR molten iron pretreatment, 150t converter smelting, LF refining and RH vacuum degassing treatment are adopted to produce high-purity molten steel, and then 370-glass 450mm extra-thick continuous casting billets are produced by continuous casting; and then, covering, stacking, slowly cooling and hydrogen diffusion treatment are carried out on the continuous casting billets, and the stacking, slowly cooling time is more than or equal to 72 hours.
(2) Heating the continuous casting slab to 1100-1220 ℃, preserving heat for 160-200 min, fully dissolving alloy elements in the steel to ensure the uniformity of performance, and descaling and removing iron oxide scales by using high-pressure water after discharging.
(3) After descaling by high-pressure water, carrying out two-stage rolling, wherein the first stage is a rough rolling stage, the initial rolling temperature is 1000-1150 ℃, the accumulated comprehensive reduction rate is more than or equal to 50%, the average reduction rate of the last three single passes is more than or equal to 20%, the full refinement of the prior austenite grain size is ensured, and the total control is within 25 mu m; the second stage is a finish rolling stage, the initial rolling temperature is 820-880 ℃, the cumulative pass reduction rate is more than or equal to 65%, the non-recrystallization zone adopts a low-temperature large-reduction rolling process to enable austenite to be fully flattened, and the flattening width is controlled within 10-16 mu m; carrying out ACC quick cooling on the rolled steel plate, wherein the water outlet temperature is 300-400 ℃; continuing air cooling after water is discharged; the advantages of a continuous casting billet with the thickness of 450mm are exerted, and the rolling under high pressure is fully realized, and the prior austenite grains are refined.
(4) The off-line tempering temperature is 580-680 ℃, the in-furnace time is 3.5-4.0 min/mm, and the furnace is discharged and then air-cooled.
Aiming at the steel plate for the easily welded pipe fitting used under the low temperature condition, the invention uses low-carbon and low-crack sensitivity coefficient component design, high-purity steel smelting, 370-doped 450mm thick super-thick continuous casting plate blank as raw materials, and adopts the method of controlled rolling, controlled cooling and off-line tempering to produce the steel plate for the easily welded pipe fitting with the yield strength of more than 500MPa, and the performance of the steel plate can meet the requirement of X70 grade steel for the pipe fitting used under the low temperature condition of minus 60 ℃.
The invention emphasizes the advantages of component design, realizes the aim of low crack sensitivity by reasonably matching micro alloy elements such as Nb, Ti, Ni and the like on the basis of reducing the alloy elements such as carbon, Mn and the like, has Pcm not more than 0.18 percent, can realize the aim of no need of preheating under the condition of high heat input welding and ensures the stable performance of a heat affected zone after welding; on the basis, the rolling advantage of the 370-pass 450mm thick large-thickness continuous casting billet is exerted, the rough rolling large reduction is realized in the recrystallization zone, the accumulated pass reduction rate reaches more than 50 percent, the later three single pass reduction rates are more than or equal to 20 percent, the full refinement of the prior austenite grain size is ensured, and the total control is within 25 mu m; meanwhile, in the stage of a non-recrystallization zone, due to the advantages of an ultra-thick continuous casting billet, on the premise of meeting the requirements of rough rolling and large compression ratio, the accumulated pass reduction rate in the finish rolling stage is enabled to be more than or equal to 65%, the rolling process at low temperature and high pressure in the non-recrystallization zone enables austenite to be fully flattened, the flattening width is controlled within 10-16 mu m, the full flattening can provide more nucleation positions for the formation of subsequent bainite, the grain refinement of the bainite is realized, the bainite structure is obtained after accelerated cooling, and the grain size is within 3-6 mu m; in the off-line tempering treatment, the bainite internal dislocation density is reduced to some extent, and fine carbides are precipitated, but because the grains are fully refined, the bainite structure is still formed on the structure, the strength is reduced to some extent, the toughness is improved to some extent, and the bainite steel can be used at the temperature of minus 60 ℃.
Compared with the prior art, the invention has the advantages that: (1) the invention adopts the design of low-carbon and low-crack sensitivity components, C is 0.03-0.08%, and the cold crack sensitivity coefficient Pcm is less than or equal to 0.18; ensuring good weldability of the steel plate. (2) The steel plate provided by the invention is produced by using a controlled rolling and controlled cooling and offline tempering process, the advantages of 370-doped large-thickness continuous casting billets with the thickness of 450mm are fully exerted, the rolling compression ratio is improved, and the austenite grain size is fully refined, so that the Charpy impact energy of 1/4-thickness parts of the steel plate at-60 ℃ is more than or equal to 200J when the steel plate is used at the low temperature of-60 ℃, and the steel plate has good low-temperature toughness.
Drawings
FIG. 1 is a photograph of a microstructure of a 40mm thick steel plate according to the present invention;
FIG. 2 is a photograph showing the microstructure of a 60mm thick steel plate according to the present invention.
Detailed Description
The present invention will be described in further detail with reference to examples.
Examples 1 to 2:
according to the chemical component range and the manufacturing method, the steel plate for the low-crack-sensitivity pipe fitting used under the low-temperature condition of 60 ℃ below zero is manufactured through the process steps of KR molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting blank continuous casting with the thickness of 450mm, covering and slow cooling of the continuous casting blank, checking and cleaning of the continuous casting blank, heating of the casting blank, high-pressure water descaling, rolling, ACC cooling, air cooling, straightening and offline tempering.
The specific process of the heating, rolling and slow cooling stage comprises the following steps: heating a continuous casting slab with the thickness of 450mm to 1200 ℃, preserving heat for 200min (examples 1 and 2), and descaling the continuous casting slab by using high-pressure water after the continuous casting slab is discharged; then two-stage rolling is carried out, the first stage is rough rolling, the rolling temperature is 1080-; the second stage is finish rolling, the initial rolling temperature is 830 ℃, the cumulative pass reduction rate is 66.7 percent (example 1) or 820 ℃, the cumulative pass reduction rate is 66.7 percent (example 2), and the final steel plate thickness is 40mm (example 1) and 60mm (example 2); after rolling, the ACC is rapidly cooled, and the effluent temperature is 370 ℃ (example 1) and 330 ℃ (example 2); then air cooling is carried out, and off-line tempering treatment is carried out after air cooling, wherein the tempering temperature is 620 ℃, the tempering heat preservation time is 140min (example 1), the tempering temperature is 660 ℃, and the tempering heat preservation time is 210min (example 2).
The mechanical properties of the test steel sheet are shown in Table 2, and the microstructure of the steel sheet is shown in FIGS. 1 and 2, the microstructure is a uniform tempered bainite structure, and the grain size is in the range of 3 to 6 μm.
Table 1 chemical composition (wt.%) of steel sheet for pipe fittings in examples 1 and 2
| Examples of the invention | C | Si | Mn | P | S | Al | Nb+Ti+V | Pcm | Ceq |
| 1 | 0.06 | 0.25 | 1.50 | 0.008 | 0.0006 | 0.023 | 0.10 | 0.17 | 0.418 |
| 2 | 0.07 | 0.24 | 1.53 | 0.007 | 0.0005 | 0.025 | 0.12 | 0.18 | 0.422 |
Ceq＝C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
Pcm＝C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B
Table 2 mechanical properties of steel sheets for pipe fittings in examples 1 and 2
In addition to the above embodiments, the present invention also includes other embodiments, and any technical solutions formed by equivalent transformation or equivalent replacement should fall within the scope of the claims of the present invention.
Claims (2)
1. A method for producing a steel sheet for a pipe fitting having low crack sensitivity, which is used under low temperature conditions, characterized by comprising: the production process flow comprises KR molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting of a continuous casting billet with the thickness of 450mm in 370-plus-cover slow cooling of the continuous casting billet, inspection and cleaning of the continuous casting billet, heating of the casting billet, high-pressure water descaling, rolling, cooling, straightening and off-line tempering,
the method comprises the following main steps:
(1) a steelmaking process, namely KR molten iron pretreatment, 150t converter smelting, LF refining and RH vacuum degassing treatment are adopted to produce high-purity molten steel, and then 370-glass 450mm extra-thick continuous casting billets are produced by continuous casting; then, covering, stacking, slowly cooling and hydrogen diffusion treatment are carried out on the continuous casting billets, and the stacking, slowly cooling time is more than or equal to 72 hours; the steel plate comprises the following chemical components, by mass percent, 0.03-0.08% of C, 0.2-0.35% of Si, 0.9-1.6% of Mn0.02-0.04% of Al, less than or equal to 0.15% of Nb + V + Ti, less than or equal to 0.5% of Cr, less than or equal to 0.5% of Ni, less than or equal to 0.4% of Mo, less than or equal to 0.3% of Cu, 0.01-0.02% of Ti0.01, and less than or equal to 0.18% of Pcm; the weld crack susceptibility Pcm = C + Si/30+ (Mn + Cu + Cr)/20+ Ni/60+ Mo/15+ V/10+ 5B; the balance of Fe and inevitable impurities;
(2) heating the continuous casting slab to 1100-1220 ℃, preserving heat for 160-200 min to ensure that alloy elements in the steel are fully dissolved in a solid manner so as to ensure the uniformity of performance, and descaling and removing iron oxide scales by using high-pressure water after discharging;
(3) after descaling by high-pressure water, carrying out two-stage rolling, wherein the first stage is a rough rolling stage, the initial rolling temperature is 1000-1150 ℃, the accumulated comprehensive reduction rate is more than or equal to 50%, the average reduction rate of the last three single passes is more than or equal to 20%, the full refinement of the prior austenite grain size is ensured, and the total control is within 25 mu m; the second stage is a finish rolling stage, the initial rolling temperature is 820-880 ℃, the cumulative pass reduction rate is more than or equal to 65%, the non-recrystallization zone adopts a low-temperature large-reduction rolling process to enable austenite to be fully flattened, and the flattening width is controlled within 10-16 mu m; carrying out ACC quick cooling on the rolled steel plate, wherein the water outlet temperature is 300-400 ℃; continuing air cooling after water is discharged; the advantages of a continuous casting billet with the thickness of 450mm are exerted, the rolling under high pressure is fully realized, and the prior austenite grains are refined;
(4) the off-line tempering temperature is 580-680 ℃, the in-furnace time is 3.5-4.0 min/mm, and the furnace is discharged and then air-cooled.
2. The method for manufacturing a steel sheet for a low-crack-susceptibility pipe used under low-temperature conditions according to claim 1, wherein: the thickness of the steel plate is 40-60mm, the steel plate can be used at the low temperature of-60 ℃, the Charpy impact energy of the steel plate 1/4 at the thickness of-60 ℃ is more than or equal to 200J, and the requirement of the steel for X70-level pipe fittings can be met.
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