Production method of BJ770 steel strip and method for preparing straight seam boom steel pipe from same

Abstract

A method for producing BJ770 steel strip and a method for preparing straight seam boom steel pipes thereof belong to the field of metallurgy technology. The production method of BJ770 steel strip includes smelting, continuous casting, heating, hot rolling, and cooling and coiling processes; in the hot rolling process, the total reduction rate of rolling below 950°C is 35-50%, and the final rolling temperature is 830-850°C; in the cooling and coiling process, laminar cooling adopts rear-stage water cooling, the starting cooling temperature is 640-670°C, and the coiling temperature is 510-570°C. After the obtained BJ770 steel strip is formed and pipe-formed, the weld or the entire welded pipe is normalized at 920-950°C, and after cooling, it is heated to 540-580°C and tempered for 20-60 minutes. The BJ770 steel strip of the present invention has good forming performance and is not prone to forming cracking and weld cracking during welding. The straight seam boom steel pipe produced has a low crack sensitivity coefficient and higher toughness and fatigue performance than seamless steel pipes of the same grade.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and particularly relates to a method for producing a BJ770 steel strip and a method for preparing a straight seam boom steel pipe thereof. Background Art

[0002] Crane steel structure is one of the main components of the crane, and the boom is an important family member of the crane steel structure. The crane boom pipe is the main member of the boom family. The crane boom pipe was originally made of seamless steel pipe, and the standard "GB 30584-2014 Seamless Steel Pipe for Crane Boom" was implemented. Among them, ultra-high strength BJ770 is the main material for lightweight crane boom. BJ700 yield strength R eL / R p0.2 : ≥770MPa, tensile strength: 820~1000MPa, elongation after fracture A≥14%, Charpy V-notch impact energy of longitudinal full-size specimen of steel pipe at -20℃ ≥55J.

[0003] Compared to seamless steel pipe, ERW straight seam steel pipe has a more uniform wall thickness, a lower carbon equivalent, a lower crack sensitivity coefficient, and higher toughness and fatigue performance. With advances in hot-rolled steel strip and ERW welded pipe technology, the development of high-strength welded pipe to replace seamless steel pipe has become possible. However, the BJ770 material has a very high yield strength, resulting in significant springback during the forming process, reaching or even exceeding the design strength limit of the forming unit. This results in unstable weld quality. Ultra-high-strength steel is generally very sensitive to cut quality during longitudinal shearing. Poor cut quality in the steel strip after longitudinal shearing can easily lead to microcracks in the steel strip, which can easily cause cracking in the forming process and weld cracking. The welded pipe qualification rate is generally only 80-90%, a technical challenge that has limited the use of welded pipe in place of seamless pipe in the production of crane boom pipes. Summary of the Invention

[0004] To address the issues of high strength, microcrack sensitivity, and high rebound force during forming that can lead to cracking and poor weld quality in hot-rolled steel strip, the present invention provides a method for producing BJ770 steel strip and a method for manufacturing straight seam boom steel pipe. This invention utilizes controlled rolling, controlled cooling, and tempering precipitation-strengthening steel strip. The microstructure of the BJ770 hot-rolled steel strip is primarily composed of fine polygonal ferrite and acicular ferrite, with a small amount of bainite. By lowering the coiling temperature to suppress the precipitation of secondary precipitate particles in the steel, the steel strip's strength is controlled near the strength valley, resulting in excellent cold forming properties. After forming and pipemaking, the weld seam (or the entire strip) is normalized and the welded pipe is tempered as a whole to reduce residual stress in the pipe and improve performance uniformity. Tempering also promotes the precipitation of 2-5 nanometer-sized secondary phase particles in the ferrite phase, improving the overall strength and fatigue resistance of the straight seam steel pipe.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is:

[0006] A production method for a BJ770 steel strip comprises the steps of smelting, continuous casting, heating, hot rolling, and cooling and coiling. In the hot rolling step, the total reduction rate during rolling is 35-50% at a temperature below 950° C., and the final rolling temperature is 830-850° C. In the cooling and coiling step, laminar cooling adopts rear-stage water cooling, the start-up temperature is 640-670° C., and the coiling temperature is 510-570° C.

[0007] The chemical composition of the BJ770 steel strip and its mass percentage are: C: 0.04-0.07%, Mn: 1.20-1.80%, S≤0.006%, P≤0.015%, Si: 0.20-0.30%, Als: 0.015-0.050%, Nb: 0.020-0.035%, Ti: 0.08-0.10%, Cr: 0.10-0.60%, Mo: 0.15-0.30%, N≤0.0050%, and the balance is iron and unavoidable impurities.

[0008] The heating process is carried out by heating in a walking beam heating furnace at a heating temperature of 1260-1290°C.

[0009] The thickness of the BJ770 steel strip is 3.0-7.0 mm; the microstructure is polygonal ferrite, acicular ferrite and bainite, wherein the area proportion of bainite is ≤15%; the yield strength R p0.2 : 660~760MPa, tensile strength R m : 760~880MPa, elongation at break A≥22%.

[0010] The carbon equivalent Ceq of the BJ770 steel strip is 0.41-0.45%, the welding crack sensitivity coefficient Pcm≤0.25%, and the alloy equivalent Aeq≥1.00%.

[0011] Ceq=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14;

[0012] Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B;

[0013] Aeq=0.15Si+0.35Mn+0.4Cr+0.6Mo+0.18Ni+B+2 (Nb+V+Ti) ≥1.00%.

[0014] The method for preparing a straight seam boom steel pipe using the above-mentioned BJ770 steel strip includes: after the BJ770 steel strip is formed and pipe-formed, the weld or the welded pipe is subjected to normalizing heat treatment as a whole, and then the welded pipe is subjected to tempering treatment as a whole; the normalizing temperature is 920-950°C, air cooling to ≤350°C, and then water cooling; the tempering temperature is 540-580°C, and the tempering time is 20-60 minutes.

[0015] The yield strength R of the straight seam boom steel pipe p0.2 : 770 ~ 900MPa, tensile strength: 820 ~ 950MPa, elongation after fracture A ≥ 14%, -20 ℃ Charpy V-notch impact energy of full-size longitudinal specimen of steel pipe ≥ 55J.

[0016] The design idea of the present invention:

[0017] (1) The addition of Cr, Mo, Nb and Ti alloy elements to the BJ770 steel strip of the present invention can reduce the critical cooling rate of bainite formation. C: 0.04% to 0.07%, which can form ferrite + bainite structure after normalizing the weld (or the whole) of the steel pipe; the total reduction rate of the hot-rolled steel strip is 35 to 50% when rolled below 950℃, and the final rolling temperature is 830 to 850℃, which is beneficial to the transformation of deformation-induced ferrite, increasing the ferrite phase ratio and refining the ferrite grain size; at the same time, the laminar cooling adopts the rear water cooling, and the cooling temperature is 640 to 67 0℃, which increases the ferrite transformation time of the hot-rolled steel strip after rolling, which is beneficial to increase the ferrite ratio and make the ferrite phase ratio ≥85%. At the same time, the rear water cooling avoids the nose temperature where the ferrite phase transformation is fastest, which is beneficial to improve the hot-rolled plate shape control; the post-rolling laminar cooling adopts rear water cooling, and the cooling temperature of 640~670℃ can avoid the pearlite transformation, and then to the composition of fine polygonal ferrite + acicular ferrite + a small amount of bainite. Such a structure has good forming properties, is not prone to micro cracks when shearing, and can effectively avoid forming cracking and weld cracking.

[0018] (2) The billet rolling heating temperature is 1260-1290℃, which is conducive to the solid solution of strong carbides of alloying elements such as Nb, Ti, and Mo. The coiling temperature is 510-570℃, which can inhibit the precipitation of secondary phase particles of strong carbides, so that the strength of the steel strip is controlled near the strength valley between the two peaks of phase transformation strengthening and precipitation strengthening, avoiding the steel strip from rebounding due to excessively high yield strength, affecting the quality of cold bending and increasing the energy consumption of the unit during the forming process.

[0019] The beneficial effects of adopting the above technical solution are:

[0020] 1. The carbon equivalent (Ceq) of the BJ770 hot-rolled steel strip of the present invention is 0.41% to 0.45%, and the weld crack sensitivity coefficient (Pcm) is ≤ 0.25%, which helps ensure welding quality. The steel strip is less susceptible to microcracks during shearing, has moderate strength control, low resistance to cold bending deformation, and minimal forming springback, resulting in excellent welding quality, improved weld pipe qualification rate, and reduced energy consumption of the welding unit.

[0021] 2. The yield strength of BJ770 straight seam welded boom steel pipe produced by the present invention is R p0.2 : 770 ~ 900MPa, tensile strength: 820 ~ 950MPa, elongation after fracture A ≥ 14%, -20 ℃ Charpy V-notch impact energy of the longitudinal full-size specimen of the steel pipe ≥ 55J, the product performance meets the standard requirements of BJ770 grade in GB "30584-2014 Seamless Steel Pipe for Crane Boom".

[0022] 3. The BJ770 ERW straight seam boom steel pipe produced by the present invention has a more uniform wall thickness than the seamless steel pipe of the same grade, a lower carbon equivalent, a lower crack sensitivity coefficient, and higher toughness and fatigue performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a microstructure diagram of the BJ770 steel strip of the present invention;

[0024] Figure 2 This is a microstructure diagram of the straight seam boom steel pipe matrix of the present invention;

[0025] Figure 3 This is a microstructure diagram of the straight seam boom steel pipe weld of the present invention. DETAILED DESCRIPTION

[0026] The production process of the BJ770 steel strip and straight seam boom steel pipe of the present invention is as follows: molten iron desulfurization → converter smelting → LF furnace refining → RH vacuum treatment → slab continuous casting → heating → hot rolling → cooling → coiling. The resulting BJ770 hot-rolled steel coil undergoes longitudinal slitting → cold bending → ERW welding → internal and external burr removal → weld (or overall) normalizing heat treatment → air cooling → water cooling → precision shaping → cutting to length → tempering → straightening → flaw detection inspection → packaging, and the finished straight seam boom steel pipe is obtained. The specific process parameters are controlled as follows:

[0027] Heating process: heating in a walking beam furnace at a temperature of 1260-1290°C;

[0028] Hot rolling process: the total reduction rate of rolling below 950℃ is 35-50%, and the final rolling temperature is 830-850℃;

[0029] Cooling and coiling process: Laminar cooling adopts rear water cooling, the cooling temperature is 640~670℃, and the coiling temperature is 510~570℃.

[0030] After forming and pipe making, the obtained BJ770 steel coil is subjected to normalizing heat treatment of the weld or the entire welded pipe, and then the entire welded pipe is tempered; the normalizing temperature is 920-950°C, air cooling to ≤350°C, and then water cooling; the tempering temperature is 540-580°C, and the tempering time is 20-60 minutes.

[0031] For Examples 1-6, the chemical (melting analysis) composition, production process parameters, mechanical properties, and microstructure of the BJ770 steel strip are shown in Tables 1-3, and the heat treatment process parameters and mechanical properties of the straight seam boom steel pipe are shown in Tables 4 and 5.

[0032] Table 1. Chemical composition and content (wt%) of BJ770 steel strips in various examples

[0033]

[0034] Note: V is a residual element.

[0035] Table 2. Production process parameters of BJ770 steel strip in various examples

[0036]

[0037] Table 3. Mechanical properties and microstructure of BJ770 steel strips in various examples

[0038]

[0039] Table 4. Heat treatment process parameters of straight seam boom steel pipes in various embodiments

[0040]

[0041] Table 5. Mechanical properties of straight seam boom steel pipes in various examples

[0042]

[0043] Depend on Figure 1 Analysis of the microstructure of the steel strip shows that the structure is composed of fine polygonal ferrite + acicular ferrite + a small amount of bainite, the banded structure is level 1, and the structure is uniform.

[0044] Depend on Figure 2 Analysis of the matrix structure of the welded pipe after normalizing shows that the structure is composed of fine polygonal ferrite + pearlite, the banded structure is level 0.5, and the structure is uniform.

[0045] Depend on Figure 3 Analysis of the weld structure of the welded pipe after normalizing shows that the structure is composed of fine polygonal ferrite + pearlite, with a banded structure of level 0.5. The difference between the weld and the matrix structure is small, and the structure is uniform.

Claims

1. A method for producing BJ770 steel strip, characterized in that: Including smelting, continuous casting, heating, hot rolling, cooling and coiling processes; In the hot rolling process, the total reduction rate of rolling is 35-50% at a temperature below 950°C, and the final rolling temperature is 830-845°C; In the cooling and coiling process, laminar cooling adopts rear water cooling, the cooling temperature is 640-670°C, and the coiling temperature is 510-570°C; The chemical composition and mass percentage of BJ770 steel strip are: C: 0.04-0.06%, Mn: 1.20-1.55%, S≤0.006%, P≤0.015%, Si: 0.20-0.30%, Als: 0.015-0.050%, Nb: 0.020-0.035%, Ti: 0.08-0.10%, Cr: 0.10-0.60%, Mo: 0.15-0.30%, N≤0.0050%, the balance is iron and unavoidable impurities; The thickness of the BJ770 steel strip is 3.0-7.0 mm; the microstructure is polygonal ferrite, acicular ferrite and bainite, wherein the area proportion of bainite is ≤15%; the yield strength R p0.2 : 660~760MPa, tensile strength R m : 760~880MPa, elongation after fracture A≥22%; carbon equivalent Ceq: 0.41~0.45%, welding crack sensitivity coefficient Pcm≤0.25%, alloy equivalent Aeq≥1.00%.

2. The method for producing the BJ770 steel strip according to claim 1, characterized in that: The heating process is carried out by heating in a walking beam heating furnace at a heating temperature of 1260-1290°C.

3. A method for preparing a straight seam boom steel pipe using the BJ770 steel strip according to claim 1 or 2, characterized in that: After BJ770 steel strip is formed and made into pipe, the weld or the whole welded pipe is normalized and then the whole welded pipe is tempered; The normalizing temperature is 920-950° C., followed by air cooling; the tempering temperature is 540-580° C., and the tempering time is 20-60 minutes.

4. The method for preparing a straight seam boom steel pipe according to claim 3, characterized in that: The air cooling is performed to ≤350°C, and then water cooling is performed.

5. The method for preparing a straight seam boom steel pipe according to claim 3 or 4, characterized in that: The yield strength R of the straight seam boom steel pipe p0.2 : 770 ~ 900MPa, tensile strength: 820 ~ 950MPa, elongation after fracture A ≥ 14%, -20 ℃ Charpy V-notch impact energy of full-size longitudinal specimen of steel pipe ≥ 55J.

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