Method for controlling dendritic structure of high-strength and high-toughness steel forge piece

Abstract

The invention relates to a dendritic structure control method for a high-strength and toughness steel forging, which comprises the following steps of: performing electrode blank rare earth treatment on the forging produced by 4 percent Ni-Cr-Mo-V high-strength and toughness steel; the method comprises the following steps of: mixing a molten steel ingot with an electroslag remelting slag system, melting speed and cooling into a whole, refining a uniform as-cast structure, reducing the secondary dendritic crystal spacing of the as-cast structure, reducing dendritic crystal segregation of elements such as Cr, Mo, V, Mn, Si and Ni, performing closed extrusion on the high-temperature homogenized steel ingot, increasing the transverse and longitudinal deformation degree, breaking transverse and longitudinal columnar crystals, changing the direction of the columnar crystals, refining the uniform structure, and obtaining the high-temperature homogenized steel ingot. The intermediate refined annealing blank is uniformly forged by a precision forging machine to further smash secondary dendritic crystals and refine the structure, in application detection, the high-strength and high-toughness forging manufactured by the method has macroscopic dendritic crystal residue of less than 5%, the actual grain size of 8.5 grade, the normal-temperature yield strength of 1375Mpa-1401Mpa, the low-temperature impact of-40 DEG C of 29.3 J-33.6 J, and the impact difference value of the same test piece is below 6J.

Description

Technical Field

[0001] The invention belongs to the technical field of material forming, and specifically relates to a method for controlling the dendrite structure of a high-strength and tough steel forging, in particular to a method for controlling the dendrite structure of a 4% Ni-Cr-Mo-V high-strength and tough steel forging. Background Art

[0002] 4% Ni-Cr-Mo-V high-strength and tough steel has ultra-high room temperature strength and high low-temperature impact toughness. It is used to manufacture heavy equipment parts. Its products need to be formed by hot working of electroslag remelted steel ingots. Because the alloy content reaches more than 7%, the dendritic structure and component segregation of the electroslag remelted steel ingots are aggravated. The traditional free forging deformation makes the distribution of deformed dendrites remaining in the forgings uneven, and there is a high proportion of residual cast dendritic structure. Due to the strong inheritance of the steel structure, the inherited residual dendrites and uneven distribution on the forgings are difficult to solve by heat treatment methods, resulting in uneven product structure and unstable product performance, especially affecting the low-temperature impact toughness. The dendritic structure distribution of products manufactured by traditional methods is uneven in cross-section, as shown in the attached figure. Figure 1 . Summary of the invention

[0003] The invention provides a method for controlling dendrite structure of high-strength and toughness steel forgings, which solves the problems of high residual ratio of as-cast structure, uneven structure and unstable performance in the traditional method of manufacturing 4% Ni-Cr-Mo-V high-strength and toughness steel forgings.

[0004] In order to solve the above technical problems, the present invention provides a method for controlling dendrite structure of high-strength and toughness steel forgings, which is characterized by comprising the following steps:

[0005] S1. Treat electrode blanks with rare earth for forgings produced from high-strength and tough steel;

[0006] S2, electroslag remelting

[0007] S3, electroslag remelting steel ingot is deformed by upsetting and extrusion

[0008] S4. Intermediate fine grain annealing of deformed billet

[0009] S5. The annealed bar blank is forged evenly through a precision forging machine to form a forging.

[0010] Furthermore, after quenching and tempering, the grain of the cross-section test piece is ≥7.0 level, and the residual low-magnification dendrite structure is ≤10%.

[0011] Furthermore, S1 specifically comprises: after the high-strength and toughness steel is vacuum treated, a ferrocerium rare earth alloy wire wrapped in a steel jacket is fed into the ladle, and the total amount of rare earth added is 0.15kg / t to 0.20kg / t of molten steel.

[0012] Furthermore, the electroslag remelting uses electroslag remelting slag of 99% by weight of hexane slag system + 1% SiO2, and the electroslag remelting enters the remelting steady-state period: the temperature difference between the inlet and outlet water of the crystallizer is ≤8°C, the temperature difference between the inlet and outlet water of the low water tank is ≤6°C, and the melting rate is ≤ the average diameter of the crystallizer in mm × 0.7, unit: kg / h.

[0013] Furthermore, in S3, the electroslag remelted steel ingot is subjected to upsetting and extrusion composite deformation to form a solid bar blank, specifically: the electroslag remelted steel ingot is heated to a temperature of 1260±10°C, kept warm for ≥4.5h / 100mm, and then formed into a solid billet on a billet making press with an upsetting ratio of ≥1.8; on the extruder, the upset solid billet is extruded and modified into an extruded rod blank in an extrusion barrel according to the parameters of a primary deformation degree of ≥55% and an extrusion speed of ≤30mm / s, and the proportion of low-magnification dendrites in the cross-section test piece of the rod blank is ≤50%.

[0014] Furthermore, the upsetting blank is returned to the furnace for heating at a temperature of 1210° C. to 1230° C. for a holding time of 6 hours, and is extruded in an extrusion barrel.

[0015] Further, in S4, the extruded bar blank is air-cooled to a surface temperature of 400°C-450°C, and is loaded into a furnace with a temperature of 600°C-650°C, and subsequently subjected to high temperature annealing.

[0016] Further, in S5, the annealed bar blank is heated to 1050±10°C and kept warm for ≥1h / 100mm. After the holding period is completed, the bar blank is deformed and stretched in two or more passes on a finishing forging machine, the total forging ratio is ≥2.5, and the blank feed speed is 3.5-4.0m / min.

[0017] Furthermore, in S5, the reduction amount of each pass is ≥60 mm, and the surface temperature after forging is ≥750°C-910°C.

[0018] Beneficial effects: The present invention realizes the homogenization control of forging structure, effectively solves the shortcomings of the traditional process of high-strength and tough steel forging, and realizes a major innovation in the preparation technology of high-strength and tough steel forging blanks. The invention greatly improves the uniformity of forging structure and creates organizational conditions for improving the mechanical properties of high-strength and tough steel forgings.

[0019] The present invention achieves a significant improvement in the uniformity of the structure of 4% Ni-Cr-Mo-V high-strength and tough steel forgings through the forging low-magnification dendrite structure control technology. The low-magnification dendrite residue is less than 5%, the actual grain size is 8.5, the yield strength at room temperature is between 1375Mpa and 1401Mpa, the low-temperature impact at -40°C is between 29.3J and 33.6J, and the impact difference of the same test piece is less than 6J. No abnormality was found during the trial production process. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1: Dendrite structure of traditional forgings;

[0021] Figure 2 : Quenched and tempered structure of extrusion forgings;

[0022] Figure 3 : Actual grain size of extrusion forgings after quenching and tempering;

[0023] Figure 4 : Dendrite structure control method 1# forging low-magnification dendrite;

[0024] Figure 5 : Dendrite structure control method 2# forging low-magnification dendrite.

[0025] Figure 1 Among them, the dendrite structures of 1#-5# are: no obvious solidification structure, residual cross dendrites, coarse cross dendrites, columnar crystals, and deformed columnar crystals. DETAILED DESCRIPTION

[0026] In order to make the purpose, content and advantages of the present invention more clear, the specific implementation methods of the present invention are further described in detail below.

[0027] The present invention proposes a method for controlling the dendrite structure of a high-strength and tough steel forging, comprising the following steps:

[0028] S1. Performing rare earth treatment on electrode blank of 4% Ni-Cr-Mo-V high-strength and tough steel after vacuum treatment;

[0029] After 4% Ni-Cr-Mo-V high-strength and tough steel is smelted, refined and vacuum treated in an electric furnace, ferrocerium rare earth alloy wire wrapped in a steel sleeve is fed into the ladle. The total amount of rare earth added is 0.15kg / t~0.20kg / t of molten steel. The wire is fed evenly in two times with an interval of 1min.

[0030] S2, electroslag remelting

[0031] Using electroslag remelting slag of 99% by weight of hexane slag system + 1% SiO2, the temperature difference of the inlet and outlet water of the crystallizer during the remelting steady state period is ≤8°C, the temperature difference of the inlet and outlet water of the low water tank is ≤6°C, and the melting rate of the electroslag ingot is ≤ the average diameter of the crystallizer in mm × 0.75 (unit: kg / h);

[0032] S3, electroslag remelting steel ingot is deformed by upsetting and extrusion

[0033] The electroslag remelting steel ingot is heated to 1260±10℃, and kept warm for ≥4.5h / 100mm. Then, the steel ingot is upset into a solid billet using a closed upsetting cylinder on a billet press, with an upsetting ratio of ≥1.8. The upset billet is returned to the furnace for heating to 1210℃~1230℃, and kept warm for 6h. It is then extruded in a 1100 series extrusion cylinder, with the big end of the ingot facing downward, and a Q235 material Φ1130±5×Φ380±5×110±5mm pad is used as the pressure balance. The diameter of the formed bar is φ520mm, the length is about 2570mm, and the extrusion ratio is 4.63.

[0034] On the extruder, the upset solid billet is extruded and modified into an extruded rod billet in the extrusion barrel according to the parameters of primary deformation degree ≥55% and extrusion speed ≤30mm / s, and the proportion of low-magnification dendrites in the cross-section test piece of the rod is ≤50%.

[0035] S4. Intermediate fine grain annealing of deformed billet

[0036] The extruded bar billet needs intermediate fine grain annealing. After extrusion, the billet is air-cooled to a surface temperature of 400℃-450℃, and the furnace temperature is 600℃-650℃. After entering the furnace, it is kept at 620±10℃ for 8h, and then high temperature annealing is carried out.

[0037] S5. The annealed bar blank is forged into a forging by rotating the fine forging machine to form a uniform structure.

[0038] The billet is charged to the furnace at a temperature of ≤550℃, kept at 500℃-550℃ for 3h, and then preheated to 650℃-850℃, and kept at this temperature for not less than 6h. Then the extruded bar billet is heated to 1050±10℃, and the holding time is ≥1h / 100mm. The deformation and elongation are carried out on the fine forging machine in ≥2 passes, and the total forging ratio is ≥2.5. The billet feeding speed is 3.5-4.0m / min, and the surface temperature after forging is in the range of 750℃-910℃.

[0039] The present invention integrates the "electrode billet rare earth treatment and electroslag remelting slag system, melting rate and cooling" for controlling the dendrite structure of electroslag remelting, and subsequently adopts the combination of "upsetting and extrusion composite large deformation" to break up the coarse cast dendrite structure and "precision forging machine rotary forging" to uniformly refine the dendrite structure, so as to achieve the control of low-magnification dendrite structure of large forgings of 4% Ni-Cr-Mo-V steel, greatly improve the uniformity of the structure, the grain of the cross-section test piece treated by quenching and tempering is ≥6.0 grade, the proportion of low-magnification dendrite structure of the forging is ≤10%, and the problems of low uniformity of the structure and great difficulty in improving the low-temperature impact strength are solved.

[0040] The invention can be used to produce large forgings of different specifications with an outer diameter of 200-370 mm and a length of ≤9000 mm, laying a foundation for improving the uniformity of material organization and performance.

[0041] Example:

[0042] Forging size: Φ320*1300mm+Φ290*2240mmΦ240*3300mm

[0043] 1) Raw material electrode blank

[0044] 4% Ni-Cr-Mo-V steel is smelted, refined and vacuum treated in an electric furnace. After vacuum treatment, ferrocerium rare earth wire is fed into the ladle. The wire is fed twice evenly with an interval of 1 minute and a total amount of 0.18kg / t of molten steel. After soft blowing time ≥20 minutes, it is cast into a 6.75t electrode blank.

[0045] 2) Electroslag remelting

[0046] Electroslag remelting is carried out in an atmosphere protection remelting furnace using an 8t (average diameter φ730) crystallizer.

[0047] 6.0t electroslag ingot, slag is 622 pre-melted slag + 1% SiO2, slag quantity is 240kg, melting rate in steady state stage is controlled at 520kg / h±8%, in remelting steady state period, the temperature difference of inlet and outlet water of crystallizer is ≤8℃, the temperature difference of inlet and outlet water of low water tank is ≤6℃,

[0048] 3) Upsetting and extruding bar blank

[0049] The ingot is heated to 1260℃~1270℃ and kept at this temperature for 30h. After the ingot is taken out, the surface of the ingot is dephosphorized and lubricated with water. Then, it is upset in a φ1100mm upsetting cylinder on a 150MN upsetting billet making machine (with the big end of the ingot facing downwards), and the upsetting ratio is 2.6. The upsetting billet is returned to the furnace for heating at 1210℃~1230℃ and kept at this temperature for 6h. It is extruded in a 1100 series extrusion cylinder with the big end of the ingot facing downwards, and a Q235 material Φ1130±5×Φ380±5×110±5mm pad is used as the pressure surplus. The diameter of the formed bar is φ520mm, the length is about 2570mm, and the extrusion ratio is 4.63.

[0050] 4) After extrusion, the billet is air-cooled to a surface temperature of 400°C-450°C, and the furnace temperature is 600°C-650°C. After entering the furnace, it is kept at 620±10°C for 8 hours, and then high-temperature annealing is performed.

[0051] 5) Forming forging with precision forging machine

[0052] The billet is charged to the furnace at a temperature of ≤550℃, kept at 500℃-550℃ for 3h, and then heated to 650℃-850℃ for preheating, and the retention time at this temperature is not less than 6h. Then it is heated to 1040℃~1060℃, kept for 5h, and then forged. The final forging temperature is controlled at 750-910℃℃. R180 hammer is used, and the pressing from Φ520mm to Φ315mm is divided into three passes, and the pressing amount of each pass is not less than 70mm. The rest are forged one by one according to the step size of Φ290mm and Φ240mm, and the forging ratio is 2.72~5.58. After forging, the record is printed and loaded into the furnace, and annealed according to the specified annealing curve.

[0053] 6) Detection:

[0054] (1) High-magnification tissue analysis

[0055] After rough machining, the forgings were subjected to quenching and tempering heat treatment. A total of 8 forging blanks were tested. The grain size and structure analysis was performed on both ends. The structure was tempered troostite level 2, and the actual grain size at the 1 / 2 radius of both ends was 8.5. The structure is shown in the attached Figure 2 、Grain size see attached Figure 3 .

[0056] (2) Low-magnification dendrite detection: Two transverse test pieces were selected for low-magnification dendrite corrosion detection. The residual solidification structure accounted for 4.39% and 0% respectively. The distribution is shown in the attached Figure 4 , Attachment Figure 5 .

[0057] (3) Performance testing and analysis

[0058] The invention treats the forgings produced by 4% Ni-Cr-Mo-V high-strength and tough steel with rare earth in electrode blanks, integrates the electroslag remelting slag system, melting speed and cooling, refines the uniform cast structure, reduces the secondary dendrite spacing of the cast structure, and reduces the dendrite segregation of elements such as Cr, Mo, V, Mn, Si and Ni. The steel ingot is homogenized at high temperature and is extruded through closed extrusion to increase the transverse and longitudinal deformation degree, break the transverse and longitudinal columnar crystals, change the direction of the columnar crystals, and refine the uniform structure. The intermediate refined annealed blank is uniformly forged by a fine forging machine to further break the secondary dendrites and refine the structure.

[0059] After rough machining and tempering treatment, the forgings were tested for performance at both ends. The residual low-magnification dendrites were less than 5%, the actual grain size was 8.5, the yield strength at room temperature was between 1375Mpa and 1401Mpa, the low-temperature impact at -40℃ was between 29.3J and 33.6J, and the impact difference of the same test piece was less than 6J.

[0060] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for controlling dendrite structure of high-strength and tough steel forgings, characterized in that: The steps include: S1. Treat electrode blanks with rare earth for forgings produced from high-strength and tough steel; S2, electroslag remelting S3, electroslag remelting steel ingot is deformed by upsetting and extrusion S4. Intermediate fine grain annealing of deformed billet S5. The annealed bar blank is forged evenly through a precision forging machine to form a forging.

2. A method for controlling dendrite structure of high-strength and tough steel forgings according to claim 1, characterized in that: After quenching and tempering, the grain of the cross-section test piece is ≥7.0 level, and the residual low-magnification dendrite structure is ≤10%.

3. The method for controlling dendrite structure of high-strength and toughness steel forgings according to claim 1, characterized in that: S1 is specifically as follows: after high-strength and toughness steel is vacuum treated, a ferrocerium rare earth alloy wire wrapped in a steel jacket is fed into the ladle, and the total amount of rare earth added is 0.15kg / t to 0.20kg / t of molten steel.

4. A method for controlling dendrite structure of high-strength and toughness steel forgings according to claim 3, characterized in that: The feeding line is fed twice evenly, with an interval of 1min.

5. The method for controlling dendrite structure of high-strength and toughness steel forgings according to claim 3, characterized in that: The electroslag remelting uses electroslag remelting slag of 99% by weight of hexane slag system + 1% SiO2. The electroslag remelting enters the remelting steady-state period: the temperature difference between the inlet and outlet water of the crystallizer is ≤8°C, the temperature difference between the inlet and outlet water of the low water tank is ≤6°C, and the melting rate is ≤ the average diameter of the crystallizer in mm × 0.7, unit: kg / h.

6. A method for controlling dendrite structure of high-strength and toughness steel forgings according to claim 3, characterized in that: In S3, the electroslag remelted steel ingot is formed into a solid bar blank by upsetting and extrusion composite deformation, specifically: the electroslag remelted steel ingot is heated to a temperature of 1260±10℃, kept warm for ≥4.5h / 100mm, and then formed into a solid billet on a billet making press with an upsetting ratio of ≥1.8; on the extruder, the upset solid billet is extruded and modified into an extruded bar blank in an extrusion barrel according to the parameters of a primary deformation degree of ≥55% and an extrusion speed of ≤30mm / s, and the proportion of low-magnification dendrites in the cross-section test piece of the bar blank is ≤50%.

7. A method for controlling dendrite structure of high-strength and toughness steel forgings according to claim 6, characterized in that: The upsetting blank is returned to the furnace for heating at a temperature of 1210°C to 1230°C for 6 hours and is extruded in an extrusion barrel.

8. The method for controlling dendrite structure of high-strength and toughness steel forgings according to claim 3, characterized in that: In S4, the extruded bar blank is air-cooled to a surface temperature of 400°C-450°C, and is loaded into a furnace with a temperature of 600°C-650°C, and subsequently subjected to high temperature annealing.

9. A method for controlling dendrite structure of high-strength and toughness steel forgings according to claim 3, characterized in that: In S5, the annealed bar blank is heated to 1050±10°C and kept warm for ≥1h / 100mm. After the holding period, it is deformed and stretched in two or more passes on a finishing forging machine. The total forging ratio is ≥2.5 and the blank feed speed is 3.5-4.0m / min.

10. A method for controlling dendrite structure of high-strength and tough steel forgings according to claim 9, characterized in that: In S5, the reduction in each pass is ≥60mm, and the surface temperature after forging is ≥750℃-910℃.

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