Steel material for large breaker hammer piston shaft and manufacturing method thereof

Abstract

The invention relates to a steel material for a piston shaft of a large-scale breaker hammer and a manufacturing method thereof. The steel material is based on iron and contains the following chemical components by mass percentage: C: 0.85-1.10%, Si: 0.15-0.70%, Mn: 0.25-1.20%, Cr: 1.05-1.60%, S≤0.020%, P≤0.020%, Ni≤0.30%, Cu≤0.25%, Mo≤0.10%, Al: 0.015-0.050%, Ca≤0.0010%, Ti≤0.0030%, O≤0.0010%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%; the piston shaft meets the following requirements: O≤6ppm, Ti≤15ppm, and large-particle non-deformable inclusions are in accordance with ISO 4967-2013 rating ≤ 1.0, product surface hardness ≥ 61HRC, core hardness ≥ 40HRC, surface hardened layer thickness not less than 10mm, hardness within the surface hardened layer thickness not less than 58HRC. Piston shaft masterbatch manufacturing (continuous casting and rolling + structural pretreatment) - piston shaft processing - heat treatment (turning + quenching, tempering + grinding).

Description

Technical Field

[0001] The present invention belongs to the technical field of special steel materials, and in particular relates to a first type of high-hardness shaft steel material and a manufacturing method thereof. Background Art

[0002] The breaker hammer is a new type of high-efficiency impact vibration machine. It is a supporting working part of an excavator or loader. When working, it uses the high-pressure oil output by the working mother machine to realize the high-frequency reciprocating motion of the breaker piston in the cylinder, thereby striking the drill rod and performing external work. It is used in many fields such as mining, metallurgical industry, municipal engineering, construction, highways and railways. Among them, the piston is a key part of the breaker hammer. The breaker hammer piston has continuous high-speed relative reciprocating motion with the cylinder during work. There is severe friction between the reciprocating motions, and the lower end face frequently hits the drill rod and bears impact stress. Therefore, the quality of the design and manufacturing of the breaker hammer piston directly affects the reliability and life of the breaker hammer, and the reliability and life of the work are closely related to the manufacturing material of the piston. The selection of materials is a key issue in piston design. It determines the success or failure of piston design and the life of parts. At present, breaker pistons are mostly made of low-carbon alloy carburizing steel and medium-carbon alloy carburizing steel. They have the disadvantages of low tensile strength and low core strength after heat treatment. Under the strong impact of large and medium-sized hydraulic breakers, the piston is prone to end face depression and end upsetting.

[0003] Patent document with patent publication number CN 101574769 A discloses a surface treatment method for a piston shaft, in which the material is carburized to a depth of about 3 mm, and then the piston circumference and striking surface are removed by 1.5 mm and 2 mm respectively. This method has high heat treatment cost and long cycle. The hardness layer after surface processing is relatively shallow and the wear resistance is not strong, which affects the life of the piston shaft and is prone to strain or even cracking.

[0004] Patent document with patent publication number CN102796964A discloses a method of using 18Cr2Ni4WA as the raw material of the piston shaft, and adopting forging, normalizing and high temperature tempering, machining, carburizing, quenching, cryogenic treatment, low temperature tempering and finishing. The material contains a large amount of precious alloys such as Ni, Cr, and W, and the production cost is high. The processing and heat treatment process is complicated, and the actual core hardness of the product is only 30-40HRC. The disadvantage of low hardness of the core of the large piston shaft is not solved, and it is easy to cause end face depression and end upsetting. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a steel material for a large breaker hammer piston shaft and a manufacturing method thereof in response to the above-mentioned prior art. The steel material does not rely on the addition of precious alloys in terms of elemental composition, has economical raw material costs, simple heat treatment, and can meet the requirements of large breaker hammer piston friction resistance and good impact performance.

[0006] The technical solution adopted by the present invention to solve the above problems is: a steel material for a piston shaft of a large breaker hammer, which is based on iron and contains the following chemical components in percentage by weight: C: 0.85-1.10%, Si: 0.15-0.70%, Mn: 0.25-1.20%, Cr: 1.05-1.60%, S≤0.020%, P≤0.020%, Ni≤0.30%, Cu≤0.25%, Mo≤0.10%, Al: 0.015-0.050%, Ca≤0.0010%, Ti≤0.0030%, O≤0.0010%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, and Pb≤0.002%.

[0007] The piston shaft manufactured from the steel described in this application meets the following requirements: O≤6ppm, Ti≤15ppm, large-particle non-deformable inclusions rated ≤1.0 according to ISO 4967-2013, product surface hardness ≥61HRC, core hardness ≥40HRC, surface hardened layer thickness not less than 10mm, and hardness within the surface hardened layer thickness not less than 58HRC.

[0008] Compared with the chemical composition of the steel disclosed in publication number CN102796964A, the present application does not need to add precious metals Ni and W, and adopts a high-carbon and high-manganese design.

[0009] The following is an explanation of the effects and dosage of the components in the present invention:

[0010] 1) Determination of C content: The piston of the breaker hammer has continuous high-speed relative reciprocating motion with the cylinder body during operation, and there is severe friction during the reciprocating motion. C is the most economical and basic strengthening element in steel, forming a solid solution structure and a carbide structure with the matrix to improve the strength and wear resistance of the material surface. The present invention controls its content to be 0.85-1.10%.

[0011] 2) Determination of Si content: Si improves strength, elastic limit and hardenability. It reduces the diffusion rate of C in ferrite, making it difficult for carbides precipitated during tempering to aggregate, increasing the tempering stability of the material, and reducing the internal stress in the material while maintaining high hardness when the steel is tempered at a lower temperature. In addition, Si can reduce the oxidation of the piston shaft during reciprocating friction and heat generation, increase the cold deformation hardening rate of the material, and thus improve the wear resistance of the material. The present invention controls the Si content to be 0.15-0.70%.

[0012] 3) Determination of Mn content: Mn can improve the hardenability of steel. The lower end surface of the breaker piston frequently hits the drill rod during operation and is subjected to large impact stress. To avoid end concavity and upsetting, the Mn content of the present invention is controlled at 0.25-1.20%.

[0013] 4) Determination of Cr content: Since the present invention increases the amount of C, Cr, as a carbide-forming element, can improve the hardenability, wear resistance and corrosion resistance of steel. Part of the Cr in this material replaces iron to form alloy cementite, which improves the tempering stability of the steel; part of it dissolves into ferrite to produce solid solution strengthening, which improves the strength and hardness of ferrite. In order to effectively ensure the surface hardness and heat treatment hardness layer depth of large-sized pistons, and avoid cracking during impact due to low surface hardness layer, the range of Cr content in the present invention is determined to be 1.05-1.60%.

[0014] 5) Determination of Al content: Al is added as a deoxidizing element in steel. In addition to reducing the dissolved oxygen in the molten steel, it also has the effect of refining the grains. Al and N form dispersed and fine aluminum nitride inclusions, which can refine the grains. This significantly improves the impact toughness of the steel and reduces the cold brittleness tendency and aging tendency.

[0015] The present invention also provides a manufacturing process for a breaker piston shaft, and the entire process steps include: piston shaft masterbatch manufacturing (continuous casting and rolling-tissue pretreatment)-piston shaft processing, heat treatment (turning+quenching, tempering+grinding).

[0016] Preparation method of piston shaft masterbatch: prepare smelting raw materials according to the above chemical composition, adopt molten iron pretreatment technology to strictly control the composition of crude molten iron, carry out low phosphation and low titanium of crude molten steel in converter (or electric furnace), improve material purity and ensure stable composition through ladle refining, implement full protection casting after vacuum degassing, and the billet is made into material by continuous rolling; the cross-sectional area of ​​the billet is not less than 150000mm 2 , high temperature heating temperature 1150-1300℃, high temperature heating time ≥5 hours, start rolling temperature ≥1100℃, final rolling temperature ≥800℃, rolling reduction ratio greater than 6S, rolling specification ≥φ155mm. Intermediate billet pit cooling, pit temperature ≥550℃, slow cooling time not less than 48 hours. The rolled product structure is lamellar pearlite, O≤6ppm, Ti≤15ppm, large particle non-deformable inclusions are rated ≤1.0 according to ISO 4967-2013.

[0017] In order to reduce the quenching stress of large piston shafts during heat treatment, it is necessary to pre-treat the above-mentioned steel materials for homogenization in advance. At the same time, in order to improve the process efficiency, the piston shaft masterbatch innovatively adopts a two-stage isothermal spheroidizing annealing process. First, the temperature is raised to Ac1-Accm with the furnace, preferably 810℃-825℃, and kept warm for 5-6 hours. At this temperature range, the cementite in the lamellar pearlite begins to dissolve locally, so that a piece of cementite is broken into several fine point-like cementite, which is dispersed on the austenite matrix; the isothermal temperature in the second stage should be 20℃ lower than the Ar1 line and kept warm at 710℃-730℃ for 9-11 hours, and then cooled to below 450℃ with the furnace and air-cooled. Due to the low heating temperature and incomplete dissolution of cementite, the austenite composition is uneven, and the organization is centered on the original fine carbide points, or new carbide cores are generated from the carbon-rich areas of the austenite, forming uniform and fine granular carbides. The spheroidized structure finally meets the GB / T 18254 rating 2-4, with a hardness of 179-207HBW. Compared with ordinary spheroidizing annealing, this process can break through the limit size of the conventional spheroidizing annealing process, make the spheroidized structure of the large-size piston shaft section ≥φ155mm uniform, and strictly control the hardness after annealing.

[0018] The above base material is further processed into the piston shaft through the following steps:

[0019] Step 1: Turning: Use room temperature machining to process the piston shaft masterbatch to the rough size of the breaker, and the minimum machining is expected to be no less than 0.5mm.

[0020] Step 2: Quenching treatment: Place the piston shaft rough parts in a heating furnace with a protective atmosphere and heat them at a temperature of 840℃-860℃ for 4-6 hours. Oil quench them after they are taken out of the furnace. The surface hardness is ≥61HRC, the core hardness is ≥40HRC, and the thickness of the surface hardened layer is not less than 10mm (the hardness of the hardened layer structure is not less than 58HRC).

[0021] Step 3: Tempering treatment: The tempering temperature is 200℃-260℃, and the tempering time is not less than 3 hours to fully eliminate the stress of the surface hardened layer caused by quenching.

[0022] Step 4: Grinding: Use ultra-precision grinding to adjust the dimensional accuracy of the piston shaft.

[0023] Compared with the prior art, the advantages of the present invention are:

[0024] (1) According to the composition design adopted in the present invention, the cost of raw material alloy is reduced. According to the composition characteristics, the corresponding piston shaft heat treatment omits the process of high-temperature carburizing and deep cryogenic treatment, making the manufacturing process simpler.

[0025] (2) The present invention adopts continuous casting and continuous rolling to replace the die casting and forging process, which significantly improves the yield rate, improves the dimensional accuracy of the piston shaft masterbatch, reduces the processing allowance, and greatly shortens the production cycle and improves production efficiency.

[0026] (3) The present invention increases the depth of the hardened layer on the surface of the piston shaft of a large-sized breaker hammer and increases the hardness of the core of the material by adding Mn and Cr elements without adding alloy elements, especially alloy elements such as Mo that are beneficial to hardenability, which can effectively prevent the phenomenon of the end face being dented and the surface being easily scratched when the material is used. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a low-magnification organization diagram of the cross section of the piston shaft of the present invention;

[0028] Figure 2 This is the cross-sectional hardness curve of the φ213 piston shaft after heat treatment of the present invention; DETAILED DESCRIPTION

[0029] The present invention is further described in detail below in conjunction with the accompanying drawings. The embodiments are exemplary and are intended to be used to explain the present invention, but should not be understood as limiting the present invention. The text description in this embodiment corresponds to the accompanying drawings, and the description of the orientation is also based on the description of the accompanying drawings, which should not be understood as limiting the scope of protection of the present invention.

[0030] Examples 1-3 respectively exemplify the chemical composition and manufacturing method of the large piston shaft breaker of the present invention.

[0031] The chemical composition (wt%) of each example is shown in Table 1 and Table 2.

[0032] Table 1

[0033] Example C Si Mn S P Cr Ni Cu A1 Mo The present invention 1 0.95 0.28 0.57 0.002 0.012 1.52 0.02 0.03 0.023 0.02 The present invention 2 0.97 0.25 0.57 0.003 0.015 1.48 0.02 0.03 0.020 0.02 The present invention 3 0.97 0.25 0.55 0.001 0.009 1.51 0.02 0.03 0.025 0.02 Alternatives 4 0.43 0.25 0.80 0.05 0.017 0.83 1.68 0.05 0.025 0.23

[0034] Table 2

[0035]

[0036]

[0037] According to the chemical composition of the present invention, the smelting raw materials are prepared, and the crude molten iron composition is strictly controlled. The crude molten steel is low-phosphorized and low-titanized in the converter (or electric furnace). The tapping end point C of the three embodiments is controlled at 0.17-0.25%, and the end point P is ≤ 0.015%. The purity of the material is improved and the composition is stable through ladle refining. After vacuum degassing, the whole process protection casting is implemented, and the continuous casting superheat is controlled within 15-25°C. The actual mass O ≤ 4.5ppm, Ti ≤ 19ppm, and the application requirements of high-end large piston shafts are met.

[0038] The rolling and slow cooling processes of each embodiment are shown in Table 3.

[0039] Table 3

[0040] Example Rolling specifications Heating temperature Heating time Rolling temperature Finishing temperature Lower pit temperature Slow cooling time 1 φ220mm 1250℃ 5.5h 1150℃ 840℃ 570℃ 50h 2 φ215mm 1245℃ 5.7h 1140℃ 830℃ 560℃ 50h 3 φ200mm 1252℃ 5.7h 1145℃ 810℃ 571℃ 50h

[0041] The ingot is sent to the heating furnace for high-temperature diffusion and then rolled into the target steel. The specific rolling process is: set the preheating section temperature to 650-900℃, the heating section temperature to 1150-1300℃, and control according to the upper limit temperature. The high-temperature heating time cannot be less than 5 hours to improve the effect of diffusion annealing, so that carbon atoms can fully diffuse and migrate, thereby providing good basic conditions for subsequent organizational pretreatment. The rolling start temperature is controlled at 1000℃-1150℃, and the final rolling temperature is controlled at above 800℃. After rolling, the steel should be placed in a slow cooling pit for slow cooling, so that the rolling and cooling stresses are fully released to avoid stress fractures.

[0042] The properties of the steel materials after heat treatment in each embodiment are shown in Table 4

[0043] Table 4

[0044]

[0045] The steel structure pretreatment adopts a two-stage isothermal spheroidizing annealing process. First, the temperature is raised to 810℃-825℃ with the furnace, and then it is kept for 5-6 hours. The second stage is kept at 710℃-730℃ for 9-11 hours, and then it is cooled to below 450℃ with the furnace and air-cooled. At this time, the steel has a spheroidized structure and a hardness of 179-207HBW. The steel is turned to remove the surface iron oxide scale and decarburized layer by normal temperature mechanical turning. Then the steel rough parts are placed in a heating furnace with a protective atmosphere for heating. The heating temperature is 840℃-860℃, and the temperature is kept for 4-6 hours. After being taken out of the furnace, it is oil quenched; after quenching, it is treated at a tempering temperature of 240℃ for 210min. The surface hardness is ≥61HRC, the core hardness is ≥40HRC, the surface hardened layer thickness is not less than 10mm (the hardness of the hardened layer is not less than 58HRC), and the performance of the steel section can be seen in the low-power organization diagram of the piston shaft cross section and the hardness curve of the cross section after heat treatment. Ultra-precision grinding is used to adjust the dimensional accuracy of the piston shaft. The dimensional accuracy reaches ±0.18mm, and the piston shaft can be manufactured.

[0046] As can be seen from Tables 1, 2, and 4, the large-scale breaker in the above embodiments of the present invention has significantly better control levels of harmful elements such as oxygen, titanium, and non-metallic inclusions than the piston shaft made of traditional medium-carbon alloy steel. In particular, in terms of heat treatment performance, the surface hardness of the material of the present invention is increased by 8-9HRC, which significantly improves the wear resistance of the piston shaft running in the piston sleeve. In addition, due to the increase in hardenability, it not only solves the problem of easy flesh loss on the surface of the original piston shaft, but also increases the hardness of the core, making it easy for the piston to have a concave end face and an end upset phenomenon under a strong impact on large and medium-sized hydraulic breakers. Traditional large-scale piston shafts are added with precious alloys such as Ni and Mo, and the cost is also significantly lower than that of the present invention.

[0047] In addition to the above embodiments, the present invention also includes other implementation modes. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the protection scope of the claims of the present invention.

Claims

1. A steel material for a large breaker hammer piston shaft, characterized by: The mass percentage of other chemical components contained in iron is as follows: C: 0.85-1.10%, Si: 0.15-0.70%, Mn: 0.25-1.20%, Cr: 1.05-1.60%, S≤0.020%, P≤0.020%, Ni≤0.30%, Cu≤0.25%, Mo≤0.10%, Al: 0.015-0.050%, Ca≤0.0010%, Ti≤0.0030%, O≤0.0010%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%; the piston shaft manufactured by the steel material meets the following requirements: O≤6ppm, Ti≤15ppm, and large-particle non-deformable inclusions are in accordance with ISO 4967-2013 rating ≤ 1.0; the surface hardness of the steel product is ≥ 61HRC, the core hardness is ≥ 40HRC, the thickness of the surface hardened layer is not less than 10mm, and the hardness within the thickness of the surface hardened layer is not less than 58HRC; The manufacturing method of the steel material comprises the following steps: preparing smelting raw materials according to chemical composition, adopting molten iron pretreatment technology, carrying out low phosphation and low titanium treatment of crude molten steel in a converter or electric furnace, improving material purity and ensuring component stability through ladle refining, implementing full-process protective pouring after vacuum degassing of molten steel, and continuously rolling the billet cast by molten steel into a product; the cross-sectional area of ​​the billet is not less than 150,000 mm 2 , high temperature heating temperature 1150-1300℃, high temperature heating time ≥5 hours, start rolling temperature ≥1100℃, final rolling temperature ≥800℃, rolling reduction ratio greater than 6, rolling specification ≥φ155mm; the obtained rolled product is slowly cooled in the pit, the pit temperature ≥550℃, and the slow cooling time is not less than 48 hours; the rolled product structure is lamellar pearlite, O≤6ppm, Ti≤15ppm, and large-grained non-deformable inclusions are rated ≤1.0 according to ISO 4967-2013; The rolled material is subjected to a pretreatment for homogenization of the structure, and a two-stage isothermal spheroidizing annealing process is adopted. First, the temperature is raised to Ac1-Accm with the furnace, and the temperature is kept for 5-6 hours. At this temperature range, the cementite in the lamellar pearlite begins to partially dissolve, so that each piece of cementite is broken into a number of fine point-shaped cementite, which is dispersed on the austenite matrix. The second stage isothermal temperature is 20°C lower than the Ar1 line, and the temperature is kept for 9-11 hours. Then, the material is cooled to below 450°C with the furnace and air-cooled. Due to the low heating temperature and incomplete dissolution of cementite, the austenite composition is uneven, and the structure is centered on the original fine carbide points, or a new carbide core is generated from the carbon-rich area of ​​the austenite, forming a uniform and fine granular carbide. The spheroidized structure finally meets the GB / T 18254 rating 2-4 level and the hardness is 179-207HBW. The piston shaft base material obtained by the method is processed into the piston shaft through the following steps:

1. Turning: Process the piston shaft masterbatch to the rough size of the breaker by machining at room temperature, and the minimum machining excess shall not be less than 0.5mm; 2. Quenching treatment: Place the piston shaft rough parts in a heating furnace with a protective atmosphere for heating at a temperature of 840℃-860℃ for 4-6 hours. After being taken out of the furnace, oil quench them to achieve the target hardness requirements.

3. Tempering treatment: eliminate the stress of the surface hardened layer caused by quenching; 4. Grinding: Use ultra-precision grinding to adjust the piston shaft size accuracy.

2. The steel material for a large breaker hammer piston shaft according to claim 1, characterized in that: In the two-stage isothermal spheroidizing annealing process, the rolled material is heated to 810°C-825°C along with the furnace; the isothermal temperature of the second stage is 710°C-730°C.

3. The steel material for a large-scale breaker hammer piston shaft according to claim 1, characterized in that: In step 3, the tempering treatment is performed at a temperature of 200°C to 260°C and a tempering time of not less than 3 hours.

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