Steel for working cylinder of automobile shock absorber and short-process production method of steel

Abstract

The invention discloses steel for a working cylinder of an automobile shock absorber and a short-process production method of the steel, and belongs to the technical field of manufacturing of automobile parts. The steel comprises the following components in percentage by mass: 0.13-0.16% of C, 0.05-0.15% of Si, 0.50-0.60% of Mn, less than or equal to 0.015% of P, less than or equal to 0.005% of S, more than or equal to 0.010% and less than or equal to 0.050% of Als, 0.10-0.20% of Cr, more than or equal to 0.015% and less than or equal to 0.025% of Ti, less than or equal to 0.0020% of O, less than or equal to 0.0040% of N and the balance of Fe and impurities. During production, the working cylinder of the shock absorber is manufactured by adopting the processes of casting blank heating, hot continuous rolling, cooling, acid pickling, single-stand rolling soft reduction finishing treatment, steel plate splitting, straight seam high-frequency welding, heat treatment, cold drawing, pipe cutting and the like. The steel for the shock absorber working cylinder with the strength meeting the requirement can be obtained by accurately controlling steel components, adopting conventional raw materials and a conventional process without a complex surface treatment process and adding high-cost alloy, the cost is saved, and when the automobile shock absorber working cylinder is manufactured, the production efficiency is high, and the production cost is low. And the production cost of the whole industrial chain is further reduced.

Description

Technical Field

[0001] The invention relates to the technical field of automobile parts manufacturing, and in particular to steel for automobile shock absorber working cylinder and a short-process production method thereof. Background Art

[0002] Shock absorbers are vulnerable parts during the use of automobiles. The performance of shock absorbers will affect the stability and safety of the car's driving. Shock absorber steel pipes (including working cylinders and oil storage cylinders) are important components of shock absorbers, and the quality of the steel pipes directly affects the use of shock absorbers.

[0003] Due to the particularity of the shock absorber movement, there are also special requirements for the working cylinder that carries it. The main factors affecting the shock absorber working cylinder are the composition of the working cylinder material and the processing method. At present, the materials used for shock absorber working cylinders are mostly cast iron and seamless steel, and the surface quality (especially surface roughness) and mechanical properties of the finished products are insufficient. In order to improve the material performance, it is often necessary to increase the alloy cost of the material, resulting in increased production costs and waste of resources. Therefore, choosing the right chemical composition and processing method is crucial for the preparation of high-performance shock absorber working cylinders.

[0004] Normally, the processing methods are direct smelting followed by die casting or seamless pipe production technology. However, with the market demand, the production cost control technology requirements for the steel pipes used in the shock absorber working cylinder are becoming higher and higher. The performance of ordinary steel pipes can no longer meet the market demand. The performance here mainly refers to the surface hardness, surface roughness, mechanical properties and other indicators of the steel pipe. If the technical requirements are to be met, the production cost will increase significantly.

[0005] The invention patent "A high-performance shock absorber steel pipe and its processing method (CN 202010477377.1)" adopts "a diffusion layer is infiltrated into the surface of the low-carbon steel pipe, a white bright layer is infiltrated into the diffusion layer, and an oxide film is infiltrated into the white bright layer to form a nitrogen-carbon-oxygen composite infiltration layer structure", which solves the problems of wear resistance, corrosion resistance and fatigue resistance of existing shock absorber steel pipes, but it has the problems of low production efficiency and high unit cost.

[0006] The invention patent "A shock absorber working cylinder and its processing method (CN 201711437052.1)" designs an alloy steel with a mass percentage composition of "C: 0.02-0.2%, Si: 0.5-0.8%, Cr: 2.5-3.5%, Mn: 0.12-0.16%, N: 0.25-0.30%, nano WC: 0.08-0.25%, nano CBN: 0.3-0.5%, potassium tetratitanate: 0.08-0.25%, the remainder is Fe and inevitable impurities". It has good wear resistance, high temperature resistance, corrosion resistance and other properties, but the production efficiency of the die-casting molding process is low, resulting in high alloy cost and production cost, and it has no promotion value. Summary of the invention

[0007] In order to overcome the above-mentioned deficiencies of existing shock absorber steels, the technical problem to be solved by the present invention is to provide an intensive and low-cost production of automobile shock absorber working cylinder steel and a short-process production method thereof.

[0008] The technical solution adopted by the present invention to solve its technical problem is:

[0009] A steel for a working cylinder of an automobile shock absorber comprises the following components in percentage by mass: C: 0.13% to 0.16%, Si: 0.05% to 0.15%, Mn: 0.50% to 0.60%, P≤0.015%, S≤0.005%, 0.010%≤Als≤0.050%, Cr: 0.10% to 0.20%, 0.010%≤Ti≤0.025%, O≤0.0020%, N≤0.0040%, and the balance is Fe and unavoidable impurities.

[0010] A short-process production method for a working cylinder of an automobile shock absorber, using the above-mentioned steel for the working cylinder of the automobile shock absorber, further comprising the following steps:

[0011] Step 1: sequentially carry out hot metal desulfurization, converter, and LF refining to obtain molten steel that meets the chemical composition design requirements;

[0012] Step 2: Send the molten steel obtained in step 1 to a continuous casting machine for continuous casting. The requirements for the roll gap of the two streams of the continuous casting machine are that the roll gap of the straightening section does not exceed ±1mm compared with the standard value, and the roll gap of other sections does not exceed ±2mm compared with the standard value, and the difference between the maximum and minimum roll gaps of two adjacent rolls is ≤2.5mm, and the total number of exceeding the standard points is ≤3 points; the superheat of the molten steel in the tundish is ≤25℃, the reduction is ≥3.0mm, and the secondary cooling water of the runner adopts strong cooling;

[0013] Step 3: heating the slab obtained in step 2 at a temperature of 1250±15°C for a heating time of 180 to 250 minutes, wherein the holding time is ≥30 minutes;

[0014] Step 4: hot rolling the heated slab, wherein the intermediate slab thickness at the rough rolling outlet is 36±1 mm, the finishing rolling inlet temperature is 1050-1080° C., the final rolling temperature is 860-900° C., and the thickness of the slab after finishing rolling is 2.2-2.5 mm;

[0015] Step 5: coil the finished steel strip, control the coiling temperature at 600-630°C, and then place it in a slow cooling pit to slowly cool it to room temperature;

[0016] Step 6: Cut appropriate materials according to the diameter of the shock absorber working cylinder, weld them into a tubular structure, and then perform heat treatment and cold rolling to obtain the automobile shock absorber working cylinder.

[0017] Furthermore, in step 4, after the rough rolling is completed, the intermediate billet is firstly curled and scaled by a hot coil box, and then uncoiled for finish rolling.

[0018] Furthermore, before curling the steel strip in step five, the steel strip is cooled by rapid laminar cooling in the front section and air cooling in the back section, with the laminar cooling rate being ≥40°C / s, and the entire cooling time is ensured to meet the subsequent curling temperature requirements.

[0019] Furthermore, before making the shock absorber working cylinder in step 6, the steel strip is pickled once, and the steel strip is firstly subjected to a tensile straightening and descaling treatment by a straightening equipment, and the tensile straightening elongation is 1.5% to 1.8%, and then pickled. The target value of the oil coating amount on one side of the steel strip after pickling is 500mg / m 2 control.

[0020] Furthermore, after pickling, the steel strip is subjected to a skin-finishing treatment by single-stand reciprocating rolling, requiring the roller roughness Ra≤0.4μm, the total deformation to be 0.3-0.5mm, and the rolling passes to be no less than 2 and no more than 4.

[0021] Furthermore, in step six, when welding, straight seam high frequency welding technology is adopted, which requires a high frequency heating temperature of 1350±5°C, a metal streamline angle of the welding joint of 50° to 60°, and a cooling rate of 20 to 30°C / s after welding.

[0022] Furthermore, when the working cylinder after welding is heat treated, the heating temperature is required to be 600±5°C, the temperature is kept for 10±1min, and then cooled to room temperature at a cooling rate of 30-35°C / min. Carbon monoxide gas protection is used during the heat treatment process.

[0023] Furthermore, when the working cylinder is cold-rolled and shaped, the steel pipe is sized and smoothed by a Sendzimir rolling mill to obtain a steel pipe with a roughness of 0.3 to 0.5 μm, a yield strength of 520 to 630 MPa, a tensile strength of 620 to 700 MPa, and an elongation after fracture of A 50 ≥12%.

[0024] The beneficial effects of the present invention are as follows: by precisely controlling the steel components, using conventional raw materials, conventional processes and conventional energy consumption, it is possible to obtain steel for the shock absorber working cylinder with strength that meets the requirements without the need for complex surface treatment processes and the addition of high-cost alloys, thereby saving costs; when manufacturing the shock absorber working cylinder, a manufacturing method is adopted in which steel plates are first welded into steel pipes and then the steel pipes are cold-rolled and shaped, which has a simple process, convenient operation, high production efficiency and low cost. DETAILED DESCRIPTION

[0025] The present invention is further described below in conjunction with embodiments.

[0026] The steel for a working cylinder of an automobile shock absorber provided by the present invention comprises the following components in percentage by mass: C: 0.13% to 0.16%, Si: 0.05% to 0.15%, Mn: 0.50% to 0.60%, P≤0.015%, S≤0.005%, 0.010%≤Als≤0.050%, Cr: 0.10% to 0.20%, 0.010%≤Ti≤0.025%, O≤0.0020%, N≤0.0040%, and the balance is Fe and unavoidable impurities.

[0027] The role of each element component is:

[0028] C: must have a certain content to promote the formation of carbide strengthening phase, thereby ensuring the strength level of the steel strip. When its content is too high, the cementite and pearlite content are too high, the plasticity is reduced, resulting in reduced cold working performance; when its content is too low, the strength does not meet the design requirements. The present invention designs the C content to be ≤0.16%, preferably 0.13% to 0.16%.

[0029] Si: has a solid solution strengthening effect. However, when the Si content is high, Fe2SiO4 red rust is easily generated on the surface of the steel strip, which is difficult to remove during pickling. In order to improve the surface quality of the steel strip and reduce the difficulty of industrial production, the present invention designs a low Si composition scheme, i.e. ≤0.30%, preferably 0.05% to 0.15%.

[0030] Mn: It is a solid solution strengthening element. According to the strength requirements of this steel grade, when the content is too low, the strength will not meet the design requirements; when the content is too high, the proportion of pearlite will increase, and high Mn content will easily lead to the segregation of Mn components in the ingot, which is one of the main reasons for the appearance of banded structure in the steel strip, and the banded structure will cause the hole expansion performance of the steel strip to drop sharply. At the same time, for carbon manganese steel, the interaction between C and Mn has a certain influence on the formation and distribution of pearlite structure. When the content of both is too high, coarse pearlite and banded structure are easily generated. Therefore, under the premise that the strength meets the design requirements, the C content and Mn content are designed to be as low as possible. According to the C content, the present invention designs the Mn content ≤1.00%, preferably 0.50% to 0.60%.

[0031] P: As the strongest element for strengthening ferrite, it has strong solid solution strengthening and cold working hardening effects. When used as an alloying element, it can improve its strength and atmospheric corrosion resistance of the steel strip. However, when the content is too high, the cold stamping performance of the steel strip will drop sharply. At the same time, there will be segregation of the components of the continuous casting slab, causing uneven structure, increased temper brittleness, and significantly reduced plasticity and toughness of the steel strip, so that the steel strip will show "cold brittleness" phenomenon and brittle cracking during cold working. The present invention designs the P content to be ≤0.015%.

[0032] S: It is an impurity element in steel and is easy to form MnS in steel. The amount and form of sulfides in steel directly affect the hole expansion performance of the steel strip. In particular, strip-shaped sulfide inclusions are prone to cracks during deformation. Therefore, when designing the chemical composition, the S content must be reduced, that is, below 0.005%, preferably ≤0.003%.

[0033] Al: Its basic function in the present invention is mainly deoxidation in the steelmaking process. The Als content in steel is generally lower than 0.010%. At the same time, if the Als content exceeds 0.060%, its grain refining effect is weakened. Therefore, the present invention designs the Al content to be between 0.010% and 0.050%, preferably 0.020% to 0.035%.

[0034] Cr: It can delay the pearlite transformation, significantly improve the hardenability of steel, facilitate the formation of bainite structure, and refine the structure to achieve a strengthening effect. Too high Cr content will deteriorate the processing and formability of the material, so the present invention designs the Cr content to be ≤0.30%, preferably 0.10% to 0.20%.

[0035] Ti: plays the role of precipitation strengthening and fine grain strengthening in steel. Micro-titanium treatment can improve the welding performance of steel. The Ti content of the present invention is designed to be ≤0.30%, preferably 0.010% to 0.025%.

[0036] O: As a harmful element in steel, it is also an unavoidable element in the steelmaking process. The present invention realizes ultra-low oxygen content control through double RH vacuum treatment. Therefore, the present invention designs the O content to be ≤0.0020%, preferably ≤0.0015%.

[0037] N: can combine with alloy elements to produce non-metallic inclusions, reducing the role of alloy elements and is considered a harmful element. Under controlled rolling and controlled cooling conditions, N, C and Ti combine to form nano-precipitated phase Ti (C, N). The present invention requires N≤0.0040%, preferably ≤0.0030%.

[0038] The present invention provides a short-process production method for a working cylinder of an automobile shock absorber, using the above-mentioned steel for the working cylinder of the automobile shock absorber, and further comprising the following steps:

[0039] Step 1: sequentially carry out hot metal desulfurization, converter, and LF refining to obtain molten steel that meets the chemical composition design requirements;

[0040] Step 2: Send the molten steel obtained in step 1 to a continuous casting machine for continuous casting. The requirements for the roll gap of the two streams of the continuous casting machine are that the roll gap of the straightening section does not exceed ±1mm compared with the standard value, and the roll gap of other sections does not exceed ±2mm compared with the standard value, and the difference between the maximum and minimum roll gaps of two adjacent rolls is ≤2.5mm, and the total number of exceeding the standard points is ≤3 points; the superheat of the molten steel in the tundish is ≤25℃, the reduction is ≥3.0mm, and the secondary cooling water of the runner adopts strong cooling;

[0041] Step 3: heating the slab obtained in step 2 at a temperature of 1250±15°C for a heating time of 180 to 250 minutes, wherein the holding time is ≥30 minutes;

[0042] Step 4: hot rolling the heated slab. According to the size requirements of the shock absorber working cylinder, the thickness of the intermediate slab at the rough rolling outlet is 36±1mm, the finishing rolling inlet temperature is 1050-1080°C, the final rolling temperature is 860-900°C, and the thickness of the strip after finishing rolling is 2.2-2.5mm.

[0043] Step 5: coil the finished steel strip, control the coiling temperature at 600-630°C, and then place it in a slow cooling pit to slowly cool it to room temperature;

[0044] Step 6: Cut appropriate materials according to the diameter of the shock absorber working cylinder, weld them into a tubular structure, and then perform heat treatment and cold rolling to obtain the automobile shock absorber working cylinder.

[0045] In step 4, after the rough rolling is completed, the intermediate billet is firstly curled and scaled with a hot coil box, and then uncoiled for finish rolling, which can remove the iron oxide scale.

[0046] Before coiling the steel strip in step 5, the steel strip is cooled by rapid laminar cooling in the front section and air cooling in the back section. The laminar cooling rate is ≥40℃ / s, and the entire cooling time ensures that the subsequent coiling temperature requirements are met. To ensure that the coiling temperature is controlled at 600~630℃ under the action of the strong hardenability element Cr, a stable ferrite + pearlite structure can be obtained. According to the test, the mechanical properties of the steel plate after coiling are R eL ≥330MPa, R m ≥440MPa, A 50 ≥28%.

[0047] Before making the shock absorber working cylinder in step 6, the steel strip is pickled once. The steel strip is first straightened and scaled by the straightening equipment. The straightening elongation is 1.5% to 1.8%, and then pickled. The target value of the oil coating on one side of the steel strip after pickling is 500mg / m 2Control. After pickling, the steel strip can be polished by single-stand reciprocating rolling. The roughness of the roll of the single-stand cold rolling mill directly affects the surface roughness of the steel strip after the steel coil is rolled, that is, the smaller the surface roughness of the roll, the smaller the roughness of the steel coil after rolling. In order to ensure that the Ra of the inner wall of the working cylinder is ≤0.6μm, the factors that increase the roughness due to stripping, welding and other processes must be considered, so the roll roughness Ra≤0.4μm is required. During rolling, small deformation rolling is carried out according to the requirements of raw material thickness, finished product thickness, and the technical requirements of mechanical properties and surface roughness of the finished product, and the total deformation is controlled to be 0.3-0.5mm, and the number of rolling passes is not less than 2 times and not more than 4 times, preferably 3 times.

[0048] In step six, straight seam high-frequency welding technology is used for welding. The quality of the weld is a key factor affecting whether the subsequent cold drawing will "break the tube" and whether the shock absorber working cylinder can work stably. To ensure the quality of the weld, the high-frequency heating temperature is required to be 1350±5℃, the metal streamline angle of the welding joint is 50°~60°, and the cooling rate after welding is 20~30℃ / s.

[0049] Due to the large internal stress and poor plasticity of the welded joint, it is not suitable for cold deformation processing. Therefore, the weld must be stress-relieved to improve the performance of the welded joint. When heat treating the working cylinder after welding, the heating temperature is required to be 600±5℃, the heat preservation is 10±1min, and then cooled to room temperature at a cooling rate of 30-35℃ / min. Carbon monoxide gas protection is used during the heat treatment process to prevent the surface of the steel pipe from being oxidized and causing increased roughness.

[0050] Finally, when the working cylinder is cold-rolled and shaped, the Sendzimir rolling mill is preferably used to perform sizing and finishing on the steel pipe to obtain a steel pipe with a roughness between 0.3 and 0.5 μm, a yield strength of 520 to 630 MPa, a tensile strength of 620 to 700 MPa, and an elongation after fracture of A. 50 ≥12%. After the steel pipe is made, it needs to be cut, tested and inspected before the shock absorber working cylinder can be made.

[0051] The present invention is further described below in conjunction with embodiments.

[0052] Example:

[0053] The present invention provides three groups of materials for comparative tests, the chemical compositions of which are shown in Table 1, and the remainder is Fe and unavoidable impurities.

[0054] Table 1

[0055]

[0056] The process parameters such as billet heating, rough rolling, finishing rolling, laminar cooling, coiling, and the indicators of microstructure and properties are shown in Tables 2 and 3.

[0057] Table 2

[0058]

[0059]

[0060] Table 3

[0061]

[0062] According to the comparison of the above embodiments, it can be seen that the automobile shock absorber working cylinder finally obtained by adopting the steel composition and production process provided by the present invention has good yield strength and tensile strength, meeting the performance requirements of the automobile shock absorber working cylinder.

Claims

1. A steel for a working cylinder of an automobile shock absorber, characterized in that: The invention comprises the following components in mass percentage: C: 0.13% to 0.16%, Si: 0.05% to 0.15%, Mn: 0.50% to 0.60%, P≤0.015%, S≤0.005%, 0.010%≤Als≤0.050%, Cr: 0.10% to 0.20%, 0.010%≤Ti≤0.025%, O≤0.0020%, N≤0.0040%, and the balance is Fe and unavoidable impurities.

2. A short-process production method for a working cylinder of an automobile shock absorber, characterized in that: The method of using the steel for automobile shock absorber working cylinder as claimed in claim 1 further comprises the following steps: Step 1: sequentially carry out hot metal desulfurization, converter, and LF refining to obtain molten steel that meets the chemical composition design requirements; Step 2: Send the molten steel obtained in step 1 to a continuous casting machine for continuous casting. The requirements for the roll gap of the two streams of the continuous casting machine are that the roll gap of the straightening section does not exceed ±1mm compared with the standard value, and the roll gap of other sections does not exceed ±2mm compared with the standard value, and the difference between the maximum and minimum roll gaps of two adjacent rolls is ≤2.5mm, and the total number of exceeding the standard points is ≤3 points; the superheat of the molten steel in the tundish is ≤25℃, the reduction is ≥3.0mm, and the secondary cooling water of the runner adopts strong cooling; Step 3: heating the slab obtained in step 2 at a temperature of 1250±15°C for a heating time of 180 to 250 minutes, wherein the holding time is ≥30 minutes; Step 4: hot rolling the heated slab, wherein the intermediate slab thickness at the rough rolling outlet is 36±1 mm, the finishing rolling inlet temperature is 1050-1080° C., the final rolling temperature is 860-900° C., and the thickness of the slab after finishing rolling is 2.2-2.5 mm; Step 5: coil the finished steel strip, control the coiling temperature at 600-630°C, and then place it in a slow cooling pit to slowly cool it to room temperature; Step 6: Cut appropriate materials according to the diameter of the shock absorber working cylinder, weld them into a tubular structure, and then perform heat treatment and cold rolling to obtain the automobile shock absorber working cylinder.

3. A short-process production method for a working cylinder of an automobile shock absorber as claimed in claim 2, characterized in that: In step 4, after the rough rolling is completed, the intermediate billet is firstly curled and scaled by a hot coil box, and then uncoiled for finish rolling.

4. A short-process production method for a working cylinder of an automobile shock absorber as claimed in claim 2, characterized in that: Before curling the steel strip in step 5, the steel strip is cooled by rapid laminar cooling in the front section and air cooling in the back section. The laminar cooling rate is ≥40°C / s, and the entire cooling time ensures that the subsequent curling temperature requirements are met.

5. A short-process production method for a working cylinder of an automobile shock absorber as claimed in claim 2, characterized in that: Before making the shock absorber working cylinder in step 6, the steel strip is pickled once. The steel strip is first straightened and scaled by the straightening equipment. The straightening elongation is 1.5% to 1.8%, and then pickled. The target value of the oil coating on one side of the steel strip after pickling is 500mg / m 2 control.

6. A short-process production method for a working cylinder of an automobile shock absorber as claimed in claim 5, characterized in that: After pickling, the steel strip is polished by single-stand reciprocating rolling, requiring the roller roughness Ra ≤ 0.4 μm, the total deformation of 0.3 to 0.5 mm, and the rolling passes of not less than 2 and not more than 4.

7. A short-process production method for a working cylinder of an automobile shock absorber as claimed in claim 2, characterized in that: In step six, straight seam high frequency welding technology is used for welding, which requires a high frequency heating temperature of 1350±5°C, a metal flow line angle of 50° to 60° at the welding joint, and a cooling rate of 20 to 30°C / s after welding.

8. A short-process production method for a working cylinder of an automobile shock absorber as claimed in claim 2, characterized in that: When heat treating the working cylinder after welding, the heating temperature is required to be 600±5℃, the temperature is kept for 10±1min, and then cooled to room temperature at a cooling rate of 30-35℃ / min. Carbon monoxide gas protection is used during the heat treatment process.

9. A short-process production method for a working cylinder of an automobile shock absorber as claimed in claim 2, characterized in that: When the working cylinder is cold rolled, the steel pipe is sized and smoothed by the Sendzimir rolling mill to obtain a steel pipe with a roughness of 0.3-0.5 μm, a yield strength of 520-630 MPa, a tensile strength of 620-700 MPa, and an elongation of A. 50 ≥12%.

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