Finishing method for controlling martensite layer on surface of medium carbon steel bar

Abstract

The invention relates to the technical field of finishing of medium carbon steel bars, and particularly discloses a finishing method for controlling a martensite layer on the surface of a medium carbon steel bar. According to the method, through accurate control over the boundary dimension and curvature of supplied materials, matching design of the granularity of a grinding wheel and grinding steps, optimized regulation and control of the emulsion concentration and standardized setting of grinding operation parameters, formation of a surface martensite layer in the fine stripping process is synergistically reduced, and the depth of the martensite layer is effectively reduced to be smaller than 2 micrometers; the problem of shear cracking caused by a surface martensite layer when the medium carbon steel bar is sheared and blanked by a user is solved, and batch, stable and efficient production and high-quality delivery of products are realized.

Description

Technical Field

[0001] This invention belongs to the field of finishing technology of metal materials, and particularly relates to a finishing method for medium carbon steel bars (such as gear steel, hub steel, crankshaft steel and other high-end steels), specifically involving the technology of optimizing and controlling the surface martensite layer through fine peeling process parameters. Background Technology

[0002] Medium carbon steel bars are key materials in the automotive and machinery manufacturing industries, and their surface quality directly affects subsequent processing performance. In actual production, users often use shearing for processing. However, after hot rolling and traditional finishing, a martensitic layer (usually over 25 μm thick) easily forms on the surface of medium carbon steel bars. This martensitic layer has high hardness and brittleness, making it prone to cracking during shearing, resulting in batches of scrap and quality complaints.

[0003] In existing technologies, improvements to the finishing of medium carbon steel bars mainly focus on a single step, such as simply adjusting the grinding wheel grit size or cooling parameters, but these methods have the following drawbacks: Insufficient control of incoming materials: Traditional processing has lax control over the dimensional accuracy (such as ovality) and curvature of hot-rolled bars, resulting in uneven force during grinding, local over-grinding and high temperature, which promotes the formation of martensite; Inappropriate selection of grinding wheels: Using only a single grit size grinding wheel, although coarse grit grinding wheels can ensure grinding volume, they are prone to surface burns, while fine grit grinding wheels can improve surface finish but have insufficient grinding volume, and cannot achieve both. Cooling parameters mismatch: The emulsion concentration is not precisely controlled. Too low a concentration can easily cause grinding and quenching (martensite formation), while too high a concentration can lead to surface oil stains and environmental problems. There are no standard operating parameters: parameters such as grinding wheel pressure and motor current are set based on experience, which can easily lead to problems such as grinding wheel cracking and insufficient grinding amount, further aggravating the formation of martensite layer.

[0004] Therefore, there is an urgent need for an integrated method to coordinate and control the incoming material, grinding wheel, cooling, and operating parameters, so as to reduce the surface martensite layer from the source and solve the problem of shear cracking. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a finishing method for controlling the martensite layer on the surface of medium carbon steel bars, which is an improvement over the above-mentioned prior art. By optimizing the parameters of multiple links, the depth of the surface martensite layer is reduced, the shearing cracking problem during shearing and blanking is solved, and stable product delivery is achieved.

[0006] The technical solution adopted by this invention to solve the above problems is: a finishing method for controlling the martensite layer on the surface of medium carbon steel bars, comprising the following steps: (a) Incoming material pretreatment Dimensional accuracy control: The dimensions of the hot-rolled bars used meet the national standard level (tolerance ≤ ±0.1mm), and the ovality is ≤ 0.05mm; Bending control: The incoming material is straightened, and the bending degree of the bar is <4mm / m after straightening; for the serpentine bending at the end of small-sized bars, the bending length is reduced by optimizing the diameter of the exit guide of the finished mill (reduced by 5-10%), and the bending section is removed by subsequent sawing; for the wavy bending in the middle, straightening grids are set in the 950℃-650℃ section of the hot rolling cooling bed to reduce bending caused by uneven cooling.

[0007] (II) Grinding Parameter Design The grinding process employs a two-step method: rough grinding followed by fine grinding. Rough grinding stage: Select L46 mesh grinding wheel (grit size 40-50μm), grinding wheel bond is ceramic bond V, hardness is N grade (medium hard), ensure grinding amount (single grinding amount 0.3-0.5mm). Fine grinding stage: Select F60 mesh grinding wheel (grit size 0.5-1mm), the grinding wheel bond is ceramic bond V, hardness is P grade (medium hard), improve surface finish (Ra≤1.6μm) and reduce surface burn.

[0008] (III) Regulation of Emulsion Cooling System Concentration control: The concentration of the emulsion (a mixture of emulsified oil and water) should be controlled at 12-15% (by mass), with a minimum of 10% and a maximum of 20%. Regular maintenance: Clean the grinding machine sedimentation tank every 10-15 days to remove sediment and prevent blockage; replenish the emulsion regularly to ensure stable concentration.

[0009] (iv) Standardization of grinding operation parameters Grinding wheel pressure: The normal grinding wheel pressure is set to about 1 kg, and the pressure of a newly replaced grinding wheel is adjusted to 0.5 kg (to avoid cracking). Motor current: The current of the 22kW grinding motor during feed is controlled at 40-45A. The current change is monitored in real time. If it exceeds 45A, the pressure needs to be reduced to ensure stable grinding.

[0010] Compared with the prior art, the present invention has the following beneficial effects: The depth of the martensite layer is significantly reduced: through multi-stage coordinated control, the depth of the surface martensite layer is reduced from more than 25μm in the traditional process to less than 2μm, completely solving the problem of cracking during shearing. Improved processing stability: Control of incoming material curvature and dimensional accuracy ensures grinding uniformity; two-step grinding wheel method and parameter standardization avoid over-grinding or under-grinding; motor current and pressure control reduce the risk of equipment failure (such as grinding wheel breakage). Balancing economic efficiency and environmental friendliness: Optimized emulsion concentration (12-15%) avoids grinding and quenching, reduces oil residue (no thick smoke when heating for users), and reduces emulsion oil consumption (cost is reduced by 15-20% compared to 20% concentration). Mass production adaptability: Standardized process parameters enable mass processing of medium carbon steel bars (increasing capacity by 10-15%), and the pass rate of re-inspection is increased from 80% to over 98%. Attached Figure Description

[0011] Figure 1: Thermodynamic diagram of martensite transformation during fine peeling in this invention (showing the temperature-time relationship of martensite formation under different grinding parameters); Figure 2: Martensite transformation diagram of the steel surface of the present invention (comparison of martensite formation trend under different emulsion concentrations). Figure 3: Martensite layer depth map of traditional process (thickness 25μm); Figure 4: Martensite layer depth map of the process of the present invention (thickness 2μm). Detailed Implementation

[0012] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention.

[0013] I. Experimental Materials Φ50mm medium carbon steel bar (material 45# steel) is selected. The initial dimensional tolerance after hot rolling is ±0.2mm, ellipticity is 0.1mm, curvature is 6mm / m, and the initial thickness of the surface martensite layer is 28μm.

[0014] II. Implementation Steps Incoming material pretreatment Hot-rolled bars are straightened using a multi-roll straightener, and the curvature is controlled to 3 mm / m after straightening. The dimensional accuracy is checked, and the rolling mill parameters are adjusted through internal control to ensure that the dimensional tolerance of the incoming material meets the national standard set (±0.1mm) and the ovality is reduced to 0.04mm. Sawing off the 100mm serpentine bend at the end, replacing the 950℃-650℃ area of ​​the cooling bed with a straightening grid, and eliminating the wavy bend in the middle.

[0015] Grinding wheel preparation Coarse grinding wheel: L46 mesh, ceramic bond V, hardness N grade, diameter 500mm; Fine grinding wheel: F60 mesh (0.8mm abrasive grains), ceramic bond V, hardness P grade, diameter 500mm.

[0016] Emulsion preparation Mix emulsified oil (model: M-300) with tap water at a ratio of 1:7 (concentration 12.5%), pour into the grinding machine water tank, and clean the sedimentation tank (12 days after the last cleaning) to ensure no sediment buildup.

[0017] Grinding operation Rough grinding: Grinding wheel pressure set to 1kg, 22kW motor feed current 42A, grinding depth 0.4mm; Fine grinding: Replace with an F60 grit grinding wheel, adjust the pressure to 1kg (not a new grinding wheel), motor current 43A, grinding depth 0.1mm.

[0018] III. Effect Testing Surface martensite layer depth: measured using metallographic microscopy, the result is 1.8 μm; Surface finish: Ra 1.2 μm; Shear test: 100 samples were sheared and cut, and none of them cracked, with a pass rate of 100%.

Claims

1. A finishing method for controlling the martensitic layer on the surface of medium carbon steel bars, characterized in that, Includes the following steps: (1) Incoming material pretreatment: control the incoming dimensions of hot-rolled bars to meet the national standard level 1, with ellipticity ≤0.05mm; the bending degree of the straightened bars per meter <4mm / m, and remove the end serpentine bending and the middle wavy bending by optimizing the mill exit guide and the cooling bed straightening grid; (2) Grinding wheel: Two-step grinding is adopted. For rough grinding, L46 mesh grinding wheel is selected, and for fine grinding, F60 mesh grinding wheel with 0.5-1mm abrasive grain is selected. The grinding wheel bond is ceramic bond V, and the hardness is N or P grade. (3) Emulsion cooling: The emulsion concentration is controlled at 12-15%, and the grinding mill sedimentation tank is cleaned every 10-15 days; (4) Control of operating parameters: The grinding wheel pressure is normally 1kg, and 0.5kg when the grinding wheel is newly replaced; the feed current of the 22kW motor is controlled at 40-45A.

2. The finishing method for controlling the martensitic layer on the surface of medium carbon steel bars according to claim 1, characterized in that, In step (1), the diameter of the mill exit guide is reduced by 5-10%, and a straightening grid is set in the 950℃-650℃ area of ​​the cooling bed.

3. The finishing method for controlling the martensitic layer on the surface of medium carbon steel bars according to claim 1, characterized in that, In step (2), the single grinding amount of rough grinding is 0.3-0.5 mm, and the surface finish of fine grinding is Ra≤1.6μm.

4. The finishing method for controlling the martensitic layer on the surface of medium carbon steel bars according to claim 1, characterized in that: In step (3), the emulsion is a mixture of emulsified oil and water, and the concentration is maintained at 12-15% by periodically replenishing the emulsified oil.

5. The finishing method for controlling the martensitic layer on the surface of medium carbon steel bars according to claim 3, characterized in that: In step (4), the motor current is monitored in real time. If it exceeds 45A, the grinding wheel pressure is reduced until the current returns to 40-45A.

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