| The effects of zinc layer on advanced high-strength steel stamping process galling behavior between three kinds of 600 MPa grade advanced high-strength steel sheet(DP590 hot-dipped galvanized steel and it’s bare board, HSLA340 hot-dipped galvanized steel) and the quenched and tempered DC53 cold work die steels were studied on a self-developed friction tester. And the DP590 hot-dipped galvanized steel galling dynamic evolution process was analyzed by scanning electron microscopy(SEM) and the initiation point was determined by data transition. Finally, based on the study of the thickness, hardness, porosity and phase constitution for the two surface modification layers of nitriding treatment and TD treatment(vanadinizing), the effect of two surface modifications on DP590 hot-dipped galvanized steel galling property was studied. The major conclusions can be drawn as follows:1. Under the same condition of friction coupling with deformation, the average friction coefficient of DP590 hot-dipped galvanized steel is larger than the bare board. The galling wear of DP590 hot-dipped galvanized steel is more severity. And the main wear mechanism of the bare board is abrasive wear.2. The work- hardening exponent n of DP590 steel is larger than H340 LAD steel in the low strain region. So this early work-hardening behavior causes the deformation of DP590 is smaller than H340 LAD steel. Therefore, due to the deformation of H340 LAD steel is larger, lead to the zine layer of specimen surface fall off, result in the galling is more severer. Under the same condition of friction coupling with deformation, the average friction coefficient of H340 LAD steel is larger than the DP590 hot-dipped galvanized steel.3. Galling initiation could be categorized into three stages. At first, a giant number of micro cracks appeared on the stretched zinc layer under the tension of bending deformation, which led to the surface protrusion easier to fall off and transfer to the tool surface in sliding friction. Local wear began to occur on the sheet surface and the difference of the surface roughness maintained increasing in this stage. Subsequently, repeating sliding operations resulted in accumulation of the transferred zinc on the die surface, which gradually formed a thin film covering the tool surface. At last, protuberant lump and adherence burl formed from the accumulated zinc adhering on the die surface, thus resulted in serious surface scratch on the sheet strip.4. A quantitative index of galling initiation and the corresponding test method were proposed in this paper. According the geometric characteristics of galling choose roughness variation ΔRy as the quantitative evaluation parameters, then verify the reliability of the method by surface micro topography analysis. The ΔRy is about 8μm when the galling appears between the DP590 hot-dipped galvanized steel and the quenched and tempered DC53 die steel under the condition of friction coupling deformation.5. The nitrding layer on DC53 steel surface are consist of Fe3 N and Fe4 N, with a thickness of about 180μm, surface hardness is about 990HV0.05; the TD coating is consist of a single phase VC with a thickness of about 8μm, and the surface hardness is about 2200HV0.05. Compared with the quenched and tempered DC53 steel, the nitriding treatment can reduce the galling tendency significantly. And the galling mechanisms are abrasive wear and slight adhesive wear. Since VC layer has well anti-adhesion ability, reducing the galling tendency. Therefore, compared to the nitriding treatment, mold surface with TD treatment have the excellent galling resistance property. And the main galling mechanism is abrasive wear. |
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