Microstructural evolution of Mo?W?V alloyed hot-work die steel during high-temperature tempering
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摘要: 為適應熱沖壓技術的發展需求，開發了一種新型高熱導率高耐磨性能熱沖壓用模具鋼材料。采用掃描電鏡（SEM）、透射電鏡（TEM）等檢測手段對鉬鎢釩合金化新型模具鋼的高溫回火性能與組織特征進行了研究。闡明了新型熱沖壓模具鋼回火過程碳化物析出與演變規律。實驗結果表明：實驗用鉬鎢釩合金化模具鋼材料具有良好的回火二次硬化性能，在500～600 ℃溫度區間回火時，回火組織硬度上升；在600 ℃回火出現二次硬化峰值；當回火溫度超過600 ℃后，組織軟化程度明顯，回火硬度開始下降。實驗模具鋼在高溫回火過程中的硬度變化與其合金碳化物的偏聚、析出和聚集長大密切相關。當在560 ℃以下回火時，實驗鋼組織中未有合金碳化物析出；當回火溫度大于560 ℃時，回火組織中開始析出M2C型碳化物；當回火溫度高于600 ℃后開始析出MC型碳化物；當在620 ℃長時間回火后M2C型碳化物轉化為M6C型碳化物，此時實驗鋼硬度開始明顯下降；而當回火溫度高于660 ℃時，新型實驗鋼組織中主要為M6C和MC型合金碳化物。Abstract: Hot-work die steels are widely used to meet the requirements of industrial applications in which the steels must endure high temperature and mechanical loads, such as the hot stamping of very-high-strength steel. In the field of hot-stamping technology applications, the tool materials must have excellent high-temperature performance, such as the high-temperature stability of the microstructure. Research on hot-stamping die materials began somewhat late in China because high-quality die steel products had typically been imported. A new type of hot-stamping die steel with high thermal conductivity and high wear resistance was developed to meet the requirements of hot-stamping technology. In this study, we used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to determine the high-temperature tempering performance and microstructural characteristics of this new Mo?W?V alloyed hot-work die steel. Based on the results, we derived the precipitation and evolvement rules of the carbides in the new type hot-stamping die steel during the tempering process, which indicate that the new Mo?W?V alloyed test steel has an excellent secondary hardening property. We find the hardness of the microstructure to increase after tempering at 500 ℃–600 ℃; however, at tempering temperatures above 600 °C, the matrix obviously softens and the hardness of the test-steel microstructure decreases. The hardness of the test die steel is strongly linked to the segregation, precipitation and growth of the alloy carbides in the matrix. No alloy carbide precipitation is observed at tempering temperatures below 560 ℃; however, M2C-type carbide precipitation is observed at tempering temperatures higher than 560 ℃. MC-type alloy carbide is observed in the test-steel matrix at tempering temperatures up to 600 ℃. At tempering temperatures above 620 ℃, the M2C-type alloy carbides transform into M6C-type alloy carbides and the hardness curve of the test steel sharply declines. The MC-type and M6C-type alloy carbides are the main carbides in the matrix of the new Mo?W?V alloyed hot-work die steel.
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Key words:
- die steel /
- heat treatment /
- alloy carbide /
- microstructure /
- transmission electron microscopy
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圖 2 不同熱處理后組織掃描電鏡照片。（a）淬火態；（b）560 ℃回火；（c）600 ℃回火；（d）620 ℃回火；（e）640 ℃回火；（f）700 ℃回火
Figure 2. SEM images of microstructure after heat treatment: (a) after quenching; (b) tempering at 560 ℃; (c) tempering at 600 ℃; (d) tempering at 620 ℃; (e) tempering at 640 ℃; (f) tempering at 700 ℃
圖 3 回火組織掃描電鏡照片。（a）600 ℃；（b）640 ℃；（c）700 ℃
Figure 3. SEM images of tempering microstructure: (a) 600 ℃; (b) 640 ℃; (c) 700 ℃
圖 4 520 ℃回火保溫60 h后的基體組織的透射電鏡照片。（a）明場；（b）暗場相和對應衍射斑點
Figure 4. TEM images of microstructure after tempering for 60 h at 520 ℃: (a) bright-field image; (b) dark-field image and diffraction patterns
圖 5 560 ℃回火保溫60 h后的基體組織的透射電鏡照片。（a）明場；（b）暗場像和對應的衍射斑點
Figure 5. TEM images of microstructure after tempering for 60 h at 560 ℃: (a) bright-field image; (b) dark-field image and diffraction patterns
圖 6 600 ℃回火保溫4 h后的基體組織的M2C型碳化物透射電鏡照片。（a）明場；（b）暗場像和對應的衍射斑點
Figure 6. TEM images of M2C carbides in microstructure after tempering for 4 h at 600 ℃: (a) bright-field image; (b) dark-field image and diffraction patterns
圖 7 600 ℃回火保溫4 h后的基體組織的MC型碳化物透射電鏡照片。（a）明場；（b）暗場像和對應的衍射斑點
Figure 7. TEM images of MC carbides in microstructure after tempering for 4 h at 600 ℃: (a) bright-field image; (b) dark-field image and diffraction patterns
圖 8 620 ℃回火保溫4 h后的基體組織中碳化物透射電鏡照片。（a）明場；（b）暗場像和對應的衍射斑點
Figure 8. TEM images of carbides in microstructure after tempering for 4 h at 620 ℃: (a) bright-field image; (b) dark-field image and diffraction patterns
圖 9 620 ℃回火保溫20 h后的基體組織的M6C型碳化物透射電鏡照片。（a）明場；（b）暗場像對應的衍射斑點
Figure 9. TEM images of M6C carbides in microstructure after tempering for 20 h at 620 ℃: (a) bright-field image; (b) dark-field image and diffraction patterns
圖 10 660 ℃回火保溫4 h后的基體組織的M6C型碳化物透射電鏡照片。（a）明場；（b）暗場像和對應的衍射斑點
Figure 10. TEM images of M6C carbides in microstructure of test steel after tempering for 4 h at 660 ℃: (a) bright-field image; (b) dark-field image and diffraction patterns
表 1 實驗鋼成分（質量分數）
Table 1. Composition of the test steel
% C Si Mn Cr Mo W V P S Fe 0.48 0.10 0.08 0.13 2.99 1.70 0.86 0.01 0.02 Bal. 
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