Heat treatment with different temperature ,holding time and cooling rate was conducted for typical steel 07MnNiMoDR used in hazardous chemicals storage tanks to simulate the fire process .Hardness and metallographic structures of 07MnNiMoDR after different heat treatment processes were tested and inspec -ted.The results show that , critical temperature range for the hardness and metallographic structures of 07MnNiMoDR steel to change rapidly after fire is 650~750 ℃,the hardness range is 190~220 HV10 and the effect of holding time on hardness and metallographic structures is not obvious when the temperature is lower than 650 ℃.The hardness under water cooling is always higher than that under air cooling at the same temperature and holding time .Metallographic structures provide evidences for the hardness change law.When the temperature is higher than the critical value and at the holding time of 2~12 h,the hard-ness decreases with temperature under air cooling conditions , and the maximum relative hardness is re-spectively reduced by 10%and 25%at 750 ℃and 850 ℃;under water cooling conditions ,hardness de-creases at first and then increases with temperature ,and it reaches the lowest value when the temperature is 750 ℃where the maximum relative hardness is reduced by 5%,however the maximum relative hard-ness is increased by 30%when the temperature reaches 850 ℃.%对危化品大型储罐典型用钢07 MnNiMoDR进行不同热暴露温度、保温时间和冷却速率的热处理来模拟火灾发生时的受火过程，对经历不同热处理工艺后的硬度和金相组织进行了测定和检验。结果表明，07 MnNiMoDR钢受火后其硬度和金相组织急剧变化的温度临界值范围为650～750℃，650℃以下时硬度值范围为190～220 HV10且保温时间对硬度和金相的影响不明显；在相同的温度和保温时间下，水冷时的硬度总高于空冷时的硬度；显微组织为硬度的变化规律提供了依据。当保温时间为2～12 h和热暴露温度高于温度临界值时，空冷时硬度随温度升高而不断下降，750和850℃时最大的硬度相对百分数分别降低了10％和25％；水冷时硬度随温度先下降后升高，750℃时硬度达到最小值，此时最大的硬度相对百分数下降了5％，当温度达到850℃时最大的硬度相对百分数增加了30％。
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