Effect Of Composition Fluctuation And Nano Precipitates On The Properties Of DH36 Steel

A steel company in Hebei Province proposed to further improve the quality of DH36 Structural steel.In this paper,I researched the relationship between composition fluctuation of conventional component of C,Si,Mn,P,S,Alt with impact energy,yield strength,tensile strength and contraction ratio of section.I also researched nano-precipitates strengthening mechanism and strengthening model in the case of different sulfur contents.In the current fluctuation range of conventional elements,the C content is 0.08-0.17%,the Si content is 0.27-0.46%,the Mn content is 1.27-1.50%,the S content is 0.002-0.022%,and the Alt content is 0.02-0.042%.The mathematical model of the relationship between P,S content and mechanical properties such as impact energy,yield strength,tensile strength and reduction of area was established.It was found through model the mechanical properties of the low sulfur end with sulfur content less than 100ppm was appeared the phenomenon of polymerization,and also will be the highest value.With the increase of sulfur content,the mechanical properties began to diverge with the fluctuation of element concentration.The greater the sulfur content,the more serious the divergence.The content of P was between 0.019 and 0.020%,and the impact energy,yield strength,tensile strength in the study area had a great value.Based on BP neural network with 288 commercial data.Putting 278 sets of data as test samples,which is training data.Through the verification of 10 sets of data,it was found that using genetic algorithm to optimize BP neural network,there is a good response to find The nonlinear relationship between the main components and the mechanical properties provides a valuable reference for the prediction of the production process performance.Thermodynamic and kinetic analysis of MnS precipitation and pinning effect in DH36 structural steels with 20 ppm,70 ppm,150 ppm and 380 ppm S content.It was found that when the S content is 20 ppm and 70 ppm,MnS precipitated in high-temperature ferrite and its size was also larger.When the S content decreased from 380ppm to 20ppm,the maximum nucleation rate of MnS was 1465 ℃ and the fastest Precipitation temperature of 1480℃ to 900 ℃ and 920 ℃,respectively.Resulting in a drastic reduction of precipitated MnS size and a large amount of nano-sized MnS with a strong pinning effect on nucleation,which can make it possible to control the pre-rolling austenite The grain size is 71.23μm,the grain size after controlled rolling and controlled cooling is 8.7μm,far superior to the mechanical properties under high S content.Under the TEM observation,the types,morphologies,precipitates size,content and volume fraction of nano-scale secondary precipitates in the as-rolled low-sulfur DH36 steel were statistically analyzed.The effects of four strengthening mechanisms were analysised.I found that a large number of the second phase precipitates in the as-rolled DH36 had a regular square morphology.The nano-scale precipitates consisted of carbonitrides of Ti and Nb and a small amount of Cr carbon nitride.The volume fraction of carbon nitride precipitated in DH36 steel was 0.00957%.The contribution of precipitation strengthening was 211.24MPa and the contribution of solid solution strengthening was 122.93MPa.The contribution of fine grain strengthening and dislocation strengthening were 212.30MPa and 134.43MPa.The study on the strengthening mechanism of DH36 steel shows that the contribution of each strengthening to yield strength can not be simply summed.It should be the square root of twice the product of solid solution strengthening and dislocation strengthening plus the product of fine grain strengthening and precipitation strengthening of the square root,which theoretical results and the actual value is very close,but this calculation model also needs further tests to prove.Finally,using DH36 data of enterprises under production conditions,the above conclusions were analyzed by QC quality diagram and histogram analysis.It was found that the mechanical properties of shipbuilding steel when w(S)Small fluctuations in the vicinity,with good stability,for the conventional elements C,Si,Mn,P,S,Alt composition is no longer sensitive to fluctuations.The mechanical properties observed from the histogram showed the largest number of samples falling in the small range around the CL line at w(S)≤40ppm,and the DH36 ship plate steel had more excellent mechanical properties.