Defect Evolution And Microstructure Homogenization Control Of Large Section Special Steel Forging

The diameter or side length of large section forging is commonly greater than 250mm. They have been widely used as the raw material of critical components and parts of large military or civilian equipments, so there are very strict requirements for defects and microstructure homogenization for them. However, defect and inhomogeneous microstructure are two serious issues trouble both steel mills and their customers over a long period of time. Just under such a background, medium carbon steel 27SiMn and low carbon steel Q345E were studied in this thesis around defects detected in ultrasonic inspection and microstructure homogenization. Integrated control technology for defects and microstructure was proposed and the problems in industrial practice were solved. The technology had been applied in industry.Systematic study was firstly carried out on the defects in large section forgings detected in ultrasonic inspection. The defects were identified as follow:1) Voids (common defects),2) Inner cracks, accounting for 54.3%(the amount of forgings with inner cracks/total amount of forgings X 100%),3) Slags and inclusions larger than 100?m, accounting for 9.7%(the amount of forgings with slags or inclusions/total amount of forgings X 100%). Based on the above, the control direction for the defects was pointed out clearly. Though the effect of pattern segregation on ultrasonic flaw detection was not obvious, it had been also considered in the study.Numeral simulation software DEFORM-3D was used to simulate the evolution of voids and pattern segregation during forging process. The critical control point for voids closure was the total deformation in a single direction and the deformation mode. By studying the effect of hot deformation parameter on the microstructure, it was clear that the pass deformation and deformation temperature was crucial for microstructure homogenization. The integrated control technology was put forward for voids, pattern segregation and microstructure homogenization as follows:The forging direction in the first and second pass was along the height direction of the ingot, pass deformation was 35%, total deformation was 58%, intermediate billet size was 300mm X 620mm, stopped forging and cooled the billet to the specific temperature (Q345E:850?,27SiMn:900?), and then started the third and forth pass forging. The forging direction in the third and forth pass was along the width direction of the ingot, pass deformation was 30%, total deformation was 51%.It was clear that the inner cracks had formed during slow cooing. For Q345E, the cracks in pearlite and bainite had formed below 607? and 450? respectively. For 27SiMn, the cracks in pearlite and bainite had formed below 635? and 425?, respectively. The new slow cooling technology was proposed and the rate of inner crack (the amount of forgings with inner cracks/total amount of forgingsX 100%) was reduced from 54.3% to 2.3%.Through the study of the effect of heating temperature and time on austenite growth, the sensitive heating temperature for austenite uneven growth was found as 1050?. In industry, it should be avoid to heat the ingot at 1050? for long time. The rational heating temperature for Q345E and 27SiMn was 1180? ?1200?,1200??1220?, respectively.By the study on the effect of heat treatment on banded structure, the evolution of banded structure during heat treatment was revealed, the formation mechanism of coarse and fine grain band was illuminated, the effect of carbon atoms diffused along grain boundaries was analysed, the optimal normalizing temperature was specified as 880?, the -40? impact value had been improved to 100J?130J.Integrated control technology was proposed for the comprehensive quality of the large section special steel forging. Forging quality should be controlled in such aspects:Ingot metallurgy quality, heating, forging, slow cooling and heat treatment. The cleaness and uniformity of the ingot composition should be improved. The inhomogeneous growth of austenite grain should be avoided in heating. Controlled forging should be used to integratedly control the voids, pattern segregation and microstructure in forging. The internal residual stress should be relieved thoroughly. The key point for banded structure control and low temperature toughness for Q345E was the normalizing temperature. The qualified ratio of flaw inspection of the forging in industry was improved from 18.8%?25.0% to 87.0%?96.0% by using integrated control technology.