Steel recrystallization annealing principle

 
2015-07-14 14:33:02  

Steel recrystallization annealing principle

First, the cold plastic deformation of steel without heating the steel at normal temperature by rolling, drawing, extrusion and other processes, resulting in deformation can not restore its original shape and size. Description suffered steel processing pressure is greater than the elastic limit of the steel, causing the plastic deformation of steel, a process called steel cold plastic deformation.
1. Changes in the organizational structure. carbon steel strip during rolling, the size and shape of the change is the sum of the internal grain deformation during rolling, the various aspects of the grains along the rolling stretch breaking flattened fibrous formation, deformation degree is large, the crushing and elongated grains there were many very small pieces, often referred to this structure as the sub-structure, which is called the grain subgrain.
2. The internal stresses. In cold plastic deformation of metallic materials, various factors have led to uneven deformation, so that deformation energy applied in 10% to 15% of the elastic form can remain in the metal inside. The specific form is the metal elastic distortion and internal stress.
3. cold plastic deformation and deformation of tissue. In cold plastic deformation, as the deformation increases, the orientation of each grain is substantially in line, leaving the grain as a result of this modification has a preferred orientation of the organization, called the deformable structure. Example: the formation of the strip by the reduction rate of about 20% of cold rolled grain structure is extended and hardening, tensile strength up to 680mpa above, and product standards 260 ~ 350mpa, this strip is hardly any further processing , is completely incompatible with the product requirements, this must be adjusted to recover the desired grain structure of plastic, get the standard required mechanical properties and good formability.
 
Second, the recrystallization annealing of cold-rolled steel strip. By cold plastic deformation of the metal, heated to above the recrystallization temperature, after cooling the heat treatment process called thermal annealing for recrystallization. After cold rolling metal internal organization which is elongated grain crushing and crystal defects abound phenomenon, spontaneous conversion to trend stable organization, but at high temperatures, the kinetic energy of a metal atom small, poor dispersal ability, diffusion is slow. This spontaneous tendency can not be achieved, must be applied to the driving force of this driving force is to strip heated to a certain temperature, the atomic energy change. As the temperature changes in the microstructure and properties of three phases: recovery, recrystallization, grain growth.
1. reply. When the heating temperature, cold deformation of the metal in a significant reduction of microscopic stress, but little change in strength hardness, ductility and toughness increased slightly, no significant change in the microstructure of the new grain does not appear that this change is called Reply .
Example: from room temperature to 400 ℃, strip inside the organization did not change significantly, the rolling process is elongated grains just get recovery, has not yet formed recrystallization.
2. recrystallization. Cold deformation of the metal when heated to a higher temperature, the formation of a number of different grains and deformation and internal defects less equiaxed small grains, these small grains continued to expand around the deformation of the metal until the metal is completely cold deformation organization disappeared, a process known as recrystallization. Strip heating from 400 ℃ to 723 ℃ the following section is recrystallized formative stages, so that the temperature range of the heating rate must be controlled.
3. The grain growth. After recrystallization completed, the temperature continues to rise or extend the holding time, the grain will continue to grow. Grain growth is a spontaneous process, which reduce grain boundary energy decreased tissue becomes stable.
4. The carbon deposition. Upon cooling, the solubility of carbon in ferrite is decreased with the decrease in temperature after heating steel. Thus there have been saturated carbon deposition, but slower precipitated, if
About 300 ℃ rapid cooling to room temperature, saturated carbon have had to stay in the ferrite, continued precipitation from ferrite at room temperature, increased hardness, produce age-hardening. Therefore, in the 320 or so must slow cooling, the use of carbon sufficiently precipitate from ferrite. Cooled from 600 ℃ to 320 ℃ have the time required to call an effective cooling time. In order from the high temperature to room temperature, the shortest time, leaving the carbon precipitates as far as possible, using the first rapid cooling to overaging temperature, carbon in ferrite reached saturation, and in the over-aging temperature insulation, so that carbon deposition, and then again rapidly cooled to ambient temperature.
 
Third, the recrystallization annealing process selection
1. recrystallization annealing temperature. The greater the degree of deformation during cold rolling, the higher the stress, the more unstable, the lower the recrystallization temperature; the higher the carbon content of sulfur and phosphorus impurities in the material, the higher the recrystallization temperature; heating faster, higher recrystallization temperature.
2. overaging temperature. Overaging temperature is too high, the carbon too rapid cooling after the second ferrite body, the material still has age hardening phenomenon; overaging temperature is too low, then the energy is not enough carbon precipitated too long. Generally use 350 ~ 450 ℃, heat about 20 ~ 300s, different kinds of steel chosen different temperatures and times