There are many varieties of wire rods.Carbon steel wire rod in the low carbon steel wire commonly known as soft wire, medium and high carbon steel wire commonly known as hard wire.The wire rod is mainly used as blank for wire drawing, and can also be directly used as building materials and processed into machine parts.Stainless steel wire is used to manufacture stainless steel wire, stainless steel spring steel wire, stainless steel top forging steel wire and stainless steel wire rope.With the progress of production technology, there have been square,
Hexagonal, fan-shaped and other irregular sections of wire rod;The upper limit of diameter has been extended to 38 mm;The plate weight has been increased from 40 to 60 kg to 3000 kg.Due to the development of new post-rolling heat treatment technology, the oxide scale on the surface of the rod is obviously thinned and the microstructure and properties are greatly improved.
Effect of composition segregation of continuous casting billet
(1) The influence of central segregation
On the cross section of steel billet sample, use Ø 5. 5 mm drill bit, respectively in four from each side, middle, taking 1 core sample, carbon, sulfur content is analyzed with infrared carbon sulfur, and silicon, manganese, phosphorus is analyzed with chemical method.The results show that the mass fraction of carbon in the center is up to 1.06%, and the segregation coefficient of carbon in the center is 1.33, which is far beyond the standard requirements (the segregation coefficient of carbon in the center of billet for steel cord is less than or equal to 1.05). The segregation degree of other elements is small.The wire rod rolled by the continuous casting billet is easy to break when the user is drawing, and the fracture is cup and cone shape.A V-shaped crack was found in the center of the fracture specimen. Metallographic examination showed that there was a netlike or semi-netlike cementite in the center.
The cementite itself is not easy to deform and is reticulated or semi-reticulated at the grain boundary, which is harmful to the quality of the rod.The wire rod is easy to crack along the grain boundary during drawing and brittle fracture in further drawing.The reason for this is that continuous casting billet, the center carbon segregation itself exists while rolling wire the organization in the austenitizing state, and the rolling temperature is higher, but because of the heat preservation time is limited, the heart of carbon segregation is difficult to eliminate, thus destroyed the homogeneity of wire matrix organization, leads to the formation of a wire in the process of drawing a cup cone fracture and fracture.Center segregation is a typical defect of continuous casting billet. The solution is to use large rectangular billet rolling, because large size billet can increase the rolling ratio during rolling to eliminate the above defects, so it is generally not suitable to use less than 120mm ×120mm continuous casting billet, especially the wire with the mass fraction of rolled carbon greater than 0.75%.In addition, the superheat of molten steel is strictly controlled, electromagnetic stirring and light pressure down technology are used in continuous casting, and the cooling intensity of the secondary cooling section of continuous casting is strengthened to reduce the central segregation.
(2) the influence of surface carburization
The broken wire sample taken from the user has a part of the nib fracture and a series of fish scale cracks on the side of the sample.According to previous experience, it should be caused by uneven local surface tissue.Metallographic examination of the specimen with broken filament shows that there are massive cementite and network cementite in the structure near the fishy scale crack.
This is because in the process of continuous casting, improper operation, the graphite carbon in the protective slag with molten steel into the mold, resulting in continuous casting billet surface local carburization.
Influence of nonmetallic inclusions
It is found that the non-metallic inclusions of 82B wire rod are generally C and D inclusions, and there are many C inclusions, up to C4.5E, and D inclusions are generally 1 ~ 1.5 grade.The maximum width of the observed inclusions is 30 μm, far exceeding the standard requirements (standard requirements of class C ≤1, class D ≤ 0.5).Non-metallic inclusions exist in the wire rod, which are harmful to the subsequent processing of the wire rod in the following aspects:
(1) drawing and twisting deformation, destroy the continuity of the steel wire matrix, resulting in stress concentration, once subjected to the action of tensile stress or shear stress, rupture along the direction of inclusion, resulting in steel wire drawing twist easy fracture, and irregular fracture;
(2) Non-metallic inclusions reduce the mechanical properties of the wire, especially its transverse mechanical properties, which reduces the plasticity of the wire. It is easy to fracture under high deformation, and the bending and torsion values decrease. Non-metallic inclusions become the fatigue fracture source of the wire, resulting in the reduction of the fatigue resistance limit of the wire.
(3) in the heat treatment of steel wire, because the expansion coefficient of non-metallic inclusion and steel wire matrix is different, in the steel wire matrix continuity cut, play a role of local gap, resulting in the formation of steel wire heat treatment process microcracks, in the continuing drawing, twist microcrack expansion makes the steel wire fracture.
3. Influence of gas content
By oxygen and nitrogen analysis of 82B wire rod, it is found that the mass fraction of oxygen in wire rod is (55 ~ 85) ×10-6, with an average of 68.42 ×10-6, and the mass fraction of nitrogen is (50 ~ 60) ×10-6, with an average of 54.09 ×10-6, which is far higher than the standard requirements (WO ≤25 ×10-6, WN ≤30 ×10-6).High nitrogen content in steel will cause deterioration of steel quality, nitrogen can increase the effective hardening of steel, improve the strength and hardness of steel, plastic, impact resistance and toughness decreased significantly.Oxygen in steel also has adverse effects on the mechanical properties of steel, and the degree of influence is related to the concentration of oxygen and the type, distribution and quantity of oxygen-containing inclusions.Hydrogen in steel is very harmful, with the increase of hydrogen content in steel, plasticity and toughness decreased significantly, especially for 82B such high carbon steel, in the process of cooling, this phenomenon is more serious.
4. Influence of rolling process
Because the rolling of high wire rod is completed in the prescribed pass system, the deformation conditions are basically fixed, and the deformation parameters of each pass have been determined, the actual production is mainly through the control of rolling temperature, namely temperature control rolling to achieve.The main purpose of temperature control rolling is to refine the grain: through low temperature rolling, can control the size of original austenite grain;The growth of austenite grains can be prevented by reducing the final rolling temperature.Through the rapid water cooling of the wire after finishing rolling to reach the set spinning temperature, not only the deformed austenite can be transformed into supercooled austenite quickly, which makes full preparation for the transformation of the microstructure, but also the grain size of the supercooled austenite can be controlled.However, it should be noted that due to the limit of rolling mill equipment load, the opening rolling temperature should not be too low, otherwise the equipment is prone to accidents.In addition, if the opening rolling temperature is too low, it is easy to cause uneven heating of billet, uneven austenitization, insufficient carbide dissolution, porosity and other defects in the billet can not be completely eliminated, resulting in poor performance of the wire rod and final abnormal structure.Through the austenite grain size test of 82B wire rod, it is found that the grain size of 82B wire rod produced by Qingshan Iron and Steel Co., Ltd is 6 ~ 7 grade. Compared with that of Wuhan Iron and Steel Co., Ltd., Baoshan Iron and Steel Co., Ltd. (8 grade), the grain size of 82B wire rod is coarse.
The austenite grain size is 7 at the opening temperature of 990 ~ 1 010 ℃.The opening rolling temperature is controlled between 1 000 ℃ and 1 050 ℃, and the austenite grain size is 6 grades, which indicates that the opening rolling temperature has a great influence on the austenite grain size. Under the premise of normal production, the opening rolling temperature should be controlled as low as possible.
5. Influence of control cooling process after rolling
The main purpose of cooling control is to control the degree of supercooling and cooling rate to obtain the soxenite structure needed for strengthening and toughening.According to the principle of metal heat treatment, accelerating the cooling rate can make the continuous cooling curve move to the lower right, and the faster the cooling rate is.Forced-air cooling, the standard cooling mode of Styrmore, is impossible to reach the critical transition temperature for martensite formation.The faster the air cooling rate is, the easier it is for austenite to be transformed into soxenite. Therefore, large air volume is used in the actual cooling to achieve rapid cooling. First, the precipitation amount of ferrite can be controlled, and second, the degree of supercooling can be increased.The strength of 82B wire rod with special requirements can be improved by setting higher spinning temperature and increasing cooling rate.
With the increase of the spinning temperature, the tensile strength of the wire rod increases obviously, which seems to be in contradiction with the theory that the higher the spinning temperature, the coarser the grain size, and the lower the spinning temperature, the finer the grain size.In fact, this kind of steel after micro-alloying, and in the rolling process of temperature control rolling, after recovery and recrystallization, deformation austenite grain has been quite fine, even if the increase of the spinning temperature, the degree of grain coarsening is relatively small, so the impact on the strength can be ignored.It should be noted that the spinning temperature should not be too high, otherwise due to the limitation of air cooling equipment, the structure transformation of high carbon steel wire can not be completely ended in the cooling control line, not only can not get the expected structure, but also easy to form a thick oxide iron sheet on the surface of the wire rod, which is not conducive to drawing, so that the comprehensive performance of the wire rod is reduced.It is necessary to reasonably control the spinning temperature and cooling rate after rolling to obtain the microstructure of fine sorbite and make the finished product have higher strength and good plasticity.
Organization of 82 b wire rod inspection, found that as the S + P, F group, conform to the requirements of the standard, but general sorbite rates are around 85%, with the standard requirement, there is still a gap of 95% or more, its heart of sorbite rate compared with shagang group, baosteel's low, only about 65% (shagang group 75%, 85% baosteel), the pearlite group is bigger, grain is relatively bulky, therefore, controlled cooling after rolling process of 82 b wire rod also need to be further optimized
standard export packing (inside:water proof paper,outside:steel covered with strips and pallets)