Different steel grades, different slab specifications, the use of different rolling mill types, rolling at different rolling speeds, for rolling different thicknesses of finished products, the use of different billet heating temperature and billet heating time.
Based on the crystal phase structure of the finished product at different temperatures, combined with the thermal stress analysis of stainless steel rolling, and then refer to the iron carbon phase diagram, the final rolling temperature of different thicknesses of the finished product is formulated, and then the baking temperature of the slab is calculated by establishing a thermal model of the rolling process, so as to provide heating basis for various forms of heating and rolling.
1. The basis for determining the heating temperature of stainless steel
For the pressure processing of metals, the heating before metal rolling is to obtain good plasticity and less deformation resistance, and the heating temperature is mainly determined by the plasticity and deformation resistance of the metal according to the processing process requirements. Different thermal processing methods have different heating temperatures.
The plasticity and deformation resistance of a metal mainly depend on the chemical composition, microstructure, temperature and other deformation conditions of the metal. Among them, the general situation of temperature influence is that, with the increase of temperature, the plasticity of the metal increases, and the deformation resistance decreases, which is because the temperature rises, the thermal motion of the atoms increases, and the binding force between the atoms weakens, so the deformation resistance is reduced, and a new slip system can be added, and the thermal deformation process is accompanied by the recovery and recrystallization softening process, which improves the plastic deformation ability of the metal. However, as the temperature rises, the plasticity of the metal does not rise in a straight line, because the phase state and grain boundary also change, and this change has an impact on the plasticity.
The heating temperature of the steel cannot be too low, and it must be ensured that the steel can still maintain a certain temperature (that is, the final rolling temperature) at the end of the pressure processing. Due to the best plasticity of the austenite structure, if processed in the single-phase austenite region, the deformation resistance of the metal is minimal, and the residual stress after processing is minimal, and defects such as cracks will not occur. For carbon steel, this area is 30-50 °C above AC3 in the iron carbon balance chart and 100-150 °C below the solid phase line, and the minimum heating temperature of the steel can be determined according to the final rolling temperature and then consider the heat loss of the steel in the process of production and processing. The final rolling temperature of steel has a great influence on the microstructure and properties of steel, the higher the final rolling temperature, the greater the tendency of grain agglomeration and growth, the coarser the grain of austenite, the lower the mechanical properties of steel. Therefore, the final rolling temperature can not be too high, according to the iron carbon phase diagram is best at about 850 ° C, it is best not to exceed 900 ° C, nor less than 700 ° C.
The heating temperature of the metal generally needs to be determined comprehensively with reference to the metal's state phase diagram, plasticity diagram and deformation resistance diagram. The heating temperature of the rolling is determined according to the solid phase line, because the phenomenon of overburning is related to the temperature at which the metal begins to melt. If there is segregation and non-metallic inclusion in the steel, it will cause the melting point to decrease. Therefore, the maximum temperature of heating should be 100-150 ° C lower than the solid phase line.
Stainless steel belongs to a kind of high-alloy steel, the steel contains more alloying elements, alloying elements also have a certain impact on the heating temperature of steel, one is the influence of alloying elements on the austenite region, and the other is the influence of carbide generation.
For alloying elements in stainless steel such as nickel, copper, cobalt, manganese, etc., they all have the same face center cubic lattice as austenite, which can be dissolved in an infinite amount in austenite, so that the austenite region is expanded, the final rolling temperature of steel can be correspondingly lower, and because the solid phase line is improved, the rolling temperature (that is, the maximum heating temperature) can be appropriately increased. For high-alloy steels such as stainless steel, the heating temperature should not only refer to the phase diagram, but also be determined according to the plasticity diagram, deformation resistance curve and metallographic structure.
The rolling process also has certain requirements for heating temperature. The more times the rolling passes, the greater the temperature drop in the middle, the heating temperature
Should be slightly higher. When the cross-sectional size of the steel is larger, the rolling mill bites into the more difficult, the rolling number of passes must be more, so for the steel billet with a large section or difficult to bite into, the heating temperature should be correspondingly higher. The processing method is different, and the heating temperature is also different. For hot-rolled sheets, the heating temperature should not be too high, otherwise it is easy to appear adhesion during the rolling process.
The alloy state diagram is an important basis for selecting the heating temperature. Taking some binary alloy state diagrams as an example, the solid phase line determines the upper limit of the heating temperature, in order to prevent the metal from overheating and overheating, the upper limit temperature is 100-200 °C lower than the melting point, which is equivalent to 0.8-0.9 times the melting point of the alloy. The lower limit of the heating temperature is determined by the final rolling temperature. For alloys in the completely solid solution state, there will be no solid phase change with the decrease of temperature, and the final rolling temperature is generally equivalent to 0.6-0.7 times the melting point of the alloy, which can ensure the plasticity and deformation resistance required by thermal processing. However, there are exceptions, some alloys are brittle and hard in the single-phase region, and the plasticity is poor, while the plasticity is better in the two-phase area, and the heating temperature is set in the two-phase area is better. It can be seen that the alloy state diagram can only give a general temperature range, whether it is appropriate, but also refer to the plasticity diagram of the metal.
Plasticity diagram is the main basis for determining the heating temperature, it gives the highest temperature range of metal plasticity, and the upper limit of the heating temperature should be taken near the area with the highest plasticity.
After determining the heating temperature range according to the state diagram and plasticity diagram, it is also necessary to use the deformation resistance map (deformation resistance change curve with temperature) to correct it to ensure that the entire thermal process is completed within the range of the minimum metal deformation resistance.
2. Stainless steel is different from the characteristics of carbon steel in the heating process
1) Essential coarse grain steel in 700-800 degrees when the grain begins to grow, but the essence of fine grain steel in the 930-950 degrees temperature is not enough to grow, only after exceeding this temperature began to coarse, and with the temperature continues to rise, its growth trend is greater than the essence of the coarse grain.
2) For the grain of steel coarse, heating temperature and time has a decisive role, alloying elements increase the tendency of grain growth, according to the degree of influence of the order of mn, P, C, reduce the grain growth tendency is V, Ti, Ai, Zr, W, Mo, Cr, Si, Ni, most alloy steel structures of the overheating sensitivity is lower than carbon steel.
3) Ferritic stainless steel carbon content is generally lower than 0.12%, containing 12%-30% of Cr, then higher than martensitic, its structure is basically ferrite, it is heated to a higher temperature only a small part of the transformation into austenite, most of them are still ferritic, chromium content in the heating process generally does not occur phase change, the higher the chromium content, the plasticity and corrosion resistance is improved, but its annealing or normalizing after the tissue is composed of ferrite and a small amount of carbide, the higher the carbon content, the higher the hardness and wear resistance. When the chromium content of ferritic stainless steel exceeds 17%, brittleness, б phase brittleness and high temperature brittleness occur at 475 degrees.
4) Stainless steel is most likely to produce intergranular corrosion when the temperature of the heat affected zone is 600-800 degrees when welding.
5) Austenitic stainless steel belongs to the face center cubic structure, the expansion coefficient is about 1.5 times that of carbon steel, the thermal conductivity is about 1/3 of carbon steel, and the specific resistance is about 4 times that of carbon steel. The thermal conductivity of high-network stainless steel is about 1/2 of carbon steel compared with carbon steel, and the specific resistance is about 3 times that of carbon steel. Martensitic stainless steels need to be preheated before heating, because of their low thermal conductivity and the hard and brittle surface heat affected area. When ferritic stainless steel is heated to 900 °C, the grains in the hot area become significantly thicker, making their elongation and toughness at low temperatures worse, and cracks are easy to occur after cooling.
6) Low-carbon chromium stainless steel with chromium content greater than 14%, any chromium stainless steel with carbon content containing 27% of chromium dry, and stainless steel with molybdenum, titanium, niobium, silicon, aluminum, tungsten, vanadium and other elements added on the basis of the above components, the chemical composition of the elements that form ferrites is absolutely dominant, and the matrix structure is ferrite. The structure of such steel in the quenched (solid solution) state is ferrite, and a small amount of carbides and intermetallic compounds can be seen in the annealed and effective state of the tissue. Ferritic stainless steel because of the high chromium content, corrosion resistance and oxidation resistance are better, but the mechanical properties and process performance are poor, mostly used for acid-resistant structures with little stress and used as antioxidant steel.
7) Martensitic steel
Such steels are in the y-phase zone at normal quenching temperatures, but their y-phase is only stable at high temperatures, and the M-point is generally around 3OO °C, so it is transformed into martensitic when cooling.
The mechanical properties, corrosion resistance, process properties and physical properties of martensitic stainless steel are similar to those of ferritic-martensitic stainless steels containing 12 to 14% chromium. Since there is no free ferrite in the tissue, the mechanical properties are higher than those of the above steel, but the overheating sensitivity during heat treatment is low.
8) Martensitic-carbide stainless steel
The carbon content of the fe-C alloy and the decomposition point is 0.83%, and in stainless steel, steel containing 12% chromium and more than 0.4% carbon due to the left shift of the S point by chromium, and steel containing 18% chromium and more than 0.3% carbon are all hyperionic steels. This type of steel is heated at normal quenching temperature, and the secondary carbide cannot be completely dissolved in austenite, so the structure after quenching is martensitic and carbide.
There are not many stainless steel grades belonging to this category, but some stainless steels with relatively high carbon content, and 3Crl3 steel with an upper carbon content is quenched at a lower temperature, and such a structure may also occur. Due to the high carbon content, although the steel contains more chromium, its corrosion resistance is only comparable to that of stainless steel containing 12 to 14% germanium. The main uses of this type of steel are parts that require high hardness and wear resistance, such as cutting tools, bearings, springs and medical devices.
9) Ferritic stainless steel
Ferritic stainless steel is a stainless steel with a ferritic structure in the state of use. The chromium content is 11% to 30%, and it has a body-centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements, this kind of steel has a large thermal conductivity, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance and other characteristics, mostly used to manufacture parts that are resistant to atmosphere, water vapor, water and oxidative acid corrosion. This type of steel has shortcomings such as poor plasticity, post-weld plasticity and corrosion resistance are significantly reduced, thus limiting its application.
11) Martensitic stainless steel
Martensitic stainless steel can adjust its mechanical properties through heat treatment of stainless steel, colloquially speaking, is a type of hardenable stainless steel. The hardness is high after pure fire, and different tempering temperatures have different combinations of strength and toughness.
3. The necessity of establishing a mathematical model of the stainless steel thermal process
1, ferritic stainless steel rolling development is more difficult, which is mainly related to ferritic steel in the heating process, grain growth tendency. Grain growth, grain boundaries become less, the binding performance deteriorates, will deteriorate the thermal processing performance, therefore, it is necessary to establish a mathematical model of the heating and rolling process, its highest heating temperature and rolling system is strictly controlled. If stainless steel is also heated like ordinary alloy steel, various product defects will occur in thermal processing.
2. Through the mathematical model, the temperature curve of the stainless steel slab in the furnace and the time in the furnace are optimized, and under the premise of maximizing the output, the heating process of the slab adopts the method of slowing down and then fasting before and after the slab to prevent the overheating and overcoating of the slab in the furnace and the phenomenon of coarse grain size.
3. According to the statistics on the production process of stainless steel band crimping loss, product quality mainly occurs in the edges and head and tail of the coil.
In the rolling process of the roughing mill, the strip needs to be rolled, the heat dissipation area of the edge of the strip is larger, after the roughing mill is rolled, the temperature difference between the head and tail, the edge and the middle of the plate is generally above 50 degrees, after the finishing of the finishing mill, this phenomenon is more obvious, the temperature difference between the head and the middle of the plate is generally above 150 degrees, the temperature difference between the edge and the middle of the plate belt is generally above 100 degrees, the temperature of the head and tail of the plate, the edge is low, its plastic deformation is poor, and the rolling process is prone to problems. This requires the establishment of a mathematical model of the rolling process and the accurate control of the rolling temperature of the strip.
4. In order to control the heating temperature of both ends of the slab in the heating furnace, according to the layout and temperature characteristics of the lower furnace burner, the cloth rules of the slab and the spacing between the front and back slabs are reasonably formulated. At the same time, according to the position of the discharge end of the furnace head slab close to the discharge furnace door, the discharge end wall is not arranged with a heating nozzle, its temperature is low, and its position is equipped with a camera and laser locator, resulting in a lower furnace temperature at the place. In order to prevent the temperature of the heated slab in the furnace from decreasing here, the position of the furnace head blank should be reasonably controlled, and the measures of the slab retreat to avoid the low temperature area should be taken when the rolling is stopped to ensure the normal heating of the furnace head slab. At the same time, the hot air flow of the lower furnace chamber nozzle is floated up, which is easy to pass through the gap between the slabs, resulting in a high furnace gas temperature in the area with a large gap, and a low furnace gas temperature in the area with a small gap, which is easy to cause uneven heating of the slab.
5. Reasonably control the atmosphere in the furnace to reduce oxidative burn loss. The iron oxide sheet of steel containing Ni3% to 6% is "inter-embedded" with the metal. Because Ni is more difficult to oxidize than Fe, during the formation of the scale of fe-Ni alloys, Fe in the solid solution is preferentially diffused (outward and oxidized), thus making Ni partially enriched in the inner surface of the oxide layer. The iron around Ni is also oxidized after all at the hot working temperature, so the metal matrix and the oxide scale are seriously "interspersed" in the state of enriching Ni, and the iron oxide skin is difficult to fall off.
In order to solve the problem of nickel-containing mild steel oxide peeling off and reducing oxidative burn loss, the method of less oxidative heating is adopted, that is, the open flame type oxidation-free heating furnace is used to heat the slab, at which time the generated iron oxide sheet is difficult to fall off, but because the iron oxide sheet is very thin, the surface quality of the plate is not affected after pickling.
6. The mathematical model is used to accurately control the various parameters in the production process of the heating furnace, make full use of the equipment rolling time, fixed repair time, rolling line cleaning time, and steel change time, check and adjust the control accuracy of the equipment in time, ensure that the equipment function is perfect, the control is accurate, the water distribution is reasonable, the rolling model is calculated accurately, the plate tracking and positioning is accurate, the tension control is reasonable, the plate shape is good in the plate rolling process, the width and thickness control are accurate, and the abnormal phenomenon of no plate head and tail deviation and sleeve or pulling off occurs.
7. Study the transition between stainless steel and carbon steel and other steel grades before and after entering the furnace. Before loading the stainless steel slab behind the carbon steel billet, according to the different heating characteristics of carbon steel and stainless steel, there should be a certain vacancy between carbon steel and stainless steel, and after the stainless steel is loaded, a certain vacancy should also be left in the furnace before loading carbon steel, so as to provide heating time for subsequent rolled carbon steel.
8, the bending roll force and string roll position of the finishing mill has a greater impact on the plate shape, and must be optimized and adjusted; the plate and strip side blowing adjustment is very important for the temperature drop in the rolling process, which is related to the level of the surface temperature of the strip, and the surface cooling water of the strip should be blown as the benchmark, and the phenomenon of excessive adjustment or uneven distribution cannot be produced; the amount of pressure of the last pass of the fine rolling should be appropriately reduced to ensure the plate shape of the finished strip.
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