What is steel ingot?
What is steel ingot?
The molten steel is poured into the mold through ladle to form ingot. After the molten steel is smelted in the steelmaking furnace, it must be cast into ingots or billets of a certain shape before processing. The technological process of casting ingot with mould is called ingot for short.
After smelting, except for a few direct castings, most of the steel should be cast into ingots first, and then rolled into various steels, such as plates, bars, pipes, strips, flanges, etc. Heat treatment is required for the manufacture of tools and certain machine parts. Steel ingot is still the main raw material for steel rolling. The quality of steel ingot, the condition of ingot shape and its weight play an important role in steel rolling.
Casting process of steel ingot
Ingot casting can be divided into upper casting method (Figure 2) and lower casting method (Figure 3). Generally, the internal structure of the upper ingot is better, the inclusions are less, and the operation cost is lower; the surface quality of the lower ingot is good, but the inclusions in the steel increase because of the middle injection pipe and the channel.
Fig.2 Upper casting method
Figure.2 Lower casting method
The size of ingot depends on many factors, such as the capacity of steelmaking furnace, the ability of blooming mill, the size of steel and the characteristics of steel grades. The ingots used to produce bars and profiles are generally square sections (called square ingots); the ingots used to produce plates are generally rectangular sections (called flat ingots); the ingots used to produce forging materials are square, circular and polygonal.
After the liquid steel enters into the ladle, it needs to be left for a period of time, so that the slag or other impurities mixed in the steel can be floated and removed during tapping, and the casting temperature can be adjusted at the same time.
Casting temperature shall be strictly controlled. If the casting temperature is too low, the surface of the ingot will solidify immediately after the liquid steel is put into the mold, which will cause the surface defects of the ingot, or even the liquid steel will start to solidify in the steel ladle, resulting in metal loss or scrap of the whole furnace of steel; if the casting temperature is too high, the formation time of the ingot surface will be delayed, resulting in hot cracks of the ingot. For killed steel, the casting temperature is generally controlled at 40-60 ℃ higher than the liquidus temperature of the steel. In order to ensure the uniform temperature of the whole ladle, argon can be blown into the ladle to stir the molten steel.
The down casting method generally requires proper casting speed to ensure the stable rise of molten steel in the mold and adjust the casting temperature. Slow injection is used when the casting temperature is too high and fast injection is used when the casting temperature is too low. The casting speed should be controlled to reduce splashing.
During casting, oxygen in the atmosphere will enter the ingot, which will cause the secondary oxidation of the molten steel and reduce the quality of the steel. When casting high quality steel, inert gas argon is needed to protect the steel flow contacting with air, and synthetic solid slag powder is used to protect the rising steel level in the mold.
The volume shrinkage caused by the solidification of the killed ingot body needs to be supplemented by the liquid steel in the cap, so the casting time of the cap can be appropriately prolonged. Generally, the injection speed of hat head is about twice as slow as that of spindle body.
Defects of steel ingots
Shrinkage cavity: the shrinkage cavity formed when the molten steel contracts in the steel mold is called shrinkage cavity.
Transverse crack: refers to the transverse crack on the surface of the ingot, generally on the ingot, generally in a shallow depth, which can be removed by fine grinding.
Longitudinal crack: refers to the longitudinal crack on the surface of the ingot, generally in the upper part and corner of the ingot, and the crack on the upper part is very deep, which is difficult to be eliminated by grinding.
Scab: scab refers to the splashing and sticking of steel ingot surface, shell like or rumen like metal, and the scab often appears in the lower part of ingot.
Double skin: at the edge of the low power test piece, when there is an irregular dark loose color, a large amount of oxide inclusions (mainly ferrous oxide) gather around it, which is called overturning.
Surface inclusion: refers to the non subtractive inclusion embedded on the surface of ingot.
Surface porosity: refers to the small holes exposed to the naked eye on the surface of the ingot, which are mostly found in the middle and lower part of the ingot, generally not deep, and can be removed by finishing.
Cracking: the cracking on the surface of a ingot when it is extremely cold. The cracking is often accompanied by noise, so it is called cracking.
Rising: the head of the ingot is irregularly raised. This defect is called rising or rising.
Reticulation: the reticulation bulge on the surface of ingot is called reticulation.
Pockmarks: pockmarks on the surface of ingots are called pockmarks.
Double pouring: obvious traces of reconnection around the steel ingot surface.
Flash: the metal sheet which is perpendicular to the surface of ingot at the head or tail of ingot is called flash.
Flying wing: the thin sheet formed on the surface of the ingot perpendicular to the ingot surface is called flying wing.
Bubble: bubble is the defect of ingot or just caused by poor degassing and deoxidization of molten steel or wet raw materials of steel injection system. It is generally divided into subcutaneous bubble and internal bubble. (prevention method: the molten steel shall be well oxidized, boiled, deoxidized and deoxidized, the furnace and all refractory materials in contact with the molten steel shall be dry, the tapping tank, the bun, the handboard, the mold, etc. shall be dry, the rust spots of the mold shall be cleaned, and the oil shall be thinly coated.)
White spot: white spot is actually a kind of tiny crack. It is a zigzag crack with irregular radioactivity on the transverse low-power sample just now. It is a silver bright spot of circular or elliptical star on the longitudinal low-power sample, so it is called white spot. (the main cause is hydrogen)
Classification of steel ingots
Generally, ingots can be divided into the following categories:
UN clean steel ingot
The ingot has only been deoxidized in the most basic (minimum) way. During ingot solidification, a metal layer close to pure iron is formed around and at the bottom of ingot mold, while carbon, sulfur and phosphorus are deflected near the central shrinkage hole. The oxygen in the metal will form carbon monoxide coil and generate pores in the metal, but the pores will disappear in the hot rolling process. The biggest advantage of unclean steel is that it can obtain a few flawless steel surfaces, which are mostly pure iron. The carbon content of most of the raw steel is less than 0.1%.
Cap steel ingot (cap steel ingot)
The oxygen in the liquid causes the deflection near the shrinkage center of the ingot, which is lower than that of the raw steel, with beautiful surface, uniform internal chemical composition and better mechanical properties.
Fully clean steel ingot
The oxygen in the liquid is completely removed before solidification to avoid the edge effect of unclean steel. The deoxidization process is to add Fe Si alloy to the molten steel to make the oxygen in the molten steel reverse generate slag and obtain uniform molten metal.
Semi clean steel ingot
The semi clean steel is between the UN cleaned steel and the fully cleaned steel. A small amount of iron silicon or aluminum is added into the molten steel as the deoxidizer, which is just enough to remove the edge effect of the UN cleaned steel and make the oxygen get full decomposition.
Vacuum degassing steel ingot
By vacuum treatment, no deoxidizing elements are added, so that the liquid steel can get full deoxidizing effect and no non-metallic intermedium can be formed in the steel. The treatment process is to increase the carbon content in the steel first, and then pour the liquid steel in vacuum. At this time, carbon and oxygen will react to form carbon monoxide, which will gradually reduce the carbon and oxygen in the liquid steel to the specified level. The steel is very clean because no deoxidizing elements are added in the process, which will generate solid oxides.
In addition, due to the difference of oxygen content in molten steel before casting, ingots can be divided into three basic types: killed steel, rimmed steel and semi killed steel.
Sedation steel ingot
It is also called fully deoxidized steel. It is a steel whose oxygen content in the molten steel is so low that it will not react with carbon in the steel to form carbon monoxide bubbles. Before casting, the molten steel must be deoxidized sufficiently, such as deoxidizing with silicon and aluminum. The silicon content in steel is about 0.3%, and the aluminum content is (0.02-0.06%). There are shrinkage holes in the killed ingot, which must be cast in the ingot mould with heat preservation cap. After rolling, the billet rate of ingot is 85-89%. This kind of ingot is required for steel with uniform composition and dense structure. The mould with heat preservation cap is used for killed steel. In recent years, heating and heat insulation cap and heat insulation plate heat insulation cap are widely used to improve the rate of green.
Rimmed steel ingot
The liquid steel has high oxygen content (0.02-0.04%), strong carbon oxygen reaction and carbon monoxide bubble formation in ingot mold, which makes the liquid steel boil in the mold. As soon as the steel solidifies, bubbles form and float up. The surface of the ingot solidified into a pure shell containing iron. When the surface layer reaches the required thickness, a cover plate is added on the top of the ingot to make the top solidify and prevent the bubbles from escaping; deoxidization such as ferrosilicon and aluminum can also be added on the top for chemical capping; a bottle mouth type ingot mold can also be used for capping. Another method is to add aluminum to the whole molten steel to deoxidize after the molten steel solidifies into a surface layer. This kind of ingot is called external boiling internal steel. The boiling steel is generally made of a bottle mouth mold with a small upper and a large lower opening. The billet rate of rimming ingot is as high as 90-92%, which is mainly used for low carbon steel.
Semi killed steel ingot
Steel grade between killed steel and rimmed steel. This kind of steel has less internal gas and its structure is close to that of killed steel. In the initial stage of semi killed steel casting, there is no bubble. When the top is naturally solidified and capped (the bottle mouth mold can be used to promote capped), due to the enrichment of carbon and oxygen in the molten steel and the decrease of temperature, a small amount of carbon monoxide bubble is generated at the top of the ingot, filling the solidification shrinkage space of the whole molten steel. Therefore, the billet rate of ingot similar to that of rimming steel can be obtained. Semi killed steel is mainly used for medium carbon content and medium quality structural steel, and the mold used is generally open type with small upper and large lower.
The ingot after demoulding and casting can only be demoulded after the internal solidification is completed. For alloy ingots with strong crack sensitivity, after demoulding, they shall be put into the slow cooling pit in the hot state (> 900 ℃) for heat preservation and slow cooling, or hot into the soaking pit or heating furnace in the steel rolling workshop at a temperature not lower than 750 ℃.
Use of steel ingots
Ingots are widely used. According to the types of products forged and rolled, ingots can be divided into the following categories.
Electric power steel
Electric power steel includes nuclear power, thermal power, wind power, hydropower and other steel.
Nuclear power: for the core components of nuclear island, such as evaporator, main pipeline, core support plate, elbow plate, low-pressure rotor of generator set and other large components, in addition to the requirements of high strength, high toughness, high homogeneity, some also require nuclear radiation resistance, most of which are made of electroslag Remelting Ingot, with the maximum ingot weight of 715t.
Hydropower: the rotor, frame and lock gate of the generator used in large-scale hydropower station are required to be of high strength, toughness and cleanliness.
Thermal power: the steam drum, generator base and rotor of supercritical thermal power generator require high temperature strength, high cleanliness and creep resistance.
Wind power: used for the manufacture of base, pole and wind turbine shaft.
Steel for machinery manufacturing
Machinery manufacturing industry is the largest user of ingots, especially China has become the world’s largest machinery manufacturing country. Large machinery not only needs to meet domestic demand, but also needs to be exported on a large scale. Among them, the large-scale mining crusher, ball mill, large-scale blast furnace shell, converter supporting ring, rotating shaft and transmission system, working roll, supporting roll and universal joint shaft of wide and thick plate mill, main motor shaft, floating mandrel and limiting mandrel of seamless steel tube mill and rolls, bearings and bearing blocks of other kinds of mills are all forged by steel ingots; the large-scale mining machinery The same is true for excavator bucket, shovel arm, crane boom, etc.
High rise building steel
Emerge in an endless stream with high-rise buildings such as Burj Dubai and Shanghai center, with the development of construction industry. In order to meet the needs of earthquake resistance, wind resistance and fire protection, high-rise buildings over 200m are mostly made into the main frame of steel structure, whose structure can be made of extra thick plates rolled from steel ingots, which are cut, processed and welded, requiring high strength, earthquake resistance and Z-direction performance.
What is the purpose of annealing ingots?
Instant annealing of ingot to eliminate internal stress
Some of the high carbon martensitic steel and ledeburite steel ingots with high crack sensitivity must be put into the annealing furnace immediately to eliminate the internal stress annealing, in addition to the direct opening of the red rotary heating furnace, so as to prevent cracks.
The ingot is softened and annealed for finishing
The surface defects of ingot need to be removed, the hardness can be reduced by annealing, and it is easy to finish and peel.
High temperature diffusion annealing of ingot makes the structure and composition uniform
Through high temperature diffusion annealing, the homogeneity of structure and chemical composition in ingot can be solved. The main solution here is to solve the microsegregation and dendrite segregation in the solidification process of steel, which can be improved by diffusion annealing.
Conventional cooling process of steel ingot
It is suitable for high-quality steel and alloy steel ingots without hot delivery, especially for some kinds of ingots which are easy to harden and have high thermal stress and structural stress when they are cooled. The purpose is to eliminate or slow down the thermal stress and structural stress of the ingot, improve the coarse and uneven as cast structure of the ingot, reduce the hardness of the ingot, and facilitate the surface finishing treatment of the ingot.
There are three methods for slow cooling of ingot: heap cooling, mould cooling and pit cooling. The reasonable selection can be made according to the steel properties, ingot size and production conditions.
Stacking cooling is to stack the steel ingot after demoulding in a dry and windproof place and cool it naturally in the air. The cooling speed is fast, and it is suitable for pearlitic steel ingots with carbon content less than 0.3% and ferrite steel and austenitic steel ingots without structural transformation during cooling, such as 08-20, 10-20mn, 15-20cr, 12-20crmo, 15-20nimo, 1Cr18Ni9Ti, 4Cr14Ni14W2Mo and other steel grades.
Mould cooling means that after the ingot is decapitated, it is stacked together with the mould in a dry and windproof place for cooling. When the ingot body is cooled to below 300 ℃, it is demoulded and air cooled. The cooling rate of die cooling is smaller than that of air cooling, generally 120-140 ℃ / h for small ingots below 1t and 80-120 ℃ / h for 1-5t ingots, which is suitable for pearlitic steel with carbon content less than 0.3% and more than 0.5%, such as some larger ingots such as carbon structural steel, spring steel, ball steel, carbon tool steel and some ferritic steel, and ball steel with carbon content less than 0.3% and ingot weight less than 600kg in alloy structural steel Smooth steel, etc.
Pit cooling refers to the rapid demoulding after the ingot is solidified, and the ingot is vertically stacked in the slow cooling pit above 700 ℃, which is immediately covered with slow cooling, and the ingot can be discharged only when the ingot body is cooled to below 300 ℃. The average cooling speed of pit cooling is smaller. Generally, the average cooling speed of small ingots below lt is 30-50 ℃ / h, and that of 1-5t ingots is 20-30 ℃ / h. Pit cooling can replace mould cooling and stack cooling and is suitable for any steel grade. However, the investment in the construction of steel-making workshop with pit cooling is large, and the passing capacity of ingot is low. Generally, the pit cooling time of ingots is 16-24h (the upper limit of large ingots). Each electric furnace is usually equipped with 6 slow cooling pits, and each pit can be equipped with a furnace of ingots. For the medium carbon pearlitic steel and ingot with cracking tendency and ingot weight more than 600 kg, the semi martensitic steel with low carbon content must be pit cooled.
Large ingots of semi martensitic steel with low carbon content, martensitic steel, hypereutectoid steel with high carbon content and ledeburite steel ingots need to be annealed. For some steel grades, the small ingot does not need to be annealed, while the large ingot needs to be annealed. For example, the ingot with t10-t13 more than 1100kg and the ingot with 30-35crmnsi more than 1600kg need to be annealed.
Ingot annealing is divided into hot delivery annealing and cold delivery annealing. Hot delivery annealing is that the ingot is put into the annealing furnace with the furnace temperature above 650-700 ℃ after demoulding, and it is suitable for martensitic steel which is particularly sensitive to cracks, such as 3Cr2W8V, 30-37crni3, 25Cr2MoV, etc. Cold delivery annealing is to install annealing furnace in a short time (generally no more than 3 days) after the ingot is cooled by die or pit cooling; it is suitable for semi martensitic steel and small martensitic steel ingots with small crack tendency, such as Cr12MoV, W9Mo3Cr4V, 20cr2ni4, etc. The annealing system of ingot depends on the nature of steel.
New process of stress relief annealing for alloy steel ingot:
- A. The two-stage annealing process is used for medium and high carbon high alloy steel ingots. The process is as follows: slowly heat the ingot to 800-860 ℃ for 2-4h, then cool it to 720-780 ℃ for 1.5-3h, then cool it to 300 ℃.
- B. A one-stage low-temperature annealing process is used for 650kg to 6T chromium nickel high hardenability ingot. The process is: slowly heat the ingot to 620-680 ℃ for one-stage heat preservation for 5-10h, and cool it to 300 ℃ for discharging.
- C. The process of two-stage annealing for low and medium alloy tool steel ingot is as follows: slowly heat the ingot to 780-850 ℃ for 3-6h, then cool it to 650-720 ℃ for 2-4h, then close the furnace and cool it to 300 ℃.
- D. For 540kg to 2T stainless steel and heat-resistant steel ingots, a low-temperature annealing process is used. The process is as follows: slowly heat the ingot to 800-860 ℃ for 2-6h, and then cool it in closed furnace to 300 ℃.
- E. For 1.6-10t low alloy structural steel ingots, the pseudo two-stage annealing process is used. The process is as follows: slowly heat the ingots to 780-860 ℃ for 2-5h, then cool them to 700-760 ℃ for 2-6h at 50 ℃ / h, and finally cool them to 500 ℃.
Slow cooling annealing process of low carbon alloy steel ingot:
After demoulding, the ingot is put into the furnace and heated up. When the temperature is raised to 800-900 ℃, the ingot is kept warm and cooled in the furnace. When the temperature is reduced to 760 ± 15 ℃, the ingot is kept warm; when the temperature is reduced to 650 ± 15 ℃, the ingot is put out of the furnace and the temperature is ≤ 200 ℃.
As the temperature of ingot demoulding is increased, the ingot will be demoulded before it is precipitated along the crystal ferrite. After demoulding, the ingot will be heated and isothermal to eliminate the stress caused by the temperature difference between inside and outside, and then it will be gradually isothermal or cooled in the annealing furnace until the room temperature, so as to significantly reduce the thermal stress and structural stress in the process of ingot cooling, so that it will not generate large stress all the time, avoiding The crack is avoided.
Steel ingot cracking and prevention methods?
Cracking of steel ingots
After the molten steel is poured into the ingot mold, it will crystallize and solidify for cooling. After the ingot is demoulded, it will be cooled in the workshop. Because the ingot size is large, the temperature difference between the inside and outside of the ingot will be larger as the temperature keeps decreasing. Because the cooling speed of each part is different, the temperature difference between the inside and outside of the ingot will form thermal stress. When the ingot is cooled below the critical point, a series of phase transformation processes will occur, such as pearlitic fraction Solution, bainite transformation, martensite transformation, etc. these transformations cause volume expansion. If the temperature inside and outside the ingot is different, the transformation will not be carried out at the same time. The transformation will cause volume change, and the cooling will not be isochronous, which will produce structural stress. The superposition of thermal stress and structural stress constitutes the internal stress in the ingot. This stress generally increases with the content of alloy elements in the steel and the quality of the ingot Add and increase. For alloy steel, especially bainite steel, as well as high alloy steel, such as high chromium steel, high speed steel and other steel ingots, even if they are cooled slowly after pouring, bainite transformation and martensite transformation will still occur during the cooling process, resulting in great structural stress. With the decrease of ingot temperature, the internal stress becomes larger and larger. If the cooling process is improper, or annealing is not carried out in time after cooling to eliminate internal stress, there is a risk of self cracking or even explosion during cooling or storage, resulting in serious accidents and economic losses.
Many alloy steel ingots need to be annealed to eliminate internal stress, prevent cracking, reduce hardness and facilitate grinding and cleaning of ingot surface. In the past, the ingot annealing process was old, the heating temperature was too high, the heating time was too long, the energy consumption was large and the productivity was low. Therefore, a new annealing process for ingots has been developed.
Development of steel ingot annealing process
The thermal stress caused by the temperature difference inside and outside the ingot can be calculated. For example, if 700kg ingot is heated to 910 ℃ at the heating speed of 100 ℃ / h, the maximum temperature difference within the range of 400 ~ 500 ℃ is calculated to be 160 ℃. The estimated thermal stress is σ = 291mpa. When the ingot is below 550 ℃, this stress will not lead to cracking. Therefore, it is suitable to heat the ingot at the speed of 50 ~ 100 ℃ / h.
It is also very important to control the cooling rate of annealing. The cooling rate of annealing is generally furnace cooling or controlled slow cooling. The cooling rate is mostly 50 ~ 70 ℃ / h.
According to the transformation kinetics curves of various steels, the time required for transformation can be determined, and the annealing holding time can be selected. Generally speaking, the transformation recrystallization time of various steels above the critical point is very short, and the holding time does not need to be very long.
With the increase of annealing temperature, the activity of atoms increases, the dislocation density decreases with the movement of dislocations, the twins decrease until they disappear, and the internal stress decreases with the recovery and recrystallization.
With partial or complete recrystallization, the second and the third kind of internal stresses have disappeared. The first kind of internal stress, namely macro internal stress, is also gradually relaxed. With the increase of heating temperature, the stress of ingot changes from elastic body to plastic body, and it will relax continuously. After transformation recrystallization, the internal stress can be eliminated by 80% ~ 90%.
The selection of ingot annealing temperature is mainly based on the principle of eliminating internal stress. The internal stress can be eliminated by heating in low, medium and high temperature areas. Because reducing the surface hardness of ingot to facilitate grinding and cleaning is also one of the purposes of annealing, so the annealing temperature is selected in the high temperature zone. At the same time, a little higher temperature is beneficial to eliminate the internal stress and improve the productivity. However, the heating temperature is not necessary to be too high. The annealing temperature is near the critical point. If AC1 is slightly up or down, or heated to AC3 first, and then cooled to below AC3 rapidly in the furnace, the temperature of AC1 is isothermal.
The elimination of internal stress is a process of stress relaxation, which is essentially a process of high temperature creep. The speed of stress relaxation depends on the temperature. With the increase of heating temperature, the rate of stress relief increases. For example, ordinary cast iron and high alloy cast iron can completely eliminate the internal stress in the as cast state at 700 ℃. When the temperature is kept between 600 ~ 700 ℃, the effect of holding for 1H and 48h is almost the same, and it is unnecessary to hold for too long.
Therefore, when the steel is annealed at 650 ~ 700 ℃, the internal stress can be basically removed in about 1H. More data show that the quenching internal stress of any steel can be eliminated by more than 90% only after tempering at 550 ℃.
How to prevent the crack in steel ingot?
The hot crack produced in the solidification process of ingot can not be welded after forging and rolling, which shows the defects of axial intergranular crack, hairline, white spot and so on.
The preventive measures for the cracks in the billet are as follows.
Prevention of intergranular crack of axis
Axial intergranular cracks are common in austenitic and martensitic stainless steels. Most of them are found in steel billets with serious dendrite structure. The cracks develop radially from the center to the outside along the main dendrites. When the cracks are serious, they connect with each other in a network.
The main reason for this kind of defect is that the pouring temperature of ingot is too high and the solidification speed of molten steel is too fast, which leads to the hot crack caused by large internal stress. At the same time of reducing pouring temperature, the ratio of height to width of ingot should be reduced or the taper of ingot should be increased to slow down the solidification speed of ingot and reduce the thermal stress.
Prevention of hairline and white spot
The essence of hairline and white spot is the huge stress produced by the desolvation and precipitation of hydrogen in steel, which makes the steel produce micro crack clusters. Hair lines are large in size and visible to the naked eye. The size of white dots is small and the cluster distribution is white bright spots. This kind of defect is common in alloy structural steel, which brings great harm to the plasticity and transverse mechanical properties of steel.
Measures for prevention of billet hairline and white spot:
- 1) Measures shall be taken to reduce the hydrogen content in the steel.
- 2) Strengthen the red turning or slow cooling of ingot after demoulding.
- 3) The ingot is annealed at high temperature.
- 4) When conditions permit, ladle argon blowing dehydrogenation can be carried out.
What is the cause of longitudinal cracking in steel ingot forging?
The causes of longitudinal cracking of ingots are as follows.
Ingot shrinkage extending to ingot body
When the ingot is poured, the temperature is too high, the injection speed is fast, and the shrinkage cavity is very deep. The longitudinal crack in the center occurs when the forging is deformed.
Too much hammering force in the development of ingot columnar crystal
The ferrite steel with developed columnar crystal should be operated by light beating and quick striking at the beginning of forging. If the deformation is too large, it will produce longitudinal cracking.
Forging heating temperature too high
The heating temperature of ingot is too high, resulting in overburning. At high temperature, grain coarsening loses strength and embrittlement.
In a word, it should be noted that the high temperature holding time should be short, the forging temperature should be low, the hammering force and deformation should be small first, and then increase gradually. Only in this way can longitudinal cracking of ingot be reduced.
What is the cause of transverse cracking in ingot forging?
The causes of transverse cracking of ingots are as follows.
Ingot surface defects are not removed
The surface defect of ingot is the source of transverse crack during forging. In the process of forging elongation, the defects produce transverse cracking under tensile stress. The removal of surface defects is an important measure to reduce transverse cracking.
Low thermoplasticity of steel
When a large number of returned materials are added into the smelting process, the purity of the steel decreases and the impurity content increases. At last, the thermoplasticity of steel is decreased, and the longitudinal tensile stress under forging condition makes the ingot appear transverse crack. This kind of situation is common in the forging of high alloy ingots. By controlling the proportion of returned material, the occurrence of transverse cracking can be reduced.
Influence of ingot heating atmosphere
When the fuel with high sulfur content is used in forging furnace and the alloy with high nickel content is heated, the low melting point eutectic formed between sulfur and nickel in ingot will become the source of forging crack. It eventually leads to transverse cracking. In order to avoid this phenomenon, high nickel alloy should not be heated in coal and heavy oil fuel heating furnace.
In a word, the transverse cracking in ingot forging mostly occurs in high alloy steel. The main reason of cracking is the internal quality of steel.
Source: China Alloy Steel Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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What is a qualified steel ingot?
The ingot smelted completely meets the following conditions, which is called qualified ingot. Otherwise, it is unqualified ingot.
(1) Chemical composition of finished steel is qualified
In the smelting of finished steel, the content of all elements shall meet the standard requirements. The chemical composition is qualified.
If one of the elements is unqualified, it is allowed to re sample any ingot from the same heat number for re inspection. When the re inspection is qualified, the re inspection result shall prevail, otherwise the element shall be determined as unqualified.
If the upper or lower limit of analytical content of one element is edged, it shall be judged whether it is qualified according to relevant national standards.
(2) The external dimension of ingot is qualified
If the height of ingot without heat preservation cap is less than half of the specified height, the ingot shall be judged as scrap or unqualified.
(3) The shrinkage cavity and surface quality of ingot are qualified
1) When the shrinkage cavity of ingot extends into 1 / 3 of ingot body, it shall be judged as unqualified product.
2) If the surface quality of ingot cannot be used after peeling treatment, it shall be judged as unqualified.
What is the function of steel ingot surface finishing?
(1) Meaning of steel ingot surface finishing
There are some defects on the surface of ingot, such as crack, scab, peeling and slag inclusion. Removing these defects is the work of ingot finishing, which is usually called ingot finishing.
(2) The function of finishing work
Defects on ingot surface must be removed completely. Especially in the production of precision alloy, stainless steel and other wire and strip, more attention is paid to the finishing of products with high surface quality requirements.
The surface defects of ingots will continue to pass on to the next process and finally to the finished product. The lighter one will reduce the yield of the process, the heavier one will cause waste products, which will cause great economic losses.
(3) Ingot finishing method
According to the different level of product quality requirements, the finishing methods of ingots are also different. Generally, there are two ways.
1) Grinding wheel dressing method. Use the ingot wheel to repair the mill and remove the surface defects of the steel in all directions.
During grinding, pay attention to cooling with water to avoid new cracks due to high temperature.
2) Steel ingot peeling method
After annealing, the square section ingot is peeled on the cesium bed; after annealing, the round section ingot is peeled on the lathe.
The above is a general ingot finishing method for high alloy special steel.
How to improve the inhomogeneity of carbide in steel ingot?
High carbon and high chromium tool steel, die steel, high speed tool steel and other ledeburite steel. In the process of ingot solidification, the eutectic carbide precipitates in an uneven network. When the carbide heterogeneity is serious, it will reduce the hot working plasticity of steel, make the steel crack during heat treatment and crack in use.
The main reasons for the inhomogeneity of carbides are the slow solidification speed of molten steel and the small ratio of height to width of ingot.
The measures to improve the carbide heterogeneity are as follows.
(1) Replacing square ingot with flat ingot
The cooling rate of liquid steel in flat ingot is faster than that in square ingot, which is beneficial to improve the uniformity of carbide.
The carbide uniformity of high-speed tool steel is improved obviously by using flat ingot with flatness of 2.5 ~ 3.0.
(2) Replacing large ingots with small ingots
The solidification speed of small ingot is faster than that of large ingot, and the uniformity of carbide is naturally better than that of large ingot. Small ingots should be used to produce high chromium and high carbon tool steels and high speed tool steels.
(3) Alloy elements with fine carbide added to steel
Adding about 0.20% rare earth elements into high-speed tool steel and die steel can refine carbide and improve its heterogeneity.
(4) Improvement of carbide inhomogeneity by electroslag remelting
The refinement and inhomogeneity of carbide in ledeburite alloy tool steel can be significantly improved by using the characteristics of ESR, such as fast cooling speed and solidification at the shallow edge of molten pool.
(5) Casting process of control steel
When pouring ledeburite alloy tool steel, the principle of low temperature and fast pouring should be followed. The superheat temperature of molten steel should be reduced as much as possible, and pouring should be controlled at 60 ~ 80 ℃ above the liquid phase point. The liquid steel entering into the ingot mould is cooled rapidly to reduce the heterogeneity of carbide.
How to prevent air bubble in steel ingot?
When the air bubbles in ingot extend into billet, there are two main forms: subcutaneous air bubbles and honeycomb air bubbles.
(1) Subcutaneous air bubble
The subcutaneous bubbles are regularly distributed around the skin of billet, and their depth is generally 5-20 mm. The cracks extending from the surface to the inside are observed from the cross section of billet.
Hypodermic bubble in high alloy steel is mainly caused by hydrogen. The preventive measures are as follows.
1) Necessary measures should be taken to reduce the hydrogen content of molten steel.
2) The moisture in the coating on the inner wall of ingot mould shall be removed. Avoid the hydrocarbon or moisture in the coating and the gas produced by the interaction of the liquid steel, and the formation of steel ingot subcutaneous bubbles, resulting in billet defects.
(2) Honeycomb bubble
Honeycomb bubble is usually produced in the billet, which is difficult to find and has great harm to the steel.
In low carbon high alloy steel, the formation of honeycomb bubble is mainly caused by nitrogen. When the nitrogen content in the steel exceeds the limit solubility, the excess nitrogen will precipitate and form honeycomb bubbles during the solidification process. When the ingot is opened, it is hidden in the billet.
When smelting high alloy steel containing nitrogen, it is necessary to master the balance between the nitrogen brought in by the furnace charge, the nitrogen absorbed from the atmosphere in the smelting process, and the nitrogen absorbed during pouring, which is equal to the nitrogen content of the required components in the steel. Avoid nitrogen content in steel exceeding its ultimate solubility.
In order to avoid honeycomb bubbles in the ingot of smelting steel, check the nitrogen content of smelting steel in combination with the data in the table to ensure that the nitrogen is completely dissolved without bubbles.