Measures to reduce forging cracks in backup roll steel ingots

Abstract: In view of the serious forging crack problem that occurs in backup roll steel ingots, the factors affecting the surface quality of the steel ingot and the causes of forging cracks are analyzed from the aspects of composition, smelting, ingot casting, demoulding, hot feeding, heating and forging. Measures to reduce forging cracks in steel ingots were also proposed.

Keywords: Backup roll steel ingot; Forging crack; Stress

During the forging process of backup roll steel ingots, visible cracks on the surface of the ingot body have always been a problem that troubled technicians. If the crack defects are minor, they can be saved by oxygen blowing or flame peeling. If the crack defect is severe, it will be scrapped directly, causing serious economic losses. This article conducts a preliminary analysis of the factors leading to surface cracks on the backup roll steel ingot from a theoretical perspective and seeks for effective control methods.

Crack overview

Back-up roller steel ingots often appear smooth cracks during upsetting, transverse cracks occur during KD, and even cracks appear when the nip is pressed. Since the chamfering process of large steel ingots has been cancelled, the ingot body has not yet been forged before upsetting. In order to determine whether the cracks were caused by the steel ingot itself or by forging or heating, we carefully inspected the surface quality of the steel ingot during the entire process from demoulding, hot transport, to forging. It was found that some steel ingots had fine transverse cracks or smooth cracks on the surface after being demoulded. Some steel ingots that had no surface cracks before entering the heating furnace had fine straight lines or fine horizontal cracks on the surface of the ingot body when the nip was pressed. It can be determined that some of the cracks that occurred during the forging process of the steel ingots exist in the steel ingots themselves, but have not been discovered. Because there is no chamfering, most of the cracks that exist at the press nip mouth are hidden under the oxide scale. At the same time, some of the cracks are thin and short, making it difficult to confirm.

It is certain that the crack shapes and timing of occurrence are different, and the causes must be different. The factors affecting the surface quality of steel ingots and the causes of forging cracks are analyzed one by one from the composition, smelting, ingot casting, demoulding, hot feeding, heating and forging, and measures to reduce forging cracks in steel ingots are proposed.

Manufacturing process analysis

Chemical composition

Under mass production conditions, cracks are concentrated on the backup roll steel ingots made of YB-65, YB-70, and YB-75. YB-75 has the highest rate of cracks, followed by YB-65. YB-70 and YB-75 are made of Cr5, YB-65 is made of Cr4, the carbon content of YB-65 and YB-75 is 0.45% ~ 0.55%, and the carbon content of YB-70 is 0.48% ~ 0.58%. These three materials can be regarded as adding alloy elements to medium carbon steel. The hot cracking resistance of the as-cast structure of medium carbon steel materials with a carbon content between 0.4% and 0.6% is relatively low. Medium-carbon steel ingots of materials such as 42CrMo and 45 with the same carbon content in this range will also have serious cracks during forging, but the production batch is small and does not attract attention. Actual composition statistics show that the carbon content of YB-70 is higher than that of YB-65 and YB-75, and the Mn content of YB-65 is higher than that of YB-75. This shows that C and Mn elements affect the hot cracking resistance of these three backup roller materials and have a certain impact on the formation of steel ingot cracks. Therefore, under the premise that the composition specification range remains unchanged, using computational materials science methods to design and adjust the alloy composition will help reduce the crack tendency of the back-up roller steel ingot. In addition, Cu, Pb, Sn, As and other elements and their compounds are enriched in the grain boundaries, reducing the grain boundary strength. This is also one of the reasons for inducing cracks. The content of the above elements brought in by raw materials and scrap steel should be strictly controlled.

Smelting and ingot casting

Smelting

The high content of hydrogen, oxygen, nitrogen, phosphorus, sulfur and non-metallic inclusions in steel will reduce the strength and plasticity of steel to varying degrees. Therefore, ores, fluorspar and ferroalloys must be baked well, and ferrosilicon powder, aluminum powder, calcium silicate powder, electrode powder, and carbon powder must be dried, and strict requirements are required in the rainy season. In particular, the lime must be burned thoroughly, and care should be taken to prevent moisture during transportation and storage to ensure that the time out of the kiln is no more than 24 hours. Strengthen the oxidation operation to ensure that the decarburization is not less than 40% to ensure the decarburization intensity and form a good molten pool, which is conducive to the floating and elimination of inclusions. In addition, the amount of aluminum added must be strictly controlled and the nitrogen content should be reduced as much as possible. Too much aluminum will cause the AlN precipitation surface to crack during forging.

Ingot casting

(1) Pouring temperature

Through data statistics, it was found that when the pouring speed is constant and the pouring temperature is too high, the steel ingot is prone to cracks. This is because on the premise that the cooling capacity of the steel ingot mold remains unchanged, the increase in pouring temperature causes the chill layer of the ingot to become thinner. Under the action of the static pressure of the melt, there are problems such as cracking and the escape of inclusions in the molten steel. At the same time, if the pouring temperature is too high, the steel ingot is easily welded to the steel ingot mold, making demolding difficult and the steel ingot prone to cracks. Secondly, when the pouring temperature is high, the temperature gradient on the solidification front is large, so the solid-liquid two-phase area is narrow, the primary dendrites in the two-phase area are short, and the spacing between primary dendrites is also small. Therefore, the linear shrinkage of the side surface caused by the inward shrinkage of the entire steel ingot shell is large. If the longitudinal shrinkage is hindered, it is easy to produce transverse cracks. When the transverse shrinkage is hindered, the tendency of smooth cracks is greater. In addition, if the pouring temperature is too high, the refractory material will be seriously eroded and the inclusions in the steel will increase.

(2) Pouring speed

Killed steel is generally quite sensitive to surface cracks, so it should be poured slowly and appropriately, but the floating of non-metallic inclusions should be ensured. The pouring speed should be adjusted according to the pouring temperature. When the pouring temperature is high, slow pouring is used. On the contrary, when the pouring temperature is low, pouring should be done quickly. This is because slow pouring is equivalent to lowering the temperature of the molten steel, and fast pouring is equivalent to increasing the temperature of the molten steel in the mold. In order to control the injection speed within the specified range, it is necessary to prevent the nozzle from expanding. Steel ingots made of support rollers that are sensitive to cracks must be poured evenly and slowly to thicken the cooling layer of the steel ingot, weaken the convection circulation of the steel ingot, slow the static pressure of the molten steel, and prevent the occurrence of hot cracks.

(3) Gating system

The entire pouring system must be kept clean, and baking must be strengthened to prevent the liquid steel from being involved in non-metallic inclusions such as refractory materials. When there are concentrated non-metallic inclusions on the surface and subsurface of the steel ingot, the threat of cracks in the steel ingot is greatest.

The following measures can be taken for the entire ingot:

1) Use refractory materials with good slag resistance to build the ladle.

2) After the steel ingot mold is lowered into the vacuum chamber, ensure that the center position is aligned to avoid local thinning of the chill layer caused by eccentric pouring of the steel ingot.

3) Strictly control the injection temperature and injection speed, truly achieve slow injection at high temperatures and fast injection at low temperatures to keep the molten steel rising steadily.

4) In summer, the temperature of the steel ingot mold before pouring should not be too high, and at the same time, try to avoid excessive differences in the thickness of the quench layer of the steel ingot caused by uneven temperatures in the steel ingot mold.

5) Place asbestos rope between the insulation cap and the steel ingot mold to prevent the steel from penetrating into the gap and causing hanging and hindering the longitudinal shrinkage. Under the action of the tensile force of the ingot weight, the riser end of the steel ingot body will pull the weak part of the chill layer to crack, resulting in transverse cracks.

6) Pay attention to the grinding of the bottom surface of the steel ingot mold to avoid oxygen burning at the nozzle of the steel ingot, so as to avoid transverse cracks at the nozzle end of the ingot body due to the loose connection between the ingot mold and the chassis and the penetration of molten steel to form flash that hinders the longitudinal condensation shrinkage of the steel ingot.

7) Strengthen the grinding of the inner wall of the ingot mold. If it is too rough and has serious cracks and pits, it will hinder the shrinkage of the steel ingot and cause cracks.

Cooling and demoulding of steel ingots

When the steel ingot cools, the inner and outer layers do not solidify at the same time. When the outer layer solidifies into a shell, the unsolidified molten steel inside exerts static pressure on the outer steel shell, trying to tear the shell apart. At the same time, during the solidification process of the steel ingot, because the temperature of the surface layer drops faster than that of the inside, there is a large temperature difference between the inner and outer layers. This difference causes the volume shrinkage of the inner and outer layers of the steel ingot to be inconsistent. The part of the surface layer that shrinks first must be hindered by the unshrunk part of the inner layer to generate thermal stress. In addition, when steel is cooled to 650°C, there is a structural transformation, accompanied by a sudden expansion of volume. Due to the existence of the temperature difference between inside and outside, this volume expansion will inevitably not occur at the same time. For example, the surface tissue has been transformed and the volume shrinks according to the normal cooling process, while the internal tissue transformation is taking place and the volume expands. The expanded inner layer will inevitably give the surface layer an obvious tensile stress. When the sum of these three stresses exceeds the grain boundary strength, cracks will occur in the steel ingot. Cracks often appear under the skin of the steel ingot and will expand into surface cracks during forging. Therefore, it is necessary to avoid premature demolding of steel ingots, and eliminate thermal stress caused by volume shrinkage when the ingot cools and the temperature drops, and structural stress caused by structural transformation through slow cooling. Measures that can be taken include:

(1) A reasonable demoulding time should be specified to allow the steel ingot to be demoulded after phase change.

(2) Avoid being exposed to rain or moisture after demoulding.

Hot delivery and heating of steel ingots

In actual production, when large steel ingots are hot-transported to the furnace, the surface temperature of the steel ingot (one-third of the distance from the riser end of the ingot body) is difficult to exceed 650°C, and the surface layer of the steel ingot can basically be identified. In particular, the structural transformation has occurred at the nozzle end of the steel ingot. During heating, surface cracks will be limited to the depth of the austenite grain boundaries and will not penetrate deep into the interior along the grain boundaries where inclusions are enriched. Steel ingots with a surface temperature greater than 550°C can be directly heated in a high-temperature furnace. In the early stage of heating, the radiation and convection heat transfer in the furnace are particularly strong, and the surface temperature of the steel ingot rises quickly. In the later stage, as the surface temperature of the steel ingot increases, the radiation and convection heat transfer decreases, and the rise rate of the surface temperature of the steel ingot will slow down. Since the nozzle end of the hot-feeding steel ingot is 100~150°C lower than the riser end, and the diameter of the nozzle end is small, the nozzle end heats up the fastest and endures the largest thermal stress caused by the temperature difference. When the steel ingot is heated to about 750°C, phase transformation will occur, causing structural stress. At the same time, the plasticity of the steel ingot is low at 550-750°C. If the heating rate is too high, huge thermal stress and structural stress will be superimposed, causing smooth cracks on the surface of the steel ingot. When the steel ingot is heated to 750-1200°C, the plasticity of the metal increases sharply, and the structural stress and thermal stress disappear. Measures that can be taken include:

(1) Hot delivery of steel ingots and loading of furnaces to avoid delays.

(2) When the surface of the steel ingot is heated to 750°C, the rate should be as slow as possible to avoid excessive stress.

(3) The hot-transported steel ingot is heated after phase transformation to reduce the hazards of inclusions and microscopic shrinkage cavities by improving the recrystallized columnar crystals.

(4) The heating temperature of the upsetting fire is increased to 1270°C to fully dissolve the precipitated phase, improve micro-segregation, homogenize the structure, and improve the as-cast structure plasticity of the steel ingot. However, it is necessary to control the maximum heating temperature (furnace temperature) not to exceed 1270°C to avoid overburning and to avoid staying at high temperature for too long.

Forging

For back-up roller steel ingots whose material is prone to cracks, the chamfering should be restored at the press nip. Through small deformation surface deformation, welding may exist defects such as subcutaneous micro-cracks on the surface of the steel ingot, and at the same time, the surface metal is changed from the cast state to the forged state, and the high-temperature plasticity is significantly improved, so that the surface of the steel ingot will not produce cracks during upsetting. Chamfering can also remove the iron oxide scale from the ingot body and detect cracks hidden under the iron oxide scale as soon as possible. If there are smooth cracks on the surface with a length greater than 100 mm, they must be removed one by one by blowing oxygen to prevent the cracks from expanding and deepening during the upsetting process.

During the upsetting process, cracks often appear on the concave surfaces between the edges of the steel ingot. The crack extension direction is straight or oblique at about 45°, or a combination of both. This is because after the steel ingot is poured, the edges of the steel ingot have good heat dissipation conditions and rapid cooling. The chill layer is thicker than the concave surface between the edges, which acts as a “reinforcement rib”. During upsetting of some steel ingots, cracks have already appeared before the drum shape is obvious. The cracks are mostly at the nozzle end of the ingot body, followed by the middle part of the ingot body (the maximum diameter of the drum shape), and cracks rarely appear at the riser end of the ingot body.

Due to the influence of friction between the nozzle end surface of the steel ingot and the upsetting cover plate (or anvil) during upsetting and the difference in the cross-sectional radius of the steel ingot, the steel ingot deforms unevenly during upsetting, and a large compressive stress is generated in the axial direction of the steel ingot. Due to uneven deformation in the peripheral deformation zone on the side surface of the steel ingot, hoop (tangential) tensile stress appears. The stress acts perpendicular to the crystallization direction and becomes larger closer to the surface. At the same time, the radial compressive stress weakens and becomes smaller closer to the surface. Upsetting deformation is carried out above the equal strength temperature of the metal, and the grain boundary deformation is large. At the same time, due to the well-developed columnar grains of steel ingots, a large number of metallurgical defects, non-metallic inclusions and microscopic shrinkage holes often accumulate on the grain boundaries, which separates the columnar grain boundaries and greatly reduces the bonding ability between grains. The macroscopic manifestation is that the plasticity of the columnar crystal region is low. When the tensile stress is large enough, columnar grain boundary inclusions will cause intergranular cracking, forming microcracks, which then expand and connect to form large cracks, which are perpendicular to the surface. If the upsetting deformation speed is slow, cracks can only expand in the columnar zone and cannot extend to the high plasticity zone on the surface. During upsetting, there is the maximum shear stress in the direction at an angle of 45° to the axis, so the low plasticity region rich in inclusions on the surface of the steel ingot is prone to oblique cracks at an angle of 45°. In addition, the stress cracks existing in the steel ingot itself will also expand outward to the surface of the steel ingot under the action of additional tensile stress. As the amount of reduction increases, the drum shape produced will inevitably gradually increase, resulting in increasing circumferential tensile stress on the side surface. The stress state at the existing micro-defects on the outside of the drum’s largest section reaches a critical value, microscopic cracks appear, and continued upsetting causes longitudinal cracks on the side surface. The following measures can be taken:

(1) Since the purpose of upsetting is to increase the forging ratio for the next step of elongation and compaction, the compaction effect is very small. When formulating the forging process, it is best not to adopt upsetting deformation, and give priority to the ingot type with a steel ingot length and forging ratio greater than 3, such as formulating a process of producing two pieces from one ingot.

(2) The steel ingot is returned to the furnace for heating when the temperature is uneven. Because the temperature of each part is uneven, the plasticity of each part is different, and the alignment effect during deformation causes local tearing.

(3) When formulating the forging process, consider that the upsetting reduction amount should be smaller than the allowable range of material plasticity.

(4) The steel ingot is heated to the maximum temperature allowed by the material to reduce the pressure required for upsetting.

(5) When upsetting, the pressing speed should be as slow as possible, and the maximum capacity of the hydraulic press should not be used for upsetting.

(6) Use concave upsetting cover for upsetting. During the upsetting process of the concave upsetting cover plate, the increase in the end diameter is almost entirely caused by the flipping of the side metal, and the additional tensile stress on the outer surface of the nozzle end is smaller than that of the flat upsetting cover plate.

When KD is pulled out, transverse cracks appear on the side surface. It can be considered that when the feed amount and reduction amount are large, the deformation of the axis center part is large, and the side surface is subject to greater tensile stress along the axial direction. Since the front and rear sides of the deformation zone are not in contact with the anvil surface, the impact of friction resistance is very small. If this is the low plasticity zone where inclusions are relatively concentrated, it is easy to cause surface transverse cracks. The main countermeasure is to appropriately control the reduction amount and reduction rate, and cannot use the maximum capacity of the hydraulic press to pull out the length. During KD, if the surface of the steel ingot itself has good plasticity, or even has local transverse cracks, the cracks can be blunted and flattened through local plastic deformation.

Since the tip radius of the crack tip tends to zero and the tip stress tends to infinity, the crack will expand rapidly during the subsequent forging deformation process. Therefore, whether it is a transverse crack or a longitudinal crack, if the crack is relatively sharp, it must be removed in time to avoid the development of cracks in continued forging, causing the steel ingot to be severely torn and scrapped.

Conclusion

Forging cracks in backup roll steel ingots are in most cases caused by a combination of factors. The fundamental reason for cracks is that the stress at the source of the crack exceeds the strength limit of the steel at that location and under the working conditions. Therefore, in order to prevent cracks, management must be strengthened, starting from the composition design and control of steel, strictly controlling the inclusions in the steel, strictly controlling the pouring, steel ingot cooling and heating processes, and formulating a reasonable forging process.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

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