A method for improving grain boundary cracks on hot rolled coil surface

Abstract

This invention discloses a method for improving grain boundary cracking on the surface of hot-rolled coils. The method comprises controlling the carbon equivalent, residual Sn and Cu contents during the steelmaking process, the continuous casting process, and the heating of the ingot. Specifically, during the steelmaking process, the carbon equivalent (Cp) is designed to avoid the range of 0.15% to 0.18%, and the residual element contents (Sn) and (Cu) are ≤ 0.05% and ≤ 0.15%, respectively. This method effectively controls the occurrence of grain boundary cracking on the surface of hot-rolled coils, achieving unexpectedly positive results, improving product quality, and enhancing economic benefits and brand recognition.

Description

Technical Field

[0001] The invention belongs to the field of iron and steel metallurgy, and in particular relates to a method for improving grain boundary cracks on the surface of a hot-rolled coil. Background Art

[0002] During the hot rolling production process, surface grain boundary cracks generated during the heating or rolling process of micro-alloyed steels, especially special steels such as austenitic steels with high manganese and high aluminum contents, are common and difficult to control surface defects of hot-rolled coils, which seriously restrict the improvement of product quality and hot rolling yield of such steel grades.

[0003] Long-term production practice and theoretical research have shown that the abnormally coarse austenite grains in the ingot are the essential cause of grain boundary cracks on the surface of hot-rolled coils. The main reasons are as follows.

[0004] 1) The surface of the continuous casting billet of special steels such as austenitic steel with high manganese and high aluminum content needs to be fully ground. The surface chill layer is ground away, and the grain boundaries of the coarse austenite grains become heating oxidation channels, causing grain boundary oxidation. The oxidized grain boundaries become crack sources during hot rolling.

[0005] 2) Crack sensitivity of coarse austenite grains themselves: The bonding force between coarse austenite grains is weaker than that between fine grains, and smaller strain energy may drive grain boundaries to move and produce microcracks.

[0006] 3) Coarse austenite grain size determines the precipitation of grain boundary precipitates: Surface grain boundary cracks caused by the enrichment of residual elements in the second brittle zone during solidification, and surface grain boundary cracks caused by the precipitation of carbonitrides in the third brittle zone, are both related to coarse austenite grains. Research suggests that the critical austenite grain size is 1 mm. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for improving grain boundary cracks on the surface of a hot-rolled coil, so as to improve the grain boundary crack defects on the surface of the hot-rolled coil.

[0008] In order to solve the above technical problems, the technical solution provided by the present invention is:

[0009] A method for improving grain boundary cracks on the surface of a hot-rolled coil comprises controlling the carbon equivalent and residual element content during the steelmaking process, the continuous casting process, and the slab heating process. The carbon equivalent Cp is designed to avoid the range of 0.15% to 0.18% during steelmaking composition design, and the residual element contents Sn and Cu are ≤ 0.05% and ≤ 0.15%, respectively.

[0010] The carbon equivalent Cp calculation formula of the present invention is as follows:

[0011] Cp=w[%C]+0.02w[%Mn]+0.7w[%N]+0.023w[%Ni]+0.0414w[%P]-0.037[%Si]-0.7w[%S]-0.0189w[%Mo]-0.0254w[%Cr]-0.0276w[%Ti].

[0012] When the carbon equivalent Cp is 0.15% to 0.18%, the austenite grain size on the surface of the ingot is larger, especially when the carbon equivalent is 0.17%, the austenite grain size is the largest.

[0013] Residual Sn and Cu in the steel segregate at grain boundaries during solidification, weakening the bond at those boundaries and leading to a decrease in plasticity. Furthermore, copper has a lower melting point than iron, and due to the high temperature on the ingot surface, it becomes liquid and aggregates. If cracks occur in the ingot, copper can enter the steel along the cracked grain boundaries, causing damage.

[0014] Furthermore, in the continuous casting process of the present invention, the secondary cooling needs to be uniform, and the lateral temperature difference of the casting billet is ≤50°C.

[0015] The main causes of coarse austenite grains are high superheat and uneven cooling. Excessive superheat thins the billet shell and coarsens the structure. Uneven cooling leads to uneven growth of the billet shell, resulting in wrinkles, or severe shrinkage in certain areas, forming depressions. Poor heat transfer at the bottom of the depressions allows the austenite grains to grow, reaching a size several times that of the primary solidified structure. Furthermore, depressions in the billet inevitably cause bulging. During the bulging process, the static pressure of the molten steel causes existing microcracks to expand or new cracks to form. The more severe the bulging, the more severe the cracks.

[0016] Furthermore, the continuous casting process of the present invention avoids straightening of the ingot in the two-phase region, and the straightening temperature is ≥ the austenite start transformation temperature Ar3 temperature + 50°C.

[0017] Most existing studies believe that the precipitates of second-phase particles (occurring in the third low-temperature brittle zone at 600-900°C) reduce the strength of the grain boundaries, thereby causing grain boundary fracture during the straightening deformation process. Therefore, the straightening of the ingot must avoid the third low-temperature brittle zone.

[0018] The present invention corrects this theoretical bias, believing that the main issue is phase transformation. When austenite transforms to ferrite, a surrounding ferrite film is generated at the austenite grain boundary. When strain occurs, it is concentrated on the soft phase of this ferrite film, causing stress concentration and tearing of the grain boundary, resulting in surface defects in the ingot. The precipitation of second-phase particles only worsens this process. Therefore, it is necessary to first consider avoiding the area where the phase transformation occurs. The straightening machine should deform and straighten at a higher temperature as much as possible to avoid straightening in the two-phase region. As long as the straightening is 50°C above the austenite, the ingot will not have straightening defects. Although the as-cast microstructure performance is poor, the plasticity is more than sufficient for a strain of 1-3%.

[0019] Furthermore, in the continuous casting process of the present invention, the hot delivery temperature of the ingot is controlled to be ≤ the ferrite transformation completion temperature Ar1 temperature - 100°C.

[0020] The existing technology believes that hot delivery can be done when the surface temperature is lower than the temperature at which ferrite transformation is completed. However, this is not the case in practice. To avoid hot delivery cracking, the surface temperature of the ingot should be lowered to below 500℃.

[0021] The present invention believes that there is a temperature gradient from the surface to the inside of the high-temperature ingot. For example, at a surface temperature of 600°C, it seems that the phase transformation to full ferrite has been completed. However, this is only achieved in a very shallow area of ​​the surface. A large volume of the interior is still in the two-phase region, especially the sub-surface two-phase region, which is in the temperature range of 600-900°C. After entering the heating furnace, the surface metal is rapidly heated and expanded, generating tensile stress on the sub-surface, thereby tearing the grains in the two-phase region apart. This crack may extend to the surface, forming a surface crack.

[0022] Research shows that if the decomposition of austenite is not complete before charging, the hot charging or warm charging process cannot achieve effective grain refinement through dual phase transformation. When the furnace is charged below the phase transformation completion temperature, there is actually still residual austenite phase. After re-austenitization, a large number of small new γ grains appear, and at the same time there are coarse grains obtained by the growth of residual γ. When heating continues, the mixed crystal structure in the latter evolves in an abnormal growth manner, causing the γ grains to coarsen significantly. The pre-rolling microstructure is coarser than the initial γ before cooling.

[0023] Furthermore, in the ingot heating process described in the present invention, an incomplete combustion method is adopted in the heating furnace to obtain a weak oxidizing atmosphere, and the weak oxidizing atmosphere in the heating furnace is a furnace atmosphere with a residual oxygen value of 1.6% to 3.0%; when the residual oxygen value is less than 1.6%, the air volume in the heating section is increased, and when the residual oxygen value is greater than 3.0%, the air volume in the heating section is reduced; during the ingot heating process, the excess air coefficient in the heating furnace is controlled at 0.9 to 1.2, and is controlled in sections, with the heating section being the largest, the preheating section being the second, and the soaking section being the smallest, that is, the heating section>preheating section>soaking section, so as to reduce the degree of high-temperature oxidation of the steel ingot during heating and control grain boundary oxidation.

[0024] The beneficial effects of adopting the above technical solution are:

[0025] The present invention is based on the fact that abnormally coarse austenite grains in the ingot are the essential cause of grain boundary cracks on the surface of the hot-rolled coil. By controlling the carbon equivalent and residual element content in steelmaking, optimizing the process of continuous casting to refine the austenite grains, and adopting a weak oxidizing atmosphere in the hot rolling heating process to control grain boundary oxidation, the occurrence of grain boundary cracks on the surface of the hot-rolled coil is effectively controlled, achieving unexpected good results, improving product quality, and enhancing the economic benefits and quality brand effect of the enterprise. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. Example 1

[0027] In this embodiment, the method for improving the surface grain boundary cracks of W780QX peritectic high-aluminum high-manganese microalloyed steel is as follows:

[0028] The steel composition and its mass percentage are shown in Table 1.

[0029]

[0030] Substitute the content of each component into the carbon equivalent calculation formula:

[0031] Cp=w[%C]+0.02w[%Mn]+0.7w[%N]+0.023w[%Ni]+0.0414w[%P]-0.037[%Si]-0.7w[%S]-0.0189w[%Mo]-0.0254w[%Cr]-0.0276w[%Ti]

[0032] The calculated Cp=0.116%, avoiding the range of 0.15% to 0.18%.

[0033] After testing, the residual element content is Sn=0.01%<0.05%, Cu=0.03%<0.15%.

[0034] (2) Control the continuous casting process and carefully check the condition of the secondary cooling nozzle before casting to ensure uniform cooling of the billet and a lateral temperature difference of ≤50°C.

[0035] Calculation: The steel's Ar3 temperature = 771°C, Ar1 temperature = 524°C

[0036] The slab straightening temperature is ≥ the austenite start transformation temperature Ar3 temperature + 50°C = 771 + 50 = 821°C, specifically ≥ 850°C.

[0037] The hot delivery temperature of the ingot is ≤ the ferrite transformation completion temperature Ar1 temperature - 100°C = 524-100 = 424°C, specifically ≤ 400°C.

[0038] The heating process of the ingot is controlled, and the weak oxidizing atmosphere in the heating furnace is a furnace atmosphere with a residual oxygen value of 1.6% to 3.0%; the excess air coefficient in the heating furnace is controlled in sections, specifically: 1.1≤excess air coefficient of the heating section≤1.2, 0.95≤excess air coefficient of the preheating section≤1.05, 0.9≤excess air coefficient of the heating furnace<0.95.

[0039] The incidence of grain boundary cracks on the steel of this embodiment after rolling was reduced from the original 27.3% to 0, indicating that the technical solution of the present invention can avoid the occurrence of grain boundary cracks on the surface of the steel plate after rolling. Example 2

[0040] In this embodiment, the method for improving the surface grain boundary cracks of 22MnB5 medium carbon hot-formed steel is as follows:

[0041] (1) Steel composition and its mass percentage are shown in Table 2.

[0042]

[0043] Substitute the content of each component into the carbon equivalent calculation formula:

[0044] Cp=w[%C]+0.02w[%Mn]+0.7w[%N]+0.023w[%Ni]+0.0414w[%P]-0.037[%Si]-0.7w[%S]-0.0189w[%Mo]-0.0254w[%Cr]-0.0276w[%Ti]

[0045] The calculated Cp=0.24%, avoiding the range of 0.15% to 0.18%.

[0046] After testing, the residual element content is Sn=0.02%<0.05%, Cu=0.04%<0.15%.

[0047] (2) Control the continuous casting process and carefully check the condition of the secondary cooling nozzle before casting to ensure uniform cooling of the billet and a lateral temperature difference of ≤30°C.

[0048] After calculation: the steel Ar3 temperature = 738℃, Ar1 temperature = 568℃

[0049] The slab straightening temperature is ≥ the austenite start transformation temperature Ar3 temperature + 50°C = 738 + 50 = 788°C, specifically ≥ 850°C.

[0050] The hot delivery temperature of the ingot is ≤ the ferrite transformation completion temperature Ar1 temperature - 100°C = 568-100 = 468°C, specifically ≤ 400°C.

[0051] (3) The heating process of the ingot is controlled. The weak oxidizing atmosphere in the heating furnace is a furnace atmosphere with a residual oxygen value of 1.6% to 3.0%. The excess air coefficient in the heating furnace is controlled in sections, specifically: 1.1≤excess air coefficient of the heating section≤1.2, 0.95≤excess air coefficient of the preheating section≤1.05, 0.9≤excess air coefficient of the heating furnace<0.95.

[0052] The incidence of grain boundary cracks on the steel of this embodiment after rolling was reduced from the original 21.5% to 0, indicating that the technical solution of the present invention can avoid the occurrence of grain boundary cracks on the surface of the steel plate after rolling. Example 3

[0053] In this embodiment, the method for improving the surface grain boundary cracks of W590X peritectic microalloyed steel is as follows:

[0054] (1) Steel composition and its mass percentage are shown in Table 3.

[0055]

[0056] Substitute the content of each component into the carbon equivalent calculation formula:

[0057] Cp=w[%C]+0.02w[%Mn]+0.7w[%N]+0.023w[%Ni]+0.0414w[%P]-0.037[%Si]-0.7w[%S]-0.0189w[%Mo]-0.0254w[%Cr]-0.0276w[%Ti]

[0058] The calculated Cp=0.115%, avoiding the range of 0.15% to 0.18%.

[0059] After testing, the residual element content is Sn=0.01%<0.05%, Cu=0.02%<0.15%.

[0060] (2) Control the continuous casting process and carefully check the condition of the secondary cooling nozzle before casting to ensure uniform cooling of the billet and a lateral temperature difference of ≤20°C.

[0061] Calculation: The steel's Ar3 temperature = 742°C, Ar1 temperature = 552°C

[0062] The slab straightening temperature is ≥ the austenite start transformation temperature Ar3 temperature + 50°C = 742 + 50 = 792°C, specifically ≥ 850°C.

[0063] The hot delivery temperature of the ingot is ≤ the ferrite transformation completion temperature Ar1 temperature - 100°C = 552-100 = 452°C, specifically ≤ 400°C.

[0064] (3) The heating process of the ingot is controlled. The weak oxidizing atmosphere in the heating furnace is a furnace atmosphere with a residual oxygen value of 1.6% to 3.0%. The excess air coefficient in the heating furnace is controlled in sections, specifically: 1.1≤excess air coefficient of the heating section≤1.2, 0.95≤excess air coefficient of the preheating section≤1.05, 0.9≤excess air coefficient of the heating furnace<0.95.

[0065] The incidence of grain boundary cracks on the steel of this embodiment after rolling was reduced from the original 18.7% to 0, indicating that the technical solution of the present invention can avoid the occurrence of grain boundary cracks on the surface of the steel plate after rolling.

[0066] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for improving grain boundary cracks on the surface of a hot-rolled coil, characterized in that: This includes controlling the carbon equivalent and residual elements Sn and Cu content during the steelmaking process, the continuous casting process, and the heating process of the ingot; wherein, when designing the composition of the steelmaking process, the carbon equivalent Cp is kept away from the range of 0.15% to 0.18%, and the residual element content Sn is ≤ 0.05%, and Cu is ≤ 0.15%; The continuous casting process avoids straightening of the slab in the two-phase region, the straightening temperature is ≥ the austenite start transformation temperature Ar3 temperature + 50°C; the slab hot delivery temperature is ≤ the ferrite transformation completion temperature Ar1 temperature - 100°C; The carbon equivalent Cp calculation formula is: Cp=w[%C]+0.02w[%Mn]+0.7w[%N]+0.023w[%Ni]+0.0414w[%P]-0.037[%Si]-0.7w[%S]-0.0189w[%Mo]-0.0254w[%Cr]-0.0276w[%Ti].

2. The method for improving grain boundary cracks on the surface of a hot-rolled coil according to claim 1, characterized in that: In the continuous casting process, the secondary cooling needs to be uniform, and the lateral temperature difference of the casting billet is ≤50°C.

3. The method for improving grain boundary cracks on the surface of a hot-rolled coil according to claim 1, characterized in that: During the heating process of the ingot, the weakly oxidizing atmosphere in the heating furnace is a furnace atmosphere with a residual oxygen value of 1.6% to 3.0%. During the heating process of the ingot, the excess air coefficient in the heating furnace is controlled at 0.9 to 1.2, and is controlled in sections, with the heating section being the largest, the preheating section being the second, and the soaking section being the smallest, that is, the heating section>preheating section>soaking section.

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