Method for controlled rolling and controlled cooling of Q500 grade low alloy structural steel bar

Abstract

The present invention discloses a Q500 steel grade low alloy structural steel bar and a rolling method thereof with controlled cooling during rolling. The chemical components (wt%) of the bar are C 0.05%~0.18%, Si 0.20%~0.40%, Mn 1.00%~1.60%, P≦0.030%, S≦0.030%, Cr≦0.20%, Ni≦0.20%, Mo≦0.10%, Cu≦0.15%, V 0.02%~0.10%, Nb≦0.05%, Ti≦0.02%, B≦0.004%, N 0.005%~0.012%, Al 0.005%~0.012%, and Al 0.005%~0.012%. 0.02%~0.04%, CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≦0.45%, and the starting rolling temperature of this method is 1000~1050℃. This method can be used to achieve uniformity in the rolled material structure.

Description

[Technical field]

[0001] The present invention relates to a Q500 steel grade low alloy structural steel bar and a rolling method thereof, which belongs to the field of rolling cooling control. [Background technology]

[0002] Q500 steel is a steel for bridges. With the application of high-strength bridge steel, the brittle fracture accidents of steel structures occur frequently, which causes greater economic losses and human casualties. At the same time, high-strength bridge steel cannot meet the demands of modern bridge design in terms of mechanical properties such as toughness, which greatly restricts its large-scale widespread use in bridges.

[0003] At present, in order to ensure that the strength of Q500 steel grade materials meets the requirements, the general practice in the component design is to add a large amount of alloys such as Ni and Mo, or to add Nb, V, Ti and other refined grain elements in combination, and then carry out tempering treatment to achieve the performance requirements, which results in high production costs.

[0004] The rolling cooling control method is a method for significantly improving the mechanical performance of steel by controlling the rolling temperature and cooling. By adopting rolling cooling control, the production costs of steel can be significantly reduced, and high strength and low temperature impact toughness can be obtained in low-alloy high-strength structural steel with a low carbon equivalent (low CEV), thereby significantly reducing production costs.

[0005] Therefore, it is of great research significance to adopt rolling cooling control research to improve the comprehensive mechanical properties of steel. Summary of the Invention [Problem to be solved by the invention]

[0006] In order to solve the problem of high production costs of Q500 steel grade materials in the prior art, the present invention provides a rolling method with controlled rolling cooling, which can effectively improve the strength and impact toughness of steel materials, so that low-alloy structural steel with low carbon equivalent obtains high strength and low-temperature impact toughness, while at the same time greatly reducing the production cost. [Means for solving the problem]

[0007] In order to achieve the above object, the present invention adopts the following technical solutions. A Q500 steel grade low alloy structural steel bar, the chemical composition of the low alloy structural steel bar in mass percent includes: C 0.05%~0.18%, Si 0.20%~0.40%, Mn 1.00%~1.60%, P≦0.030%, S≦0.030%, Cr≦0.20%, Ni≦0.20%, Mo≦0.10%, Cu≦0.15%, V 0.02%~0.10%, Nb≦0.05%, Ti≦0.02%, B≦0.004%, N 0.005%~0.012%, Al 0.02%~0.04%, and CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≦0.45%.

[0008] In the low alloy structural steel bar, the C content in the low alloy structural steel bar is 0.10% to 0.18% by mass (for example, 0.11%, 0.12%, 0.14%, 0.15%, 0.16%, 0.17%).

[0009] In the conventional technology, in order to ensure that the strength of Q500 steel grade materials meets the requirements, large amounts of alloys such as Ni and Mo are generally added during the component design, or Nb, V, Ti and other refinement elements are added in combination, and further tempering treatment is performed to achieve the performance requirements, which results in high production costs.

[0010] The present invention does not add Ni, Mo, Nb, Ti elements, and offsets the addition of noble heavy alloys by increasing the C content, and designs the C content to be 0.05%-0.18%, preferably 0.10%-0.18%, and uses low-alloy structural steel with low carbon equivalent to obtain high strength and low-temperature impact toughness. At the same time, based on the chemical composition of Q500 steel class in the GB / T1591-2018 standard, the present invention does not add noble heavy alloys such as Ni, Mo, Nb, Ti, etc., so that the production cost is reduced.

[0011] In the above low alloy structural steel bar, as a preferred embodiment, the specification (diameter) of the Q500 steel class low alloy structural steel bar is 20 to 120 mm.

[0012] GB / T1591-2018 standard requires that the CEV of Q500 steel grade bar is 0.47% when the bar specification is 63mm or less, and that the CEV of Q500 steel grade bar is 0.48% when the bar specification is >63mm. In the present invention, the CEV is designed to be 0.45% or less, and the rolling cooling control method is used to make the Q500 steel grade obtain high strength and low temperature impact energy at low CEV.

[0013] Generally, the CEV affects the welding performance of materials. The lower the CEV, the better the weldability. However, the CEV also affects the strength of the material, and under the same conditions, the lower the CEV, the lower the strength. By adopting the technical solution of the present invention, the steel material with low CEV can obtain high strength and at the same time has good low-temperature impact toughness.

[0014] In the above-mentioned low alloy structural steel bar, as a preferred embodiment, the CEV is 0.40 to 0.45% (for example, 0.41%, 0.42%, 0.43%, 0.44%).

[0015] In the above-mentioned low alloy structural steel bar, as a preferred embodiment, the performance of the Q500 steel class low alloy structural steel bar is R p0.2 ≧470MPa, R m is 600~750MPa, A≧25%, -40℃ longitudinal KV2≧100J.

[0016] The present invention also provides a rolling method for controlling the cooling of the above-mentioned low alloy structural steel bar, which adopts the following technical solution:

[0017] The rolling method of the above-mentioned Q500 steel grade low alloy structural steel bar with controlled rolling and cooling includes the steps of heating, rolling, cooling, KOCKS rolling, and secondary cooling. In the rolling step, the heated billet is rolled, and the initial rolling temperature is 1000 to 1050°C (for example, 1010°C, 1020°C, 1030°C, 1040°C, 1045°C).

[0018] In the above rolling method with rolling cooling control, as a preferred embodiment, the rolling method with rolling cooling control uses a circular billet with a diameter of Φ100 mm to 390 mm as a raw material, or a square billet with a diameter of (100 mm to 300 mm) x (100 mm to 400 mm) as a raw material, and the final rolled finished bar has a specification (diameter) of 20 mm to 120 mm (e.g., 40 mm, 60 mm, 80 mm, 100 mm, 110 mm).

[0019] In the rolling method with controlled rolling cooling, as a preferred embodiment, the soaking temperature of the billet in the heating step is 1100 to 1199°C (for example, 1120°C, 1140°C, 1150°C, 1160°C, 1180°C, 1195°C).

[0020] In the prior art, the general soaking temperature is 1200-1260℃, the present invention adopts a relatively low soaking temperature, mainly considering two aspects: 1. Low temperature soaking meets the demand of the billet high temperature stage and does not affect the subsequent rolling temperature; 2. Considering the initial temperature of rolling cooling control, low temperature soaking is easy to obtain the desired final rolling temperature during the rolling cooling control process.

[0021] In the rolling method with controlled rolling cooling, as a preferred embodiment, in the rolling step, the final rolling temperature of the two-roll rolling is 850 to 900° C. (for example, 860° C., 870° C., 880° C., or 890° C.).

[0022] In the rolling method of the above rolling cooling control, as a preferred embodiment, in the rolling step, the rolling method is two-roll rolling.

[0023] In the rolling method of the above rolling cooling control, as a preferred embodiment, in the rolling step, the rolled material obtained by the two-roll rolling is used as a master batch for KOCKS rolling.

[0024] In the above-mentioned rolling method with controlled rolling cooling, as a preferred embodiment, in the rolling process, the two-roll rolling adopts a short stress wire rolling machine, which can effectively improve the rolling stability and ensure the deformation of the rolled material; and according to the selected blank dimensions, the two-roll rolling can achieve a large compression ratio (10-30), which can improve the uniformity of the core structure of the rolled material, and is favorable for improving the uniformity of the performance of the rolled material.

[0025] In the rolling method of the above-mentioned rolling cooling control, as a preferred embodiment, in the cooling step, the cooling is performed by using spray cooling to cool the rolled material obtained after the final rolling to 800 to 849°C (e.g., 810°C, 820°C, 830°C, 840°C, 845°C), and then KOCKS rolling is performed.

[0026] In the present invention, if the cooling rate is too fast, the temperature after cooling is too low, so that the deformation resistance of the bar obtained by the rolling of the present invention increases, and the risk of the rolling roll of the KOCKS rolling machine breaking increases significantly. Therefore, in the present invention, the rolled material obtained after the final rolling is cooled to 800 to 849°C using spray cooling, and then the KOCKS rolling is performed.

[0027] In addition, the present invention adopts spray cooling, and uses a cooling device to rapidly cool the surface of the steel material, and reduces the core temperature of the rolled material through heat conduction, thereby gradually reducing the temperature difference between the core temperature and the surface temperature of the rolled material.

[0028] In the present invention, the rolled material is cooled to 800 to 849° C. in order to promote grain refinement by deformation at the final rolling temperature near the two-phase region temperature, thereby achieving the effect of homogenizing the rolled material structure.

[0029] In the present invention, the tank cooling passage to be selected in the spray cooling process is determined based on the dimensions of the rolled material obtained in the rolling process. In the spray cooling process, the tank cooling passage parameters (i.e., tank passage diameter) can be selected according to the following table.

[0030] TIFF0007621556000001.tif45160

[0031] The spray cooling rate can be calculated based on the number of tanks and the cooling water pressure. In the spray cooling process, the spray cooling method with gradually decreasing cooling intensity can be adopted to prevent the head of the rolled material from bending and causing production failure.

[0032] In the rolling method of the rolling cooling control, as a preferred embodiment, in the KOCKS rolling process, the cooled rolled material is KOCKS rolled, and the deformation amount is 20% to 100% (e.g., 25%, 30%, 50%, 70%, 80%, 90%), preferably, the KOCKS rolling uses three KOCKS rolling mills, more preferably, the distribution of the three rolling mills is alternating between forward Y and reverse Y, and the reduction in diameter of the bar is achieved by utilizing the rolling range where the three rolls are at 120° to each other. In the KOCKS rolling process, the larger the deformation amount, the more advantageous it is for improving the grain size and mechanical properties of the final material obtained.

[0033] In the present invention, KOCKS rolling adopts three-roll rolling technology, which has the following advantages compared with the traditional two-roll rolling technology:

[0034] The deformation efficiency of three-roll rolling is much higher than that of two-roll rolling. In three-roll groove type, the rolling force acts on the rolled material from three sides toward the core, and in three-roll groove type, the deformation is transformed into elongation more, the temperature rise during rolling (i.e. the rise in the surface temperature of the rolled material during the rolling process) is reduced, which is favorable for the temperature-controlled rolling of the rolled material, and in three-roll groove type, the rolled material is uniformly deformed along the interface, and a uniform metallographic structure and consistent grain size can be obtained.

[0035] In the above-mentioned rolling method of rolling cooling control, as a preferred embodiment, the secondary cooling process cools the KOCKS rolled material, and the secondary cooling includes a second spray cooling and a cold bed cooling.

[0036] In the rolling method of the above-mentioned rolling cooling control, as a preferred embodiment, the second spray cooling is performed by second spray cooling of the rolled material after KOCKS rolling, and the cooling rate is set to 20°C / min to 35°C / min (e.g., 23°C / min, 25°C / min, 30°C / min, 33°C / min), ensuring a cooling intensity sufficient to realize the upper cold bed temperature.

[0037] In the rolling method of the above rolling cooling control, as a preferred embodiment, the cold bed cooling is performed by cold bed cooling the bar obtained after the second spray cooling, and the upper cold bed temperature is ≦570°C, preferably 530 to 570°C (e.g., 540°C, 550°C, 560°C), and the lower cold bed temperature is 200 to 250°C (e.g., 210°C, 220°C, 230°C, 240°C).

[0038] The bar material manufactured using the technical solution of the present invention has a sorbite (S) structure near the surface of the bar material structure, and a ferrite + pearlite structure (F + P) structure inside. The bar material manufactured by the conventional process has a ferrite + pearlite structure.

[0039] Sorbite is a structure in a tempered state. In the present invention, the speed of the second spray cooling is relatively high, especially after the near-surface cooling, the temperature reaches below the martensite transition temperature (Ms) to form martensite, but the internal temperature is relatively high. After spraying, the internal temperature is led outward, and finally returns to the upper cold bed at 530-570°C. After warming, it corresponds to the self-tempering process, and the martensite changes to sorbite.

[0040] In the present invention, the above technical features can be freely combined to form a new technical solution when they do not conflict with each other.

[0041] The technical solutions not described in detail in the present invention may adopt conventional techniques in the art.

[0042] Compared with the prior art, the beneficial effects of the present invention are: 1. The present invention adopts a relatively low soaking temperature, which meets the requirements of the high temperature stage in the billet heating process, ensures the uniformity of billet heating, and does not affect the subsequent rolling temperature. The adoption of low-temperature soaking makes it easier to obtain the desired final rolling temperature in the rolling cooling control process. 2. The rolling cooling control method of the present invention can realize the uniformity of the rolled material structure and improve the uniformity of the rolled material performance. 3. By adopting the technical solution of the present invention, the yield strength and low temperature impact performance of steel with the same chemical composition can be significantly improved with a lower start rolling temperature, a lower final rolling temperature, a higher spray intensity and a lower upper cold bed temperature. [Brief description of the drawings]

[0043] [Figure 1] FIG. 1A is a metallographic image at a magnification of 100 and 500 times, taken at 5 mm subcutaneously (below the surface) in the same location of a steel material produced in Example 1 of the present invention, where FIG. 1A is a metallographic image at a magnification of 100 and FIG. 1B is a metallographic image at a magnification of 500. [Diagram 2] 100x and 500x metallographic micrographs of the same location 15 mm below the skin surface of the steel material produced in Example 1 of the present invention, where (c) is a 100x metallographic micrograph and (d) is a 500x metallographic micrograph. [Diagram 3] FIG. 1 shows metallographic micrographs at 100x magnification and 500x magnification at 25 mm subcutaneously in the same location of the steel material produced in Example 1 of the present invention, where (e) is a metallographic micrograph at 100x magnification and (f) is a metallographic micrograph at 500x magnification. [Figure 4] 100x and 500x metallographic micrographs of the same location 5 mm below the skin of the steel material produced in Comparative Example 1, where (g) is a 100x metallographic micrograph and (h) is a 500x metallographic micrograph. [Diagram 5] 100x and 500x metallographic micrographs of the same location 15 mm below the skin of the steel material produced in Comparative Example 1, where (i) is a 100x metallographic micrograph and (j) is a 500x metallographic micrograph. [Figure 6]100x and 500x metallographic micrographs of the same location 25 mm below the skin of the steel material produced in Comparative Example 1, where (k) is a 100x metallographic micrograph and (l) is a 500x metallographic micrograph. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] The following provides a clear and complete description of the technical solutions according to the embodiments of the present invention based on the drawings of the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, and are not all of the embodiments. All other embodiments obtained based on the embodiments of the present invention under the premise that those skilled in the art have not performed creative labor, belong to the scope of protection of the present invention. The technical solution of the present invention will be described in more detail below with reference to the drawings and examples.

[0045] Example 1 This embodiment provides a rolling method for Q500 steel grade low alloy structural steel bar with controlled rolling and cooling, using a round billet with a diameter of 390 mm as raw material and rolling it to obtain a finished rolled bar with a diameter of 100 mm. The chemical composition of this billet (in mass%, wt%) is C: 0.16, Si: 0.30, Mn: 1.40, P: 0.020, S: 0.008, Cr: 0.06, V: 0.06, Al: 0.030, N: 0.0090, the balance being iron and unavoidable impurities, and the CEV is 0.42.

[0046] The rolling method of the rolling cooling control includes the steps of heating, rolling, spray cooling, KOCKS rolling, and cooling in sequence. The specific steps are as follows:

[0047] (1) Heating: Using stage heating technology, the billet is placed in the heating furnace and heated. The preheating temperature is 680℃, the first stage heating temperature is 1030℃, the second stage heating temperature is 1130℃, and the soaking temperature is 1130℃.

[0048] (2) Rolling: A two-roll rolling method was adopted, the initial rolling temperature was 1020°C, and the final rolling temperature was 860°C. A rolled material with a specification (diameter) of 115 mm was obtained in the final rolling.

[0049] (3) Spray cooling: The rolled material obtained after the final rolling is spray cooled, and the rolled material passes through the 1#, 2#, and 3# tanks in sequence. The specific spray cooling process parameters are shown in Table 1. After cooling to 830°C, KOCKS rolling is performed. Table 1 shows the spray cooling process parameters in this embodiment.

[0050] TIFF0007621556000002.tif30129

[0051] In this embodiment, the tank cooling passage including the nozzle is selected based on the size of the rolled raw material (i.e., the final rolled rolled material). In this embodiment, the cooling intensity of the spray cooling is gradually reduced to prevent production failure due to bending of the head of the rolled material.

[0052] (4) KOCKS rolling: The rolled material obtained after spray cooling was subjected to KOCKS rolling. The process parameters of the KOCKS rolling in this example are shown in Table 2. The deformation amount is the deformation amount of the cross-sectional area. Deformation amount = (raw material cross-sectional area - finished product cross-sectional area) / raw material cross-sectional area * 100%.

[0053] TIFF0007621556000003.tif35154

[0054] In the present invention, KOCKS rolling adopts three-roll rolling technology, which has the following advantages compared with the traditional two-roll rolling technology:

[0055] The deformation efficiency of three-roll rolling is much higher than that of two-roll rolling. In three-roll groove type, the rolling force acts on the rolled material from three sides toward the core, and in three-roll groove type, the deformation is transformed into elongation more, the temperature rise during rolling is reduced, which is favorable for the temperature-controlled rolling of the rolled material, and in three-roll groove type, the rolled material is uniformly deformed along the interface, and a uniform metallographic structure and consistent grain size can be obtained.

[0056] (5) Cooling process: includes second spray cooling and cold bed cooling. Second spray cooling: The second spray cooling is carried out on the rolled material after KOCKS rolling, and the rolled material passes through the 4#, 5# and 6# tanks in order. The process parameters are shown in Table 3.

[0057] TIFF0007621556000004.tif29127Note: The second cooling needs to achieve a strong cooling effect, so the tank opening degree must be maximized. However, the 6# cooling tank is adjacent to the shearing equipment, and excessive cooling intensity will cause uneven shear stress distribution in the shearing process and cause the rolled material to curve, so the rolled material must pass through the 6# tank at a low cooling rate.

[0058] Cold bed cooling: The bar obtained after spray cooling is cooled in a cold bed, with the upper cold bed temperature being 570°C and the lower cold bed temperature being 230-250°C. The final bar has a specification of Φ100mm, and its performance is shown in Table 4.

[0059] As can be seen from Table 4, the steel bar obtained by the rolling cooling control of the present invention has an impact of more than 100 J at -40°C, and meets the performance requirements for Q500 steel grade (three numbers: Q500MC, Q500MD, and Q500ME) in the GB / T1591-2018 standard.

[0060] TIFF0007621556000005.tif115164

[0061] The structure of the bar produced in this example is shown in FIGS. Figure 1 shows the metallographic structure of the same site of the rod produced in this example, 5 mm subcutaneously (subsurface), at different magnifications, where Figure 1A shows the metallographic structure at 100 times magnification and Figure 1B shows the metallographic structure at 500 times magnification. From Figure 1 (Figure 1A and Figure 1B), it can be seen that the structure of the rod is uniform, the structure is sorbite (S), and the grain size is class 9.

[0062] Figure 2 shows the metallographic structure of the bar produced in this example at 15 mm below the skin surface at different magnifications, where (c) shows the metallographic structure at 100 times magnification and (d) shows the metallographic structure at 500 times magnification. From Figure 2, it can be seen that the structure of the bar is uniform, the structure is ferrite + pearlite (F+P), and the grain size is class 9.

[0063] Figure 3 shows the metallographic structure of the same part of the bar produced in this example at 25 mm below the skin surface at different magnifications, where (e) is the metallographic structure magnified 100 times, and (f) is the metallographic structure magnified 500 times. From Figure 3, it can be seen that the structure of the bar is uniform, the structure is F+P, and the grain size is class 8.

[0064] Example 2 This embodiment provides a rolling method for controlling the rolling and cooling of Q500 steel-class low alloy structural steel bar. A circular billet with a specification of 390 mm is used as the raw material. The chemical compositions of this billet (in mass%, wt%) are C: 0.11, Si: 0.35, Mn: 1.45, P: 0.018, S: 0.005, Cr: 0.18, V: 0.08, Al: 0.030, N: 0.0100, the balance being Fe and unavoidable impurities, and the CEV is 0.404.

[0065] Using the rolling method of rolling and cooling control similar to that of Example 1, including heating, rolling, spray cooling, KOCKS rolling, and cooling in order, the final bar specification obtained was Φ100mm, and its performance is shown in Table 4. As can be seen from Table 4, the steel bar obtained by the rolling and cooling control of the present invention had an impact of more than 100J at -40℃, and met the performance requirements for Q500 steel grade (three numbers: Q500MC, Q500MD, and Q500ME) of the GB / T1591-2018 standard.

[0066] The rod material produced in this example has a structure of sorbite (S) at 5 mm subcutaneously in the same location, a crystal grain size of class 9, and a uniform rod material structure. The structure of the rod material at 15 mm subcutaneously in the same location is F+P, a crystal grain size of class 9, and a uniform rod material structure. The structure of the rod material at 25 mm subcutaneously in the same location is F+P, a crystal grain size of class 8, and a uniform rod material structure.

[0067] Example 3 This embodiment provides a rolling method for controlling the rolling and cooling of Q500 steel-class low alloy structural steel bar. The raw material is a billet with a specification of 300×400 mm, and the chemical composition of this billet is the same as that of the billet in embodiment 1.

[0068] The rolling method of this rolling and cooling control includes the steps of heating, rolling, spray cooling, KOCKS rolling, and cooling, and obtains a finished rolled material with a specification (diameter) of 50 mm. The specific steps are as follows:

[0069] (1) Heating: Using stage heating technology, the billet is placed in the heating furnace and heated. The preheating temperature is 650℃, the first stage heating temperature is 1080℃, the second stage heating temperature is 1180℃, and the soaking temperature is 1180℃.

[0070] (2) Rolling: A two-roll rolling method was adopted, the initial rolling temperature was 1045°C, and the final rolling temperature was 890°C, and a rolled material with a specification (diameter) of 60 mm was obtained.

[0071] (3) Spray cooling: The rolled material obtained after the final rolling is spray cooled to 830° C., and then KOCKS rolling is performed. Table 5 shows the process parameters of the spray cooling in this example.

[0072] TIFF0007621556000006.tif31126

[0073] In this embodiment, the tank cooling passage with a spray ring diameter of Φ110 is selected based on the size of the rolling raw material (i.e., the final rolled rolled material). In this embodiment, the intensity of the spray cooling is gradually reduced to prevent production failures caused by bending of the head of the rolling material.

[0074] (4) KOCKS rolling: The rolled material obtained after spray cooling was subjected to KOCKS rolling. The process parameters of the KOCKS rolling in this example are shown in Table 6. The deformation amount is the deformation amount of the cross-sectional area. Deformation amount = (raw material cross-sectional area - finished product cross-sectional area) / raw material cross-sectional area * 100%.

[0075] TIFF0007621556000007.tif35141

[0076] In the present invention, KOCKS rolling adopts three-roll rolling technology, which has the following advantages compared with the traditional two-roll rolling technology:

[0077] The deformation efficiency of three-roll rolling is much higher than that of two-roll rolling. In three-roll groove type, the rolling force acts on the rolled material from three sides toward the core, in three-roll groove type, deformation is more converted to elongation, the temperature rise during rolling is reduced, which is favorable for temperature-controlled rolling of the rolled material, and in three-roll groove type, the rolled material can be uniformly deformed along the interface, and a uniform metallographic structure can be obtained, and the grain size is consistent.

[0078] (5) Cooling process: includes second spray cooling and cold bed cooling. Second spray cooling: The second spray cooling was carried out on the rolled material after KOCKS rolling, and the process parameters are shown in Table 7.

[0079] TIFF0007621556000008.tif32129

[0080] Cold bed cooling: The bar obtained after spray cooling is cooled under a cold bed, with the upper cold bed temperature being 540°C and the lower cold bed temperature being 220-240°C. The specifications of the finally obtained bar are Φ50mm, and its performance is shown in Table 4. As can be seen from Table 4, the steel bar obtained by the rolling cooling control of the present invention has an impact of more than 100J at -40°C, and meets the performance requirements for Q500 steel grade (three numbers: Q500MC, Q500MD, and Q500ME) of the GB / T1591-2018 standard.

[0081] The rod material produced in this example has a structure of sorbite (S) at 5 mm subcutaneously in the same location, a crystal grain size of class 9, and a uniform rod material structure. The structure of the rod material at 15 mm subcutaneously in the same location is F+P, a crystal grain size of class 9, and a uniform rod material structure. The structure of the rod material at 25 mm subcutaneously in the same location is F+P, a crystal grain size of class 8, and a uniform rod material structure.

[0082] Comparative Example 1 In this comparative example, a circular billet with specifications of 390 mm was used as the raw material, and a finished rolled material with specifications of 100 mm was obtained using a conventional rolling method. The chemical composition of this billet (wt%) was C: 0.16, Si: 0.30, Mn: 1.40, P: 0.020, S: 0.008, Cr: 0.06, V: 0.06, Al: 0.030, N: 0.0090, with the remainder being iron and unavoidable impurities, and the CEV was 0.43.

[0083] This conventional rolling method includes the steps of heating, rolling and spray cooling in order to obtain a finished rolled material with a specification of 100 mm. The specific steps are as follows:

[0084] (1) Heating: Using stage heating technology, the billet is placed in the heating furnace and heated. The pre-heating temperature is 650℃, the first stage heating temperature is 1080℃, the second stage heating temperature is 1240℃, and the soaking temperature is 1240℃.

[0085] (2) Rolling: A two-roll rolling method was adopted, the initial rolling temperature was 1120°C, and the final rolling temperature was 950°C, and a rolled material with a specification of 100 mm was obtained.

[0086] (3) Spray cooling: The rolled material obtained after the final rolling is spray cooled. Table 8 shows the process parameters of the spray cooling in this example.

[0087] TIFF0007621556000009.tif24127

[0088] Cold bed cooling: The bar obtained after spray cooling is cooled in a cold bed, with the upper cold bed temperature at 750°C and the lower cold bed temperature at 300-350°C. The final bar specification is Φ100mm, and its performance is shown in Table 4. As can be seen from Figure 5, for the same chemical composition, the yield strength, tensile strength and impact energy of the bar produced by the conventional method meet the standard requirements, but they are all low.

[0089] The structure of the bar produced in this example is ferrite + pearlite (F+P) at 5 mm below the skin at the same location, the grain size is class 7, and the bar structure is uniform (as shown in Figure 4). The structure of the bar at 15 mm below the skin at the same location is F+P, the grain size is class 6.5, and the bar structure is uniform (as shown in Figure 5). The structure of the bar at 25 mm below the skin at the same location is F+P, the grain size is class 6, and the bar structure is uniform (as shown in Figure 6).

[0090] To sum up the above analysis and consideration, the technical solution of the present invention can obviously improve the yield strength and low temperature impact performance of steel with the same chemical composition.

[0091] The above examples are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Those skilled in the art may change and modify these examples without violating the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical ideas disclosed in the present invention should be covered by the claims of the present invention.

Claims

1. A method for controlled rolling and controlled cooling of a Q500 steel grade low alloy structural steel bar, the chemical composition of the low alloy structural steel bar being, in mass%, C 0.05%-0.18%, Si 0.20%-0.40%, Mn 1.00%-1.60%, P≦0.030%, S≦0.030%, Cr≦0.20% (excluding 0%), Ni≦0.20%, Mo≦0.1 0%, Cu≦0.15%, V0.02%-0.10%, Nb≦0.05%, Ti≦0.02%, B≦0.004%, N0.005%-0.012%, Al0.02%-0.04%, CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≦0.45%, the remainder being Fe and unavoidable impurities, The C content in the low alloy structural steel bar is 0.10% to 0.18% by mass, The diameter of the low alloy structural steel bar is 20 to 120 mm; The CEV is 0.40% to 0.45%. The controlled rolling and controlled cooling method uses a circular billet having a diameter of 100 mm to 390 mm as a raw material, or a square billet having a diameter of (100 mm to 300 mm) x (100 mm to 400 mm) as a raw material, and the diameter of the final rolled finished bar is 20 mm to 120 mm; The properties of the Q500 steel grade low alloy structural steel bar are R p0.2 ≧470 MPa, R m is 600-750 MPa, A (elongation at break) ≧25%, and −40° C. longitudinal KV 2 ≧100 J; The controlled rolling and controlled cooling method includes the steps of heating, rolling, cooling, three-roll rolling, and secondary cooling, The three-roll rolling process is carried out using three three-roll rolling mills, the distribution of the three three-roll rolling mills is alternating between forward Y and reverse Y, and the reduction in diameter of the bar is realized by utilizing the rolling range in which the three rolls are at 120° to each other. In the heating step, the billet is heated at a soaking temperature of 1100 to 1199°C. In the rolling step, the heated billet is rolled, the initial rolling temperature is 1000 to 1050° C., the final rolling temperature is 850 to 900° C., and the rolling method is two-roll rolling; In the cooling step, spray cooling is used for the cooling, and the rolled material obtained after the final rolling is cooled to 800 to 849 ° C., and then the three-roll rolling is performed. In the spray cooling, a spray cooling method is used in which the cooling intensity is gradually reduced; In the three-roll rolling step, the cooled rolled material is rolled by the three-roll rolling machine, and the deformation amount of the cross-sectional area is 20% to 100% (excluding 100%), In the secondary cooling step, the rolled material rolled in the three-roll rolling step is cooled, and the secondary cooling includes a second spray cooling and cold bed cooling, and in the second spray cooling, the rolled material after the three-roll rolling is spray cooled for the second time, and the cooling rate is 20°C / min to 35°C / min, and in the cold bed cooling, the bar material obtained after the second spray cooling is cold bed cooled, and the upper cold bed temperature is ≦570°C and the lower cold bed temperature is 200 to 250°C. A method for controlled rolling and controlled cooling of Q500 grade low alloy structural steel bar material.

2. In the rolling step, the rolled material obtained from the two-roll rolling is used as a master batch for the three-roll rolling step. The method for controlled rolling and controlled cooling of Q500 steel grade low alloy structural steel bar according to claim 1.

3. In the rolling step, the two-roll rolling is performed using a short stress wire rolling machine. The method for controlled rolling and controlled cooling of Q500 steel grade low alloy structural steel bar according to claim 1.

4. The upper cold bed temperature is 530 to 570 ° C. The method for controlled rolling and controlled cooling of Q500 steel grade low alloy structural steel bar according to claim 1.

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