Method for producing HRB500E hot-rolled ribbed steel bar with yield ratio larger than or equal to 1.28 by adopting direct rolling process

Abstract

The invention discloses a method for producing an HRB500E hot-rolled ribbed steel bar with the yield ratio larger than or equal to 1.28 through a direct rolling technology, and the method comprises the steps that molten steel used for producing the HRB500E hot-rolled ribbed steel bar is provided, and the molten steel used for producing the HRB500E hot-rolled ribbed steel bar comprises, by mass, 0.21%-0.25% of C, 0.40%-0.60% of Si, 1.30%-1.50% of Mn, smaller than or equal to 0.045% of P, smaller than or equal to 0.045% of S, 0.042%-0.080% of V, 0.010%-0.030% of Nb, 0.012%-0.022% of N and the balance iron; molten steel used by the HRB500E hot-rolled strip steel bar is subjected to continuous casting, and a continuous casting steel billet is prepared; the continuous casting steel billet is subjected to direct rolling treatment, the direct rolling treatment comprises the steps that rough rolling, intermediate rolling, pre-finish rolling controlled cooling treatment and finish rolling are sequentially conducted on the continuous casting steel billet, and the hot-rolled ribbed steel bar is obtained; and the hot-rolled ribbed steel bar is subjected to cooling bed cooling, flush treatment, shearing and shaping treatment, and the HRB500E hot-rolled ribbed steel bar with the yield ratio larger than or equal to 1.28 is obtained.

Description

Technical Field

[0001] The present application belongs to the technical field of steel manufacturing, and in particular, relates to a method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio of ≥1.28 by using a direct rolling process. Background Art

[0002] HRB500E hot-rolled ribbed steel bars have high strength, good seismic performance and wide application. GB / T1499.2-2018 "Steel for reinforced concrete Part 2: Hot-rolled ribbed steel bars" clearly stipulates that the strength-to-yield ratio Rm / Rel of HRB500E hot-rolled ribbed steel bars must be ≥1.25. High-speed bars adopt advanced control and cooling technology, and the steel bar dimensions produced are precisely controlled, the surface quality is excellent, and the final rolling speed can reach 45m / s, which has the advantages of high yield, high quality and low cost. At present, the common high-speed bar production line mainly produces HRB500E hot-rolled ribbed steel bars with a diameter of Φ12mm~Φ25mm. Compared with the hot-rolled ribbed steel bars of the same specification produced by the conventional bar production line, the HRB500E hot-rolled ribbed steel bars produced by the high-speed bar line are more easily affected by the grain size, which makes the strength-to-yield ratio of the steel bars produced by the high-speed bar line lower. The problem of strength-to-yield ratio Rm / ReL being lower than 1.25 often occurs during the production process, and the smaller the specification, the more prominent the problem of unqualified strength-to-yield ratio.

[0003] The prior art discloses a method for producing HRB500E coil by direct rolling, which eliminates the heating relief of the heating furnace, reduces the temperature drop by controlling the continuous casting speed, controlling the direct rolling insulation roller conveying, controlling the rolling temperature and roller speed, and improves the uniformity of the rolling temperature and the performance stability. Summary of the invention

[0004] The embodiment of the present application provides a method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio ≥ 1.28 using a direct rolling process, which can ensure that the strength-to-yield ratio of HRB500E hot-rolled ribbed steel bars with a diameter of Φ12mm to Φ25mm is ≥ 1.28, effectively reducing fuel consumption costs and improving the product competitiveness of small and medium-sized HRB500E hot-rolled ribbed steel bars.

[0005] In a first aspect, the present application provides a method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio of ≥1.28 by a direct rolling process, comprising: providing molten steel used for producing HRB500E hot-rolled ribbed steel bars, wherein the composition of the molten steel used for the HRB500E hot-rolled ribbed steel bars is, by mass percentage, C: 0.21% to 0.25%, Si: 0.40% to 0.60%, Mn: 1.30% to 1.50%, P≤0.045%, S≤0.045%, V: 0.042% to 0.080%, Nb : 0.010%~0.030%, N: 0.012%~0.022%, the balance is iron; the molten steel used for HRB500E hot-rolled strip steel bar is continuously cast to obtain a continuously cast steel billet; the continuously cast steel billet is directly rolled, and the direct rolling treatment includes sequentially performing rough rolling, intermediate rolling, pre-finishing rolling controlled cooling treatment, and finishing rolling on the continuously cast steel billet to obtain a hot-rolled ribbed steel bar; the hot-rolled ribbed steel bar is cooled on a cooling bed, end-dressed, sheared, and finished to obtain a HRB500E hot-rolled ribbed steel bar with a strength-to-yield ratio ≥1.28.

[0006] According to an embodiment of the present application, molten steel for producing HRB500E hot-rolled strip steel bars is provided, including: smelting molten steel in a converter, so that the requirements of the molten iron entering the furnace must meet the following requirements: S≤0.040Wt.%, bottom blowing nitrogen is adopted throughout the converter smelting process, the flow rate of blowing nitrogen is increased in the later stage of blowing, and the stirring of the molten pool is strengthened; the converter smelting end point is controlled to be C≤0.15Wt.%, and both P and S are ≤0.040Wt.%, to obtain converter molten steel; and LF refining is performed on the converter molten steel to obtain LF refined molten steel.

[0007] According to an embodiment of the present application, for HRB500E hot-rolled ribbed steel bars with a specification of Φ12mm~Φ18mm, the composition of the LF refined molten steel of the HRB500E hot-rolled ribbed steel bars at the LF refining end point is, by mass percentage: C: 0.21%~0.25%, Si: 0.40%~0.60%, Mn: 1.30%~1.50%, P≤0.045%, S≤0.045%, V: 0.042%~0.067%, Nb: 0.010%~0.030%, N: 0.012%~0.021%, and the remainder is iron and unavoidable impurities.

[0008] According to an embodiment of the present application, for HRB500E hot-rolled ribbed steel bars with a specification of Φ20mm~Φ25mm, the composition of the LF refined steel liquid of the HRB500E hot-rolled ribbed steel bars at the LF refining end point is, by mass percentage: C: 0.21%~0.25%, Si: 0.40%~0.60%, Mn: 1.30%~1.50%, P≤0.045%, S≤0.045%, V: 0.055%~0.080%, Nb: 0.010%~0.030%, N: 0.013%~0.022%, and the remainder is iron and unavoidable impurities.

[0009] According to an embodiment of the present application, a continuous casting machine is used to continuously cast the molten steel used to produce HRB500E hot-rolled strip steel bars, and the casting speed of the continuous casting machine is 3.2 m / min to 3.6 m / min.

[0010] According to an embodiment of the present application, the direct rolling process for the continuous casting steel billet adopts full continuous rolling.

[0011] According to an embodiment of the present application, the equipment used for direct rolling processing includes 6 roughing mills, 4 intermediate mills, 6 pre-finishing mills, 4 finishing mills and 2 finishing mills; among them, the finishing mills and the finishing mills both adopt top-intersecting 45° cantilever module mills, adopting a 4+2 layout.

[0012] According to the embodiments of the present application, double-line rolling is adopted for HRB500E hot-rolled ribbed steel bars with specifications of Φ12~Φ25mm; wherein, when rolling HRB500E hot-rolled ribbed steel bars with specifications of Φ14mm~Φ16mm, the finishing rolling unit is empty for more than 2 frames; when rolling HRB500E hot-rolled ribbed steel bars with specifications of Φ18mm~Φ20mm, the finishing rolling unit is not put into use; when rolling HRB500E hot-rolled ribbed steel bars with specifications of Φ22mm~Φ25mm, the pre-finishing rolling unit is empty for more than 2 frames, and the finishing rolling unit is not put into use.

[0013] According to the embodiments of the present application, the continuous casting steel billet is subjected to rough rolling, intermediate rolling, pre-finishing controlled cooling treatment, and finishing rolling in sequence, including: adopting controlled cooling treatment before and after finishing rolling, controlled cooling treatment after finishing controlled cooling recovery, and cooling intensity of cooling by graded control of cooling bed, wherein at least 20m controlled cooling device is adopted after the pre-finishing mill.

[0014] According to the embodiment of the present application, for HRB500E hot-rolled ribbed steel bars with specifications of Φ12mm~Φ18mm, the finishing rolling temperature is 980℃~1040℃, the temperature entering the finished product rack is 940℃~1000℃, and the temperature of the upper cooling bed is 920℃~980℃.

[0015] According to the embodiment of the present application, for HRB500E hot-rolled ribbed steel bars with a specification of Φ20mm~Φ25mm, the finishing rolling temperature is 940℃~1000℃, the temperature entering the finished product rack is 920℃~980℃, and the temperature of the upper cooling bed is 900℃~960℃.

[0016] According to the embodiment of the present application, HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio ≥ 1.28 need to meet the following requirements: the grain size of pearlite is 7.3 μm to 12.0 μm, and the amount of pearlite accounts for 35% to 42%.

[0017] The embodiment of the present application adopts a method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio of ≥1.28 by a direct rolling process. By increasing the Mn content in the steel and adding the V element to combine with the N element in the steel, the yield strength and tensile strength of the steel are improved at the same time, and the N element in the steel is controlled within a reasonable range, so that most of the N is combined with the V to form the compound VN, and the free N in the steel is reduced; and the Nb element is added to the steel. The Nb content and the high manganese content work together to significantly improve the hardenability of the steel, so that the CCT curve of the steel shifts to the right, and pearlite is formed in advance under the same cooling conditions, so that the HRB500E hot-rolled ribbed steel bars with a yield ratio greater than 1.28 are obtained, which solves the problem that the strength-to-yield ratio is easily lower than 1.25 when the HRB500E earthquake-resistant steel bars are produced by direct rolling of high-speed bar lines. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solution of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0019] Figure 1 It is a flow chart of a method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio ≥ 1.28 by using a direct rolling process provided in an embodiment of the present application.

[0020] Figure 2 This is a metallographic image of the cross section of HRB500E hot-rolled ribbed steel bar with a specification of Φ16 mm prepared in comparative example 3, magnified 500 times.

[0021] Figure 3 This is a metallographic image of the cross section of the HRB500E hot-rolled ribbed steel bar with a specification of Φ16 mm obtained in Example 3, magnified 500 times. DETAILED DESCRIPTION

[0022] The features and exemplary embodiments of various aspects of the present application will be described in detail below. In order to make the purpose, technical solutions and advantages of the present application clearer, the present application will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain the present application, rather than to limit the present application. For those skilled in the art, the present application can be implemented without the need for some of these specific details. The following description of the embodiments is only to provide a better understanding of the present application by illustrating the examples of the present application.

[0023] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the statement "include..." do not exclude the presence of other identical elements in the process, method, article or device including the elements.

[0024] As described in the background technology section, the HRB500E hot-rolled ribbed steel bars with a specification of Φ12-25 mm produced by the existing high-speed bar production line are more easily affected by the grain size than the HRB500E hot-rolled ribbed steel bars with the same specification produced by the conventional bar production line: Figure 2 As shown in the figure, under the same composition system, the grain size of HRB500E hot-rolled ribbed steel bars of the same specification obtained in the high-bar line is smaller (generally 5.5μm to 9.5μm), and the proportion of pearlite is small (generally 32% to 39%), which makes the strength-to-yield ratio of HRB500E hot-rolled ribbed steel bars produced by the high-bar line low, that is, the strength-to-yield ratio Rm / ReL of the produced HRB500E hot-rolled ribbed steel bars is lower than 1.25. And the smaller the specification, the more prominent the problem of unqualified strength-to-yield ratio.

[0025] In order to solve the problems of the prior art, the embodiment of the present application provides a method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio ≥ 1.28 by direct rolling process. The method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio ≥ 1.28 by direct rolling process provided by the embodiment of the present application is first introduced below.

[0026] like Figure 1As shown, the method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio of ≥1.28 by direct rolling process comprises: providing molten steel used for producing HRB500E hot-rolled ribbed steel bars, wherein the composition of the molten steel used for the HRB500E hot-rolled ribbed steel bars is as follows, by mass percentage: C: 0.21% to 0.25%, Si: 0.40% to 0.60%, Mn: 1.30% to 1.50%, P≤0.045%, S≤0.045%, V: 0.042% to 0.080%, Nb: 0.0 10%~0.030%, N: 0.012%~0.022%, the balance is iron; the molten steel used for HRB500E hot-rolled strip steel bar is continuously cast to obtain a continuously cast steel billet; the continuously cast steel billet is directly rolled, and the direct rolling treatment includes sequentially performing rough rolling, intermediate rolling, pre-finishing rolling controlled cooling treatment, and finishing rolling on the continuously cast steel billet to obtain a hot-rolled ribbed steel bar; the hot-rolled ribbed steel bar is cooled on a cooling bed, end-dressed, sheared, and finished to obtain a HRB500E hot-rolled ribbed steel bar with a strength-to-yield ratio ≥1.28.

[0027] The present application adopts a direct rolling process to produce a HRB500E hot-rolled ribbed steel bar with a strength-to-yield ratio of ≥1.28. The method increases the Mn content in the steel and adds the V element to combine with the N element in the steel, thereby increasing the yield strength and tensile strength of the steel at the same time, controlling the N element in the steel within a reasonable range, so that most of the N is combined with the V to form a compound VN, and reducing the free N in the steel; and adding the Nb element to the steel. The Nb content and the high manganese content work together to significantly improve the hardenability of the steel, shift the CCT curve of the steel to the right, and form pearlite in advance under the same cooling conditions, so as to obtain the HRB500E hot-rolled ribbed steel bar with a strength-to-yield ratio greater than 1.28, thereby solving the problem that the strength-to-yield ratio is easily lower than 1.25 when producing HRB500E earthquake-resistant steel bars by direct rolling of high-speed bar lines.

[0028] The present application adopts a method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio ≥ 1.28 by direct rolling process. The principle of improving the strength-to-yield ratio of HRB500E hot-rolled ribbed steel bars is as follows: through the design of steel composition, the hardenability of steel is slightly improved, the CCT curve of steel is shifted to the right, the content of Mn element that contributes more to tensile strength is controlled according to the upper limit, and a weaker controlled rolling and controlled cooling process is used in the process of direct rolling HRB500E hot-rolled ribbed steel bars to form pearlite in advance, so as to obtain pearlite with relatively large grain size and suitable grain size, increase the grain size of pearlite to 7.3μm~12.0μm, and make the proportion of pearlite to 35%~42%, so that HRB500E hot-rolled ribbed steel bars with a diameter specification of Φ12mm~Φ25mm and a strength-to-yield ratio Rm / Rel greater than 1.28 can be obtained. It should be noted that pearlite is a mechanical mixture composed of alternating ferrite and cementite lamellae.

[0029] In the embodiments of the present application, the molten steel used to prepare the steel has a reasonable C and Mn content. The C element in the composition is the main element that effectively improves the strength and strength-to-yield ratio of the steel, and is also the cheapest alloying element. The inventors of the present application found that for every 0.01wt.% increase in the C element in the steel, the yield strength is increased by about 10MPa, and the contribution of the C element to the strength-to-yield ratio is greater than 1.25. In order to give full play to the role of the C element, the carbon element smelting composition is preferably 0.21wt.% to 0.25wt.%. The Mn element is also the main element that effectively improves the strength and strength-to-yield ratio of the steel. It is dissolved in ferrite and cementite in the steel, which can improve the stability of the supercooled austenite and improve the hardenability of the steel, but it is limited by the carbon equivalent of the steel bar. When the C element is 0.25wt.%, the Mn element content must be limited to 1.5wt.%. In order to give full play to the role of the Mn element, the Mn content is preferably 1.3wt.% to 1.5wt.%.

[0030] In the embodiments of the present application, by making the molten steel for preparing steel have reasonable V and N contents, V and N combine to produce VN to improve the strength of the steel, and the precipitated VN and V(C, N) particles serve as nucleation particles of ferrite, promoting the formation of intracrystalline ferrite. The amount of ferrite increases with the increase of precipitates such as VN, V(C, N), which is obviously beneficial to improve the strength of the steel bar. However, although excessive N elements have a strengthening effect, they also reduce the strength-to-yield ratio. The inventors of the present application have found through practice that it is more reasonable to control the V content in the steel at 0.042% to 0.080% and the N content in the steel at 0.012wt.% to 0.022wt.%.

[0031] In the embodiment of the present application, the molten steel for preparing HRB500E hot-rolled ribbed steel bars has a reasonable Nb content. The trace amount of Nb dissolved in austenite can delay the precipitation of proeutectoid ferrite and increase the time for austenite to start decomposing and separating pearlite. Figure 3 As shown, the amount of pearlite in HRB500E hot-rolled ribbed steel bars is increased to 35% to 42%, and the grain size of the pearlite is made to reach 7.3μm to 12.0μm.

[0032] In the embodiments of the present application, since Nb increases the recrystallization temperature of austenite, rolling of austenite in the recrystallization zone can be achieved at a higher temperature, and the steel is controlled to enter the finishing mill and the finished product unit for rolling at a higher temperature, which can avoid the adverse effects of fine grain strengthening on the strength-to-yield ratio, and is also beneficial to the formation of iron oxide scale on the surface of the steel bar after rolling.

[0033] In some embodiments of the present application, a continuous casting machine is used to continuously cast the molten steel used for HRB500E hot-rolled ribbed steel bars, and the pulling speed of the continuous casting machine is 3.2m / min to 3.6m / min. Exemplarily, the pulling speed of the continuous casting machine is 3.2m / min, 3.3m / min, 3.45m / min, 3.5m / min, and 3.6m / min. The present application controls the pulling speed of the continuous casting machine so that the obtained continuous casting steel billet is directly sent to the direct rolling process at a temperature that meets the direct rolling requirements for rolling. If the pulling speed is low, the temperature of the continuous casting slab is low, which does not meet the rolling temperature requirements, and it is difficult to obtain the required HRB500E hot-rolled ribbed steel bars.

[0034] The main processes of the method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio ≥ 1.28 by direct rolling process provided in the embodiment of the present application include: blast furnace molten iron smelting, molten iron desulfurization treatment, converter molten steel smelting, LF refining, slab continuous casting, rough rolling, intermediate rolling, pre-finishing rolling controlled cold cutting into two lines, double-line finishing rolling, controlled cooling after double-line rolling, double-line disc multiple-length shearing, double-line braking device, cooling on cooling bed, cold shearing to length, finishing, packaging, weighing, and warehousing. Among them, the pre-finishing rolling controlled cold treatment is to use the pre-finishing rolling mill to control the cooling to cut the continuous casting billet after rough rolling and intermediate rolling into a double-line intermediate rolling billet, and then use the finishing mill to carry out double-line finishing rolling on the intermediate rolling billet cut into two lines, and control cooling after double-line finishing rolling to obtain finished rolled steel; and use the finished rolling mill to roll the finished rolled steel into HRB500E hot-rolled ribbed steel bars. It should be noted that double-line finishing rolling can also adopt single-line finishing rolling, and this application does not make clear limitations on this.

[0035] In some embodiments of the present application, the direct rolling of the continuous casting billet uses full continuous rolling, and the equipment used for the direct rolling includes 6 roughing mills, 4 intermediate mills, 6 pre-finishing mills, 4 finishing mills and a finished product mill. Among them, in order to achieve hot machine rolling, the finishing mill and the finished product mill both use a top-crossed 45° cantilever module mill, and adopt a 4+2 arrangement. Among them, the finished product mill is a finished product unit that reduces the diameter of the hot-rolled ribbed steel bars.

[0036] The present application adopts a method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio of ≥1.28 by a direct rolling process. The above-mentioned composition control and full-continuous rolling direct rolling process are adopted. Appropriate cooling control process parameters are used in the hot machine rolling stage, as well as a weak water penetration process before and after the finish rolling, to avoid surface overcooling caused by concentrated cooling, which makes the size of pearlite precipitated in the austenite smaller and the content less, thereby improving the strength-to-yield ratio of the rolled HRB500E hot-rolled ribbed steel bars.

[0037] In an embodiment of the present application, the arrangement order of the rolling mill is: 6 roughing mills + 4 intermediate rolling mills + 6 pre-finishing mills → temperature-controlled 1# water tank → recovery section → 3# flying shear → 4*2 finishing mills → temperature-controlled 2# and 3# water tanks → recovery section → 4# flying shear → 2 finished product mills → temperature-controlled 4# water tank → recovery section → temperature-controlled 5# water tank → recovery section → pinch rollers → high-speed multiple-length flying shear → high-speed steel feeding system → cooling on cooling bed.

[0038] In the embodiment of the present application, the temperature control 1# water tank is mainly used to control the temperature of the steel entering the finishing mill at 940℃~1040℃. The temperature control 2# water tank and the temperature control 3# water tank are mainly used to control the temperature of the steel entering the finished rolling mill after the finishing rolling mill at 920℃~1000℃, and the temperature control 4# water tank and the temperature control 5# water tank are used to control the temperature of the steel on the cooling bed at 900℃~980℃. It should be noted that the weak water penetration process before and after the finishing rolling refers to the use of the temperature control 1# water tank and the temperature control 2# and 3# water tanks before and after the finishing rolling mill to cool the hot-rolled ribbed steel before and after the finishing rolling mill, so that the rolled hot-rolled ribbed steel reaches the required temperature before and after the finishing rolling mill. The finishing mill 4*2 and the finished product unit*2 respectively represent that the finishing mill and the finished product unit arranged after the pre-finishing mill are respectively provided with two rolling production lines.

[0039] In some embodiments of the present application, the roughing mill and the intermediate rolling mill both adopt micro-tension rolling, and vertical loops are set between the pre-finishing mill, the finishing mill and the finished rolling mill to implement tension-free controlled rolling to ensure the dimensional accuracy of the rolled parts.

[0040] In some embodiments of the present application, the rolled pieces are evenly sheared by the flying shear after the pre-finishing mill, and are respectively pre-water-cooled and then enter the finishing mill of each rolling line for rolling. According to different product specifications, they are rolled by the finishing mill and then enter the water cooling box. After recovery, they are finally rolled in the finished product mill, and then sent to the high-speed multiple-length flying shear for segmented shearing. After the speed of the rolled pieces is reduced to 4m / s by the clamping tail brake, the rolled bars, i.e., HRB500E hot-rolled ribbed steel bars, are sent to the stepping rack type cooling bed for cooling treatment through the drum steel feeding device. The cooling method of the cooling bed is natural cooling.

[0041] In the embodiment of the present application, both the finishing mill and the finished product mill adopt a modular rolling mill with an intersection angle of 45°, i.e., a tungsten carbide or high-speed steel roller ring rolling mill, and the rolled parts are subjected to unidirectional non-twisting micro-tension rolling to roll the rolled parts into products with high dimensional accuracy and high surface quality. The cone box and roller box of the modular rolling mill can be flexibly interchanged, and the motor can be turned off when part of the rolling mill is idle, which can effectively improve the utilization rate of the rolling rolls and reduce power consumption and roller consumption.

[0042] In the embodiments of the present application, HRB500E hot-rolled ribbed steel bars with specifications of Φ12 to Φ25 mm are all double-line rolled, wherein, when rolling HRB500E hot-rolled ribbed steel bars with specifications of Φ14 to Φ16 mm, the finishing rolling unit is empty for more than 2 frames; when rolling HRB500E hot-rolled ribbed steel bars with specifications of Φ18 to Φ20 mm, the finishing rolling unit is not put into use; when rolling HRB500E hot-rolled ribbed steel bars with specifications of Φ22 to Φ25 mm, the pre-finishing rolling unit is empty for more than 2 frames, and the finishing rolling unit is not put into use.

[0043] In the embodiment of the present application, the route of the direct rolling process is: blast furnace molten iron smelting → molten iron desulfurization treatment → converter molten steel smelting → LF refining → slab continuous casting → direct continuous rolling → cut to length → inspection packaging and warehousing; wherein, the process characteristics of each stage are: converter molten steel smelting, the molten iron entering the furnace must meet the requirements: S≤0.040Wt.%; during the converter smelting process, nitrogen is blown throughout the bottom, and the ladle bottom blowing flow rate is controlled at 50Nm3 / h~100Nm3 / h, and the nitrogen blowing flow rate is increased in the later stage of blowing. amount, strengthen the stirring of the molten pool; control the converter smelting end point C≤0.15Wt.%, P, S are both ≤0.040Wt.%, to obtain converter molten steel; LF refine the converter molten steel to obtain LF refined molten steel; continuously cast the LF refined molten steel to obtain continuous casting slab; the pouring temperature of the tundish is 1525℃~1545℃, the tundish uses ordinary covering agent, uses ordinary slab protection slag, and the single-stream pulling speed of the continuous casting billet is 3.2m / s~3.6m / s to obtain the continuous casting billet;

[0044] The continuous casting steel billet is subjected to direct rolling treatment on a high-speed bar line: the starting rolling temperature is 880°C to 980°C, a 22-stand hot rolling mill is used, and a cooling control device is installed and used before finishing rolling, before the last two stands of finishing rolling, and after finishing rolling. For example, the starting rolling temperature of the direct rolling treatment of the continuous casting steel billet is 880°C, 890°C, 895°C, 900°C, 905°C, 910°C, 915°C, 920°C, 930°C, 935°C, 945°C, 950°C, 960°C, 975°C, and 978°C.

[0045] In the embodiments of the present application, the continuous casting steel billet is subjected to rough rolling, intermediate rolling, pre-finishing controlled cooling treatment, and finishing rolling in sequence, including: controlled cooling treatment before and after finishing rolling, controlled cooling treatment after recovery of controlled cooling in finishing rolling, and cooling intensity of cooling by graded control of cooling on the cooling bed, wherein at least 20m controlled cooling device is used after the pre-finishing mill, which can realize the weak water penetration process after rolling, avoid the surface overcooling caused by the concentrated cooling of the steel during the rolling process of HRB500E hot-rolled ribbed steel bars, thereby preventing the strength-to-yield ratio of HRB500E hot-rolled ribbed steel bars from not meeting the requirements.

[0046] In some embodiments of the present application, the finishing mill exit speed of the hot rolling mill is not less than 10 m / s, and the maximum design rolling speed is 45 m / s.

[0047] In some embodiments of the present application, the roughing mill, intermediate mill and pre-finishing mill of the high-speed rolling line adopt short stress line mills, which have high rigidity and small bounce, which is conducive to ensuring the dimensional accuracy of the rolled HRB500E hot-rolled ribbed steel bar products; the roller bearings have a long life and can be replaced as a whole frame, the roller changing time is short, and the operating rate is high.

[0048] In some embodiments of the present application, a 22-stand hot rolling mill is used in the rolling process, and cooling control equipment is installed and used before finishing rolling, before the last two stands of finishing rolling, and after finishing rolling. The starting rolling temperature (the temperature out of the rough rolling mill) is 880°C to 980°C.

[0049] In the embodiment of the present application, the HRB500E hot-rolled ribbed steel bar produced by the direct rolling process with a strength-to-yield ratio of ≥1.28 has a structure mainly composed of ferrite and pearlite, and the content of bainite and martensite in the cross section is less than 4%.

[0050] In some embodiments of the present application, for HRB500E hot-rolled ribbed steel bars with specifications of Φ12mm~Φ18mm, the composition of the LF refined steel liquid of the HRB500E hot-rolled ribbed steel bars at the end point of LF refining is, by mass percentage: C: 0.21%~0.25%, Si: 0.40%~0.60%, Mn: 1.30%~1.50%, P≤0.045%, S≤0.045%, V: 0.042%~0.067%, Nb: 0.010%~0.030%, N: 0.012%~0.021%, and the remainder is iron and unavoidable impurities.

[0051] In some embodiments of the present application, for HRB500E hot-rolled ribbed steel bars with specifications of Φ12mm~Φ18mm, the finishing rolling temperature is 980℃~1040℃, the temperature entering the finished product rack is 940℃~1000℃, and the temperature of the upper cooling bed is 920℃~980℃.

[0052] In some embodiments of the present application, for HRB500E hot-rolled ribbed steel bars with a specification of Φ20mm~Φ25mm, the composition of the LF refined steel liquid of the HRB500E hot-rolled ribbed steel bars at the end point of LF refining is, by mass percentage: C: 0.21%~0.25%, Si: 0.40%~0.60%, Mn: 1.30%~1.50%, P≤0.045%, S≤0.045%, V: 0.055%~0.080%, Nb: 0.010%~0.030%, N: 0.013%~0.022%, and the remainder is iron and unavoidable impurities.

[0053] In some embodiments of the present application, for HRB500E hot-rolled ribbed steel bars with specifications of Φ20mm~Φ25mm, the finishing rolling temperature is 940℃~1000℃, the temperature entering the finished product rack is 920℃~980℃, and the temperature of the upper cooling bed is 900℃~960℃.

[0054] In some embodiments of the present application, the strength-to-yield ratio of HRB500E hot-rolled ribbed steel bars with a specification of Φ12mm~Φ25mm produced by the direct rolling process with a strength-to-yield ratio ≥1.28 is increased to 1.28 or above, and the tensile strength reaches above 700MPa.

[0055] The method of producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio ≥ 1.28 using a direct rolling process in the embodiment of the present application is suitable for high-speed direct rolling production of bars. Depending on the specifications, the final rolling speed (exit speed of the finishing rolling unit) is 2 to 3 times higher than that of ordinary bar products of the same specifications.

[0056] In order to have a clearer understanding of the technical solution of the present application, further explanation is now provided through embodiments and comparative examples.

[0057] Examples 1-5

[0058] Molten steel used for producing HRB500E hot-rolled ribbed steel bars as shown in Table 1 below is provided, and the molten steel used for HRB500E hot-rolled ribbed steel bars is continuously cast to obtain a continuously cast steel billet, and its specific composition is shown in Table 3.

[0059] Table 1

[0060]

[0061] The continuous casting billet is subjected to direct rolling treatment, which includes rough rolling, intermediate rolling, pre-finishing controlled cooling treatment and finishing rolling in sequence to obtain rolled billets; wherein, the hot rolling unit for direct rolling treatment is arranged as follows: 6 roughing mills, 4 intermediate rolling mills, 6 pre-finishing mills, temperature-controlled 1# water tank, recovery section, 3# flying shear, 4 finishing mills, temperature-controlled 2# and 3# water tanks, recovery section, 4# flying shear, 2 finished product units, temperature-controlled 4# water tank, recovery section, temperature-controlled 5# water tank, recovery section, pinch rollers, high-speed multiple-length flying shear, high-speed steel feeding system, cooling on cooling bed, finishing, packaging, weighing and warehousing. Since the rolling speed of rods and wires is fast and the temperature is increased, the cooling intensity should be controlled in a graded manner, that is, the cooling should be controlled in five stages to avoid the surface overcooling caused by concentrated cooling, which will lead to smaller main pearlite grain size and less content, thereby increasing the pearlite grain size and the proportion of pearlite.

[0062] Rolling process and organizational control: Φ12mm~Φ25mm uses continuous casting billets with different compositions in Table 3 and different steel direct rolling production processes in Table 2, adopts the appropriate starting rolling temperature in Table 4, controls the higher finishing rolling temperature and the temperature entering the finished rolling mill, weakens the adverse effect of fine grain strengthening effect on the strength-to-yield ratio, and increases the pearlite content.

[0063] Table 2 Related rolling parameters

[0064]

[0065] Table 3 Chemical composition of HRB500E hot-rolled ribbed steel bars of the embodiments and comparative examples (content: wt.%)

[0066]

[0067] According to the mechanical property testing method: GB / T18900, reinforced concrete was tested for mechanical properties of HRB500E hot-rolled ribbed steel bars prepared in Examples 1-5 and Comparative Examples 1-5 using the test method just described, and the test results are recorded in Table 4 below.

[0068] Table 4 Comparison of specific process parameters and mechanical properties of Examples 1-5 and Comparative Examples 1-5

[0069]

[0070] From the comparison between Table 3 and Table 4, it can be seen that the HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio of ≥1.28 prepared by the direct rolling process according to Examples 1 to 5 of the present application all have a strength-to-yield ratio of ≥1.28 for Φ12mm to Φ25mm. Compared with the HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio of Φ12mm to Φ25mm produced by direct rolling using the existing HRB500E hot-rolled ribbed steel bars method in Comparative Examples 1-5, the strength-to-yield ratio is significantly less than 1.28, and it is difficult for industrial production to meet the standard requirement of steel bar seismic resistance ≥1.25. Figure 2 As shown in the metallographic image of the cross section of the HRB500E hot-rolled ribbed steel bar produced in Comparative Example 3, the grain size of the pearlite reaches 5.5 μm to 9.5 μm, and the amount of pearlite reaches 32% to 39%. Figure 3As shown, in the metallographic diagram of the cross section of the HRB500E hot-rolled ribbed steel bar produced in Example 3, the grain size of the pearlite reaches 7.3μm to 12.0μm, and the amount of pearlite reaches 35% to 42%. From the comparison of the above contents, it can be concluded that the method of producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio ≥ 1.28 by direct rolling process in the embodiment of the present application can quickly prepare HRB500E hot-rolled ribbed steel bars that meet the requirements of GB / T 1499.2-2018 "Steel for reinforced concrete Part 2: Hot-rolled ribbed steel bars" and have a strength-to-yield ratio of ≥ 1.28 for HRB500E hot-rolled ribbed steel bars with a diameter of Φ12mm to Φ25mm, and the strength-to-yield ratio of HRB500E hot-rolled ribbed steel bars will not be less than 1.25 due to different specifications of HRB500E hot-rolled ribbed steel bars.

[0071] The above is only a specific implementation method of the present application. Those skilled in the art can clearly understand that for the convenience and simplicity of description, the specific working process described above can refer to the corresponding process in the aforementioned method embodiment, and will not be repeated here. It should be understood that the protection scope of the present application is not limited to this. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed in the present application, and these modifications or replacements should be included in the protection scope of the present application.

Claims

1. A method for producing HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio of ≥1.28 by direct rolling process, characterized in that: include: Provide molten steel used for producing HRB500E hot-rolled strip steel bars. The composition of the molten steel used for HRB500E hot-rolled ribbed steel bars is as follows: C: 0.21%-0.25%, Si: 0.40%-0.60%, Mn: 1.30%-1.50%, P≤0.045%, S≤0.045%, V: 0.042%-0.080%, Nb: 0.010%-0.030%, N: 0.012%-0.022%, and the balance is iron. Continuously casting the molten steel used for the HRB500E hot-rolled steel strip to obtain a continuously cast steel billet; The continuous casting steel billet is subjected to direct rolling treatment, wherein the direct rolling treatment comprises sequentially performing rough rolling, intermediate rolling, pre-finishing rolling controlled cooling treatment, and finishing rolling on the continuous casting steel billet to obtain hot-rolled ribbed steel bars; The hot-rolled ribbed steel bars are cooled on a cooling bed, end-dressed, sheared and shaped to obtain HRB500E hot-rolled ribbed steel bars with a strength-to-yield ratio of ≥1.

28.

2. The method according to claim 1, characterized in that The molten steel used for producing HRB500E hot-rolled steel strips comprises: During converter steelmaking, the requirements for molten iron entering the furnace must meet the following requirements: S≤0.040Wt.%, nitrogen is blown from the bottom of the converter throughout the process, the nitrogen flow rate is increased in the later stage of blowing, and the stirring of the molten pool is strengthened; the converter smelting end point is controlled to be C≤0.15Wt.%, and both P and S are ≤0.040Wt.%, to obtain converter steel; The converter molten steel is subjected to LF refining to obtain LF refined molten steel.

3. The method according to claim 2, characterized in that For HRB500E hot-rolled ribbed steel bars with specifications of Φ12mm to Φ18mm, the composition of LF refined molten steel at the end point of LF refining of HRB500E hot-rolled ribbed steel bars is as follows: C: 0.21% to 0.25%, Si: 0.40% to 0.60%, Mn: 1.30% to 1.50%, P≤0.045%, S≤0.045%, V: 0.042% to 0.067%, Nb: 0.010% to 0.030%, N: 0.012% to 0.021%, and the balance is iron and unavoidable impurities; For HRB500E hot-rolled ribbed steel bars with specifications of Φ20mm~Φ25mm, the composition of LF refined molten steel at the end point of LF refining is, calculated by mass percentage: C: 0.21%~0.25%, Si: 0.40%~0.60%, Mn: 1.30%~1.50%, P≤0.045%, S≤0.045%, V: 0.055%~0.080%, Nb: 0.010%~0.030%, N: 0.013%~0.022%, and the remainder is iron and unavoidable impurities.

4. The method according to any one of claims 1 to 3, characterized in that: The molten steel used for producing HRB500E hot-rolled strip steel bars is continuously cast using a continuous casting machine, and the casting speed of the continuous casting machine is 3.2 m / min to 3.6 m / min.

5. The method according to claim 1, characterized in that The direct rolling process for the continuous casting steel billet adopts full continuous rolling.

6. The method according to claim 5, characterized in that The equipment used for the direct rolling process includes 6 roughing mills, 4 intermediate mills, 6 pre-finishing mills, 4 finishing mills and 2 finished product mills; wherein the finishing mills and the finished product mills both adopt top-intersecting 45° cantilever module mills, and the finishing mills and the finished product mills adopt a 4+2 layout.

7. The method according to claim 6, characterized in that For HRB500E hot-rolled ribbed steel bars with specifications of Φ12~Φ25mm, double-line rolling is adopted; among them, when rolling HRB500E hot-rolled ribbed steel bars with specifications of Φ14mm~Φ16mm, the finishing rolling unit is empty for more than 2 frames; when rolling HRB500E hot-rolled ribbed steel bars with specifications of Φ18mm~Φ20mm, the finishing rolling unit is not put into use; when rolling HRB500E hot-rolled ribbed steel bars with specifications of Φ22mm~Φ25mm, the pre-finishing rolling unit is empty for more than 2 frames, and the finishing rolling unit is not put into use.

8. The method according to claim 6, characterized in that The continuous casting steel billet is subjected to rough rolling, intermediate rolling, pre-finishing rolling controlled cooling treatment, and finishing rolling in sequence, including: The cooling intensity is achieved by adopting controlled cooling treatment before and after finishing rolling, controlled cooling treatment after controlled cooling recovery of finishing rolling, and graded control of cooling on the cooling bed. Among them, at least 20m controlled cooling device is used after the pre-finishing mill.

9. The method according to claim 8, characterized in that For HRB500E hot-rolled ribbed steel bars with specifications of Φ12mm~Φ18mm, the temperature before finishing rolling is 980℃~1040℃, the temperature before entering the finished product rack is 940℃~1000℃, and the temperature of the upper cooling bed is 920℃~980℃; For HRB500E hot-rolled ribbed steel bars with specifications of Φ20mm~Φ25mm, the finishing rolling temperature is 940℃~1000℃, the temperature entering the finished product rack is 920℃~980℃, and the temperature on the cooling bed is 900℃~960℃.

10. The method according to claim 1, characterized in that The HRB500E hot-rolled ribbed steel bar with a strength-to-yield ratio of ≥1.28 needs to meet the following requirements: the grain size of pearlite is 7.3 μm to 12.0 μm, and the amount of pearlite accounts for 35% to 42%.

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