A method for preventing cold bending cracks in hot-rolled ribbed steel bars

Abstract

This invention belongs to the field of steel rolling technology, specifically relating to a method for preventing cold bending cracks in hot-rolled ribbed steel bars. This invention rapidly resolves cold bending cracks in steel bars caused by cold working stress at the bottom of the transverse ribs by limiting the angle of the transverse ribs during the steel bar die design stage, adjusting the rolling speed and temperature during the rolling process, and adjusting the roll cooling.

Description

Technical Field

[0001] This invention belongs to the field of steel rolling, and specifically relates to a rolling method for preventing cold bending cracks in hot-rolled ribbed steel bars. Background Technology

[0002] Hot-rolled ribbed steel bars, also known as rebar, are a key material for construction steel both domestically and internationally, widely used in reinforced concrete structures. With the rapid development of China's steel industry, the production capacity and market demand for rebar have continued to grow. Cold bending performance is one of the important indicators for measuring the processing performance of rebar, directly affecting the bending quality and safety of the bars during construction.

[0003] Cold bending cracks in reinforcing bars refer to the bending and reverse bending tests conducted on the finished reinforcing bars after rolling, according to the specified mandrel diameter in various countries. These tests primarily examine the processing performance and resistance to cold working stresses of the reinforcing bars. Hot-rolled rebar, due to the presence of transverse ribs on its surface, is prone to stress concentration at the root of these ribs during rolling. Cracks often appear first at the bottom of the transverse ribs during bending tests, which is a common cause of failure.

[0004] Several solutions have been proposed in the existing technology to address cold bending cracks in rebar. Chinese Patent CN 111191338 A discloses a method to prevent cracking of transverse ribs in equal-ribbed high-strength rebar due to stress concentration. This method reduces the oblique angle (α) and the spacing (E) of the transverse ribs at their ends, and increases the arc transition at the intersection of the transverse ribs and the inner diameter surface of the rebar, thereby reducing the bending and shear stress at the root of the transverse ribs and alleviating stress concentration, thus reducing the tendency for cold bending cracking. However, this technical solution is limited to the geometric optimization of equal-ribbed high-strength rebars and is not applicable to crescent-shaped transverse ribs. Chinese Patent CN 110358892 A addresses bamboo-joint rebar by modifying the roll die shape and adding a transition arc at the intersection of the transverse rib roots. Simultaneously, it coordinates with the control of heating and finishing temperatures to form a tempered martensitic layer on the rebar surface. These methods require high precision in roll processing, and the optimization of the arc radius lacks differentiated design for different rebar specifications, making implementation difficult. Furthermore, the applicability to the widely used crescent-ribbed rebar is not verified. Chinese patent CN 109304370 A uses a special chamfering milling cutter to chamfer the bottom width of the transverse rib in order to increase the contact area between the bottom width of the transverse rib and the base circle, thereby reducing stress concentration. This method requires replacing the special chamfering milling cutter head and resetting the cutter after normal rib milling, which increases the number of rolling mill processing steps and the difficulty of precision control. In addition, the frequent replacement of the milling cutter head may affect the processing efficiency.

[0005] Therefore, it is necessary to develop a universal method that is easy to operate and low in cost to prevent cold bending cracks in rebar. Summary of the Invention

[0006] The purpose of this invention is to address the above-mentioned technical problems by providing a method for preventing cold bending cracks in hot-rolled ribbed steel bars.

[0007] To achieve the above objectives, this invention provides a method for preventing cold bending cracks in hot-rolled ribbed steel bars, quickly solving the problem of cold bending cracks in steel bars. Based on conventional production processes, the method optimizes the die design and rolling process.

[0008] I. The hole design stage shall be carried out in accordance with the following steps: Step S1: Design the transverse rib α angle according to the minimum α angle value in the corresponding steel reinforcement standard +10°; Step S2: Design the transverse rib height according to the maximum transverse rib height in the corresponding steel bar standard, i.e., the milling groove depth.

[0009] Here, the α angle refers to the angle between the transverse rib and the steel matrix interface. Appropriately reducing this angle helps decrease stress concentration. However, due to high-speed rolling, the transverse rib undergoes a groove removal process after forming, resulting in a certain deviation in the α angle. Therefore, it is set to the minimum α angle in the standard +10°. This allows for a certain margin while reducing stress concentration. It also helps to improve the fullness of the transverse rib and ensure that the rib height meets the requirements.

[0010] II. The rolling stage shall be carried out in accordance with the following steps: Step S10: Heating of steel billets for steel bar production adopts the original high-temperature heating process according to the type of steel, such as the temperature requirement of the soaking zone: 1180℃-1280℃; Step S20: The rolling speed of the steel bar production is set according to the following principles: the initial rolling mill speed is set to the lowest value within the rated range, and the rolling speed of each stand is adjusted by cascading matching.

[0011] High-temperature heating and reduced rolling speed are beneficial to improving the plasticity of the steel matrix and reducing the stress on the rib surface during the degrooving process of the transverse ribs, thereby reducing deformation stress.

[0012] Step S30: Final rolling process - reduce or shut off the roll cooling water.

[0013] In rolling processes, conventional methods involve spraying water / water mist to cool the working surfaces of the rolls. This reduces the temperature of the roll working surfaces, prevents aging of the roll grooves, ensures the surface quality of the steel, and extends the service life. In this solution, the cooling water to the rolls is reduced or shut off, effectively "burning the roll surface." This unconventional "burning the roll surface" operation increases wear on the roll groove working surfaces, rapidly dulling them and thus reducing stress concentration caused by irregular angles during the formation of transverse ribs. This results in faster rolling of threaded steel bars with a lower risk of cold bending cracking.

[0014] This invention effectively reduces the risk of cold bending cracks in hot-rolled ribbed steel bars by optimizing the angle and height of the transverse ribs in the roll die design and by reducing or shutting off the cooling water in the final rolling stage. Compared with the existing technology that generally uses the reduction of the transverse rib angle and height to alleviate stress concentration, this invention unexpectedly achieves a significant reduction in the cold bending crack rate, providing a new approach to improving the cold bending performance of rebar.

[0015] This invention improves metal flow by increasing the angle and height of the transverse ribs, ensuring full filling at the root of the ribs and reducing microcracks and folding defects during rolling. It also increases the transition radius between the transverse ribs and the substrate, effectively alleviating stress concentration during bending.

[0016] Reducing or shutting off the cooling water for the rolls during the final rolling stage allows the roll grooves to passivate quickly, reducing stress concentration caused by irregular angles during the transverse rib forming process.

[0017] Based on the existing hot-rolled ribbed steel bar production process, this invention eliminates the need for complex chamfering or rounding of the rolls. It effectively prevents cold bending cracks simply by designing the transverse ribs and optimizing the cooling process during the final rolling. The invention is simple to operate, easy to promote and apply on existing production lines, and has good economic benefits. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of stress concentration cracking at the bottom of the transverse rib during a cold bending test.

[0019] Figure 2 This is a 20x magnified image of stress concentration cracking at the bottom of the transverse rib during a cold bending test, taken under a stereomicroscope.

[0020] Figure 3 This is a schematic diagram of the transverse cross section of the crescent-shaped ribbed steel bar; in the diagram: α - oblique angle of the transverse rib, b - top width of the transverse rib, h - height of the transverse rib. Detailed Implementation

[0021] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and examples. These embodiments do not constitute a limitation on the present invention. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this application.

[0022] Unless otherwise specified, the process steps in the following embodiments of the present invention are conventional processes in the art, and the transverse ribs are all crescent-shaped.

[0023] Example 1

[0024] For B500B rebar, the design value of the β angle is 55°, the design value of the transverse rib spacing is 10.2mm, and the design value of the transverse rib end gap is 4.8mm. The design value of the transverse rib top width is 1.1mm. Based on the original design, the transverse rib design in the hole shape design has been optimized. The transverse rib height and α angle hole shape design data are shown in Table 1.

[0025] Table 1. Hole design data for Example 1

[0026] standard Brand Specifications / mm Standard α angle requirement α angle design value Standard cross rib height Design value of transverse rib height DIN 488 B500B 20 ≥40° 50° 1.1-2.3 2.3

[0027] Optimization of heating and rolling cooling process: The original high-temperature heating process is adopted, with a soaking zone temperature of 1250℃ and a high-temperature heating time of 38min; the final rolling speed is 7m / s, the final rolling temperature is 1113℃, and the cooling water of the final rolling mill is turned off.

[0028] The pass rate for rib-type inspection was 100%, and the pass rate for cold bending test was 100%.

[0029] Example 2

[0030] GR60 rebar has a design β angle of 55°, a design transverse rib spacing of 15.5mm, and a design transverse rib end gap of 7.4mm. The design top width of the transverse rib is 2.0mm. Based on the original design, the transverse rib design in the die shape has been optimized. The transverse rib height and α angle data are shown in Table 2.

[0031] Table 2, Hole Pattern Design Data for Example 2

[0032] standard Brand Specification Standard α angle requirement α angle design value Standard cross rib height Design value of transverse rib height ASTM A706 GR60 11

[36] ≥45° 55° 1.8-3.6 3.6

[0033] Optimization of heating and rolling cooling process: The original high-temperature heating process is adopted, with a soaking zone temperature of 1260℃ and a high-temperature heating time of 42min; the final rolling speed is 4m / s, the final rolling temperature is 1105℃, and the cooling water of the final rolling mill is turned off.

[0034] The pass rate for rib-type inspection was 100%, and the pass rate for cold bending test was 100%.

[0035] Comparative Example 1

[0036] B500B rebar, standard die type design, β angle design value 55°, transverse rib spacing design value 10.2mm, transverse rib end gap design value 4.8mm. Transverse rib top width design value 1.1mm, transverse rib height and α angle data are shown in Table 3:

[0037] Table 3. Hole design data for Comparative Example 1

[0038] standard Brand Specifications / mm Standard α angle requirement α angle design value Standard cross rib height Design value of transverse rib height DIN 488 B500B 20 ≥40° 42° 1.1-2.3 2.3

[0039] Optimization of heating and rolling cooling process: The original high-temperature heating process is adopted, with a soaking zone temperature of 1200℃ and a high-temperature heating time of 36min; the final rolling speed is 9.5m / s, the final rolling temperature is 1050℃, and the cooling water of the final rolling mill is turned on with a water pressure of 0.8MPa.

[0040] To reduce the risk of cold bending cracks, the design allowance for the irregular angle α of the transverse ribs was too low, resulting in a high proportion of finished products exceeding the acceptable limits. The rib inspection pass rate was 40%, while the cold bending test pass rate was only 56%.

[0041] Comparative Example 2

[0042] GR60 rebar, standard die type design, β angle design value 55°, transverse rib spacing design value 15.5mm, transverse rib end gap design value 7.4mm. Transverse rib top width design value 2.0mm, transverse rib height and α angle data are shown in Table 4:

[0043] Table 4. Hole design data for Comparative Example 2

[0044] standard Brand Specification Standard α angle requirement α angle design value Standard cross rib height Design value of transverse rib height ASTM A706 GR60 11

[36] ≥45° 55° 1.8-3.6 3.6

[0045] Optimization of heating and rolling cooling process: The original high-temperature heating process is adopted, with a soaking zone temperature of 1260℃ and a high-temperature heating time of 41min; the final rolling speed is 6m / s, the final rolling temperature is 1055℃, and the cooling water of the final rolling mill is turned on with a water pressure of 0.8MPa.

[0046] The rib inspection pass rate was 100%. However, due to the setting of the rolling speed and the opening of the final rolling roll cooling, the wear and aging of the rolling groove was slow, and the stress concentration at the α angle of the transverse rib was obvious, resulting in a cold bending test pass rate of 46.7%.

[0047] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preventing cold bending cracks in hot-rolled ribbed steel bars, characterized in that, This includes increasing the transverse rib angle α and the maximum design transverse rib height during the die design stage, and reducing the rolling speed and cooling rate during the rolling stage.

2. The method for preventing cold bending cracks in hot-rolled ribbed steel bars according to claim 1, characterized in that, The hole design phase is implemented according to the following steps: Step S1: Design the transverse rib α angle according to the minimum α angle value in the corresponding steel reinforcement standard +10°; Step S2: Design the transverse rib height according to the maximum value of the transverse rib height in the corresponding steel reinforcement standard.

3. The method for preventing cold bending cracks in hot-rolled ribbed steel bars according to claim 1, characterized in that, The rolling stage is carried out according to the following steps: Step S10: High-temperature heating of the steel billet; Step S20: The rolling speed of the steel bar production shall be set according to the following principle: the speed of the primary rolling mill shall be set to the lowest value of the rated range, and the rolling speed of each stand shall be adjusted by cascading matching. Step S30: Final rolling process - reduce or shut off the roll cooling water.

4. The method for preventing cold bending cracks in hot-rolled ribbed steel bars according to claim 3, characterized in that, In step S10, high-temperature heating is performed according to the type of steel.

5. The method for preventing cold bending cracks in hot-rolled ribbed steel bars according to claim 3, characterized in that, In step S30, the rolling mill cooling water is turned off.

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