WO2023040581A1

WO2023040581A1 - Q500-grade low-alloy structural steel bar, and controlled rolling and controlled cold rolling method therefor

Info

Publication number: WO2023040581A1
Application number: PCT/CN2022/113683
Authority: WO
Inventors: 郭士北; 汪洋; 彭峰; 王占忠; 张越; 马建祎; 杜正龙; 陈君
Original assignee: 大冶特殊钢有限公司
Priority date: 2021-09-17
Filing date: 2022-08-19
Publication date: 2023-03-23
Also published as: CN114058944B; CN114058944A

Abstract

Disclosed is a Q500-grade low-alloy structural steel bar and a controlled rolling and controlled cold-rolling method therefor. The chemical composition (wt%) of the bar comprises: 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％, CEV＝C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15≤0.45％. The initial rolling temperature of the present method is 1000-1050°C. Using the present method, uniformity of texture of a rolled piece can be achieved.

Description

A kind of Q500 steel grade low-alloy structural steel bar and its controlled rolling and controlled cold rolling method

technical field

The invention relates to a Q500 steel grade low-alloy structural steel bar and a rolling method thereof, belonging to the field of controlled rolling and controlled cooling of steel rolling.

Background technique

Q500 steel is a kind of bridge steel. With the application of high-strength bridge steel, brittle fracture accidents of steel structures have occurred frequently, and the economic losses and casualties caused by them have become more and more serious. At the same time, high-strength bridge steel cannot meet the design requirements of modern bridges in terms of toughness and other mechanical properties, which largely restricts its large-scale promotion and use in bridges.

At present, in order to ensure that the strength of Q500 steel grade materials meets the requirements, a large amount of alloys such as Ni and Mo are generally added to the composition design, or Nb, V, Ti and other fine grain elements are added in combination, and even quenching and tempering treatment is required to meet the performance requirements. high.

Controlled rolling and controlled cooling method is a method to significantly improve the mechanical properties of steel by controlling rolling temperature and controlled cooling. The use of controlled rolling and controlled cooling can greatly reduce the production cost of steel, and make low-carbon equivalent low-alloy high-strength structural steel obtain high Strength and low temperature impact toughness, greatly reduced production.

Therefore, it has important research significance to improve the comprehensive mechanical properties of steel by using controlled rolling and controlled cooling.

Contents of the invention

In order to solve the problem of high production cost of Q500 steel grade materials in the prior art, the present invention provides a controlled rolling and controlled cold rolling method to effectively improve the strength and impact toughness of steel, so that low carbon equivalent low alloy structural steel can obtain high strength At the same time as low temperature impact toughness, the production cost is greatly reduced.

To achieve the above object, the present invention adopts the following technical solutions:

A Q500 steel grade low-alloy structural steel bar. According to the mass percentage, the chemical composition of the low-alloy structural steel bar 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%, CEV=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15≤0.45%.

The above-mentioned low-alloy structural steel bar, in the low-alloy structural steel bar, according to the mass percentage, the C content is 0.10% to 0.18% (for example, 0.11%, 0.12%, 0.14%, 0.15%, 0.16%, 0.17% ).

In the existing technology, in order to ensure that the strength of Q500 steel grade materials meets the requirements, a large amount of alloys such as Ni and Mo are generally added in the composition design, or Nb, V, Ti and other grain refinement elements are added in combination, and even quenching and tempering treatment is required to meet the performance requirements , high production cost.

The present invention does not add Ni, Mo, Nb, Ti elements, offsets the addition of precious alloys by increasing the C content, designs the C content to be 0.05% to 0.18%, preferably 0.10% to 0.18%, and adopts low carbon equivalent low alloy structural steel High strength and low temperature impact toughness are obtained, and at the same time, based on the chemical composition of Q500 steel grade in the GB/T1591-2018 standard, since the invention does not add precious alloys such as Ni, Mo, Nb, Ti, etc., the production cost is reduced.

As a preferred embodiment of the above-mentioned low-alloy structural steel bar, the specification (diameter) of the Q500 steel-grade low-alloy structural steel bar is 20-120 mm.

The GB/T1591-2018 standard requires that the CEV of Q500 steel grade bars shall be ≤0.47% when the specification is ≤63mm, and the CEV≤0.48% when the specification is >63mm. The invention designs CEV≤0.45%. Under the condition of low CEV, the Q500 steel grade can obtain high strength and low-temperature impact energy through controlled rolling and controlled cooling.

Generally speaking, the level of CEV affects the welding performance of materials. On the one hand, it is weldability, the lower the CEV, the better the weldability, but on the other hand, CEV also affects the strength of the material. Under the same conditions, the lower the CEV, the lower the strength. By adopting the technical scheme of the invention, the low-CEV steel can obtain high strength and at the same time have good low-temperature impact toughness.

As a preferred embodiment of the above-mentioned low-alloy structural steel bar, the CEV is 0.40-0.45% (for example, 0.41%, 0.42%, 0.43%, 0.44%).

The above-mentioned low-alloy structural steel bar, as a preferred embodiment, the performance of the Q500 steel-grade low-alloy structural steel bar is as follows: R _p0.2 ≥ 470MPa, R _m is 600-750MPa, A ≥ 25%, - Longitudinal KV ₂ ≥100J at 40°C.

The present invention also provides a controlled rolling and controlled cold rolling method for the above-mentioned low-alloy structural steel bar, which adopts the following technical scheme:

A controlled rolling and controlled cold rolling method for the above-mentioned Q500 steel grade low-alloy structural steel bar, the controlled rolling and controlled cold rolling method includes the following steps in sequence: heating, rolling, cooling, KOCKS rolling and secondary cooling; In the rolling step, the heated billet is rolled, and the rolling start temperature is 1000-1050°C (for example, 1010°C, 1020°C, 1030°C, 1040°C, 1045°C).

In the above-mentioned controlled rolling and controlled cold rolling method, as a preferred embodiment, the controlled rolling and controlled cold rolling method uses ∮100mm～390mm steel billet round billet as raw material, or (100mm～300mm)×(100mm～400mm) The steel billet is used as the raw material, and the specification (diameter) of the finished bar obtained by final rolling is 20mm-120mm (for example, 40mm, 60mm, 80mm, 100mm, 110mm).

In the above controlled rolling and controlled cold rolling method, as a preferred embodiment, in the heating step, the soaking temperature of the billet is 1100-1199°C (for example, 1120°C, 1140°C, 1150°C, 1160°C, 1180°C , 1195°C).

In the prior art, the general soaking temperature is 1200-1260°C. The present invention adopts a lower soaking temperature, mainly considering two aspects: 1. The low-temperature soaking meets the requirements of the high-temperature section of the billet, and has no influence on the subsequent rolling temperature; 2. Considering the initial temperature of controlled rolling and cooling, low temperature soaking is easier to obtain the desired final rolling temperature in the process of controlled rolling and controlled cooling.

In the above controlled rolling and controlled cold rolling method, as a preferred embodiment, in the rolling step, the finishing rolling temperature of the two-roll rolling is 850-900°C (for example, 860°C, 870°C, 880°C, 890°C).

In the above controlled rolling and controlled cold rolling method, as a preferred embodiment, in the rolling step, the rolling method is two-roll rolling.

In the above controlled rolling and controlled cold rolling method, as a preferred embodiment, in the rolling step, the rolled piece obtained by the two-roll rolling is used as a masterbatch for KOCKS rolling.

In the above controlled rolling and controlled cold rolling method, as a preferred embodiment, in the rolling step, the two-roll rolling adopts a short stress line rolling mill, which can effectively improve the rolling stability and ensure the deformation of the rolled piece At the same time, according to the size of the selected billet, two-roll rolling can achieve a large compression ratio (10-30), improve the uniformity of the core structure of the rolled material, and help improve the uniformity of the rolled material's performance.

In the above-mentioned controlled rolling and controlled cold rolling method, as a preferred embodiment, in the cooling step, the cooling adopts spray cooling, and the rolled piece obtained after the final rolling is cooled to 800-849°C (for example, 810°C , 820°C, 830°C, 840°C, 845°C) followed by KOCKS rolling.

In the present invention, because the cooling rate is too fast, the temperature after cooling will be too low, so that the deformation resistance of the bar obtained by rolling in the present invention is increased, and the risk of roll breakage in the KOCKS rolling mill is greatly increased. Therefore, the present invention adopts spray cooling to cool the rolled piece obtained after finishing rolling to 800-849° C. before performing KOCKS rolling.

In addition, the present invention adopts spray cooling, uses a cooling device to rapidly cool the steel surface, and reduces the temperature of the core of the rolled piece through heat conduction, so that the temperature difference between the core temperature and the surface temperature of the rolled piece is gradually reduced.

In the present invention, the purpose of cooling the rolled piece to 800-849° C. is to promote grain refinement through deformation at the final rolling temperature near the temperature in the two-phase region, so as to achieve uniform structure of the rolled piece.

In the present invention, according to the size of the rolled piece obtained in the rolling step, the water tank cooling channel that should be selected in the spray cooling step is determined. In the spray cooling step, the parameters of the cooling channel of the water tank (that is, the diameter of the channel of the water tank) can be selected according to the following table.

终轧得到的轧件直径(mm)Rolled piece diameter obtained by final rolling (mm)	∮20～50∮20～50	∮50～55∮50～55	∮55～75∮55～75	∮75～105∮75～105	∮105～120∮105～120
水箱通道直径(mm)Water tank channel diameter (mm)	∮70∮70	∮90∮90	∮110∮110	∮125∮125	喷嘴nozzle

The spray cooling speed can be calculated according to the number of water tanks put into use and the pressure of cooling water. In the spray cooling step, the spray cooling method in which the cooling intensity is gradually reduced can prevent production failure caused by bending of the head of the rolled piece.

In the above-mentioned controlled rolling and controlled cold rolling method, as a preferred embodiment, in the KOCKS rolling step, the cooled rolled piece is subjected to KOCKS rolling; the amount of deformation is 20% to 100% (for example, 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 positive Y and inverted Y alternately, using three rolls The rolling range of 120° mutually realizes the reduction and sizing of bars. In the KOCKS rolling step, a larger amount of deformation is more beneficial to the improvement of the grain size and mechanical properties of the finally obtained material.

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

The deformation efficiency of three-roll rolling is much higher than that of two-roll rolling. In the three-roll pass, the rolling force acts on the rolled piece from three sides, the deformation in the three-roll pass is more converted into extension, and the temperature rise of rolling (that is, the temperature rise of the rolled piece surface during the rolling process rise) and lower, which is beneficial to the temperature-controlled rolling of the rolled piece; the uniform deformation along the interface of the rolled piece in the three-roll pass can obtain a uniform metallographic structure with consistent grain size.

In the above controlled rolling and controlled cold rolling method, as a preferred embodiment, the secondary cooling step cools the KOCKS rolled piece, and the secondary cooling includes secondary spray cooling and cooling bed cooling.

In the above-mentioned controlled rolling and controlled cold rolling method, as a preferred embodiment, the second spray cooling is performed on the rolled piece after KOCKS rolling, and the cooling rate is 20°C/min～35°C. °C/min (eg, 23°C/min, 25°C/min, 30°C/min, 33°C/min); ensure sufficient cooling intensity for achieving upper cooling bed temperature.

In the above-mentioned controlled rolling and controlled cold rolling method, as a preferred embodiment, the cooling bed cooling performs cooling bed cooling on the bar obtained after the second spray cooling, and the temperature of the upper cooling bed is ≤570°C, preferably 530°C ~570°C (for example, 540°C, 550°C, 560°C), the temperature of the lower cooling bed is 200-250°C (for example, 210°C, 220°C, 230°C, 240°C).

For the bar prepared by adopting the technical scheme of the present invention, the near-surface layer of the bar structure is a sorbite (S) structure, and the inside is a ferrite+pearlite structure (F+P). However, the rod prepared in the conventional process has a structure of ferrite + pearlite.

Sorbitite is a quenched and tempered structure. In the present invention, the second spray cooling rate is relatively large, especially after cooling near the surface, the temperature reaches below the martensitic transformation temperature (Ms) to form martensite, but the internal temperature Relatively high, after spraying, the internal temperature conducts outward, and finally returns to 530-570°C on the cooling bed. After returning to temperature, it is equivalent to a self-tempering process, and martensite transforms into sorbite.

In the present invention, under the condition of not conflicting with each other, the above technical features can be freely combined to form a new technical solution.

For technical solutions not described in detail in the present invention, conventional techniques in the art may be used.

Compared with prior art, the beneficial effect of the present invention is:

1. The present invention adopts a lower soaking temperature to meet the requirements of the high-temperature section in the billet heating step, ensure uniform heating of the billet, and have no effect on the subsequent rolling temperature; and the low-temperature soaking is easier to obtain in the controlled rolling and cooling process. The desired finishing temperature.

2. By adopting the controlled rolling and controlled cooling method of the present invention, the uniformity of the structure of the rolled piece can be realized, and the uniformity of the performance of the rolled piece can be improved.

3. By adopting the technical solution of the present invention, the yield of steel products with the same chemical composition can be significantly improved at relatively low starting and finishing temperatures, relatively high spray intensity and relatively low upper cooling bed temperature. strength and low temperature impact properties.

Description of drawings

Fig. 1 is the 100-fold and 500-fold metallographic structure diagrams of the steel produced in Example 1 of the present invention at the same site at 5 mm subcutaneously, wherein Fig. 1A is a 100-fold metallographic structure diagram, and Fig. 1B is a 500-fold metallographic structure diagram picture.

Fig. 2 is the 100-fold metallographic structure diagram and the 500-fold metallographic structure diagram of the steel produced in Example 1 of the present invention at the same position at 15 mm subcutaneously, wherein (c) is the 100-fold metallographic structure diagram, (d) It is a metallographic structure map at 500 times.

Fig. 3 is the 100-fold metallographic structure diagram and the 500-fold metallographic structure diagram of the steel produced in Example 1 of the present invention at the same position at 25 mm subcutaneously, wherein (e) is a 100-fold metallographic structure diagram, (f) It is a metallographic structure map at 500 times.

Fig. 4 is the 100-fold and 500-fold metallographic structure diagrams of the steel produced in Comparative Example 1 at the same site at 5 mm subcutaneously, wherein (g) is a 100-fold metallographic structure diagram, and (h) is a 500-fold metallographic structure diagram .

Fig. 5 is the 100-fold metallographic structure diagram and the 500-fold metallographic structure diagram of the steel produced in Comparative Example 1 at the same position at 15 mm subcutaneously, wherein (i) is a 100-fold metallographic structure diagram, and (j) is a 500-fold metallographic structure diagram Metallographic histogram.

Fig. 6 is the 100-fold metallographic structure diagram and the 500-fold metallographic structure diagram of the steel produced in Comparative Example 1 at the subcutaneous 25mm place in the same position, wherein (k) is a 100-fold metallographic structure diagram, and (l) is a 500-fold metallographic structure diagram Metallographic histogram.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The technical solution of the present invention will be further described in detail below with examples in conjunction with the accompanying drawings.

Example 1

This embodiment provides a controlled rolling and controlled cold rolling method for Q500 steel grade low-alloy structural steel bars, using a 390 mm steel billet round billet as a raw material to roll a finished product with a specification of 100 mm; the chemical composition of the steel billet (by mass percent, wt%) includes: C: 0.16; Si: 0.30; Mn: 1.40; P: 0.020; S: 0.008; Cr: 0.06; V: 0.06; Iron and unavoidable impurities, and CEV: 0.42.

The controlled rolling and controlled cold rolling method sequentially comprises the following steps: heating, rolling, spray cooling, KOCKS rolling, and cooling. Specific steps are as follows:

(1) Heating: Using a segmented heating process, the steel billet is placed in a heating furnace for heating. The preheating temperature is 680°C, the heating temperature of the first stage is 1030°C, the heating temperature of the second stage is 1130°C, and the soaking temperature is 1130°C.

(2) Rolling: Two-roll rolling method is adopted, the starting rolling temperature is 1020°C, the final rolling temperature is 860°C, and the final rolling obtains a rolled piece with a specification (diameter) of 115mm;

(3) Spray cooling: the rolled piece obtained after final rolling is sprayed and cooled, and the rolled piece passes through 1#, 2# and 3# water tanks successively, and the process parameters of specifically sprayed cooling are as shown in Table 1; KOCKS rolling is carried out after 830°C. Table 1 lists the process parameters of spray cooling in this embodiment.

The process parameter of spray cooling in the embodiment 1 of table 1

1#水箱冷却速率1# water tank cooling rate	2#水箱冷却速率2# water tank cooling rate	3#水箱冷却速率3# water tank cooling rate
20℃/min20℃/min	15℃/min15℃/min	10℃/min10℃/min

In this embodiment, the cooling channel of the water tank containing the nozzle is selected according to the size of the rolling incoming material (ie, the rolled piece obtained from the final rolling). In this embodiment, the cooling intensity of spray cooling is gradually reduced to prevent production failure caused by bending of the head of the rolled piece.

(4) KOCKS rolling: KOCKS rolling is performed on the rolled material obtained after spray cooling, and Table 2 lists the process parameters of KOCKS rolling in this embodiment. Among them, the deformation amount is the deformation amount of the cross-sectional area. Deformation = (cross-sectional area of incoming material - cross-sectional area of finished product) / cross-sectional area of incoming material * 100%.

The process parameter of KOCKS rolling in the embodiment 1 of table 2

The deformation efficiency of three-roll rolling is much higher than that of two-roll rolling. In the three-roll pass, the rolling force acts on the rolled piece from three sides to the center, and the deformation in the three-roll pass is more transformed into extension, and the rolling temperature rise is reduced, which is beneficial to the temperature-controlled rolling of the rolled piece; In the roll pass, the uniform deformation along the interface of the rolled piece can obtain a uniform metallographic structure and a consistent grain size.

(5) Cooling step: including the second spray cooling and cooling bed cooling.

The second spray cooling: the rolled piece after KOCKS rolling is sprayed and cooled for the second time;

The process parameter of spray cooling for the second time in the embodiment 1 of table 3

4#水箱冷却速率4# water tank cooling rate	5#水箱冷却速率5# water tank cooling rate	6#水箱冷却速率6# water tank cooling rate
40℃/min40℃/min	35℃/min35℃/min	25℃/min25℃/min

Note: The second cooling needs to achieve a strong cooling effect, so the opening of the water tank must be the largest; but the 6# cooling water tank is adjacent to the shearing equipment, and excessive cooling intensity will cause uneven shear stress distribution during the shearing process The rolled piece is evenly bent, so a lower cooling rate should be used when the rolled piece passes through the 6# water tank.

Cooling bed cooling: Cooling the rods obtained after spray cooling, the temperature of the upper cooling bed is 570°C; the temperature of the lower cooling bed is 230-250°C. The final bar size is ∮100mm, and its properties are shown in Table 4.

It can be seen from Table 4 that the impact of the steel bar obtained by the present invention through controlled rolling and controlled cooling has reached more than 100J at -40°C, meeting the GB/T1591-2018 standard on Q500 steel grades (three grades of Q500MC, Q500MD and Q500ME) performance requirements.

Table 4 Q500ME steel grade performance requirements and performance of the steel bar obtained after controlled rolling and controlled cooling in Example 1

The structure of the bar prepared in this embodiment is shown in Figures 1-3.

Figure 1 is the metallographic structure of the bar prepared in this example at different magnifications at the same part of the subcutaneous 5mm, wherein Figure 1A is the metallographic structure magnified 100 times, and Figure 1B is the metallographic structure magnified 500 times. From Figure 1 (Figure 1A and Figure 1B), it can be seen that the structure of the bar is uniform, and its structure is sorbite (S) with a grain size of 9 grades.

Fig. 2 is the metallographic structure of the bar prepared in this embodiment at different magnifications at the subcutaneous 15mm of the same part, wherein (c) is the metallographic structure magnified 100 times, and (d) is the metallographic structure magnified 500 times . It can be seen from Figure 2 that the structure of the bar is uniform, and its structure is ferrite + pearlite (F+P), with a grain size of 9 grades.

Fig. 3 is the metallographic structure of the bar prepared in this embodiment at different magnifications at the subcutaneous 25mm of the same part, wherein (e) is the metallographic structure magnified 100 times, and (f) is the metallographic structure magnified 500 times . It can be seen from Figure 3 that the structure of the bar is uniform, its structure is F+P, and the grain size is 8 grades.

Example 2

This embodiment provides a controlled rolling and controlled cold rolling method for Q500 steel grade low-alloy structural steel bars. The steel billet round billet with a specification of 390mm is used as a raw material. The chemical composition of the steel billet (by mass percentage, wt%) includes : 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; 0.404.

Using the same controlled rolling and controlled cold rolling method as in Example 1, the following steps are successively included: heating, rolling, spray cooling, KOCKS rolling, and cooling. The final bar specification is ∮100mm, and its properties are shown in Table 4. shown. It can be seen from Table 4 that the impact of the steel bar obtained by the present invention through controlled rolling and controlled cooling has reached more than 100J at -40°C, meeting the GB/T1591-2018 standard on Q500 steel grades (three grades of Q500MC, Q500MD and Q500ME) performance requirements.

The bar prepared in this embodiment has a structure of sorbite (S) at the subcutaneous 5mm place in the same position, with a grain size of 9 grades, and the bar structure is uniform; The grain size is grade 9, and the structure of the rod is uniform; the structure at 25mm subcutaneous in the same part is F+P, the grain size is grade 8, and the structure of the rod is uniform.

Example 3

This embodiment provides a controlled rolling and controlled cold rolling method for Q500 steel grade low-alloy structural steel bars, using a steel billet with a specification of 300 × 400mm as a raw material, and the chemical composition of the billet is the same as that of the billet in Example 1. The chemical composition is the same.

The controlled rolling and controlled cold rolling method sequentially includes the following steps: heating, rolling, spray cooling, KOCKS rolling, and cooling to obtain a finished rolled material with a specification (diameter) of 50 mm. Specific steps are as follows:

(1) Heating: Using a segmented heating process, the steel billet is placed in a heating furnace for heating. The preheating temperature is 650°C, the heating temperature of the first stage is 1080°C, the heating temperature of the second stage is 1180°C, and the soaking temperature is 1180°C.

(2) Rolling: Two-roll rolling is adopted, the rolling start temperature is 1045°C, and the final rolling end temperature is 890°C to obtain a rolled piece with a specification (diameter) of 60mm;

(3) Spray cooling: the rolled piece obtained after final rolling is spray cooled; after cooling to 830°C, KOCKS rolling is carried out. Table 5 lists the process parameters of spray cooling in this embodiment.

The process parameter of spray cooling in the embodiment 3 of table 5

1#水箱冷却速率1# water tank cooling rate	2#水箱冷却速率2# water tank cooling rate	3#水箱冷却速率3# water tank cooling rate
15℃/min15℃/min	12℃/min12℃/min	8℃/min8℃/min

In this embodiment, a water tank cooling channel with a spray ring diameter of ∮110 is selected according to the size of the incoming rolling material (ie, the rolled piece obtained from final rolling). In this embodiment, the intensity of spray cooling is gradually reduced to prevent production failure caused by bending of the head of the rolled piece.

(4) KOCKS rolling: KOCKS rolling is performed on the rolled material obtained after spray cooling, and Table 6 lists the process parameters of KOCKS rolling in this embodiment. Among them, the deformation amount is the deformation amount of the cross-sectional area. Deformation = (cross-sectional area of incoming material - cross-sectional area of finished product) / cross-sectional area of incoming material * 100%.

The process parameter of KOCKS rolling in the embodiment 3 of table 6

The deformation efficiency of three-roll rolling is much higher than that of two-roll rolling. In the three-roll pass, the rolling force acts on the rolled piece from three sides to the center. In the three-roll pass, more deformation is converted into extension, and the rolling temperature rise is reduced, which is beneficial to the temperature-controlled rolling of the rolled piece; In the roll pass, the uniform deformation along the interface of the rolled piece can obtain a uniform metallographic structure and a consistent grain size.

(5) Cooling step: including the second spray cooling and cooling bed cooling.

The second spray cooling: carry out the second spray cooling on the rolled piece after KOCKS rolling; the process parameters are shown in Table 7:

The process parameter of spray cooling for the second time in the embodiment 3 of table 7

4#水箱冷却速率4# water tank cooling rate	5#水箱冷却速率5# water tank cooling rate	6#水箱冷却速率6# water tank cooling rate
30℃/min30℃/min	25℃/min25℃/min	15℃/min15℃/min

Cooling bed cooling: Cooling the rods obtained after spray cooling, the temperature of the upper cooling bed is 540°C; the temperature of the lower cooling bed is 220-240°C. The finally obtained bar size is ∮50mm, and its properties are shown in Table 4. It can be seen from Table 4 that the impact of the steel bar obtained by the present invention through controlled rolling and controlled cooling has reached more than 100J at -40°C, meeting the GB/T1591-2018 standard on Q500 steel grades (three grades of Q500MC, Q500MD and Q500ME) performance requirements.

The bar prepared in this embodiment has a structure of sorbite (S) at the subcutaneous 5mm place in the same position, with a grain size of 9 grades, and the bar structure is uniform; The grain size is grade 9, and the structure of the rod is uniform; the structure at 25mm subcutaneous in the same part is F+P, the grain size is grade 9, and the structure of the rod is uniform.

Comparative example 1

In this comparative example, a steel billet round billet with a specification of 390 mm is used as a raw material, and a finished product with a specification of 100 mm is obtained by rolling by a conventional rolling method; the chemical composition (wt%) of the steel billet includes: 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 is iron and inevitable impurities, and CEV: 0.43.

The conventional rolling method includes the following steps in sequence: heating, rolling, and spray cooling to obtain a finished rolled material with a specification of 100 mm. Specific steps are as follows:

(1) Heating: Using a segmented heating process, the steel billet is placed in a heating furnace for heating. The preheating temperature is 650°C, the heating temperature of the first stage is 1080°C, the heating temperature of the second stage is 1240°C, and the soaking temperature is 1240°C.

(2) Rolling: Two-roll rolling is adopted, the rolling start temperature is 1120°C, and the final rolling end temperature is 950°C to obtain a rolled piece with a specification of 100mm;

(3) Spray cooling: the rolled piece obtained after final rolling is subjected to spray cooling. Table 8 lists the process parameters of spray cooling in this embodiment.

The process parameters of spray cooling in Table 8 Comparative Example 1

4#水箱冷却速率4# water tank cooling rate	5#水箱冷却速率5# water tank cooling rate	6#水箱冷却速率6# water tank cooling rate
35℃/min35℃/min	30℃/min30℃/min	20℃/min20℃/min

Cooling bed cooling: Cooling the rods obtained after spray cooling, the temperature of the upper cooling bed is 750°C; the temperature of the lower cooling bed is 300-350°C. The final bar size is ∮100mm, and its properties are shown in Table 4. It can be seen from Figure 5 that under the same chemical composition, the yield strength, tensile strength and impact energy of the bar prepared by the conventional method are relatively low although they meet the standard requirements.

The rod prepared in this embodiment has a structure of ferrite+pearlite (F+P) at 5 mm below the skin at the same site, with a grain size of 7 grades, and the rod structure is uniform (as shown in Figure 4); The tissue at 15mm below the skin is F+P, the grain size is 6.5, and the bar structure is uniform (as shown in Figure 5); the tissue at 25mm below the skin at the same site is F+P, the grain size is 6, and the bar The tissue is uniform (as shown in Figure 6).

Based on the above analysis, the technical scheme of the present invention can significantly improve the yield strength and low-temperature impact performance of steel products with the same chemical composition.

The above-mentioned embodiments only illustrate the principles and effects of the present invention, but are not intended to limit the present invention. Those skilled in the art can make changes and modifications to these embodiments without departing from 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 shall still be covered by the claims of the present invention.

Claims

A controlled rolling and controlled cold rolling method for Q500 steel grade low alloy structural steel bars, according to the mass percentage, the chemical composition of the low alloy structural steel bars includes: C 0.05% to 0.18%, Si 0.20% to 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%, CEV＝C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15≤ 0.45%;

The controlled rolling and controlled cold rolling method comprises the following steps in sequence: heating, rolling, cooling, KOCKS rolling and secondary cooling; wherein,

In the heating step, the soaking temperature of the billet is 1100-1199°C;

In the rolling step, the heated steel billet is rolled, the starting rolling temperature is 1000-1050°C; the final rolling temperature is 850-900°C; the rolling method is two-roll rolling;

In the cooling step, the cooling adopts spray cooling, and the rolled piece obtained after the final rolling is cooled to 800-849°C before KOCKS rolling; in the spray cooling, the spray cooling method with gradually decreasing cooling intensity is adopted ;

In the KOCKS rolling step, KOCKS rolling is performed on the cooled rolled piece; the amount of deformation is 20% to 100%;

The secondary cooling step cools the KOCKS rolling stock, and the secondary cooling includes spray cooling and cooling bed cooling for the second time; Spray cooling, the cooling rate is 20°C/min～35°C/min; the cooling bed cooling is performed on the rods obtained after the second spray cooling, the temperature of the upper cooling bed is ≤570°C, and the lower cooling bed The temperature is 200-250°C.
The controlled rolling and controlled cold rolling method for Q500 steel grade low-alloy structural steel bars according to claim 1, characterized in that, in the low-alloy structural steel bars, the C content is 0.10% to 0.18% by mass percentage %.
The controlled rolling and controlled cold rolling method for Q500 steel grade low-alloy structural steel bars according to claim 1, characterized in that the diameter of the low-alloy structural steel bars is 20-120 mm.
The controlled rolling and controlled cold rolling method for Q500 steel grade low alloy structural steel bars according to claim 1, characterized in that the CEV is 0.40%-0.45%.
According to the controlled rolling and controlled cold rolling method of Q500 steel grade low alloy structural steel bar described in any one of claims 1-4, it is characterized in that, the performance of described Q500 steel grade low alloy structural steel bar is: R p0.2 ≥470MPa, R m 600~750MPa, A≥25%, longitudinal KV 2 ≥100J at -40°C.
The controlled rolling and controlled cold rolling method of Q500 steel grade low alloy structural steel bar according to claim 1, characterized in that,

The controlled rolling and controlled cold rolling method uses ∮100mm～390mm steel billet round billet as raw material, or takes (100mm～300mm)×(100mm～400mm) steel billet billet as raw material, and the diameter of the finished bar obtained by final rolling is 20mm～120mm.
The controlled rolling and controlled cold rolling method for Q500 steel grade low-alloy structural steel bars according to claim 1, characterized in that, in the heating step, the soaking temperature of the billet is 1100-1199°C.
The controlled rolling and controlled cold rolling method of Q500 steel grade low alloy structural steel bar according to claim 1, characterized in that,

In the rolling step, the rolled piece obtained by the two-roll rolling is used as a masterbatch for KOCKS rolling.
The controlled rolling and controlled cold rolling method of Q500 steel grade low alloy structural steel bar according to claim 1, characterized in that,

In the rolling step, the two-roll rolling adopts a short stress line rolling mill.
The controlled rolling and controlled cold rolling method of Q500 steel grade low alloy structural steel bar according to claim 1, characterized in that,

The KOCKS rolling uses three KOCKS rolling mills.
The controlled rolling and controlled cold rolling method of Q500 steel grade low alloy structural steel bar according to claim 1, is characterized in that,

The distribution of the three rolling mills is positive Y and inverted Y alternately, and the rolling range of the three rolls is 120° to realize the reduction and sizing of the bar.
The controlled rolling and controlled cold rolling method of Q500 steel grade low alloy structural steel bar according to claim 1, characterized in that,

The temperature of the upper cooling bed is 530-570°C.