CN118854165A - A production method for low yield ratio 500MPa bridge structure steel plate - Google Patents

Abstract

A production method of a 500 Mpa-level bridge structural steel plate with low yield ratio belongs to the technical field of low alloy steel production. The chemical components of the steel are C=0.06%～0.08%,Si=0.20%～0.35%,Mn=1.60%～1.70%,P≤0.015%,S≤0.003%,Al=0.02%～0.05%,Ti=0.008%～0.020%,Nb=0.040%～0.050%,Cr=0.15%～0.35%,Ni=0.70%～0.90%,Mo=0.70%～0.90%, weight percent, and the balance is Fe and unavoidable impurity elements; the high-strength bridge steel with the thickness specification of 10-80 mm is produced, the yield strength of the steel is more than or equal to 500Mpa, the tensile strength is more than or equal to 630Mpa, the yield ratio is less than or equal to 0.80, the impact at the low temperature of-60 ℃ at the positions of 1/4 and 1/2 of the plate thickness is more than or equal to 220J, and the high-strength bridge steel has the advantages of high strength, good toughness, excellent low-temperature toughness, excellent processing performance and excellent welding performance.

Description

A production method for low yield ratio 500MPa grade bridge structure steel plate

Technical Field

The invention belongs to the technical field of low alloy steel production, and relates to a production method of a low yield ratio 500Mpa grade bridge structure steel plate.

Background Art

Structural steel is commonly used in load-bearing structures such as buildings, bridges, ships, and vehicles, and it must have certain strength and toughness. As the technical requirements of engineering projects become higher and higher, the requirements for the strength and toughness of structural steel are also getting higher and higher. For example, the standard for structural steel for bridges, GB/T 714-2015, clearly requires that the low-temperature impact value of steel plates of different strength levels must reach 220J or more. For bridge structures, steel plates with a lower yield strength ratio can provide a higher safety threshold, which can further significantly improve the safety and stability of the project.

With the improvement of bridge construction and metallurgical technology, 420Mpa grade and above bridge structural steel has been widely used, among which 500Mpa grade, as the fifth generation of new bridge steel, has been favored by owners and design institutes. At present, many large Yangtze River Bridges, Yellow River Bridges and key projects in China use 500Mpa grade bridge steel, such as representative projects such as the Hutong Yangtze River Bridge, Changtai Yangtze River Bridge, and Tongling Yangtze River Bridge. Low yield ratio and high strength and toughness are the typical characteristics of this steel. At present, most of the domestic 500Mpa grade bridge steel requirements are yield ratio ≤ 0.86 and -40℃ impact value ≥ 220J, but considering different bridge construction methods, higher requirements are also put forward for yield ratio and low temperature toughness.

Summary of the invention

The purpose of the present invention is to provide a production method of a low yield ratio 500Mpa grade bridge structure steel plate, which can produce high-strength bridge steel with a thickness specification of 10 to 80mm, the yield strength of the steel is ≥500Mpa, the tensile strength is ≥630Mpa, the yield ratio is ≤0.80, and the low-temperature -60°C impact at 1/4 and 1/2 of the plate thickness is ≥220J. The steel has high strength, good toughness, excellent low-temperature toughness, excellent processing performance and excellent welding performance.

The technical solution of the present invention:

A method for producing a low yield ratio 500 MPa grade bridge structure steel plate, wherein the chemical composition of the steel is C=0.06%-0.08%, Si=0.20%-0.35%, Mn=1.60%-1.70%, P≤0.015%, S≤0.003%, Al=0.02%-0.05%, Ti=0.008%-0.020%, Nb=0.040%-0.050%, Cr=0.15%-0.35%, Ni=0.70%-0.90%, Mo=0.70%-0.90%, and the remainder is Fe and unavoidable impurity elements; the key process steps are:

1) Billet heating: heating temperature 1180～1220℃, furnace time ≥ billet thickness mm×1.0min/mm, furnace discharge temperature 1180～1200℃;

2) Rough rolling: The rough rolling end temperature is 1000-1050℃, the cumulative rough rolling reduction rate after widening is 50%-55%, the last two passes reduction rate is ≥15%, and the rough rolling end thickness is 2.5-4t (t is the finished product thickness in mm);

3) Finishing rolling: The starting temperature of finishing rolling is 770-920℃, the rolling is 9-13 times, and the effective pass reduction rate is 10%-15%;

4) Cooling: The water entry temperature of the steel plate is 710-750°C, Mulpic accelerated cooling, cooling rate is 5-10°C/s, and the final cooling temperature is 550-650°C;

5) Tempering and rapid cooling: After controlled rolling and controlled cooling, the steel plate is subjected to tempering + rapid cooling treatment;

6) Finishing and warehousing: trimming and warehousing according to the designed size.

Furthermore, in the step 5), the tempering temperature is controlled at Ac1+ (60-70)°C.

The present invention is tempered in the two-phase region Ac1+ (60-70)°C, and the temperature range should not exceed this range. According to the lever principle, the proportion of austenite in this temperature range is about 8-10%, and then the MA+GB+AF complex phase structure is formed by tempering and rapid cooling. This structure finally achieves the strength-toughness matching of yield ratio ≤ 0.80 and -60°C impact ≥ 220J. When the temperature range is below Ac1+60°C, the proportion of austenite is insufficient, resulting in a small proportion of MA+GB after rapid cooling, low tensile strength, and the yield ratio cannot be stably controlled below 0.80. When the temperature range is above Ac1+70°C, the proportion of austenite is large, resulting in an increase in the number and size of rapidly cooled MA+GB, which has an adverse effect on the low-temperature toughness of the steel plate, and it is difficult to ensure -60°C impact ≥ 220J.

Furthermore, in the step 5), the tempering and heat preservation time is 25 to 35 minutes.

The tempering and holding time of the present invention can control the size of carbide precipitation and the proportion of the organization. When the tempering and holding time is less than 25 minutes, the carbide precipitation is insufficient, and the support for the organization yield strength is insufficient, so that the yield strength of the steel plate cannot be stably controlled to be above 500Mpa. As the tempering and holding time is extended, the amount of carbide precipitation gradually increases, and it has a strong pinning effect on the climb fusion of dislocations, resulting in an increase in the yield strength and yield strength ratio of the steel plate. When the tempering and holding time is greater than 35 minutes, the yield strength ratio cannot be stably controlled below 0.80. Therefore, the more suitable tempering and holding time is 25 to 35 minutes.

Furthermore, in the step 5), after the tempering, the steel is rapidly cooled at a cooling rate of 15 to 20°C and a final cooling temperature of below 180°C.

The cooling rate of rapid cooling after tempering in the present invention is more important for the strength and toughness matching of the steel plate. When the cooling rate is less than 15°C, the amount of MA in the austenite low-temperature transformation product is insufficient, the tensile strength is low, and the yield strength ratio cannot be stably controlled below 0.80. When the cooling rate is greater than 20°C, the amount and size of MA in the austenite low-temperature transformation product increase, which has an adverse effect on the low-temperature toughness of the steel plate, and it is difficult to ensure that the -60°C impact is ≥220J.

The design principle of the steel grade components described in the present invention is:

Carbon is an effective element for increasing the strength of steel, but as the carbon content increases, the toughness, plasticity and weldability of bridge structure steel will decrease. Generally, when the carbon content exceeds 0.11%, its welding performance will deteriorate significantly. However, if the carbon content is too low, the strength of bridge steel will be affected, and smelting will be difficult for industrial production. Therefore, the present invention adopts a low-carbon design and determines the carbon content range to be 0.06% to 0.08%.

Silicon dissolves in ferrite in steel, which can increase the strength and hardness of steel, but reduce its plasticity and toughness. As a deoxidizer, silicon can form silicate slag with FeO in molten steel with low density and be removed, so silicon is a beneficial element for improving the purity of molten steel. However, too high silicon content is not good for the surface quality of steel and the control of inclusions, and is also not good for the plasticity, toughness and weldability of steel. The present invention determines that the silicon content ranges from 0.20% to 0.35%.

Manganese is an important element of strength and toughness, which can reduce the Ar ₃ temperature of the phase transition from γ to α in steel, thereby promoting bainite transformation. Mn causes solid solution strengthening by dissolving into ferrite, thereby improving the strength of steel. Manganese promotes grain refinement, so it can improve the toughness of steel while improving strength. However, manganese is also prone to segregation, and combines with S to form banded structures such as MnS, which affects low-temperature impact and thickness tensile properties. The present invention determines that the range of manganese content is 1.60% to 1.70%.

In general, phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates welding performance, and reduces plasticity. Therefore, the present invention adopts a low-phosphorus design, limiting the phosphorus content to no more than 0.015%.

Sulfur is a harmful impurity in steel, which reduces the ductility, toughness and weldability of steel. High-sulfur steel is prone to brittle cracking when subjected to pressure processing at high temperatures. In addition, S can also form MnS segregation with Mn, which seriously affects the comprehensive properties of steel. Therefore, the present invention adopts a low-sulfur design, limiting the sulfur content to no more than 0.003%.

Aluminum is a deoxidizer in steel. The combination of appropriate amounts of Al and Ca is beneficial to reducing the number of inclusions, changing the form of inclusions, and is beneficial to the internal quality, plasticity, and toughness of bridge steel. Al has a certain effect of refining grains, improving impact toughness, and reducing the tough-to-brittle transition temperature of steel. The present invention determines that the aluminum content ranges from 0.02% to 0.05%.

Titanium can refine the grain structure of steel, thereby improving the strength and toughness of steel, reducing aging sensitivity and cold brittleness, and improving welding performance. TiN formed by micro-titanium treatment can effectively pin austenite grain boundaries, which is beneficial to controlling the growth of austenite grains. However, when the titanium content is high, large-sized liquid TiN is easily generated, affecting the mechanical properties of steel. The present invention determines that the titanium content ranges from 0.008% to 0.020%.

Niobium: Niobium is a microalloying element with two effects: solid solution strengthening and grain refinement. Solid solution strengthening means that after it is dissolved in austenite, it can significantly improve the hardenability of steel, and improve the strength and impact toughness of steel. In addition, Nb can be combined with C and N to generate high melting point and highly dispersed Nb (C, N). This carbide has the effect of strongly inhibiting the recrystallization of austenite, and has the characteristics of precipitating in γ to pin the austenite grain boundary and refine the original austenite grains, so that the material has a significantly refined structure after phase transformation and a smaller grain size. The present invention determines the niobium content to be 0.040-0.050%.

Chromium plays a variety of important roles in bridge steel. It can form relatively stable and fine chromium carbides, which are evenly distributed in the steel volume, and play a role in refining the structure, improving strength and wear resistance. Chromium and nickel form stable compounds, which can play a role in resisting oxidation and corrosion, and have a good effect on protecting the surface quality of bridge steel. The present invention determines that the range of chromium content is 0.15-0.35%.

Nickel can form an infinite solid solution with iron and is an austenite stabilizing element. It can significantly improve the strength and low-temperature toughness of steel. In order to reduce the adverse effects of the brittle phase caused by the addition of molybdenum, nickel and manganese elements interact with each other to further stabilize the austenite phase region, thereby improving the hardenability of steel and stabilizing the low-temperature toughness of steel. The present invention determines that the nickel content ranges from 0.70% to 0.90%.

Molybdenum combined with nickel and chromium can significantly improve the hardenability of steel, can play a role in grain refinement, and can comprehensively improve the comprehensive performance of steel, especially the tensile strength and hardness of steel. In the present invention, the hardenability of molybdenum is utilized to transform austenite into MA and low-carbon GB structures as much as possible during the tempering and controlled cooling process. In addition, the carbides formed by molybdenum and carbon are controlled to precipitate during the tempering process, which can not only improve the yield strength of steel, but also further improve the low-temperature toughness of steel. Due to the addition of nickel, the harmfulness of molybdenum is greatly reduced, so the molybdenum content in this solution is selected to be 0.70% to 0.90%.

The present invention utilizes the synergistic effect of C/Nb/Mo/Ni to optimize the strength-toughness matching. After the ingot is re-austenitized by relatively low temperature heating, a reasonable reduction system is provided to ensure the homogeneous and refined crystallization control in the thickness direction of the steel plate. The controlled rolling and controlled cooling process adopts Tnr-Ar3 temperature range rolling + medium-high temperature cooling transformation, retains the rolled structure to be composed of QF+AF+GB, and then performs two-phase temperature limited tempering + limited speed rapid cooling transformation. After a series of organizational adjustments, the heat-treated steel plate structure is obtained to be composed of MA (4-5%) + GB (4-5%) + AF (70-80%) + QF (10-22%), thereby achieving the performance optimization of ultra-low yield ratio + low-temperature toughness.

The beneficial effects of the present invention are as follows: the bridge structure steel plate with a thickness of 10 to 80 mm produced by the method has a yield strength of ≥500Mpa, a tensile strength of ≥630Mpa, a yield strength ratio of ≤0.80, and a low-temperature -60°C impact of ≥220J at 1/4 and 1/2 of the plate thickness. It has technical characteristics such as high strength, good toughness, and a high safety threshold, and can well meet the needs of various large-span bridge projects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical microscope photograph of a steel plate at 1/4 thickness of Example 3 of the present invention.

FIG. 2 is an optical microscope photograph of the steel plate at 1/2 thickness of Example 3 of the present invention.

DETAILED DESCRIPTION

The present invention is further described below with a group of embodiments and a group of comparative examples.

Example 1: Production of 10 mm thick Q500qF steel plate

The chemical composition of the steel is shown in Table 1. The thickness of the continuous casting billet is 180 mm, the thickness of the steel plate is 10 mm, and the measured Ac1 = 729 ° C. The key process steps are:

1) Billet heating: heating temperature 1220℃, furnace time 185min, furnace temperature 1200℃;

2) Rough rolling: the rough rolling end temperature is 1050℃, the rough rolling cumulative reduction rate after widening is 55%, the last two passes reduction rates are 22% and 24%, and the rough rolling end thickness is 60mm;

3) Finish rolling: Finish rolling start temperature is 920℃, rolling is 9 times, and the effective pass reduction rate is 13-15%;

4) Cooling: The water entry temperature of the steel plate is 740-750°C, Mulpic accelerated cooling, cooling rate is 10°C/s, and the final cooling temperature is 600-650°C;

5) Tempering and rapid cooling (TCC): After controlled rolling and controlled cooling, the steel plate is tempered and rapidly cooled. The tempering temperature is controlled at 789-799°C, the tempering holding time is 25 minutes, and the steel plate is rapidly cooled after being tempered. The cooling rate is 20°C, and the final cooling temperature is 150-180°C.

6) Finishing and warehousing: trimming and warehousing according to the designed size.

Example 2: Production of 42mm thick Q500qF steel plate

The chemical composition of the steel is shown in Table 2. The thickness of the continuous casting billet is 260 mm, the thickness of the steel plate is 42 mm, and the measured Ac1 = 727 ° C. The key process steps are:

1) Billet heating: heating temperature 1200℃, furnace time 261min, furnace temperature 1190℃;

2) Rough rolling: The rough rolling end temperature is 1020℃, the cumulative rough rolling reduction rate after widening is 52%, the last two passes have a reduction rate of 18% and 20%, and the rough rolling end thickness is 110mm;

3) Finish rolling: The starting temperature of the finishing rolling is 840°C, the rolling is 11 times, and the effective pass reduction rate is 12-14%;

4) Cooling: The water entry temperature of the steel plate is 730-740°C, Mulpic accelerated cooling, cooling rate is 8°C/s, and the final cooling temperature is 580-620°C;

5) Tempering and rapid cooling (TCC): After controlled rolling and controlled cooling, the steel plate is tempered and rapidly cooled. The tempering temperature is controlled at 787-797°C, the tempering holding time is 30 minutes, and the steel plate is rapidly cooled after being tempered out of the furnace. The cooling rate is 18°C, and the final cooling temperature is 130-170°C.

6) Finishing and warehousing: trimming and warehousing according to the designed size.

Example 3: Production of 80mm thick Q500qF steel plate

The chemical composition of the steel is shown in Table 1. The thickness of the continuous casting billet is 350 mm, the thickness of the steel plate is 80 mm, and the measured Ac1 = 728 ° C. The key process steps are:

1) Billet heating: heating temperature 1180℃, furnace time 352min, furnace temperature 1175℃;

2) Rough rolling: The rough rolling end temperature is 1000℃, the cumulative rough rolling reduction rate after widening is 50%, the last two passes have a reduction rate of 15% and 16%, and the rough rolling end thickness is 200mm;

3) Finish rolling: The finishing rolling temperature is 770℃, 13 passes, and the effective pass reduction rate is 10-12%;

4) Cooling: The water entry temperature of the steel plate is 710-720°C, Mulpic accelerated cooling, cooling rate is 5°C/s, and the final cooling temperature is 550-580°C;

5) Tempering and rapid cooling (TCC): After controlled rolling and controlled cooling, the steel plate is tempered and rapidly cooled. The tempering temperature is controlled at 788-798°C, the tempering holding time is 35 minutes, and the steel plate is rapidly cooled after being tempered. The cooling rate is 15°C, and the final cooling temperature is 100-130°C.

6) Finishing and warehousing: trimming and warehousing according to the designed size.

Impact tests were carried out on the steel plates of the above three embodiments. The mechanical properties thereof are shown in Table 2. The single value and the average value of the -60°C impact test were both above 220J.

The detection methods of various parameters refer to standards GB/T 222-2002 and GB/T 229-2007.

Table 1 Chemical composition of steel grades in the examples (%)

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Table 2 Mechanical properties of the steel plates of the examples

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Comparative Example 1:

This comparative example is the same as Example 1 except that the tempering temperature in step 5) is controlled at 780°C.

Comparative Example 2:

This comparative example is the same as Example 1 except that the tempering temperature in step 5) is controlled at 810°C.

Comparative Example 3:

In this comparative example, except for the tempering holding time of 20 min in step 5), everything else is the same as Example 1.

Comparative Example 4:

In this comparative example, except for the tempering holding time of 40 min in step 5), everything else is the same as that in Example 1.

Comparative Example 5:

This comparative example is the same as Example 1 except that the cooling rate is 12°C after tempering in step 5).

Comparative Example 6:

This comparative example is the same as Example 1 except that the cooling rate is 25°C after tempering in step 5).

The performance of Q500F of Comparative Examples 1 to 6 above was tested, and the results are shown in Table 3.

The detection methods of various parameters refer to standards GB/T 222-2002 and GB/T 229-2007.

Table 3 Mechanical properties test results of Q500F steel plate obtained in comparative example

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Claims (4)

1. A production method of a 500 Mpa-grade bridge structural steel plate with low yield ratio comprises the following steps of enabling chemical components of steel to be C=0.06%～0.08%,Si=0.20%～0.35%,Mn=1.60%～1.70%,P≤0.015%,S≤0.003%,Al=0.02%～0.05%,Ti=0.008%～0.020%,Nb=0.040%～0.050%,Cr=0.15%～0.35%,Ni=0.70%～0.90%,Mo=0.70%～0.90%, in percentage by weight and enabling the balance to be Fe and unavoidable impurity elements; the thickness of the steel plate is 10-80 mm, the yield strength is more than or equal to 500Mpa, the tensile strength is more than or equal to 630Mpa, the yield ratio is less than or equal to 0.80, and the impact at the low temperature of-60 ℃ at the positions of 1/4 and 1/2 of the plate thickness is more than or equal to 220J; the key process steps are as follows:

1) Heating a casting blank: the heating temperature is 1180-1220 ℃, the furnace time is more than or equal to the thickness of the slab, the thickness is multiplied by 1.0min/mm, and the furnace discharging temperature is 1180-1200 ℃;

2) Rough rolling: the finishing temperature of rough rolling is 1000-1050 ℃, the cumulative rolling reduction of rough rolling after stretching is 50-55%, the rolling reduction of last two passes is more than or equal to 15%, and the finishing thickness of rough rolling is 2.5-4 t (t is the thickness mm of a finished product);

3) Finish rolling: the initial rolling temperature of the finish rolling is 770-920 ℃, the rolling is 9-13 times, and the effective pass reduction is 10-15%;

4) And (3) cooling: the water entering temperature of the steel plate is 710-750 ℃, mulpic is cooled in an accelerated way, the cooling speed is 5-10 ℃/s, and the final cooling temperature is 550-650 ℃;

5) Tempering and quick cooling: tempering and quick cooling the steel plate after rolling and cooling control;

6) Finishing and warehousing: and trimming and warehousing according to the design size.

2. The method for producing the 500 Mpa-level bridge structural steel plate with low yield ratio according to claim 1, wherein the method comprises the following steps: in the step 5), the tempering temperature is controlled to be Ac < 1+ > (60-70 ℃).

3. The method for producing the 500 Mpa-level bridge structural steel plate with low yield ratio according to claim 1, wherein the method comprises the following steps: in the step 5), tempering and heat preserving time is 25-35 min.

4. The method for producing the 500 Mpa-level bridge structural steel plate with low yield ratio according to claim 1, wherein the method comprises the following steps: in the step 5), the tempering is carried out after the tempering is carried out, the cooling speed is 15-20 ℃, and the final cooling temperature is below 180 ℃.