Low-Yield-Ratio, Easy-To-Weld and Weather-Resistant Bridge Steel and Manufacturing Method Therefor

The present disclosure relates to the field of metal materials, and in particular to a low-yield-ratio, easy-to-weld and weather-resistant bridge steel and a manufacturing method therefor.

Since steel has advantages of high strength, light weight, good plasticity and toughness, long-span bridges spanning rivers, lakes, seas, and deep mountains and canyons are all made of steel. Since 1990, a series of studies on practicality, reliability and the like of high-performance steel in bridge engineering have been carried out at abroad. Study results show that the high-performance steel is much superior to ordinary steel in low-temperature toughness, brittle fracture resistance, fatigue resistance, endurance strength and the like.

Ordinary steel bridges are prone to rust, seriously affecting their safety during their service periods. Therefore, anti-corrosion coating for steel bridges is very important, and costs of repeated coating during bridge manufacturing and service are enormous. Data show that during a 40-year service period of a steel bridge, costs of ordinary steel and repeated coating for three times are more than twice a cost of using a bare weather-resistant steel bridge. It can be seen that use of uncoated weather-resistant steel can greatly reduce a life cycle cost of a bridge and brings huge economic benefits. Therefore, development of high-performance weather-resistant bridge steel has become a trend of bridge development at home and abroad.

In order to ensure safety and a long service life of a bridge structure, steel plates not only need to have a certain strength, plasticity and toughness, but also need to have a small yield ratio and good Z-direction property. Bridge steel used in special environments also needs to have good weather resistance. With an increasing of bridge steel structures, it is imperative to improve welding efficiency of bridge steel structure manufacturing enterprises, which requires bridge steel to have excellent welding property. Therefore, development of bridge steel with comprehensive technical characteristics such as high strength and toughness, low yield ratio, weather resistance and good welding property has become an urgent need for development of new green bridges.

However, complexity and diversity of technical characteristics and indicators significantly increase difficulty in development of low-yield-ratio, easy-to-weld and weather-resistant bridge steel with high strength and toughness. Firstly, an increase in steel plate strength enlarges rolling deformation resistance, limits a pass deformation rate, and doubles difficulty in grain refinement and control. An increase in a thickness of a steel plate will lead to an increase in a temperature gradient across a thickness zone, and worsen cooling and structure homogeneity in a thickness direction. Secondly, requirements for the low yield ratio of steel plates also intensify a strength-toughness contradiction, and further increase difficulty of property control. Moreover, bridge steel needs to have good corrosion resistance and welding property, and components need to be further optimized. Therefore, how to achieve the comprehensive technical characteristics of high-strength bridge steel such as the high strength and toughness, the low yield ratio, the weather resistance and the good welding property is a key problem to be solved by the present disclosure.

At present, China and other countries have carried out some studies on weather-resistant bridge steel plates with high strength and toughness. Some patents and documents have been found through search, but technical solutions recorded therein are quite different from the technical solution of the present disclosure in terms of components, production methods, property, product categories, and the like.

A Chinese invention patent with the application No. CN 107385358 A discloses a TMCP type bridge steel plate with a yield strength of 420 MPa and a production method therefor. The steel plate of this invention includes the components in percentages by mass: C: 0.07-0.09%, Si: 0.25-0.50%, Mn: 1.40-1.60%, P<0.015%, S<0.005%, Ni: 0.15-0.25%, Cr: 0.10-0.20%, Nb: 0.020-0.030%, Al: 0.030-0.050%, V: 0.030-0.040%, and a balance of iron and inevitable impurities. The production method includes the steps of smelting, continuous casting, heating, rolling and cooling. Chemical components of the steel plate of this invention mainly include a low content of C, Nb and V microalloying elements, supplemented by Ni, Cr and other alloying elements to ensure the strength and toughness of the steel plate. A maximum thickness of the steel plate can reach 70 mm. However, the steel plate of this invention has the disadvantages such as lack of weather resistance and failure to meet requirements for use of the bare bridge steel.

A Chinese invention patent with the application No. CN 109797342 A discloses a high-strength, high-toughness and atmospheric corrosion-resistant steel plate for steel structure manufacturing, and a manufacturing method therefor. The steel plate of this invention includes the components by wt %: C: 0.03-0.10%, Si: 0.30-0.50%, Mn: 1.10-1.50%, P<0.010%, S<0.003%, Cr: 0.45-0.70%, Cu: 0.25-0.40%, Ni: 0.30-0.40%, Alt: ≥0.030%, Ti: 0.006-0.030%, V: 0.040-0.080%, Mo: 0.02-0.08%, Ca: 0.0010-0.0030%, N: 0.0020-0.0080%, B: 0.0002-0.0030%, Ce: 0.001-0.010%, with a balance of Fe and inevitable impurities, and an atmospheric corrosion resistance index I is greater than 6.5, CEV is less than 0.54, and Pcm is less than 0.27. In this invention, a high-performance steel plate with a bainite structure made of specific chemical components is obtained through a steel steel plate modulation process, and can be used for steel structure manufacturing of bridges, high-rise buildings and the like. However, the steel plate of this invention has the disadvantages such as lack of a low yield ratio, poor safety performance, and smelting difficulty due to addition of rare elements such as Ca, B and Ce, thereby increasing production costs. Moreover, the steel plate has an excessive carbon equivalent and poor welding property.

A Chinese invention patent with the application No. CN 102534384 A discloses a Cr-free, high-performance and weather-resistant bridge steel and a preparation method therefor. The steel plate of this invention includes the components in percentages by mass: C: 0.02-0.05%, Si: 0.20-0.30%, Mn: 0.6-1.00%, P≤0.02%, S≤0.010%, Cu: 0.20-0.40%, Ni: 0.30-0.80%, Nb: 0.04-0.07%, Ti: 0.005-0.015%, Al≤0.02%, and a balance of iron and inevitable impurities. The Cr-free, high-performance and weather-resistant bridge steel of this invention has excellent comprehensive mechanical properties, welding property and resistance to atmospheric and marine corrosion, and does not contain a toxic element Cr. Due to addition of a considerable amount of Ni alloy, the bridge steel of this invention has a higher cost and a higher yield ratio, and cannot meet safety performance requirements for bridges.

A Japanese invention patent with the application No. JP1992173920 (A) discloses a low-yield-ratio, thick and high tension plate, which contains a high content of Ni and B elements, leads to a high cost, has poor weather resistance, and cannot meet relevant bridge engineering requirements.

To sum up, studies on low-yield-ratio, easy-to-weld and weather-resistant bridge steel in the prior art are still insufficient. Most bridge steel does not have a low yield ratio or weather resistance, and cannot meet relevant engineering application requirements.

An objective of the present disclosure is to provide a low-yield-ratio, easy-to-weld and weather-resistant bridge steel and a manufacturing method therefor, so as to overcome the defects of the prior art. A product produced based on chemical components and production process requirements of the bridge steel of the present disclosure has high strength and toughness, high plasticity, a low yield ratio, excellent weather resistance, welding property and lamellar tearing resistance. The technical means employed by the present disclosure are as follows:

A low-yield-ratio, easy-to-weld and weather-resistant bridge steel comprises the following components in percentages by mass: C: 0.051%-0.080%, Si: 0.20%-0.50%, Mn: 1.20%-1.50%, P≤0.010%, S≤0.003%, Cr: 0.30%-0.60%, Ni: 0.20%-0.50%, Cu: 0.20%-0.50%, Mo: 0%-0.20%, Nb: 0.02%-0.06%, V: 0%-0.070%, Ti: 0.005%-0.025%, Al: 0.010%-0.040%, and a balance of iron and inevitable impurities, and has a carbon equivalent (CEV)≤0.46%, a weld cracking parameter (Pcm)≤0.20%, and an atmospheric corrosion resistance index I≥6.2.

Further, a maximum thickness of the steel plate is 100 mm, a yield strength thereof is ≥345 MPa, a tensile strength thereof is ≥500 MPa, an elongation after fracture thereof is ≥22%, a yield ratio thereof is ≤0.80, a Z-direction area reduction thereof in a thickness direction is ≥60%, and an impact energy of a base material of the steel plate at −40° C. is ≥200 J.

Further, in a preset atmospheric corrosion environment, a thickness corrosion rate in a 168 h alternate immersion corrosion test is 0.74-1.20 g/m·h.

Further, a weather resistance of the steel plate is more than twice that of an ordinary bridge steel with a yield strength of 345 MPa.

Further, an impact energy of a heat affected zone in welding at −40° C. is ≥100 J.

In order to achieve the above objective, the present disclosure further provides a technical solution, i.e., a method for preparing a low-yield-ratio, easy-to-weld and weather-resistant bridge steel. The method includes process steps of melted iron pretreatment, converter smelting, external refining, continuous casting, rolling, cooling, straightening, and heat treatment. Prior art can be adopted for the process steps of melted iron pretreatment, converter smelting and external refining. Main specific process steps of the present disclosure are as follows:

1) The continuous casting process: a casting superheat temperature of a continuous casting billet is 10-25° C., and a thickness of the continuous casting billet is greater than 6 times a thickness of a finished steel plate. Quality defects of the continuous casting billet can be effectively reduced through control of the casting superheat temperature and a drawing speed of the continuous casting billet.

2) The rolling process:

Heating of the continuous casting billet: a heating zone temperature of the continuous casting billet is 1210-1250° C., a soaking zone temperature is 1190-1230° C., and a soaking time is not less than 60 min; the heating process can satisfy solubility requirements of alloy elements particularly including Nb and V, and prevents excessive growth of austenite grains; and the heating time can ensure a temperature uniformity of the continuous casting billet.

The rolling process includes rough rolling and finish rolling.

An initial rolling temperature of the rough rolling is 1070-1120° C., and a finish rolling temperature of the rough rolling is 1020-1070° C. The rolling temperature and deformation process in the rough rolling stage enable the austenite grains to recrystallize and inhibit growth of the grains. It should be ensured that each deformation rate of at least the last two passes is greater than 15% and a pass interval does not exceed 15 s in the rough rolling stage. A cumulative deformation rate in the rough rolling stage is ≥50%. In a final stage of the rough rolling, a high reduction and a short interval can be used to reduce an equipment load of bridge steel, and an deformation superposition effect with a multi-pass high reduction rate can be leveraged to promote recrystallization of the austenite grains and achieve the goal of grain refinement, which is suitable for the production of weather-resistant bridge steel plates of the present disclosure. A grain size can be effectively controlled by increasing a compression ratio between the continuous casting billet and the finished steel plate, with a total compression ratio≥6, where the total compression ratio is a ratio of a thickness of the continuous casting billet to a thickness of the finished steel plate.

A THICKNESS Of an intermediate billet at holding temperature is 2.5-3.5 t, where t is a thickness of the finished steel plate. An initial rolling temperature of the finish rolling is 830-900° C., and a finishing rolling temperature of the finish rolling is 770-830° C. An appropriate thickness of the intermediate billet at temperature holding and pass deformation rate can not only satisfy requirements for austenite deformation and accumulation of deformation energy in a non-recrystallization zone, but also can ensure that a sufficient deformation rate can be obtained in the rough rolling stage under the condition of a certain thickness of an original casting billet to achieve the purpose of grain refinement. A low finish rolling temperature is conducive to promoting accumulation of austenite deformation energy and induced precipitation of fine precipitates of Nb, V and Ti, and increasing nucleation sites. In a final stage of the finish rolling, sufficient deformation near a temperature of a phase transformation point is conducive to formation of fine ferrite, which can reduce an effective grain size and significantly improve low-temperature toughness.

3) The cooling process: a rolled steel plate is subjected to accelerated water cooling. After completion of the rolling, the steel plate is subjected to the temperature holding. An initial temperature of water cooling is 680-740° C., a self-tempering temperature is 400-600° C., and a water cooling rate is 10-25° C./s. Subsequently, the water-cooled steel plate is subjected to thermal straightening and air cooling. By controlling the initial temperature of water cooling of the steel plate, the present disclosure enables to mitigate excessive stress of the steel plate during cooling, maintain a shape of the steel plate, form fine proeutectoid ferrite, and make the grain size of the steel plate more uniform, thus further reduce the yield ratio of the steel plate. An appropriate final cooling temperature can promote formation of bainite and refine M/A islands, thus improve a strength of the steel plate.

In order to further ensure a uniform grain size of the steel plate and improve plasticity and toughness of the steel plate, step 4) of the heat treatment is also required: the rolled steel plate is subjected to high-temperature tempering, where a heating temperature is 580-690° C., and a total holding time is 2.5-4.5 min/mm. The steel plate is air-cooled to room temperature after taken out of a furnace. An object of the high-temperature tempering is to eliminate residual stress of the steel plate so as to make the property of the steel plate more uniform, which is beneficial to the subsequent processing and manufacturing. Another object is to generate some ferrite structures through two-phase zone tempering to reduce the yield ratio. A third object is to further refine grains to improve the plasticity and low-temperature impact resistance of the steel plate.

Further, a deformation rate of each pass is not less than 10% in the finish rolling.

Further, the rolled steel plate is stacked for slow cooling after the finish rolling. A stacking temperature for slow cooling after is ≥300° C., and a stacking time is ≥24 h.

Further, a final microstructure of the steel plate is one of composite structures of ferrite+pearlite, ferrite+pearlite+bainite, and ferrite+bainite, where a volume percentage of the ferrite is 20%-70%.

Selection principles and content design reasons of each chemical component of the present disclosure are as follows:

An element C in the present disclosure can play a strengthening effect through an interstitial solid solution, and can also react with Nb and other alloying elements to form fine carbonized precipitates. Precipitation of the carbonized precipitates before rolling deformation or austenite transformation hinders grain growth, improves a nucleation rate, and refines structures. Moreover, the precipitation can also obstruct dislocation movement, effectively increase the tensile strength and reduce the yield ratio. Therefore, C content should not be too low. However, an increase of C adversely affects toughness, particularly having material impact on low-temperature toughness. Moreover, the increase of C will deteriorate the welding property of the steel plate. Therefore, the C content cannot be too high. For the present disclosure, it is appropriate to control the C content to be 0.051%-0.080%.

Si is one of deoxidizing elements of steel, which is capable of improving corrosion resistance of steel. Moreover, Si has a strong solid solution strengthening effect. Si is capable of increasing retained austenite in steel and reducing the yield ratio of the steel plate. However, excessive Si will enlarge a grain size of bainite, and deteriorate the toughness and welding property of steel. Therefore, it is appropriate to control the Si content in the present disclosure to be 0.20%-0.50%.

Mn is capable of effectively improving the strength and hardenability of steel. Mn can lower an austenite phase transformation temperature, inhibit the growth of phase transformation grains before accelerated cooling of the steel plate, refine the grains, and enhance the strength of the steel plate. However, an excessively high Mn content will easily inhibit ferrite transformation, and affect the yield strength of steel, which is not conducive to reducing the yield ratio. The excessively high Mn content will also induce segregation, and worsen structure homogeneity and lamellar tearing resistance of the steel plate, which is not conducive to welding. In the present disclosure, it is appropriate to control the Mn content to be 1.20%-1.50%.

In the present disclosure, P and S are harmful impurity elements, and lower contents thereof are advised. An excessively high P content will cause structure segregation and adversely affect low-temperature toughness. In the present disclosure, the P content is controlled to be ≤0.010%. An increase of a S content will promote formation and growth of inclusions, and deteriorate low-temperature property and thickness direction property. Therefore, the S content is preferably controlled to be ≤0.003%.

In the present disclosure, Cr is a main element for improving the weather resistance. An increase of a Cr content is beneficial to refining α-FeOOH, and Cr is capable of replacing Fein α-FeOOH to form amorphous α-(Fe1-XCrX)OOH. A dense rust layer of α-FeOOH and δ-FeOOH generated on a surface of the steel plate is capable of protecting steel from further corrosion. Due to compound addition of Cr and Cu elements, a denser rust layer can be formed, such that the weather resistance can be significantly improved. Cr also has good hardenability, which can improve a rate of cooling of a core of the steel plate during the accelerated cooling of the rolled steel plate, refine a structure of the core of the steel plate, and improve the low-temperature toughness and Z-direction property of the steel plate. However, the excessively high Cr content will cause deterioration of hot workability of the steel plate. Therefore, the Cr content is limited to be 0.30%-0.60%.

Ni does not significantly enhance the strength of steel, but is capable of maintaining good plasticity, low-temperature toughness and corrosion resistance of steel, and has anti-rust and heat resistance at high temperatures. The compound addition of Ni, Cr and Cu not only significantly improves the low-temperature toughness, but also enhances stability of the rust layer and significantly improves the corrosion resistance of steel. Austenite stabilizing elements such as Ni and Mn also enable to obtain an appropriate amount of fine and stable reverted austenite during the heat treatment in the two-phase zone, which is beneficial to improving the plasticity of steel and reducing the yield ratio. However, when a Ni content is excessively high, a large amount of iron oxide scale that hardly falls off is easily generated on the surface of the steel plate, with an increase in cost. In the present disclosure, it is appropriate to control the Ni content to be 0.20%-0.50%.

Cu is capable of improving the hardenability of steel and significantly increasing a core strength of thick steel plate. Cu is also an important element for improving the weather resistance of steel. During slow cooling of thick steel plate, an appropriate amount of Cu is capable of precipitating ε-Cu through self-tempering, enhancing the strength of the steel plate. An excessively high Cu content will degrade a surface quality and plasticity of the steel plate. In the present disclosure, it is appropriate to control the Cu content to be 0.20%-0.50%.

Mo is capable of stabilizing the rust film layer, which can effectively improve the corrosion resistance of the steel plate and significantly enhance the resistance to pitting and crevice corrosion particularly in a chloride-containing environment. Moreover, Mo helps to refine the austenite grains during the rolling and improves the stability of the high-temperature tempering of the steel plate. An excessively high Mo content will reduce the welding property of the steel plate. Moreover, Mo is a precious element, thereby significantly increasing the cost of steel. Therefore, for the steel of the present disclosure, the Mo content is limited to be 0%-0.20%.

In the present disclosure, functions of Nb include: (1) achieving the solid solution strengthening; (2) achieving precipitation during the rolling and before the accelerated cooling, pinning grain boundaries, promoting the nucleation, effectively refining the grains, thereby enhancing the strength and toughness of steel; (3) lowering the austenite phase transformation temperature, refining the grains, and enhancing a high-temperature strength of steel through precipitation of NbC particles at a high temperature or precipitation of a second phase in combination with V and Mo. However, an excessively high Nb content will deteriorate the toughness of welds and the heat affected zone and increase the cost. In the present disclosure, it is appropriate to control the Nb content to be 0.02%-0.06%.

Since having a low total solid solution temperature, V is basically fully solutionized during the soaking. The solutionized V during the rolling is capable of effectively improving the hardenability and raising a recrystallization temperature. During rapid water cooling, V is capable of forming fine carbonitride precipitates, and significantly enhancing the strength of the steel plate. During the high-temperature tempering, the carbonitrides of the solutionized V will also be precipitated, ensuring a high-temperature tempering strength of the steel plate. V also has effects of strengthening the solid solution and reducing the yield ratio. An excessively high V content significantly enhances the strength, but deteriorates the low-temperature toughness and welding property. In the present disclosure, it is appropriate to control the V content to be 0%-0.070%.

Ti has an N-fixing effect and form a precipitated phase mainly composed of TiN, which can inhibit the growth of austenite grains under high-temperature conditions and improve the toughness of the heat-affected zone after welding. During the welding, TiN particles prevent the growth of grains in a coarse-grained heat-affected zone, and enhance a low-temperature toughness of welded joints. Further, due to low solid solubility, Ti easily appears in the form of interphase precipitation during transformation from the austenite to the ferrite, thereby enhancing the high-temperature strength. However, excessive Ti will reduce the toughness of steel. In the present disclosure, it is appropriate to control the Ti content to be 0.005%-0.0025%.

Al is a strong deoxidizing element that can also be combined with N to form AlN, which can refine grains, improve a low-temperature impact toughness, and lower a brittle transition temperature of steel. Al further has oxidation resistance and corrosion resistance. Through combination of Al with Cr and Si, high-temperature scale resistance and high-temperature corrosion resistance of steel can be significantly improved. However, an excessively high Al content is detrimental to weldability. In the present disclosure, it is appropriate to control the Al content to be 0.010%-0.040%.

The carbon equivalent (CEV) and a welding crack sensitivity index (Pcm) can be used to predict a tendency of cold cracking in steel. A smaller value of the tendency indicates a smaller tendency of cracking in the steel during welding, and better a welding property of the steel. Calculation formulas are as follows: CEV %=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15; and Pcm %=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B. In the present disclosure, CEV≤0.46% and Pcm≤0.20, indicating excellent welding property of the steel.

I represents the atmospheric corrosion resistance index, and its calculation formula is as follows: I=26.01(% Cu)+3.88(% Ni)+1.20(% Cr)+1.49(% Si)+17.28(% P)−7.29(% Cu)(% Ni)−9.10(% Ni)(% P)−33.39(% Cu). When I≥6.0, the steel can be deemed as a corrosion-resistant steel. When normally exposed to the air, the steel can be used as bare steel (unpainted). The steel plate of the present disclosure has an I≥6.20, having good atmospheric corrosion resistance.

The present disclosure has the following advantages:

To make the objectives, technical solutions, and advantages of the present invention clearer, the following embodiments and drawings are set forth to clearly and completely describe the technical solutions in the embodiments of the present invention. Apparently, the described embodiments are merely some rather than all of the embodiments. Based on the embodiments of the present invention, all the other embodiments obtained by those of ordinary skill in the art without inventive effort are within the protection scope of the present invention.

Chemical components of the present disclosure in examples are shown in Table 1. Processes of smelting, continuous casting and casting billet heating according to corresponding examples are shown in Table 2. Rough rolling according to corresponding examples is shown in Table 3. Finish rolling according to corresponding examples is shown in Table 4. Cooling and heat treatment according to corresponding examples are shown in Table 5. Properties and microstructure ratios according to corresponding examples are shown in Table 6. Corrosion rates according to corresponding examples are shown in Table 7. The metallographic structure diagrams of Example 1, 4 and 7 are shown in, respectively.