High strength steel bars are called the backbone of the construction industry and also the skeleton of the construction industry.
At present, there are five main directions in the development of high-strength reinforcement varieties:
First, strengthen the research, development, promotion and application of high-strength steel bars of 500MPa and above;
Second, strengthen the production and application of seismic reinforcement;
Third, strengthen the R&D, promotion and application of corrosion resistant reinforcement;
Fourth, strengthen the research, development, promotion and application of low-cost high-performance reinforcement;
Fifthly, strengthen the research on the application technology of high-strength reinforcement.
This post only briefly introduces the properties and production process of high strength steel bars and seismic steel bars of 500 MPa grade and above for buildings.
2. Production process of 500 MPa and above high strength reinforcement
2.1 Production process of 500MPa high strength reinforcement
The main production process of 500MPa high strength steel bars is to add microalloyed element vanadium on the basis of low alloy steel 20MnSi, and make full use of cheap nitrogen to achieve precipitation strengthening, so that the steel strength can reach 500MPa.
Vanadium microalloying technology has the characteristics of economic and reasonable composition design, stable reinforcement performance, high strength yield ratio, and good low-temperature performance and welding performance.
It is a better production process for producing 500MPa high strength reinforcement.
2.1.1 Composition design and mechanical properties
GB l499.2 (revised in 2016) stipulates that the chemical composition and carbon equivalent of HRB500 should meet the requirements of Table 1, and vanadium, niobium, titanium and other elements can also be added to the steel as required.
Table 1 GB1499.2 (Revised in 2016) Chemical Composition and Mechanical Property Requirements for 500MPa High Strength Reinforcement
|Chemical composition, mass%||Brand||HRB500||HRBFS00||HRBSODE||HRBFSOOE|
|Mechanical property||Yield strength RtL, MPa||500|
|Tensile strength R, MPa||630|
|Elongation after fracture A%||15||–|
|Total secondary length ratio of maximum force A%||7.5||9|
2.1.2 Technical route
The technical routes of 500MPa high strength steel bars are waste heat treatment after rolling, ultra-fine grain and microalloying.
The first two technologies use the composition of low alloy steel 20MnSi, and the microalloying technology adds vanadium, niobium, titanium and other microalloying elements on the basis of 20MnSi.
Microalloying technology is to improve the mechanical properties of steel by adding microalloying elements on the basis of 20MnSi steel through metallurgical methods.
The strengthening mechanism is that microalloying elements and carbon and nitrogen atoms in steel form high melting point, high hardness carbides and nitrides.
On the one hand, it precipitates on the austenite grain boundary and is not easy to melt into austenite during heating, which can prevent the growth of austenite grains and cause fine grain strengthening;
On the other hand, these carbides and nitrides can also be precipitated during or after the transformation of austenite into ferrite, which will hinder dislocation movement in the iron lattice and cause precipitation strengthening.
2) Ultrafine grain technology
Ultrafine grain technology does not need to add micro alloy elements, and is a modern production technology combining controlled rolling and controlled cooling.
The premise of the implementation of controlled rolling and controlled cooling process is the computer control of the whole process temperature of the steel rolling production line, and the specific steel rolling process system needs to be determined according to different varieties and specifications.
The recrystallization controlled rolling, non recrystallization controlled rolling, deformation induced ferrite transformation and ferrite dynamic recrystallization mechanism are comprehensively utilized to ensure the control of grain size and microstructure, and finally realize the fine grain strengthening of steel.
3) Residual heat treatment after rolling
The post rolling waste heat treatment technology does not need to add microalloy elements.
It organically combines the hot rolling and heat treatment processes, that is, the steel bars are directly quenched online after hot rolling for surface cooling, and then the steel core waste heat is used to temper the surface layer of the steel bars, so that the surface structure of the steel bars is transformed into tempered sorbite structure that retains the martensite orientation, and the core is refined ferrite plus pearlite structure.
And the relative content of pearlite increased, and finally the 20MnSi steel reached 500MPa level through microstructure strengthening.
Although the post rolling heat treatment and ultra-fine grain technology do not need to add microalloy elements, the equipment cost is high, and the strength yield ratio of the product is low, aging phenomenon is obvious, so the mechanical connection mode of welding and damaging the outer surface is not suitable.
The microalloying technology does not need to add auxiliary equipment for temperature control on the steel rolling production line, with the lowest equipment cost, high strength yield ratio, low aging sensitivity and good welding performance.
By comparing the product performance and production cost, it can be seen that microalloying is the best technical route to produce 500MPa high strength steel bars.
Table 2GB1499.2 (Revised in 2016) Chemical Composition and Mechanical Property Requirements for 600MPa High Strength Reinforcement
|Chemical composition, mass%||Spleen number||HRB600|
|Mechanical property||Yield strength RL, MPa||600|
|Tensile strength Rm/MPa||730|
|Elongation after fracture%||14|
|Total elongation of maximum force A%||7.5|
2.2 Production process of 600MPa high strength reinforcement
2.2.1 Composition design and mechanical properties
At present, Shagang, Chenggang, Jigang and other steel plants in China have the experience of successfully producing 600MPa hot rolled deformed bars.
Table 2 shows the requirements of GB l499.2 (revised in 2016) on the chemical composition and mechanical properties of 600MPa high strength reinforcement HRB600.
2.2.2 Technical route
At present, many steel plants in China can produce 600MPa grade high-strength steel bars, which have been applied in construction projects.
However, the current research on the chemical composition, phase transformation and microstructure evolution of 600MPa high strength steel bars and the relationship between rolling and cooling production processes is not deep enough in China, so the micro alloying technology and controlled rolling and cooling process are not properly matched, which results in the waste of expensive alloy elements on the one hand, and the mechanical properties of steel bars can not meet the requirements on the other hand.
At home, Shagang, Chenggang, Jigang and other steel plants that have successfully achieved HRB600 production mainly adopt the technical route of vanadium alloying, that is, by adding vanadium to greatly improve the strength.
At present, it is rare to produce 600MPa high-strength steel bars through niobium, titanium and process control.
In fact, vanadium alloying technology has become the main technical route for developing high-strength weldable steel bars in the world.
There are usually two ways of process control, namely, controlled rolling and controlled cooling and post rolling heat treatment.
High strength steel bars are produced by controlled rolling and controlled cooling, mainly through low temperature rolling and rapid cooling, so as to reduce the grain size as much as possible and improve the strength.
Using the alloying method to produce 600MPa high-strength steel bars according to the same production process as the current medium and low strength steel bars.
On the one hand, it can avoid the transformation of the production line, as well as the problems caused by the transformation of a series of equipment and cost input;
On the other hand, it can also help the rapid production and promotion of HRB600 new products in a large scale.
However, because only relying on alloy elements to improve the strength, the cost of the alloy will increase, and higher alloy content is also easy to cause structural abnormalities.
To sum up, the current relatively reasonable process route of 600MPa high-strength reinforcement is: mainly alloying, supplemented by process control.
Especially in the initial stage, the production process of 600MPa high-strength reinforcement should be as close as possible to that of medium and low strength reinforcement, so as to facilitate its popularization and application.
3. Production process of anti-seismic high-strength reinforcement
With the increasing requirements of the Chinese construction industry on the performance of steel bars, the safety and seismic resistance of building structures have aroused widespread concern.
3.1 Composition design and mechanical properties
In the GB 1499.2-2007 standard, the seismic performance index of reinforcement is included in the national standard for the first time, and three representative indexes of seismic reinforcement are specified: strength yield ratio (R ˚ m /R ˚ eL), super bending ratio (R ˚ eL/ReL) and total elongation of maximum force (Agt).
See Table 3 and Table 4 for the chemical composition and mechanical property indexes obtained from the multi sample inspection of HRB400E and HRB500E seismic reinforcement of a domestic steel plant.
Table 3 Chemical Composition of HRB400E and HRB500E Seismic High Strength Reinforcement %
Table 4 Mechanical Property Inspection of HRB400E and HRB500E Seismic High Strength Reinforcement
3.2 Technical route
3.2.1 Microalloying technology
The high strain low cycle fatigue performance is the main seismic index of steel bars.
The main way to improve the high strain low cycle fatigue performance of seismic steel bars is microalloying.
Micro alloying technology is widely used at home and abroad to improve the comprehensive properties of steel bars by refining grains and precipitation strengthening.
When selecting microalloying elements in China, vanadium is preferred, and a small amount of nitrogen is added at the same time as vanadium, which increases the number of V (C, N) precipitated phases, thus giving full play to the role of precipitation strengthening and fine grain strengthening, and significantly improving the seismic performance of steel.
In addition, some researchers have successfully developed 600MPa grade fine grain high-strength anti-seismic reinforcement by using Cr+V microalloying process.
In this study, vanadium is used to form V (C, N) compounds in steel, which greatly improves the strength of steel.
At the same time, a certain amount of chromium is added to improve the seismic performance of the reinforcement, and the final mechanical properties meet the requirements of 600MPa fine grain high-strength seismic resistance.
The metallographic structure of the reinforcement is: the edge and the center are “ferrite+pearlite”, and there is no bainite and edge tempering structure that affect the service performance.
3.2.2 Fine crystallization technology
Japan has long been studying the fine crystallization technology, combining large deformation rolling with dynamic recrystallization to refine the grains, and has developed 685-980MPa ultra-high strength seismic reinforcement using the fine crystallization technology, representing the international advanced level.
In addition to the use of strong deformation and dynamic recrystallization, China focuses on the combination of deformation and phase transformation to achieve the goal of grain refinement.
It is pointed out that fine grained steel bars have a wide range of cyclic plastic deformation, and the probability of crack cracking during material deformation is low.
Moreover, fine grained steel bars have higher cyclic toughness and low cycle fatigue life than waste heat treated steel bars.
At the same time, ultra-fine grained steel has better weldability than ferrite pearlite steel.
However, although the fine grained steel bars have excellent performance, there are still shortcomings in practical application.
For example, the requirements for equipment and workpiece size are relatively strict;
The deformation and uneven cooling of large-sized bars cause uneven microstructure and internal and external differences in properties;
When the grain size is too small, the yield strength increases more than the tensile strength, resulting in a decrease in the strength yield ratio, which cannot meet the performance requirements of seismic reinforcement.
In addition, fine grain reinforcement has low corrosion resistance due to its fine grain, more grain boundaries.
Therefore, the fine crystallization technology needs further development.
Microalloying, fine crystallization and waste heat treatment are commonly used to produce high-strength steel bars.
Compared with the other two processes, the microalloyed steel bar has the advantages of stable performance, low strain aging sensitivity and good welding performance;
The waste heat treated steel bars are quenched by rolling steel bars at high temperature.
After the waste heat treatment, the strength is improved.
The resource and energy consumption are low and the production cost is low.
The fine grained reinforcement can meet the requirements of strength and toughness of seismic reinforcement at the same time.
However, there are still some problems in the above three processes, for example:
- The production cost of micro alloying technology is high;
- The ductility, weldability, mechanical connection performance and construction adaptability of waste heat treatment reinforcement are low;
- The fine crystallization technology is complex, and the strength yield ratio of reinforcement is low.
Therefore, in the production of high-strength steel bars, microalloying, fine crystallization and waste heat treatment technologies should be effectively combined according to the actual application needs and based on cost-effectiveness, which can not only reduce the addition of alloy elements, save production costs, but also greatly improve the mechanical properties of steel bars.