The new thread standard is about to take effect, silicon manganese has been bought out by the steel mill! This is about the "water penetration process" of the strongest nuclear technology paste!

February 22, SMM8: on February 6, 2018, the State Administration of quality Supervision, Inspection and Quarantine and the State Standardization Administration Committee approved and issued "Steel for reinforced concrete part 2: hot rolled ribbed bars" with notice No. 2 of 2018. The standard is numbered GB/T 1499.2 / 2018, replacing the old standard: GB/T 1499.2 / 2007, with a specific implementation date of November 1, 2018.

Among them, it is particularly worth noting the addition of metallographic inspection regulations, macro metallography, cross-section Vickers hardness, microstructure and inspection methods, in order to prevent water-piercing steel bars from impersonating hot rolled steel bars from circulating in the market.

For the requirements of steel bar strength, the state has implemented high strength steel bar since the end of 2012. Steel mills can improve the strength of steel bar in two ways, one of which can produce high strength steel bar through "water penetration technology". The other is that the strength of steel bar can be improved by adding manganese-vanadium alloy. According to the relevant data, at present, most of the state-owned steel mills and large private steel enterprises in China adopt the production process of adding manganese-vanadium alloy, while some small and medium-sized private steel mills adopt "water penetration process".

What is "water penetration process"?

Commonly known as "water-piercing" steel bar is rolled by water cooling process. The water cooling process is the section steel controlled cooling through the water cooling device immediately after the steel bar is rolled out of the finishing mill. After rolling, the controlled cooling of steel can be divided into three stages: "primary cooling", "secondary cooling" and "air cooling". Most of the primary cooling is through water cooling, industry insiders believe that the production cost of "water piercing" steel bar is low.

What is the basic principle of "water penetration process"?

When the steel bar comes out of the finishing mill, it is continuously treated by quenching and tempering on the surface, resulting in a thin layer of martensite structure on the surface and reaching a certain depth, and the core of the steel bar is still austenitic.

When a certain thickness of martensite structure is formed on the surface of the steel ring, the water cooling is stopped. When the steel bar leaves the water quenching zone, the temperature gradient formed on the cross section of the bar will produce heat flow from the center to the surface. It is during this period that the heat flow in the heart tempers the Markov layer and equalizes the temperature between the surface and the center.

Through-water cooling site

Finally, the steel bar is transported to the cold bed or stayed in the cold bed, and the isothermal transformation from Austenite to Ferrite is carried out all the time, and a fine layered pearlite is formed in the boundary. The transition heat released at this stage not only compensates for the loss of temperature of the steel bar on the cold bed, but also provides additional tempering for the martensitic wheel.

The steel bars produced by the "water penetration process" show only two concentric cross sections: a tempered martensite flange and a pearlescent core. It is the combination of tempered martensite, ferrite and pearlite that is the main reason for the high yield strength, good bending and heat resistance of steel bars.

Why can the addition of manganese-vanadium alloy improve the strength of steel bar?

First of all, manganese is a common element in structural steel and a weak deoxidizer. Appropriate amount of manganese can significantly improve the cold embrittlement properties of steel, and increase its yield strength and tensile strength. At the same time, it does not reduce the plasticity and impact toughness too much, and plays a certain role in removing the content of harmful element oxygen in steel. Manganese can also react with sulfur in steel at high temperature to synthesize manganese sulfide with a very high melting point (about 1600 ℃, much higher than the general processing temperature of steel), so the harmful effect of sulfur can be reduced. The "hot embrittlement" phenomenon of cracks caused by sulfur during hot working of steel is reduced. However, excessive manganese content can also make steel brittle and reduce its plasticity.

Why do steel mills like to use thread steel water penetration process?

There are two processes to improve the strength of steel bar, but why choose "water penetration process"?

Through piercing water after rolling, threaded steel can also improve the properties of steel bar and reduce the content of alloy elements, so as to reduce the production cost and increase the economic benefit. The relevant data show that if the thread steel bar with the same strength grade is produced by water-piercing cooling process, the content of manganese can be reduced properly. According to statistics, if the threaded steel bar with the same strength is used, the manganese-carbon alloy can be saved about 20 to 30 yuan per ton. According to the steel plant's annual output of 100t threaded steel bar, about 25 million economic benefits can be created every year, and the increase in steel plant cost can be almost ignored.

However, the production cost will be much higher by adding mn-V alloy. Therefore, many steel mills will use "water penetration technology" to make threaded steel bars.

Why should the water piercing process stop rolling steel bars completely?

With the sustained and rapid development of China's economy, the construction industry, as one of the pillar industries of our national economy, has also been rapid development. At present, the construction in our country is mainly in the form of reinforced concrete structure, and the threaded steel bar is the steel product with the largest consumption in the construction industry, and its mechanical properties directly determine the safety and durability of the building structure. The steel bar products rolled by water penetration process mainly have the following problems:

First of all, the steel bar after piercing water reduces the physical property plasticity index, although the strength can meet the standard, the production cost is also relatively saved a lot, but the steel bar surface oxide film is destroyed, rust is inevitable, and the strength is easy to fail.

Secondly, after the welding of "water-piercing" threaded steel, it will affect the performance of the steel, and it is not easy to weld.

Thirdly, the strength of "water-piercing" rebar is timely and the strength is very easy to fail.

Finally, the surface crack of rebar is serious. The steel bar after passing through water is prone to edge crack, which originates from the surface and extends to the matrix, usually as deep as 0.15-0.25mm. In the process of terminal use, it is easy to produce the phenomenon of stress concentration, and the crack is further deepened or diffused, which seriously affects the performance of steel bar.

Some research practices show that when the thickness of rust layer on the surface of steel bar is very thin (such as 20 ~ 40 μ m), it will lead to the cracking of steel bar in concrete. Therefore, concrete cracking caused by steel bar corrosion is very easy to occur. When the steel bar (including prestress) is in the stress state, it may break suddenly when it is corroded. This kind of accident is more common in our country, such as the sudden collapse of reinforced concrete bridges and the sudden fracture of buildings. This kind of accident is more common in our country, such as the sudden collapse of reinforced concrete bridges, the sudden fracture of buildings and so on. This kind of situation is the result of the mutual promotion of corrosion and stress. Stress can cause microcracks on the surface of steel bars, corrosion along the cracks, and stress can promote the development of cracks. It went on and on until it suddenly broke.

The durability of building concrete is a major problem in the world today, and the corrosion of steel bar is the most important factor affecting the durability of concrete. There is a wide range of corrosion environment in China, especially in the industrial environment of buildings, the corrosion of steel bars is very common. In order to improve the construction safety standard of our country, the relevant departments of the state have clearly stipulated that grade III rebar should gradually replace grade II rebar with high strength grade III rebar. However, in order to thoroughly improve all aspects of the properties of rebar, and solve the problems of easy rust, difficult welding and strength failure, the key is to stop the water piercing process in an all-round way.

Why increase the metallographic examination?

In the newly released new industry standard of hot rolled steel bar, the metallographic organization inspection regulation has been added. What is the effect of increasing metallographic examination? Why add this test method?

First of all, the main principle of rolling water-through cooling of rebar is that the thread steel bar has a certain hardenability. After finishing rolling, the temperature is above Ae3, and under the control of verified water-through cooling process, the pearlite and medium temperature bainite transformation region is avoided. The martensitic transformation occurs within a certain thickness of the steel bar surface. However, under the condition of water cooling for a short time, the core matrix structure can not occur martensitic transformation, but only diffusion pearlite transformation, forming fine grain size of equiaxed ferrite and pearlite structure. The final rolling of threaded steel bar is not cooled through water, and the microstructure after slow cooling is mainly uniform ferrite and pearlite.

Therefore, through the metallographic examination, the thread steel with water-piercing process and non-water-piercing process can be accurately identified, and it can also be used as the final identification and inspection method.

However, the new standard puts forward the requirements for metallographic inspection. If the metallographic inspection is enforced, the production of thread steel by water piercing process can not meet the requirements of the new standard. Comprehensive now the country in the quality policy implementation is strict and the ground bar steel and the small and medium-sized steel plant elimination, the national implementation of the new standard to stop the water penetration process is a high probability.

What is the impact on steel mills and raw materials?

In the future, after the implementation of the new standard of rebar in the iron and steel industry, the rebar produced by the "water penetration" process will gradually lose its market space, and small and medium-sized steel mills will have to adopt the method of adding vanadium to produce rebar, which will have a certain impact on steel mills. And it will also bring a certain boost to the demand for manganese alloy in the later steel bar.

1. Increase in production cost of steel mill

Ma Ren, an analyst at SMM Steel, said in an article entitled "increase in production cost of New Peugeot Steel Plant" that Pinggang is actively seeking to resume production, as the new target of screw thread approaches, the production cost of steel mill will further increase, or push up the market price.

First of all, the weight deviation is strict, the production cost of billet adjustment enterprises increases. The weight deviation of thread steel in the standard φ 12 is ±7%, and this time it is also increased to ±6%. Billet rolling enterprises are used to offset rolling costs with negative tolerances, which means that the production cost increases for some billet rolling enterprises which are on the edge of profit and loss.

Secondly, the circulation of water-piercing steel bars is prohibited, forcing steel enterprises to increase the amount of manganese-vanadium alloy.

According to the investigation of SMM iron and steel, more than 50% of the steel mills are still using the water piercing process. With the approach of the implementation time of the new thread standard, the steel mills begin to try rolling the new standard products one after another, in which the cost caused by alloy increment is about 120 to 200 yuan / ton.

2. Demand for Si-mn alloy will increase

Liu Yuqiao, an analyst with SMM manganese industry, believes that after the water penetration process is banned, the amount of manganese (Mn) and vanadium (V) in steel bars will be significantly increased.

According to SMM, more than 50% of steel mills are still using water-piercing process, and the average difference of manganese content between strong water-piercing and non-water-piercing thread steel is about 0.3%.

In 2017, the total output of rebar per unit of CISA is 207.24 million tons, based on the monthly output of 17 million tons of rebar, of which 8.5 million rebar uses strong water penetration process, the monthly demand for silicon and manganese will be at least 40, 000 tons. Considering the extensive application of water penetration technology in iron and steel production in China, the actual level of new silicon and manganese demand may be significantly higher than this figure. For example, some steel mills that use strong water penetration say that after production according to the new standards, the actual amount of silicon and manganese has increased by about 80%.

Liu Yuqiao, manganese analyst at SMM, pointed out in the article "[SMM Analysis] New Screw Standard stimulating Alloy demand for Silicon and Manganese New production capacity ready to be generated" that with the entry into force of the new standard for rebar steel, the new demand for silicon and manganese is considerable. New demand will be more than 40,000 tons a month. It is true that the new production capacity of silicon and manganese in the second half of the year is relatively dense, but after taking into account factors such as delays in the project period and commissioning and production, the actual production output is often on the small side. In the second half of the year to the end of the year, the new monthly output is up to 40,000 tons. In fact, it will be lower, so at least there will not be a serious surplus caused by intensive production, and the supply and demand pattern of the silicon-manganese industry is expected to remain tight and balanced in the second half of the year.

On the whole, after the implementation of the new thread standard, it will play a positive role in improving the quality of hot rolled ribbed bar products, promoting energy saving and emission reduction, and eliminating backward production capacity. And can better meet the needs of hot rolled ribbed bars in the fields of building, bridge, railway, highway and so on. At the same time, it is expected that the supply and demand pattern of steel market and manganese-vanadium alloy market will also be significantly improved.