Application Research of High Boron High Speed Steel Alloy Composite Roll in Rebar Production

1 Introduction

Rolls are the largest consumable parts in the metallurgical industry, and the consumption cost of rolls is about 5%-10% of the production cost of steel rolling, which is directly related to the production efficiency, product quality and production cost of steel production enterprises. Most of the rolls currently in use are high-nickel-chromium-molybdenum infinite chilled cast iron rolls developed by the United States in the 1930s, and the working layer is scrapped as a whole after use. A roller sleeve-type roll structure is adopted so that about 60% of the roll can be recycled. At the same time, the outer layer of the roll is made of high-alloy material and properly heat-treated, so as to achieve the ideal use effect, greatly improve the service life of the roll, and reduce production costs.

Roller sleeves and rolls are a long-term research topic for the metallurgical industry and roll manufacturers. How to improve their safety and stability and selecting appropriate structures and materials is the key to solving the problem. In the past, traditional cast iron rolls were used in steel rolling. Among them, the finishing rolling unit used centrifugal casting high-nickel-chromium acicular bainitic ductile iron rolls (hereinafter referred to as high-nickel-chromium nodular iron rolls). The production specifications and process conditions are different, generally only 300–500t/groove, especially when the slitting process is used for rolling, due to the pre-slitting and slitting pass The reduction load is large and the slitting wedge is sharp, so wear and damage It is more serious, and the amount of steel passing through a single channel is less.

This means that a shift with an output of about 800t needs to replace the rolling groove 2-3 times, resulting in many roll and groove changes and a long time. Accidents and production process accidents, which lead to a decrease in the operating rate of the entire production line and poor yield indicators, have become an important factor restricting production capacity and index optimization.

At present, in order to greatly improve the service life of rolls, the roll materials that have been researched and put into use mainly include:

(1) Tungsten carbide alloy roll ring.

The research on tungsten carbide alloy roll rings is the earliest and most mature. Although it is very wear-resistant and has high hardness, it is brittle and is not suitable for rolling bars with large impact loads. It is usually used for rolling high-speed wire rods.

(2) Steel-based cemented carbide combined roll (hereinafter referred to as ACS roll)

Aiming at the cutting and pre-cutting design of the splitting and rolling threaded steel bar, the splitting hole has the characteristics of an extremely large impact load and large thermal stress. Therefore, this product has good wear resistance, uniform wear, and impact resistance toughness. It has the advantages of good thermal fatigue resistance, etc. Its steel passing capacity in a single groove is more than 10-15 times that of cast iron rolls. During the rolling process, it can control the cutting material type.

(3) High-boron high-speed steel alloy combined roll

Using bimetal composite extrusion casting technology, compact structure, high red toughness, wear resistance and impact resistance, it is not easy to produce pitting, meat loss and other phenomena during use, The amount of steel passing is relatively high, and its brittleness index is better Compared with tungsten carbide, the wear resistance is worse than the above two materials, but its investment cost is much lower than the first two materials.

In the previous year, our factory carried out research on the application of steel-based cemented carbide composite rolls and achieved good results. The service life was increased by 14 times. However, due to the high price of the rolls, the inventory of spare parts takes up a lot of funds. Based on the overall production and operation needs, the research group decided to choose high-boron high-speed steel alloy combination rolls for application in rebar production, in order to achieve the goal of improving the service life of the rolls and keeping the roll inventory funds small.

2 Principles of Applied Research

2.1 Principle overview

In the production of hot-rolled bar wire, the roll is a tool for plastically deforming the rolled piece at high temperatures. It is in direct contact with the high-temperature rolled piece, and it needs to overcome the rolling pressure, the impact load at the moment the rolled piece bites, and the rolling piece The transfer of heat to the roll leads to a decrease in temperature and hardness and accelerated wear, and it is difficult to keep the shape of the pass in line with the design requirements for as long as possible.

Usually, to ensure these performance requirements, it is necessary to adjust the chemical composition and processing technology of the roll so that the roll has high hardness, high strength, and certain plasticity. At the same time, reasonable cooling methods are used as much as possible during use to reduce rolling The transmission of the temperature of the workpiece to the roll reduces the temperature rise of the roll to ensure that the change in hardness is small and the wear resistance is stable. The manufacturing method and process route of the high-boron high-speed steel alloy composite roll are shown in Figure 1.

2.2 Performance comparison between high-boron high-speed steel alloy combined roll and high-nickel-chromium ductile iron roll

2.2.1 Analysis of the difference in material properties of the two rolls

The low-magnification structures of the two rolls are shown in Figure 2 and Figure 3

It can be seen from Figure 2 and Figure 3 that high-nickel-chromium ductile iron is composed of Fe3C, matrix and graphite, and high-boron high-speed steel alloy is composed of fine alloy carbide and matrix.

High-nickel-chromium ductile iron is composed of a large amount of Fe3C, and Fe3C is not high in hardness and has a skeleton-like distribution. It is not wear-resistant when used as a roll material. At the same time, because Fe3C is distributed in a skeleton-like distribution and splits the matrix, it is easy to fall off when rolling steel. Phenomenon.

The high-boron high-speed steel alloy is composed of a large number of alloy carbides, has a high carbon content, and contains a large amount of W, Mo, V, Cr, Co, B and other alloying elements, and the alloy content accounts for about 40%.

Carbon

Carbon is an essential element in obtaining high hardness and wear-resistant carbide particle reinforcement phase, and part of the carbon is dissolved in the matrix to improve the hardenability and hardenability of the matrix. The amount of carbon added is related to the carbide-forming elements such as chromium, molybdenum, tungsten and vanadium.

Boron

Boron has low solubility in steel and has a strong interaction with crystal defects (dislocations, grain boundaries, vacancies…), and is easy to form inclusions or precipitates of O, C, S, etc., in order to further improve the hardenability of the body It can improve the high-temperature strength of steel and strengthen the effect of the grain boundary. Since boron delays the nucleation process of ferrite (but does not affect the thermodynamic properties of austenite or ferrite matrix, that is to say, boron can reduce the nucleation rate of ferrite, but does not affect its growth rate And the formation speed of pearlite and martensite), which delays the phase transformation of γ→α+β, shifts the S curve of steel to the right, and improves the hardenability of steel. At the same time, due to the small solid solubility of boron in iron, and because the atomic diameter ratio of boron and iron is 0.7, which is significantly larger than the upper limit size of 0.59 for the formation of interstitial solid solution, and significantly smaller than the lower limit size of 0.86 for the formation of substitutional solid solution, no matter the boron atom In any form of solid solution in the matrix, it will cause large lattice distortion and increase the hardness of the matrix.

Chromium, molybdenum, tungsten, vanadium

The main purpose of adding chromium, molybdenum, tungsten and vanadium is to obtain high-hardness wear-resistant carbides. In addition to forming carbides, chromium, molybdenum, tungsten and vanadium also partly dissolve into the matrix to improve the hardenability and red hardness of the matrix. , The chromium dissolved into the matrix can also improve the oxidation resistance of the material. Alloy carbides have high hardness and fine distribution, so they have excellent wear resistance; molybdenum can greatly enhance the hardenability of boron, and the two have a compound effect, especially when the molybdenum and boron content ratio is appropriate, the steel can be used in a considerable The bainite structure can be obtained through controlled cooling within a wide range of cooling rates. There are fewer microscopic pores on the strong and tough matrix, and there is no splitting effect of the skeleton carbide on the matrix, so it also has high impact toughness, and at the same time, it does not have too high requirements for cooling water quality.

2.2.2 Comparison of the mechanical properties of the two rolls

The difference analysis of the mechanical properties of the two rolls is shown in Table 1.

Table 1 Comparison of main technical indicators between high-boron high-speed steel alloy composite rolls and high-nickel-chromium ductile iron rolls

It can be seen from Table 1 that the indicators of hardness, tensile strength, compressive strength and impact toughness of high-boron high-speed steel alloy composite rolls are better than those of high-nickel-chromium ductile iron rolls, and the quenching of high-boron high-speed steel alloy composite rolls The depth of the hard layer is deeper, which is beneficial to the guarantee of the hardness layer at the hole slot and the heavy vehicle.

3 Trial plan

3.1 Roll combination structure scheme

The high-boron high-speed steel alloy combined roll adopts a unique interference sleeve combination, that is, the high-boron high-speed steel alloy steel with good red hardness, wear resistance and hardenability is used as the working layer of the roll (that is, the roll sleeve) and the strength and toughness. Excellent 12Cr2Ni4 forged steel, cast steel or high-nickel-chromium ductile iron rolls provided by the manufacturer are used as roll core materials (roller cores) and are nested together by interference shrink-fitting, and the assembly tolerance is within 10 It is about one wire, and bears the compressive stress when driving the torque, so as to ensure that the combined roll can be used safely. Its combination is shown in Figure 4.

1. Roller core 2. Shrink-fit interference layer 3. Alloy working layer (roller ring)

Fig.4 Schematic diagram of cross-section of high-boron high-speed steel alloy composite roll

3.2 Test variety sorties and target selection

From the perspective of the normal production process, the varieties with fast wear of the pass are mainly concentrated in the pass of the finishing mill for the slit rolling of Ф12-Φ18 small-sized rebar, especially the pre-slit, slit pass, and finished product front hole. why.

Through understanding, due to the good wear resistance of this alloy combined roll, the amount of steel passing through a single hole can be higher than that of nickel-chromium ductile iron rolls, and the 12 three-cut pre-cut, Before the finished product, the 14 two-cut pre-cut and the 18 two-cut pre-cut are tested on several most critical and fastest-wearing parts. These parts are located in the finish rolling unit, with a nominal diameter of Ф320mm and a maximum roll diameter of Ф350mm. According to the other dimensions of the originally designed roll and the situation in Table 1, the thickness of the alloy roll ring of the roll designed and tested this time is selected as 65mm. Test target: The amount of steel passing through a single hole is more than three times that of high-nickel-chromium ductile iron rolls, that is, 1200-1500 tons/hole·time.

3.3 Roll pass cooling scheme

High-boron high-speed steel alloy composite rolls have higher requirements for cooling water than cast iron rolls. The water pressure of the cooling water must reach about 0.4-0.5Ma, and the water temperature is ≤40°C. During production, the cooling water must face the rolling groove and must be checked frequently. Otherwise, the roll ring will fall off and break. For this reason, a special cooling system for high-boron high-speed steel alloy composite rolls is specially designed and manufactured.

Test Results and Analysis

4.1 The results and analysis of the single pass steel volume of the single hole of the trial roll

See Table 2 for the results of comparison with the steel passing capacity of the single groove of the high-nickel-chromium ductile iron roll.

Table 2 Comparison table of steel passing capacity of high-boron high-speed steel alloy combined roll and high-nickel-chromium ductile iron roll in a single groove

It can be seen from Table 2 that, compared with the high-nickel-chromium ductile iron roll, the single-hole steel passing capacity of the two rolls is more than three times that of the high-nickel-chromium ductile iron roll, and the service life is longer. The range was extended, exceeding the expected goal of more than 3 times.

Brief analysis: From the results, it can be shown that the high-boron high-speed steel alloy composite roll has higher hardness and wear resistance than the high-nickel-chromium ductile iron roll, and its composition ratio, processing technology, and assembly structure technology are reasonable.

4.2 Save time after trial high-boron high-speed steel alloy combination roller

After the trial of high-boron high-speed steel alloy combined rolls, compared with the rolling conditions of high-nickel-chromium ductile iron rolls, the time of process accidents in the finishing rolling unit behind the trial stand was significantly reduced, and the number and time of hole replacement were greatly reduced. significantly reduced, see Table 3 for details.

Table 3 Comparison table of accident time, hole changing time and intermediate rolling waste of split rolling process before and after high-boron high-speed steel alloy composite roll trial

It can be seen from Table 3 that the use of high-boron high-speed steel alloy composite rolls reduces accident time and hole replacement time, reduces intermediate rolling waste, and lays the foundation for smooth rolling lines and improved technical indicators.

Brief analysis: Due to the enhanced wear resistance of high-boron high-speed steel alloy combined rolls, the stability of the material shape makes the rolling process stable, the rolling accidents are reduced, and the time for process accidents is reduced; the number of hole changes is reduced, and the number and time of debugging are reduced. The rolling accidents were caused by improper debugging and the resulting reduction of mid-rolling scrap.

4.3 Cost Analysis

The average rolling tonnage of each pair of high-boron high-speed steel alloy combined rolls and high-nickel-chromium ductile iron rolls: 6 passes are made for each pair of rolls, and each pair of rolls is repaired 6 times. The manufacturing tonnage is 1992.5×6×6=71730 tons, and the rolling tonnage of each pair of high-nickel-chromium ductile iron rolls is 500×6×6=18000 tons.

The price of a pair of high-boron high-speed steel alloy combined rolls is 26,500 yuan, and the price of a pair of nickel-chromium ductile iron rolls is 10,300 yuan. From this, we can get:

Rolling 71,730 tons of finished products, using a pair of high-boron high-speed steel alloy composite rolls, the input cost is 26,500 yuan; the number of roll pairs used for high-nickel-chromium ductile iron rolls: 71730÷18000=3.985 pairs

Input cost: 10300×3.985=41045 yuan

Cost saving: 41045－26500=14545 yuan

Trial 4 pairs of rolls saved a total cost of 14545×4=58180 yuan

5 Economic benefit analysis

5.1 Reduce the consumption of steel rolls per ton to create economic benefits

High-boron high-speed steel alloy composite rolls have low one-time investment, and their cost performance is better than high-nickel-chromium ductile iron rolls, see Table 4.

Table 4 Comparison of the use of high-boron high-speed steel alloy composite rolls and high-nickel-chromium ductile iron rolls

According to the data in Table 4, the steel consumption per ton of high-boron high-speed steel alloy composite rolls is 0.203 yuan lower than that of high-nickel-chromium ductile iron rolls. Based on the annual production of 80,000 tons of Ф12-18 rebar, the annual roll consumption can be reduced:
80,000 tons × 0.203 yuan/ton = 16,240 yuan

5.2 Reduce accident time and hole replacement time to create economic benefits

After the high-boron high-speed steel alloy combined rolls are used for rebar split rolling, the daily accident time is reduced by 0.3 hours, and the hole replacement time is saved by 0.4 hours. The total production time can be saved by 0.7 hours/day, and the annual production of Ф12-18 rebar 8 10,000 tons, the average hourly output is 60 tons/hour, the annual time can be saved by about 39 hours, and 2340 tons of finished products can be produced. Calculated on the basis of 340.28 yuan per ton of steel, the annual benefit can be created: 2340 tons × 340.28 yuan/ton = 4.3896 million yuan.

5.3 Reduce intermediate rolling waste and create economic benefits

Calculated according to the data in Table 3, based on the annual production of 80,000 tons of Ф12-18 rebar, the average hourly output is 60 tons/hour, the total production time is about 55 days, and the intermediate rolling waste is reduced by 0.5 tons/day. If the price difference between rolling scrap and the finished product is 1997.74 yuan/ton, then the annual benefit can be created by reducing mid-rolling scrap: 0.5 tons/day×55 days×1997.74 yuan/ton=275,700 yuan.

5.4 Years Roll Cost Savings

The cost saved by using 4 sets of high-boron high-speed steel alloy combination rollers:

(41045-26500) × 4 = 58,180 yuan

5.5 Costs

Cost of direct materials and spare parts invested in the project: 114,000 yuan

5.6 Create economic benefits throughout the year

20,300 yuan + 4,389,600 yuan + 275,700 yuan + 58,180,000 yuan – 114,000 yuan = 4,629,800 yuan

6 Existing problems and future plans

(1) In the actual trial of the cooling circulating water system of the finishing mill, the water pressure is only about 0.35-0.4Ma, and the water temperature is 40°C-45°C, which is still far from the trial requirements of the roll material.

(2) Due to the high hardness of this high-boron high-speed steel alloy combined roll, the processing of the pass is slightly more difficult, so some better tools and a more rigid roll lathe must be selected for processing.

(3) Since the high-boron high-speed steel alloy combination roller is reused as a mandrel, it is necessary to increase the inspection of the bearing position to prevent wear and tear during repeated use, which will cause the roller neck to be too small and burn out the bearing.

7 Conclusion

(1) Through the comparative analysis of various domestic alloy combination rolls, the high-boron high-speed steel alloy combination rolls are tried on the pre-slitting and slitting passes with heavy rolling load and fast wear, and the corresponding passes are designed and implemented Cooling and other schemes have been tried out, and the effect has reached the expected goal, which shows that the scheme selection and supporting measures are reasonable and feasible.

(2) The trial of high-boron high-speed steel alloy combination rolls in the production of 12-18 rebars has increased the service life of the rolls by 3-4 times, reduced the number and time of changing holes and rolls, stabilized the rolling material shape and process, and reduced accidents Time, reduce the consumption of guide spare parts, reduce rolling waste, improve the yield, and create certain economic benefits for the factory.

(3) According to the results of the subject research, an effective way has been found for the factory to solve the problems of non-abrasion-resistant and unstable material shapes of small-sized split-rolled rebars in the future.

(4) In view of the low investment of this kind of high-boron high-speed steel alloy combined roll and the obvious advantages of comprehensive cost performance, it is recommended to use it in batches in the production line.