A brief analysis of the surfacing technology of continuous casting rollers in the sectoral section of the continuous casting machine

Abstract: This article mainly introduces the working conditions of continuous casting rollers, the process characteristics of open arc welding and submerged arc welding, and the material composition of the surfacing layer. The continuous casting roller welding scheme and the on-site use effect of the submerged arc welding roller are described, and the open arc welding process is analyzed.

Keywords: continuous casting roller; open arc welding; submerged arc welding; life

The two 2150mm continuous casting machines of the first phase project of Zhanjiang Iron and Steel were designed by Siemens VAI as two-machine, two-flow, vertically bending continuous casting machines. Using continuous straightening and bending technology, the basic radius of the continuous casting machine is 9500mm, the length of the casting machine is 37.044m, the thickness of the production slab is 230mm, and the width is 900 ~ 2150mm. It was put into use on September 27, 2015. There are a total of 88 online installed capacity and spare parts of the sector sections of the two 2150mm continuous casting machines, and the installed number of rollers is huge. The long online maintenance and replacement of the sector section affects the smooth running of the casting plan and production. The wear and tear of the continuous casting rollers in the sector section and the gap between the rollers directly affect the accuracy of the casting machine and the quality of the cast slab. In the early days of the continuous casting machine being put into operation, submerged arc surfacing technology was used in the manufacturing of new continuous casting rollers installed in the sector section. When producing steel types that are sensitive to the accuracy of the caster roll gap, there was a phenomenon of low-magnification failure of the cast slab. In particular, the service life of the continuous casting rollers from sector 7 to sector 12 is shorter than that of continuous casting machines of the same type. This greatly reduces the online service life of the sector section, seriously affects the production of various types of steel, and at the same time requires an increase in the turnover number of continuous casting rollers and increases maintenance costs. To this end, this article briefly analyzes the open arc welding and submerged arc welding processes of continuous casting rollers to find ways to improve the online service life of the rollers. Compared with open arc surfacing technology, the technical characteristics of submerged arc surfacing technology limit its application scope, making it difficult to repair continuous casting roll workpieces such as medium and high carbon alloy steel and high chromium cast iron. The characteristic of open arc cladding is that it reduces the heat input of the workpiece while reducing the deformation of the workpiece. It is especially suitable for the three-section roller cladding designed by VAI. At the same time, it is suitable for the continuous casting roller sleeve processed by submerged arc welding.

Continuous casting roller operating environment

The surface temperature of the continuous casting billet from the crystallizer to the horizontal section is as high as 1000°C. Under the working environment of the casting billet and mist cooling, the continuous casting roller is subjected to high temperature quenching and extrusion wear. The surface temperature of the roller changes around 420 ~ 480oC during one rotation. The surface of the roller sleeve bears the bulging force of the billet and the clamping force of the sector section, especially in the area where the light pressing technology is implemented in the continuous casting machine. The continuous casting production process uses secondary cooling mist cooling. The continuous casting rollers are subject to hot and cold fatigue stress and high temperature wear, and are exposed to secondary cooling spray water containing fluoride ions, which is corrosive. The surface quality of the continuous casting roller is directly in contact with the high-temperature cast slab and is worn for a long time, which leads to the deterioration of the surface quality of the continuous casting roller. This is the key factor in the expansion of the roll gap in the sector-shaped section.

In view of the wear factors of the continuous casting roller, the surface of the continuous casting roller must have the properties of resistance to oxidation, corrosion, high humidity and heat fatigue. For this reason, hard surfacing technology is used on the surface of new and repaired continuous casting roller sleeves to achieve the mechanical properties of the continuous casting roller.

Chemical composition of surfacing layer

The cladding layer of the continuous casting roll contains a variety of chemical elements such as C, N, Cr, Ni, Mo and Fe. The analysis is based on the characteristics of the relevant elements and the characteristics of the cladding layer.

The C element is a key factor in hard surface materials. The increase in carbon content helps to improve the strength, hardness and wear resistance of the surfacing layer. At the same time, the toughness and welding performance of the workpiece are reduced. For this reason, the carbon content should be controlled to the smallest range possible.

The N element has little effect on the welding performance of the workpiece. Controlling an appropriate amount of nitrogen content can help improve the hardness of the surfacing layer. In high-temperature working environments, N combines with strong nitride elements to form fine and uniform nitrides and plays a secondary hardening effect, promoting the workpiece to improve its aging resistance.

The Cr element increases the electrode potential of α-Fe in steel. Under high-temperature working environments, Cr carbides or nitrides are formed on the surface of the workpiece to improve the oxidation resistance and hardenability of stainless steel.

Ni has a higher effect than C, and the Ni element content increases by about 4%, which promotes the workpiece to obtain a better martensite structure. At the same time, Ni can reduce the delta ferrite content of the base material and increase the hardness. At the same time, Ni can improve the tempering resistance of martensitic stainless steel and increase the high-temperature toughness of the weld layer.

Mo increases the tempering stability and strengthens the secondary hardening effect. At the same time, it increases the strength of the steel without reducing the toughness, and improves the high-temperature strength of the steel. The content is controlled at 0.5% to 1.0%. The Mo content will cause the formation of delta ferrite.

At the same time, the selection of surface cladding materials for continuous casting roll sleeves must take into account the above factors. The chemical composition of the cladding material is not exactly the same as that of the steel cladding layer. The material of the surfacing layer is alloyed with its components, and there are varying degrees of alloy burning loss and segregation during the surfacing process.

According to relevant information, the acceptance criteria for alloy element content on the surface of the finished roll sleeve of a continuous casting roll adopts the Schaeffler curve method. The acceptance range is the surfacing alloy element content ranging from the surface of the completed roll sleeve and below to 3.5mm. The main control indicators are Cr content 12.5% ~ 13.5% and delta ferrite content 0 ~ 9%.

Analyze and compare objects

The roller type used by Zhanjiang Iron and Steel’s 2150mm continuous casting machine is mandrel + roller sleeve type, as shown in Figure 1.

A brief analysis of the surfacing technology of continuous casting rollers in the sectoral section of the continuous casting machine

figure 1

Comparison of open arc welding and submerged arc welding processes

Open arc welding

The base material of the surfacing roller sleeve is 42CrMo, and the welding is divided into three layers.

First layer: The base welding wire is made of 430-O. The specific composition is shown in Table 1.

Table 1 430- Welding wire composition requirements

CCrMnFeother
0.06170.5Remaining1

The diameter of the welding wire is D2.4mm; the thickness of the surfacing layer is 2~5mm; the purpose of surfacing is to reduce the C content, increase the Cr content, and improve the anti-oxidation and thermal fatigue resistance.

Second layer: transition layer. Use 414N-O welding wire; the material of the welding wire and the specific composition requirements after welding are shown in Table 2.

Table 2 Specific composition requirements of 414N-O welding wire after welding

CCrMnNiFeNother
0.08131.04Remaining0.122

The diameter of the welding wire is D2.4mm; the thickness of the overlay welding layer is 2~5mm; in order to ensure that the Cr content is greater than 12.5 within the required range, the Ni content is guaranteed to be around 4.0, C is controlled at 0.06~0.07, and N is at 0.1~0.12. Ensure that the material composition meets the above requirements so that the hardness, wear resistance, and thermal fatigue resistance of the material surface meet the usage requirements of the working conditions.

The third layer: working layer. The welding wire material is 414N-O. The specific composition requirements after welding are shown in Table 3.

Table 3 Specific composition requirements of 414N-O welding wire after welding

CCrMnNiFeNother
0.08131.04Remaining0.122

The diameter of the welding wire is D2.4mm; the thickness of the cladding layer is 2~5mm; the cladding meets the hardness, wear resistance and thermal fatigue resistance of the material surface to meet the usage requirements of the working conditions. Leave a machining allowance for roller machining.

After welding, the surface and below the surface of the finished cladding layer are 1.5mm, with a chromium content of 12.5% to 13.5% and a delta ferrite content of 0 to 9% (according to the Schaeffler method). The hardness is 44-50HRC (average of three measurements at each measurement point).

Submerged arc welding

The base material of the surfacing roller sleeve is: 42CrMo.

The first layer: welding wire CY430+WLDC, protective agent composition SJ604; wire diameter D3.2mm; surfacing layer thickness 1.5±0.5mm. The purpose of surfacing is to dilute the C content of the base metal, prevent Cr depletion in the working layer, and improve the thermal fatigue resistance of the work.

Second layer: transition layer, welding wire CY430+WLDC, protective agent composition SJ604; wire diameter D3.2mm;

The thickness of the surfacing layer is 1.5±0.5mm; the purpose of surfacing is to ensure the wear resistance and thermal fatigue resistance of the working layer under its working conditions.

The third and fourth layers are working layers, the welding wire is CY430+WLDC, the protective agent composition is SJ604; the welding wire diameter is D3.2mm; the surfacing layer thickness is 1.5±0.5mm. Surfacing welding ensures the wear resistance and thermal fatigue resistance of the workpiece under its working conditions, and ensures that the machining allowance flows out when the roller surface is processed. Hardness reached after welding: 38-42HRC (average of three measurements at each measurement point).

Effect

The continuous casting rollers in the sector section have used 500,000 tons of steel and are to be replaced. The rollers are offline and inspected as follows:

The surface quality of the submerged arc welding roller is not ideal. After 500,000 tons of excess weight, the middle segmented roller wears 1 to 2 mm on one side. There are clear thermal fatigue cracks on most of the roller surface, and some parts have begun to peel off. There are annular cracks at the weld overlap, which cannot meet the use requirements and cannot continue to be used.

The continuous casting rollers are repaired by open arc welding. When the repair rollers are put into service, they can meet the steel processing capacity of the arc-shaped rollers to reach 600,000 tons. The straightening section and horizontal section rollers can pass 1 million tons of steel, which greatly improves the wear resistance of the roller sleeve surface and meets the functional accuracy requirements of the production process.

A brief analysis of the surfacing technology of continuous casting rollers in the sectoral section of the continuous casting machine

Figure 2

Conclusion

(1) From the perspective of processing technology characteristics, the two surfacing technologies of open arc welding and submerged arc welding can meet the processing requirements of continuous casting rollers and can improve the hardness and wear resistance.

(2) Select 414N-O open arc welding wire as the working layer material of the continuous casting roller cladding welding, and N is in the working layer of the continuous casting roller of low carbon martensitic nickel-chromium stainless steel. Effectively improve the toughness, thermal fatigue resistance and tempering resistance of the weld layer structure.

(3) According to the analysis and comparison of the use conditions of continuous casting rollers, submerged arc welding rollers and open arc welding repair rollers, open arc surfacing continuous casting rollers are more effective in extending the life of the rollers, increasing wear resistance, and reducing continuous casting roller maintenance costs.

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