Analysis of the causes of vertical cracks on the surface of profile blanks

Abstract: Based on the production and quality control experience of Maanshan Iron and Steel’s special-shaped billet continuous casting machine, the causes of longitudinal cracks on the surface of special-shaped billets are analyzed, measures and methods to inhibit the occurrence of cracks are found, and the occurrence of longitudinal cracks on the surface of continuous-cast billets is controlled.

Keywords: crack; profile blank; continuous casting

Introduction

Masteel’s special-shaped billet continuous casting machine was put into operation in August 1998. It is the first special-shaped billet continuous casting machine in China. It introduced SMS-Concast three-machine three-flow special-shaped billet/rectangular billet continuous casting machine. Its design production capacity is 630,000 t/a. Since it was put into operation, the continuous casting machine’s production capacity has been continuously released, reaching a production level of 1.2 million t/a, and new steel types have been continuously developed. At present, it mainly produces ordinary carbon structural steel, low alloy structural steel, weathering steel and other series of steel types, which are mainly used in the construction of buildings, bridges, offshore oil platforms, automobile girders, railway vehicles, etc. With the continuous development of varieties, due to its unique cross-sectional shape and complex continuous casting process, the incidence of longitudinal cracks on the surface of profile blanks is high, and there are many cracked products after rolling. The following is an introduction to the analysis of the causes of longitudinal cracks on the surface of Maanshan Iron and Steel’s profile blanks, as well as production practices and process research on reducing longitudinal cracks on the surface of profile billets.

Main technical parameters of profile billet continuous casting machine

The main technical parameters of Masteel’s special-shaped billet continuous casting machine are shown in Table 1.

Table 1 Main technical parameters of continuous casting machine

modelFull arc type
Number of streams3 machines 3 streams
arc radius10m,15m,30m
Section and drawing speed750×450×120 (large size): 0.7~0.9m/min500×300×120 (small size): 0.9~1.1m/min
Metallurgical length7100mm
Crystallizer length700mm

Characteristic analysis of longitudinal cracks on the surface of profiled blanks

Macroscopic analysis of longitudinal cracks on the surface of profile blanks

(1) The longitudinal cracks on the surface of the profile blank mainly occur in three areas: the web, the junction between the web and the R angle, and the flange. The cracks are shown in Figure 1. Among them, there are many cracks in the web and the junction between the web and the R corner.

(2) The longitudinal cracks on the surface of the profile blank can be divided into two types: wide longitudinal cracks and small longitudinal cracks according to the size.

Figure 1 Schematic diagram of crack location

(3) The incidence rate of longitudinal cracks in the inner arc is higher than that in the outer arc.

(4) The incidence rate of longitudinal cracks in microalloy steel is higher than that in ordinary carbon steel.

Microscopic analysis of longitudinal cracks on the surface of profile blanks

In order to understand the microscopic characteristics of the longitudinal cracks on the surface of the special-shaped billet, samples of the surface longitudinal cracks were taken, and scanning electron microscopy and energy spectrum analysis were used to observe and analyze the defect morphology and the morphology of inclusions in the defects.

Inclusion analysis

Through scanning electron microscopy observation, there are a large number of inclusions in and near the longitudinal cracks, and energy spectrum analysis mainly shows sulfides and silicates. The large-size silicate inclusions are shown in Figure 2, and the energy spectrum analysis results of some inclusions are shown in Table 2. It can be seen that the contents of Na and K are relatively high, indicating that they should originate from the copper mould tube. Some inclusions contain higher S content. Many small oxidized particles were also found in the longitudinal cracks and on both sides of the edges (Figure 3). Table 3 shows the energy spectrum component analysis results of such particles. It can be seen that this type of inclusion is a composite oxide of Mn, Si and Fe, and also contains a small amount of sulfur. According to the opinion of Document U, this oxidation particle should be the product of the secondary oxidation reaction. It shows that oxygen is transferred to the inside of the cast slab through the longitudinal cracks on the surface at high temperature, and diffuses in the cast slab matrix, causing a secondary oxidation reaction. However, due to their small size, it can be inferred that when they were generated, the surrounding matrix was a high-temperature solid.

Figure 2 Silicate inclusions in longitudinal cracks of profile blank

Table 2 Chemical composition of silicate inclusions in longitudinally cracked specimens/%

numberMnAlSiMgSCaKNa
128.23249.4620.74713.5324.6852.210
27.5474.80646.8985.1951.58626.8480.3980.009
39.99311.95452.3801.3440.42718.1951.0830.233
48.5327.46643.58620.26211.7752.6930.432

Figure 3 Oxidized particles on one side of the longitudinal crack

Table 3 Chemical composition of oxidized particles

elementSiSMnFe
%30.391.3536.1532.11

Cross-section morphology

Break the longitudinally cracked sample along the direction of the crack, observe the morphology of the longitudinal cracked section, and compare the morphology of the original section with the new section formed by breaking it apart. Figure 4 shows the new cross-sectional morphology. The new section is significantly smoother than the old section. This difference can be explained by the fact that the old section is formed at high temperature. At this time, the grain boundaries are enriched with low melting point substances (such as sulfides) and inclusions, and the strength is low. The cast slab breaks along the grain boundaries under the action of stress, and the section is uneven. The new cross section is at low temperature, the sample is broken along the crack, the grain boundary strength is high, the crack expands transgranularly, and the cross section is relatively smooth. To sum up, longitudinal cracks are caused by various stresses (including thermal stress, stress caused by solidification shrinkage and static pressure, structural stress caused by phase change, etc.) during the solidification process of the molten steel mold. When these stresses exceed the high-temperature strength of the billet shell, small cracks will appear on the surface of the billet shell. As the continuous casting billet continues to cool, the cracks will expand and develop into large cracks. Inclusions such as sulfide in steel weaken the grain boundaries, resulting in a reduction in its high-temperature strength, which is also one of the causes of longitudinal cracks.

Figure 4 The new cross-sectional morphology formed by splitting the longitudinally cracked sample along the crack direction.

Analysis of causes of vertical cracks on the surface of profile blanks

The generation of longitudinal cracks on the surface of the profile blank is the result of a combination of factors. The following analyzes and discusses the molten steel composition, protective slag, cooling process, superheat and other aspects.

Composition of molten steel

The influence of the composition of molten steel on cracks is, firstly, the carbon content in the steel, and secondly, some microalloying alloy elements and S, etc.

Effect of carbon content on surface longitudinal cracks

The carbon content of steel types cast by profile billets is mostly between 0.09% and 0.16%, which is in the crack-sensitive area. Among them, steel with a carbon content of 0.10% to 0.12% is most sensitive to slab cracks. This is mainly because when C=0.10%, the steel begins to undergo a peritectic reaction, and the primary green shell undergoes a phase transformation, accompanied by the largest linear shrinkage (0.38%). Therefore, the gap between the green billet shell and the hot surface of the mold is the largest, making the shrinkage of the billet shell near the meniscus and the corner region very irregular during the continuous casting process. The shell grows unevenly and coarse columnar crystals are easily formed. The shell cannot solidify uniformly, and depressions are often formed where coarse columnar crystals are formed. At the same time, the temperature at which molten steel forms single-phase austenite during the condensation process is high, the austenite grains of the as-cast steel are coarse, the plasticity of the cast slab is reduced, and the tendency of surface cracks is increased.

Effect of sulfur and manganese-sulfur ratio on surface longitudinal cracks

The increase of sulfur element in steel and the decrease of manganese-sulfur ratio will greatly reduce the high-temperature strength and plasticity of steel, making the high-temperature shell prone to cracks. Increasing the manganese-sulfur ratio in steel to above 30 can significantly reduce the incidence of longitudinal cracks, see Figure 5.

Figure 5 Effect of Mn/S ratio on surface longitudinal cracks

Influence of microalloying elements

Microalloyed steel is used because Nb, V, etc. easily form compounds with carbon and nitrogen. During the continuous casting process, when the Nb and V-containing microalloy steel billet is cooled to the austenite low temperature region, the fine Nb and V compounds in the billet precipitate along the austenite grain boundaries, making the ductility of the steel worse. As a result, cracks are prone to occur on the surface of the cast slab. It can be seen from Figure 6 that as the Nb content increases, the longitudinal cracks on the surface of the profile become more severe.

Figure 6 Effect of Nb on surface longitudinal cracks

Mold powder performance

The performance of mold powder has an important influence on the surface longitudinal cracks. Mold powder mainly affects the occurrence of cracks from two aspects: heat transfer and uniform slag film thickness. The performance of the mold powder must ensure that the thickness of the slag film between the shell and the crystallizer wall is uniform. The thermal resistance of the mold powder should be appropriately larger, that is, the heat transfer is slower, which is beneficial to inhibiting cracks. Therefore, based on the performance of the original mold powder, a new type of mold powder was developed, mainly to appropriately increase the crystallization rate of the mold powder to reduce the heat transfer rate of the mold powder, as shown in Figure 7. The temperature difference between the inlet and outlet water of the crystallizer using new mold powder is about 1°C lower than that of the old mold powder. The incidence of longitudinal cracks on the surface of the profile blank is also greatly reduced, see Figure 8.

Figure 7 Comparison of the temperature difference between the inlet and outlet water of the old and new mold powder crystallizers

Figure 8 Effect of mold powder type on surface longitudinal cracks

Cooling process

The primary cooling must be appropriate, especially for profiled billets with irregular cross-sections and irregular shapes. During the continuous casting process, different parts of the billet shell receive different forces. The primary cooling should ensure uniform thickness of the billet shell as much as possible to avoid excessive thinning of the local billet shell. If the secondary cooling is too strong or uneven, small cracks generated in the crystallizer can expand into large cracks. The faster the cooling, the greater the temperature difference between the inside and outside of the cast slab, the greater the resistance to volume change, the greater the structural stress and thermal stress, and the greater the tendency of the cast slab to produce surface longitudinal cracks. In the crystallizer, as the superheat of the molten steel and the flow rate of the molten steel increase, the thickness of the initial solidified shell decreases and the unevenness of the solidified shell increases. The use of slow cooling in the crystallizer can effectively prevent uneven solidification caused by the superheat of the molten steel and the flow of the molten steel. Therefore, strong cooling should not be used for the primary and secondary cooling.

Using the weakest possible primary and secondary cooling is beneficial to reducing the crack index on the surface of the slab, especially for steel types that are sensitive to cracks. This can also be seen from the results in Figures 7 and 8.

Pouring temperature

The effect of casting temperature on the surface longitudinal crack index is shown in Figure 9. It can be seen that when the superheat is low and high, the crack index increases. When the casting temperature is high, the thickness of the shell becomes thinner and cracks are easy to form. When the casting temperature is low, the performance of the mold slag used is affected, resulting in uneven slag film thickness, which affects the uniform heat transfer of the crystallizer and increases longitudinal cracks on the surface.

Figure 9 Relationship between tundish casting temperature and surface longitudinal crack index

Measures to control the occurrence of longitudinal cracks on the surface

Optimization of molten steel composition

(1) When selecting the steel type composition, consider the influence of the C content of the molten steel on the longitudinal cracks of the continuous casting billet, and try to avoid the peritectic reaction zone.

(2) Reduce the S content of molten steel as much as possible (≤0.020%) and increase Mn/S to above 30.

Optimize and adjust the performance of mold powder

Select protective powder with appropriate properties for different steel types. The main considerations for protective powder include alkalinity, viscosity and heat transfer rate. In particular, the crystallization rate of the protective powder should be appropriately increased to achieve weak cooling.

Optimization of primary and secondary cooling in continuous casting

Optimize primary cooling, use weak cooling, control the cooling water flow below 230m³/hour, and control the temperature difference between the inlet and outlet water of the crystallizer at 5~6°C. Weak cooling is also used for secondary cooling. The specific water volume of secondary cooling is controlled below 0.65l/kg steel, and the distribution of water volume in each section is optimized at the same time.

Optimization of superheat in molten steel

According to statistical results, controlling the superheat of the molten steel in the tundish package at 20~25°C can effectively reduce the incidence of longitudinal cracks on the surface of the profile blank.

Conclusion

(1) Longitudinal cracks on the surface of the profile billet originate from the mold and expand in the secondary cooling section; their occurrence is the result of a combination of factors. The main influencing factors include molten steel composition, mold slag, cooling process, superheat, etc.

(2) After taking the above measures to improve, the longitudinal cracks on the surface of the continuous casting billet have been greatly reduced, the incidence of longitudinal cracks has dropped by about 50%, and the cracked finished products have also been significantly reduced.

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