Analysis of Drawing Fracture Defects of SWRH82B Wire Rod

SWRH82B线材拉拔断裂缺陷分析

内容指南

用户在使用SWRH82B拉拔钢绞线时，出现批量拉拔断裂的问题。经过对原盘条和半成品的化学成分、夹杂物、冶金学分析，认为原盘条存在划伤、缩孔、组织异常等情况，而且用户的生产工艺也不合理。通过工艺改进，避免了断批的发生。

The steel strand is a steel product composed of multiple steel wires twisted. It uses high-carbon steel wire rods, which are cold-drawn into steel wires after surface treatment, and then a certain number of steel wires are twisted into strands according to the structure of the steel strands. stabilized. The main characteristics of prestressed steel strands are high strength, good relaxation performance, and relatively straight when unfolded. Therefore, the performance requirements are relatively high, and generally require good surface quality and core quality, sufficient strength and proper organization, and excellent drawing performance. Commonly used materials such as SWRH77B, SWRH82B, etc.

Recently, some users have successively experienced batch drawing fracture defects in the production process, which has seriously affected the normal delivery of steel products. Entrusted by the manufacturer, the cracking was systematically analyzed by means of chemical composition, inclusions and metallographic methods.

1. Crack Analysis and Discussion

1.1 Morphological characteristics of cracks

The specifications of defective wire rods are 12.5 and 13.0 mm. The user’s production process is: wire rod – physical phosphorus removal – phosphating – saponification (to improve the lubrication of wire drawing) – wire drawing (drawing to 5.05 mm) – twisting (seven strands in total) – finished product (steel strand), Drawing fracture occurs in the process of wire rod opening and straightening, rough drawing and strand twisting. If the wire rod breaks when it is cut and straightened, it can continue to be produced by welding; if it breaks when it is twisted, it cannot continue to be produced by welding and can only be divided.

There are two main types of steel wire fractures: cup-cone fractures and oblique fractures. There are significant differences in the two types of fracture morphology, and the causes are also significantly different (Fig. 1).

Fig.1 Samples of raw material brittle fracture and coarse wire fracture: (a) oblique fracture; (b) cup-cone port

The process layout of the user’s production site, the production of steel wire is not completed in one drawing, but a separate process of drawing spiral ribs one by one. According to user feedback, wire breakage at the welded part of the steel wire is common during spiral rib processing.

Macroscopic observation of the oblique fracture (Fig. 2) shows that the crack source is located on the outer surface of the wire, where the cracks are radial and lead to overall tearing. Macroscopically, the crack source area, fiber area and expansion area appear, and the fracture morphology of the crack is cleavage. The radial stripes are in the shape of a herringbone, and the tip of the herringbone points to the edge of the sample, indicating that the fracture originates from the outer surface of the wire, and the outer surface of the sample has a “V”-shaped defect, which is consistent with the drawing direction. The crack sources of the samples all originate from the outer surface of the wire. Careful observation of the outer surface shows that there are obvious concave and crack defects near the fracture source of individual oblique fractures, which can be identified as rubbing and scratch defects after observation. However, the low-magnification inspection of the cup-cone fracture shows that there are obvious porosity defects in the core. The preliminary inspection has exposed some causes of defects, and further systematic analysis is necessary to further confirm the causes of defects.

Figure 2 Macroscopic morphology of defect samples: (a) source of oblique fracture crack; (b) hole defect in the center of cup-cone fracture

1.2 Chemical Composition Inspection

The samples were taken for spectrum and gas content analysis, and there was no abnormality in each component, all of which met the requirements of BYBZ008-2011. The contents of P, S, O, and N in molten steel have reached the normal range, and the purity level of molten steel is acceptable. The specific components are shown in Table 1.

Table 1 Chemical composition test results (mass fraction) %

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1.3 Inspection of non-metallic inclusions, etc.

Inclusions and decarburization layer microstructure inspections were carried out on the samples (Table 2). The inclusions were all thin and the level was not too high. Therefore, the inclusions were not the direct cause of the SWRH82B drawing fracture. In addition, there is no obvious decarburization layer on the surface, and the inspection result is normal.

Table 2 Levels of non-metallic inclusions

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1.4 Metallographic examination results

The cracked samples were sampled and analyzed according to the type, and the matrix structure of the wire rod was sorbite + pearlite + a small amount of carbide. Among them, the metallographic structure of the sample matrix with cup-cone fracture is normal, but there are different degrees of looseness and shrinkage defects in the center (Fig. 3).

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Fig.3 Core tissue of cup-cone fracture: (a) loose defect; (b) shrinkage defect

During the continuous casting process of the billet, due to the difference in the cooling conditions of the molten steel in the mold, a “crystal bridge” is formed in the center of the longitudinal section of the billet to separate the molten steel, and the part under the bridge solidifies the latest. , Porosity or shrinkage cavity is formed in the center due to lack of supplementation by other liquids. Porosity and shrinkage cavities cannot be welded during the rolling process of the slab, which reduces the deformation resistance of the steel wire during the drawing process and becomes the source of fracture during the drawing process of the steel wire, causing a cup-cone fracture.

However, during the inspection of the oblique fracture sample, it was found that there was a martensite structure on the surface, with a thickness of 20-50 μm. The existence of this structure induced the fracture phenomenon during the drawing process. The appearance of high-carbon martensite on the surface is mainly due to the instability of the casting speed and the liquid level of the mold during the steelmaking and casting process, which causes the mold to entangle slag, thereby causing carburization (carburization) on the surface of the continuous casting slab. Due to the local carburization of the surface, the carbon content of this part is much higher than that of the matrix, so that the cooling “C” curve of the metal at this part shifts to the right, which increases the stability of the supercooled austenite, and the metal at this part is also cooled at the same cooling rate High-carbon martensite structure can be obtained; and the cooling rate of the surface itself is faster, which ensures the conditions for the formation of high-carbon martensite structure on the surface and promotes the formation of high-carbon martensite structure. In addition, there is an obvious mixed crystal phenomenon in the core, as shown in Figure 4, the size and size of the steel wire core are mixed. In the later drawing process, the severe mixed crystal part of the steel wire structure produces huge internal stress, thereby accelerating the occurrence of drawing fracture. On the other hand, due to the work hardening degree of the steel wire after drawing and the flow direction deformation of the grains, the stress concentration of the welded part is higher than that of the base metal. The corresponding weld fracture frequency is also higher than that of the base metal.

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Fig.4 Heart tissue (a) and surface tissue (b) of oblique fracture

2. Analysis of the causes of defects and solutions

2.1 Cause Analysis of Defects

Through the macroscopic and microscopic analysis of the sample, it is considered that the reason for the drawing fracture is as follows.

(1) Abnormal martensite appears on the surface of the wire rod, which is more obvious in winter and spring when the ambient temperature is low. Moreover, there are scratches on the surface of the base metal, and the defects are inherited to the surface of the steel wire, which is more likely to generate internal stress concentration at this part, forming a source of fracture, and inducing the oblique fracture of the wire rod during the straightening or rough drawing process. In addition, due to the aging and damage of the wire drawing die or the unsatisfactory lubrication effect of the wire drawing lubricant, the steel wire may also be broken.

(2) The main reason for the cup-cone fracture in the wire rod drawing process is that the core of the slab is loose and the shrinkage cavity is defective. After the wire rod is rolled, it cannot be effectively welded, which induces the core of the steel wire to break.

(3) There are certain problems in the process layout and setting of the customer’s production process, the production process is not continuous, and the repetitive welding operation increases the probability of wire breakage during the drawing process of the welding part. This is mutually confirmed with customer feedback on the parts where broken wires frequently occur.

2.2 Process improvement measures

According to the cause analysis, the following measures were taken in continuous casting, rolling and other processes.

(1) Scratch defects on the surface of wire rods may appear in all aspects of wire rod production, storage and transportation, and later use and operation. For the prevention and control of surface defects such as wire rod scratches in the “cold” state in the factory, corresponding operation and management regulations have been formulated and on-site physical sampling inspections have been strengthened. The prevention and control of other links need to be further strengthened and paid attention to. In addition, it is also necessary to strengthen the protection of the surface quality of the wire rod during transportation and post-processing.

(2) From the perspective of chemical composition control, the release index of C content in SWRH82B wire rod is appropriately reduced to achieve the purpose of alleviating the hardening performance of wire rod.

(3) Strengthen the control and management of the solidification structure of the slab, and optimize the ratio of superheat, electromagnetic stirring and casting speed. Ensure that the cooling system of the casting machine operates normally, and the billet is sprayed and cooled evenly. To ensure the stability of the solidification structure of the slab, the low-magnification inspection of the slab is implemented as a constraint condition for the release of the slab for rolling.

2.3 Effect of Quality Improvement

经过强化现场操作规程，后续用户生产使用过程取得了良好的效果，批量拉丝断裂现象得到有效改善。在提高劳动生产率的同时，也避免了外界异议的发生。

三、结论

(1)拉拔斜断缺陷是由于线材表面异常组织马氏体和表面划伤造成的。

(2)拉拔杯圆锥形断裂是板坯芯部存在松动、缩孔的缺陷，盘条轧制后不能有效焊接，从而诱发钢丝芯部拉拔断裂。

(3)通过工艺优化，可以有效避免此类缺陷。