Cause analysis of surface defects of 316L stainless steel cold rolled plate

Aiming at the linear defects on the surface of 316L stainless steel cold rolled plate, the chemical composition analysis, metallographic analysis, scanning electron microscope and energy spectrum detection of continuous casting billet, hot rolled black coil and cold rolled plate were analyzed. The results show that the surface of 316L stainless steel continuous casting billet with defects has deep vibration marks, pits and microcracks; There are linear defects on the surface of hot rolled plate, and there is a large area of oxidation area near the defects; No inclusions with large size were found at the defects of cold rolled plate, and the chemical composition met the requirements. Therefore, it is judged that the linear defect source of cold rolled plate comes from continuous casting slab. During the heating process of heating furnace, the original solidification defect of continuous casting slab or too rough trace after grinding are seriously oxidized and inherited to cold rolled plate, resulting in surface linear defects. Through the adjustment of continuous casting process parameters and the fine grinding operation of continuous casting slab surface, the surface quality of continuous casting slab is effectively improved and the linear defects on the surface of cold rolled plate are greatly reduced.

316L stainless steel is a more corrosion-resistant stainless steel developed on the basis of 304 stainless steel, with the addition of 2% ~ 3% mo. the addition of molybdenum greatly improves the pitting corrosion resistance of 316L stainless steel to dilute sulfuric acid, phosphoric acid, various organic acids, urea and chloride. It is mainly used in the Petroleum and petrochemical industry and is widely used in pressure vessels, transportation, metal products, machinery manufacturing, decoration Welded pipe and other purposes. The surface quality of 316L stainless steel cold rolled sheet is required to be high, and there shall be no defects such as scratch, roughness, pit scar and stripe. It must undergo strict surface inspection before leaving the factory.

The production process of 316L stainless steel cold rolled sheet is EAF + AOD + LF + CC + slab grinding + hot rolling + cold rolling. The capacity of EAF, AOD and LF furnaces is 180t, and the slab size is 200 × Mainly 2040m, the thickness range of hot-rolled plate is 2.5 – 14.0mm, and the thickness range of cold-rolled plate is 0.3 – 4.0mm. The type with the highest surface defect rate of cold plate is linear defect, which often leads to surface nonconformity of cold plate, which has a great impact on product quality and yield.

Inspection contents

In order to determine the causes of linear defects on the surface of 316L stainless steel cold rolled plate, the surface and subcutaneous quality of continuous casting slab, the rolling direction cross section of hot rolled black coil, the defects and rolling direction cross section of cold rolled plate were analyzed by metallography, scanning electron microscope and energy spectrum, and the chemical composition of the finished product was analyzed at the same time.

Inspection results

Chemical composition

Pure or sub pure austenitic stainless steel does not occur or rarely occurs during solidification δ Phase transformation, the concentration of sulfur between dendrites is enriched, which makes the intergranular adhesion and plasticity extremely weak. Under this condition, due to the wide solid-liquid two-phase interval and the highest solidification shrinkage rate, longitudinal microcracks are generated in the vibration trace trough, s is enriched at the grain boundary, and the inclination of microcracks during hot rolling will also increase. Therefore, 316L stainless steel is required to contain low S content.
At the same time, there is an obvious relationship between the crack sensitivity of stainless steel and the Cr Ni equivalent ratio. When the Cr Ni equivalent ratio is more than 1.5, the crack sensitivity disappears, because when the Cr Ni equivalent ratio is greater than 1.5, phase transformation will occur in the solidification process, and the growing sulfide will be cut off and divided into point sulfide, so as to inhibit the harmfulness of sulfur.
Based on the above reasons, firstly, the composition of cold-rolled sheet with unqualified surface quality is tested, mainly focusing on the s content and chromium nickel equivalent ratio in steel. See Table 1 for the specific composition. It can be seen from table 1 that the content of s in 316L stainless steel is controlled at a low level, Creq / Nieq > 1.5. Therefore, the relationship between liquid steel composition and linear defects of cold rolled sheet shall be excluded first.

Table 1 chemical composition of defective sample /%

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Surface defect inspection of continuous casting slab

After inspection, it is found that the surface defects of 316L stainless steel continuous casting billet mainly include deep vibration marks (the depth is about 1.5mm), irregular and disordered vibration marks, slag pits at the edge of the wide surface, etc., as shown in Figure 1.

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Fig. 1 surface defects of 316L stainless steel continuous casting billet

Taking samples from deep vibration marks and slag pits for metallographic analysis, it is found that there are microcrack defects on the surface of continuous casting slab, as shown in Figure 2.

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Fig. 2 subcutaneous defects of 316L continuous casting slab

Through a large number of sampling, detection and analysis, it is found that there are basically microcrack defects under the surface of 316L stainless steel continuous casting slab as shown in Figure 2. For this defect, the surface of continuous casting slab should be polished. Excessive pressure or continuous local grinding will continue to expand the crack. Therefore, cracks still exist on the surface and secondary surface of the polished continuous casting slab through the metallographic microscope, as shown in Fig. 3 (a), and the grinding roughness of the surface of the continuous casting slab is uneven by naked eye, as shown in Fig. 3 (b), with an average roughness Ra of 250 μ m.

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Fig.3 surface quality of continuous casting slab after grinding (a) microcrack (b) surface roughness

Defect detection of hot rolled black coil

After further taking the hot rolled black coil sample without pickling, it is found that there are linear defects on the surface. Scanning electron microscope observation and energy spectrum analysis are carried out at the cross section of the defect, as shown in Figure 4. The composition in the figure is the composition of point (a).

It can be seen from Figure 4 that the linear defects on the surface of the hot rolled black coil form cracks downward along the surface, and a large area of oxidation area is formed around the crack. After the composition detection, it is found that the oxidation area contains high Si and O elements. Therefore, it is inferred that the large area of oxidation area is heated by the surface cracks of continuous casting billet in the hot rolling furnace for a long time, The oxidizing atmosphere in the furnace penetrates into the matrix through the crack and combines with the oxygen loving material Si in the matrix, forming a large area of oxidation area around the crack. After rolling, it leads to linear defects on the surface of hot-rolled black coil, and the unclean pickling of oxidation area will be inherited to the cold-rolled plate, resulting in surface defects of cold-rolled plate.

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Fig. 4 scanning electron microscope observation and energy spectrum analysis of defects in pickled hot rolled coil

Detection and analysis of surface defects of cold rolled sheet

The linear defects on the surface of the cold-rolled sheet are distributed parallel to the rolling direction, and the parts on both sides are densely distributed, with a length of about 10mm and a width of about 1mm, as shown in Fig. 5. The defects were observed by scanning electron microscope and detected by composition. The chemical composition of point (a) in Figure 6 was analyzed by energy spectrometer, which was the oxide scale composition under the normal matrix structure, and no large-size inclusions were found, as shown in Figure 6.

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Fig. 5 macro morphology of surface defects of 316L stainless steel cold rolled plate

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Figure. 6 Analysis of chemical composition with energy spectrometer

There is a corresponding relationship between the defects of cold rolled plate and the distribution of surface defects of continuous casting slab. Through the analysis of the defects of pickled hot rolled black coil, it is proved that the source of linear defects of cold rolled plate is the surface microcrack defects of continuous casting slab.

Improvement measures

Light grinding operation of continuous casting slab surface

Because the microcrack defect on the surface of 316L stainless steel continuous casting slab will lead to the linear defect of cold rolled plate, it is found that the depth of microcrack on the surface of continuous casting slab is generally no more than 300 μm. On the premise of ensuring fine grinding operation, defects can be removed by light grinding with more than 20# grinding wheels. The improvement of grinding process parameters is shown in Table 2. The subcutaneous quality of the improved surface grinding quality of continuous casting billet is shown in Figure 7 (a), and the surface quality of continuous casting billet is shown in Figure 7 (b).

Table 2 improvement of grinding process parameters of 316L stainless steel continuous casting billet

Process parameters Grinding wheel model Single grinding amount / mm Transverse feed rate of grinding wheel / mm Forward speed of grinding wheel / m.s-1 Grinding type
Before improvement 16# 1.5 80 80 90 degree grinding
After improvement 20# 1 60 70 45 degree grinding

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Fig. 7 surface condition of continuous casting slab after improved grinding process (a) subcutaneous condition (b) surface condition
As can be seen from Fig. 7, the surface roughness Ra of 316L stainless steel continuous casting billet after grinding is measured by the roughness instrument by using 20# grinding wheel for fine grinding operation, and the surface roughness Ra is increased from 250 μ M reduced to 180 μ m. Flat and smooth without microcrack and other defects.

Quality control of continuous casting slab

The fundamental to completely eliminate the surface linear defects of 316L stainless steel cold rolled plate is to improve the surface quality of continuous casting slab. After observation, the main surface defects of 316L stainless steel continuous casting slab actually produced include deep vibration marks, slag pits and microcracks.
Vibration mark is the inevitable product of mold vibration. During negative slippage, the primary shell of meniscus bends to the liquid steel side by the pressure of mold and the extrusion of slag ring; During the positive slippage, the primary green shell adheres to the copper wall under the static pressure of molten steel, and the molten steel overflows to the front end of the solidified shell to form vibration marks. The vibration mark trough is often the birthplace of slab transverse crack and the accumulation of slag particles and bubbles. With the increase of vibration mark depth, the probability of defects increases. The mold vibration form is the main factor affecting the vibration mark depth. As we all know, using the vibration form of high frequency and small amplitude to minimize the negative slip time is an effective measure to reduce the vibration mark and reduce the surface defects under the vibration mark under the condition of ensuring the smooth demoulding of the slab.
At the initial stage of solidification, 316L stainless steel has large volume shrinkage, uneven shell growth, local stress concentration at the weak part of the shell, and uneven heat transfer of slag film in the mold, resulting in poor lubrication between the mold and the slab, resulting in pits on the surface of the continuous casting slab, and cracks may appear at the bottom of some pits. To control the generation of surface pits, uniform cooling should be maintained in the mold. The main control factor is the uniform heat transfer of protective slag film, which requires a thin and uniform protective slag film, which is closely related to the melting point, viscosity and consumption of protective slag.
With the increase of slab section size, the flow of molten steel increases, and the non-uniformity of flow field and temperature field in the mold increases. The 316L stainless steel with defects is mainly ultra wide slab, and the slag metal interface temperature at the edge of slab is more prone to non-uniformity. If the temperature is too low, the melting of protective slag is unfavorable, which is easy to produce slab surface defects, such as excessive fluctuation of liquid level in the mold, Therefore, the optimal liquid steel flow state in the mold plays an important role in the production of high surface quality continuous casting billet. The liquid steel flow field is affected by the insertion depth, shape, drawing speed and other factors of submerged nozzle.
Based on the above theory, the composition of mold powder is improved and the continuous casting production process parameters are strengthened. The composition of mold powder before and after improvement is shown in Table 3, and the process parameters before and after improvement are shown in Table 4.

Table 3 improvement of characteristic parameters of mold flux for 316L stainless steel continuous casting

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Table 4 improvement of 316L stainless steel continuous casting process parameters

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The surface quality of 316L stainless steel continuous casting billet produced after improving the continuous casting process parameters is shown in Figure 8.

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Fig. 8 surface condition of 316L stainless steel continuous casting billet
The surface vibration marks of 316L continuous casting billet produced after improving the continuous casting process parameters are stable. There are a few depth of 0.9 – 1.0mm, most of the vibration marks are near 0.6mm, and there are few surface defects such as slag pit vibration marks disorder.
At present, the surface of 316L stainless steel continuous casting billet is graded and confirmed. Grade A and B billets with good surface quality can be rolled without grinding. Grade C and D billets with poor surface quality can also avoid the occurrence of linear defects of cold-rolled plate after light grinding with 20# grinding wheel. The nonconformity rate of linear defects of cold-rolled plate can be reduced from 5% to less than 1%.

Conclusion

  • 1) Through the morphology and energy spectrum analysis of the surface defects of 316L stainless steel continuous casting slab, hot rolled black coil and cold rolled plate, the results show that the linear defects come from the continuous casting slab.
  • 2) The micro cracks on the surface of continuous casting slab are heated at high temperature for a long time in the heating furnace to form a large area of oxidation area, which is inherited to the surface of cold plate after rolling, resulting in linear defects on the surface of cold plate.
  • 3) Defects such as slag pits and microcracks on the surface of continuous casting slab can ensure the surface quality of cold rolled plate after fine grinding with 20# grinding wheel.
  • 4) By improving the process parameters of continuous casting, the surface quality of 316L stainless steel continuous casting billet and cold rolled plate are greatly improved.

Source: China 316L Plates Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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