Macro defect analysis and improvement measures of medium carbon steel

This paper analyzes the defects of medium carbon steel 42CrMoA, S38MnSiV and other non-conforming steels. Four types of defects such as oxide inclusion, central crack, internal slag inclusion and slag inclusion with nozzle were analyzed by flaw detection, low power acid leaching and scanning electron microscopy. According to the morphology and internal composition of the defects, the causes of defects were analyzed and corresponding improvement measures were formulated for different types of defects.

With the continuous progress of the steel industry, the product quality requirements of the raw materials used are increasing, which includes medium carbon steel used for automotive crankshafts, connecting rods, etc. The varieties of steel must have a high degree of purity in order to ensure the performance of the finished product and inclusions and other indicators. 42CrMoA and S38MnSiV are the two main varieties of automotive steel crankshafts produced in the plant at present. fracture and UT flaw detection and other macroscopic inspection methods to identify quality defects, and then use scanning electron microscopy to accurately analyze and finally characterize the defects and propose effective improvement measures to improve the qualified rate of finished products. The reasonable use of macroscopic defect analysis can effectively avoid quality accidents, and at the same time, according to the characteristics of macroscopic defects can determine quality problems in advance, improve production efficiency, which is of great significance to the smooth production of enterprises.

1. Test process

1.1 Brief description of the test

The internal quality of steel has a decisive role in the processing of finished products, so the internal quality needs to be strictly controlled in the production process, the plant production process is converter → LF → RH → continuous casting → heating and rolling → slow cooling → finishing flaw detection → packaging into storage. Ultrasonic flaw detection is one of the main detection methods for the internal quality of large materials. Combined with the production capacity of the site, this variety of steel >Φ150mm bar is inspected by manual flaw detection method, and Φ65-Φ150mm bar is inspected by automatic line flaw detection. In this paper, we summarize the qualified internal flaw detection of 42CrMoA and S38MnSiV materials for automotive crankshaft produced by large materials in the first half of 2017 in combination with the actual production situation (see Table 1), take the metal material and analyze the defects affecting their internal quality disagreement, and analyze and organize the defects obtained.
Table.1 Ultrasonic flaw detection

Steel grade	Flaw detection	Total number of flaw detection	Qualified quantity	Number of non conformal (longitudinal wave)	(Longitudinal wave) qualification rate
S38MnSiV	Manual flaw detection	1770	1362	408	76.95%
	Automatic line flaw detection	8426	8197	229	97.28%
42CrMoA	Manual flaw detection	9503	9486	17	99.82%
	Automatic line flaw detection	6043	5817	226	96.26%

1.2 Test material

Test material sampling and numbering rules are shown in Table 2.
Table.2 test material and numbering rules

Sample No.	Steel grade	Heat number	Specification (mm)	Source
Sample 1	S38MnSiv	1473	φ160	Manual flaw detection
Sample 2	S38MnSiV	457	φ160	Manual flaw detection
Sample 3	S38F	3240	φ135	Automatic line longitudinal wave
Sample 4	42CrMoA	3219	φ145	Automatic line longitudinal wave

Sample 1 – sample 4 are the defective specimens from the positioning anatomy of the flaw, respectively, representing four different types of defects, each type of defect is analyzed more than five specimens, the results of the specimens listed in the text are representative of the results of various types of defects.

1.3 Analysis method

1.3.1 Probe positioning anatomy method

• (1) First of all, the probe does not fit the steel for manual secondary calibration, found defects on its probe positioning, mark, according to the location of the mark cutting samples.
• (2) Manual flaw detection on the sample piece again, the defects are accurately located, marked.
• (3) The marked position for cutting, grinding and processing into metallographic specimens, analysis of defects.

1.3.2 Fracture analysis method

• (1) First manual calibration of the steel that does not fit the flaw detection, the defect is found to be located, marked, and the sample is cut in accordance with the marked position.
• (2) Acid dip treatment of the sample pieces, corrosion of the surface, determine the location of defects, shape.
• (3) Marking the defects and using a punching machine to break the specimen at the marked position so that the longitudinal direction of the defect is exposed.
• (4) Processing specimens, the location of the defect for electron microscopic scanning analysis.

Due to the high requirements of the anatomical method of flaw detection positioning accuracy, and the deviation in the specimen processing process has a greater impact on the exposure of defects, is not easy to be detected, the success rate of inspection and analysis is low, so the analysis of the results of this paper uses the fracture analysis method.

2. Test results

2.1 Macroscopic test results

2.1.1 Sample 1-sample 4 low-frequency acid leaching results (as shown in Figure 1)

Figure.1 Acid leaching low-power test results

Figure.2 Macroscopic shape of the fracture

2.1.2 Macroscopic appearance of the fracture

The macroscopic shape of the fracture of sample 1 – sample 4 is shown in Fig. 2.
The macroscopic inspection results of acid dip and fracture are shown in the figure above. The macroscopic results of sample 1 – sample 4 show four different macroscopic morphologies of defects, and the morphologies and tissues presented after acid dip and fracture are different for different defects. From the macroscopic results can be seen, sample 1 defects in the subcutaneous R / 3 distribution, the shape is more regular, the fracture organization and the matrix organization differences are obvious, the general defects at the regular long strip; sample 2 defects are mainly distributed in the subcutaneous and heart location, the defect is larger, irregular shape, generally through the sample, acid dip or interrupt the mouth of the defect location has obvious color differences; sample 3 defect location is located in the heart of the steel, and shrinkage shape similar to the presence of obvious open cracks, fracture organization, color and matrix organization, but the shape is different; sample 4 defects are distributed in the subcutaneous or heart location, the defect is larger, similar to the slag defects, irregular shape, fracture shape is lamellar, color and matrix organization is different.
Table.3 sample 1 electron microscopy results

Component	Percentage of alloy in defect
0K	26.71
AlK	21.37
FeK	51.92

Figure.3 Sample 1 electron microscope picture

2.2 SEM electron microscope sample preparation and microscopic analysis results

(1) Sample 1 analysis results are shown in Table 3 and Figure 3
From the SEM composition can be judged, the main characteristic components are [O] and [Al], the defect can be determined as oxide inclusions. The oxide inclusions defect is mainly Al2O3, the formation mechanism for the deoxidation process reaction 2Al + 3 [O] = Al2O3 generated by oxide inclusions, the formation period is mainly one is generated by the pre-refining deoxidation reaction, failed to get sufficient uplift, deposited in the steel; the second is generated by the late refining or pouring process protection is not sufficient, secondary oxidation [2].
Table.4 Sample 2 electron microscopy results

Component	Percentage of alloy in defect
O	36.77
F	11.22
Na	6.15
Si	12.96
K	0.6
Ca	12.42

Figure.4 Sample 2 electron microscopy pictures
(2) The analysis of sample 2 is shown in Table 4 and Figure 4
From the scanning electron microscopy results, the defect is mainly characterized by [O], [F], [Na], [Si], [K], [Ca], combined with the main components of the casting protection slag in Table 7 below, it can be determined that the defect is a slag trap, not a shrinkage. The size of the slag defect is large, the number is not much, but the damage is great, the main reason is caused by the continuous casting casting process in the crystallizer protection slag is caught in the steel; the second is due to the continuous casting billet tail billet is not cut clean, with slag billet rolling into material.
Table.5 Sample 3 electron microscopy results

Component	Percentage of alloy in defect
C	7.2
O	8.51
Si	3.85
Mn	5.75
Fe	74.69

Figure.5 Sample 3 electron microscope picture
(3) Sample 3 analysis results are shown in Table 5 and Figure 5
From the scanning electron microscopy results can be seen, the defect of the main characteristics of the composition of the steel matrix in addition to [O], combined with its macroscopic morphology can be determined that the defect is the center of the crack. The formation of a central crack or shrinkage is the result of the interaction of heat transfer, mass transfer and stress. The action of various forces during the operation of a high-temperature cast billet with a liquid core in a casting machine is the external cause of cracking, while the sensitivity of the steel to cracking is the internal cause of cracking. One of the reasons for the formation of shrinkage holes is the “solidification crystal bridge” [3].
(4) Sample 4 analysis results are shown in Table 6 and Figure 6
From the scanning electron microscope results can be seen, the defect of the main characteristics of the composition is similar to sample 3, but highlights the [Zr] component, this component is one of the main components of the continuous casting immersion water mouth, combined with the composition of the protective slag can be determined that the defect is a slag with water mouth. Protective slag with water mouth defects are similar to the causes of slag defects, the difference is that the poor material of the immersion spout or long service cycle, so that in the pouring process of water mouth slag line location of the refractory material by the protective slag erosion, and then part of the spalling, with the protective slag mixed into the crystallizer steel, along with the steel flow solidified in the billet, the size of such defects and slag defects, but the distribution of relatively no fixed location, and the crystallizer vibration and agitation are related.
Table.6 Sample 4 electron microscopy results

Component	Percentage of alloy in defect
O	40.61
Na	0.89
Al	17.31
Si	7.29
K	0.43
Ca	1.1
Zr	5.82

Figure.6 Sample 4 electron microscope images
Table.7 Protective slag used in the production process

3. Improvement measures and results verification

3.1 Improvement measures

In summary, the defects are caused by the lack of purity of the steel, secondary oxidation and the quality of the resistant material; for such cases, the corresponding solution measures are formulated.

• (1) Planning process, improve the purity of the molten steel, enhance the effective deoxidation measures in the early stage, stirring sufficiently to speed up the gathering of inclusions in the ladle float, refining late to ensure sufficient weak blowing argon time to promote the further floating of inclusions.
• (2) The pouring process to strengthen gas protection measures and equipment cleaning measures to avoid secondary oxidation of the steel and the formation of secondary oxides.
• (3) ladle, intermediate ladle and water spout selection of high-quality resistant materials, production in a timely manner to observe the loss of resistant materials, abnormalities occur in a timely manner; and in the daily production of timely replacement, maintenance of resistant materials, to prevent the erosion of resistant materials into the steel.
• (4) Pay attention to the choice of protective slag material, while fully planning the amount of protective slag added before production and timely observation of the melting and crusting of the crystallizer protective slag in the production process, timely handling of abnormalities.
• (5) In the casting process, strictly control the ratio of superheat and pulling speed, crystallizer vibration frequency, crystallizer cooling water and second cooling water control according to the characteristics of the steel grade should be properly adjusted to reduce the generation of shrinkage and central cracking.
• (6) Strictly control the head and tail removal length of the continuous casting billet, according to the characteristics of the steel species and combined with the actual situation of pouring to develop a corresponding suitable head and tail removal length.

3.2 Verification of results

Through 4 months of analysis and rectification, the passing rate of medium carbon steel flaw detection has been greatly improved.
Authors: Zhang Zhixing, Lin Limin, Zhao Yuguang

Source: Tubesheet Manufacturer: www.epowermetals.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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