1.4112 steel has high C and CR content, so it has high hardenability and good wear resistance [1], so it is widely used in harsh environments. Due to its high carbon content and low toughness, it is prone to brittle fracture [2]. In China, the steel has been developed as a cutting-edge material for aerospace. In recent years, with the popularization and application of this steel grade in the stainless steel, bearing, and tool industries [3], domestic special steel enterprises have carried out a small amount of production, mostly used to manufacture parts or tools with less impact load [4]. Since 14112 steel is a high-carbon martensitic stainless steel, which has poor plasticity, a narrow malleable temperature range [5], and is prone to carbide cracking during production, so that the steel has to be scrapped or forged. Since the change of heating medium in 2017, the pass rate of flaw detection has been fluctuating, and there has been a large decline compared with before. Recently, it dropped to 47% and there were visible holes, which seriously affected the company's contract delivery rate. Therefore, improving the pass rate of flaw detection has become the main research direction of this paper.
According to the specifications of the finished product, 14112 Steel Production Process:
1) Smelting and die casting 1000 t precision forging timber (referred to as 1000 straight timber) or 1800 t fast forging + 1000 t precision forging timber (referred to as 1800 + 1000 composite timber) to produce 120 mm peeling timber.
2) Smelting and die casting 1800 t precision forging timber (referred to as 1800 straight timber), producing 121 150 mm diameter peeling timber.
3) Smelting, die casting 3150 t fast forging 1800 t precision forging timber (referred to as 3150 + 1800 composite lumber) or 1800 t precision forging + 3150 t fast forging + 1800 t precision forging (referred to as 1800 + 3150 + 1800 composite lumber), the production of 151 300 mm diameter peeling timber.
4) Smelting and die casting 3150 t fast forging (referred to as 3150 straight steel) to produce 301 mm peeling material.
No.2 Steelmaking Plant: The main equipment includes 1 30 t non-vacuum induction furnace, 1 30 t ordinary power electric arc furnace, 2 30 t LF furnaces, 1 30 t VOD VHD furnace, 1 30 t VD furnace and 5 trolley pit casting lines. According to the requirements of the forging plant, the primary molten steel can be poured after being degassed by the LF refining furnace and VD furnace. 1 t or t or t or 10 13 t adjustable spindle type.
Forging plant: divided into old line and new line. The main equipment of the old line includes 1000 t precision forging machine, 2000 t fast forging machine and 3500 t fast forging machine. The new line was established in 2010, and the main equipment includes 1800 t precision forging machine and 3150 t fast precision forging machine. The heating equipment includes ring heating furnace and chamber heating furnace, and the heating medium was changed from water gas to natural gas in October 2017.
Mode of transport: car with thermal cover.
1.4112 steel grade is equivalent to domestic 9Cr18MOV steel grade, which can be used to manufacture stainless steel chips, mechanical cutting tools and shearing tools and other materials. In order to improve the cutting performance, most users require the sulfur mass fraction in the steel to be 0015%～0.030% control. The chemical composition is shown in Table 1.
There are two main types of quality problems that occur in the new line:
One is the tail of the steel ingot, which refers to the transverse cracking and falling of the ingot tail during the forging process, commonly known as tail dropping, see Figure 1.
Figure 1Tail dropping occurred at the beginning of forging.
One is the flaw detection incompatibility of the finished material, and the flaw wave is mostly below 20 dB, and even the bottom wave disappears, that is, the flaw wave (f b). In the central area of the section of the finished timber, there are holes visible to the naked eye, see Figure 2.
Figure 2There is a visible hole in the center of the end face of the finished product.
Take the low magnification section of the incompatible material for inspection, see Figure 3. The low magnification qualitative is the central hole. It can be seen from the low magnification that the central defect on the low magnification becomes more and more severe as the injury wave increases. Fig. 3(a) specimen has small holes visible to the naked eye; Figure 3(b) and (c) have very obvious holes in the central area of the specimen, consistent with the defects in Figure 2.
Figure 3Low magnification of qualitative films with different injury waves: (a) 14 dB; (b) f=b test piece 1; (c) F=B test piece 2
The low-magnification test piece (Fig. 4) of the flaw detection incompatibility was cut, and the defective part was made into a small sample for high-magnification analysis. Fig. 4(a) shows a specimen with a light injury wave, which is cut transversely and has a small hole in the central area. Fig. 4(b) shows a specimen with a heavy injury wave, which is cut longitudinally. At high magnification, holes can be observed, and there are carbides in the vicinity of the holes, in which the reticulated carbides crack along the carbides.
Figure 4High magnification of qualitative films with different injury waves: (a) 14 dB; （b）f=b
In 2015, when the large spindle type was used, the proportion of tail shedding accounted for 657%, which affects the overall yield [6].
The tail of the steel ingot falls off abnormally, mainly because the tail of the steel ingot produces internal stress, which cracks from the inside to the outside. The reason for the analysis of internal stress is that the ingot demoulding time is too late, and the temperature of the ingot tail is low after demoulding. When the temperature of the center is cooled below the phase transition temperature point (ar1), if the cooling continues and the cooling rate is too large, a large tensile stress will be generated in the center of the ingot, which will cause the tail to fall off [6].
Through further data statistics and research analysis, it is believed that the second reason for tail drop is the low waiting temperature of the heating furnace. Since this grade is a high-carbon martensitic steel, a martensitic structure has been acquired during demoulding. Due to the insufficient preheating of the ingot, after the high-temperature ingot is put into the heating furnace, due to the low temperature of the furnace temperature, it does not play the role of heat preservation or heating on the ingot, but further decreases the temperature of the center of the ingot, which increases the stress of the core. The third reason is that after the average temperature of the ingot, in the process of heating up at low temperature, the temperature of the ingot is too fast, due to the poor thermal conductivity of the steel, the internal and external stress is further increased, forming greater thermal stress and tissue stress, resulting in internal transverse cracking. In the early stage of forging, under the action of external force, the steel ingot or billet becomes a crack to the surface after processing and deformation, and the tail falling problem occurs in serious cases, see Figure 1.
In view of the incompatibility of flaw detection, combined with the ingot type and process route, the pass rate of flaw detection is calculated according to the number of finished materials. The old line uses 071 t spindle type and new line with 9The qualified rate of flaw detection is very good for the finished materials produced by 0 t spindles; And the new line is adopted. 1 and 5The pass rate of flaw detection of the finished material produced by the 2 t spindle type is not good, especially the first two ingot types, the pass rate of flaw detection is even lower, see Table 2.
Therefore, in order to find out the real reasons affecting the pass rate of flaw detection, combined with the high and low magnification inspection results, the on-site production records, images and other information and variety process regulations were collected for technical analysis. Through comparison, the high and low magnification defects of the samples in this study are the same as those of previous flaw detection, as shown in Fig. 3 and Fig. 4.
Further investigation and analysis show that there is such a pattern:
1) The ingot holds for too long. Affected by factors such as production organization and equipment failure, the holding time of steel ingots is longer than that required by the process.
2) There are differences in the understanding of the holding time of the on-site personnel. Some personnel believe that when the third stage of the steel ingot out of the annular furnace enters the fourth stage (holding section), the timing of the holding time begins, rather than when the furnace temperature reaches the requirements of the regulations.
3) The re-burning time of the intermediate billet is too long. Exceeding the required time.
4) The insulation meets the requirements of the regulations, but the flaw detection is still inconsistent. The uniform temperature and holding time of the whole furnace fully meet the requirements of the process specification. When the production conditions are available, the flaw detection of the steel ingots produced by the first half furnace is all qualified. However, due to the large number of small ingots and the long forging time, the steel ingots that are released after the furnace are left for too long in the high temperature section, and the qualified rate of the flaw detection of the produced materials is greatly reduced.
5) Due to equipment failure, in order to ensure the continuity of production, the on-site operator usually heats and insulates at the lower limit temperature required by the regulations. However, because the specific time of fault removal cannot be determined, in order to improve production efficiency, the operator does not carry out cooling treatment as required, which artificially leads to too long holding time.
6) There are unexpected failures in the process of ingots loading, red delivery and furnace loading, resulting in low temperature of the ingots entering the furnace and tail dropping. During the forging process, the cross-sectional crack at the end of the drop extends inward, resulting in flaw detection failure.
7) After the heating medium is changed from water, coal and steam to natural gas, the incidence of flaw detection incompatibility is higher than before.
In order to improve the cutting performance, the sulfur mass fraction in the steel is required to be 0015%～0.030% control. Combined with the flaw detection, there is no obvious correspondence with the manganese-sulfur ratio, and the test 030% mn and 060% MN, all of them have the occurrence of flaw detection incompatibility, and the occurrence of incompatibility meets the common problems mentioned above. However, in order to ensure the deoxidation and material properties of molten steel, the manganese content should not be too low.
In addition to the difference in the size of the ingot, the holding time of the ingot, and the specification of the finished product, other process parameters, such as chemical composition control, ingot heating and furnace temperature, re-firing temperature of the intermediate billet, and the deformation of the intermediate billet, are not much different between the heat and the production batch.
Through the above discussion and analysis, in order to solve the flaw detection inconsistency, the adjustment direction should be the ingot holding time and the re-burning time of the intermediate billet.
Due to the change of heating medium, the calorific value of natural gas is higher than that of water and gas, and the heating rate is also faster than that of water gas. If the heating is still carried out according to the original process, it is inevitable that the flaw detection will be incompatible for the varieties that are prone to overheating, poor thermal conductivity and perforation.
Through the above analysis and discussion, the production process of the new line is optimized after the new heating medium is adopted
1) In order to avoid tail dropping, continue to use high temperature red delivery, open up a green channel, first ensure that this variety is put into the furnace, and the waiting temperature is increased from the original 550 600 °C to 650 700 °C.
2) If there is an accident in the process of red feeding, resulting in the temperature of the ingot entering the furnace being lower than the conventional temperature, the waiting temperature (t1) of the heating furnace should be adjusted to meet the average temperature of the ingot tail (t2): t1=t2+(150 10) °C and the waiting temperature should not exceed 700 °C.
3) After the furnace is averaged, the heating rate of the low temperature section is controlled by 50 °C h.
4) Optimization of ingot heating process: the holding temperature is 1140 1160 °C, and the holding time is 4 8 h.
5) Optimization of the heating process of the billet: the re-firing temperature is 1140 1160 °C, and the re-firing time is 15 3 h control.
6) Requirements for production organization: due to equipment failure or other reasons, the thermal shutdown can not be baked on time, adjust to the lower limit of heat preservation. When the total holding time reaches the upper limit, when it is still unable to be baked for production, the temperature is lowered to 800 1000 °C; When the production conditions are met, the temperature rises and the heating speed is not limited, and the material temperature reaches the thermal insulation temperature required by the process, and the thermal insulation time is produced according to the original process. When the temperature is cooled to 800 1000 °C and the production is still unable after 8 hours, the relevant management regulations shall be followed.
According to the above optimization scheme, the appropriate ingot type is used to ensure the processing ratio, and a total of 5 furnaces of steel are smelted. The production process strengthens the compactness of the red delivery. During the forging process, there was no tail drop problem, and a total of 125 pieces of material were produced, and all of them were qualified. For the first time since October 2017, the new line has achieved a 100% pass rate for batch flaw detection, as shown in Table 3.
1) For the change of heating medium, for special varieties with high carbon and high alloy ratio, the red feeding and heating process should be adjusted to avoid defects such as tail dropping, overheating holes, and flaw detection incompatibility.
2) The steel grade contains high CR and contains Mo and V, which are all elements that are easy to form carbides. In order to avoid the occurrence of overheating holes, in addition to reducing the heating temperature, it is also necessary to pay attention to the holding time, especially after the heating medium or furnace has changed.
3) The steel grade contains higher C and Cr and contains Mo and V, which increases the internal stress of the ingot. In order to avoid tail dropping due to stress, the high temperature should be ensured, and the temperature of the furnace should be controlled at 650 700 °C.
4) This steel grade is high-carbon high-alloy steel, in order to ensure the solidification quality, the pouring temperature should be strictly controlled, and the hot turnover package smelting should be used to prevent quality defects such as ingot deviation and center looseness caused by high-temperature pouring.
5) Quality is produced. For the varieties that are prone to flaw detection, in addition to scientific analysis from a professional perspective and strengthening technical support from process planning, in terms of production organization, it is necessary to seriously implement the process regulations and accurately understand the concept of holding time.
6) For this steel grade, the manganese content is not the influencing factor that causes the flaw detection incompatibility, and it is feasible to reduce the manganese content or low manganese-sulfur ratio in order to save costs.
7) This steel grade is not suitable for cold ingots. When using cold ingot production, metallurgists need to further study the heating process of cold ingots in combination with the description in this paper.
References. 
Article** — Metal World.