Formation Causes of Piercing Cracks in 304L Seamless Steel Tube by Two-Roll Cross Rolling

Abstract : Using optical microscope, thermodynamic calculation software, scanning electron microscope, etc. , the reasons for the cracks in the blank pipe during the two-roll cross-rolling piercing process of 304L seamless steel pipe were analyzed . The results show that there are about 3% (volume fraction) of 8- ferrite phase in the billet for piercing , and the uneven deformation in the piercing process is the main cause of cracks; The tensile stress and shear stress that exceed the fracture strength of the metal promote the formation and propagation of cracks.
Key words : 304L seamless steel pipe; perforation; crack;
the 1960s in China , the 18-8 stainless steel seamless pipe has been produced by the two-roll cross-rolling piercing process . Corrosion-resistant alloy) steel pipe production . However, in the two-roll skew rolling piercing process, when factors such as billet quality, billet heating system, tool design, equipment adjustment and operation are not well controlled, waste products such as wall thickness, internal cracks, internal scratches, external cracks and Surface scratches, etc. . / 202 mm X 23 mm 304L produced by Zhejiang Jiuli Special Materials Technology Co., Ltd. using the two-roll cross-rolling piercing process
(00C19N10) Seamless steel tubes provide tube blanks for cold rolling mills, but in a batch of products, almost every empty tube has sporadic cracks along the axial direction. The characteristic of the crack is that the inner and outer surfaces of the empty tube are basically complete, but the solution ( mixed solution of 4 g NaOH , 4 g KMnO 4 and 100 mL HZ O ) along the circumferential direction on the cross section of the tube is boiled, and the corrosion time is about 10 min ; use a portable ferrite analyzer to measure the ferrite content , so as to compare with the quantitative metallographic calculation results; use a Hitachi3400N scanning electron microscope ( SEM) to observe the microstructure of 304L steel billet and perforated waste pipe , and use Its attached energy spectrometer analyzes the phase composition .
1.1 Chemical composition
It can be seen from Table 1 that the chemical composition of billet and waste pipe is basically the same .
Table 1 Chemical composition ( mass fraction ) of steel billets and waste pipes
Tab. 1 Chemical compositions of billet and pierced billet (mass) %

sample	C	Cr	Ni	Mo	Si	mn	S	P	Fe
billet	0 026	18 49	8 14	0 108	0 33	0 64	0 019	0 033	Remain
waste management	0 021	18 64	8 14	0 108	0 33	0 63	0 023	0 035	Remain

1.2 Microstructure Generally speaking , the quality defects of the blank tube in the two-roll cross-rolling piercing
There are both internal reasons ( the chemical composition of the billet , the type and distribution of inclusions, etc. ) and external factors ( the billet heating temperature and system , tooling, etc. ). In this 304L seamless steel pipe piercing process, the piercing temperature of the steel billet is 1 100 ~ 1 140 ° C , and the water is cooled immediately after the piercing is completed . Under the same piercing process , no perforation cracks were found in billets of different furnace numbers and different batches , but only one furnace number
30 pieces of steel billets in total , and there are sporadic cracks in almost every waste pipe . It is speculated that the cracks are probably mainly caused by the quality of the billet . Therefore , the microstructure of the billet and waste pipe was analyzed .
from Figure 2 that in the axial direction of the tube blank, a large number of slender black substances are distributed along the grain boundaries . Through quantitative metallographic calculation, the area fraction of the elongated strip-shaped black substance is 2% to 3% .
It can be found that there are a large number of fine cracks along the axial direction , and the local crack width reaches 0.1 mm ; at the same time , a large number of thin and long strips are distributed along the grain boundaries , and most of the fine cracks are associated with the thin and long strips . In addition , according to the analysis of the slender material by energy spectrometer , it is known that it is a nickel-poor and chromium-rich phase ( 26.2% chromium , 3.6% nickel ). It can be seen that the appearance of cracks inside the perforated waste pipe is closely related to this slender strip substance , how to determine and eliminate this substance is of great significance for improving the quality of the perforated waste pipe .
1.3 Thermodynamic phase calculation
It can be seen that when the molten steel is cooled, the body-centered cubic ( BCC) structure S phase is first precipitated from the liquid phase, and the peritectic reaction occurs at about 1 450 C , and the liquid phase is completely transformed into 5 phases; when the temperature continues to drop, the phase The austenite phase ( phase ) with face-centered cubic ( FCC) structure is precipitated in the medium , and the S phase is completely transformed into the Y phase at about 1 170 C ; when the temperature continues to drop to 840 ~ 720 C , M 23 is precipitated from the Y phase C 6 and. phase, when the temperature drops below about 680 °C , a ferrite phase with BCC structure is precipitated from the Y phase . According to the perforation process of 304L , it can be judged that it is impossible for M23C6 phase and a phase to appear in large quantities . Therefore , the thin strips may be the 5th phase .

2 Analysis and Discussion
Generally speaking , there are two reasons for the appearance of 5 phases : one is that the ingot is directly precipitated from the liquid phase when the ingot is solidified , and it remains in the alloy during the subsequent ingot billet opening and thermal processing ; the other is due to the heating temperature of the billet On the high side and for a long time, resulting in the precipitation of 8 phases from the Y phase . In the piercing process of 304L seamless steel pipe, the piercing temperature is 1 100 ~ 1 140 C , the piercing time is very short, and the water is cooled immediately after piercing . It can be seen that the perforation process is unlikely to lead to the generation of 8- phase . In addition, under the same process, no cracks were found inside the 304L perforated blank tubes of other furnace numbers and batches . It can be deduced that the 8 Correspondingly, it was inherited from billet .
It can be seen that there are a large number of elongated strip- like 8 phases along the billet axis . Quantitative metallographic determination of its volume fraction is 3 % to 4 % . In addition , the results of the measurement by a portable ferrite meter also show that the ferrite content ( volume fraction ) is about 3 % .

According to the above analysis , it can be deduced that the elongated substance mentioned above is the 8 phase . Combined with the piercing process , it is believed that the appearance of 8 phases is closely related to the smelting of billets .
It can be seen from Table 1 that the mass fraction of molybdenum in 304L (00Cr19Nil0) billet is about 0 . 11 % , there is no such element in the standard ; while the mass fraction of nickel is only 8 . 14 % , which is the lower limit of the standard value . Molybdenum element is the forming element of ferrite . Generally speaking , in order to obtain a stable austenite structure , the content of nickel must be appropriately increased in molybdenum-containing steel to balance the effect of molybdenum . Therefore , in the 304L billet with low nickel and molybdenum content , if there is segregation of chromium and molybdenum elements, it is easier to form 8 phases . In addition , during the ingot solidification process , if the cooling control is not good , the 8- phase will not fully dissolve back into the matrix , and the structure genetic phenomenon will occur in the later processing , thus affecting the thermal processing of the billet .
Two-roll cross-rolling piercing is a complex and uneven metal deformation process . The distribution of deformation strength of the cross-section along the diameter at each stage of the piercing deformation zone can be expressed by [(U1+W ) + 2U Z ] .

In the piercing preparation area , the outer layer of the billet in contact with the roll deforms violently , with fine grains , and the deformation near the center of the billet is small , the grains are coarse, and the deformation intensity is U- shaped along the diameter direction , that is, the U1 area . This zone continues such a process with increasing diameter reduction . When the reduction is further increased , the deformation of the outer layer of the tube blank in contact with the roll is large , and the deformation of the central area is also large , while the deformation of the transition area between the two is small , and the grains on the inner and outer surfaces of the tube blank are finer. , while the grains in the transition zone are coarse, and the deformation strength is distributed in a W shape along the diameter direction, that is, the W zone . This shape continues to the nose of the perforated plug , but the grains continue to refine , and the grain size difference on the same cross section gradually decreases . In the piercing section and rolling section , the outer surface in contact with the roll has a large amount of deformation , and a grain refinement layer appears . At the same time, the deformation of the inner surface of the blank tube in contact with the plug is also relatively large , and a grain refinement layer appears . However , the deformation is milder and the grains are slightly larger in the middle area of the blank tube wall thickness . In this way , on the plug side , the deformation intensity is distributed in a U shape along the radial direction , and there are two such deformation zones in the radial direction, that is, 2U2 zone 口-11 ] .

In such a state of uneven deformation , the metal on the outer layer deforms violently , and the metal must flow longitudinally , transversely and tangentially , while twisting , and the metal tends to increase in circumference and buckle , so that it transitions between the deformation zone of the outer layer and the wall thickness. The deformation zone produces additional tensile stress and shear stress . When this additional tensile stress exceeds the strength and plastic deformation capacity of the metal , the metal will crack , which is the reason for the cracking of the outer layer of the waste tube . Outer fissures usually occur in the U1 and U2 regions . In the W area, the metal grains are refined. When the metal flows through the surface of the plug , it is rolled by the plug , and the metal on the inner surface of the tube blank undergoes severe deformation , and the grains continue to refine . At the same time , the metal produces tangential , longitudinal flow and twist . As a result , longitudinal , tangential, and transverse tensile stresses are generated in the inner surface layer and the middle transition zone . During the rolling process of the metal , when the tensile stress exceeds the fracture strength of the metal , an inner layer will be generated between the inner surface layer and the middle layer. Cracks, internal cracks are usually generated in the U2 area .

The 5- phase has a body-centered cubic structure, while the Y -phase has a face-centered cubic structure . The deformation capabilities of the two are inconsistent , especially the thermoplasticity of the interface between the two phases is lower than that of the Y- phase matrix. When the two phases coexist and deform, cracks are prone to occur 9 . Venugopal et al. used constant temperature thermal compression tests to study the hot workability of 304L steel and found that five phases are likely to be formed during high temperature and high speed deformation and lead to a decrease in the high temperature plasticity of the alloy. When the deformation temperature is 1200 C and the strain rate is 100 s^ 1 , the formation About 0. 7% ( volume fraction ) of the 5- phase , resulting in processing instability [ 14 In the process of oblique rolling and piercing of thick-walled pipes , since the deformation process is not easy to reach the center layer of the pipe wall , a stress state of double -drum deformation occurs along the deformation zone , so the tensile stress and shear stress of the center layer at this time are sharp increase . In this high tensile stress state , cracks are more likely to occur on the interface between phase 5 and phase Y in the central layer ,

To sum up , it can be seen that the non-uniform deformation state and the existence of five phases in the piercing process of 304L stainless steel two-roll cross-rolling are the main causes of cracks , and the tensile stress and shear stress that exceed the metal fracture strength due to non-uniform deformation are the promotion. Cracks occur . Therefore , in the two-roll cross-rolling piercing of austenitic stainless steel thick-walled pipes , it is first necessary to control the quality of raw materials to avoid the generation of five phases ; at the same time , it is necessary to reasonably select the piercing process parameters , such as billet heating temperature , rolling angle , Feed angle and roll speed , minimize the uneven deformation of the piercing deformation zone and the additional tensile stress generated .
in conclusion

1. 304L stainless steel billet, about 3% ( volume fraction ) of the 5- ferrite phase is produced in the billet , and the uneven deformation during piercing is the main reason for cracking .
2. During the hot piercing process of 304L stainless steel two-roll cross-rolling, the deformation strength is unevenly distributed along the cross-sectional direction , and the deformation ability of the 5 phase and the Y phase is inconsistent , and the tensile stress and shear stress higher than the metal fracture strength are generated at the interface of the two phases , thereby promoting Microcracks form and propagate .

---