Causes and Improvement Measures of Tensile Fracture Delamination of A131Gr.AH36 Ship Plate Steel

With the help of metallographic microscope, scanning electron microscope and energy dispersive spectrometer , the tensile fracture of Al31GrAH36 high-strength ship plate steel was detected and analyzed. Through the discussion of internal mechanism and external incentives, it was pointed out that the tensile fracture delamination, inclusions , center segregation, etc. The factors are closely related , and the corresponding improvement measures are put forward , the fracture delamination rate of the tensile specimen is significantly reduced , and obvious results have been achieved .
Keywords : fracture delamination ; inclusions ; central segregation

With the substantial growth of shipbuilding and the continuous increase of ship types , the development trend of large-scale , light-weight and specialized ships is prominent . The demand for low-alloy high-strength ship plates is increasing day by day , and the requirements for physical quality are more stringent . With the variety expansion and grade improvement of ship plates , the phenomenon of fracture delamination of samples occasionally occurs . For this reason, a series of analysis and research work has been carried out , and effective technological measures have been taken . The defect rate of fracture delamination of samples is significantly reduced , with good results . This article will systematically describe and analyze the fracture delamination of A131GrAH36 high-strength ship plate tensile specimens on the basis of previous work .
1 Fracture layered morphology
Fracture layers have different shapes , different openings , and different penetrations . Neither ship regulations nor national standards have given a clear definition . It can only be grasped by the experience of inspectors , which is not conducive to the improvement of ship plate quality . The typical fracture layered morphology is shown in Fig. 1 .
Abnormal fracture tissue appears in the center of the fracture thickness direction , showing bright white color , while the normal fracture position is gray . There are multiple groups of nearly parallel discontinuous cracks in the middle of some fractures , making the center of the sample thickness a multilayer structure [ Fig . 1 (a)]. There are two layers [ Fig. 1(b) ], and the cracks of some fracture samples extend to the side [ Fig . 1 (c) ]. The fracture of the normal tensile sample is shown in Figure 1 (d). There is neither crack on the parallel rolling surface nor a bright white fracture area on the fracture of the sample . The macroscopic morphology of the entire fracture area has good uniformity .
2 Analysis of test results
2-1 Metallographic analysis
The fracture part of the tensile sample is taken as sample 1, the clamping section is sample 2, ground and polished , eroded by 4% nitric acid ethanol solution for 15 s, observed on a GX71 inverted universal material microscope, the microstructure is mainly ferrite body + pearlite . Metallographic structure of the surface and center of sample 1 in the thickness direction . The grain size of the surface structure is fine and uniform, and the grain size is 12 grades ; the central structure is coarser than the surface structure, the grain size is 11 grades, the banded structure is obvious, and the amount of pearlite increases significantly , indicating that there is carbon segregation, which leads to the carbon content in the central area increase .
Observing the central structure of the sample at a higher magnification ( 500 times ) , it is found that there are microcracks in both the fracture site and the clamping section, and the elongated light gray MnS inclusions can be observed at the edge of the crack .

on the core of the sample 2. The average microhardness of ferrite was 217.8 HV0.1 , the average microhardness of pearlite was 276.1 HV0.1, and the average microhardness of the crack area and the abnormal structure nearby 467. 6 HV0- 1 is far beyond the proper microhardness of pearlite ( Fig. 4, Table 1 ) . Judging from the microhardness value , the abnormal structure can be confirmed as a mixed structure of bainite and martensite.

Table 1 Microhardness HV at load of 100g

organize Hardness HV	ferrite	Pearlite	Pearlite-bainite junction zone	bainite martensite zone
point 1	204.0	276.4	292.0	401.2
point 2	216.0	284.1	316.6	461.2
point 3	230.7	285.2	338.2	477.8
point 4	220.5	258.6	326.0	530.3
average	217.8	276.1	318.2	467.6

The results of metallographic analysis show that the fracture delamination of the tensile sample is caused by the strip-shaped MnS inclusions and the mixed structure of bainite and martensite in the central zone . Due to the high manganese content of high-strength ship plate steel ( above 1.2 % ), the solid-liquid two-phase region is widened , and the time for molten steel to remain liquid is prolonged , and MnS is precipitated in the two-phase region in the later stage of solidification, causing slab segregation and loosening exacerbated . In addition , the shrinkage coefficient of MnS is larger than that of the matrix , so when the cooling rate is fast or the cooling is uneven , the shrinkage of MnS inclusions is larger than that of the matrix , which makes it easy to generate cracks and voids around the inclusions . The segregation of central manganese causes the CCT curve to move to the right in the central area of the steel plate thickness . During the austenite cooling process in the rolling process , the central part is easy to form bainite and martensite structures , and their hardness and volume changes are different from those of During the cooling process , the cooling rate of the surface and the center of the steel plate is different , and thermal stress is easily generated inside the steel plate . The effect of two internal stresses, tissue stress and thermal stress , makes it easy to produce microcracks under the condition of inclusions in the center .

 

2-2 Based on in-situ analysis , resolution and extraction technology, spark micro-beam ( probe ) technology and continuous excitation without pre-combustion synchronous scanning positioning technology are used . Based on this, millions of original content and state information of each element corresponding to the original position of the material are obtained , and the segregation degree of the material is quantitatively characterized by the method of statistical analysis . This technology can obtain accurate information on the position distribution , state distribution and quantitative distribution of each component in a large scale range of metal materials .

Internal Mechanism and External Incentive Discussion
internal mechanism
The central segregation of the continuous casting slab is the result of flow heat transfer and solute redistribution of molten steel during solidification . During the continuous casting process , the columnar grains of the continuous casting slab develop uniformly towards the center , so that the molten steel that finally solidifies in the center of the continuous casting slab is rich in solute elements such as carbon , manganese , phosphorus , sulfur, etc. [2] . The center segregation phenomenon will seriously affect the structure and performance of the center of the rolled plate .
When the continuous casting slab is heated , the expansion of alloying elements and other impurity elements

The dispersion coefficient is low , and homogenization is difficult , so that segregation still exists . The inhomogeneity of alloying elements , on the one hand, leads to the dense distribution of sulfide inclusions; on the other hand, it makes the Ar3 temperature difference between different regions , because the alloying elements such as manganese increase the stability of austenite , resulting in austenite A medium-temperature transformation occurs directly , and a bainite structure appears in the heart . Since the segregation of sulfur and other alloying elements is not easy to eliminate during the heating process of the continuous casting slab , the sulfides are distributed in strips along the rolling direction during rolling , and become the core of ferrite nucleation during cooling , which promotes the formation of core bands. Formation of shaped tissue . The existence of banded structure , sulfide inclusions and loose holes makes the central position of the plate thickness along the rolling direction the weakest place of the steel plate . The strip-shaped sulfide inclusions act as crack sources when the steel plate is subjected to tensile force .
During the tensile test , the steel plate undergoes plastic deformation along the tensile direction . Since the banded bainite in the center of the steel plate and the surrounding ferrite structure have a large difference in plastic deformation resistance , the poorly plastic bainite structure will hinder the deformation of the ferrite zone , and it is easy to deform in the banded bainite zone. stress concentration occurs . The cracking state observed at the fracture is formed due to the less plastic deformation of the banded bainite in the center of the thickness of the steel plate and the larger plastic deformation of the ferrite on both sides of them . The cracks and pores in and around the banded bainite have expanded and united during fracture , thus forming a macroscopically visible fracture cracking and delamination phenomenon . 3-2 External Incentives
The deformation process of the entire tensile test needs to go through several stages of elastic deformation , yield deformation , uniform plastic deformation and local concentrated plastic deformation caused by necking . When there is no necking in the tensile sample , under the action of the tensile load , the sample undergoes uniform elastic-plastic deformation , and each part of the sample is subjected to uniform axial stress . As the tensile load continues to increase , The sample will be necked down . In Figure 8 , Stress concentrations occur at the smallest cross-section adjacent to the narrow diameter . The force at the constriction can be calculated by the following formula :
1 ,1+a — r 2 , , ],
a — Q — Width • yln ( 2r ) ( 1 )
fish = 6 + a — a [1 + *ln( — r _ ) ] (2)
In the formula, a is the radius of the minimum section of a certain instantaneous necking; R is the radius of curvature of the outer surface contour of a certain instantaneous necking ; r is the distance from the center of the sample; a is the axial stress; a is the radial stress ; a is the tangential stress .
Visible , After the necking of the tensile specimen occurs , The stress distribution at the constriction has changed significantly . In Fig. 8 , the distance r from the sample center not only bears unidirectional tensile stress' but is in a state of triaxial tensile stress under the joint action of axial stress a , radial stress a and tangential stress a , Cracks are easier to initiate , aggregate and grow . In the central region of the constriction , since the distance r from the center of the sample is almost zero, it can be seen from the formulas (1) and 2) that q , a , and q reach the maximum in this region, which is more conducive to the continuous initiation and aggregation of cracks And grow up , leading to the appearance of a macroscopic crack in the center of the specimen .

4 Epilogue
Scanning electron microscopy and energy spectrometer analysis of the fracture found that there were manganese sulfide inclusions gathered in the 20 Hm area of the defect on the inner wall of the tube, indicating that during the solidification process of the billet, the sulfide with a low melting point was enriched in the loose area of the final solidified center , and the metallographic The inspection results found that there is a decarburized structure around the iron oxide in the crack of the defect site , indicating that the central loose part with inclusions inside the billet is exposed to the inner wall of the tube during the process of passing through the tube . And contact with air to form oxidative decarburization , so it cannot be welded during the rolling process to form inner wall folding defects .

The internal quality of the continuous casting slab, such as inclusions and central segregation , is the internal factor causing the tensile fracture delamination of high-strength ship plate ; when the tension is loaded , the stress state in the central area of the sample is the external cause of the tensile fracture delamination . Production practice has proved that increasing the purity of molten steel and improving the internal quality of the slab can effectively reduce the delamination rate .

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