Study on Fault Delamination of Low Alloy Steel Plate

The corresponding relationship between delamination along the crystal form and phosphorus segregation bands in the fracture of two low-alloy steel sheets (coils) was investigated. As a result, Nong Ming, the embrittlement of segregation zone is the source of fracture along the crystal structure, which has the characteristics of reversible tempering pickling. When the plate (coil) passes through the temper brittleness sensitive temperature zone at a small cooling rate, the impurities and alloying elements with higher concentration on the segregation zone regenerate at the grain boundary (original austenite grain boundary or ferrite grain boundary). Segregation, causing intergranular brittleness. In addition, the heterogeneous segregation of sulfides on the segregation zone also aggravates the susceptibility.

With the development of modern surface analysis technology, the theory that impurity and alloy elements segregation in steel cause high temperature reversible temper brittleness is increasingly accepted by people. In recent years, the band segregation of phosphorus in rolled steel and its brittleness It has attracted the attention of many researchers. It has been noticed that the banded segregation of phosphorus often causes intergranular brittle fracture along the segregation zone in the fracture. However, the understanding of this phenomenon is not consistent. Fracture has nothing to do with temper brittleness, it is a local brittle phenomenon, while some researchers believe that this is a temper brittle phenomenon that occurs in the segregation zone 3". Although the steels they studied are different in chemical composition , but the discussion on the brittleness mechanism of phosphorus segregation bands in steel is common. Since band segregation of phosphorus and other elements often exists in general hot-rolled low-alloy steels, it is important to conduct in-depth research on the brittle nature of phosphorus segregation bands. significance.
During production inspection, delamination often occurs on the fracture surface. Delamination often occurs in the middle of the fracture, the cracks are parallel to the rolling direction, and the separation surface is parallel to the rolling surface (Fig. 1 ). When studying the causes of delamination, it is often found that Fracture delamination is related to the segregation zone, and the delamination plane is just on the segregation zone. Obviously, the occurrence of this type of delamination is a reflection of the brittleness of the segregation zone in steel. In order to understand the brittle nature of the segregation zone in hot-rolled steel and its impact on fracture Influenced by morphology, we choose two kinds of hot-rolled low-alloy steel plate (coil) fractures (chemical composition as shown in Table 1), and use metallographic techniques to reveal the relationship between fracture delamination cracks and phosphorus segregation bands, and use scanning electron microscopy to observe the delamination The fracture morphology of the segregation zone was investigated by using different heat treatment systems, and the micro-area composition of the segregation zone and the intergranular fracture surface was analyzed by electron probe and ion probe. According to the above tests, the brittleness of phosphorus segregation zone in steel Have a brief discussion.
-Chun-Tempered steel in real value analysis zone and fracture analysis
1 Delamination fracture morphology and segregation zone
Take the 80X100X300 mm fracture* sample from the steel plate of steel I and carry out the quenching and tempering treatment of 920t water quenching + 6101C tempering. The microstructure is tempered sorbite . The notch should be broken at room temperature without dropping the hammer. The layered part of the fracture was manually sawed and observed under the scanning electron microscope. The observation of many samples showed that the fracture morphology of the main fracture was dimples and along the sulfide The "trench" formed by the separation. The intergranular fracture and brittle separation of sulfide inclusions are mixed on the wall of the delamination crack (Fig. 2), and the intergranular fracture zone is distributed in a band along the rolling direction.
with Obehoffer reagent on the metallographic grinding surface perpendicular to the fracture surface, it was found that the delamination cracks corresponded to the phosphorus segregation zone, and the delamination plane was on the phosphorus segregation layer. It can be seen that the edge of the

 

Table 1 The chemical composition of the test steel (wt%)

steel number	c	Si	mn	S	P	Cr	Ni	Mo	Cu	Plate thickness (mm)
I	0.280	0.450	1.20	0.020	0.025	0.90	—	0.70	one	80
I	0.088	0.358	0.353	0.026	0.088	0.41	0.361	-	0.316	12 ( steel coil)

Grain breakage is related to the banded segregation of phosphorus. When the reagent composed of saturated picric acid aqueous solution + hydrochloric acid + seagull detergent is used to erode the grinding surface to reveal the original austenite grain boundary, it can be seen that the austenite grain boundary on the segregation band is obviously larger than that of Coarse on the non-segregation zone (Figure 4). Since the reagent is sensitive to the local concentration gradient of impurity phosphorus, it indicates that the austenite grain boundaries on the segregation zone are enriched by phosphorus.
2 Composition analysis of segregation zone and intergranular fracture surface
Determination of micro-domain composition of segregated and non-segregated bands by electron probe

(take the average value of ten measured points for each band), and found that Cr , Mn , Si , Mo , S , P and C element segregation existed on the segregation band , and the segregation ratio was between 1.10 and 1.29 . For the analysis of impurity elements with low content in steel, P , S and As in the non-segregated zone cannot be detected under the same sensitivity , while the relative intensities of the secondary ion currents of P , S and As in the segregated zone are 1.6.6 .8 and 6.1, the segregation coefficient is relatively large.
C , P , S, Cr , Mn , Mo and As elements were determined . In order to judge the segregation degree of elements on the intergranular fracture surface, the mass spectrum peak height ratio of the above elements in the matrix was also measured, and the analysis results are listed in Table 2. It can be seen that Mn , Cr , S , P , Mo , C , As contents are all higher than the matrix, and there is obvious grain boundary segregation. When the ion sputtering method was used to analyze the grain boundary depth distribution of P element, it was found that the P concentration decreased rapidly with the distance from the grain boundary, indicating that the enrichment range of phosphorus at the grain boundary was very thin.
The results of micro-composition analysis show that the intergranular fracture on the delamination plane is caused by the enrichment of impurities and alloying elements at the grain boundary, and the banded segregation creates favorable conditions for the grain boundary segregation.
3 Effect of heat treatment system on fracture morphology
Although it is impossible to conduct a reproducible test on the fracture delamination, since the fracture delamination corresponds to the thickness section of the steel plate, samples are taken from the delamination fracture and processed into thick impact fracture samples, so that the notch is at the thickness of the steel plate. Middle part. Scanning electron microscope observation of thick impact fracture shows that in addition to some dimples and inclusion "grooves" in the fracture morphology, there are still intergranular fracture areas distributed in bands along the rolling direction. Metallographic corrosion of vertical fractures It still shows the corresponding relationship between the intergranular fracture zone and the phosphorus segregation zone. It can be seen that the intergranular fracture zone in the thick fracture is the same phenomenon as the intergranular fracture zone on the above-mentioned fracture delamination surface.
various heat treatments as listed in Table 3 were carried out with the thickness impact specimen . Observation of the microscopic morphology of the fracture after heat treatment shows that the non-embrittling treated fracture is a mixture of tough dimples and separation along the sulfide, basically no intergranular fracture. After embrittlement treatment, the brittle separation zone along the sulfide In addition, the proportion of intergranular fractures is greater than 60%, and the intergranular fractures are mostly distributed in bands. Restoration treatment reduces the proportion of intergranular fractures of embrittled samples to less than 10 foils. At this time, most of the original intergranular fracture zones have disappeared. Instead of quasi-cleavage and dimple morphology, occasionally a few intergranular fracture "islands" are always adjacent to the characteristics of plastic rheology, indicating that the grain boundary decoupling has basically disappeared (Fig. 6 ) . After the secondary embrittlement treatment, the proportion of intergranular fracture increased to more than §0% .

Table 2 Ion probe analysis results of intergranular composition

Analysis site	C+/Fe+	P+/Fe+	S+/Fe+	Mn + / Fe +	Cr + /Fe +	Mo+/ Fe +	As+/Fe+
Along crystal plane	4.2	4.7	10.0	1800.0	520.0	14.7	13.3
Substrate	3.0	3.0	4.2	100.0	177.0	6.2	7.2
Segregation pecks	1.43	1.53	1.53	18.0	2 , 94	2.37	1.85

Table 3 Effect of heat treatment on fracture morphology

processing method	Heat treatment system	Intergranular Fracture (Percent Quantity *)
non-brittle	920t X 4 hours water quenching + 610tX 7 hours water cooling	<2%
embrittlement _	920t X 4 hours water quenching + 610t X 7 hours furnace cooling	>50%
recover	Embrittlement + 6801CX 2 hours water cooling	<10%
secondary embrittlement	Recovery + 610^X7 hours furnace cooling	>50%

* There is a large proportion of brittle separation zones along the sulfide inclusions on the fracture.

1 Fracture delamination and exact segregation zone
I with higher phosphorus content is a 12 mm thick hot-rolled strip steel, the microstructure is ferrite + pearlite (grain 9 ~ 10 grade), and the normal temperature impact fracture often appears delamination. The gold layer perpendicular to the fracture surface After the phase grinding surface was corroded with Obehoffer reagent, it was found that the stratification was mostly on the phosphorus segregation zone (Fig. 6). The hardness of ferrite on the segregation zone and non-segregation zone was measured by a microhardness tester, and the average value on the segregation zone was 166kg/ mm\ The average value on the non-segregation zone is HOkg/mm .
P , Cr , Mn , Ni , and element segregation existed on the segregation zone ; the phosphorus content was sometimes as high as 0.18~0.30%,
2 Fracture Morphology
A large number of steel coil fracture delaminations have been observed, and the fracture morphology varies with the sampling location. The fracture delamination fracture morphology of the steel doctor head sampled is an alternating arrangement of intergranular fracture and brittle separation of sulfide inclusions along the strip. The grain fracture zone is distributed in bands along the rolling direction (Fig. 7 ).
The layering of the fracture in the middle of the steel coil is a mixed morphology of intergranular fracture, cleavage fracture and sulfide inclusion separation. The river pattern on the cleavage surface is generally less and shallow, and the cleavage surface is generally larger than the intergranular fracture surface; This may be due to the smaller dislocation difference between adjacent grains, and the smaller turning point when the river crosses the grain boundary. In addition, some corrugated slips are often observed on some cleavage sections and intergranular fracture surfaces Streaks (Figure 8). This may be the slip trace that the main fracture slides out on the delamination cleavage plane or along the crystal plane due to the deformation of the sample during the fracture process after the delamination crack is generated.
The fracture morphology of the fracture layer sampled at the tail of the steel coil is mainly cleavage + a small amount of intergranular fracture. The fracture morphology along the brittle separation of sulfides is significantly less than that of the head and middle samples.
The ion probe analysis of the above intergranular fracture region shows that there are obvious segregation of P , S , As and Cr on the intergranular fracture surface.
Statistics on the fracture delamination of steel I found that the proportion and severity of delamination increased in the order of the tail, middle and head of the steel coil. This is consistent with the fluctuation of phosphorus content in each part of the steel coil (the average phosphorus content of the head is 0.104 % , the average tail is 0.081%) is related to the cooling rate of each part of the steel coil after curling. Because the brittle tendency is sensitive to phosphorus content and cooling rate.
Three Pairs of Definitive Discussions
The above observations on the fracture delamination of quenched-tempered steel and ferritic-pearlite steel reveal the corresponding relationship between delamination and phosphorus segregation bands in the cocoon, and the embrittlement of segregation bands is the root cause of this type of delamination .In addition, since the segregation-band is formed after the rolling of the interdendritic segregation when the steel ingot is solidified, the dendritic segregation zone is also the segregation zone of the I- type sulfide manganese, so the intergranular fracture on the delamination surface is often related to the sulfide along the This brittle separation along the sulfide inclusions also aggravates the brittleness of the segregation zone.

In this test, the intergranular fracture surface on the fracture layer of the two steels has the enrichment of impurities (mainly phosphorus) and alloy elements, and their embrittlement is related to the cooling rate after tempering or rolling. Among them, the quenching and tempering test Although the sample is water-cooled after tempering, the cooling rate of the core is relatively small due to the large cross-sectional size of the sample; the ferritic pearlitic steel is curled after rolling, and the cooling rate of the middle and head of the steel coil is relatively small. When the tempering brittleness sensitive temperature zone passes through the tempering brittleness sensitive temperature zone at a small cooling rate, the impurities and alloying elements with a higher concentration in the segregation zone re-segregate at the grain boundaries of the original austenite or ferrite, causing intergranular brittleness. Because the impurity elements segregate to the grain boundary In addition to being affected by temperature, the speed and number of grain boundaries are also related to the ratio of the composition and concentration of the system to the grain boundary area. The research work records that the reversible temper brittleness of phosphorus-containing steel is very sensitive to the phosphorus content. With the increase of phosphorus content, the degree of brittleness increases—the interaction between some alloying elements and impurities can promote the grain boundary segregation of impurities, Mn and Cr Co-segregation can occur with phosphorus at the grain boundary . Although Si does not segregate at the grain boundary, it promotes the segregation of phosphorus to the grain boundary by increasing the activity of phosphorus. For this reason, the higher concentration of impurities and alloy content on the segregation zone caused Favorable conditions for boundary segregation.
Since the temper brittleness tendency of ferrite-pearlite steel is smaller than that of tempered martensitic steel "•", a certain amount of cleavage fractures are often accompanied by a certain number of cleavage fractures on the fracture delamination surface in the middle of steel I coil except for intergranular fracture. This shows that The degree of grain boundary segregation on some delamination planes fluctuates to a certain extent among the grains, which makes the difference in microscopic fracture strength between some grains and grain boundaries small, and cracks propagate along the cleavage plane in some grains. At the tail of the steel coil, because the fast cooling rate inhibits the segregation of impurities to the grain boundary, the fracture morphology of the delamination is dominated by transgranular cleavage. This shows that the local phosphorus segregation does not necessarily lead to intergranular fracture. Ji Jingwen The research work of et al. also proves that the water-cooled fractures of high-phosphorus steel with a phosphorus content of 0.1% are cleavage fractures from room temperature to low temperature after heating to 7001C for heat preservation. Literature [11] pointed out that with the increase of phosphorus content in steel, the hardness gradually increases, the twine crystals are easy to start, and the proportion of cleavage in the fracture increases. Therefore, it can be imagined that whether the crack propagates in the transgranular mode or in the intergranular mode , will be dominated by the microscopic yield stress around the fracture pathway. When the brittle elements on the segregation zone are enriched at the grain boundaries, the fracture mode is intergranular, and if these brittle elements reside in the matrix, the brittle fracture occurs in the form of transgranular cleavage.
1. The intercrystalline delamination in the fracture of low-alloy steel plate (coil) corresponds to the phosphorus segregation zone in the steel. This delamination is the reflection of the brittleness of the segregation zone in the fracture. When there is a serious segregation zone under the main fracture surface, due to the three-dimensional stress state under the notch during the fracture process, the thickness stress causes the thickness separation of the brittle segregation zone, forming delamination cracks.
2 The intergranular fracture on the delamination plane is caused by the enrichment of impurities and alloying elements on the grain boundary in the segregation zone, which is related to the cooling rate through the embrittlement temperature zone, and is essentially the same as the reversible temper embrittlement.

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