200 ( CrMrM ) series austenitic hot-rolled sheet Influencing factors and control measures

Through the analysis of the edge crack defects of the intermediate billet and steel coil, it is found that the main reason for the edge crack of the austenitic stainless steel hot-rolled plate is that the high heating temperature leads to a high ferrite content and a large amount of ferrite in the material. microscopic air bubbles. Aiming at the above problems, the process optimization measures have been formulated, and obvious results have been achieved.
Key words: CMnN series steel; hot-rolled sheet; rolling

1 Several manifestations of edge cracks in stainless steel hot-rolled plates
a factory adopts a steel coil mill to produce 200 ( CMnN ) series austenitic hot-rolled plates, the edge cracks are serious for a period of time . As shown in Figure 1( a ) , when the intermediate billet is in the second and third passes of rough rolling, cracks appear in the strips on both sides . Judging from the edge crack shape of the sample, the edge cracks are triangular in shape along the rolling direction . Pulled length up to 55 mm , The edge crack extends inward from the edge to a width of 20 mm ; it can be seen from Fig. 1( b ) , The edges of both sides of the steel coil sample are jagged . From the end face of the whole volume, it looks like a broken city wall , and the two sides have fallen off seriously . Viewed from the exposed outer ring, the widest point is 80 mm , and the longest point is 300 mm .
There are edge cracks on both sides of the sample roll , mainly concentrated in the outer ring of the roll , and the phenomenon of partial shedding and falling off occurs .

Cause Analysis of Edge Cracks in Austenitic Stainless Steel Hot Rolled Sheet
Influence of High Heating Temperature and Poor Coordination of Rolling Deformation on Plasticity of Hot Rolled Sheet
Spectral analysis was carried out on the samples , and the results are shown in Table 1 .
No inclusions were found at the main crack of the intermediate billet sample, the extension line of the secondary crack was severely oxidized, the crack cracked along the strip a phase, and the end of the crack was connected to the a phase ( the other end had been cracked to the end of a phase ) , the content of a phase The average is 47% .

No inclusions were found at the main crack of the 1 # steel coil sample , the extension line of the secondary crack was severely oxidized , and the end of the crack cracked intermittently along the strip-shaped a- phase , and cracks were formed when they connected together . The average a -phase ( area ) content was 15 ． 93% .
1 # steel coil sample after heat treatment at room temperature and at different temperatures is shown in Figure 5 , and the content of phase a is shown in Table 2 Phase a at room temperature

distributed in a large number of strips; when the heat treatment temperature is 1 200C , the a phase in the sample structure is distributed in an elliptical and circular shape and the number is small; when the heat treatment temperature is 1260C and 1300C , The sample was corroded by copper sulfate, and the a phase was gray and irregular in shape, and the number was large .
Judging from the heat treatment , the sample has a tendency to increase the a- phase content as the temperature increases ( the a -phase content at room temperature is only for comparison ). The measured a -phase content of the steel coil sample at room temperature is higher than that of the 1260C heat -treated sample, and the actual heating temperature of the raw material billet of the steel coil sample should be higher than 1260C .
Table 1 Measured value of chemical composition of hot-rolled sheet ( mass fraction, % )
Table 1 M - easured value of hot rollrig plate chan ical can position fractiorb % )

	C.	Si	Mn	P.		S.		Cr	Ni	Cu	N.
midterm exam	0 . 09	0 . 53	8 . 65	0 .	034	0 .	006	15 . 04	1 . 14	1 . 08	0 .	112
1 steel coil sample	0 . 04	0 . 45	9 . 67	0 .	035	0 .	003	15 . 16	1 . 10	1 . 42	0 .	106

Table 2 1 Phase a content in wet roll samples (% )
Table 2 a phase content of 1# strp coil sample(%)

	1	2	3	4	5	6	7	8	9	10	average value
room temperature	15 3	16 1	11 8	15 4	17 6	17 1	15 9	15 4	18	16 7	15 93
1100C	0 2	0 2	0 1	0 1	0 3	0 2	0 6	0 3	0 3	0 4	0 27
1200C	1 7	1 3	1 4	1 2	1 2	0 6	0 7	0 8	0 9	0 9	1 07
1260C	14 1	15 4	8 7	9 7	12 5	13 1	16 2	19 6	16 2	10 7	13 62
1300C	35 4	27 2	33 5	39 9	24 9	26 4	26 1	27 5	31 7	23 7	29 63

Room temperature 1100 1200 1260 1300
Analysis of the cause of side cracks : the cracks in the middle billet and 1 # steel coil sample cracked along phase a , the average content of a phase in the middle billet sample was 47% , and the average content of phase a in 1 pasted coil sample was 593% . Ferrite is related to heating temperature , The ferrite content gradually increases with the heating temperature (1 100~1 300C ). The cracks at the edge of the intermediate billet sample are triangular in shape along the rolling direction . The edge of the intermediate billet sample is cracked , falling off and falling off seriously , that is to say, the rolling force on the material exceeds the allowable fracture strength of the material . This is because austenite has a face-centered cubic structure , high density , Ferrite has a body-centered cubic structure with a low density . So , When austenite transforms to ferrite , volume expands , produce expansion stress , When the stress exceeds the strength limit of steel , microcracks are generated at the two-phase interface . At the same time , due to the large difference in strength and plasticity between ferrite and austenite , Therefore , During rolling deformation, the original micro-cracks rapidly expand into cracks visible to the naked eye . This kind of crack mainly appears on the edge of the board . Cracks are formed . Combined with the results obtained from the high-temperature plastic thermal simulation test of J4 continuous casting slab, it can be known that the plasticity of the material decreases linearly at 1 000 ~ 1 050C , and 1 050C is the lowest point . The ferrite in the material is brittle and extremely poor in plasticity at 1050C . When rolling at this temperature, the deformation of the pass exceeds the allowable deformation of the material and it will crack . From this we can see that , Higher heating temperature leads to higher ferrite content , And the unreasonable rolling temperature is the main reason for the formation of edge crack defects .

The gas of the sample was analyzed , and the results are shown in Table 3 . The content of oxygen and nitrogen in the sample is high , which shows that the purity of the molten steel is not high , and the control of the smelting process is poor .
According to electron microscope energy spectrum analysis , the round and long strip particles in the 2 # steel coil edge crack sample are a large number of air bubbles ; the normal parts of micro-area analysis mainly include: Fe , CrMn , Cu , Ni S and a small amount of Si . No inclusions were found in the pores , and the microanalysis results were almost the same as those of the matrix . The micro-area analysis results are shown in Table 4 .
J4 fracture surface analyzed by electron microscope Auger spectrometer , and it is the result of Auger spectrometry detection . Table 5 is the atomic concentration at the grain boundary . According to the detection results of Auger spectroscopy , the composition at the grain boundary is mainly CNO Cr Fe Mn , Moreover, the content of CN and O far exceeds the average content of the matrix . The O content is more than 3000 times that of the matrix .
According to the test analysis , the hot rolling process of the material is basically reasonable and coincides with the plastic change of the material . However, there are many bubbles in the sample , especially the dense arrangement of bubbles at the edge cracks , and individual bubbles connect to form cracks . The content of 0 and N gas in the sample is high, resulting in the existence of a large number of microscopic bubbles in rows , forming cracks during the rolling process . According to the analysis of the fracture Auger spectrum detection results of the J4 sample, the total content of C , N and O at the grain boundary accounts for 41%, and the enrichment of gas elements at the grain boundary seriously affects the plasticity of the grain boundary , resulting in the material Cracks form during rolling .

surface	3 2 N , O gas analysis results of paste coil samples
Table 3 The analysis resu It of r itroger
	and oxvgen n 2fi str b coil san d le
sample	N	0
1	1690X10 —6	141 . 4X10—6 _
2	1708X10—6 _	166 ． 6X10—6 _
3	1708X10—6 _	147 . 8X10—6 _

Precaution
Reasonable Coordination of Heating Temperature and Rolling Pass Deformation
On the premise of a reasonable Cr/Ni equivalent ratio, setting a reasonable heating and rolling temperature can effectively reduce the a -phase content in the material . The maximum heating temperature should be controlled at 1 180 ~ 1 24C o 3 . 2 Strengthen the control of smelting process and reduce the gas content in steel
In general, the N content should be controlled within 1 500X10 -6 and the O content should be controlled within 35X10T .

Table 4 energy spectrum micro-area analysis results ( mass fraction , % )
TabQ4 The analysis result of pectriun n icro area (mas fracton %)

spectrogram	in state	Si	V	Cr	mn	Fe	Ni	Cu	Total
Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4 Spectrum 5 Spectrum 6	yes yes yes yes	0 . 68 0 . 30 0 . 54	0 . 39	17 . 77 15 . 20 16 . 33 16 . twenty three 15 . 39 16．50	10．59 9．00 9．14 8．52 9．14 9．16	71．65 72．51 72．26 73．09 73．11 72．45	0．89 1．01 0．74 0．87 0．82	1．71 0．97 1．03 0．96 1．08	100．00 100．00 100．00 100．00 100．00 100．00

Tabte5 Alon ic concentratan at J4 gran boundary(% )

File	FileName	C1	N1	01	C2	Mn1	Fe3
1	1．SPE	7．72	8．50	20．95	11．65	5．08	46．10
2	2．SPE	6．63	6．20	23．84	9．96	4．01	49．37
3	3．SPE	6．85	5．42	26．94	9．94	2．22	48．62
4	4．SPE	6．91	5．82	28．58	9．39	0．97	48．33
5	5．SPE	7．32	5．29	25．13	9．47	4．32	48．47
6	6．SPE	7．98	7．25	27．74	9．31	0．57	47．15
7	7．SPE	6．50	7．20	31．33	8．71	3．13	43．13
8	8．SPE	8．76	5．33	26．67	9．44	3．31	46．49
9	9．SPE	13．97	6．42	28．44	7．36	2．63	41．19
10	10．SPE	8．07	6．38	26．62	9．47	2．92	46．54

In addition , the oxygen content is closely related to the carbon content . Carbon content 0 . 08 % is the mutation point , lower than 0 . 08 % molten steel is prone to peroxidation , and measures must be improved to reduce the oxygen content .

apply effects
Through the systematic anatomical analysis and detection of the 200 series hot-rolled plates , the basic process parameters of this series of steels were understood ; the appropriate heating temperature and time were found out through the heat treatment tests of the initially produced continuous casting slabs at different temperatures , and according to different temperatures By changing ferrite content and impact toughness , the heating and rolling process was optimized ; the basic composition ratio was determined, and the N content was controlled within 1 500 X 10 — 6 , and the O content was within 35X 10 — 6 ; combined with the high temperature The plastic thermal simulation test determined the rolling temperature and the deformation of each pass . Through a series of measures to solve the edge cracking problem , The qualification rate of the plate is obviously improved .

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