560 MPa Grade Automobile Girder Steel

Aiming at the problem of unqualified batch elongation of 560 MPa level P560L automobile frame steel, this paper analyzes the reasons for the unqualified elongation by means of metallographic analysis, and points out the fluctuation of coiling temperature in the hot rolling process As a result, the coiling temperature of some steel coils or local positions of the steel coils is low, and the phase transformation of the structure enters the bainite transformation zone, and a certain thickness of bainite structure layer is formed on the surface of the strip, which results in the low elongation of P560L immediate cause . In production, after technical measures such as increasing the Mn content, increasing the thickness of the intermediate blank, and increasing the coiling temperature, the situation of unqualified elongation of P560L has been significantly improved, and the qualified rate has been greatly improved .
【 Key words 】 automobile frame steel coiling temperature microstructure mechanical properties

 With the increase of China's automobile production year by year, the market demand for hot-rolled steel strips (plates) for automobile beams is also increasing, which has huge market development potential . Automobile girder plates are mainly used to manufacture automotive longitudinal beams , cross beams , front and rear axles , bumpers and other structural parts . The girder is the main load-bearing part of the truck, which bears almost all the weight of the cargo . The quality of the beam affects the service life and driving safety of the vehicle . The stamping process is generally used in the manufacture of automobile girders, and the deformation method is mainly bending. Therefore, the girder plate has high requirements on formability, that is, the automobile girder steel plate must have good comprehensive properties, sufficient strength and toughness, and good fatigue resistance. , Cold formability . With the development of the automobile industry, high-strength steel sheets that reduce weight and improve safety, and new products with excellent strength and formability are the main body of the development needs and development directions of automobile sheets . In order to improve the carrying capacity of automobiles, prolong the service life of automobiles and meet the requirements of energy saving , material saving and safe driving, it has become the development trend of automobile frame steel to produce automobile frame with low alloy high strength and ultra high strength steel plate .
In order to meet market demand and expand production varieties, Panzhihua Iron and Steel has successfully developed high-strength automobile frame heat sinks with yield strengths of 510 MPa , 560 MPa , 590 MPa and 610 MPa through a micro-alloy process route and a reasonable controlled rolling and controlled cooling process. Rolled steel . Panzhihua Iron and Steel’s automobile frame steels are all ferrite + pearlitic low-alloy high-strength steels, whose main chemical composition is low carbon , slightly high manganese, and an appropriate amount of silicon elements, as well as microalloying elements such as niobium and vanadium added individually or in combination , using strengthening mechanisms such as solid solution strengthening , precipitation strengthening and grain refinement to obtain the required strength and ductility, and the mechanical properties and processing properties of the product meet the relevant standards and user requirements . But at present, the 560 MPa grade ( P560L ) automobile frame steel in industrialized production has a problem of unqualified batch elongation. This paper uses metallographic analysis and other means to analyze the reasons for the unqualified elongation and proposes improvements. measures .

1. Cause Analysis and Improvement Measures

2 . 1 Reason Analysis
Recently, Panzhihua Iron and Steel produced a total of 7,195 tons of P560L high-strength automobile girder plates , and the preliminary pass rate of mechanical properties was 80.18% . See Table 1 for statistics on the mechanical properties of P560L before improvement . It can be seen from Table 1 that the pass rate of yield strength of P560L is 100% , and the tensile strength and elongation are not up to standard . The distribution of tensile strength and elongation of P560L before improvement is shown in Figure 1. It can be seen from Figure 1 that the proportion of tensile strength lower than the lower limit of 560 MPa required by the standard is 0.9% , and the qualified rate is 99.1% , mainly distributed between 570 and 630 Within the range of MPa , the ratio is 82.88% . The ratio of elongation lower than the lower limit of the standard requirement is 18.02% , and its pass rate is 81.98% . Therefore, the low elongation rate is the main reason affecting the pass rate of P560L performance .
Table 1 Statistics of mechanical properties of P560L before improvement

category	standard requirement	minimum value	maximum value	average value
Rel / MPa	$ 450	455	600	487
Rm/MPa	560 ~ 680	555	675	586
A/%	$ 23	17	31	24 . 3

P560L metallographic structure with different elongation are shown in the table
2. See the typical metallographic structure. The structure of the sample with high elongation is ferrite + pearlite, and the grain size level is 10.0 ; Ferrite + pearlite structure, ferrite grains tend to be needle-like, and mixed crystals are serious; the other side is ferrite + bainite structure, with a thickness ranging from 2.5 to 4.8 mm , accounting for 1/4 of the entire steel plate above .
Table 2 Analysis results of metallographic structure of P560L with different elongation before improvement

sample	Elongation	organize	Grain Size Bainite Layer Thickness
serial number	/%		/ level	degree /mm
Sample one	28 . 5	ferrite + pearlite	10 . 0	/
Sample two	23 . 5	ferrite + pearlite	10 . 0	/
Sample three	21 . 5	One side ferrite + pearlite, the other side ferrite + bainite	/	About 2 . 5
Sample four	17 . 0	One side ferrite + pearlite, the other side ferrite + bainite	/	About 3 . 0

The mechanical properties of different positions of the steel coil are shown in table 3. From the results in table 3 , it can be seen that the mechanical properties of different parts of the steel coil are very different, the elongation of the tail is the lowest, and the elongation of the head is the highest. Very large, the elongation on the operating side is about 4% lower than on the drive side . The coiling temperature of 1 # and 2 # steel coils, the coiling temperature of the two steel coils fluctuates greatly, the coiling temperature of the tail is lower, and the coiling temperature of the head is higher .
Table 3 Mechanical properties of steel coils at different positions before improvement

Coil No.	Location	Rel / MPa	Rm/MPa	A/%
	head	505	595	26 . 0
1 #	middle	510	600	20 . 0
	tail	495	595	20 . 0
	head	490	595	25 . 0
2 #	middle	500	600	24 . 0
	tail	490	595	22 . 0

Figure 2 Typical metallographic structure of P560L with different elongation

From the above analysis results, it can be seen that among the three mechanical performance indicators of P560 , the elongation rate is the most different from the standard, which is the main indicator affecting the pass rate of P560L performance . The physical reason for the low elongation is that there is a certain thickness of bainite in the metallographic structure of the relevant samples. The specific reason is that the coiling temperature of P560L is on the edge of the bainite transformation temperature range. It is difficult to control the coiling temperature of the product, which causes some steel coils or local coils to have a lower coiling temperature, enter the bainite transformation zone, and undergo bainite transformation to form a bainite structure .
2 . 2 technical measures
In order to avoid the appearance of bainite structure in P560L , increase its elongation rate, and thus improve the performance pass rate of P560L , the following technical measures have been taken:

1. Properly increase the target coiling temperature of P560L to avoid abnormal structures such as bainite .
2. Mn is increased by 0.15% - 0.

20% , to make up for the loss of yield strength and tensile strength caused by the increase of coiling temperature, so as to ensure the strength of P560L .

1. The thickness of the intermediate billet is controlled by M 38 mm , and the cumulative strain in the rolling process is appropriately increased to increase the nucleation core of ferrite, thereby refining the ferrite grains .
2. Improvement effect

3.1 Mechanical properties
After improvement according to the above technical measures, the statistics and distribution of the mechanical properties of the improved P560L are shown in Table 4 . It can be seen from Table 4 that the range of yield strength is 465-540 MPa and the range of tensile strength is 590-640
MPa , the range of elongation is 23% ~27% , and the pass rate is 100% .
Table 4 Statistics of mechanical properties of P560L after improvement

category	standard requirement	minimum value	maximum value	average value
Rel / MPa	450	465	540	509
Rm/MPa	560 ~ 680	590	640	615
A/%	twenty three	twenty three	27	25.3

The impact value of the P560L series was tested on the improved steel coil sampling . The sample size is 7.5 x 10 x 55 mm . The test results are shown in Figure 4 . From the results in Figure 4 , it can be seen that the transverse and longitudinal impact values of the test steel are relatively high, the impact toughness of the head , middle and tail is relatively stable, and the ductile-brittle transition temperature is - Below 40T .
3.2 Metallographic structure
The metallographic inspection results of the improved P560L are shown in Table 5 , and the typical photo of the metallographic structure is shown in Figure 5 . As can be seen from Table 5 and Figure 5 , the P560L
Table 5 Inspection results of typical metallographic structure after improvement

Volume number	Elongation /%	sampling Location	Organization Percentage	grain size / level
5#	23.5	tail	93% ferrite + 7% pearlite	11.5
6#	24.0	tail	92% ferrite + 8% pearlite	11.0
	25.0	head	91% ferrite + 9% pearlite	11.5
7#	23.0	middle	97% ferrite + 3% pearlite + trace bainite	11.5
	26.0	tail	93% ferrite + 7% pearlite	11.5
	23.0	head	99% ferrite + 1% pearlite + a small amount of bainite	11.0
8#	23.0	middle	98% ferrite + 2% pearlite + a small amount of bainite	11.5
	23.5	tail	97% ferrite + 3% pearlite + trace bainite	11.5

The grain size grade of the test steel is 11.0-11.5 . The metallographic structure of the steel coil with higher elongation is ferrite + pearlite, and the steel coil with lower elongation has less pearlite content, and a small amount or a small amount of bainite structure is sporadically distributed .

1. in conclusion
2. From the metallographic structure analysis of the P560L sample with low elongation, it can be seen that in the structure of the sample with low elongation before improvement, in addition to the ferrite + pearlite structure, a certain thickness of bainite layer appears on one side of the steel plate . However, after the improvement, the edge and core structures of the steel plate are consistent, and there is no bainite layer at the edge, but a small or trace amount of bainite structure is scattered throughout the entire section, and the main structure is still ferrite + Pearlite, which shows that pearlite transformation occurs after increasing the coiling temperature, which inhibits the transformation of bainite .
3. After technical measures such as increasing the Mn content, increasing the thickness of the intermediate billet , and increasing the coiling temperature, the unqualified elongation of P560L has been significantly improved, and the pass rate of its mechanical properties has increased from 80.18% to 100% . more significant .

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