Stamping performance control of St37-2G cold-rolled sheet

According to the characteristics of bell annealing production process, the effects of chemical composition, cold rolling reduction rate and annealing process on the structure and properties of St37-2 G cold-rolled sheet were systematically studied, and the plastic strain of St3 7-2 G cold-rolled steel was discussed. Ratio control method, put forward the industrial technical scheme of producing St3 7-2 G cold-rolled sheet with weighted average value of high plastic strain ratio ( r ) . The industrially produced St37-2G cold-rolled steel plate has excellent stamping and forming properties.

Key words: cold-rolled sheet; St37-2G carbon structural steel; stamping performance; plastic strain ratio; strain hardening index

S372G is a cold-rolled carbon structural steel plate with a tensile strength of 360 MPa in the DN1623 (2) standard , and is generally used for simple forming structural parts of automobiles . Because of its low cost, it is widely used in the domestic miniature car manufacturing industry , mainly used for structural parts such as frame , pillars , and rear beams . In addition to being used for simple deformation parts , it is also used for some parts formed by deep drawing , so higher requirements are put forward for the formability .
Generally speaking , in carbon steel , with the increase of carbon and manganese content , the strain hardening exponent ( n value ) and plastic strain ratio (! value ) decrease significantly . Therefore , in order to improve its formability , usually from increasing Starting with the elongation after fracture, the commonly used technical measures are: ① reduce the content of residual elements such as phosphorus and sulfur, and improve the purity of steel; ② reduce the content of carbon and manganese . Adopting the first option will increase the production cost of the steelmaking process ; when adopting the second option , in order to ensure the tensile strength , continuous annealing is required for production , resulting in relatively high yield strength , which is not conducive to forming .
According to the characteristics of bell annealing production process , the author Taking carbon structural steel with typical components in industrial production as the test material , Laboratory cold rolling and annealing tests were carried out to study the effects of chemical composition , cold rolling reduction and annealing process on its microstructure and properties . Accordingly , A technical scheme for producing high-value S37-2G cold-rolled steel plates was proposed . The industrially produced St37-2G cold-rolled steel plates have excellent formability .
1 Test materials and methods

1. 1 test material

The test material is industrially produced carbon structural steel plate, and the main chemical composition is shown in Table 1 . See Table 2 for hot rolling process conditions . Among them, the test steel
1 2 and 4 are hot-rolled samples; while test steel 3 is a cold-rolled sample, and the cold-rolled reduction rate is 58 . 6 % . Test steels 1 and 2 were hot-rolled at a higher coiling temperature, around 680 °C ; while test steels 3 and 4 were hot-rolled at a lower coiling temperature, around 620 ° C .

Table 1 Main chemical composition of test steel
Table 1 Main chan ical compositions of the experimental steels %

serial number		C	st	mn		P	the s	A Is	Remark
1	0	13	0 . 02	0 . 34	0 .	022	0 . 013	0 . 06	Used to study the influence of chemical composition on the properties of cold-rolled sheet
2	0	19	0 . 04	0 . 90	0 .	012	0 . 005	0 . 03	Used to study the influence of chemical composition on the properties of cold-rolled sheet
3	0	16	0 . 02	0 . 60	0 .	018	0 . 013	0 . 06	Used to study the effect of annealing temperature on the properties of cold rolled sheet
4	0 .	15	0 . 05	0 . 78	0 .	017	0 . 008	0 . 05	Used to study the effect of cold rolling reduction rate and annealing heating rate on the properties of cold rolled sheet

Table 2 Test steel hot rolling process parameters
Table 2 Ho — oiled process s parameters of he

	experimental steels	°C
serial number	Finishing temperature	coiling temperature
1	860±20	680±20
2	860±20	680±20
3	860±20	620±20
4	860±20	620±20

1 2 Test method
The cold rolling test was carried out on a 0300 mm single-stand four-high rolling mill in the laboratory . The cold rolling reductions of test steels 1 and 2 were 65 %, while the cold rolling reductions of test steel 4 were 40 % , 50 % , 60 % and 70 % , respectively .
test steels 1 and 2 in a laboratory medium-temperature box-type resistance furnace
The samples were protected with anti-oxidation coatings . During annealing, the sample is heated with the furnace , and after reaching the annealing temperature, it is kept for a certain period of time and cooled with the furnace . The annealing temperature is set to 600 , 650 , 690 , 720C , The holding time is 3 hours
test steels 3 and 4 was carried out in a vacuum protective atmosphere furnace. The annealing process of test steel 3 was as follows: the sample was heated to 400 C at a heating rate of 360 C/h, and then heated to 630 C at a heating rate of 40 C/h. , 650 , 680 , 710°C, the holding time is 0 h and 5 h respectively , and finally cooled to less than 150 ° C with the furnace ; Heating to 670C at a heating rate of 20 , 40 and 60C /h , holding time for 5 hours , and finally cooling to less than 50C with the furnace .
After annealing, the sample is prepared according to GB/T2975 , and then according to GB/T228 , GB/T5027 and GB/T5028 , the mechanical properties (R , Rabbit and A ), plastic strain ratio weighted average ( Le ) and strain hardening index are weighted Mean (horse ) test . Observe the structure with an optical microscope, and evaluate the microstructure according to GB /T6394 .
2 test results

1. 1 Effect of chemical composition on mechanical properties

annealing temperature on mechanical properties of test steel 1 and 2

It can be seen that for test steel 1, only when the annealing temperature is below 650 C , the mechanical properties can meet the requirements of St37-2G , and the margin of tensile strength is not large; for test steel 2 compared with test steel 1 , although due to the increase The strength of carbon and manganese content is obviously increased, but the elongation after fracture is also significantly reduced; it is worth noting that when the annealing temperature is increased from 690 C to 720 C , although the strength decreases, the elongation after fracture also decreases .
Considering that carbon has a greater adverse effect on plasticity than manganese, from the perspective of strong plasticity, the carbon content of St37-2G is controlled at the middle limit ( ie about 0 Zhao ) required by the standard , and the manganese content is controlled at 0 . Around 9 % is more suitable .
Microstructure of test steel 1 and 2 samples after annealing at different temperatures .
It can be seen that when the annealing temperature is 600°C , the test steel 1
and 2 did not undergo complete recrystallization ; when the annealing temperature rose above 650C , test steel 1 and 2 were completely recrystallized, and the recrystallized ferrite grains were equiaxed, and the annealing temperature had an effect on the ferrite grain The influence of grain size is small, among which the ferrite grain size of test steel 1 is around 80 , while the ferrite grain size of test steel 2 is 10 . Level 5 or so .

Comparing the mechanical properties, it can be seen that the increase in the strength ( Re L , Rn ) of test steel 2 compared with test steel 1 is mainly caused by the refinement of ferrite grains caused by the increase of carbon and manganese content ; The decrease in plasticity of steel 1 is mainly caused by the increase of pearlite content caused by the increase of carbon and manganese content ; while the strength of the sample decreases with the increase of annealing temperature mainly due to the coarsening of the second phase ( pearlite and cementite ) caused by .

2.2 Effect of annealing temperature on mechanical properties The effect of annealing temperature on mechanical properties of test steel 3 is given . Figure 3 shows that when the annealing temperature is above 660° C , even if the annealing holding time is 0h , the elongation after fracture of the test steel 3 returns to more than 30 % , and as the annealing temperature increases , the strength gradually decreases ; When the annealing holding time is extended to 5 h , the strength at each annealing temperature decreases . And when the annealing temperature is higher , In addition , the test further shows that when the annealing temperature is 710 C , the elongation after break is lower than that when the annealing temperature is 680 C , and as the annealing time prolongs , while the strength decreases , the elongation after break The length is also reduced .
In summary , From the perspective of ensuring strong plastic fit , The suitable annealing temperature is 660-680C , and the holding time should ensure that the cold spot temperature of the steel coil reaches about 660C during bell annealing .
2.3 Effect of cold rolling reduction rate and annealing heating rate on mechanical properties
cold rolling reduction rate on the mechanical properties of test steel 4 .
n and I values of test steel 4 when the annealing heating rate is 20 40 60 C / h .
Therefore , If the cold-rolling reduction rate of test steel 4 is controlled at about 50% , and the heating rate is controlled below 40C/h , a cold-rolled carbon structural steel plate with a high r value can be obtained , and the overall performance is good .

test steel 4 under different process conditions .

 

the annealed microstructure of test steel 4 with a hot-rolled coiling temperature of about 620 C is the same as that of a hot-rolled coiled temperature of 680
1 and 2 around C are completely different, which shows that the coiling temperature of hot rolling has a significant effect on the annealed microstructure and properties . To obtain cold rolled carbon structural steel with high r value , Lower coiling required for hot rolling

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