Microstructure and properties of DP 600 dual-phase steel produced by 1 880 mm hot rolling

Abstract : The simple composition design of SiMn series is adopted to give full play to the intensive cooling capacity and coiling capacity of the 1 880 mm hot continuous rolling mill. Through the three-stage cooling mode of intensive water cooling-air cooling - water cooling and low-temperature coiling, the 1 880 mm 600 MPa- grade hot-rolled dual-phase steel was pilot-produced by the mm unit . The results show that the test steel can obtain a dual-phase structure with a suitable ratio of ferrite and martensite through reasonable cooling rate, intermediate waiting temperature and waiting time, and the mechanical properties can meet the design requirements of 600 MPa dual-phase steel; at the same time The test found that the cooling and coiling process significantly affected the microstructure and properties of the dual-phase steel, and the cooling conditions at the head and tail of the coil were inconsistent due to the change of rolling speed, resulting in large fluctuations in the mechanical properties of the coil at different positions .
Key words: dual-phase steel ; microstructure ; strength ; phase transformation

the increasing requirements for energy saving and weight reduction in industries such as automobiles and construction machinery , the development of high-strength steel has attracted attention from all aspects and has become a hot spot in the field of steel research . Micro-alloy and low-alloy high-strength steel ( HSLA steel ) has been developed by means of solid solution strengthening , dislocation strengthening , and precipitation strengthening.
The strength of steel is significantly improved, but it is usually accompanied by deterioration of plasticity and poor formability . In order to coordinate the strength and plasticity , the design concept of composite materials was introduced , and the dual-phase steel was developed . Due to its low yield ratio , high work hardening rate , excellent strength and ductility , it has become a new type of high-strength stamping with good formability. Steel is widely used in the automotive industry . According to the production method, dual-phase steel is divided into hot-rolled dual-phase steel and cold-rolled dual-phase steel . Because cold-rolled dual-phase steel requires post-rolling annealing heat treatment , the production process is relatively long . In order to simplify the process , many companies are committed to researching hot-rolled dual-phase steel. At present, Europe, America and Japan have successfully developed hot-rolled dual-phase steel products . Limited by laminar cooling capacity and coiling capacity, the hot-rolled dual-phase steels produced by conventional hot-rolling mills are all added C*M to varying degrees . and other alloying elements, produced by medium temperature coiling . In order to reduce costs, economical dual-phase steels use SiMn as the main alloying element, and add less or no expensive alloying elements such as Ck Mo through precise control of the cooling process. Therefore, the equipment of the hot rolling mill and the controlled rolling and cooling process are strictly required . The 1 880 mm hot continuous rolling mill is equipped with dense laminar flow cooling device , Cooperating with the sub-stage cooling control mode , it is suitable for the production of various advanced high-strength steels ( AHSS ) such as dual-phase steel and multi-phase steel .
Laboratory research on dual-phase steel has been carried out in the early stage . The composition system and hot-rolling process of economical dual-phase steel have been basically determined (4). This paper studies the microstructure and properties of DP600 hot -rolled dual-phase steel trial- produced in a 1 880 mm hot-rolling mill. Process control parameters are provided for reference .
1 Test materials and methods
SiMn- based DP600 dual-phase steel used in the test was smelted in a 250 t top-bottom combined blowing converter . After ladle refining and vacuum degassing , The chemical composition is shown in Table 1 .
Table 1 The mass fraction of the chemical composition of the test DP600 dual-phase steel

Table 1 Chemical composition of he DP600 test steel %
W c	w Si	wxya _	W P	W s	w Al	w n
0. 079 0	1 000 0	1 520 0	0.015 0	0. 004 9	0.023 0	0. 003 7

Fig.1 Schematic curves of rolling and cooling of DP600 dual phase steel
Fig 1 Schanatic illustration of the mlling and cooling process for DP600

6 m away from the end of the outer ring of the steel coil , and the transverse and longitudinal tensile properties are tested at the same time . The tensile results are shown in Table 3 . It can be seen from Table 3 that the yield strength of the trial-produced dual-phase steel is basically above 400 MPa , and the yield strength of the 15 mm -thick test dual-phase steel is high, exceeding the upper limit of 470 MPa ; the tensile strength of both the transverse and longitudinal specimens exceeds 700 MPa The strength is not much different , no obvious directionality ; The yield-to-strength ratio is basically 0.55 to 0.60 . Test the elongation rate of 80 mm calibration distance , the elongation index of the longitudinal sample is above 19% , It meets the standard requirements ; the elongation of some test steel coils in the transverse direction is lower than 19% . Visible , There is obvious directionality in elongation , The elongation of the longitudinal specimen is obviously better than that of the transverse specimen .

The test steel was cast into a slab with a thickness of 230 mm , and the rolling test was carried out in a 1 880 mm hot rolling mill . The slab heating temperature is 1150 ~ 1200C , the amount of deformation and the deformation temperature are controlled, and the final rolling temperature is controlled between 840 ~ 880C . After rolling, the cooling mode is adopted for cooling in sections, that is, the steel strip is first rapidly cooled to the intermediate waiting temperature and then air-cooled After a certain period of time, it is rapidly cooled to below 2O0C and coiled .
Table 3 Tensile properties of test hot-rolled dual-phase steel
Table 3 Tensile properties of the DP600 test steel

Thickness /mm	Specimen direction	R p 0. 2 MPw	R m MPa	A 0 %	R p 0.2 R
3 5	portrait	443	762	20	0. 581
	horizontal	459	778	16	0. 590
3 0	portrait	404	717	twenty three	0. 563
	horizontal	408	723	20	0. 564
2 5	portrait	413	737	twenty three	0. 560
	horizontal	394	721	19	0. 546
2 0	portrait	401	725	twenty three	0. 553
	horizontal	402	720	twenty one	0. 558
1 5	portrait	472	709	19	0. 666
	horizontal	561	727	17	0. 772
				.	/

Test the finished thickness of hot-rolled dual-phase steel 1 . 5 ~ 3 . 5mm , _ Prepare JS13A tensile specimens with a gauge length of 80 mm, and test the tensile properties of each specification test steel coil ; at the same time, cut the specimens next to the tensile specimens, prepare metallographic specimens by conventional mechanical grinding and polishing, and pass through 4% Nitrate was etched, and the microstructure was observed with a metallographic microscope .

1. test results

2 . 1 Tensile properties
2 1 1 Tensile properties of steel coils with different thicknesses
The mechanical property standard requirements of DP600 hot-rolled dual-phase steel longitudinal specimens are shown in Table 2 . Tensile samples were taken from the outer ring of the test steel coil , and samples
Table 2 Mechanical property requirements of DP600 dual-phase steel
Table 2 Mechanical property requisites OrDP600 steel

Specimen direction	R p 0. 2 MPa	R m MPa	A80 %
portrait	330 ~ 470	>580	>19

2 1 2 Tensile properties of steel coils with the same thickness at different positions
Select a 30 mm thick test steel coil, take the first test plate at a distance of 6 m from the end of the outer ring of the steel coil , and then take a test plate at an interval of 60 m , and take a test plate at a distance of 6 m from the end of the inner ring of the steel coil. For the last test panel, a total of 12 test panels were taken, and the test panels were numbered sequentially to check the longitudinal tensile properties of each test panel, and analyze the performance distribution of different positions of the steel coil length. The results are shown in Table 4. Different positions of the steel coil There are large differences in mechanical properties, and the yield strength is generally high. Only the yield strength of the sample near the outer ring of the steel coil meets the standard requirements, and the yield strength of the rest of the samples exceeds the upper limit of the standard; the tensile strength is relatively stable , basically 700-750 MPa The elongation rate fluctuates greatly, the elongation rate of the outer ring sample is higher, reaching 23%, the elongation rate of the middle ring and inner ring samples gradually decreases , and the elongation rate of some samples is less than 19 % .

22 microstructure
Test the metallographic structure of various thickness specifications of duplex steel . The outer ring samples of the steel coils in each thickness test are ferrite and martensite structures , the ferrite is equiaxed , and the martensite is mostly island-shaped , isolated in the ferrite grain boundary , and the number of martensite slightly different .
Table 4 Tensile properties of 30mm thick test dual-phase steel coils at different positions
Table 4 Tensile properties of ihe test DP steel coil (3 0 mm thick) at various locations

Sample No	R p 0.2 MPa	R m MPa	As . %	Rp0-2Rm _ _ _ _
No 1 ( outer circle )	410	709	twenty four	0.578
No 2	481	696	twenty three	0.691
No 3	447	716	twenty three	0.624
No 4	493	736	17	0.670
no 5	535	754	16	0.710
no 6	501	725	19	0.691
no 7	488	716	twenty-one	0.682 _
no 8	580	767	16	0.756
no 9	524	752	16	0.697
not 10	491	727	twenty	0.675
No 11	526	748	17	0.703
No 12 ( inner circle )	549	725	16	0.757

Fig 2
Observing the metallographic structure of the sample at different positions of the 30 mm steel coil , it is obvious that the structure at different positions of the length of the steel coil has significant differences , which is different from the typical dual-phase structure of ferrite and martensite in the outer ring . No. 4 sample ( about 200 m away from the end of the outer ring of the steel coil ) began to appear a small amount of granular structure, and the shape of the granular structure was point-shaped or short rod-shaped particles distributed in the ferrite grain . No. 4- No. 11 sample Granular structure appears more or less , some samples even all are granular . structure , but the amount of ferrite is very small , scattered in a polygonal shape , and the rest are large bainite structures .
3 Analysis and Discussion
31 Effect of finishing temperature

The excellent strength and plasticity of dual-phase steel are inseparable from the structure and relative quantity of ferrite and martensite ; hot rolling process parameters , such as final rolling temperature , rolling deformation, etc., will affect the ferrite phase.

The final performance is the difference in performance indicators such as strength and elongation . Research by Liu Jiantao and others believes that large final rolling deformation and low temperature final rolling are beneficial to dual-phase steel with low yield ratio and high elongation [5] ; however, low temperature large deformation rolling with final rolling temperature below 8O0C It will cause the rolling force to rise rapidly , and the practical application is limited . Takashi et al. [6] pointed out that the yield strength ratio of dual-phase steel is related to Si content and finish rolling temperature, as shown in Figure 4 . For dual-phase steels with a Mn content of about 1.4% composition system, if no Si is added, it needs to be finished rolling below 800C , and the yield ratio can be reduced to less than 0.60 ; When the added Si content exceeds 0.60% , the requirements for the finish rolling temperature are greatly reduced, and the final rolling in a wide range of 760 ~ 900C is expected to control the yield ratio below 0.60 . The target finish rolling temperature of the dual-phase steel in this test is set at 860C , The actual average final rolling temperature is slightly higher than the target final rolling temperature. The final rolling temperature of 35mm and 15mm thick steel coils is about 900C , The final rolling temperature of steel coils of other specifications is about 870C . From the test results , The outer rings of test steel coils of various specifications are ferrite-martensitic dual-phase structure , and the yield strength ratio is basically 0.55 to 0.60 ; Compared with other samples, the number of martensite in this sample is more , the strength is higher , and the yield ratio is also slightly higher . The test results show that since the Si content in the test steel is close to 10%, there is no need for low-temperature finish rolling , Even if the finishing temperature is close to 900C , A dual-phase structure with excellent performance can also be obtained , which is convenient for the implementation of the hot rolling process .

 

3 . 2 Effect of cooling process on microstructure and performance The post-rolling cooling system , including cooling speed , intermediate waiting temperature , waiting time, etc. , directly affects the formation and quantity of ferrite and martensite , thus determining the performance of dual-phase steel . The outer ring of the test coil is ferrite and martensite ( Fig. 2) , which shows that the water cooling rate after rolling has avoided the pearlite phase transformation region ; The intermediate heating temperature corresponding to the outer ring of the steel coil is about 680C , and the air cooling time is about 3 s . The combination of the heating temperature and the heating time is conducive to the formation of a large amount of ferrite . The remaining untransformed austenite transforms into martensite avoiding the bainite transformation zone during the third stage of cooling .
Due to the accelerated rolling , the rolling speed of the tail ( corresponding to the outer ring ) is about 50% higher than that of the head ( corresponding to the inner ring ) , so that the middle waiting temperature and waiting time are uneven . This results in significant differences in the structure and performance of steel coils at different positions . The rolling speed at the head position is slower , so the cooling rate after rolling is higher , and the intermediate waiting temperature is relatively low . The actual intermediate waiting temperature at the head position is about 600C . Although the waiting time has also been extended , However, sufficient ferrite was not formed . Even since the intermediate temperature is lower than the ferrite transformation temperature range , Ferrite transformation fails , austenite directly forms granular structure

1. ), the corresponding yield strength increases and elongation decreases . 3 . 3 Influence of coiling temperature

The coiling temperature determines the transformation behavior of retained austenite , Different from the martensitic transformation of hot-rolled dual-phase steel added with CrMo alloy after coiling, the martensitic transformation of SiMn- based dual-phase steel occurs before coiling, so the coiling temperature must be lowered to the M s point Below . After measuring the temperature of the outer ring of the steel coil, the coiling temperature of the test steel coil is lower than 200C, and the microstructure in Figure 2 also confirms that the remaining austenite is transformed into martensite . But in order for the steel strip to bite into and coil smoothly , The coiling temperature of the inner ring is about 4O0C , and martensite cannot be formed when coiling at this temperature, as shown in Figure 3

1. , and due to the lack of martensite structure, the yield strength of the sample at the corresponding position is high, and it does not show the low yield ratio that dual-phase steel should have. 4 Conclusion
2. The 1 880 mm hot continuous rolling mill is equipped to produce joints whose connection strength and failure pressure are greater than the specified values after eccentric wear .

Table 4 Hydrostatic pressure and failure internal pressure test load
Table 4 The test load of hydrostatic test and failure inner pre s ure

Sample No.	Hydrostatic load ( holding pressure for 30 min ) ／ MPa ( psi )	failure internal pressure / MPa ( psi )
1z _	68. 95 (10 000)	125.1 (18 138)
2Z _	67. 99 (9 861)	132. 6 (19 233)
3z _	68. 02 (9 865)	140. 6 (20 388)

1. The sample was subjected to the initial make-up and break-out test (3 times of make-up and 2 times of break-out ) according to the provided torque, and no sticking and sealing surface damage occurred .
2. For the produced ① 139.7X9.17 mm P110 drilling casing, after ( 120 ± 0.15 ) mm partial wear on the coupling, the tensile failure load is greater than the minimum value of joint connection strength specified in API BUL 5C2 , and the failure within The pressure value is greater than the minimum inner yield strength specified in API BUL 5C2 .
3. The produced ① 139. 7X9. 17 mm P110 drilling casing has passed the casing drilling test in Jilin Oilfield of PetroChina and meets the requirements of casing drilling at a depth of 1 160 m .

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