550 MPa grade cold-rolled material with DC04 steel

Abstract : The strength of DC04 steel is improved by cold rolling deformation, stress relief annealing is carried out in the partly recrystallized area, and the holding time is reasonably controlled. The yield strength of steel 02 $ 550 MPa is successfully produced, and the cold bending performance d = 0 , 80 ° is qualified The cold plate and galvanized sheet can be widely used in special protective boxes, light steel structure houses and other fields.
Key words: DC04 ; high-strength cold-rolled material; annealing

A container manufacturer produces protective boxes for mines . Due to the high price and long delivery cycle of imported cold-rolled plates , it is hoped that WISCO can use ordinary carbon steel to produce similar products to reduce costs . The technical condition of cold-rolled sheet is that the yield strength ^ pc.2 > 550 MPa ; d — 0 , 80° cold bending without cracking , which meets the processing requirements . At that time, the common carbon steel cold - rolled plates produced by WISCO had a yield strength of less than 300 MPa. If low-alloy steel H420LAD steel was used, the yield strength was only 420-520 MPa, and the price was relatively expensive. Therefore, ordinary carbon steel was developed . Research and development work for producing 550 MPa grade cold-rolled materials .
1 Test materials and methods
11 Selection of test steel types
The analysis results of the imported cold-rolled plate show that the raw material is ordinary carbon steel with a W (C ) of 0.06 % , and the performance test results are shown in Table 1 .
With reference to the chemical composition and mechanical properties of imported cold-rolled sheets , and in order to meet the customer’s requirements for cold-bending performance of high-strength cold-rolled sheets with a thickness of 10 to 15 mm , it was decided to use DC04 with a low C content as the test material. steel . The test steel type is DC04 chilled plate produced on site , and the chemical composition and specifications of the sample are shown in Table 2 .
12 test equipment
The test uses SL - 384 high-temperature annealing furnace ( Shanghai Electric Furnace Factory ), SUN10 thin plate tensile testing machine (Italy GALDA - BINI company ), WB-1000kN plate bending testing machine ( Jilin Jinli Company ), BRIN 200D - TL type Luo Hardness tester ( Foundrax , UK ), PME3 optical metallurgical microscope ( OLYMPUS , Japan ).

Table 1 Test results of mechanical properties of imported cold-rolled sheets
steel type	Specifications /mm	R P0 2 /MPa	R m/ MPa 4 80mm / % cold-formed 4 =0,180 .	Zinc layer adhesion	Microstructure
Imported cold rolled sheet	0 5	640	680 2 5 qualified	qualified	Deformation organization
Table 2 Chemical composition and specifications of DC 04 chilled hard plate samples
steel coil	C	Si	Chemical CompositionW b /% Mn P S Al s	Sample size /mm	Cold rolling reduction / %
A	< 0.06 _	< 01	< 0 5 0 013 0 01 < 0 06	110X250X1. 5	63.98

1.3 Test method
From the composition and properties of imported cold-rolled materials , it can be seen that the cold-rolled plate adopts the deformed structure produced by cold-rolling of ordinary carbon steel to increase the strength , and anneals below the recrystallization temperature to eliminate part of the internal stress , so that its cold-bending performance meets the requirements of processing and forming. Requirements , so the test should measure its recrystallization temperature and the influence of annealing process on its performance . The determination of the recrystallization temperature is to keep the sample at different temperatures for 30 minutes ［ 1 ］ , but since more and more chilled coils adopt the continuous annealing method , the annealing time is only a few minutes , so it is necessary to know the recrystallization temperature under short-time annealing conditions. crystallization temperature . According to the literature [ 2 ] , the core temperature of the 0.8 mm thick steel plate reaches the furnace temperature for only 12 s , so the holding time is selected as 10 mn, and two rounds of experiments are carried out :

1. the sample into a high-temperature annealing furnace and keep it warm at 400-650 ° C for 10 minutes , then cool it down to room temperature with the furnace ; , cooled to room temperature with the furnace . The heat-treated samples were tested for strength , cold bending performance , hardness and metallographic structure .

2 Test results and analysis

1. Determination of recrystallization temperature

The relationship curve between the Rockwell hardness of the sample and the annealing temperature is shown in Figure 1 . According to the measurement method of recrystallization temperature [ 1 ] , it can be calculated from Figure 1 that the recrystallization temperature of DC04 steel is about 580 C, slightly higher than the recrystallization temperature 550 C 3 mentioned in the literature , which may be related to the sample The reduction rate of cold rolling , the heating rate of the annealing furnace and the holding time are related .

Fig.1 The relationship curve between Rockwell hardness and annealing temperature of sample

1. Effect of annealing temperature on mechanical properties

sample strength , elongation and annealing temperature are shown in Fig . 2 and Fig. 3 respectively .
The effect of annealing temperature on strength shown in Figure 2 is more complex . Taking the sample's yield strength "0.2" as an example, when the annealing temperature is less than or equal to 400 C , R p0 .2 remains at 700 MPa, which is not much different from the strength of the chilled plate; when the annealing temperature increases to 450 C , R P0.2 0.2 decreased to 650 MPa ; when the annealing temperature increased to 575 C , the strength changed drastically R p0.2 decreased to 300 MPa . The annealing temperature had a similar effect on the tensile strength R m of the sample . Figure 3 shows the annealing temperature on the test The influence of sample elongation is much simpler . When the annealing temperature rises above 525 C , the elongation will increase significantly .

1. Effect of annealing time on mechanical properties

It can be seen that when the annealing temperature is 500 and 520 °C , the strength of the sample first decreases slowly with the extension of the holding time , and then tends to be stable , and its elongation does not change much . The sample annealed at this temperature was cold-bent at d — 0,180° , and the result was unqualified ( shown in Figure 6a ). However, when the annealing test was carried out at 540 °C , the strength of the sample decreased slowly with the extension of the holding time , and the elongation gradually increased ; when the holding time was extended to 20 min , the cold bending performance d — 0, 180° qualified (Fig. 6b ) , at this time -R p0.2 is 570 MPa ; when the holding time is extended to 30 min , Rp 0.2 is 540 MPa . Tests have shown that when held at 540°C for 20 minutes , a product with a reasonable match in strength and cold bending properties can be obtained .

1. Effect of annealing process on metallographic structure

The metallographic structure of the sample at different annealing temperatures is shown in Figure 7 , and the metallographic structure of the sample at 540 C for different holding times .

 It can be seen that with the increase of annealing temperature or the prolongation of annealing time , the metallographic structure of the sample has experienced the change of " deformation structure - deformation structure + recrystallized ferrite - recrystallized ferrite" , and the change of this process and its strength is also corresponding .
3 Results and analysis of industrial trial production
According to the above test results , trial production was carried out in the bell annealing furnace and galvanizing line of the No. 1 branch of Wuhan Iron and Steel Cold Rolling Plant .

1. Industrial trial production and analysis of Lengpu board

Cold general steel plate is produced in bell type furnace, and steel coils A , B , C , and D are put into No. 1 and No. 2 bell type annealing furnaces respectively for production . Influenced by performance , the holding temperature of No. 1 bell-type furnace is set to be less than or equal to 540 C , the holding temperature of No. 2 bell-type furnace is set to be less than or equal to 530 C, and the holding time is properly controlled . The test results are shown in tables 3 and 4 respectively .
For the A steel coil ( i.e. the sample ) in the No. 1 bell-type furnace , when the laboratory process is 540 C , the product with qualified strength and cold-bending performance can be obtained , while the holding temperature drops to Below 540C . _ This is mainly due to the low heating rate and long holding time of the bell annealing furnace, which makes the recrystallization temperature of DC04 chilled plate drop . Generally speaking , as the heating rate slows down , the recrystallization temperature of cold deformed metals also decreases [ 5-6 ] . However, the B steel coil with a larger reduction rate has undergone a large degree of recrystallization at this annealing temperature , and its strength has dropped sharply . Usually , the amount of deformation ( that is, cold rolling reduction rate ) will significantly affect the recrystallization process , so that the recrystallization process can be completed in advance [ 5 , 7-8 ] . Therefore, the annealing temperature should be appropriately lowered for the chilled plate with a large reduction rate . It is a good proof that the steel coil with a large reduction rate can be annealed at a lower temperature in the No. 2 furnace .

1. Industrial trial production and analysis of galvanized sheet

During the production of galvanized sheet, the temperature of the strip steel entering the zinc pot should be about 460 C , and the strip steel has a natural temperature drop of about 100 C in the cooling section of the continuous annealing furnace . In order to ensure the temperature of the strip steel entering the zinc pot , the uniform hot section

Table 3 Trial production results of No. 1 bell-type furnace ( T C540 C )
Coil No.	Specifications /mm	Cold rolling reduction / %	R eL/ MPa	Rm / MPa _	4 80mm / %	Cold bending 4 = 0 , 180°	Metallographic
A	15	63 98	605	635	45	qualified	Deformation organization
B	12	68 40	365	425	16 5	qualified	Ferrite + free cementite
Table 4 Trial production results of No. 2 bell-type furnace ( T C530 C )
Coil No.	Specifications /mm	Cold rolling reduction / %	R eL / MPa	Rm / MPa _	4 80mm / %	Cold bending 4 = 0 , 180°	Metallographic
C	10	66 40	610	625	10 5	qualified	Deformed structure + recrystallized ferrite
D.	10	66 40	610	625	8	qualified	Deformed structure + recrystallized ferrite

The strip temperature is controlled above 560C . So to do 550
For the continuous withdrawal production with a time of only a few minutes , the unit should be appropriately reduced

MPa- grade galvanized sheets, it is advisable to use DC04 chilled coils with relatively small cold rolling reductions as raw materials to increase the recrystallization temperature. At the same time, the heating rate of the continuous annealing furnace is very fast , which can further increase the recrystallization temperature . At the same time , reducing the speed of the unit appropriately has a very important effect on the heat preservation speed and the cold bending performance of the finished product . Therefore, the heat preservation temperature of the continuous annealing furnace is increased to above 560 C , and the temperature is appropriately reduced.
Slow down the running speed of the unit , and also produce qualified high-strength galvanized
board , the product properties are shown in Table 5 .

Table 5 Results of trial production of galvanized line ( T >560 °C )

Coil No.	Specifications /mm	Cold rolling reduction / %	R p0 2 / MPa	Rm / MPa _	4 80mm / %	Cold bending 4 = 0 , 180°	Zinc layer adhesion	Metallographic
E.	14	62 89	620	660	9	qualified	qualified	Deformed structure + recrystallized ferrite

to the properties of cold rolled steel coils A , C , D and galvanized steel coil E , the yield strength R p0.2 is greater than 550 MPa, and the cold bending performance 4 =0 , 180 . Qualified , meeting the performance requirements . The forming and welding properties are very satisfactory after the user's use , which shows that the production of 550 MPa grade cold-rolled products with DC04 steel can fully meet the user's requirements . At present, more than 1 800 tons have been produced stably , which are widely used in special protective boxes , light steel structure houses , large outdoor advertisements and other fields , and have been highly recognized by the market .

1. The recrystallization temperature of DC04 steel with a cold rolling reduction rate of 6398% is about 580 °C ;
2. The recrystallization temperature is not only related to the annealing time , but also related to cold rolling reduction rate , heating rate and other factors ;
3. The DC04 cold -rolled coil with a cold rolling reduction rate of 6398% is held at 540° C for 20 minutes to obtain a yield strength R pt) .2 ^ 550 MPa, and a qualified product with cold bending performance 4 = 0,180° ;
4. DC04 steel is used to produce 550 MPa cold-rolled products , the cold-rolled reduction rate will affect the product performance . The steel coils with the same cold-rolled reduction rate should be put into the same bell annealing furnace for production , and the galvanized sheet production , chilled coils with a smaller reduction rate should be used , and the operating speed of the unit should be appropriately reduced .

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