Study on High Temperature Mechanical Properties of D36 Ship Plate Steel

D36 ship plate steel were studied by using Gleeble-3500 thermal simulation testing machine , the fracture morphology was observed by scanning electron microscope, and the embrittlement mechanism was analyzed.
brittle temperature zone of the cast slab without vanadium is greater than 1 350 ℃; the reduction of area is greater than 80% at 1 350 ~ 950 ℃ , and has good high temperature plasticity; the second brittle temperature zone is 950 ~ 600 ℃ , and the reduction of area of the sample is at 41.7% ~ 64% . The first brittle temperature zone of the cast slab containing vanadium is from melting point to 1250 ℃ ; in the range of 1 250 ~ 950 ℃, the plasticity is better; its second brittle temperature zone is 950 ~ 600 ℃ , and the reduction of area is 34% at this time ~ 73% . In order to prevent cracks in the process of billet straightening, the straightening temperature should be controlled above 950 °C .
Key words: ship plate steel; high temperature mechanical properties; continuous casting slab; microalloying

D36 Ship Plate Steel introduction:
The high-temperature mechanical properties of continuous casting slab determine the load and critical strain it can withstand at high temperature, which is directly related to the crack formation in the process of solidification, straightening and rolling of the slab. For Nb-Ti-V microalloyed high-strength ship plate steel, dispersed carbonitrides will be formed in the steel, which can refine the grain and strengthen the precipitation, but at the same time, it will also increase the brittleness of the slab and cause cracks. Increased incidence.
Nb-Ti-V microalloyed ship plate steel billets were studied by sampling the D36 ship plate steel continuous casting slabs in a certain factory , and using the Gleeble-3500 thermal simulation testing machine to test their high temperature mechanical properties. At the same time, the fracture morphology of tensile samples was observed by scanning electron microscope and the embrittlement mechanism was analyzed. Therefore , the high temperature plastic zone of D36 ship plate steel is determined, which provides a basis for reducing and preventing cracks in the actual continuous casting production process and formulating a reasonable secondary cooling process.

1. experiment method

sample is taken from the continuous casting slab of D36 ship plate steel. The cross-sectional size of the slab is 300 mm x 2 000 mm .
1521T . _ The sampling direction is perpendicular to the drawing direction, and the sample size after processing is 0.10 mm X 120 mm .

Table 1 Main chemical compositions of the D36 steel for ship plate %

serial number	C	Si	mn	P	S	Alt	Ti	Ni	Cu	V	Nb
373 #	0． 11	0． twenty two	1． 52	0． 007	0． 002	0． 039	0． 015	0． 25	0． 28		0． 024
374#	0． 11	0． 22	1． 48	0． 01	0． 002	0． 036	0． 016	0． 24	0． 25	0． 060	0． 024

When using the Gleeble -3500 thermal simulation testing machine to test the high temperature mechanical properties of the slab, the flow rate is 0 . 1 L/min of argon protection, the sample was heated to 1 350 °C at a rate of 10 °C/s and kept for 1 min , and then dropped to the test temperature ( in the range of 600 to 1 350 °C ) at a rate of 3 C/s Make a test temperature every 50 °C ), hold the temperature for 1 min , and stretch the sample at a strain rate of 1 X 10 -3 /s until it breaks. Immediately after the sample is broken, the area near the fracture is cooled by spraying water to maintain the original appearance of the fracture. The fracture morphology is observed with a scanning electron microscope, and the fracture mode and reason are analyzed.

The reduction of area and tensile strength of steel at different temperatures are important indicators to measure its high-temperature mechanical properties . The reduction of area ( RA ) was calculated according to the formula ( 1 ) by measuring the original diameter of the sample and the diameter after breaking . Calculate the tensile strength ( Rm ) from the maximum tensile force reached when the sample is broken according to formula ( 2 ) .
R 4X10-3F _ _ _
m = fly factory
In the formula, Ra —reduction of area, % ;
D o —original diameter of sample, mm ;
Dj —diameter after breaking, mm . R m —Tensile strength, N / mm2 ;
F —Maximum tension value, kN .

1. test results
2. 1 The graph of section shrinkage is the effect of temperature on the section shrinkage of ship plate steel. The first brittle temperature zone of 373# slab without V is greater than 1 350 C , and no high temperature brittle zone was detected under the test conditions; its high plastic platform zone When the temperature is 950 - 1 350 C , the reduction of area is maintained above 80% ; when the temperature is lower than 950 C , the plasticity begins to decrease rapidly, and reaches the lowest at 850 C , which is 41.7% ; as the temperature continues to decrease, the reduction of area begins to rise again It rises to 65% at 700 C , rises to 73 % at 650 C , and then drops slightly; from the plasticity - temperature curve, the third brittle temperature zone of 373 # billet, that is, the "trough zone" is 700-950 C. The first brittle temperature zone of 374# slab containing V is from 1 200 °C to melting point; its high plastic plateau range is 1 000 ~ 1 200 °C , and the reduction of area is maintained above 90% ; the temperature is lower than 1 000 °C , the plasticity began to decrease rapidly, and the plasticity was the lowest at 850 C , which was 34.4% . With the decrease of temperature, the plasticity was in the trough area, and it rose to about 73% at 650 °C , and then began to decrease; from the curve, 374# There are two brittle temperature zones in the slab: the first brittle temperature zone (high temperature brittle zone) from 1 200 °C to the melting point, and the third brittle temperature zone from 650 to 1 000 °C . The ship plate steel containing V is narrower than the ship plate steel without V , and the brittle trough area is wider, which will lead to relatively higher crack sensitivity. From the high temperature plasticity curves of the two, the correction of the ship plate steel The direct temperature should be above 950 °C .

 Tensile strength of D36 ship plate steel :
temperature pair Effect of D36 on the tensile strength of ship plate steel.
In the range of 1 350 ~ 600 ° C , the tensile strength of 373 # >374# slabs decreased gradually with the increase of temperature . °C all drop to about 13 MPa .

Analysis and Discussion on High Temperature Mechanical Properties of D36 Ship Plate Steel
brittle temperature zone of the slab containing V is from 1 200 C to the melting point, and the first brittle temperature zone of the slab without V is from 1 350 C to the melting point. The overmelt fracture in the first brittle temperature zone is caused by the presence of brittle inclusions or low-melting impurity elements at the grain boundaries. When the temperature rises to a certain level, the low-melting point substance and some grain boundaries begin to melt, the plasticity of the sample decreases sharply, and it is directly broken along the grain boundary under the action of tensile stress.
The high plasticity temperature zone of the slab containing V is 1 000 ~ 1 200 C , and the high temperature plasticity zone of the slab without V is 950 ~ 1 350 C .
In the test, the th dish brittle temperature zone of D36 ship plate steel is 950 ~ 600 C. From the Fe-FeC 3 phase diagram, it can be seen that the th plate brittle temperature zone occurs in the Y single-phase low temperature zone and the Y + a two-phase zone. There are generally two reasons for its brittle fracture: one is the embrittlement of the austenite single-phase low temperature zone, which is mainly caused by the precipitation of the second phase particles in the steel, such as TiC , NbC , VC and other grain boundaries7-8 ; The second is embrittlement in the high-temperature zone of the Y + a two-phase transformation zone, that is, the network-shaped a -ferrite precipitates at the austenite grain boundary, which has low strength and small deformation resistance, and becomes a stress concentration location under the action of external force, resulting in fracture at grain boundaries.
In order to prevent cracks during the straightening process of the cast slab, it is necessary to control the straightening temperature in the plastic temperature zone as much as possible, and avoid the third brittle temperature zone of the steel. The straightening temperature of the continuous casting ship plate steel should be controlled above 950 °C .

D36 ship plate steel research conclusion :

1. D36 ship plate steel in the temperature range of 600 ~ 1 350 C , the tensile strength decreases with the increase of temperature. The tensile strength of the slab is 220-250 MPa at 600 C , and drops to about 13 MPa at 1350 C.
2. Under the test conditions, the high temperature brittle zone was not detected in the 373 # slab without V , and its high plasticity plateau ranged from 950 to 1 350 C , and fell to the lowest at 850 C , and the reduction of area was 41.7%, and recovered at 700 C to about 65% . The third brittle temperature zone of 373# slab, that is, the "trough zone" is 700 ~ 950 C.
3. The high plasticity plateau range of 374 # billet containing V is 1 000 ~ 1 200 C , the plasticity is the lowest at 850 C , the reduction of area is 34.4% , and it rises to about 73% at 650 C , there are two The first brittle temperature zone: the first brittle temperature zone (high temperature brittle zone) from the melting point to 1 200 C and the third brittle temperature zone from 1 000 to 650 C.
4. with the ship plate steel without V , the plate steel with V has narrower high-temperature plasticity platform range and wider brittle trough area, which will lead to relatively higher crack sensitivity. From the high-temperature plasticity curves of the two, in order to prevent cracks during the straightening process of the slab, the straightening temperature of the ship plate steel should be controlled above 950 °C .
5. D36 ship plate steel. One is that the second phases such as fine Nb ( CN ) and V ( CN ) formed after microalloying are precipitated at the grain boundary of the austenite single phase region, resulting in stress concentration Embrittlement occurs; the second is that the low strength of reticulated ferrite precipitated at the grain boundary of prior austenite leads to embrittlement.

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