Analysis of Factors Affecting Elongation of Ship Plate Steel

Elongation mainly refers to the ability of metal to deform. All factors that affect the continuity of metal and hinder its deformation can lead to a decrease in elongation. The important measure is to increase its resistance to deformation. Therefore, elongation is a measure of high-strength ship plate. Important indicators of steel AH32 .

• Factors Affecting the Elongation of Steel
• Effect of Chemical Elements on Elongation

If only the effects of grain size, pearlite content, solid solution elements, and segregation elements on elongation in steel are considered, the total strain when the material breaks can be expressed by the following formula:
e T=1. 3-0. 02 (% pearlite ) +0. 3 ( %Mn ) +0. 2 ( %Si ) -3.4 ( %S ) -4.4 ( %P ) -0.29 ( %Sn ) + 0.015d-1/2 ( 1 )
from formula ( 1 ) that high overall plasticity depends on low carbon content, fine ferrite grains and minimum brittle solute elements, among which P and tin are the most harmful (in the grain boundary precipitation), the harm of S lies in the influence of sulfide inclusions; Si and Mn are ferrite solid solution elements, except for the formation of inclusions affecting plasticity, the solid solution of Si and Mn in ferrite has little effect on plasticity, on the contrary due to Lowering the austenite transformation temperature is beneficial to the refinement of ferrite grains.

• Effect of metallographic structure on elongation
• Effect of Tissue Type on Elongation Rate

High-strength ship plate steel AH32 belongs to low-carbon steel. For low-carbon steel, the general structure state is ferrite + pearlite, and most of the structure is ferrite. Since pearlite is composed of ferrite and infiltration Composed of carbon, ferrite is a plastic phase, cementite is a brittle phase, and the brittle phase is not easily deformed. Therefore, the size and shape of pearlite are the main factors affecting the elongation performance.
Reducing the amount of pearlite can effectively improve the plasticity and toughness, but it is inevitable to reduce the tensile strength; in addition, thinning the interlamellar spacing of pearlite can effectively make the cementite sheet thinner, making it easy to deform, but at the same time it will cause pearlite The increase in the total amount of volume (increasing the amount of pseudo-pearlite) is not suitable for low carbon steel.

• Effect of grain size and its uniformity on elongation The smaller the grain size, the easier the metal is to deform, and its elongation refers to

The standard is also about high, but while considering the influence of grains on elongation, the uniformity of grains must be considered. When the mixed crystal phenomenon is serious, regardless of other effects, metal fracture generally occurs at the largest grain position.

• Influence of banded structure on elongation Since the tensile properties of general tests are transverse specimens, the banded structure

The generation of weaving is along the rolling direction. Therefore, the banded structure seriously destroys the continuity of metal deformation. With the increase of the banded level, the elongation index also shows a downward trend. Therefore, the banded structure in steel must be strictly controlled.

• Effect of inclusions in steel

The existence of inclusions severely damages the continuity of the matrix and affects the improvement of elongation. Combining the elongation test method and the deformation degree of various inclusions, the main inclusions that affect the elongation index are sulfide inclusions and silicate inclusions. Because such inclusions are plastic inclusions, they extend and deform along the rolling direction during the rolling process, while tensile test specimens are generally transverse, so longitudinally extended and deformed inclusions will seriously affect continuity, resulting in a decrease in transverse tensile elongation .
1.3 Effect of controlled rolling process on elongation of low carbon microalloyed steel

• Heating temperature with low influence on elongation is beneficial to obtain fine prior austenite grains

grains, thereby creating conditions for obtaining the final fine ferrite grains, which is conducive to the improvement of elongation. However, for steel with high alloy content, low heating temperature will lead to a decrease in the solid solution of the alloy, which will affect subsequent precipitation and grain refinement. Therefore, the heating temperature should be appropriately increased due to comprehensive consideration.

• Effect of finish rolling temperature on elongation At present, low-carbon microalloyed steel generally adopts two-stage controlled rolling process. With the decrease of finish rolling temperature, grain deformation bands increase during non-zone rolling, and two situations will occur. One is that the increase of the deformation zone is beneficial to the nucleation of subsequent ferrite and the refinement of ferrite grains, which is beneficial to the improvement of plasticity; the second is that the final inhomogeneity of the grain becomes larger with the increase of the deformation zone , leading to a decrease in plasticity, so only in terms of the finishing temperature, there is an optimal temperature range for plasticity for steels with different components. Below this temperature, the elongation decreases due to the significant influence of grain inhomogeneity; This temperature also leads to a drop in elongation due to ineffective grain refinement.
• 2-4 passes before final rolling on the elongation of the low-temperature final-rolled steel is also greater. The deformation of the 2-4 passes before the final rolling also has a greater impact on the elongation . It will refine the grains and induce the growth of some grains, resulting in the appearance of coarse grains, which seriously affects the plasticity and toughness.
• Influence of cooling rate and final cooling temperature As for the cooling rate and final cooling temperature of the steel plate after rolling, as long as there is no medium and low temperature structure, in principle, the greater the cooling rate and the lower the final cooling temperature, the finer the ferrite grains of the steel plate , the smaller the band structure, the higher the final plasticity and toughness.
• Experimental Research

2.1 Macro Analysis
Observe and analyze the macroscopic morphology characteristics of the specimen fracture.
2.2 Metallographic analysis
Intercept metallographic samples for metallographic testing. Mainly for organization and inclusion analysis.

• Result analysis

3.1 Test results of mechanical properties
Table 1 Mechanical properties of ship plate steel AH32

standard	material plate Number	Upper Yield Strong degree /MPa	Tensile strength
			degree /MPa	Elongation A/%
LR specification requirements	AH32	Work 315	440 - 570	Work 22
measured value	AH32	400	550	19

( 1 ) The sulfide inclusions gathered in the central area are plastic inclusions, which extend and deform along the rolling direction during the rolling process, while the tensile test specimens are generally transverse, so the longitudinally extended and deformed inclusions will seriously affect the continuity, resulting in One of the reasons for the decrease in transverse tensile elongation.
( 2 ) The higher content of Mn and C in the central segregation area will lead to a certain deviation in the phase transition temperature between the central area and other parts. Mn and C belong to elements that expand the austenite zone and reduce the phase transition temperature, so the central area will have Bainite structure, but the edge is still pearlite. The central structure is abnormal, there is more bainite, and other parts of the structure are normally pearlite, and the bainite structure is an unfavorable factor that reduces the central toughness.

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