Effects of Composition and Heating Rate on Thermal Expansion of Mild Steel

The volume of any material, especially steel, will change with temperature, which will lead to changes in the geometric dimensions of the material, resulting in deviations in width and thickness. Therefore, the thermal effect (ie, thermal expansion) of the volume of steel materials will directly affect the accuracy, product qualification rate and yield of hot-rolled products, and accurate measurement of the thermal expansion coefficient of the strip material is the premise to correct the dimensional errors caused by thermal effects. The thermal expansion curve of each steel grade is determined according to the thermal expansion curve drawn to determine the expansion coefficient of the steel grade at a certain temperature, and then the corresponding width change can be obtained according to the model to achieve precise control of the strip width. With the change of temperature, the volume of the material will expand or contract. The physical property parameter used to represent the amount of material strain caused by unit temperature change is the thermal expansion rate, which is one of the basic thermophysical parameters of the material. Accurately measuring the thermal expansion rate of the material plays a key role in determining the geometric scale of the length gauge block and accurately correcting the geometric scale value of the length gauge block due to temperature changes. It is also very important in the fields of industry, national defense, basic scientific research, and new material development. meaning.

Influencing factors of thermal expansion
In addition to changing with temperature, the coefficient of expansion is also affected by many other factors. Alloy composition, phase transformation, crystal defects, crystal anisotropy, ferromagnetic transitions, and heating and cooling rates will affect the thermal expansion coefficient.

The effect of alloy composition
The solute elements and their contents that make up the alloy have a very obvious influence on the thermal expansion of the alloy. Usually, the thermal expansion coefficient of the single-phase solid solution is between the expansion coefficients of each component, which conforms to the law of addition; but if the solute element is a transition group element , the expansion coefficient of the alloy is more complicated. The influence of different solute atoms and contents on the thermal expansion coefficient of pure iron (0~400oC), Mn, Sn, Si and other alloying elements increase the thermal expansion coefficient of pure iron significantly, while V, Cr, Ni and other alloying elements increase the thermal expansion coefficient of pure iron. The thermal expansion coefficient is significantly reduced.

In addition, there are various structures such as martensite, ferrite, cementite, and austenite in the steel. During the heating and cooling process, there are a series of solid-state transformations. A certain amount of C, Si, Mn, etc. These alloying elements have a series of effects on the transformation temperature of steel. Therefore, the change of the thermal expansion coefficient of the actual steel material with temperature is very complicated.

Ferromagnetic transition
The coefficient of thermal expansion of most metals and alloys varies with temperature, and it expands first, then contracts and then expands, which is called normal expansion. However, for ferromagnetic metals and alloys such as iron, cobalt, nickel and some alloys, the change of expansion coefficient with temperature does not conform to the above law, and additional expansion peaks appear on the normal expansion curve. These changes are called abnormal thermal expansion. Among them, the thermal expansion peaks of nickel and cobalt are positive upward, which is called normal anomaly; while the thermal expansion peak of iron is negative downward, which is called negative anomaly. Iron-nickel alloys also have negative anomalous expansion characteristics

Alloys with negative anomalous expansion characteristics can obtain Invar alloys with zero or negative expansion coefficients, or Kovar alloys with basically unchanged expansion coefficients within a certain temperature range, so they have great significance. industrial significance.

Influence of rolling process
According to the phase transition theory and the iron-carbon phase diagram, during the heating and cooling process, the steel material will undergo a series of solid-state phase transitions. Below the equilibrium phase transition temperature, this deviation increases with the increase of the heating and cooling rate. Therefore, the actual rolling production process will also have a significant impact on the thermal expansion coefficient, and the thermal expansion curve needs to be adjusted in combination with the actual rolling process of each specific steel grade.

The chemical composition is the main factor that determines the expansion coefficient of the material. When the composition is constant, process factors such as processing and heat treatment also affect the thermal expansion, but this influence is unstable and can be eliminated after a certain process system is used.

Thermal expansion of polycrystalline and composite materials
The density of steel is related to the microstructure obtained by heat treatment. Martensite, ferrite + Fe3C (consists of pearlite, sorbite, bainite), austenite, and their density gradually increases at a time. That is, austenite has the largest density, and martensite has the smallest density. When quenched to obtain martensite, the volume of the steel will increase. This is because specific volume is the inverse of density. In this way, the order of specific volume from small to large should be martensite (varies with carbon content), cementite, ferrite, pearlite, and austenite.

When quenched steel is tempered, a volume transformation occurs with the structural transformation taking place in the steel. When martensite is tempered, the volume of the steel will shrink, and the transformation of supercooled austenite into martensite will accompany the volume expansion of the steel, and when the martensite is decomposed into troostite, the volume of the steel will shrink significantly.

It can be seen from the thermal expansion characteristics of steel that when a first-order phase transformation occurs in the heating or cooling process of carbon steel, the volume of the steel will change abruptly, and when the supercooled austenite transforms into ferrite, pearlite or martensite, the steel The volume of steel will expand; conversely, the volume of steel will shrink. This expansion property of steel is effectively used in the study of phase transformation of steel. The heating expansion curve of a general hypoeutectoid steel has been described as an example. The equilibrium structure of hypoeutectoid steel at room temperature is ferrite and pearlite. With the increase of temperature, the steel expands. When slowly heated to 727oC (A c1), eutectoid transformation occurs, and pearlite in the steel transforms into Austrian. In the case of intenite, the volume shrinks (the expansion curve begins to bend downward, forming the inflection point A c1), the temperature continues to rise, the amount of austenite gradually increases, and the volume continues to shrink until the austenite transformation is completed. After the austenite transformation is completed, the steel expands with the increase of temperature. This inflection point is A c3, and the cooling process is just the opposite.

The latest research on the expansion curve of carbon steel found that the transformation of pearlite and ferrite to austenite is not continuous when the heating rate of low carbon steel with a carbon content of 0.025%~0.35% is 7.5~200oC/min. , a non-transition interval appears in the middle, that is, the end point of A c1 transition and the start point of A c3 transition are separated, and the temperature interval can reach 80oC.

Due to the obvious volume effect of steel during phase transformation, the expansion method is currently used to determine the transformation point of steel.