(1) Thermo-Mechanical Treatment

Thermo-mechanical treatment refers to a method that combines plastic working and heat treatment. This method is used to obtain microstructures and mechanical properties that cannot be achieved through conventional heat treatment or plastic working alone.

Typically, hot working is performed in the relatively high-temperature austenite region. However, to improve the strength and toughness of steel, various treatment methods have been developed to achieve desirable fine microstructures by working in the lower-temperature austenite region, during or after phase transformation. These developments have drawn significant attention.

Such thermo-mechanical treatments are especially important for improving the strength and toughness of high-strength structural steels. Among these, controlled rolling of high-strength low-alloy (HSLA) steels is widely used in practice.

1) Thermo-Mechanical Treatment in the Stable Austenite Region

This method is similar to conventional hot working, but involves immediate quenching after hot forging or rolling to improve hardenability and thereby enhance strength and toughness.

Controlled rolling also involves working in the stable austenite region, which will be described later.

2) Strengthening by Ausforming

As a representative thermo-mechanical treatment method, ausforming involves working metastable (supercooled) austenite around 500°C, followed by rapid cooling. This greatly enhances strength without significantly impairing ductility and toughness.

The strengthening effect is due to martensite formation and cell structure refinement caused by deformation of the metastable austenite. Therefore, steels with low carbon content do not benefit much from this method.

3) Thermo-Mechanical Treatment of Maraging Steels

In carbon-free maraging steels, grain refinement by austenite deformation does not significantly increase strength but does improve ductility and toughness. In ultra-high strength maraging steels (with tensile strength over 280 kg/mm²) containing large amounts of Co, Mo, and Ti, thermo-mechanical treatment effectively improves fracture toughness through grain refinement of austenite.

4) Deformation During Martensitic Transformation

In austenitic stainless steels and high-Mn steels, deformation above the Ms temperature does not form martensite but still results in significant strengthening.

5) Deformation During Ferrite-Pearlite Transformation

In low-alloy steels with relatively low hardenability, deformation in the transformation region is called isoforming. This produces a fine-grained ferrite structure with dispersed spheroidized carbides, improving strength and toughness. This technique is employed in modern controlled rolling methods.

6) Strengthening by Cold Working of Pearlite

In eutectoid steels with 0.7–0.9% carbon content, cold working up to 70–90% reduction can result in tensile strength over 300 kg/mm².

This is applied in piano wire, which is used in wire ropes, springs, and prestressed concrete due to its high strength and excellent fatigue resistance.

Since intense cold working is required, a preliminary patenting treatment (austenitizing followed by isothermal transformation at ~500°C) is necessary. This results in a fine, uniform pearlite (sorbite) structure, improving ductility and cold workability.

To use piano wire as a spring, bluing (low-temperature heating around 350°C) is performed after cold working. This process removes localized strain, increases the elastic limit through aging, and significantly improves fatigue properties.

7) Strengthening by Controlled Rolling and Controlled Cooling

In low-carbon, normalized-free high-strength steels with small additions of Nb, V, and Ti, precise control of hot rolling and cooling processes allows the steel to achieve high strength and toughness in the rolled state.

This method is gaining attention for its potential to reduce costs by shortening the process. The strengthening mechanisms involved in controlled rolling and cooling are as follows:

1. Lower the slab reheating temperature as much as possible to refine austenite grains before rolling.
2. Deform the steel sufficiently in the lower austenite region to further refine the recrystallized austenite grains. Small additions of Nb and Ti inhibit recrystallization, promoting grain refinement.
3. Deformation just above the Ar₃ transformation temperature stretches the grains and creates deformation bands within the grains. These serve as nucleation sites for ferrite, producing very fine ferrite grains.
4. Continuing deformation into the two-phase region below the Ar₃ temperature further stretches the untransformed austenite grains and increases the density of deformation bands. Meanwhile, in transformed ferrite grains, high dislocation density sub-grains are formed, further refining the structure.
5. After controlled rolling, strength is significantly increased by controlled cooling (e.g., accelerated cooling or interrupted quenching). This is due to ferrite grain refinement and increased pearlite or bainite content.

For high-strength steels with tensile strength around 50 kg/mm², basic compositions include 0.07–0.15% C and 0.8–1.5% Mn. While there is little variation in basic chemistry among steelmakers, the selection of microalloying elements (Nb, V, Ti, REMs) and trace alloying elements (Cu, Ni, Cr, Mo), as well as process parameters such as temperature, degree of deformation, and cooling rate, differ by manufacturer.

Such steels are called TMCP (thermo-mechanical control process) steels. For these, strength increases as processing temperature decreases, but toughness reaches a peak when rolling is performed about 40°C below the Ar₃ temperature (in the dual-phase region). Nb steels, despite their lower carbon content, achieve higher strength than Si-Mn steels without Nb, particularly when rolled just below the Ar₃ temperature.

However, rolling below the Ar₃ temperature can lead to “separation,” a delamination defect in the thickness direction under high through-thickness stress. In such cases, processing temperatures must not be too low.

(2) Austempering

Austempering is a process where steel is quenched from the austenite region into a hot bath held above the Ms temperature and isothermally held until the supercooled austenite completely transforms into bainite, followed by air cooling.

No separate tempering is required for this treatment. As shown in the diagram (not provided), the center and surface of the steel reach the same temperature before the bainitic transformation, minimizing internal thermal gradients and thus reducing residual stress.

This method prevents quenching distortion and cracking. The bainitic structure formed has much better toughness than the structure formed by quenching and tempering.

In general, austempering greatly improves toughness and ductility compared to quench-and-temper treatments. However, one should not overlook the potential impact of temper embrittlement, especially with higher phosphorus content (~0.044%) and tempering temperatures around 315°C.

(3) Marquenching

Marquenching is a process in which steel is quenched from the austenite state into a hot bath just above the Ms temperature, held until the internal and external temperatures equalize, and then air cooled before the supercooled austenite begins to transform isothermally. This results in a slow, uniform martensitic transformation.

Although the cooling to the holding temperature causes surface and core temperature differences, there is no temperature difference during the subsequent martensitic transformation.

This results in slightly lower hardness compared to water-quenched martensite but significantly reduces internal stress, minimizing the risk of cracks or distortion.

If a bath below the Ms temperature is used or if slow cooling follows the bath, some martensite may be partially tempered. However, since most martensite remains untempered, a tempering treatment is required after marquenching.

This method is suitable for steels prone to quenching cracks or distortion, such as high-carbon steels, gauge steels, and bearing steels.

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