The Role of Trace Elements in Steel Heat Treatment

I. Introduction

Steel, as one of the most widely used metallic materials in industry, its performance directly determines the quality and service life of products. Heat treatment is a key process for regulating the properties of steel. Through reasonable combination of heating, holding, and cooling, the internal microstructure of steel is changed to obtain required properties such as strength, hardness, toughness, and wear resistance. In the composition system of steel, in addition to main alloying elements such as carbon, silicon, and manganese, trace elements (such as titanium, niobium, vanadium, boron, rare earth, zirconium, calcium, etc.) with a content usually below 0.1% play an irreplaceable role in the heat treatment of steel. These trace elements interact with carbon, nitrogen, oxygen, and other elements in steel, or change the phase transformation kinetics, or refine the grain, or optimize the morphology of inclusions, significantly affecting the heat treatment effect and final properties of steel, becoming the core breakthrough in the research, development, and production of modern high-performance steel.

II. The Role of Strong Carbonitride-Forming Elements in Steel Heat Treatment

Strong carbonitride-forming elements are the most representative category of trace elements in steel, mainly including titanium (Ti), niobium (Nb), vanadium (V), etc. These elements have a strong affinity for carbon and nitrogen, and are prone to form stable carbides, nitrides, or carbonitrides during the heat treatment process. They regulate the properties of steel through mechanisms such as “pinning grain boundaries” and “refining the structure”, and are key elements to improve the strength, toughness, and high-temperature properties of steel.

(A) Titanium (Ti)

The content of titanium in steel is usually controlled at 0.02%-0.05%, and its core role in heat treatment is to refine austenite grains and improve the welding performance of steel. During the heating stage (such as normalizing, annealing, or quenching heating), titanium in steel will preferentially combine with nitrogen to form TiN. The melting point of TiN is as high as 2950°C, which is much higher than the conventional heat treatment heating temperature of steel (generally 800-1200°C). These fine TiN particles will be uniformly distributed in the steel matrix, pinning on the austenite grain boundaries like “nails”, hindering the migration and growth of grain boundaries, thereby effectively inhibiting the coarsening of austenite grains. For example, in the normalizing treatment of low-carbon high-strength steel, the austenite grain size of the steel grade added with titanium can be controlled at 10-20μm, which is much smaller than 30-50μm of the steel without titanium. The refined grains lay the foundation for forming fine ferrite-pearlite structure during the subsequent cooling process, and finally increase the yield strength of the steel by 15%-25% and the impact toughness by 20%-30%.

In addition, during the quenching and tempering treatment (quenching + high-temperature tempering), titanium can also form TiC with carbon. TiC has a hardness of up to 2800HV and is not easy to dissolve during high-temperature tempering. It will precipitate in the form of dispersed particles, producing a “dispersion strengthening” effect, further improving the hardness and wear resistance of steel. At the same time, the existence of TiC can also inhibit the occurrence of temper brittleness, so that the steel maintains good toughness in the tempering temperature range of 300-500°C. This characteristic makes it widely used in engineering machinery steel, automobile girder steel, and other fields.

(B) Niobium (Nb)

The content of niobium in steel is usually 0.03%-0.06%. Its role in heat treatment has the characteristic of “two-way regulation”, which not only affects the growth of austenite grains during the heating process, but also acts on the phase transformation behavior during the cooling process. It is an important element to improve the hardenability, high-temperature strength, and low-temperature toughness of steel.

During the heating stage, Nb(C,N) formed by niobium with carbon and nitrogen has high stability and is not easy to dissolve below 1100°C, which can effectively hinder the growth of austenite grains and play a role in refining grains. Compared with titanium, the stability of niobium carbides is slightly lower, and they will partially dissolve at higher temperatures (such as 1100-1200°C). The dissolved niobium atoms will re-precipitate fine Nb(C,N) particles during the cooling process. These precipitated particles can not only improve the strength of steel through dispersion strengthening, but also change the phase transformation kinetics, delay the transformation of pearlite, and promote the formation of acicular ferrite or bainite, thereby improving the matching of strength and toughness of steel.

Taking the quenching and tempering treatment of low-alloy high-strength steel as an example, due to the existence of niobium, the hardenability of the steel grade added with niobium is significantly improved during quenching, and martensite structure can be obtained in a thicker section size; during high-temperature tempering (550-650°C), the precipitated Nb(C,N) particles can effectively inhibit the softening of martensite structure, so that the steel has good low-temperature impact toughness (-40°C impact energy ≥47J) while maintaining high strength (yield strength ≥600MPa). In addition, in the heat treatment of heat-resistant steel, the addition of niobium can significantly improve the high-temperature creep strength of steel, because Nb(C,N) can still remain stable at high temperatures (600-700°C), effectively hinder the movement of dislocations, and delay the occurrence of creep deformation. Therefore, niobium is often used in steel grades for high-temperature service scenarios such as power station boiler steel and oil pipeline steel.

(C) Vanadium (V)

The content of vanadium in steel is generally 0.05%-0.10%, and its role in heat treatment is mainly reflected in refining the structure and improving the hardness and wear resistance of steel, especially in quenching and tempering treatment and induction heating quenching.

During the heating process, VC formed by vanadium and carbon has medium stability, and will partially dissolve into austenite at the heating temperature of 800-900°C. The dissolved vanadium atoms will precipitate in the form of fine VC particles during the cooling process (such as the tempering stage after quenching). These VC particles are extremely small (usually 5-10nm) and evenly distributed in the matrix, which can produce a strong dispersion strengthening effect and significantly improve the hardness and strength of steel. For example, in the quenching and tempering treatment of 40CrV steel, after quenching at 860°C and tempering at 500°C, the hardness of the steel can reach 30-35HRC, and the yield strength exceeds 800MPa, which is much higher than that of 40Cr steel without vanadium (hardness 25-30HRC, yield strength about 700MPa).

In addition, the role of vanadium is more significant in induction heating surface quenching. Induction heating is characterized by fast heating speed and short holding time. The dissolution amount of vanadium is less in the process of rapid heating. The undissolved VC particles can hinder the rapid growth of austenite grains, so that the surface can obtain fine martensite structure after quenching, which improves the surface hardness and wear resistance, and avoids surface cracking. Therefore, vanadium is often used in steel for machine tool spindles, gears, crankshafts, and other components that require surface wear resistance and good core toughness, such as 20CrMnTiV steel, 42CrMoV steel, etc.

III. The Role of Boron (B) in Steel Heat Treatment

Boron is a trace element with extremely low content in steel (usually 0.0005%-0.003%), but its role is extremely critical. Its core role in heat treatment is to significantly improve the hardenability of steel, and it is called a “hardenability enhancer”.

The action mechanism of boron in steel is different from that of strong carbonitride-forming elements. It does not form stable compounds but segregates at austenite grain boundaries in the form of atomic state. During the cooling process, the phase transformation of steel (austenite to ferrite and pearlite) starts from the grain boundary first. The segregation of boron atoms at the grain boundary will hinder the formation and growth of ferrite nuclei and delay the transformation process of pearlite, thus creating favorable conditions for the formation of martensite. Even under a slow cooling rate (such as air cooling and oil cooling), the steel added with boron can obtain more martensite structure, while the steel without boron may form a large amount of ferrite-pearlite structure, resulting in a significant reduction in strength.

For example, in the quenching treatment of 40MnB steel, martensite structure can be obtained by oil cooling, and the hardness can reach 50-55HRC; while the 40Mn steel without boron can only obtain pearlite + ferrite structure by the same oil cooling method, and the hardness is only 20-25HRC. This characteristic of boron makes it widely used in structural steel, spring steel, and other steel grades requiring high hardenability, especially suitable for components with large cross-sectional dimensions and slow cooling rates. It can ensure that the core of components obtains the required martensite structure while reducing the quenching cooling intensity (such as replacing water cooling with oil cooling), and reduce the risk of quenching deformation and cracking.

It should be noted that the effect of boron requires high purity of steel. If there are many nitrogen, oxygen, and other elements in the steel, boron will preferentially combine with them to form compounds such as BN and BO, losing the ability to segregate at the grain boundary, resulting in “boron failure”. Therefore, in actual production, elements such as titanium and aluminum are usually added at the same time to make them preferentially combine with nitrogen to protect the effect of boron and ensure that boron can effectively improve the hardenability of steel.

IV. The Role of Rare Earth Elements (RE) in Steel Heat Treatment

The content of rare earth elements (such as lanthanum La, cerium Ce, neodymium Nd, etc.) in steel is usually 0.02%-0.10%. Their role in heat treatment has the characteristics of “diversification”, mainly including purifying molten steel, refining structure, improving the morphology of inclusions, and improving the toughness and corrosion resistance of steel. They are “multifunctional elements” to improve the comprehensive properties of steel.

(A) Purifying Molten Steel and Improving Inclusion Morphology

Rare earth elements have a strong affinity for harmful elements such as oxygen, sulfur, and phosphorus in steel. During the smelting process of steel, they will preferentially combine with these elements to form stable rare earth oxides (such as RE₂O₃), rare earth sulfides (such as RE₂S₃), or rare earth oxysulfides (such as RE₂O₂S). These compounds have low density and will float to the surface of the molten steel during the solidification process of the molten steel, be removed, or form fine and dispersed inclusions, avoiding the formation of coarse inclusions such as MnS and Al₂O₃. During the heat treatment process, these fine rare earth inclusions will not damage the structure of the steel, but can play a role in pinning the grain boundary and refining the grain.

For example, after adding rare earth to bearing steel (GCr15), the size of Al₂O₃ inclusions in the steel is reduced from the original 5-10μm to 1-3μm, and the MnS inclusions are transformed from long strip shape to spherical or short rod shape. In the subsequent spheroidizing annealing and quenching and tempering treatment, the 割裂 effect of spherical inclusions on the matrix is significantly reduced, so that the contact fatigue life of bearing steel is increased by 30%-50%, and the impact toughness is also significantly improved.

(B) Refining Structure and Improving Toughness

Rare earth elements can change the phase transformation kinetics of steel, promote the nucleation of ferrite during the cooling process, increase the amount of ferrite, and refine its grains. For example, in the normalizing treatment of low-carbon steel, the ferrite grain size of the steel grade added with rare earth can be refined from 20-30μm to 10-15μm. The refined grains increase the yield strength of the steel by 10%-15% and the impact toughness by 25%-35%. In addition, during the quenching and tempering process, rare earth elements can also inhibit the coarsening of martensite, promote the uniform formation of tempered sorbite, and reduce the occurrence of temper brittleness, so that the steel maintains a good strength-toughness matching in a wide tempering temperature range.

(C) Improving Corrosion Resistance

Rare earth elements can form a dense oxide film (mainly composed of rare earth oxides) on the surface of steel. This oxide film has good adhesion and stability, which can effectively prevent the penetration of corrosion media such as oxygen and moisture into the interior of steel, thereby improving the corrosion resistance of steel. In the heat treatment of stainless steel, after adding rare earth, the intergranular corrosion resistance of steel is significantly improved, because rare earth can inhibit the segregation of carbon at the grain boundary, reduce the precipitation of Cr₂₃C₆, avoid the occurrence of grain boundary chromium depletion, and thus enhance the corrosion resistance of stainless steel.

V. The Role of Zirconium (Zr) and Calcium (Ca) Elements in Steel Heat Treatment

(A) Zirconium (Zr)

The content of zirconium in steel is usually 0.02%-0.05%, and its role is similar to that of titanium, belonging to strong carbonitride-forming elements. However, the stability of zirconium carbide (ZrC) and nitride (ZrN) is higher, and they can still remain stable at higher heating temperatures, so they are more used in high-temperature heat-treated steel grades.

During the heating process, ZrN and ZrC can effectively hinder the growth of austenite grains. Even at high-temperature heating of 1200-1300°C, the austenite grain size of the steel added with zirconium can still be controlled below 20μm. During the cooling process, zirconium carbides can also act as the nucleation core of ferrite, refine ferrite grains, and improve the strength and toughness of steel. In addition, zirconium can also improve the high-temperature oxidation resistance of steel in heat-resistant steel, because zirconium can form stable ZrO₂ with oxygen in the steel to prevent oxygen from diffusing into the steel. Therefore, zirconium is often used in high-temperature service scenarios such as aviation engine blade steel and high-temperature furnace tube steel.

(B) Calcium (Ca)

The content of calcium in steel is extremely low (usually 0.001%-0.005%), and its main role in heat treatment is to improve the morphology of sulfide inclusions, reduce the hot brittleness tendency of steel, and improve the machinability of steel.

Sulfur in steel usually forms long strip-shaped MnS inclusions with manganese. These inclusions will extend along the rolling direction during hot rolling, resulting in a significant reduction in the transverse toughness of steel and easy occurrence of hot brittleness. After adding calcium, calcium will combine with sulfur to form spherical or short rod-shaped CaS inclusions. These inclusions are not easy to deform during hot rolling, which can effectively reduce the 割裂 effect on the steel matrix. During the heat treatment process, spherical CaS inclusions have little effect on the structure of the steel, which can make the transverse and longitudinal toughness of the steel tend to be consistent and significantly improve the mechanical property uniformity of the steel. In addition, calcium can also improve the machinability of steel. During the cutting process, CaS inclusions can play a lubricating role, reduce the wear of tools, and improve the cutting efficiency. Therefore, calcium is often used in free-cutting steel, automobile parts steel, and other steel grades requiring good processing performance.

VI. Synergistic Effect of Trace Elements and Application Precautions

(A) Synergistic Effect

In the actual research, development, and production of steel grades, the role of a single trace element is often limited. Through the composite addition of multiple trace elements, the synergistic effect of “1+1>2” can be achieved. For example, when titanium and niobium are added in combination, TiN can hinder the growth of austenite grains in the low-temperature heating stage, while Nb(C,N) can further inhibit grain coarsening in the high-temperature heating stage. The synergistic effect of the two can make the steel maintain fine austenite grains in a wider heating temperature range; when boron and rare earth are added in combination, rare earth can purify the molten steel, reduce the consumption of nitrogen on boron, ensure that boron can effectively segregate at the grain boundary, improve the hardenability of steel, and at the same time, rare earth can refine the structure and further improve the toughness of steel.

(B) Application Precautions

Although trace elements play a significant role in steel heat treatment, it is not that the higher the content, the better. Excessive addition of trace elements will lead to the formation of coarse inclusions (such as coarse TiN and ZrC) in the steel. These inclusions will become stress concentration sources, which will instead reduce the toughness and fatigue performance of the steel. For example, when the content of titanium exceeds 0.06%, TiN inclusions with a size of more than 5μm will be formed, resulting in a significant decrease in the impact toughness of the steel. Therefore, in practical applications, the content of trace elements needs to be strictly controlled, and the optimal addition amount of trace elements is determined according to the performance requirements of the steel grade and the heat treatment process.

In addition, the role of trace elements is also closely related to the matrix composition of steel and the process parameters of heat treatment (such as heating temperature, holding time, cooling rate). For example, boron has a significant effect on improving the hardenability in low-carbon steel, but the effect is weak in high-carbon steel; the dispersion strengthening effect of vanadium in quenching and tempering treatment needs to be matched with a suitable tempering temperature (generally 500-600°C). If the tempering temperature is too high, the VC particles will coarsen and the strengthening effect will be weakened. Therefore, when applying trace elements, it is necessary to carry out targeted design and optimization in combination with the specific composition of steel and the heat treatment process.