Terminology - low alloy high strength steel

Low Alloy High Strength Steel - Introduction

This is a type of weldable low carbon engineering structural steel. Its carbon content is usually less than 0.25%, higher than ordinary carbon structural steel yield point σs or yield strength σ0.2 (30 ~ 80kgf/mm2) and yield ratio σs/σb (0.65 ~ 0.95), better The hot and cold processability, good weldability, low cold and brittle tendency, notch and aging sensitivity, as well as better resistance to atmospheric, seawater and other corrosion. Its alloying element content is relatively low, generally below 2.5%, used in hot rolled state or after simple heat treatment (non-tempered state); therefore, this type of steel can be mass produced and widely used. The output of low-alloy high-strength steels in developed industrial countries accounts for about 10% of steel production (see alloy steels).

At the end of the 19th century, in the early stages of the development of low-alloy high-strength steels, the alloy designs of steels only considered the tensile strength. Higher alloying elements such as Si, Mn, Ni, and Cr are added to the steel to improve the performance in a certain area, but the main means for obtaining high strength still depends on the higher carbon content. As the steel structure develops from riveting to welding, in order to improve the brittle fracture resistance of the steel, the carbon content in the steel and the direction of composite alloying are gradually changed. In the 1950s, in order to save alloying elements, heat treatment methods were used to obtain a good match of strength and toughness. In the 1960s, a new phase called micro-alloying and controlled rolling production began, and some new steel types appeared. In the 1970s, mature micro-pearlite steel and non-pearlite steel, acicular ferrite steel, ultra-low carbon bainite steel, hot-rolled dual-phase steel, and low-carbon martensitic steel were used in oil and gas transmission pipelines and deep wells. It has been popularized and applied in the fields of tubing, automotive steel plates, etc. It is expected that in the 1980s, these steels will occupy an important position in the engineering structural materials. In 1957, China began to develop low-alloy high-strength steels, combined with China's resources to develop a series of steels such as Mn, Mn-V, Mn-Ti, Mn-Nb, and Mn-Mo, with a yield strength of 30 to 70 kgf/mm2. .

Low Alloy High Strength Steel - National Standard

"Low alloy high strength structural steel" (GB/T 1591-2008 replaces GB/T 1591-94)

Low Alloy High Strength Steel - The Role of Alloying Elements

At present, the new low alloy high strength steels are characterized by low carbon (≤0.1%) and low* (≤0.015%). The commonly used alloying elements can be classified according to their role in the strengthening mechanism of steel: solid solution strengthening elements (Mn, Si, Al, Cr, Ni, Mo, Cu, etc.); refinement of grain elements (Al, Nb, V) , Ti, N, etc.); precipitation hardening elements (Nb, V, Ti, etc.) and transformation hardening elements (Mn, Si, Mo, etc.) (see strengthening of metals).

C Form pearlite or disperse alloy carbides in the steel to strengthen the steel. In the microalloyed steel, in order to form a certain amount of carbon-nitride, the carbon content only needs 0.01 to 0.02%; so the carbon reduction is an inevitable trend of the development of such steel, which can greatly improve the toughness and welding performance of the steel.

The high Mn/C ratio of Mn is beneficial for increasing the yield strength and impact toughness of the steel. Manganese can reduce the γ→α transition temperature, favor the nucleation of acicular ferrite, increase the solubility of carbon-nitride formers in γ-Fe, and increase the carbides in ferrite during heating. Dispersion precipitation. In addition, the loss of strength of the Baosinger effect can be offset by the change in the stress/strain characteristics of the steel due to high manganese content.

Si Most low-alloy high-strength steels do not require silicon alloying, but silicon is an indispensable additive element in hot-rolled ferritic-martensite multiphase steels.

Mo Molybdenum-containing steels (~0.15% Mo) have higher strength and higher toughness than conventional ferrite-pearlite steels. Molybdenum has an inhibitory effect on the transformation of pearlite during cooling. The molybdenum content in acicular ferrite steel and ultra-low carbon bainite steel is generally 0.2 to 0.4%.

Nb, V, Ti add 0.05-0.15% Nb (or V, Ti) to low-carbon manganese steel or low-carbon manganese-molybdenum steel with obvious grain refinement and precipitation hardening. Titanium forms in the steel, improving the anisotropy and cold formability of the impact absorption work.

Rare earth (RE) trace (around 0.001%) rare earth metals do not affect the strength of the steel. Its main role is to remove *, it is also the most effective element form control elements, reducing the anisotropy of toughness, to prevent the layered tear of steel.

Other elements such as Ni, Cr, and Cu are not very effective in solid solution hardening in microalloyed steels, and are generally controlled at lower contents in non-quenched and tempered steels.

Low Alloy High Strength Steel - Classification

Low-alloy high-strength steels can be classified into high-strength steels, low-temperature steels, and corrosion-resistant steels according to their main properties and applications:

High-strength steels In addition to high strength, these steels also have good low-temperature toughness. The main features and applications are shown in the table. The output of such steel accounts for more than 80% of the output of low-alloy high-strength steel in China, among which steels with a yield strength of 35-40 kgf/mm2 account for the majority, and the most widely used steel is 16Mn. Low-alloy high-strength steels for low-temperature steel They are ferritic low-temperature steels. The toughness-brittle transition temperature is obtained by increasing the purity of the steel and reducing the content of phosphorus,* in the steel. This type of steel is mainly used for making cryogenic equipment components such as 09Mn2V (-70°C), 06MnNb (-90°C), 3.5% Ni (-100°C), and 06AlNbCuN (-120°C).

Corrosion-resistant steels These steels have a certain degree of corrosion resistance to atmospheric, seawater, *hydrogen and other environments, such as 10MnPNbRE steel resistant to marine atmospheric and seawater corrosion, used in ships, sheet piles, derricks; 12MoAlV steel suitable for the manufacture of oil refining Plant high temperature * hydrogen equipment; 10MoWVNb steel is better at 400 °C hydrogen, nitrogen, ammonia high pressure pipe.

Low alloy high strength steel - production process

Low-alloy high-strength steel can be smelted in open hearth, converter or electric furnace. Since 1979, China has produced Nb, V, Ti low-alloy high-strength steels using the processes of oxygen top-blowing converters - ladle argon blowing - continuous casting slabs - hot continuous rolling, or electric furnace - ladle dusting - heavy plate rolling mills. Steel is generally used after hot rolling. In order to obtain a uniform structure and stable performance, conventional metal heat treatment methods such as high-temperature tempering, normalizing, and tempering are usually used. Non-quenched and tempered plates with a yield strength greater than 60 kgf/mm2 can also be produced by controlled cooling after rolling.

Welding Characteristics of Low Alloy High Strength Steel and Ordinary Low Alloy High Strength Steel

Welding features are as follows:

(1) Hardening tendency of heat affected zone

The hardened tendency of the heat affected zone is one of the important characteristics of low-level steel welding. As the strength level increases, the hardenability of the heat affected zone also increases. In order to slow down the hardening tendency of the heat affected zone, a reasonable welding procedure specification must be adopted. The factors affecting the degree of hardening in the heat affected zone are:

Material and structural forms, such as the type of steel, plate thickness, joint type, weld size, etc.;

Process factors, such as process methods, welding specifications, starting temperature (air temperature or preheat temperature) near the weld.

Welding construction should avoid quenching of the heat affected zone by selecting suitable process factors such as increasing the welding current and reducing the welding speed.

(2) cracks in welded joints

Welding cracks are the most dangerous welding defects. Cold cracks, reheat cracks, hot cracks, lamellar tears, and stress corrosion cracks are common forms of welding.

Some steels tend to have a high degree of hardenability. During the post-weld cooling process, brittle martensite is produced due to the phase change, and cracking is caused by the combined effect of welding stress and hydrogen to form cold cracks. Delayed cracks are welding cold cracks that occur only after a certain period of time (hours, days, or even tens of days) after the welded joints of steel have cooled to room temperature, and therefore are very dangerous. The prevention of delayed cracking can be controlled from the selection of welding materials and strict drying, workpiece cleaning, preheating and interlayer insulation, and timely heat treatment after welding.