The introduction of some concepts related to steel is as follows:
(1) High-quality carbon structural steel (GB/T699-1999) This type of steel contains carbon (C) and a certain amount of silicon (Si) for deoxidation (generally not exceeding 0.40%) and manganese (Mn) (generally not exceeding 0.80%, but can be as high as 1.20%), and does not contain other alloying elements (except for residual elements). This type of steel must ensure both chemical composition and mechanical properties. The content of impurity elements such as sulfur (S) and phosphorus (P) is generally controlled to be below 0.035%. If controlled below 0.030%, it is called high-quality steel, and the grade should be marked with an 'A', for example, 20A; if P is controlled below 0.025% and S below 0.020%, it is called special high-quality steel, marked with an 'E' for distinction. The content of other residual alloying elements brought into the steel from raw materials, such as chromium (Cr), nickel (Ni), copper (Cu), etc., is generally controlled at Cr ≤ 0.25%, Ni ≤ 0.30%, Cu ≤ 0.25%. Some grades have manganese (Mn) content reaching up to 1.40%, known as manganese steel. This type of steel improves mechanical properties by adjusting the carbon (C) content; therefore, based on the carbon content, this type of steel can be divided into: Low-carbon steel -- generally contains less than 0.25% carbon, such as grades 10, 20; Medium-carbon steel -- generally contains between 0.25% to 0.60% carbon, such as grades 35, 45; High-carbon steel -- generally contains more than 0.60% carbon. This type of steel is usually not used for manufacturing pipes. In fact, there are no clear boundaries in their carbon content. This type of steel has a large output and wide applications; it is often rolled or forged into round, square, flat profiles, plates, and seamless pipes. It is mainly used for manufacturing general structures and mechanical structure parts as well as building structural components and pipelines for fluid transport. Depending on usage requirements, it may sometimes need to be heat-treated (normalizing or tempering) before use. (2) Carbon structural steel (GB/700-1999) This type of steel is generally smelted in converters or open-hearth furnaces with iron water plus scrap metal as its main raw materials; the sulfur and phosphorus content in this type of steel is higher than that in high-quality carbon structural steels, typically sulfur ≤ 0.050%, phosphorus ≤ 0.045%. The content of other alloying elements brought into the steel from raw materials like chromium, nickel, copper generally does not exceed 0.30%. According to composition and performance requirements, this type of steel's grades are represented by Q195, Q215A/B, Q235A/B/C/D, Q255A/B, Q275 etc. Note: 'Q' stands for the pinyin capital letter 'qu' meaning yield strength; the following number indicates the minimum yield point value (σs); subsequent symbols classify according to impurity element contents (sulfur and phosphorus) from high to low along with variations in carbon and manganese elements into four categories: A,B,C,D. This type of steel has the largest output and wide applications; it is often rolled into plates or profiles (round,square flat,I-beams,U-channels,L-shaped etc.)and also used to manufacture welded pipes.It is mainly used in buildings like factories bridges ships etc.and pipelines for general fluid transport.This type ofsteel usually does not undergo heat treatment before use. (3) Low-alloy high-strength structural steel(GB/T1591-1994) This type ofsteel contains certain amounts of silicon or manganese basic elements along with other trace elements suitable for China's resource situation,such as vanadium(V),niobium(Nb),titanium(Ti),aluminum(Al),molybdenum(Mo),nitrogen(N),and rare earth(RE).According to chemical compositionand performance requirements,this typesteel's grades are represented byQ295A,B,Q345A,B,C,D,E,Q390A,B,C,D,E,Q420A,B,C,D,E,Q460C,D,E etc.,with meanings similar to thoseofcarbonstructuralsteel. 'V,Nb,T i ,Al'andothertraceelements refine grain size.Inthis typeofsteel exceptfor A,B grade steels C,D,E grade steels should contain at least oneof these traceelements;to improve performance,A,Bgrade steels may also add oneof these.Additionally,the residual element contents Cr ,Ni ,Cu shouldnot exceed O .30 %.Q345 A ,B ,C ,D ,E are representative gradesofthis typeofsteel where A ,B grade steelsare usually referredtoas16Mn ; C gradeandabove must include morethanone traceelement which increasesoneitemlow-temperatureimpactperformanceinmechanicalproperties. This typeofsteelcomparedwithcarbonstructuralsteels has advantages suchas highstrength,greatcomprehensiveperformance,longservice life,widerangeofapplications,and cost-effectiveness.Thissteelis oftenrolledinto platesprofiles,seamlesspipes,etc.andiswidelyusedinbridges,vessels boilers vehicles,andimportantbuildingstructures. (4) Alloy structuralsteel(GB/T3077-1999) This typeofsteelis basedonhigh-qualitycarbonstructuralsteelswithappropriateadditionofoneormorealloyingelementsaimedatimprovingmechanicalproperties,toughness,andhardening ability.Products madefromthistypeofsteelusuallyrequireheat treatment(normalizingortempering);theparts producedfromit typicallyneedto undergo tempering or surface chemical treatments(carburizing,nitriding,etc.),surface hardeningorhigh-frequencyhardeningbeforeuse.Hence,basedonchemicalcomposition(mainlycarboncontent),heat treatmentprocesses,anddifferentuses,thistypeofsteelmightbe roughlydividedintocarburized,toughened,andnitridedsteels. This typeofsteelis oftenrolled(forged)into round,square,andflatprofilesandseamlesspipes,makingit suitableforproducingimportantlargerpartsandcomponentsinmechanicalproductsaswellashigh-pressurepipelines,vessels,etc.;thishigh-qualitycarbonstructuralsteelis characterizedbyexcellentcomprehensivemechanicalproperties. Seamless pipes madefromthistypeofsteelarewidelyusedinhydraulic supports,hightemperaturegas cylinders,hightemperatureboilers,fertilizer equipment,petrochemical cracking,motor vehicle half shafts,diesel engines,hose fittings,etc. (5) Stainless Steel(GB/T1220-92、GB/T1221-92) Accordingtoindustrialmainuses，stainlesssteelisdividedintotwomajorcategories：stainlessacid-resistantsteelandheat-resistantnon-scalingsteel. Austenitic stainless steel is divided into two types: stainless steel and acid-resistant steel. Steel that can resist corrosion in the air is called stainless steel; steel that can resist corrosion in various highly corrosive media is called acid-resistant steel. Stainless steel does not necessarily resist acid corrosion, but acid-resistant steel has good stainless properties. This type of steel contains five major elements (C, Si, Mn, P, S) in addition to mainly chromium and nickel alloy elements. For example, chromium steel (Cr13 type), chromium-nickel steel (18-8 type); it may also contain one or more of molybdenum, niobium, titanium, nitrogen and other elements based on chromium-nickel steel. Products made from this type of steel such as rods, plates, strips, wires, and pipes are widely used in chemical equipment, medical devices, food industry equipment as well as household kitchenware and decorations for hotels and restaurants. B. Heat-resistant non-scaling steel is also divided into heat-resistant and non-scaling types. Steel that resists medium erosion at high temperatures without scaling is called non-scaling steel; while heat-resistant steel resists medium erosion at high temperatures with sufficient strength without scaling. This type of steel contains not only the chromium and nickel elements found in stainless acid-resistant steels but also one or more alloying elements such as aluminum (Al), tungsten (W), cobalt (Co), vanadium (V). Products made from this type of steel such as rods, plates, strips, pipes are widely used in petrochemical equipment, power station boilers, gas turbines and components for industrial heating furnaces as well as components working at high temperatures. | 
