The application status and processing technology of cold extrusion forming steel for steering tie rod at home and abroad are described. The main technical indicators and the performance of the corresponding parts of ordinary quenched and tempered steel, pre-quenched and tempered steel and non-quenched and tempered cold heading steel for steering tie rod are introduced. Production practice shows that compared with ordinary quenched and tempered steel ML40Cr and pre-quenched and tempered steel ESW90, the strength and plasticity of automobile steering tie rods produced by non-quenched and tempered cold heading steel 30MnVS6 can reach the same level, but the impact toughness is significantly lower. The research progress of improving the strength and toughness of non-quenched and tempered cold heading steel is discussed from the four aspects of chemical composition, microstructure, non-metallic inclusion and segregation. By normalizing treatment, the impact energy KV2 of steel 30MnVS6 increases from hot-rolled status 62 ~67 J to 108 ~ 115 J.

The technological process  of C70S6BY  steel  for automobile  engine  connecting rod is  130 t BOF-LF-RH- CCM-Heating-Rolling.   The content   of   S   and   N   in   steel(0.064%～0.068%Sand   136×10 ^-6～142×10 ^-6N) can   be   accurately  controlled  by  using1.5~2.0  low  basicity  CaO-SiO ₂-Al ₂O ₃-MgO  slag  system  in  LF  refine,nitrogen  driving  in  RH and for 5～10 min circulation after pump withdrawal. The microstructure of the steel is pearlite and a small amount of fer- rite by slow cooling in pit after rolling,the non-metallic inclusions in the steel are mainly MnS and composite inclusions.Of 1050℃  30  min  air  coling   steel,the  tensile   strength  is 995~1018  MPa,the  yield   strength  is   586~611  MPa,the  elongation   is   11.5%～13.0%,the   reduction   of  area   is   22%~24%,and   the   HB  brinell   hardness   number   is   285~301. All   indexes meet the requirements of breaking connecting rod.

The process of developed C70S6BY non-quenched and tempered hot-rolled round steel is 80 t top-bottom combined blowing converter-LF+VD refining-240 mm×240 mm bloom continuous casting-Φ60 mm round steel continuous  rolling-slow  cooling-finishing  and  non-destructive  testing process.  The  Al ₂O ₃-CaO-SiO ₂  slag  with basicity4～8  has been  used, LF refining    time is 45~60 min; after LF, the[S]≤0.010% and [O]≤0.001%; sulphur alloying is carried out  after  VD,[S]  is  controlled  at  0.063%~0.065%.Before  VD  tapping,100～150m  of  magnesium-containing  cored  wire  is  fed, and  the  yield  of  Mg  is  11%  to17%, and  the  ratio  of magnesium  to  sulphur  content  is  approximately 1.56%.The  superheat  of  the  tundish  liquid  is   15~30  ℃,and  the  casting  speed  is 0.9  m/min. After  normalizing, the  tensile  strength  and yield  strength  of test  steel  are  957  MPa  and  563  MPa.  The material  elongation  and reduction  of area  are  12%  and  21%.After  the  magnesium  modification  treatment, the  percentage  of sulphide  inclusions  with  length-width  ratio  1~2  reaches more than 55%.

As compared with no-adding Te( 0Te ) steel, in contening 0.005 9% Te(Te/S = 0.098 )Φ38 mm bar of 46MnVS steel rolled from 0480 mm cast bloom the average equivalent diameter and average area of MnS inclusions are increased by 27% and 72% , respectively ， the number of spherical or ellipsoidal inclusions with length-width ratio of 1～3 is increased by 21% ,and the fine sulfide rating decreases from 3.0 to 2.0. Te will be dissolved in MnS to form ( Mn,Te )S, which coarsens and spheroidizes sulfides in steel, to increase ellipsoidal and spindle sulfides in rolled products. The HV10 microhardness value of MnS inclusions increased from 142. 2 to 152.1 by Te modification, to improve the ability of MnS to resist rolling deformation.

Non-quenched and tempered steel C38 + N for automobile can be used into not only crank shaft materials, but also produced engine connecting rods. By optimizing chemical composition (/% ：0.36 ~ 0.40C ,0.50 ~ 0.65Si, 1.40 ~ 1.55Mn,0.10 ~0.20Cr,0.015 ~0.020N,0.020 ~0.035S, ≤ 0.025P,0.003 ~0.015Al) and by feeding S wire to control sulfur content in steel etc, measures, the mechanical properties of steel C38 + N are Rp 0.2506 ~ 544 MPa Rm 854 ~ 879 MPa,A 12% ~23.5% and Z 26% - 30% to meet the requirement of standard.

The production process of non-quenched-tempered steel 38MnVS6 (/% ：0.36 ~ 0.40C ,0.50 ~ 0.65Si, 1.30~1.45Mn,  ≤ 0.020P, 0.045 ~0.065S, 0.10 ~0.30Cr, 0.015 ~0.030A1, ≤0.05Mo, 0.08 ~0.12V, 0.013 ~0. 019N, 0.015 ~0.025Ti) is 60 t LD-LF-VD-CC-Roll process. In order to produce 80 mm bars, the following methods would be used. The LD end point [C] is control above at 0. 01% together with [P] below at 0.015%. The basicity of LF refining slag and Al ₂O ₃ content are controlled at 3.0 ~ 4. 0 and 20% ~25% respectively. The S line would be fed after VD treatment to ensure [S] controlled at 0. 055% into molten steel. The low basicity mould fluxes (R = 0. 65) has been used in continuous casting process. It is necessary to controlled final temperature of rolling steel at 900 °C , and the 080 mm bar should be slowly cooled after rolling. Through the detection and analysis of 38MnVS6 bar, it was found that class A inclusions (thickness series) is grade 1.5,and the rating of center porosity is 1.5,and the grain size is grade 7. The performance of bar product is very good.

The new energy automobile motor shaft made of non-quenched and tempered steel 50MnSiV (/% ：0.50C, 0.52Si, 1.20Mn, 0.010P, 0.025S, 0.15Cr, 0.10V, 0.015Ti, 0.015N) by eliminating traditional carburizing heat treat¬ment, straightening and finishing process of steel 20CrMnTiH, can not only improve 95% materials utilization rate and 2 delivery days and can also reduce 25% costs. With tensile strength 1100 MPa, yield strength 858 MPa and impact energy 48 J, its static torque (4697 Nm),fatigue life ( ±1600 Nm bidirectional torsion 120 ,000 times, ± 1400 Nm bidirectional torsion 270,000 times) increased by 27% and 93% 〜140% respectively than that of carburized 20CrMnTiH motor shaft, and its performance index and 80,000 km road test results meet motor shaft technical requirements.

The 5. 0 mm strip of developed tested steel 510CL (/% : 0. 08C, 0. 07Si, 1. 25Mn, 0. 008P, 0. 005S, 0. 023Nb, 0. 018Ti) is produced by semi-steel desulphurization- 200 t top and bottom combined blowing converter-LF-230 mm x 1 650 mm slab casting-continuous rolling process. In order to ensure the better welding performance of wheel steel, the 500 MPa high strength wheel steel is developed by low carbon Nb-Ti microalloying combined TMCP ( Thermo-Mechanical Control Process) technology. The hole-expansion rate of tested steel is more than 100% , the welding performance is better, the cold-forming performance and the fatigue life of steel all meet the user technical index, and after application the test steel is up to design requirement.

The production flowsheet of developed high carbon microalloying non-quenched-tempered steel C70S6 (/% : 0. 68~0.73C, 0.15~0.25Si, 0.45~0.55Mn, ≤0.030P, 0.055~0.070S, 0.10~0.15Cr, 0.04~0.08Ni, 0.03~0.04V, 0.0120~0.0140N) is 120 t top and bottom combined blowing converter-LF-RH-240mmx240mm bloom casting-φ39 mm bar rolling. Production results show that with process measures including controlling Mn/S≥3, using basicity [(CaO )/( SiO ₂ ) ] 4.32LF refining slag (/% : 59.87CaO, 13.86SiO ₂, 5.55MgO, 16.34A1 ₂O ₃, 0.64S, 0.072MnO) , liquid superheating extent 15~30℃，casting speed 0.65 m/min, weak secondary water rate 0.5 L/t, using air mist spray cooling at M section of secondary water cooling and finishing rolling temperature at ≤880°C , the mechanical properties of high carbon microalloying non-quenched-tempered steel C70S6 are tensile strength ≥950 MPa, yield strength ≥550 MPa, elongation ≥10% , reduction of area ≥20% and HB hardness value 240~290, and each index all meets the requirement for fracture splitting connecting rod.

The production process flowsheet of steel C38N2 (/% : 0. 36 〜0.40C, 0. 50 ~0. 65Si, 1.40 ~ 1. 55Mn, ≤ 0.025P, 0.010 ~0.040S, 0. 10 ~0,25Cr, ≤ 0.01A1, 0.015 ~0.020N) is 120 t-BOF-LF-RH-410 mm x 530 mm bloom casting-continuous rolling to Φ170 mm products. The statistics of effect of refining slag basicity (2. 0 ~ 2. 5 ) , Ca content in finished steel (0 ~ 0.075%), tundish shielding agent basicity (5. 94 and 1.70), secondary cooling water (0. 14 ~0. 24 L/kg) and mould electromagnetic stirring current (250 A and 100 A) on sulfide in steel has been analyzed. With process measures including using 2 ~3 low basicity slag .in refining, controlling [ Ca]/[ S]>0. 03 , using tundish shielding agent basicity 1. 70, secondary cooling water rate 0. 24 L/kg and mould electromagnetic stirring current 100 A, the ratio of rating ≤2. 5 of A-thin series sulfide in steel increases from 80% before process improvement to 95% , and the fine, dispersed distributed spheroidal and spindle-shaped sulfide in steel increases markedly.

The microstructure and mechanical properties of V-Ti,Nb-V-Ti microalloyed non-quenched and tempered steel were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy. The results show that the impact toughness can reach 50 J when rolling ratio is greater than or equal to 10 in the Nb-V-Ti microalloyed non-quenched and tempered steel. While the similar impact toughness is achieved when the rolling ratio reach and more than 15 in V-Ti non-quenched and tempered steel. According to the same rolling ratio, it can be found that impact toughness of Nb-V-Ti microalloyed steel is better than V-Ti non quenched and tempered steel. Niobium addition could raise the coarsening temperature of austenite, effectively inhibit quick growth of austenite, and reduce the grain size and space of pearlite lamellar. The rod cemenite gradually transformed into granular of spherical cemenite. Additionally, Nb could promote the precipitation of fine-dispersed carbides in V-Ti alloyed non-quenched and tempered steel. Therefore, after non-re- crystfdlization region deformation, the Nb-V-Ti alloyed steel could obtain the fine dual phase structure with ferrite and pearlite, and the relatively 900 ℃ temperature large deformation could further improve the strength and toughness of Nb-V- Ti non-quenched and tempered steel.

The steelmaking process flowsheet of tested sulphur bearing non-quenching and tempering steel 36MnVNS4 (/% : 0. 36C, 0. 66Si, 1. 00Mn, 0. 010P, 0. 045S, 0.26V, 0. 011 ON) is hot metal + scrap- 70 t EBT EAF-LF-square billet casting-rolling. The effect of LF both slag series- 19. 82% Al ₂0 ₃, ( CaO)/( SiO ₂) = 2. 64 and 14. 63% Al ₂O ₃, (CaO)/( SiO ₂) =2. 15 on oxygen content in liquid after soft argon blowing, S yield after feeding S wire, and compound and morphology of inclusions in steel has been studied. Results show that refining by high basicity white slag is available to decrease the oxygen content in steel, but it is unfavourable to stabilize the sulphur content in steel； decreasing the refining slag basicity ( CaO)/( SiO ₂) from 2. 64 to 2. 15 is favourable to stabilized control the sulphur content in steel, and the yield of sulphur increases from 35% to 75% ； with both refining process the distribution, morphology and ingredient of inclusions in steel are basically the same. By calcium treatment in ladle the long strip MnS inclusions modify to spheroid compound inclusions.

The production flowsheet for non-quenched and tempered steel F45MnVS is 50 t UHP EAF-LF-VD-260 mm x300 mm and 180 mm x 220 mm billet casting-rolling to Φ20-Φ160 mm products. According to analysis on micro structure of steel, the hot heading crack is caused by block and lamella MnS and attached with Al ₂O ₃ -MnO-FeO compound oxide. With using the process measures including controlling Al content in steel 0.010%-0.030% , EAF end [C]≥0.20% and end [P]≤0.025% ,[Mn]/[S]＞20, LF refining slag basicity ≥3.0, after VD soft argon blowing for ≥ 12 min, ensuring homogeneous distribution of sulfur in steel, superheating extent of liquid in tundish 20-30 °C, controlling casting speed to prevent MnS segregation, and controlling finishing rolling temperature 850-1000 °C and cooling rate after rolling 2-4℃/s, the main inclusions in steel is long lath MnS, the hot heading test is all qualified without crack and other defects.

The production flowsheet of Φ35 mm bar of non-quenching-tempering steel C70S6 (/% ： 0.72C, 0.20Si, 0.58Mn, 0.015P，0.065S，0.12Cr，0.05Ni， 0.035V， 0.008Al) is 60 t UHP EAF-LF-VD-240 mm x240 mm bloom casting-rolling process. The analysis on structure, fracture and shearing technology of cracking bar during shearing process is carried out. It is obtained that the structure and hardness of cracking bar are no-obvious different to the non-cracking bar during shearing, and during shearing process by shearing stress action the shift of bar along horizontal plane occurs led to increasing the interval between flat cutter and round cutter, the deformation of bar shearing plane curving and deforming, finally causing crack at end of bar. With the measures including decreasing the interval between cutters from 0.5 mm to 0.3 mm, increasing clamping force from 3 t to 3.5 t and carried out spraying water on steel bar to enlarge coefficient of wear as installing cutters, the results of cutting 30 t steel C70S6 Φ35 mm bar products show that the rate of cutting cracking decreases to 0 from original 10% .