Non-quenched and Tempered Round Steel with High Strength, High Toughness and Easy Cutting and Manufacturing Method Therefor

Abstract

Disclosed is a non-quenched and tempered round steel with high strength, high toughness and easy cutting, comprising the following chemical elements in percentage by mass: C: 0.36-0.45%, Si: 0.20-0.70%, Mn: 1.25-1.85%, Cr: 0.15-0.55%, Ni: 0.10-0.25%, Mo: 0.10-0.25%, Al: 0.02-0.05%, Nb: 0.001-0.040%, V: 0.10-0.25%, S: 0.02-0.06%, and the balance being Fe and inevitable impurities. Also disclosed is a method for manufacturing the non-quenched and tempered round steel, comprising the steps of: S1: smelting and casting; S2: heating; S3: forging or rolling; and S4: finishing. The non-quenched and tempered round steel with high strength, high toughness and easy cutting described above has high strength, good impact toughness, elongation and cross-sectional shrinkage, and has good cutting performance and fatigue resistance, and can be used in situations requiring a high-strength steel material, such as automobiles and engineering machinery.

Claims

1. A non-quenched and tempered round steel with high strength, high toughness and easy cutting, comprising the following chemical elements in percentage by mass: C: 0.36-0.45%; Si: 0.20-0.70%; Mn: 1.25-1.85%; Cr: 0.15-0.55%; Ni: 0.10-0.25%; Mo: 0.10-0.25%; Al: 0.02-0.05%; Nb: 0.001-0.040%; V: 0.10-0.25%; S: 0.02-0.06%; and the balance being Fe and inevitable impurities.

2. The non-quenched and tempered round steel as claimed in claim 1, wherein the steel further comprises Cu, and the content of Cu is 0<Cu0.25% in percentage by mass.

3. The non-quenched and tempered round steel as claimed in claim 1, wherein among the inevitable impurities, the content of each chemical element in percentage by mass satisfies at least one of: P0.015%; N0.015%; O0.002%; Ti0.003%; and Ca0.005%.

4. The non-quenched and tempered round steel as claimed in claim 1, wherein the non-quenched and tempered round steel has a value of an ideal critical diameter for hardenability DI of 5.0-9.0; wherein the ideal critical diameter for hardenability DI is calculated according to the following formula,
DI=0.54*C*(5.10*Mn1.12)*(0.70*Si+1)*(0.363*Ni+1)*(2.16*Cr+1)*(3.00*Mo+1)*(0.365*Cu+1)*(1.73*V+1) wherein each chemical element in the formula represents the numerical value before the percentage sign of the mass percentage of the corresponding chemical element.

5. The non-quenched and tempered round steel as claimed in claim 1, wherein the non-quenched and tempered round steel has a microalloying element coefficient r.sub.M/N of 1.1-9.9; wherein the microalloying element coefficient r.sub.M/N is calculated according to the following formula,
r.sub.M/N=([Al]/2+[Nb]/7+[V]/4)/[N] wherein each chemical element in the formula represents the numerical value before the percentage sign of the mass percentage of the corresponding chemical element.

6. The non-quenched and tempered round steel as claimed in claim 1, wherein the non-quenched and tempered round steel has a carbon equivalent Ceq of 0.60-1.0%; wherein the carbon equivalent Ceq is calculated according to the following formula: Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Ni]+[Cu])/15 wherein each chemical element in the formula represents the numerical value before the percentage sign of the mass percentage of the corresponding chemical element.

7. The non-quenched and tempered round steel as claimed in claim 1, wherein the non-quenched and tempered round steel has a microstructure comprising bainite, and on any cross-section of the non-quenched and tempered round steel, the area of the bainite accounts for 85% or more of the area of the cross-section.

8. The non-quenched and tempered round steel as claimed in claim 7, wherein the non-quenched and tempered round steel has a bainite transformation temperature T.sub.B of 515-565 C.; wherein the bainite transformation temperature T.sub.B is calculated according to the following formula: T.sub.B=830270*C90*Mn37*Ni70*Cr83*Mo wherein each chemical element in the formula represents the numerical value before the percentage sign of the mass percentage of the corresponding chemical element.

9. The non-quenched and tempered round steel as claimed in claim 7, wherein the microstructure of the non-quenched and tempered round steel further comprises residual austenite and at least one of ferrite or pearlite.

10. The non-quenched and tempered round steel as claimed in claim 1, wherein the non-quenched and tempered round steel has a tensile strength R.sub.m of greater than or equal to 1000 MPa, an elongation A of greater than or equal to 12%, a cross-sectional shrinkage Z of greater than or equal to 35%, and a Charpy impact energy A.sub.ku of greater than or equal to 27 J.

11. A method for manufacturing a non-quenched and tempered round steel, comprising the following steps: S1: smelting and casting; S2: heating; S3: forging or rolling; and S4: finishing; wherein the non-quenched and tempered round steel comprising the following chemical components in percentage by mass: C: 0.36-0.45%; Si: 0.20-0.70%; Mn: 1.25-1.85%; Cr: 0.15-0.55%; Ni: 0.10-0.25%; Mo: 0.10-0.25%; Al: 0.02-0.05%; Nb: 0.001-0.040%; V: 0.10-0.25%; S: 0.02-0.06%; and the balance being Fe and inevitable impurities.

12. The manufacturing method as claimed in claim 11, wherein at least one of the following manufacturing process conditions is satisfied: in the step S2, the temperature of the heating is controlled at 1050-1250 C., and kept for 3-24 h; in the step S3, a final rolling temperature or a final forging temperature is controlled to be 800 C. or higher, and cooling is performed after the rolling or the forging.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] FIG. 1 is a microstructure metallograph of a cross section of a non-quenched and tempered round steel in example 2 under a 500-fold optical microscope; and [0057] FIG. 2 is a microstructure metallograph of a cross section of a crankshaft prepared by the non-quenched and tempered round steel in example 2 under a 500-fold optical microscope.

DETAILED DESCRIPTION

[0058] Embodiments of the present invention will be described below through particular specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this description. Although the description of the present invention will be introduced in combination with the preferred examples, it does not mean that the features of the present invention are limited to these embodiments. On the contrary, the purpose of introducing the present invention in combination with embodiments is to cover other options or modifications that may be extended based on the claims of the present invention. In order to provide a thorough understanding of the present invention, the following description will contain many specific details. The present invention can also be implemented without these details. In addition, in order to avoid confusing or obscuring the focus of the present invention, some specific details will be omitted in the description. It should be noted that the examples in the present invention and the features in the examples can be combined with each other in the case of no conflict.

Examples 1-6 and Comparative Examples 1-4

[0059] The non-quenched and tempered round steels with high strength, high toughness and easy cutting in Examples 1-6 are all prepared by the following steps: [0060] S1: performing smelting and casting according to the chemical compositions shown in following Tables 1-1 and 1-2: wherein the smelting can be performed in a 50 kg or 150 kg vacuum induction furnace, or smelting can be performed in a manner of an electric furnace smelting+refining outside the furnace+vacuum degassing; [0061] S2: heating: the temperature of the heating being controlled at 1050-1250 C., and kept for 3-24 h; [0062] S3: forging or rolling: controlling the final rolling temperature or final forging temperature to be 800 C. or higher; and performing cooling after the rolling or the forging, wherein the cooling speed is controlled to be smaller than or equal to 1.5 C./s, and the cooling manner can be air cooling or wind cooling; and [0063] S4: finishing, e.g., skinning.

[0064] It should be noted that in the step S3, when forging is performed, a steel ingot is directly forged to a final product size; while when rolling is performed, a billet can be directly rolled to a final product size, or the billet is firstly rolled to a specified intermediate billet size, and then subjected to intermediate heating and rolling to the final product size.

[0065] The specific manufacturing process of the non-quenched and tempered round steels in Examples 1-6 and the comparative steels in Comparative Examples 1-4 is as follows.

[0066] Example 1: smelting is performed on a 50 kg vacuum induction furnace according to the chemical compositions shown in the following Tables 1-1 and 1-2. Molten steel is cast into a steel ingot, and the steel ingot is heated and is forged into a billet, wherein the heating temperature is 1050 C., and then the forging is performed after holding the temperature for 3 h and a bar with a diameter of 60 mm is finally formed, wherein the final forging temperature is 910 C., and then performing air cooling after the forging.

[0067] Example 2: smelting is performed on a 150 kg vacuum induction furnace according to the chemical compositions shown in the following Tables 1-1 and 1-2. Molten steel is cast into a steel ingot, and the steel ingot is heated and is forged into a billet, wherein the heating temperature is 1100 C., and then the forging is performed after holding the temperature for 4 h and a bar with a diameter =92 mm is finally formed, wherein the final forging temperature is 1000 C., and then performing wind cooling, and skinning by turning to =90 mm.

[0068] Example 3: performing an electric furnace smelting according to the chemical compositions shown in Tables 1-1 and 1-2, LF refining and VD vacuum treatment, then casting into a 320 mm425 mm continuous casting billet. The continuous casting billet is first heated to 600 C. in a preheating section, and is continually heated to 980 C. in a first heating section and kept at this temperature, then is continually heated to 1200 C. in a second heating section and kept at this temperature for 8 h, and enters a soaking section at a temperature of 1220 C. and is kept at this temperature for 4 h, and then subjected to subsequent rolling. After leaving the heating furnace and being descaled by high-pressure water, the billet is rolled, and is finally rolled into a bar with =100 mm, wherein a final rolling temperature is 1000 C. The bar is subjected to air cooling after the rolling and is tested by ultrasonic flaw testing and magnetic particle flaw testing and the like.

[0069] Example 4: performing an electric furnace smelting according to the chemical compositions shown in Tables 1-1 and 1-2, LF refining and VD vacuum treatment, then casting into a 280 mm280 mm continuous casting billet. The continuous casting billet is first heated to 620 C. in a preheating section, then is continually heated to 950 C. in a first heating section and kept at this temperature, and is continually heated to 1150 C. in a second heating section and kept at this temperature for 6 h, and then enters a soaking section at a temperature of 1200 C., is kept at this temperature for 2 h, and subjected to subsequent rolling. After leaving the heating furnace and being descaled by high-pressure water, the billet is rolled, and is finally rolled into a bar with =80 mm, wherein the final rolling temperature is 970 C. The bar is subjected to air cooling after the rolling, and then subjected to skinning treatment with a grinding wheel, and is tested by ultrasonic flaw testing and magnetic particle flaw testing and the like.

[0070] Example 5: performing an electric furnace smelting according to the chemical compositions shown in Tables 1-1 and 1-2, LF refining and VD vacuum treatment, then casting into a 320 mm425 mm continuous casting billet. The continuous casting billet is first heated to 600 C. in a preheating section, then is continually heated to 950 C. in a first heating section, kept at this temperature, and is continually heated to 1200 C. in a second heating section, kept at this temperature for 8 h and then enters a soaking section at a temperature of 1230 C., and is kept at this temperature and is subjected to subsequent rolling. After leaving the heating furnace and being descaled by high-pressure water, the billet is rolled into an intermediate billet, wherein the first final rolling temperature is 1050 C., and the size of the intermediate billet is 220 mm220 mm. After the rolling, the billet is subjected to air cooling. Then the intermediate billet is heated to 680 C. in a preheating section, heated to 1050 C. in a first heating section, heated to 1200 C. in a second heating section, and is kept at this temperature for 6 h, then enters a soaking section with a soaking temperature of 1220 C. After leaving the furnace and being descaled by high-pressure water, the billet is rolled, wherein the second final rolling temperature is 950 C., and the specification of the finished bar is that =60 mm. The bar is subjected to air cooling after the rolling and then is tested by ultrasonic flaw testing and magnetic particle flaw testing and the like.

[0071] Example 6: performing an electric furnace smelting according to the chemical compositions shown in Tables 1-1 and 1-2, refining and vacuum treatment, then casting into a 280 mm280 mm continuous casting billet. The continuous casting billet is first heated to 680 C. in a preheating section first, then is continually heated to 900 C. in a first heating section, kept at this temperature, and is continually heated to 1180 C. in a second heating section and kept at this temperature for 6 h, and then enters a soaking section at a temperature of 1200 C., is kept at this temperature, and is subjected to subsequent rolling. After leaving the furnace and being descaled by high-pressure water, the billet is rolled into an intermediate billet, wherein the first final rolling temperature is 1000 C., and the size of the intermediate billet is 140 mm140 mm. Then the intermediate billet is preheated to 700 C., heated to 1100 C. in a first heating section, heated to 1220 C. in a second heating section, is kept at this temperature for 5 h, and then enters a soaking section with a soaking temperature of 1220 C. After leaving the furnace and being descaled by high-pressure water, the billet is rolled, wherein the second final rolling temperature is 920 C., and the specification of the finished bar is that =30 mm. The bar is subjected to air cooling after the rolling, then is subjected to skinning treatment by turning, and is tested by ultrasonic flaw testing and magnetic particle flaw testing.

[0072] Comparative Example 1: its implementation mode is the same as that of Example 1, comprising the following steps: performing an electric furnace smelting according to the chemical compositions shown in Tables 1-1 and 1-2, refining and vacuum treatment, and then continuously casting into a 280 mm280 mm square billet. The continuous casting billet is heated to 600 C. in a preheating section first, then heated to 980 C. in a first heating section, kept at this temperature, and is continually heated to 1200 C. in a second heating section and kept at this temperature, and then enters a soaking section at a temperature of 1220 C., is kept at this temperature, and is subjected to subsequent rolling. After leaving the heating furnace and being descaled by high-pressure water, the billet is continuously rolled into a bar with =90 mm, wherein the final rolling temperature is 1000 C. The bar is subjected to air cooling after the rolling, annealing treatment at 650 C., and is tested by ultrasonic flaw testing and magnetic particle flaw testing.

[0073] Comparative Example 2: its implementation mode is the same as that of Example 2, including the following steps: performing a smelting on a 150 kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. Molten steel is cast into a steel ingot, and the steel ingot is heated and is forged into a billets, wherein the heating temperature is 1100 C. the forging is performed after keeping the temperature for 4 h, wherein the final forging temperature is 1000 C., and finally a bar with a diameter =92 mm is formed, then subjected to slow cooling, and skinned by turning to =90 mm.

[0074] Comparative Example 3: its implementation mode is the same as that of Example 4, including the following steps: performing an electric furnace smelting according to the chemical compositions shown in Tables 1-1 and 1-2, refining and vacuum treatment, continuously casting into a 280 mm280 mm square billet. The continuous casting billet is first heated to 680 C. in a preheating section, then is continually heated to 900 C. in a first heating section, kept at this temperature, and is continually heated to 1180 C. in a second heating section, kept at this temperature, and then enters a soaking section at a temperature of 1200 C., is kept at this temperature, and is subjected to subsequent rolling. After leaving the heating furnace and being descaled by high-pressure water, the billet is continuously rolled into a bar with =90 mm, wherein the final rolling temperature is 960 C. The bar is subjected to air cooling after the rolling, annealing treatment at 650 C., and is tested by ultrasonic flaw testing and magnetic particle flaw testing.

[0075] Comparative Example 4: its implementation mode is the same as that of Example 5, including the following steps: performing an electric furnace smelting according to the chemical compositions shown in Tables 1-1 and 1-2, refining and vacuum treatment, and then casting into a 320 mm425 mm continuous casting billet. The continuous casting billet is heated to 600 C. in a preheating section, then is continually heated to 950 C. in a first heating section, kept at this temperature, and is continually heated to 1200 C. in a second heating section, kept at this temperature, and then enters a soaking section at a temperature of 1230 C., is kept at this temperature, and is subjected to subsequent rolling. After leaving the heating furnace and being descaled by high-pressure water, the billet is rolled into an intermediate billet, wherein the first final rolling temperature is 1050 C., and the size of the intermediate billet is 220 mmx220 mm. Then the intermediate billet is heated to 680 C. in a preheating section, heated to 1050 C. in a first heating section, heated to 1200 C. in a second heating section, is kept at this temperature, and then enters a soaking section with a soaking temperature of 1220 C. After leaving the furnace and being descaled by high-pressure water, the billet is rolled, wherein the second final rolling temperature is 950 C., and the specification of the finished bar is that =60 mm. The bar is subjected to air cooling after the rolling and is tested by ultrasonic flaw testing and magnetic particle flaw testing.

[0076] Table 1-1 lists the mass percentages of chemical elements of the non-quenched and tempered round steels with high strength, high toughness and easy cutting in Examples 1-6 and he comparative steels in Comparative Examples 1-4.

TABLE-US-00001 TABLE 1-1 (wt. %, the balance being Fe and other inevitable impurities except for P, N, O, Ti and Ca) Chemical Element No. C Si Mn P S Cr Ni Mo Cu Al V Ti Nb N 0 Ca Example 1 0.37 0.65 1.53 0.009 0.056 0.21 0.21 0.24 0.16 0.041 0.11 0.001 0.033 0.011 0.0012 0.0014 Example 2 0.44 0.66 1.52 0.006 0.037 0.16 0.19 0.21 0 0.032 0.22 0.001 0.031 0.009 0.0016 0.0021 Example 3 0.39 0.25 1.53 0.008 0.039 0.21 0.21 0.20 0.11 0.030 0.15 0.001 0.036 0.011 0.0018 0.0042 Example 4 0.37 0.68 1.49 0.005 0.040 0.21 0.19 0.19 0.02 0.027 0.13 0 0.030 0.013 0.0009 0.0009 Example 5 0.38 0.53 1.28 0.006 0.033 0.53 0.11 0.10 0.21 0.030 0.19 0.001 0.022 0.012 0.0011 0.0026 Example 6 0.41 0.64 1.72 0.007 0.022 0.26 0.23 0.23 0.01 0.045 0.14 0.002 0.011 0.006 0.0016 0.0032 Comparative 0.40 0.64 1.53 0.011 0.034 0.52 0.20 0.20 0 0.035 0.11 0.001 0.034 0.008 0.0013 0.0016 Example 1 Comparative 0.38 0.56 1.49 0.005 0.018 0.14 0.20 0.12 0.05 0.029 0.12 0.001 0 0.011 0.0014 0.0019 Example 2 Comparative 0.40 0.66 1.52 0.006 0.033 0.51 0.19 0.06 0.01 0.034 0.11 0 0.031 0.013 0.0016 0.0031 Example 3 Comparative 0.35 0.28 1.85 10.008 0.055 0.58 0.15 0.19 0.03 0.031 0.13 0.001 0.022 0.015 0.0013 0.0025 Example 4

[0077] Table 1-2 lists ideal critical diameter for hardenability DI, carbon equivalent Ceq, microalloying coefficient r.sub.M/N and bainite transformation temperature T.sub.B calculated from the mass percentages of chemical elements in the non-quenched and tempered round steels with high strength, high toughness and easy cutting in Examples 1-6 and the comparative steels in Comparative Examples 1-4.

TABLE-US-00002 TABLE 1-2 Microalloying Carbon Bainite element equivalent transformation DI coefficient Ceq temperature No. value r.sub.M/N (%) T.sub.B ( C.) Example 1 6.6 4.8 0.76 550 Example 2 7.5 8.4 0.82 539 Example 3 5.4 5.2 0.78 548 Example 4 5.8 3.9 0.74 558 Example 5 6.3 5.5 0.78 563 Example 6 8.8 9.9 0.84 519 Comparative 9.1 6.2 0.83 524 Example 1 Comparative 4.3 4.1 0.72 566 Example 2 Comparative 6.7 3.8 0.80 538 Example 3 Comparative 5.1 3.4 0.85 507 Example 4

[0078] In the table above, the DI, the microalloying element coefficient r.sub.M/N, the carbon equivalent Ceq and the bainite transformation temperature T.sub.B are calculated according to the relevant formulas listed above, respectively.

[0079] Table 2 lists the specific process parameters adopted in the manufacturing methods of the non-quenched and tempered round steels in Examples 1-6 and the comparative steels in Comparative Examples 1-4.

TABLE-US-00003 TABLE 2 Heating Final rolling Heating Final rolling temper- Tempera- temperature temperature Temperature temperature ature ture or final of keeping of Bar of cast keeping forging Size of intermediate time of intermediate speci- Smelting, refining and billet time of cast temperature intermediate billet intermediate billet fication No. casting process ( C.) billet (h) ( C.) billet ( C.) billet (h) ( C.) (mm) Example 1 50 kg vacuum induction 1050 3 910 / / / / 60 furnace + die casting Example 2 150 kg vacuum induction 1100 4 1000 / / / / 90 furnace + die casting Example 3 Electric furnace + refining + 1200 8 1000 / / / / 100 continuous casting Example 4 Electric furnace + refining + 1150 6 970 / / / / 80 continuous casting Example 5 Electric furnace + refining + 1200 8 1050 220 mm 1200 6 950 60 continuous casting 220 mm Example 6 Electric furnace + refining + 1180 6 1000 140 mm 1220 5 920 30 continuous casting 140 mm Comparative 50 kg vacuum induction 1050 3 910 / / / / 60 Example 1 furnace + die casting Comparative 150 kg vacuum induction 1100 4 1000 / / / / 90 Example 2 furnace + die casting Comparative Electric furnace + refining + 1150 6 970 / / / / 80 Example 3 continuous casting Comparative Electric furnace + refining + 1200 8 1050 220 mm 1200 6 950 60 Example 4 continuous casting 220 mm

[0080] In Table 2, in the three embodiments of Example 5, Example 6 and Comparative Example 4, in the rolling process, the steel billets are first rolled to their respective designated intermediate billet sizes, and then heated and rolled again to the final product sizes.

[0081] The non-quenched and tempered round steels obtained in Examples 1-6 and comparative steels in Comparative Examples 1-4 are sampled respectively, and test specimens are prepared with reference to GB/T 2975. Tensile tests and impact tests are carried out according to GB/T 228.1 and GB/T 229, respectively, so as to obtain the mechanical properties of steel plates in the examples and comparative examples.

[0082] The non-quenched and tempered round steels are subjected to cutting by an ordinary lathe, and chips are collected to evaluate the cutting performance of the steels: The granular chips that are easy to break are evaluated as good, while the continuous spiral chips that are not easy to break are evaluated as poor, and the chips that are presented in a C type between the foregoing two are evaluated as medium. The obtained test results of mechanical properties and cutting performances of the examples and comparative examples are listed in Table 3.

[0083] Table 3 lists the test results of the non-quenched and tempered round steels with high strength, high toughness and easy cutting in Examples 1-6 and comparative steels in Comparative Examples 1-2.

TABLE-US-00004 TABLE 3 Yield strength Tensile Cross-sectional Charpy impact R.sub.p0.2 strength R.sub.m Elongation A shrinkage Z energy A.sub.ku Hardness Cutting No. (MPa) (MPa) (%) (%) (J) (HBW) performance Example 1 599/603 1,110/1,110 21/20 41/39 42/39/38 305 good Example 2 686/669 1310/1320 17/18.5 35/37 28/34/30 303 good Example 3 601/613 1,210/1,190 18/20 36/39 32/31/38 302 good Example 4 613/619 1,020/1,050 17.5/16.5 33/31 38/38/37 296 good Example 5 591/595 1,050/1,010 16.0/17.0 33/32 40/39/38 300 good Example 6 563/563 1,190/1,190 16.0/17.5 28/27 36/35/34 330 medium Comparative 686/633 1,300/1,340 17.5/18 32/34 27/27/29 334 medium Example 1 Comparative 609/600 935/910 18.0/18.5 36/35 42/43/40 300 poor Example 2 Comparative 645/652 967/968 20.5/21 58/60 59/38/51 301 medium Example 3 Comparative 716/700 1,100/1,150 14/13.5 49/50 21/25/26 321 medium Example 4 Note: Groups of data in each column in Table 3 represent the results of two or three tests.

[0084] As can be seen from Table 3, the comprehensive properties of the non-quenched and tempered round steels with high strength, high toughness and easy cutting in Examples 1-6 of the present invention are obviously superior to those of the comparative steels in Comparative Examples 1-4. In the present invention, the non-quenched and tempered round steels with high strength, high toughness and easy cutting in Examples 1-6 have a tensile strength R.sub.m of greater than or equal to 1000 MPa, an elongation A of greater than or equal to 12%, a cross-sectional shrinkage Z of greater than or equal to 35%, and a Charpy impact energy A.sub.ku of greater than or equal to 27 J, which not only have good impact toughness and plasticity, but also have good fatigue resistance and are easy to cut, and thus can satisfy the use requirements of situations requiring a high-strength and toughness steel, such as automobiles and engineering machinery.

[0085] Continually referring to Tables 1-1, 1-2, 2 and 3, it can be seen that the chemical element composition and some related process of Comparative Example 1 all meet the design requirements of the present invention, but compared with Examples 1-6, the ideal critical diameter for hardenability DI in Comparative Example 1 is 9.1, which is not in the preferred range of 5.0-9.0, so that the impact energy in Comparative Example 1 is lower than those of the non-quenched and tempered round steels in Examples 1-6.

[0086] Furthermore, in Comparative Examples 2-4, all the three comparative examples have parameters that do not meet the requirements of the design specification of the present invention in the design process of chemical element composition. Therefore, compared with the non-quenched and tempered round steels in Examples 1-6, the comparative steel in Comparative Examples 2 and 3 have lower strength, while the comparative steel in Comparative Example 4 have lower toughness, Comparative Examples 3 and 4 have poor use effects, a crankshaft prepared from Comparative Example 3 have impact energy as low as 23 J, and a crankshaft prepared from Comparative Example 4 is not easy in chip breaking during cutting, resulting in low processing efficiency, thus being unable to meet the requirements of use.

[0087] FIG. 1 is a microstructure metallograph of a non-quenched and tempered round steel in Example 2 under a 500-fold optical microscope.

[0088] As can be seen from FIG. 1 that the microstructure of the non-quenched and tempered round steel in Example 2 is mainly bainite, and the area percentage of the bainite in the cross section of the round steel is greater than or equal to 85%. Furthermore, in Example 2, the microstructure of the non-quenched and tempered round steel also contains residual austenite and a small amount of ferrite+pearlite.

[0089] FIG. 2 is a microstructure metallograph of a cross section of a crankshaft made of the non-quenched and tempered round steel in Example 2 under a 500-fold optical microscope.

[0090] As can be seen from FIG. 2, the microstructure of the crankshaft made of the non-quenched and tempered round steel in Example 2 is bainite.

[0091] The combination manner of each technical features in the present application is not limited to the combination manner recorded in the claims of the present application or the combination manner recorded in specific examples. All technical features recorded in the present application can be freely combined or conjoined in any way, unless there are contradictions among them.

[0092] It should also be noted that the examples listed above are only specific examples of the present invention. Obviously, the present invention is not limited to the aforementioned examples, and the similar changes or variations that are made accordingly can be directly derived or easily associated by those skilled in the art from the disclosure of the present invention, and all of them belong to the claimed scope of the present invention.