Free machining and non-quenched and tempered steel and manufacturing method therefor

Abstract

A free-cutting and non-quenched and tempered steel, comprising the following chemical elements by mass percentages: C: 0.35-0.45%, Si: 0.45-0.0.65%, Mn: 1.35-1.65%, S: 0.025-0.065%, V: 0.07-0.15%, Ti: 0.01-0.018%, N: 0.012-0.017%, Al: 0.015-0.035%, Ca: 0.0008-0.0025%, with the remaining being iron and other unavoidable impurities, wherein the S and Ca elements satisfy the relationship S/Ca=20-60. A manufacturing method of the free-cutting and non-quenched and tempered steel, comprising the following steps: (1) smelting and refining; (2) casting; (3) rolling; (4) forging; and (5) two-stage cooling.

Claims

1. A free-cutting and non-quenched and non-tempered steel, comprising the following chemical elements by mass percentages: C: 0.35-0.45%, Si: 0.45-0.65%, Mn: 1.35-1.65%, S: 0.025-0.065%, V: 0.07-0.15%, Ti: 0.01-0.018%, N: 0.012-0.017%, Al: 0.015-0.035%, Ca: 0.0008-0.0025%, with the balance being iron and other unavoidable impurities; wherein the contents of S and Ca satisfy the relationship of S/Ca=20-60, wherein the steel has elongated MnS inclusions, and wherein the elongated MnS inclusions have an aspect ratio of 6.0 to 8.5.

2. The free-cutting and non-quenched and non-tempered steel as claimed in claim 1, wherein the steel has a microstructure of ferrite and pearlite.

3. The free-cutting and non-quenched and non-tempered steel as claimed in claim 1, wherein a longitudinal direction of the elongated MnS inclusions coincides with a rolling direction of the steel.

4. The free-cutting and non-quenched and non-tempered steel as claimed in claim 1, wherein a proportion of an area of the elongated MnS inclusions in a section of the steel is 1.25 to 1.85%.

5. The free-cutting and non-quenched and non-tempered steel as claimed in claim 1, wherein the steel has a tensile strength (Rm) of 900 MPa or more, a yield strength (RP0.2) of 550 MPa or more, an elongation rate (A) of 18% or more, a reduction of area (Z) of 40% or more, and an impact energy (AKv) of 30 J or more.

6. A manufacturing method of the free-cutting and non-quenched and non-tempered steel as claimed in claim 1, comprising the following steps: (1) smelting and refining; (2) casting; (3) rolling; (4) forging; and (5) two-stage cooling: cooling to 650-700° C. at a cooling rate of 20-30 ° C/min in the first stage, and then air cooling to room temperature in the second stage.

7. The manufacturing method as claimed in claim 6, wherein in the step (1), tapping temperature is controlled to 1640-1660 ° C. during the smelting.

8. The manufacturing method as claimed in claim 6, wherein in the step (2), casting start temperature is controlled to 1530-1560 ° C.

9. The manufacturing method as claimed in claim 6, wherein in the step (3), finishing rolling temperature is controlled to 950-1000 ° C.

10. The manufacturing method as claimed in claim 6, wherein in the step (4), final forging temperature is controlled to 920-960 ° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the effect of the mass percentage of V on the yield strength of the free-cutting and non-quenched and tempered steel of the present invention.

(2) FIG. 2 illustrates the effect of the mass percentage of V on the tensile strength of the free-cutting and non-quenched and tempered steel of the present invention.

(3) FIG. 3 illustrates the effect of the mass percentage of N on the tensile strength of the free-cutting and non-quenched and tempered steel of the present invention.

(4) FIG. 4 illustrates the effect of the mass percentage of N on the yield strength of the free-cutting and non-quenched and tempered steel of the present invention.

(5) FIG. 5 illustrates the effect of the mass percentage of N on the elongation rate of the free-cutting and non-quenched and tempered steel of the present invention.

(6) FIG. 6 illustrates the effect of the mass percentage of N on the impact energy of the free-cutting and non-quenched and tempered steel of the present invention.

(7) FIG. 7 shows the metallographic structure of the free-cutting and non-quenched and tempered steel of Example 3.

(8) FIG. 8 shows the metallographic structure of MnS inclusions in the free-cutting and non-quenched and tempered steel of Example 3.

DETAILED DESCRIPTION

(9) The free-cutting and non-quenched and tempered steel of the present invention and the manufacturing method thereof will be further explained and illustrated below with reference to the accompanying drawings and specific Examples. However, the explanations and illustrations do not unduly limit the technical solutions of the present invention.

Examples 1-7 and Comparative Example 1

(10) Table 1 lists the mass percentages of the chemical elements in the free-cutting and non-quenched and tempered steels of Examples 1-7 and the conventional steel of Comparative Example 1.

(11) TABLE-US-00001 TABLE 1 (wt %, the balance is Fe and other inevitable impurity elements other than P) Number C Si Mn P S V Ti N Al Ca S/Ca Example 0.35 0.55 1.65 0.01 0.051 0.13 0.015 0.012 0.02 0.0021 24.3 1 Example 0.38 0.65 1.6 0.008 0.065 0.11 0.012 0.014 0.018 0.0024 27.08 2 Example 0.41 0.5 1.5 0.008 0.037 0.12 0.018 0.013 0.032 0.0016 23.16 3 Example 0.43 0.45 1.35 0.009 0.025 0.09 0.01 0.015 0.026 0.0008 31.25 4 Example 0.45 0.47 1.4 0.007 0.044 0.1 0.015 0.017 0.034 0.001 44 5 Example 0.37 0.58 1.45 0.012 0.031 0.07 0.018 0.014 0.028 0.0012 25.83 6 Example 0.4 0.62 1.55 0.007 0.058 0.14 0.013 0.015 0.023 0.0018 32.22 7 Compar- 0.42 0.5 1.35 0.012 0.11 — 0.015 — — — ative Example 1

(12) The manufacturing method of the free-cutting and non-quenched and tempered steels of Examples 1-7 and the conventional steel of Comparative Example 1 comprises the following steps:

(13) (1) smelting and refining: tapping temperature was controlled to 1640-1660° C. during smelting;

(14) (2) casting: casting start temperature was controlled to 1530-1560° C.;

(15) (3) rolling: finishing rolling temperature was controlled to 950-1000° C.;

(16) (4) forging: final forging temperature was controlled to 920-960° C.;

(17) (5) two-stage cooling: cooling to 650-700° C. at a cooling rate of 20-30° C./min in the first stage, and then air cooling to room temperature in the second stage.

(18) Table 2 lists the specific process parameters in the manufacturing method of the free-cutting and non-quenched and tempered steels of Examples 1-7 and the conventional steel of Comparative Example 1.

(19) TABLE-US-00002 TABLE 2 Step (2) Step (4) Step (5) Step (1) Casting Step (3) Final Final Tapping start Rolling forging Cooling cooling temperature temperature temperature temperature rate temperature Micro- Number (° C.) (° C.) (° C.) (° C.) (° C./mm) (° C.) structure Example 1 1650 1552 975 958 25 680 F + P Example 2 1640 1531 955 930 22 655 F + P Example 3 1645 1542 980 948 27 678 F + P Example 4 1659 1557 995 955 21 690 F + P Example 5 1655 1535 985 928 29 650 F + P Example 6 1653 1548 967 952 24 700 F + P Comparative 1653 1552 982 958 26 690 F + P Example 1 Note: “F + P” in Table 2 represents ferrite + pearlite.

(20) Performance tests were performed on the free-cutting and non-quenched and tempered steels of Examples 1-7 and the conventional steel of Comparative Example 1, and the results are shown in Table 3.

(21) TABLE-US-00003 TABLE 3 Number RP0.2(MPa) Rm(Mpa) A(%) Z(%) AKv (J) Example 1 588 924 19.5 54 32 Example 2 574 920 20.5 52 34 Example 3 580 902 19.0 44 35 Example 4 582 918 19.5 48 32 Example 5 603 932 19.5 46 31 Example 6 596 917 18.0 42 33 Example 7 585 912 19.0 48 32 Comparative 535 855 16 38 28 Example 1

(22) As can be seen from Table 3, each of the free-cutting and non-quenched and tempered steels of Examples 1-7 has a tensile strength (Rm) of 900 MPa or more, a yield strength (RP0.2) of 550 MPa or more, an elongation rate (A) of 18% or more, a reduction of area (Z) of 40% or more, and an impact energy (AKv) of 30 J or more. The performance parameters of the steels of the Examples are superior to those of the conventional steel of Comparative Example 1.

(23) Further, the free-cutting and non-quenched and tempered steels of Examples 1-7 and the conventional steel of Comparative Example 1 were turned on the same numerically controlled machine tool at a machine speed of 400 r/min. The amount of tool loss after turning for 1 hour is shown in Table 4.

(24) TABLE-US-00004 TABLE 4 Number Tool loss (mm) Example 1-7 0.2 Comparative 0.6 Example 1

(25) As can be seen from Table 4, the average tool loss of Examples 1-7 is 0.2 mm, while the tool loss of Comparative Example 1 is 0.6 mm. The average loss of the cutting tool caused by Examples 1-7 is ⅓ of Comparative Example 1.

(26) FIG. 1 illustrates the effect of the mass percentage of V on the yield strength of the free-cutting and non-quenched and tempered steel of the present invention. FIG. 2 illustrates the effect of the mass percentage of V on the tensil strength of the free-cutting and non-quenched and tempered steel of the present invention.

(27) As shown in FIG. 1 and FIG. 2, when the mass percentage of V is 0.07-0.15%, the improvement of the yield strength and tensile strength is remarkable. Considering the manufacture cost and the improvement effect of the strength of the steel, the mass percentage in the free-cutting and non-quenched and tempered steel of the present invention is controlled to 0.07-0.15%.

(28) FIG. 3 illustrates the effect of the mass percentage of N on the tensile strength of the free-cutting and non-quenched and tempered steel of the present invention. FIG. 4 illustrates the effect of the mass percentage of N on the yield strength of the free-cutting and non-quenched and tempered steel of the present invention. FIG. 5 illustrates the effect of the mass percentage of N on the elongation rate of the free-cutting and non-quenched and tempered steel of the present invention. FIG. 6 illustrates the effect of the mass percentage of N on the impact energy of the free-cutting and non-quenched and tempered steel of the present invention.

(29) As shown in FIG. 3 to FIG. 6, when the mass percentage of N added is different, the effect of improving the performance of the steel is different. Considering the cost of addition and the effect of improvement, the inventor of the present invention limited the mass percentage of N to 0.12-0.17%. When the mass percentage of N is within this range, it is beneficial to increasing the strength of the steel, and N easily forms nitrides or nitrogen carbides with alloying elements such as V and Ti, resulting in grain refinement, which in turn enhances the toughness of steel by precipitation strengthening. In addition, void defects due to over-high mass percentage of N in the steel can be avoided.

(30) FIG. 7 shows the metallographic structure of the free-cutting and non-quenched and tempered steel of Example 3.

(31) As shown in FIG. 7, the free-cutting and non-quenched and tempered steel of Example 3 has a microstructure of ferrite+pearlite.

(32) FIG. 8 shows the metallographic structure of MnS inclusions in the free-cutting and non-quenched and tempered steel of Example 3.

(33) As shown in FIG. 8, the free-cutting and non-quenched and tempered steel of Example 3 has elongated MnS inclusions, the longitudinal direction of the elongated MnS substantially coincides with the rolling direction of the steel sheet, the elongated MnS has an aspect ratio of 6.0 to 8.5, and through calculation the proportion of the area of the elongated MnS in the section of the steel sheet of the free-cutting and non-quenched and tempered steel is 1.25 to 1.85%.

(34) It should be noted that the above is merely an illustration of specific Examples of the invention. It is obvious that the present invention is not limited to the above Examples, but has many similar variations. All variations that are directly derived or conceived by those skilled in the art from this disclosure are intended to be within the scope of the present invention.