CAST STEEL MEMBER

Abstract

A cast steel member, comprising: 0.10 to 1.00% of C, greater than 0.7% to 2.0% or less of Si, 0.3 to 2.0% of Mn, 2.0% or less of Cu, residual Fe and inevitable impurities, as represented by weight %, wherein Si (%)<=C (%)10.

Claims

1. A cast steel member, comprising: 0.10 to 1.00% of C, greater than 0.7% to 2.0% or less of Si, 0.3 to 2.0% of Mn, 2.0% or less of Cu, residual Fe and inevitable impurities, as represented by weight %, wherein
Si (%)<=C (%)10.

2. The cast steel member according to claim 1, comprising: 0.15 to 0.40% of C, 1.1% to 2.0% of Si, 0.5 to 1.5% of Mn, 0.5 to 1.5% of Cu, residual Fe and inevitable impurities, as represented by weight %.

3. The cast steel member according to claim 1, wherein an impact value (normal temperature) is 15 J/cm.sup.2 or more, a tensile strength is 680 MPa or more to less than 1000 MPa, a 0.2% yield strength is 450 MPa or more, and an elongation is 12% or more.

4. The cast steel member according to claim 2, wherein an impact value (normal temperature) is 15 J/cm.sup.2 or more, a tensile strength is 680 MPa or more to less than 1000 MPa, a 0.2% yield strength is 450 MPa or more, and an elongation is 12% or more.

5. The cast steel member according to claim 1, having a minimum thickness of 10 mm or less.

6. The cast steel member according to claim 2, having a minimum thickness of 10 mm or less.

7. The cast steel member according to claim 3, having a minimum thickness of 10 mm or less.

8. The cast steel member according to claim 4, having a minimum thickness of 1 to 6 mm or less.

9. The cast steel member according to claim 1, which is as-cast.

10. The cast steel member according to claim 2, which is as-cast.

11. The cast steel member according to claim 3, which is as-cast.

12. The cast steel member according to claim 5, which is as-cast.

13. The cast steel member according to claim 8, which is as-cast.

14. The cast steel member according to claim 1, which is for an undercarriage.

15. The cast steel member according to claim 2, which is for an undercarriage.

16. The cast steel member according to claim 3, which is for an undercarriage.

17. The cast steel member according to claim 5, which is for an undercarriage.

18. The cast steel member according to claim 8, which is for an undercarriage.

19. The cast steel member according to claim 9, which is for an undercarriage.

20. The cast steel member according to claim 7, which is formed into a steering knuckle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 A sectional perspective view showing a mold for producing Examples.

[0019] FIG. 2 A view showing a size of a tensile test piece for measuring a tensile strength in Examples and Comparative Examples.

[0020] FIG. 3 A view showing a size of an impact test piece for measuring an impact value in Examples and Comparative Examples.

[0021] FIG. 4 A view showing an example of applying a cast steel member of the present invention to a steering knuckle.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0022] Hereinafter, embodiments according to the present invention will be described. In the context of the present invention, % denotes weight % unless otherwise specified.

[0023] The cast steel member according to the embodiment of the present invention includes 0.10 to 1.00% of C, greater than 0.7% to 2.0% or less of Si, 0.3 to 2.0% of Mn, 2.0% or less of Cu, residual Fe and inevitable impurities, as represented by weight %, in which Si (%)<=C (%)10.

[0024] C (carbon) significantly increases a tensile strength and a yield strength and decreases elongation and an impact value with respect to cast steel. If the C content is less than 0.10%, there are no effects to increase the yield strength and the tensile strength. If the C content is greater than 1.00%, the cast steel becomes hard and brittle, and the impact value is decreased. Accordingly, the C content is set to from 0.10 to 1.00%.

[0025] In particular, it is preferable that if the C content be set to from 0.15 to 0.40%, the tensile strength and the rigidity can be improved without decreasing the impact value.

[0026] Si can reinforce ferrite, restrain the reduction of the elongation, and improve the yield strength, but coarsen crystal grain and decrease the impact value. If the Si content is 0.7% or less, the yield strength is not sufficiently increased. If the Si content is greater than 2.0%, the impact value is decreased. Accordingly, the Si content is set to greater than 0.7% to 2.0% or less.

[0027] In particular, if the Si content is set to 1.1% or more to 2.0% or less, the yield strength (fatigue strength) can be further improved.

[0028] In addition, if Si (%)<=C (%)10, the percentage of Si and C contained in molten metal becomes appropriate, less gas defect is generated, a stable mold is formed, both of the impact value and the tensile strength are improved, and impact properties (normal temperature) can be certainly 15 J/cm2 or more. On the other hand, if Si and C do not satisfy the above-described relationship, the gas defect is easily generated, and it is difficult to produce a normal mold product by melting in the air.

[0029] Mn is an element that promotes providing a matrix structure with a strong pearlite structure and makes crystal grain finer. While the impact value is prevented from decreasing, the tensile strength is significantly increased. However, Mn decreases the elongation. If the Mn content is less than 0.3%, an effect to increase the strength is insufficiently provided. If the Mn content is greater than 2.0%, the elongation is significantly decreased. Accordingly, the Mn content is set to from 0.3 to 2.0%.

[0030] In particular, it is preferable that if the Mn content be set to from 0.50 to 1.50%, the yield strength (fatigue strength) can be further improved.

[0031] Cu is highly effective to make crystal grain finer and dissolve into ferrite. While the elongation and the impact value are prevented from decreasing, the yield strength (fatigue strength) is especially improved. If the Cu content is greater than 2.0%, the structure other than ferrite is precipitated, which significantly embrittles the structure and cracking during manufacture easily occurs. Accordingly, the Cu content is set to 2.0% or less.

[0032] In particular, it is preferable that if the Cu content be set to from 0.5 to 1.50%, it be highly effective to make crystal grain finer, the elongation and the impact value be prevented from decreasing, and the yield strength (fatigue strength) and the tensile strength can be improved.

[0033] The Fe content is preferably 95% or more. Examples of inevitable impurities include P, S, Ni, Cr, and Al. In particular, Al can be added as a deoxidizer. In order to prevent the gas defect upon casting, the oxygen content is preferably controlled to 80 ppm by weight or less based on the total oxygen content.

[0034] Preferably, the cast steel member according to the embodiment of the present invention is cast to have a minimum thickness of 10 mm or less. In a conventional cast steel, the thicker the cast steel is, the easier the shrinkage cavity and the gas defect occur. In addition, a heat treatment is performed to homogenize segregation and the coarsened structure, to remove an internal stress, and to stabilize the structure. If no heat treatment is applied, the material properties such as the impact value and the fatigue strength are deteriorated. In particular, it is difficult to apply to a vehicle (undercarriages).

[0035] The cast steel having the above-described composition and the minimum thickness of 10 mm or less was cast. The internal stress was decreased by finer crystal grain, and the fatigue strength was successfully improved even though no heat treatment was applied. Herein, the minimum thickness refers to a minimum thickness of an as-cast product, and does not include a thickness of a cast product that is originally thick and ground later. In this case, as the as-cast product leaves a casting surface (excluding dam and deburr), the as-cast product can be distinguished from the ground cast product.

[0036] In particular, the cast steel is preferably cast to have the minimum thickness of 1 to 6 mm.

[0037] As described above, since the cast steel member according to the embodiment of the present invention has the improved fatigue strength even though no heat treatment is performed after casting, the cast steel can be used as-cast product.

[0038] The cast steel member according to the embodiment of the present invention preferably has an impact value of 15 J/cm2 or more at normal temperature, a tensile strength of 680 MPa or more to less than 1000 MPa, a 0.2% yield strength of 450 MPa or more, and an elongation of 12% or more.

[0039] In addition, the cast steel member according to the embodiment of the present invention has the Young's modulus of 190 to 210 GPa and can achieve the Young's modulus higher than the Young's modulus (about 166 GPa) of the FCD cast iron material.

[0040] Note that the tensile strength is an index of evaluating the strength, the 0.2% yield strength is an index of evaluating the fatigue strength, the elongation (breaking elongation) is an index of evaluating the ductility, the Young's modulus is an index of evaluating the rigidity, and the impact value is an index of evaluating the toughness.

[0041] The cast steel member according to the embodiment of the present invention can be produced by known methods. A melting furnace and a melting method are not especially limited. Also, a casting method is not especially limited as long as fluidity is satisfied, and a gravity casting method, a reduced pressure casting method, or the like may be used as necessary.

[0042] In addition, in order to prevent a shrinkage cavity defect upon casting, it is preferable that a cooling speed of each site of the mold be controlled for directional solidification. As appropriate, a chiller and a gate riser may be used. In this case, the chiller is set to the mold so as to rapidly and preferentially cool a thick site, and it can be controlled such that a final solidified site becomes the gate riser. The above-described method is preferable in that the thin cast steel member (preferably having the minimum thickness of 10 mm or less) can be stably cast.

[0043] According to the present invention, the balance between the strength and the toughness is excellent and high rigid and stable mechanical properties are provided as described above, which is suitable to decrease the weight of the vehicle parts. In particular, the present invention can be preferably used for an undercarriage (knuckle, upper arm, lower arm, brake caliper, trailing arm, bracket (brake support), etc.). In particular, when the present invention is applied to a steering knuckle that needs to have high strength and high toughness (impact properties), both of the strength and the rigidity are improved, and the parts can have a further light weight as compared with the material having only the improved strength is applied.

[0044] FIG. 4 shows an example of applying the cast steel member of the present invention to a steering knuckle. FIGS. 4 (a) and (b) each is a plan view and a side view of a steering knuckle 100.

[0045] Note that the sites shown by the symbols A to G are portions (holes) connecting to other parts, need working to match with the connection positions, and are therefore not as-cast. Accordingly, the minimum thickness of the steering knuckle 100 is of the as-cast site excluding the sites shown by the symbols A to G.

[0046] According to an example of an embodiment of the present invention, 100 kg of cast steel having each component composition shown in Table 1 was melted in a high frequency melting furnace, a deoxidizer was added thereto such that the total oxygen content was 80 ppm by weight or less to prepare molten metal. A Y-shaped block mold (see FIG. 1) molded by a beta set method was provided with a chiller and a gate riser for directional solidification. The molten metal was casted into the mold to cast steel 10C.

[0047] In the Figures, 2P represents a bottom portion of cast steel (thin portion having thickness of 10 mm or less); 4P represents an upper portion of cast steel (thick portion having thickness of greater than 10 mm); 10C represents a cast steel and 100 represents cast steel member (steering knuckle).

[0048] As shown in FIG. 1, the cast steel 10C had a bottom portion 2P having a thickness of 10 mm or less and a wide and thick upper portion 4P having a thickness of greater than 10 mm. The structure in each Example included ferrite and pearlite as main components (total of both components occupies 60% or more of cast steel).

[0049] The cast steel 10 was not heat-treated and as-casted. A tensile test piece and an impact test piece having a size shown in FIG. 2 and FIG. 3 in each of Examples and Comparative Examples were produced by a turning process. The tensile test piece was in accordance with JIS Z2242 and the impact test piece was a U-notched impact test piece in accordance with JIS Z2242. A cube having 10 mm on a side was cut out from the cast steel 10C, and the Young's modulus test piece was produced.

[0050] Note that each test piece was produced from an area 2R of the bottom portion 2P of the cast steel 10 in each of Examples and Comparative Examples 1, 2, 4 to 6, and the test piece was produced from an area 4R of the upper portion 4P of the cast steel 10 in Comparative Example 3. A unit of the numerical values in FIG. 1 to FIG. 3 is mm. FIG. 2 (a) and FIG. 3 (a) each shows a cross-sectional view of the test piece, and FIG. 2 (b) and FIG. 3 (b) each shows a plan view of the test piece. In FIG. 3 (b), a partial enlarged view of a U-notch is pointed by an arrow.

[0051] The following items are evaluated.

[0052] Tensile strength, 0.2% yield strength, and breaking elongation: The tensile test was performed on each tensile test piece in accordance with JIS Z2241 using the Amsler universal testing machine to measure the tensile strength, the 0.2% yield strength, and the breaking elongation.

[0053] Young's modulus: The density of the Young's modulus test piece was measured by the Archimedes method. A longitudinal wave sound speed and a transversal wave sound speed were measured by an ultrasonic pulse method. From these values, the Young's modulus was calculated. As a measurement apparatus for the ultrasonic pulse method, digital ultrasonic flaw detector UI-25 (product name) manufactured by Ryoden Shonan Electronics Corporation was used. An oscillator for longitudinal and transverse waves manufactured by Eishin Kagaku Co., Ltd. was used.

[0054] Impact value: The impact test was performed on the impact test piece in accordance with JIS Z2242 using the Charby impact tester (50J) to measure the impact value at normal temperature (25 C.).

[0055] Table 1 shows the results

TABLE-US-00001 TABLE 1 Impact Component of cast steel [wt %] 0.2% value Relationship Young's yield Tensile Elon- (normal between modulus strength strength gation temp) C Si Mn P S Cr Ni Cu Al Si (%) and C (%) [GPa] [MPa] [MPa] [%] [J/cm2] Example 1 0.25 1.00 1.00 0.020 0.008 0.031 0.02 0.038 0.034 Si (%) < C (%) 10 208 451 698 16.1 16.9 Example 2 0.20 1.50 0.80 0.020 0.008 0.031 0.02 0.031 0.025 Si (%) < C (%) 10 208 455 705 18.2 16.3 Example 3 0.40 1.00 0.60 0.020 0.008 0.031 0.02 0.031 0.025 Si (%) < C (%) 10 205 451 715 15.4 18.2 Example 4 0.21 1.00 0.35 0.019 0.008 0.033 0.02 0.03 0.029 Si (%) < C (%) 10 202 450 680 19.0 16.0 Example 5 0.20 1.00 1.95 0.019 0.008 0.031 0.02 0.03 0.033 Si (%) < C (%) 10 200 460 710 12.2 15.5 Example 6 0.40 2.00 1.00 0.019 0.008 0.031 0.02 0.031 0.035 Si (%) < C (%) 10 205 494 725 15.0 15.0 Example 7 0.40 0.75 1.00 0.019 0.008 0.031 0.02 0.031 0.035 Si (%) < C (%) 10 205 452 708 15.1 17.8 Example 8 1.00 2.00 1.00 0.019 0.008 0.031 0.02 0.031 0.035 Si (%) < C (%) 10 195 457 680 16.1 15.0 Example 9 0.20 1.50 0.90 0.024 0.007 0.043 0.01 1.11 0.021 Si (%) < C (%) 10 200 485 735 17.5 16.2 Example 10 0.21 1.10 0.95 0.020 0.008 0.035 0.02 1.50 0.025 Si (%) < C (%) 10 202 490 730 17.2 16.4 Example 11 0.20 1.60 0.80 0.021 0.008 0.031 0.02 0.51 0.030 Si (%) < C (%) 10 205 485 730 17.4 16.0 Comp- 0.23 2.50 0.70 0.019 0.008 0.031 0.02 0.031 0.035 Si (%) > C (%) 10 208 427 679 5.9 8.7 Example 1 Comp- 0.31 0.40 0.60 0.019 0.008 0.030 0.02 0.031 0.035 Si (%) < C (%) 10 206 298 605 17.1 66.9 Example 2 Comp- 0.23 2.50 0.70 0.019 0.006 0.031 0.01 0.03 0.025 Si (%) > C (%) 10 208 370 650 11.5 6.8 Example 3 Comp- 0.13 1.90 1.00 0.018 0.007 0.031 0.02 0.03 0.021 Si (%) > C (%) 10 208 349 610 7.3 5.8 Example 4 Comp- 1.00 2.00 0.20 0.019 0.008 0.031 0.02 0.031 0.035 Si (%) < C (%) 10 195 270 655 18.1 17.0 Example 5 Comp- 1.00 0.60 1.00 0.019 0.008 0.031 0.02 0.031 0.035 Si (%) < C (%) 10 195 310 665 16.1 25.0 Example 6

[0056] As shown in Table 1, in each of Examples 1 to 11 containing 0.10 to 1.00% of C, greater than 0.7% to 2.0% of Si, 0.3 to 2.0% of Mn, residual Fe and inevitable impurities and cast having the minimum thickness of 10 mm or less, all of the yield strength, the strength, and the toughness were excellent.

[0057] In particular, in Examples 1 to 7, 9 to 11 containing 0.15 to 0.40% of C, the elongation and the impact value were not decreased, and the tensile strength and the rigidity were improved as compared to Example 8.

[0058] On the other hand, in Comparative Example 1 containing greater than 2.0% of Si, the impact value was significantly decreased.

[0059] In Comparative Examples 2 and 6 containing 0.7% or less of Si, the yield strength was significantly decreased.

[0060] In Comparative Examples 1 and 4 where Si (%)>C (%)10, the elongation and the impact value were significantly lowered.

[0061] In Comparative Example 3 having the minimum thickness greater than 10 mm, the yield strength (fatigue strength), the strength, and the impact value were further lowered as compared to those in Comparative Example 1 having the same composition. It is contemplated that in the case of Comparative Example 3, the thickness is increased upon casting, the number of the internal defect is therefore increased, and segregation and the coarsened structure are generated. Accordingly, it can be concluded that the minimum thickness upon casting is preferably 10 mm or less.

[0062] In Comparative Example 5 containing less than 0.5% of Mn, the strength and the yield strength were decreased.