Multi-track laser surface hardening of low carbon cold rolled closely annealed (CRCA) grades of steels

Abstract

A multi-track laser beam process for surface hardening a low-carbon and low manganese steel. The process includes providing cold rolled close annealed (CRCA) steel sheets having in weight percentage, C: 0.03-0.07, Mn: 0.15-0.25 or 1.4, S: 0.005-0.009, P: 0.009-0.014, Si: 0.005-0.02, Al: 0.04, V: 0.001, Nb: 0.001, and Ti: 0.002 and heating the surface of the steel sheet to an austenizing temperature using a multi-track laser beam, where, upon cooling, phase transformation of the initial microstructure to a harder dual phase structure occurs. The surface temperature of the steel sheet may be controlled based on a comparison of the on-line surface temperature effect with pre-stored data representing the desired surface temperature effect to eliminate any possibility of melting the sheet. The development of the desired microstructure of the sheet, including measurement of the hardness level and the fraction of different phases, may be periodically reviewed.

Claims

1. A process for increasing tensile and fatigue strength of a cold rolled close annealed (CRCA) low carbon steel sheet, the process comprising: heating a surface of the cold rolled close annealed (CRCA) low carbon steel sheet to an austenitizing temperature using a multi-track laser beam; and rapidly cooling the steel sheet for phase transformation of an initial microstructure to a harder dual phase structure, wherein a surface temperature of the cold rolled close annealed (CRCA) low carbon steel sheet is controlled such that the surface temperature does not exceed a melting temperature of the cold rolled close annealed (CRCA) low carbon steel sheet, and wherein after cooling, a yield strength and a tensile strength of the cold rolled close annealed (CRCA) low carbon steel sheet are increased by 27-59% and 20-24%, respectively.

2. The process as claimed in claim 1, wherein tracks of the laser beam are overlapped by 0-2 mm.

3. The process as claimed in claim 1, wherein tracks of the laser beam are overlapped by 1 mm or less.

4. The process as claimed in claim 1, wherein the cold rolled close annealed (CRCA) low carbon steel sheet comprises 0.03-0.07 weight % carbon.

5. The process as claimed in claim 1, wherein the cold rolled close annealed (CRCA) low carbon steel sheet composition comprises (wt %) Carbon: 0.03-0.08, Manganese: 0.15-0.25, Sulphur: 0.005-0.008, Phosphorous: 0.009-0.024, and Silicon: 0.005-0.02, Aluminium: 0.04, Vanadium: 0.001, Niobium: 0.001, Titanium: 0.002, with the remainder Iron (Fe).

6. The process as claimed in claim 1, wherein a laser power of the multi-track laser beam is 1.8-3.5 KW.

7. The process as claimed in claim 1, wherein a scanning speed of the multi-track laser beam is 100-250 mm/s.

8. The process as claimed in claim 1, wherein a laser power of the multi-track laser beam is 2.5-3.5 KW.

9. The process as claimed in claim 1, wherein a scanning speed of the multi-track laser beam is 150-250 mm/s.

10. The process as claimed in claim 1, wherein rapid cooling of the cold rolled close annealed (CRCA) low carbon steel sheet is provided by a water cooled copper plate on which the cold rolled close annealed (CRCA) low carbon steel sheet is clamped.

11. The process as claimed in claim 1, wherein the initial microstructure of the cold rolled close annealed (CRCA) low carbon steel is ferrite.

12. The process as claimed in claim 1, wherein the cold rolled close annealed (CRCA) low carbon steel sheet has a thickness of 1 mm or less.

13. The process as claimed in claim 1, wherein, after cooling, the cold rolled close annealed (CRCA) low carbon steel sheet comprises a harder dual phase structure with a hardened layer up to a depth of 0.3 mm.

14. The process as claimed in claim 1, wherein, after cooling, the cold rolled close annealed (CRCA) low carbon steel sheet comprises a harder dual phase structure with a hardened layer depth of 200-300 μm.

15. The process as claimed in claim 1, wherein, after cooling, a fatigue strength of the cold rolled close annealed (CRCA) low carbon steel sheet is 60% of a yield strength of the cold rolled close annealed (CRCA) low carbon steel sheet.

16. The process as claimed in claim 1, wherein, after cooling, a fatigue strength of the cold rolled close annealed (CRCA) low carbon steel sheet is at least 50% of a yield strength of the cold rolled close annealed (CRCA) low carbon steel sheet.

17. A process for increasing tensile and fatigue strength of a cold rolled close annealed (CRCA) low carbon steel sheet, the process comprising: heating a surface of the cold rolled close annealed (CRCA) low carbon steel sheet to an austenitizing temperature using a multi-track laser beam; and rapidly cooling the steel sheet for phase transformation of an initial microstructure to a harder dual phase structure, wherein a surface temperature of the cold rolled close annealed (CRCA) low carbon steel sheet is controlled such that the surface temperature does not exceed a melting temperature of the cold rolled close annealed (CRCA) low carbon steel sheet, and wherein the cold rolled close annealed (CRCA) low carbon steel sheet composition comprises (wt %) Carbon: 0.03-0.08, Manganese: 0.15-0.25, Sulphur: 0.005-0.008, Phosphorous: 0.009-0.024, Silicon: 0.005-0.02, Aluminium: 0.04, Vanadium: 0.001, Niobium: 0.001, and Titanium: 0.002, with the remainder Iron (Fe).

18. The process as claimed in claim 17, wherein tracks of the laser beam are overlapped by 0-2 mm.

19. The process as claimed in claim 17, wherein tracks of the laser beam are overlapped by 1 mm or less.

20. The process as claimed in claim 17, wherein a scanning speed of the multi-track laser beam is 150-250 mm/s.

Description

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

(1) FIG. 1 is a schematic of the processing setup utilized for laser hardening of a CRCA steel sheet according to the invention (1: 1500 μm fiber carrying diode laser beam, 2: optical head for focusing laser beam, 3: 4 mm×4 mm square diode laser beam spot, 4: steel blank, 5: Clamps used for fixing steel sheet, 6: working table and 7: laser interaction region (hardened layer).

(2) FIG. 2 is a schematic representation of laser surface treatment of the steel sheet according to the present invention.

(3) FIG. 3A is a hardness profile of the laser treated surface across the laser tracks as shown in FIG. 2 for a Type 1 steel that has been laser treated according to the present invention.

(4) FIG. 3B is a hardness profile of the laser treated surface across the laser tracks shown in FIG. 2 for the Type 2 steel that has been laser treated according to the present invention.

(5) FIG. 3C is a hardness profile of the laser treated surface across the laser tracks shown in FIG. 2 for the Type 3 steel that has been laser treated according to the present invention.

(6) FIG. 4A is a Tensile Stress-Strain diagram a Type 1 base steel and a Type 1 steel that has been laser surface treated according to the present invention.

(7) FIG. 4B is a Tensile Stress-Strain diagram for a Type 2 base steel and a Type 2 steel that has been laser surface treated according to the invention.

(8) FIG. 4C is a Tensile Stress-Strain diagram for a Type 3 base steel and a Type 3 steel that has been laser surface treated according to the invention.

(9) FIG. 5A is an SEM micrograph of the surface of a base Type 1 steel sheet.

(10) FIG. 5B is an SEM micrograph of the surface of a Type 1 steel sheet that has been laser treated according to the present invention.

(11) FIG. 5C is an SEM micrograph of the surface of a base Type 2 steel sheet.

(12) FIG. 5D is an SEM micrograph of the surface of a Type 2 steel sheet that has been laser treated according to the present invention.

(13) FIG. 5E is an SEM micrograph of the surface of a base Type 3 steel sheet.

(14) FIG. 5F is an SEM micrograph of the surface of a Type 3 steel sheet that has been laser treated according to the present invention.

(15) FIG. 6A is a schematic showing the shape and dimensions for the base steel blank used in the LDH test.

(16) FIG. 6B is a schematic showing the shape and dimensions for the laser treated steel blank used in the LDH test.

(17) FIG. 6C is a photograph of the blanks of FIGS. 6A and 6B for the Type 1, Type 2, and Type 3 steels after LDH testing.

(18) FIG. 7 is a graph showing the punch load as a function of punch displacement for both base Type 1, Type 2, and Type 3 steels and Type 1, Type 2, and Type 3 steels that have been laser treated according to the present invention.

(19) FIG. 8A is photograph of a B pillar made of Type 1 steel.

(20) FIG. 8B is a photograph of a B pillar made of Type 1 steel having a tailored microstructure.

(21) FIG. 8C is a photograph of a B pillar made of Type 1 steel fully hardened by laser treatment according to the present invention.

(22) FIG. 9 is photographs of a base Type 1 steel and a Type 1 steel that has been laser treated according to the present invention.

(23) FIG. 10 is an S-N curve showing the fatigue limit of for a Type 1 base steel and a Type 1 steel that has been laser surface hardened (LSH) according the present invention.

(24) FIG. 11 is an S-N curve showing the fatigue limit of for a Type 3 base steel and a Type 3 steel that has been laser surface hardened (LSH) according the present invention.

(25) FIG. 12 is an S-N curve showing the fatigue limit of for a Type 2 base steel and a Type 2 steel that has been laser surface hardened (LSH) according the present invention.

DESCRIPTION OF THE INVENTION

(26) The process of the current invention involves laser surface hardening treatment of the cold rolled closed annealed steel sheet. Further, steel used in the current invention involves carbon in the low range. The objective of using low carbon and low manganese steel is to develop desired steel composition for use in automotive components. In an embodiment of the current invention, the carbon present in the steel is in the range of 0.04-0.07 weight % and manganese in the range of 0.15-0.25 weight %. In another embodiment of the current invention, the manganese present in the steel is equal to 1.4 weight %. Table 1 shows the chemical composition of the steel grades selected for laser surface treatment according to the current invention.

(27) TABLE-US-00001 TABLE 1 Chemical composition of the steel sheet used for experiments: Type C Mn S P Si Al V Nb Ti (CRCA) [%] [%] [%] [%] [%] [%] [%] [%] [%] Type1 0.03-0.05 0.15-0.25 0.008 0.009 0.02 0.04 0.001 0.001 0.001 Type2 0.03-0.07 0.25 0.008 0.009 0.02 0.04 0.001 0.001 0.001 Type3 0.04-0.08 1.4 0.005 0.024 0.02 0.04 0.001 0.001 0.001

(28) The selected compositions of the steel sheets were laser treated using different laser profiles to evaluate optimized processing parameters. The process of the current invention involves heating the surface of the cold rolled close annealed (CRCA) low carbon steel sheet using a multi-track laser beam to an austenizing temperature and self-quenched for phase transformation of the initial microstructure to harder dual phase structure. The process involves tracks of laser beam overlapping in the range of 0-2 mm. In the embodiment of the invention the tracks of laser beam are overlapping preferably within 1 mm. Further, rapid cooling is achieved by using a water cooled copper plate on which the cold rolled close annealed (CRCA) low carbon steel sheet is clamped.

(29) Table 3 below demonstrates the tensile property evaluation of the all laser treated samples. The laser power of the multi-track laser beam used for treating type1, type 2 and type 3 steel varies in the range of 1.8-3.5 KW. Further, the scanning speed of the multi-track laser beam is in the range of 100-250 mm/s. In an embodiment of the invention, the laser power of the multi-track laser beam is in the range of 2.5-3.5 KW and scanning speed of the multi-track laser beam is in the range of 150-250 mm/s. Further, surface temperature of the cold rolled close annealed (CRCA) low carbon steel sheet is restricted to eliminate any possibility of melting (This is achieved by evaluating effect of process parameters insitu surface temperature and post process analysis.).

(30) The type 1 and type 2 steel contains low manganese with similar carbon contents, however tensile property of base material is different and the improvement of YS for type 2 is significant (59% increase) compared to type 1 after laser surface hardening as evident from FIG. 4 and Table 3. Increase in UTS for both grades type 1 and type 2 steel was 20% after the laser surface hardening. The type 3 though has high Mn content (1.4%) and thus higher tensile strength of base material, however, it shows lesser increase in YS (27%). The increase in UTS was around (20%). The process of the current invention resulted in more increase in YS than UTS in all the cases.

(31) TABLE-US-00002 TABLE 3 Tensile property evaluation of the all laser treated samples. (LSH: Laser Surface Hardening) Type YS (MPa) UTS (MPa) EI (%) Remark Type1-Base 201 297 49 Improvement Type1-LSH 283 361 34 YS: 40% UTS: 21% Type2-Base 204 351 40 Improvement Type2-LSH 325 437 31 YS: 59% UTS: 24% Type3-Base 330 452 34 Improvement Type3-LSH 421 542 23 YS: 27% UTS: 20%

(32) The process variables for laser surface hardening have been identified as 1.8-3.5 KW of laser power and a scan speed of 100-250 mm/s. In an embodiment of the invention, laser surface hardening parameters were identified as 2.5-3.5 KW of laser power and a scan speed of 150-250 mm/s

(33) Results:

(34) Hardenability

(35) The surface microstructure of the laser treated area is illustrated in FIG. 5. At the same time, hardness profile was taken across multi-tracks of laser treated area on the surface and is presented in FIG. 3. The hardness level increased to 225-250 HV as compared to its base hardness of 90-100 HV for type1 steel, whereas, laser treated type 2 steel sheet shows 280-300 HV and type3 shows 320-350 HV as compared to base hardness of 110-120 HV and 150-160 HV respectively (FIG. 3). The SEM micrograph shown in FIG. 5 indicates the formation of hard dual phases (bainite and martensite) which are responsible for the increased hardness values.

(36) Formability

(37) Formability Test

(38) Dome test was carried out on base and laser treated blanks of three different grades: a) Type 1 b) Type 2 and c) Type 3. Blank size was 200 mm×200 mm as shown in FIG. 6. In case of laser treated blanks, the half portion of the blank was treated as shown below. Dome test was carried by a servo-hydraulic forming press. The punch speed was 1.0 mm per second and the blank holding force was 120 kN. It can be seen that the load for CMn 440 is highest followed by DQ and then EDD. This is in line with the expectation as the strengths of base material were in that order only. FIG. 7 shows the Punch force Vs-Punch displacement for laser treated blanks and it can be seen that in this case also the trend follows the same sequence. FIG. 6c shows the comparison between base and laser treated blanks for the three steel grades and it can be seen that for all the steel grades the punch load for laser treated blanks are higher compared to that of the base blank signifying the strength increase due to laser treatment.

(39) Formability Test on B-Pillar

(40) B-pillar was selected as it is one of the components which require variable strength. The forming was carried on the same double action hydraulic forming press. FIG. 8 shows the prototype of the formed component.

(41) Painting Test:

(42) Zinc phosphate treatments for the automobile industry determine the paint adhesiveness and influence the corrosion resistance of the automobile body. We have studied the Zinc phosphability and the cathodic electro deposition (CED) coating on base of Type 1 and Laser treated Type 1 steel substrate. From the different experimental analysis, it can be concluded that on base-Type 1 steel phosphating provides small crystal with uniform coverage. Whereas Laser treated type 1 steel sheet provides large-leaf shape crystal. But both the samples i.e. with and without laser treated Type 1 phosphate sheet provides almost similar performance after CED coating. In both the cases CED coated samples provide good mechanical, adhesion and corrosion resistance properties.

(43) Physical Properties of CED Coating

(44) The result on physical properties of CED films has been tabulated in table 9 i.e., no square was lifted by the cross-hatch test. Hardness of the CED film of this adduct can also be said to be good, as indicated by scratch hardness and pencil hardness as shown in table 4.

(45) TABLE-US-00003 TABLE 4 Coating properties of 3 mint CED coating at 180 V Laser-treated Type 1 Steel Type 1 Steel Parameter phosphating phosphating X-cut adhesion 5-B 5-B Pencil Hardness 5H 5H Scratch Hardness 1500 1500

(46) Salt Spray Test

(47) TABLE-US-00004 TABLE 5 Salt spray test result of CED coated sample Sample Name 7 days After 14 days After 24 days Type1 No No blister, no creepage 1-2 micro blister, 1-2 Steel change (red rust on scribe area) mm creepage on scribe area Laser- No No blister, no creepage No blister, no creepage. treated change (red rust on scribe area) Type1 Steel No Change*: No Blister, no Creepage

(48) Painted panels (base sample and laser treated samples) with scribe on the surface were exposed in ASTM B117 test chamber. At regular interval of time, panels were withdrawn from the test cabinet and visually check for any types of degradation or damage happened on coated surface. Soon after the check, panels were inserted back into the ASTM B 117 test chamber. From the salt spray test result it has been observed that, initially CED coating on type 1 steel and laser treated type 1 steel sample provide almost similar corrosion performance (FIG. 9). But after 24 days of exposure some micro blister and under film creepage was observed on scribe area. Whereas laser treated type 1 steel CED sample showing good corrosion resistance even after 24 days of exposure in SST chamber. There was no blister or under film corrosion observed on laser treated type 1 steel sample.

(49) Fatigue Property Evaluation:

(50) a) S-N Curve to Determine Fatigue Limit:

(51) High cycle fatigue tests were conducted for Type 1 base steel and the type 1 laser treated steel under the following test parameters and plotted S-N curve to evaluate the fatigue life of both the materials for comparison. R=−1, Sinus waveform, Frequency: 20 Hz

(52) No. of cycle to failure vs. the amplitude as depicted in FIG. 10 shows that type1-laser treated steel sheets have better fatigue life compared with the type1-base steel. The endurance limit for type1-base material was obtained in the stress level of 60% of its YS, i.e. 120 MPa, whereas endurance limit for type1-laser treated steel sample is in the stress level of 50% of its YS, i.e. 140 MPa. As the YS of laser treated materials are higher than the base material, the fatigue resistance of the former one is superior.

(53) Similarly, laser treated type3 grade of steel sheets show the endurance limit at stress level of 40% of YS, whereas for type3-base steel sample the same is 50% of YS (FIG. 11). Nevertheless, YS of laser surface treated material is 420 MPa, and for base material is 330 MPa. Therefore, the stress level of endurance limit of laser treated material will be marginally higher than that of base materials. These results suggest that for type3 grade of steel, fatigue resistance is not increasing as much as compared to the type1-laser treated material.

(54) S-N curve for type2 base steel sample and laser treated type2 steel was generated to evaluate its endurance limit as shown in FIG. 12. No. of cycle to failure for type 2 steel is very scattered, however, the stress level of endurance limit is the 60% of YS in the both cases. The no. of cycles to failure for laser treated type2 steel material drops sharply. YS of laser treated type2 steel increases 60% as compared to the type2 base materials.

(55) The process of the current invention offers significant advantages in light of the prior art. The process can be used for laser hardening of low carbon steel that have good formability and hence, can be used for automotive components. The process further results in increasing dent/wear resistance, overall endurance limit for fatigue of the treated steel sheets as evident from the various experimental results described above. The process further results in increasing hardening of the steel sheets and hence can be used for building components which need different strength along the length of the components.

REFERENCES

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