Method of laser cutting steel

Abstract

The present invention relates to a method for laser cutting a soft / low alloy steel with improved cutting quality suitable for laser cutting.SOLUTION: A method for laser cutting an alloy steel sheet / plate having a composition in weight% of: C:0.01 to 0.29; Mn: 0.50 to 1.35; P: max 0.04; S: max 0.05; Si: max 0.40; Cr: 0.5 to 0.75 and the rest being iron and impurities, wherein the alloy steel has no intentionally added Cu and Ni and comprises a total cumulative amount of Cu and Ni of less than 0.05%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] FIELD OF THE INVENTION The present invention relates to a method for laser cutting mild / low alloy steel, and more particularly to a method for cutting mild / low alloy steel suitable for laser cutting. In particular, the present invention relates to a method for laser cutting mild / low alloy steel with improved cut quality suitable for laser cutting. [Background technology]

[0002] Background of the Invention Laser cutting and laser precision cutting are applied to many different types of materials where complex shapes require accurate, fast, and force-free processing. Lasers achieve high-precision cuts because they create a narrow kerf (the slit created by the cut). The method results in minimal distortion, often requires no post-processing, subjects the component to only low heat input, and generally produces no cutting dross.

[0003] Nearly all metals can be laser cut: mild steel, stainless steel, and aluminum are the most common applications. Other laser-cut parts are made from wood, plastic, glass, and ceramic. Compared to alternative technologies such as die cutting, laser cutting is cost-effective for small-batch production. A major benefit of laser cutting is the localized laser energy input, which results in a small focal spot diameter, small kerf width, and high cutting speed. Essentially, laser cutting of metals occurs through localized heating of the material above its own melting point within the focal spot of the focused laser. For carbon and low-alloy steels, a jet of oxygen coaxial with the laser beam is used as an assist gas, and the exothermic reaction between the oxygen and the steel substantially contributes to the cutting action. The resulting molten / oxidized material is removed by a gas stream directed coaxially with the laser beam, forming the kerf. Specifically, oxygen is commonly used as the cutting gas for low-alloy (mild) steels.

[0004] As noted in "CO₂ laser beam cutting of steels: Material issues," Murali Manohar, Journal of Laser Applications 18, 101 (2006), minimal residual elements such as Cu, Ni, and Cr are essential to obtain clean and consistent laser cut quality in thick (20–25 mm) plates. Because minimal residual elements are essential to ensure good laser cutting in both as-rolled and shot-blasted conditions, the suitability of steel for laser cutting can be quantified by a simple "laser rapid parameter (LRP)" (defined as LRP = %Cu + %Ni + %Cr). It has also been noted that adhesion and density scales increase with increasing LRP, with the latter leveling off at LRP values ​​near 0.45%–0.5%. Manohar found that a Cu-Ni-rich layer exists at the scale-steel interface, and the enrichment increases with increasing Cu and Ni content. Manohar's findings appear to indicate that acceptable laser-ready steel must have significant amounts of Cu and Ni, and that Cr may be less important. Manohar determined that "steels containing Cu and Ni were found to cut better than those without these elements. However, in contrast to the results with rolled mill plates and shot-blasted mill plates, Cr was found to degrade the cut quality in laboratory plates, even when Cu and Ni were present." This suggests that the role played by Cu and Ni during cutting may be different from that played by Cr. Manohar proposes a cutting mechanism that depends only on Cu and Ni, but not on Cr.

[0005] Having briefly discussed the laser cutting quality of a 32 mm thick plate of Ni- and Cu-free mild steel (containing 0.84Cr), Manohar suggests that a lower-cost mild steel could be produced with Cr between 0.3 and 0.35 and just enough Ni and Cu to bring the LRP up to 0.45. There is no data to suggest that such a steel exists. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] “CO2 laser beam cutting of steels:Material issues”,Murali Manohar,Journal of Laser Applications 18,101(2006) Summary of the Invention [Problem to be solved by the invention]

[0007] There is a need in the art for a method for laser cutting laser ready mild / low alloy steel with improved cut quality. [Means for solving the problem]

[0008] Summary of the Invention The present invention is a method for laser cutting an improved laser-cuttable steel. The method includes the steps of providing a steel-cutting laser and providing a laser-cuttable alloy steel sheet / plate. The alloy steel sheet / plate has the following composition, by weight: C: 0.01-0.29; Mn: 0.50-1.35; P: max 0.04; S: max 0.05; Si: max 0.40; Cr: 0.5-0.75, with the remainder being iron and impurities, and the alloy steel contains no intentionally added Cu and Ni and less than 0.05% total cumulative Cu and Ni. The method further includes directing a laser beam from the steel-cutting laser at the laser-cuttable alloy steel sheet / plate and cutting the laser-cuttable alloy steel sheet / plate with the laser beam.

[0009] The method of the present invention cuts steels having a wide composition range: C: 0.01-0.29; Mn: 0.50-1.35; P: max 0.04; S: max 0.05; Si: max 0.40 (preferably, for thicker plates, Si: 0.15-0.40); Cr: 0.5-0.75; and the remainder being iron and impurities. Furthermore, the method of the present invention includes alloys to which Cu and Ni are not intentionally added. That is, the alloys contain only residual levels of Cu and Ni, and may contain no more. In the method of the present invention, the maximum cumulative amount of Cu and Ni is (in weight percent): Cu + Ni ≤ 0.05%. In a preferred embodiment of the method, the maximum cumulative total amount of Cu and Ni is less than 0.02%.

[0010] Preferably, the alloy of the present invention has a composition in weight percent of: C: 0.10-0.25; Mn: 0.8-1.2; Si: up to 0.15; and Cr: 0.55-0.75. Most preferably, the alloy of the present invention has a composition in weight percent of: C: 0.12-0.23; Mn: 0.8-1.05; Si: 0.02-0.14; and Cr: 0.55-0.72. DETAILED DESCRIPTION OF THE INVENTION

[0011] Detailed Description of the Invention One type of mild steel that is often cut by laser is ASTM A36 type steel for structural applications. The compositional details for A36 mild steel plate are, in weight percent: C: 0.29 max; Mn: 0.80-1.20; P: 0.04 max; S: 0.05 max; Si: 0.40 max, (preferably 0.15-0.40 for thicker plates). This steel must have a minimum yield strength of 250 MPa.

[0012] Another type of steel that can be cut by laser is ASTM A572 type steel. The compositional details for A572 are, in weight percent: C: max 0.26; Mn: 0.50-1.35; P: max 0.04; S: max 0.05; Si: max 0.40, (preferably 0.15-0.40 for thicker plates). This steel must have a minimum yield strength of 290 MPa.

[0013] The present invention is a method for laser cutting such A36 and A572 steels of improved laser-cuttable varieties. The method includes the steps of: providing a steel-cutting laser; and providing a steel sheet or plate having a broad composition range of C: 0.01-0.29; Mn: 0.50-1.35; P: max 0.04; S: max 0.05; Si: max 0.40, (preferably for thicker plates, Si: 0.15-0.40); Cr: 0.5-0.75; and the balance iron and impurities. The method also includes the steps of directing a laser beam from the steel-cutting laser at the laser-cuttable alloy steel sheet / plate; and cutting the laser-cuttable alloy steel sheet / plate with the laser beam.

[0014] The method of the present invention further provides an alloy that does not contain intentionally added Cu and Ni, i.e., the alloy contains only residual levels of Cu and Ni, and may contain no more. In the method of the present invention, the steel contains Cu and Ni in a maximum cumulative amount (in wt. %): Cu + Ni ≦ 0.05%. In a preferred embodiment, the maximum cumulative amount of Cu and Ni is less than 0.02%.

[0015] Preferably, the alloy of the present invention has a composition in weight percent of: C: 0.10-0.25; Mn: 0.8-1.2; Si: up to 0.15; and Cr: 0.55-0.75. Most preferably, the alloy of the present invention has a composition in weight percent of: C: 0.12-0.23; Mn: 0.8-1.05; Si: 0.02-0.14; and Cr: 0.55-0.72.

[0016] Examples of laser-cuttable steels useful for the method of the present invention are shown in Table 1. The compositions are given in weight percent, with Cu and Ni present only in residual amounts.

[0017] [Table 1]

[0018] Table 2 discloses the compositions, in weight percent, of three prior art (commercially available) laser-cut steel comparison samples. As shown, the prior art steels have intentionally added amounts of Cu and Ni, as well as significantly lower Cr contents than the steels of the present invention.

[0019] [Table 2]

[0020] Alloys useful in the process of the present invention and comparative example plates (25.5 mm and 19 mm) were tested for their laser cutting performance. The cutting performance was tested in both the as-hot-rolled condition and the as-hot-rolled / shot-blasted condition. Hot-rolled coils within a range of compositions up to 25 mm thick were tested in the as-rolled, shot-blasted, and pickled and oiled conditions. The laser cutting tests show that the laser cutting quality of the steel plates and coils of the present invention is better than that of comparative prior art laser-ready steel plates.

Claims

1. 1. A method for laser cutting alloy steel sheet / plate, comprising the following steps: providing a steel cutting laser; 1. A process for providing a laser-cuttable alloy steel sheet / plate, wherein the alloy steel sheet / plate comprises, in weight percent: C:0.01~0.29;Mn:0.50~1.35; P: maximum 0.04; S: maximum 0.05; Si: maximum 0.40; Cr: 0.5 to 0.75, and the remainder being iron and impurities, wherein the alloy steel contains no intentionally added Cu and Ni and a total cumulative amount of Cu and Ni of less than 0.05%; directing a laser beam from said steel cutting laser at said laser cuttable alloy steel sheet / plate; and cutting said laser-cuttable alloy steel sheet / plate with said laser beam A method comprising:

2. 2. The method for laser cutting alloy steel sheet / plate according to claim 1, wherein the steel further comprises Si: 0.15-0.

40.

3. 2. The method for laser cutting alloy steel sheet / plate according to claim 1, wherein the steel further comprises C: 0.10-0.

25.

4. 4. The method for laser cutting alloy steel sheet / plate according to claim 3, wherein the steel further comprises Mn: 0.8-1.

2.

5. 5. The method for laser cutting alloy steel sheet / plate according to claim 4, wherein the steel further comprises Si: max 0.

15.

6. 6. The method for laser cutting alloy steel sheet / plate according to claim 5, wherein the steel further comprises Cr: 0.55-0.

75.

7. 2. The method for laser cutting alloy steel sheet / plate according to claim 1, wherein the steel further comprises C: 0.12-0.

23.

8. 8. The method for laser cutting alloy steel sheet / plate according to claim 7, wherein the steel further comprises Mn: 0.8-1.

05.

9. 9. The method for laser cutting alloy steel sheet / plate according to claim 8, wherein the steel further comprises Si: 0.02-0.

14.

10. 10. The method for laser cutting alloy steel sheet / plate according to claim 9, wherein the steel further comprises Cr: 0.55-0.72.

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