Field of the Invention

The present invention relates to mild/low alloy steels and more particularly mild/low alloy steels suitable for laser cutting. Specifically, the present intention relates to mild/low alloy steels suitable for laser cutting with improved cut quality.

Background of the Invention

Laser cutting, and laser fine cutting are applied for different kinds of materials where complex contours demand precise, fast and force-free processing. Lasers create narrow kerfs (a slit made by cutting) and thus achieve high-precision cuts. This method results in minimal distortion and in many cases post-processing is not necessary as the component is subject to only little heat input and can mostly be cut dross-free.

Almost all kinds of metals can be laser cut: mild steel, stainless steel and aluminum are the most common applications. Other laser cut parts are made from wood, plastics, glass and ceramics. Compared to alternative techniques like die cutting, laser cutting is cost-efficient already for small-batch production. The big benefit of laser cutting is the localized laser energy input providing small focal diameters, small kerf widths, and high feed rate. Basically, the cutting of metals with lasers happens through the local heating of the material above its melting point in the focal point of the focused laser. In the case of carbon and low alloy steels, a jet of oxygen coaxial with the laser beam is used as the assist gas and the exothermic reaction of oxygen with the steel contributes significantly to the cutting action. The resulting molten/oxidized material is ejected by a gas flow oriented coaxially to the laser beam so that a kerf is formed. For low-alloyed (mild) steels in particular, oxygen is typically used as cutting gas.

As stated in“CO2 laser beam cutting of steels: Material issues”. Murali Manohar, Journal of Laser Applications 18, 101 (2006), usually a minimum level of residual elements such as Cu, Ni, and Cr are necessary to obtain clean and consistent laser cut quality in thick (20-25 mm) plates. Also, since a minimum level of residual elements is necessary to ensure good laser cuts in both the as-rolled as well as shot-blasted conditions, the suitability of the steel for laser cutting can be quantified by a simple "laser readiness parameter" (LRP), which was defined as LRP= %Cu+ %Ni+ %Cr. It also states that scale adherence and scale density increase with increasing LRP, with the latter leveling off at an LRP value around 0.45%-0.5%. Manohar found that a Cu-Ni- rich layer is present at the scale-steel interface, and the degree of enrichment increases with increasing Cu and Ni. Manohar seems to indicate that acceptable laser ready steels must have significant amounts of Cu and Ni and that Cr may be less important. That is, Manohar determined that:“Steels bearing Cu and Ni were found to cut better than those without these elements. However, contrary to results with as-rolled and shot- blasted mill plates, Cr was found to degrade cut quality in lab 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 by Cr. Manohar goes on to propose a cutting mechanism that only depends on Cu and Ni and less on Cr.

While briefly discussing the laser cutting quality of a 32 mm thick plate of mild steel which is free of Ni and Cu, (containing 0.84 Cr), Manohar suggests that lower cost mild steels could be made using between 0.3-0.35 Cr and just enough Ni and Cu to bring the LRP up to 0.45. No data on such a steel is presented.

There is a need in the art for laser ready mild/low alloy steels with improved cut quality.

Summary of the Invention

The instant invention is an improved laser cuttable steel. The inventive steels have a broad compositional range of C: 0.01 - 0.29; Mn: 0.50 -1.35; P: 0.04 max; S: 0.05 max; Si: 0.40 max, (preferred for thicker plates Si: 0.15 - 0.40); Cr: 0.5 - 0.75; and the remainder being iron and impurities. Further the inventive alloys are free from intentional additions of Cu and Ni. That is, the alloy may contain residual levels of Cu and Ni only, nothing higher. In the frame of the invention, the maximum cumulated amount of Cu and Ni is such that (in wt %): Cu + Ni < 0.05%. In a preferred embodiment, the maximum cumulated total amount of Cu and Ni is below 0.02%.

Preferably the alloys of the present invention have a composition in wt. % of: C: 0.10 - 0.25; Mn: 0.8 - 1.2; Si: max 0.15; and Cr: 0.55 - 0.75. Most preferably the alloys of the present invention have a composition in wt. % 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

One type of mild steel that is often cut by laser is the ASTM A36 type steel for structural applications. The compositional specifications for A36 mild steel plates is, in wt.%: C: 0.29 max; Mn: 0.80 - 1.20; P: 0.04 max; S: 0.05 max; Si: 0.40 max, (preferred for thicker plates 0.15 - 0.40). The steel must have a minimum Yield Strength of 250 MPa.

Another type of steel that can be cut by lasers is the ASTM A572 type steel. The compositional specifications for A572 is, in wt.%: C: 0.26 max; Mn: 0.50 -1.35; P: 0.04 max; S: 0.05 max; Si: 0.40 max, (preferred for thicker plates 0.15-0.40). The steel must have a minimum Yield Strength of 290 MPa.

The instant invention is an improved laser cuttable version of such A36 and A572 steels. The inventive steels have a broad compositional range of C: 0.01 - 0.29; Mn: 0.50 -1.35; P: 0.04 max; S: 0.05 max; Si: 0.40 max, (preferred for thicker plates Si: 0.15 - 0.40); Cr: 0.5 - 0.75; and the remainder being iron and impurities. Further the inventive alloys are free from intentional additions of Cu and Ni. That is, the alloy may contain residual levels of Cu and Ni only, nothing higher. In the frame of the invention, the maximum cumulated amount of Cu and Ni is such that (in wt %): Cu + Ni < 0.05%. In a preferred embodiment, the maximum cumulated amount of Cu and Ni is below 0.02%.

Preferably the alloys of the present invention have a composition in wt. % of: C: 0.10 - 0.25; Mn: 0.8 - 1.2; Si: max 0.15; and Cr: 0.55 - 0.75. Most preferably the alloys of the present invention have a composition in wt. % of: C: 0.12 - 0.23; Mn: 0.8 - 1.05; Si: 0.02 - 0.14; and Cr: 0.55 - 0.72.

Examples of the inventive laser cuttable steel are presented in Table 1. The compositions are in wt.%. Cu and Ni are only present in residual amounts. Table 1

Table 2 discloses the composition, in wt.%, of three prior art (commercially available) laser cutting steel comparative samples. As can be seen, the prior art steels have amounts of Cu and Ni added intentionally and significantly lower contents of Cr than the inventive steels.

Table 2

Plates of the inventive alloys and comparative examples (25.5mm and 19mm) were tested for their laser cutting performance. The cutting performance was tested in both the as-hot-rolled condition and the hot-rolled/shot-blasted condition. Hot rolled coils within the compositional range up to 25 mm thick were also tested in the as-rolled, shot blasted and pickled & oiled condition. Laser cut trials indicate that the laser cut quality of inventive steel plates and coils is better than the comparative prior art laser ready steel plates.