Method for manufacturing a steel sheet product

Abstract

A steel sheet product and a method for manufacturing the steel sheet product are described, the method includes the steps: providing at least two steel sheets extending in a longitudinal direction (A), cleaning longitudinal edges of the steel sheets by removing any surface oxide layers therefrom, joining the steel sheets along the cleaned longitudinal edges using butt welding without filler material to form a weld, wherein inert gas protection is applied on both a top side and a root side of the weld during welding, thereby obtaining a welded steel sheet product, removal of excess material from the weld, and hardening of the welded steel sheet product by means of heat treatment and subsequent quenching.

Claims

1. A method for manufacturing a steel sheet product, comprising: providing at least one steel slab, strip rolling of the at least one steel slab to form at least one steel strip, forming at least two steel sheets extending in a longitudinal direction (A) from the at least one steel strip, cleaning longitudinal edges of the at least two steel sheets by removing any surface oxide layers therefrom, wherein the surface oxide layers are iron oxides resulting from the strip rolling, joining the at least two steel sheets along the cleaned longitudinal edges using butt welding without filler material to form a weld, wherein inert gas protection is applied on both a top side and a root side of the weld during welding, thereby obtaining a welded steel sheet product, removing excess material from the weld, and hardening of the welded steel sheet product by means of heat treatment and subsequent quenching, thereby producing the steel sheet product.

2. The method according to claim 1, wherein each steel sheet of the at least two steel sheets has a microstructure comprising at least 80% martensite in terms of area percentages and a tensile strength of at least 950 MPa.

3. The method according to claim 1, wherein each steel sheet of the at least two steel sheets is a non-coated steel sheet.

4. The method according to claim 1, wherein the at least two steel sheets have an identical chemical composition.

5. The method according to claim 1, wherein each steel sheet of the at least two steel sheets has a chemical composition comprising, in percent by weight (wt. %): C: 0.050-0.32, Si: 0.10-0.70, Mn: 0.40-1.6, P: 0-0.025, S: 0-0.010, Cr: 0-1.5, Ni: 0-2.5, Mo: 0-0.70, Ti: 0-0.060, Al: 0-0.15, V: 0-0.070, Nb: 0-0.20, B: 0.00020-0.0050, and balance Fe and impurities.

6. The method according to claim 1, wherein the butt welding is carried out using a laser welding process.

7. The method according to claim 1, wherein the quenching is water quenching or oil quenching.

8. The method according to claim 1, wherein the step of removing any surface oxide layers is carried out using at least one of pickling, grinding, and laser ablation.

9. The method according to claim 1, wherein the at least two steel sheets have an identical thickness (t) of 1-6 mm.

10. The method according to claim 1, wherein each one of the at least two steel sheets has a width as measured in a transverse direction (B) of at least 1000 mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

(2) In the drawings:

(3) FIG. 1 is a flow chart illustrating a method according to an embodiment of the invention,

(4) FIG. 2 is a perspective view illustrating welding of a steel sheet product according to an embodiment of the invention,

(5) FIG. 3 is a diagram showing results of tensile tests,

(6) FIG. 4 is a diagram showing results of bending tests,

(7) It is to be noted that all drawings are schematic. Details may thus be omitted and the various features may not be drawn to scale.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(8) A method according to an embodiment of the invention is schematically illustrated in FIG. 1. Reference is also made to FIG. 2, schematically illustrating a steel sheet product 1 being manufactured using the method according to an embodiment of the invention.

(9) In a first step 101, at least two steel sheets 2, 3 extending in a longitudinal direction A are provided. The steel sheets 2, 3 are preferably low alloyed high strength steel sheets which are not provided with any surface coating such as a metal coating. A surface oxide layer may however be present on the steel sheets. The steel sheets 2, 3, may e.g. be produced by strip rolling of a steel slab in a hot rolling or a cold rolling process such that a steel strip is formed. The steel sheets 2, 3 are thereafter formed from the rolled steel strip, e.g. by cutting the steel strip to length. The steel sheets 2, 3 may have identical or substantially identical chemical compositions. A width w of each of the steel sheets, as measured in a transverse direction B perpendicular to the longitudinal direction A, may be at least 1000 mm, preferably at least 1250 mm. The steel sheets 2, 3 may not necessarily have the same width. A thickness t of the steel sheets 2, 3 may be 2-6 mm, such as 2-5 or 3-5 mm. A length-to-width ratio of the steel sheets 2, 3 may by way of example be between 5:1 and 10:1.

(10) In a second step 102, longitudinal edges of the steel sheets 2, 3 along which the sheets are to be joined, or portions of the steel sheets 2, 3 including those longitudinal edges and surrounding areas, are cleaned by removing any surface oxide layers therefrom. Such surface oxide layers may be iron oxides resulting from the strip rolling process, such as mill scale comprising Fe.sub.3O.sub.4 and/or rust comprising Fe.sub.2O.sub.3. Removal of the surface oxide layers may e.g. be performed using grinding, laser ablation or pickling.

(11) In a third step 103, the steel sheets 2, 3 are joined along the cleaned longitudinal edges, i.e. in the longitudinal direction A, using butt welding without filler material to form a weld 4 extending in the longitudinal direction A. Inert gas protection 5, such as He or Ar or a mixture of He and Ar, is applied on both a top side 6 and a root side 7 of the weld 4 during welding to eliminate any presence of oxygen. The welded steel sheet product 1 is thereby obtained. The butt welding may preferably be performed by means of a laser beam 8 applied in a laser welding process.

(12) In a fourth step 104, excess material is removed from the weld such as to remove sharp edges and reduce the risk of crack formation. This may be realized using e.g. grinding, laser ablation, milling or planning, or a combination of two or more of those techniques.

(13) In a fifth step 105, the welded steel sheet product 1 is hardened by means of heat treatment, i.e. annealing, and subsequent quenching to form a martensitic or mainly martensitic microstructure. The fifth step 105 is in the shown embodiment carried out after the fourth step 104. Although this order of the steps is preferred, it is also possible to first harden the steel sheet product and thereafter remove excess material. The quenching is preferably water or oil quenching, but the hardening may also be a press hardening process in which the steel sheet product is quenched within a press hardening tool.

EXAMPLES

(14) A number of steel sheets having a thickness t of 3.3 mm, a width w of 1270 mm and a length in the longitudinal direction A of 8900 mm were produced in a strip rolling process to provide a product batch S1. The steel sheets of the product batch S1 were produced from a single steel slab of a steel grade available under the trade name Hardox® 450, having a chemical composition comprising, in percent by weight (wt. %):

(15) C: max 0.26,

(16) Si: max 0.70,

(17) Mn: max 1.6,

(18) P: 0-0.025,

(19) S: 0-0.010,

(20) Cr: max 1.4,

(21) Ni: max 1.5,

(22) Mo: max 0.60,

(23) B: max 0.005,

(24) balance Fe and impurities.

(25) The steel sheets of product batch S1 were used to manufacture steel sheet products of a product batch S2 using the method according to an embodiment of the invention as described above. Laser welding was used to form the weld. Surface oxide layers were removed prior to welding using grinding. Water quenching was used in the hardening process.

(26) The steel sheets of product batch S1 were also used to manufacture reference product batches S3, S4 following the same method steps, but in one case (S3) without using inert gas protection on the root side of the weld during welding, and in one case (S4) without removing surface oxides prior to welding and without using inert gas protection on the root side of the weld during welding. Details regarding the manufactured product batches S1-S4 are summarized in table I below.

(27) TABLE-US-00001 TABLE I Product Weld Surface oxides removed Inert gas used on both top batch formed? prior to welding? and root sides? S1 No — — S2 Yes Yes Yes S3 Yes Yes No S4 Yes No No

(28) Tensile testing was performed according to standard SS-EN ISO 6892-1 2016, using on one hand test samples from the product batch S2 including the weld and on the other hand test samples from the product batch S1, without weld. Results from tensile testing is shown in FIG. 3, wherein the tensile strength R.sub.m and the yield strength R.sub.p0.2 of the samples from the product batch S1 are shown to the right and the tensile strength R.sub.m and the yield strength R.sub.p0.2 of the samples from the product batch S2, including the weld, are shown to the left. For all tested samples, the tensile strength R.sub.m was approximately 1400 MPa for samples from both batches S1 and S2. The yield strength R.sub.p0.2 was around 1150 MPa for the base material from the product batch S1 (no weld) and 1080-1150 MPa for the samples from the product batch S2, including the weld.

(29) Bending tests were performed according to standard SS-EN ISO 7438 2016, using samples from the product batches S1, S2, S3 and S4, with bending radii of 7 mm, 8 mm and 9 mm and with either the top side or the root side being in tension. Results from the bending tests are shown in FIG. 4. It can be seen that samples from the product batch S2, manufactured according to the proposed method, passed 100% of the bending tests for a bending radius of 7 mm, regardless of whether the top side or the root side was in tension. The samples from the product batch S3 had a pass rate of only 30% for a bending radius of 9 mm when the root side was in tension, and the samples from the product batch S4 had a pass rate of less than 40% at a bending radius of 8 mm. Thus, the samples from the product batch S2 produced according to the invention perform significantly better than samples from the reference product batches S3 and S4.

(30) Microscopy investigations of samples from the different product batches show that the microstructure of samples from the product batch S2 produced according to the invention is martensitic, also across the area of the weld. For samples from the product batch S4, the microstructure within the area of the weld is martensitic with a significant presence of grain boundary ferrite.

(31) It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.