T 1462/11 17-10-2013

I. In its decision dated 3 December 2010 refusing European patent application No. 05028444.7, the examining division held that the subject matter of claim 1 of the main, first and second auxiliary requests then on file lacked novelty over the technical disclosure of either documents

D1: Koh-ichi Sugimoto, T. Iida, J. Sakaguchi, T. Kashima: "Retrained Austenite Characteristics and Tensile Properties in a TRIP Type Bainitic Steel", ISIJ, vol. 40, No. 9, pages 902 to 908;

D2: EP-A-1 553 202, representing prior art pursuant to Article 54(3) EPC;

D6: Koh-ichi Sugimoto, Shun-ichi Hashimoto, Shu-shi Ikeda: "Ultra High-Strength Low-Alloy TRIP-Aided Sheet Steels with Bainitic Ferrite Matrix", International Conference on Advanced High Strength Sheet Steels for Automotive Applications Proceedings, June 6-9, 2004, Winter Park, Colorado, Association for Iron and Steel Technology, pages 63 to 70.

II. On 31 January 2011, the appellant (applicant) lodged an appeal against the decision of the examining division and paid the appeal fee on the same day. The statement setting out the grounds of appeal was received on 28 March 2011.

III. In an official communication annexed to the summons to oral proceedings, the Board gave its provisional view on the case. Specifically, the claims of all requests enclosed with the grounds of appeal were considered to lack novelty (Article 54 EPC).

IV. Oral proceedings were held on 17 October 2013.

The appellant requested that the decision under appeal be set aside and that a patent be granted on the basis of the sole request submitted during the oral proceedings on 17 October 2013.

The claim of this request read as follows:

" A high strength thin steel sheet having high hydrogen embrittlement resisting property, which comprises:

C: 0.10 to 0.25%;

Si: 1.2 to 2.5%;

Mn: 1.2 to 3.5%;

P: 0.15% or less;

S: 0.02% or less;

Al: 0.2 to 1.5%,

which optionally further comprises:

Cu: 0.003 to 0.5% and/or Ni: 0.003 to 1.0% as optional elements,

Ti and/or V: 0.003 to 1.0% as optional elements,

Mo: 1.0% or less (higher than 0%) and Nb: 0.1% or less (higher than 0%) as optional elements,

B: 0.0002 to 0.01% as an optional element,

at least one element selected from the group consisting of:

Ca: 0.0005 to 0.005%,

Mg: 0.0005 to 0.01% and

REM: 0.0005 to 0.01% as optional elements, and

Zr: 0.003 to 1.0% as an optional element,

in terms of percentage by weight, with balance of iron and inevitable impurities;

wherein the metal structure consists of:

residual austenite: 3% by area or more in proportion of the entire structure;

binary phase of bainitic ferrite and martensite with the bainitic ferrite phase acting as the main phase: 90% or more in total;

while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher, and the steel has tensile strength 1180 MPa or higher, wherein the metal structure further satisfies the requirements that:

mean length of minor axes of said residual austenite grains is 1 mym or less; and

minimum distance between the residual austenite grains is 1 mym or less."

V. The appellant's arguments relevant to the present decision are summarized as follows:

The present application aimed at providing a high strength thin steel sheet having a high resistance to hydrogen embrittlement, corrosion and an improved workability under a tensile strength of 1180 MPa or higher. This objective was achieved by the technical features of the thin steel sheet set out in claim 1. Specifically, the claimed steel sheet exhibited a binary phase structure consisting of bainitic ferrite and martensite in order to provide sufficiently high strength. In addition, the microstructure of the claimed steel sheet included lath-shaped residual austenite in the submicron order, a phase which substantially neutralized the hydrogen infiltrated from outside through atmospheric corrosion and thus contributed to the steel sheet's resistance to hydrogen embrittlement. Moreover, the Al content of the steel sheet was controlled to be within the range of 0.2 to 1.5% in order to further improve the steel sheet's resistance to hydrogen embrittlement and corrosion, as reflected in paragraphs [0046] to [0049] of the A1 publication of the application.

Document D2 disclosed ultra high-strength steel sheet having a microstructure comprising bainitic ferrite, polygonal ferrite and residual austenite. Although martensite was mentioned exemplarily as a further phase the microstructure of the known steel sheet may include, none of the examples given in D2 actually mentioned the presence of martensite. Consequently, document D2 did not explicitly disclose a high-strength steel sheet which exhibited a microstructure comprising 90% or more in total of a binary phase of bainitic ferrite and martensite. Moreover, D2 failed to disclose a steel composition comprising aluminium in the range of 0.2 to 0.5%.

Turning to document D1, the TRIP type bainitic steel sheet (TB-steel) exhibited a binary structure of bainitic ferrite and martensite structure only after austempering at the Ms-temperature or higher (450ºC in the example). By contrast, austempering below the Ms-temperature (375ºC in the example) did not result in a binary bainitic ferrite and martensite structure. However, as it was evident from Figures 7a and 8a of D1, the TB steel sheet austempered at 450ºC did not achieve a tensile strength (TS) of 1180 MPa or higher, as required by the steel sheet claimed in the present application.

Document D6 disclosed that TRIP-aided steel sheet with a bainitic ferrite matrix and inter-lath retained austenite films (called BF steel) was obtained after austempering at temperatures higher than Ms. As shown in Figure 5a of D6, the BF steel sheet austempered at 450ºC (i.e. above Ms) did not reach a TS of 1180 MPa or higher as did the claimed steel sheet. Consequently, the BF steel sheet of D6 did not achieve a sufficiently high strength which was a consequence of the different heat treatment temperatures at 950ºC or 1000ºC used in this document.

The subject-matter of the single claim submitted at the oral proceedings was therefore novel over the disclosure of any of documents D2, D1 and D6, respectively.