TECHNICAL FIELD

The invention relates to a creping blade for the detachment of a travelling paper web from a dryer cylinder. The creping blade is made of a cold rolled steel having a high tensile strength and a high hardness.

BACKGROUND OF THE INVENTION

In the paper industry, creping blades are used for the manufacture of tissue. Creping blades may be made of different materials such as steels, ceramics, composites and polymers. In addition, different types of coating may be applied in order to reinforce the working edge of the creping blade such as described in US 6,207,021 Bl . Compositions of steel alloys, which may be used for creping blades are listed in WO2012/128700 Al . US 2008/0096037 discloses a creping blade manufactured from a PM-steel comprising 1-3 %C, 4-10 %Cr, 1-8 %Mo and 2.5-10 %V.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a creping blade of steel having an improved lifetime. A further object is to provide a method of manufacturing such a creping blade.

The foregoing objects, as well as additional advantages are achieved to a significant measure by making the creping blade from a cold work tool steel with a carefully balanced composition as set out in the claims as well as by subjecting the cold rolled steel strip used for the creping blade to a specific continuous hardening and tempering treatment.

The invention is defined in the claims. DETAILED DESCRIPTION

Below the importance of the separate elements and their interaction with each other as well as the limitations of the chemical ingredients of the claimed alloy are briefly explained. All percentages for the chemical composition of the steel are given in weight % (wt. %) throughout the description.

Carbon is to be present in an amount of 1.2-1.5 wt.%, preferably 1.3-1.4 wt.% so that the steel will get the desired hardness and strength. Carbon also contributes to a good wear resistance by forming M(C,N), where M is vanadium, in the first place and other metals such as Mo in the second place.

Silicon shall be present in the steel in an amount of between 0.1-0.8 wt.%, preferably 0.2-0.6 wt.%. Silicon increases the carbon activity. By keeping the content of silicon low, it is possible to keep the carbon activity low in order to avoid or minimize the precipitation of undesired chromium-rich M23C6 particles.

Manganese contributes to give the steel the desired hardenability.

Chromium shall be present in the steel in an amount between 4.2 and 5.2 wt.% in order to give the steel a good hardenability. Cr also adds to the oxidation resistance of the alloy. However, chromium may form undesirable carbides. Molybdenum is known to have a very favourable effect on the hardenability. Mo shall be present in the steel in an amount between 3.0 and 4.0 wt.%, preferably 3.3 - 3.7 wt. %.

Molybdenum is a carbide forming element. The balanced Mo content of the present invention results in a very low amount of primary M ₆ C-carbides and in a high amount of molybdenum containing M(C,N).

Vanadium is favourable for the tempering resistance and the wear resistance of the steel, as it together with carbon form comparatively round, evenly distributed primary precipitated M(C,N) in the matrix of the steel. In the steels used for the inventive blade M is mainly V, Mo and Cr. Vanadium shall therefore be present in a content of 3.2-4.2 wt.%), preferably 3.5-3.9 wt. %>. In connection with the hardening, the primarily precipitated M(C,N)-particles will be dissolved to a certain extent depending on the austenitizing temperature At the subsequent tempering, very small vanadium-rich secondary particles of the M(C,N)-type are precipitated instead. The blade has a matrix consisting of tempered martensite containing small primary carbides of the M(C,N)-type and a high number of very small, evenly distributed secondarily precipitated M(C,N), when used for the detachment of a travelling paper web from a dryer cylinder steel.

Nitrogen is present in an amount of 0.01-0.15 wt.%>. For this reason carbo-nitrides M(C,N) may form. These will be partly dissolved during the austenitizing step and then precipitated during the tempering step as particles of nanometer size. The thermal stability of vanadium carbo-nitrides is considered to be better than that of vanadium carbides, hence the tempering resistance of the steel may be improved. Further, by tempering at least twice, the tempering curve will have a higher secondary peak.

Tungsten. In principle, molybdenum may be replaced by twice as much tungsten. However, tungsten is expensive and it also complicates the handling of scrap metal. The maximum amount is therefore limited to 0.5 wt.% and most preferably no additions are made.

Copper is an element, which may contribute to increasing the hardness of the steel. Cu may be present in an amount of up to 0.5 wt.%. However, copper may negatively influence the hot ductility of the steel. Further, it is not possible to extract copper from the steel once it has been added. This drastically makes the scrap handling more difficult. For this reason, copper shall preferably not be deliberately added. The impurity content is preferably limited to 0.25 wt. %.

Aluminium may be used for deoxidation of the steel. However, if the steel is produced by melt atomizing ,then no deliberate addition of Al is made.

Boron may be added in an amount of < 0.01 wt.%, preferably 0.0005-0.003 wt.% to further increase the hardenability.

Nickel and cobalt may be present in an amount of up to 3 wt. % each. They increase the hardenability but are expensive. A deliberate addition of these elements are therefore not necessary.

Niobium can in principle be used to replace part of the vanadium. However, Nb is not as efficient as V in that it has an atomic weight nearly twice that of V. Accordingly, Nb is normally not deliberately added.

Ti, Zr and Hf are elements, which form cubic carbides in the steel. However, these elements need not be present in the steel. P, S and O are impurities, which may be present in the steel alloy. Allowable contents are P < 0.03, S < 0.03 and O < 0.015. REM as well as Ca and Mg may be used for sulphur removal or modification of sulphide inclusions. REM may be present in an amount of up to 0.2 wt.%. Mg and Ca may be present in an amount of 0.01 wt.% each. These contents may be considered as impurity contents.

The creping blade of the present invention is used in the hardened and tempered condition. It has a tensile strength of 1800-2500 N/mm ² , preferably 1900-2400 N/mm ² and a hardness of 57-66 HRC, preferably 57-64 HRC. The reasons for these high values are the balanced steel composition in combination with the heat treatment, which results in a relatively high amount of small and uniformly distributed primary M(C,N)-particles and a very high number of nano-sized secondary precipitated M(C,N)-particles in a tempered martensitic matrix.

The austenitizing temperature is 950-1100 °C, preferably 1000-1040 °C. The tempering temperature is 500-650 °C, preferably 610-630 °C.

During austenitizing, part of the primary M(C,N)-particles will dissolve to a certain extent resulting in a martensitic matrix, which comprises 0.3-0.7 %C, preferably 0.4-0.6 %C, directly after hardening. One reason for the high dissolved carbon content may be seen in the fact that nitrogen in the steel alloy partly replaces carbon in the M(C,N)- particles. Hence, during tempering there will be a massive precipitation of secondary nano-sized M(C,N)-particles resulting in an increased tensile strength and hardness. Another reason may be seen in the fact that all carbides of the type M23C6, M7C3 and M ₆ C will dissolve during austenitizing. Hence, the content of Mo dissolved in the matrix will be increased and nano-sized Mo-rich M2C-particles and Mo-containing M(C,N)-particles may be formed during tempering. The precipitation of these very fine secondary carbides results in a marked secondary hardening effect resulting in a hardness value of 57-66 HCR and an enhanced wear resistance.

EXAMPLE

A steel melt having the composition given below was subjected to gas atomizing and HIP-ing.

C 1.43

Si 0.38

Mn 0.43

Cr 4.68

Mo 3.55 V 3.73

N 0.05

balance Fe and impurities. The steel was subjected to hot rolling to a thickness 2.75 mm and was cold rolled to a thickness of 1.25 mm in order to develop a superior surface finish. The cold rolled steel was subjected to a continuous hardening at an average austenitizing temperature of 1020 °C. Quenching occurred initially in a molten lead bath held at 320 ± 10 °C to get an even temperature distribution and then using water cooled plates to bring the material to room temperature whilst achieving the required high level of flatness. Low speed continuous tempering at 620 °C occurred twice with cooling to room temperature necessary between tempers.

The creping blade had a tensile strength of 1930 N/mm ² , an elongation A50 of 5% and a hardness of 58 HRC. The working edge had a hardness of 650 HV1.

The wear resistance was evaluated in side-by-side tests on a machine that simulates wear in a creping operation. The inventive steel blade was compared to a traditional UHB 15LM carbon steel blade (0.75 %C, 0.2 %Si, 0.73 %Mn). The result is shown in Fig. 1. This figure reveals not only that the total wear of the inventive blade is roughly 25% of the standard steel blade but also that the wear is much more linear, which is important for users so that their intervention during operation is minimized.

Hence, as a rule of thumb the service life of the inventive blade can be estimated to be at least four times that of a conventional blade.