CN111315905A - Method for producing cold-drawn wire - Google Patents

Abstract

The present application provides a method for producing cold drawn wire from granular metallurgical steel, the method comprising the steps of:-Preparing a large amount of molten metal comprising (by weight %): C 0.03-0.15, Si 0.01-1.2, Mn 0.1-1.5, Cr 15-20, Ni 5-10, A1 0.5-1.5, optionally up to 2 elements selected from the group: N, P, S, Cu, Co, W, Mo, Nb, Ti, Zr, Ta, B, Be, Bi, Se, Mg, Ca, Hf, V and REM, and using electroslag refining and atomization to provide metal powder; ‑fill the container with the metal powder and seal the container; Containers are compacted to provide full density billets; ‑ billets are hot worked and finished by wire rolling; ‑ annealed wire is cold drawn with at least 30% area reduction.

Description

Method for producing cold drawn wire

technical field

The present invention relates to a method for making cold drawn wire and wire springs of precipitation hardenable stainless steel, in particular of the type known as 17-7PH.

Background technique

In the 1940s, precipitation stainless steels were developed containing about 17% Cr, about 7% Ni and any precipitation hardening elements (usually Al). This precipitated stainless steel is disclosed in an article in Iron Age, March 1950, pages 79-83. The suitability of steel as a spring material has been suggested in this article. Good spring characteristics combined with good corrosion resistance make steel widely used as a spring material in corrosive environments. An environment of that type is an injection pump for a diesel engine, more specifically a turbo diesel engine. Springs used for this purpose must have the good corrosion resistance of 17-7PH steel, as well as high spring fatigue resistance.

Fatigue resistance largely depends on the surface of the spring wire. For the spring to have high fatigue resistance, the wire should not have any visible defects that could cause fatigue failure. The surface layer should also not contain any large slag inclusions or large areas containing large accumulations of smaller slag inclusions that could also cause failure.

US 6,383,316 discloses a method for making cold drawn wire wherein cast steel is remelted and subjected to ESR treatment. The ESR ingot is hot worked and finished by wire rolling. The rolled wire is pickled and cold drawn. ESR treatment was used to avoid large slag inclusions and large areas containing large accumulations of smaller slag inclusions. This is a great improvement over the previous process.

Detailed ways

The present invention proposes a new way to manufacture 17-7PH spring wire and wire spring. The new approach includes casting large quantities of molten metal to provide ingots; electroslag refining the ingots to provide ESR melts; atomizing the ESR melts to provide metal powders; hot isostatic pressing of the powders into billets; and processing the billets into wire. The new procedure further reduces the size of the inclusions. Furthermore, it essentially removes large areas containing large accumulations of smaller slag inclusions.

More specifically, the method includes preparing a quantity of molten metal comprising (by weight %):

optionally

Up to 2 elements selected from the group: N, P, S, Cu, Co, W, Mo, Nb, Ti, Zr, Ta, B, Be, Bi, Se, Mg, Ca, Hf, V, REM ,and

The balance Fe excluding impurities.

According to one embodiment of the invention, the steel is intentionally alloyed with a small amount of N, preferably 0.005-0.15 wt%, more preferably 0.01-0.15 wt% N. Steel can also be intentionally alloyed with small amounts of Ti, V or Nb. Ti, V or Nb are preferably in % by weight:

Ti 0.01-0.1

Nb 0.01-0.1

V 0.01-0.1

The total amount of Ti, V or Nb is preferably limited to 0.01% by weight to 0.2% by weight.

Preferably, the optional elements are limited to (by weight %):

REM includes at least one of the elements Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

Large quantities of molten metal are cast into the shape of ingots, or preferably into severed strands. Thereafter, electroslag refining of the ingot or cut strands, so-called ESR remelting, is preferably followed by hot working into the shape of the electrode.

ESR stands for Electroslag Refining, also known as Electroslag Remelting. In the ESR process, a conventional slag mixture can be used, which is used according to known techniques and forms a melt during the ESR remelting process, wherein the electrodes to be remelted are melted dropwise so that the droplets will pass through the slag melt Sink into the pool of molten metal (ESR melt) below. For example, a slag mixture can be used, which is known per se and contains approximately 30 _% each of _CaF2 , CaO, and _Al2O3 , and typically contains some amount of MgO in the lime fraction and % One or a few percent of SiO ₂ .

In the case where the melting electrode according to the invention consists of stainless 17-7PH steel containing slag inclusions of different sizes, the ESR melt will obtain a different slag image than before the remelting operation. The ESR slag appears to act as a screen for larger slag particles present in the steel prior to the remelting operation. At least this seems to be true for those slags that have been shown to have a detrimental effect on the fatigue strength of the spring wire (ie slags of the CaO, _Al2O3 and MgO types ₎ . Although at the same time the smaller slag inclusions become more evenly distributed and the possible areas of slag accumulation become smaller and thus more harmless, the amount of this type of smaller slag inclusions in the remelted material is only limited by a greater small impact.

During an ESR remelting operation, a certain amount of aluminum added during the initial preparation of the molten metal can be lost. Therefore, during the ESR remelting operation, more aluminum should be supplied to the molten pool to make up for any losses, so that the ESR melt obtained after the ESR remelting operation will contain 0.5-1.5 Al.

The ESR melt is atomized to provide metal powder. The atomization is preferably carried out by gas atomization. Gas atomization can be carried out by a jet of nitrogen and/or argon.

Preferably, the ESR melt is prepared in a melting furnace, in which the liquid metal is discharged through a discharge opening in the furnace bottom to an atomization chamber below the furnace. For example, ESR-CIG from ALD Vacuum Technologies GMBH is used, but atomized to provide metal powders instead of spray forming.

Alternatively, the ESR melt can be delivered to a melting furnace of the type described in WO2013129996, which is hereby incorporated by reference, without exposing the melt to air. In this type of furnace, the liquid metal is also discharged to the atomizing chamber below the furnace through a discharge opening in the furnace bottom. The ESR melt in the furnace can be protected by inert gas, vacuum or slag covering the melt surface.

An alternative is to have a tiltable ESR furnace and a separate tundish, both arranged in an enclosed chamber containing a protective atmosphere. The atomization chamber is arranged below the tundish. Furthermore, in this furnace and tundish combination, oxygen exposure of the molten metal is minimized.

After atomization, the atomized powder is preferably cooled in a protective atmosphere to avoid re-oxidation. Optionally, the atomized powder can be sieved to the desired powder size. For example, up to 250 μm.

Fill the container with metal powder. After filling, the container is sealed. Thereafter, the vessel is optionally compacted in a cold isostatic press (eg Asea QI 100) at a pressure of at least 1000 bar, preferably about 4000 bar. Thereafter, the vessel is optionally placed in a preheated oven in which the temperature is gradually increased to a temperature of 900°C-1250°C (eg 1130°C) without being subjected to any externally applied pressure . Thereafter, the vessel is transferred to a hot isostatic press (eg HIPen AseaQI 80) in which at least more than 500 bar (eg 1000 bar) is applied at a temperature of 900°C-1250°C (eg 1150°C) bar) pressure. Compaction of the vessel in a hot isostatic press provides a full density billet. Preferably, the temperature is controlled such that the material is consolidated in the absence of a liquid phase. The cold isostatic pressing step followed by the preheating step is mainly used for reasons of process economy and it is possible to transfer the sealed vessel directly to the hot isostatic pressing without prior cold pressing or preheating.

Thereafter, the billets from the hot isostatic press are hot worked into rods, which are then ground and hot rolled into wire rods. Thereafter, the hot rolled wire rod is descaled by mechanical descaling and/or chemical descaling (pickling).

The descaled wire is then annealed at temperatures ranging from 900°C to 1100°C for 0.5 hours to 2 hours. The annealed wire is cold drawn with at least a 30% area reduction.

The cold drawn wire can be rotated into a spring, preferably a coil-shaped spring. The springs are suitably precipitation hardened at a temperature of 450°C-500°C for 0.5 hours-2 hours and then cooled in air.

The structure of the material in the finished spring contains 50-70 vol% of tempered martensite (which contains precipitation phases (preferably _AlNi3 ) of aluminum and nickel in martensite), residual Austenite and max 5% delta-ferrite.

The cross-sectional shape of the cold drawn spring wire may be circular. However, the present invention is not exclusively limited to wires having such a cross-section, but can also be applied to wires having other shapes, ie wires having an oval cross-section, which can be provided in a finished spring rotationally shaped into a helical shape More favorable tension distribution. A rectangular cross-section is also envisaged.

According to a modification of the present invention, the new approach includes atomizing a mass of molten metal to provide a metal powder; hot isostatic pressing the powder into a billet; and processing the billet into wire, thereby providing a cold-drawn production of metallurgical steel from pellets A method of wire, the method comprising the following steps:

- producing a quantity of molten metal comprising (in % by weight):

optionally

Up to 2 elements selected from the group: N, P, S, Cu, Co, W, Mo, Nb, Ti, Zr, Ta, B, Be, Bi, Se, Mg, Ca, Hf, V, and REM ,

as well as,

The balance Fe except impurities;

- Atomizing molten metal to provide metal powder;

- filling the container with metal powder;

- seal the container;

- optionally compacting the container in a cold isostatic press;

- optionally preheating the container;

- compaction of the container in a hot isostatic press to provide a full density billet;

- hot working the billet and finishing by wire rolling;

- descaling the resulting rolled wire;

- annealing the descaled wire; and

- Cold drawn annealed wire with at least 30% area reduction.

Claims (7)

1. A method of producing a cold drawn wire from a particulate metallurgical steel, the method comprising the steps of:

-preparing a quantity of molten metal comprising (in wt%):

optionally, optionally

Up to 2 elements selected from the group: n, P, S, Cu, Co, W, Mo, Nb, Ti, Zr, Ta, B, Be, Bi, Se, Mg, Ca, Hf, V and REM,

and the number of the first and second groups,

the balance Fe except impurities;

-casting the prepared molten metal into the shape of an ingot or, preferably, into a strand that is cut;

-electroslag refining (so-called ESR remelting) the ingot or the cut strand, preferably after hot working into the shape of an electrode, thereby providing an ESR melt or for forming and remelting an ESR ingot;

-atomizing the ESR melt, thereby providing a metal powder;

-filling a container with said metal powder;

-sealing the container;

-optionally compacting the container in a cold isostatic press;

-optionally preheating the vessel;

-compacting the container in a hot isostatic press to provide a full density billet;

-hot working and finishing the blank by wire rod rolling;

-descaling the obtained rolled wire;

-annealing the descaled wire; and

-cold drawing the annealed wire with at least a 30% area reduction.

2. The method of claim 1, wherein the quantity of molten metal comprises (in weight%):

n is 0.005-0.15, preferably 0.01-0.15.

3. The method of claim 1, wherein the quantity of molten metal comprises (in weight%):

4. the method of any one of claims 1 to 3, wherein the quantity of molten metal comprises at least one of the following elements:

Ti 0.01-0.1；

Nb 0.01-0.1；

V 0.01-0.1；

and satisfies the conditions

Ti+Nb+V 0.01-0.2。

5. The method of any of claims 1-4, wherein the method further comprises:

-atomizing the ESR melt in an atomization chamber below an ESR furnace.

6. The method of any of claims 1-4, wherein the method further comprises:

-protecting the remelted ESR ingot in an inert gas, in a vacuum or by slag covering the surface of the ESR melt;

-atomizing the remelted ESR ingot by discharging the liquid metal to an atomizing chamber through a discharge opening in the bottom of a furnace containing the melt.

7. A method for producing a spring, the method comprising the steps of:

-producing a cold drawn wire according to any one of claims 1 to 6;

-rotationally forming a spring, preferably a spiral shape, from the cold drawn wire;

-precipitation hardening the spring, preferably at a temperature of 450 ℃ -500 ℃ for 0.5-2 hours.