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Title:
PROCESS FOR PRODUCING HIGH STRENGTH STEEL, AND TO A STEEL PRODUCED THEREBY
Document Type and Number:
WIPO Patent Application WO/2012/104306
Kind Code:
A1
Abstract:
Process for producing a high strength steel, wherein the process comprises producing a vacuum-degassed steel melt, wherein the target oxygen content of the melt at the end of the ladle treatment is obtained by measuring the actual oxygen content of the melt and the oxygen level is adjusted by adding Al and/or Zr whereby the excess addition of Al should be avoided and the target oxygen content of the melt at the end of the ladle treatment is at most 100 ppm. Upon casting the steel a strip or slab is obtained of ultra-low carbon steel comprising at most 0.002 % of acid soluble Al and at most 0.004 % Si and a total oxygen content of at most 150 ppm.

Inventors:
RICHARDS BERNARDUS JOHANNES (NL)
SCHAAR BENNO (NL)
TIEKINK WOUTER KAREL (NL)
Application Number:
PCT/EP2012/051566
Publication Date:
August 09, 2012
Filing Date:
January 31, 2012
Export Citation:
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Assignee:
TATA STEEL IJMUIDEN BV (NL)
RICHARDS BERNARDUS JOHANNES (NL)
SCHAAR BENNO (NL)
TIEKINK WOUTER KAREL (NL)
International Classes:
C21C7/10; C21D8/04; C21D9/48; C22C38/00
Domestic Patent References:
WO2011012242A12011-02-03
Foreign References:
EP0556834A21993-08-25
EP0505732A11992-09-30
EP1852514A12007-11-07
EP1323837A12003-07-02
JP2000144330A2000-05-26
Other References:
None
Attorney, Agent or Firm:
BODIN, Andre (CA Ijmuiden, NL)
Download PDF:
Claims:
CLAIMS

1. Process for producing a high strength steel said process comprising :

- producing a vacuum-degassed steel melt in a steelmaking step comprising a ladle treatment comprising, by weight,

o at most 0.02% carbon,

o at most 0.010% nitrogen,

o at most 0.10% phosphorus,

o at most 0.020% sulphur,

o at least 0.15% manganese,

o at most 0.0045% boron,

o at most 0.03% titanium,

o at most 0.1% niobium,

o at most 0.2% vanadium,

o at most 3% chromium,

o at most 6% nickel,

o at most 1.5% molybdenum,

o at most 0.005% calcium,

o at most 0.006% zirconium,

o at most 0.005% barium,

o at most 0.005% strontium,

o at most 0.05% in total of rare earth elements, such as cerium, o and balance iron and inevitable impurities,

- wherein a target oxygen content of the melt at the end of the ladle

treatment of the melt is obtained by measuring the actual oxygen content of the melt followed by adding a suitable amount of aluminium and/or zirconium in a suitable form to the melt to bind oxygen wherein the target oxygen content of the melt at the end of the ladle treatment is at most 100 ppm;

- casting the steel thus produced in a continuous casting process to form a slab or strip;

- wherein said process provides a slab, strip or sheet of ultra-low-carbon steel comprising at most 0.002% of acid soluble aluminium and at most 0.004% silicon and a total oxygen content of at most 150 ppm.

2. Process according to any one of the preceding claims wherein the steel slab or strip comprises

at most 0.008% carbon, preferably at most 0.0045%.

3. Process according to claim 1 or 2, wherein the steel melt comprises 0.002% carbon and/or at most 0.003% silicon and/or wherein the slab, strip or sheet comprises a total oxygen content of at most 100 ppm.

4. Process according to any one of the preceding claims, wherein the target

oxygen content of the melt at the end of the ladle treatment of the melt is at least 10 ppm. 5. Process according to any one of the preceding claims, wherein the target

oxygen content of the melt at the end of the ladle treatment of the melt is at most 70 ppm, preferably at most 60 ppm.

6. Process according to any one of the preceding claims wherein the process

provides a strip or sheet of ultra-low-carbon steel comprising at most 0.001% of acid soluble aluminium and/or at most 0.002% silicon.

7. Process according to any one of the preceding claims wherein the steel

comprises at most 3% manganese.

8. Process according to any one of the preceding claims wherein the steel slab or strip comprises

at most 5 ppm B, or wherein the steel comprises between 10 and 30 ppm B and/or

- at most 0.002% carbon and/or

between 0.0012 and 0.0030% nitrogen.

9. Process according to any one of the preceding claims wherein the steel slab or strip comprises

- hot-rolling the slab at a temperature above Ar3 to obtain a hot-rolled strip;

10. Process according to claim 9 wherein the steel slab or strip comprises

cold-rolling the hot-rolled strip with a cold rolling reduction of between 40 and 96% to obtain an intermediate cold-rolled strip;

- annealing the intermediate cold-rolled strip;

optionally subjecting the intermediate cold-rolled strip to a second cold rolling down to a final sheet thickness;

optionally cutting the strip into sheets or blanks;

Description:
PROCESS FOR PRODUCING HIGH STRENGTH STEEL, AND TO A STEEL PRODUCED THEREBY

The present invention relates to a process for producing a high strength steel and to steel produced thereby.

High strength steels generally rely on carbon in one or more strengthening mechanisms. These mechanisms vary from the formation of pearlite to increase the strength, to the transformation of carbon containing austenite into martensite or ba inite, e.g . in a heat treatment or ca rbon steels or the thermomechanical treatment of dual-phase, TRIP, complex phase steels, bainitic or martensitic steels, or to the formation of very fine carbide precipitates in HSLA steels possibly a lso resu lti ng in a very fine microstructu re as a resu lt of thermomecha n ica l rolling.

As the carbon content rises, steel has the ability to become harder and stronger through heat treating, but this also makes it less ductile. Regardless of the heat treatment, a higher carbon content reduces weldability. Welding of steels which derive their strength from a transformation product such as dual-phase and TRIP steels may be awkwa rd as the heat input from the welding process may destroy the strength of the steel.

It is an object of the invention to provide an alternative process for producing a high strength steel.

Accord ing to the first aspect a process is provided for producing a high strength steel, said process comprising :

- producing a vacuum-degassed steel melt in a steelmaking step comprising a ladle treatment comprising, by weight,

o at most 0.02% carbon,

o at most 0.010% nitrogen,

o at most 0.10% phosphorus,

o at most 0.020% sulphur,

o at least 0.15% manganese,

o at most 0.0045% boron,

o at most 0.03% titanium,

o at most 0.1% niobium,

o at most 0.2% vanadium,

o at most 3% chromium,

o at most 6% nickel,

o at most 1.5% molybdenum,

o at most 0.005% calcium, o at most 0.006% zirconium,

o at most 0.005% barium,

o at most 0.005% strontium,

o at most 0.05% in total of rare earth elements,

o and balance iron and inevitable impurities,

- wherein a target oxygen content of the melt at the end of the ladle

treatment of the melt is obtained by measuring the actual oxygen content of the melt followed by adding a suitable amount of aluminium in a suitable form to the melt to bind oxygen wherein the target oxygen content of the melt at the end of the ladle treatment is at most 100 ppm;

- casting the steel thus produced in a continuous casting process to form a slab or strip;

- wherein said process provides a slab, strip or sheet of ultra-low-carbon steel comprising at most 0.002% of acid soluble aluminium and at most 0.004% silicon and a total oxygen content of at most 150 ppm.

With the process according to the invention a steel sla b or strip ca n be produced having very clea n grain boundaries. As a result, the recrystallisation temperature of the steel is much lower than conventional ultra-low carbon steels. This phenomenon is attributed to the extremely low levels of silicon a nd acid soluble a lumi nium in the final steel strip or sheet and the presence of fi nely dispersed ma nga nese and/or iron oxide particles. A s a result of the low recrystallisation temperature of the steel the annealing temperatures can be reduced as well, leading to a more economical process as well as a reduced tendency for grain growth in the product. The reduced annealing temperatures also prevent sticking in batch annealing processes and reduce the risk of rupture in continuous annealing. A further advantage of the very clean grain boundaries is the strongly reduced susceptibility to corrosion on the grain boundaries. This is especially relevant for the application of the steel in the prod uction of battery cases. The coating systems used in the production of batteries may be leaner (e.g. thinner coating layers or fewer coating layers) when using a substrate with a better corrosion resistance. For producing a mild cold-rolled steel from the slab or stri p, the phosphoro us content shou ld be selected to be not g reater tha n 0.025wt%, prefera bly at most 0.020%. A suitable maximum for silicon is 0.003%. The manganese content is at least 0.15% to attain a minimum strength increase caused by ODS. A preferable minimum value is 0.3% where the strength increase becomes significant. The maximum content is not limited technically, only economically. A suitable maximum value for the manganese content is 4%, but preferably the manganese content does not exceed 3%.

The essential difference with the conventional process for producing an ultra-low-carbon steel strip or sheet is that the ladle treatment of the melt during the vacuum-degassing step, e.g. in an RH-process, does not target a removal of the oxygen by killing it by adding excess aluminium to form alumina particles, but a process wherein the oxygen content of the melt is monitored and controlled, and a dedicated amount of aluminium is added so as to avoid the addition of excess aluminium to the melt which would be present in the final steel as acid soluble aluminium (i.e. in the form of metallic aluminium, not as alumina). It is therefore not an aluminium killed steel in the sense of EN10130. The addition of the precise amount of aluminium ensures that all alumina formed in the ladle treatment is removed from the melt prior to solidification during continuous casting, so that the resulting steel contains hardly any or no aluminium oxide, but instead it contains very small particles which form during the solidification in the mould. These particles are believed to be MnO-MnS rich types. Very small nano- particles are created in the mould and the slab as well and these are believed to be Fe x Oy-particles combined with Mn x O y -S. The generation of these oxide- containing nanoparticles leads to the so-called oxide dispersed strengthening (ODS). There may also be a contribution of the nanoparticles to strength increase by a precipitation hardening mechanism. The degassing of the molten steel may be made by any conventional methods such as the RH method, the RH-OB method, or in a vacuum tank degasser. The oxygen content of the liquid steel may be measured using expendable oxygen sensors to measure the melt's oxygen activity.

Instead of adding aluminium to reduce the oxygen activity to the required window at the ladle treatment, any other deoxidant may be used that can reach this window, i.e. 10 and 100 ppm oxygen activity at approximately 1600 °C, e.g. Ti, Zr, Ca, Sr, Ba etc.

The absence of metallic aluminium prevents the formation of aluminium- nitride precipitates at later stages of the process and therefore provides clean grain boundaries. Moreover, the absence of AIN also prevents many problems associated with the dissolution and precipitation characteristics of AIN in the hot strip process such as inhomogeneities of the microstructure and properties over length and width of the strip as a result of the difference in thermal path of different positions of the hot rolled strip in coiled form. There is no need to dissolve the AIN in the reheating furnace of a hot strip mill so a lower furnace temperature can be used, nor is there a need to use a high coiling temperature to allow the AIN to precipitate in the coil. This in turn leads to an improved pickling ability. The chemistry of the sla b or stri p resu lts i n the formation of fi nely d ispersed oxides, comprising main ly ma nga nese oxides. Of these inclusions, relatively la rge size i ncl usions act as n uclei for the recrysta l l isation d uring annealing of cold-rolled steel, while relatively small size inclusions may act to become appropriate barriers with respect to grain coarsening caused after the recrystallisation to thereby control the grain size of the steel.

The carbon content of the steel melt is preferably limited to at most 0.02% beca use w hen a h ig her ca rbon content is used , the carbon forms carbon monoxide in the manufacturing stage during which the steel is molten, and that CO in tu rn rema ins as blow-hole defects in the solidified steel. Moreover, the boiling effect may cause operational problems during casting. It should be noted that the silicon in the solidified steel may be present as silicon oxide and/or as metallic silicon. More preferably the carbon content is limited to 0.008%. Even more preferably the carbon content is limited to at most 0.0045% (i.e. 45 ppm).

During casting very little and preferably no Al is left in the steel, and as a conseq uence the Si pick-up, which norma lly occurs according to the following reaction Al stee i + Si0 2 - Al 2 0 3 + Si s t ee i) does not occur due to the low Al-content.

A conventional process for producing an aluminium killed ultra-low-carbon steel strip or sheet results in an oxygen activity or dissolved oxygen content at the end of the ladle treatment of the melt, i.e. immediately prior to casting, of a bout 3 to 5 ppm. In the process according to the invention the target oxygen content of the melt at the end of the ladle treatment of the melt is preferably at least 10, or even more preferably 20 ppm. A preferable maximum target oxygen content of the melt at the end of the lad le treatment is 100, or even more preferably 80 ppm. It should be noted that the oxygen content of the melt may increase during the time between the end of the ladle treatment and the casting step. The total oxygen content of the slab or strip may therefore be at most 150 ppm, preferably at most 120 and even more preferably at most 100 ppm. The total oxygen content comprises oxides as well as oxygen in solution.

In an embodiment the target oxygen content of the melt at the end of the ladle treatment of the melt is at least 10 ppm. This minimum values ensures that sufficient manganese oxides are formed. To avoid too many large oxides and to avoid too much CO-formation, it is preferable that the target oxygen content is at most 100 ppm. The inventors found that a target oxygen content at the end of the ladle treatment between 10 a nd 70, provided a good compromise. A more preferable maximum value is at most 60 ppm or even at most 40 ppm. A suitable minimum target oxygen content of the melt at the end of the ladle treatment of the melt is at least 20 ppm. It is believed that the relatively high oxygen content of the steel melt prior to casting results in a low viscosity as a result of the high oxygen potential of the melt.

By steering the process on the oxygen content, rather than on the aluminium content the amount of acid soluble aluminium and the amount of silicon is as low as possible. It is preferable that the strip or sheet of ultra-low- carbon steel produced according to the invention comprises at most 0.001% or even at most 0.0005% of acid soluble aluminium and/or at most 0.003% or even 0.002% silicon . Even more prefera ble the silicon content is at most 0.001 %. Ideally, there is no acid soluble aluminium and no silicon in the solidified steel.

This process produces a slab or strip suitable for producing a high strength . Depending on the alloying additions, the mechanical properties of the steel thus produced can be tailored.

Without d ispersed oxides, norma l polygona l ferrite g ra ins form d u ring cooling from the austenite region such as on the run-out table of a hot strip mill or after a h ig h temperatu re a n nea l i ng treatment. In the presence of finely dispersed oxides such as in the steel according to the invention, the oxides act as nucleation sites for the formation of ferrite leading to acicula r ferrite and/or intragranular polygonal ferrite. This microstructure shows a significantly higher strength than the microstructure consisting of normal polygonal ferrite grains. This effect also occurs during the cooling after welding, and therefore the material to be welded together more easily retains its strength.

The acicular ferrite effect can be improved by adding elements such as Ti, Nb and V. Beside the known effects of precipitation hardening and retardation of the phase transformation, these elements will create additional oxides during solidification in the mould and slab. These oxides are small and stable. Ti, Nb and V partly use the MnO-S oxides (in the ra nge of 0.5 to 1.2 μιη) as a surface to grow on during solidification in the mould, thus changing the oxide surface of the original MnO into a surface which is very well suited for the acicular ferrite effect in the slab and hot strip mill.

Another way to make the acicular ferrite is to bring small nuclei in the liquid melt before the steel enters the tundish or add the nuclei in the tundish . This is not done by the addition of oxides but by the addition of a "deoxidiser" which is known not to create clusters : e.g . Zr, Ca, Ba, Sr, Ti, Cr, a nd/or Si. Cluster of oxides will float out of the steel and will make the process unstable in respect of the steel properties (e.g . alu mina cluster formation should be avoided) . The nuclei will act as a promoter of particle-growth during the subsequent casting and solidification into 0.5 - 1.2 μ sized particles, which can exhibit excellent acicular ferrite properties e.g. when Ba was used as the nuclei creating agent. Calcium, Ba or Sr, which are a vapour at steelmaking temperatures, can be injected by cored wire or by lance and the oxides that are formed are in the size of 0.1 to 1.2 μιη, but fine oxides (< 100 nm) can be created as well in this operation.

After solidification the majority of the total oxygen is in the form of oxides in the range of 0.6 to 1.2 μιη, having compositions which vary from MO/Mn-O-S (M = deoxidiser). The sulphur content in the steel is preferably at most 120 ppm, but it may be as low as 30 or even 20 ppm to create more pure oxides over oxy- sulphides during casting and solidification). The deoxidiser is added in the liquid steel, preferably in a n RH(-OB) where the oxygen ca n be tuned easily to the required level and Ca, Ba or Sr can be added in the RH vessel with high precision or can be added in the lance ("KTB" lance), but a simple stirring station or a ladle fu rnace ca n be used a s wel l usi ng a la nce o r cored wi re as the i nj ectio n technique. It would even be possible to do the whole operation in a tundish but smoke, dirt a nd fumes may create health problems in the tundish area of the caster, so this method is not preferable.

Cr can act as an oxide creator (ODS) but does not help very much in the micro alloying effect to strength (Cr < 0.2 wt%) . Zr and Ti have a very strong oxide effect, and the oxygen control (oxygen activity control = dissolved oxygen) in the tundish, which is usually set during the ladle treatment, needs to be controlled within strict limits (for Ti between 15 a nd 60 ppm O activity, for Zr between 5 and 25 ppm O activity) . Ti a nd Zr also create some C a nd N micro alloying properties because traces stay dissolved in the steel. Boron can be used when needed but will hardly exhibit any ODS effects as the formation of nitrides takes precedence (BN).

In an embodiment a second deoxidiser is added after the oxygen activity at the end of the ladle treatment is set to the required value; this new deoxidiser creates fine particles and, in some cases a small amount of clusters, which will float from the steel to the slag : new deoxidisers such as Zr, Ce, Ti, Ba and even Si may be used to bring the dissolved oxygen to 10 ppm or even lower (e.g. for Zr contents of 50 ppm or lower, the required oxygen activity will in some cases be 3 ppm or lower at the ladle treatment facility. The fine oxides that were formed in the liquid steel will not float because they are too small to float, and CexOy ( in combi nation with CeO-s) has the adva ntage of the h ig h density inclusions density is approximately 6 kg/I, which will prevent flotation of 1 μιη sized particles during ladle treatment. Zr will create ZrOy oxides with a density of about 4 to 5 kg/I and will show a lower tendency of flotation than e.g . alumina, titania or silica/manganese silicates. Ba can be used also to create the nano-sized particles, but Ba exhibits a too high vapour pressure to be added to the steel in a standard way.

In an embodiment the second deoxidiser is added by injecting a cored wire under high stirring conditions in a stirring station or ladle furnace treatment. The highly stirred melt in combination with the extra stirring supplied by the vaporizing alloy from the cored wire will create ideal circumstances to make very fine nano sized particles in the oxygen containing steels.

Althoug h the method of the invention can be very well performed in conventional thick slab casting (slab thickness generally between 150 and 350 mm) a thin slab caster is the preferred option to cast the high strength steels, because of the faster solidification and the temperature levelling after casting and before rolling will create optimal precipitates for strength . A calcium treatment may be avoided because the high oxygen steels do not need any help to prevent clogging at a thin slab caster. Alternatively a strip caster (cast strip thickness < 10 mm) can be used and the advantage is here the controlled high solidification rate.

The invention will now be illustrated by means of non-limitative examples. Continuously cast slabs were produced of the steel grades listed in table 1. Table 1 gives two soft ULC compositions (with and without B) which show a 50 to 100 MPa ODS strengthening after subjecting it to a conventiona l cold rolling and annealing treatment.

Table 1 : Composition in 1/1000 wt.% except C, N and B in ppm, composition in mould, except Ot, Oact_RH and Oact.

Oact_RH: oxygen activity after vacuum degassing

Oact : tundish oxygen activity

Ot: slab total oxygen content

Table 2. High strength steel composition by composition in 1/1000 wt.% except C, N and B in ppm, composition in mould, except Ot and Oact.

Oact: tundish oxygen activity

Ot: slab total oxygen content