FI87471C - Machined steel - Google Patents

Description

1 8747!

Wrought steel

The invention belongs to the field of metallurgy and relates to low-alloy modified steel, its production and use. The steel is particularly suitable for use as hot-rolled, weldable structural steel.

Hot-rolled structural steels with a minimum yield strength exceeding 690 MPa are known, which have been • heat-treated after heat-rolling. Such a steel is, for example, ASTM A S43B (AWS Welding Handbook, 10 Metals and their weldability, 7th edition, vol. 4, 1983).

Its analysis is C 0.23%

Si 0.20 - 0.40%

Mn 0.40% 15 P <0.020% S <0.020%

Cr 1.5 - 2.0%

Ni 2.6 - 4.0%

Mo 0.45 - 0.60% 20 V 0.03%

For example, this steel has the weakness of low toughness and, in addition, poor weldability.

It is a general object of the present invention to provide; . a high-strength steel with low toughness and fracture toughness properties at low temperatures for the use of steel in demanding marine and mechanical applications.

The means necessary to achieve the object of the invention are set out in the claims.

The advantage of the steel according to the invention is its high strength combined with good toughness and also weldability. The high level of strength makes it possible to reduce the weight of the structures and the amount of welding work due to the thinning of the material thickness. At the same time, the welding stresses and the deformations of the structure caused by the welding 35 are also reduced. The strength of the steel can be adjusted to the desired level by heat treatment.

The minimum yield strength of the steel is at least 790 MPa and the strength can be increased up to 1000 MPa by adjusting the heat treatment and the degree of alloying without compromising the toughness or weldability. The yield strength of 1000 MPa can be achieved up to a material thickness of 50 mm.

5 The strength and toughness properties of steel do not depend on orientation.

Steel is characterized by the following alloy combination:

Ni 4.2 - 5.0%

Mo 0.3-1.0% 10 Cr 0.5 - 1.0%

Si 0.10 - 0.60%

Mn 0.20 - 1.0%

Nickel and molybdenum have been added abundantly, but the Kro content is quite low. The silicon and manganese contents are limited relatively low despite their ability to increase hardenability. This alloy combination achieves a homogeneous microstructure as well as the desired strength / sit-keys / weldability combination.

The hardening of the steel according to the invention takes place mainly through solution hardening and not so much through a martensite change. In known steels of this strength level, the aim is in particular to increase the strength by means of microalloying with dispersion-type precipitates and the comminution of the grain size caused by them.

The steel of the invention is substantially free of microalloying such as niobium, titanium, vanadium, zirconium or boron. The concentrations of these micro-alloying elements are limited to the following maximum values:

Nb max 0.02%, preferably max 0.01% 30 Ti max 0.02%, preferably max 0.01% V max 0.03%, preferably max 0.02%

Zr max 0.02%, preferably max 0.01% B max 0.003%, preferably max 0.002%

All of the above concentrations refer to the concentrations of the final 35 product. Limiting the concentrations of the microparticle components in this way in the composition according to the invention is essential in order to achieve the desired toughness and weldability. It is also essential in the chemical composition to limit the carbon content to less than 0.12%, preferably less than 0.10%. Due to the low carbon content, both the changes in the specific volume in welding and also the deformation stresses remain small. The hardness of the hardened area of the weld 5 also does not become too high. Thus, the sensitivity to cold cracking is low and the toughness level of the change zone is high. The lower limit of the carbon content is 0.05%. This amount of carbon prevents the atomic infiltration of impurities at the grain boundaries, which, despite the low concentrations, contributes to the embrittlement of the blade-10 when it is exposed to a temperature range of about 400 to 550 ° C.

Hydrogen content should be minimized. It is preferably at most 5 ppm, preferably at most 2 ppm. In this way, there is no cold cracking in the welding and the steel breakage keys remain high.

The sulfur and iosphorus contents are very low: Smax = 0.010%, preferably 0.005%, and Pmax = 0.012%, preferably 0.010%. The nitrogen content is preferably not more than 100 ppm, most preferably not more than 60 ppm. The limitations of these impurities result in virtually no temperature or precipitation compromising toughness or weldability in any temperature range, and the steel retains its toughness after its various metallurgical histories. The low sulfur content is very important in order to prevent the formation of lamellar inclusions which impair the transverse toughness properties of the hot-rolled sheet.

Aluminum can also be used in the mixture as an antioxidant. Its content is preferably 0.02 to 0.06%.

Copper is also an impurity and preferably has a content of up to 0.20%.

Toughness can be increased by using the lowest possible silicon and manganese contents. Minimum concentrations 0.10% and resp. However, 0.20% are required to effect deoxidation.

35 The lowest possible chromium content increases toughness and weldability properties. However, a minimum content is necessary to achieve hardness up to a material thickness of 50 mm. The high nickel content improves the hardness, increases the strength through solution hardening and especially increases the toughness at low temperatures.

High molybdenum alloy increases strength. In addition, molybdenum prevents the atomic infiltration of impurities that cause the grain boundaries to become brittle.

The steel according to the invention can be produced by a two-stage smelting process, the second stage of which is carried out in a vacuum converter.

The first stage of smelting is carried out in a normal il-10 arc furnace, in which sulfur, phosphorus, silicon and manganese are removed to very small concentrations from molten steel by normal slag processes and oxygen blowing carried out beyond the conventional smelting process. At the same time, significant amounts of carbon and chromium are removed. At this stage of the arc furnace-15, the gas content of the steel, especially with respect to nitrogen and oxygen, can be very high.

In the second stage, the steel melted in the arc furnace, transferred to a vacuum converter, undergoes a short oxygen blowing step to ensure a low concentration level of harmful impurities. The concentration of the desired elements is then matched to the target limits by the addition of pure diluents. At this point, the gas content of the steel may be high.

After removal of the harmful impurities and alloying, the harmful gases are removed from the melt by suctioning the converters into a deep vacuum (less than 5 mbar, preferably less than 2 mbar) and stirring the steel melt while blowing pure argon from under the melt. During this process step, oxygen, nitrogen and hydrogen are removed from the steel to equilibrium concentrations corresponding to the above pressures, which are so small that they do not have a detrimental effect on the properties of the solidified steel.

After melt processing, the steel is cast into cast iron molds. The molds and the casting jet must be protected in such a way that the gas concentrations of the steel do not rise substantially during the casting of the coke after the vacuum treatment.

Cast steel billets are heat treated and heat treated. The heat treatment consists of austenitization followed by 87471 5 water quenching as well as discharge and subsequent air cooling.

The preforms are best formed by hot rolling so that the degree of reduction is at least 3. The pre-heating of the preform must be sufficient for the entire preform to reach the correct temperature. The preheating temperature must be 1100 ° C ± 20 ° C and the holding time 1 hour for each 25 mm of material thickness. The rolling is performed in a temperature range of 1100 ° C to 850 ° C, after which the billet can be cooled in air to room temperature.

10 After hot rolling, the steel is rejuvenated as follows: austenitization 830 ° C ± 20 ° C / water emission 520 ° C - 620 ° C / air

Retention times for both austenitization and discharge are 1 hour per inch of material strength, however, at least 1 hour. 15 The effect of emission temperature on yield strength is shown in Figure 1.

The microstructure of the final product after heat treatment is essentially slatted martensite up to a material thickness of 50 mm. The grain boundaries are virtually free of non-metallic occlusions and separations and no lamellar, roll-modified inclusions are detectable.

After annealing, the steels have an impact strength (KV) at temperatures of -40 ° C and -60 ° C, typically 100 to 250 J throughout the temperature range described above. The impact toughness is not dependent on the orientation of the notch with respect to the rolling direction.

The steel can be welded cold and does not need to be post-heat treated after welding when the welding energy is kept in the range of 10 to 30 kJ / cm. However, the steel can be welded, if necessary, both by preheating and by post-heat treatment. Post-weld flame annealing or dehydrogenation annealing is possible without compromising strength or toughness properties. When using post-heat treatment, the temperature must not exceed the previous discharge temperature. The Hit-35 sin transformation zone meets the minimum requirements for the base material and no cold cracks occur in the transformation zone if the hydrogen level of the weld itself does not exceed 10 ppm.

6 8 7 4 7 '1

The target analysis for a particularly suitable steel according to the invention is the following C 0.08%, Si 0.15%, Mn 0.45%, P 0.006%, S 0.001 I, Cu 0.10%, Cr 0.75%, Mo 0.60 %, Ni 4.50%, Al 0.035%, Zr 0.001%, B 0.0001%, V <0.005%, 5 Nb <0.005%, Ti <0.005%, N <40 ppm, H 0.7 ppm. When a steel scrubber having such a composition was settled and heat-treated (850 ° C / 3 h / water + 580 ° C / 3 h / air) as described above, the following test results were obtained: Re 890 MPa, Rm 940 MPa, A5 19% , Z 66%, CVN (-80 ° C) 180 J. The tensile test results are from samples taken perpendicular to the rolling direction. Impact tests were performed on rods with a V-notch in the transverse direction of the plate.

Claims (15)

1. Hot-formed steel, characterized in that the actual alloying elements are nickel, molybdenum and chromium, and their contents are Ni 4.2 - 5% Mo 0.3 - 1% Cr 0.5 - 1% silicon and the manganese contents are 10 Si 0.10 - 0.60% Mn 0.20 - 1.0% carbon content is C 0.05 - 0.12% The concentrations of niobium, titanium, vanadium, zirconium and boron in the micro-alloys are Nb max 0.02% Ti max 0.02% V max 0.03% Zr max 0.02% 20. max 0.003% The sulfur and phosphorus concentrations of the impurities are S max 0.010% P max 0.012% Steel according to Claim 1, characterized in that the hydrogen and nitrogen contents are: H max 5 ppm N max 100 ppm Steel according to one of Claims 1 to 2, characterized in that the concentrations of the microalloying elements 30 are Nb max 0.01% Ti max 0.01% V max 0.02% Zr max 0.01% 35. max 0.002% Steel according to one of Claims 1 to 3, characterized in that the sulfur and phosphorus contents are 8 87471 s max 0.005% P max 0.010% Steel according to one of Claims 1 to 4, characterized in that the carbon content is 5. max 0.10% Steel according to one of Claims 1 to 5, characterized in that the hydrogen and nitrogen contents are H max 2 ppm N max 60 ppm Steel according to one of Claims 1 to 6, characterized in that the reduction is at least 3. 7 87471 Steel according to one of Claims 1 to 7, characterized in that the structure of the shaped product is substantially released slatted martensite. A process for producing hot-formed steel, characterized in that the steel according to any one of claims 1 to 8 is produced by a two-stage smelting process, in which in the first step solid impurities are removed in the solidified state by mixing with vacuum and inert gas-25, after which the steel is heat treated and heat treated. Method according to Claim 9, characterized in that the vacuum is less than 5 mbar. Method according to Claim 10, characterized in that the vacuum is less than 2 mbar. Method according to one of Claims 9 to 11, characterized in that, in the second step, the steel melt is mixed by blowing argon into it from below. Process according to one of Claims 9 to 12, characterized in that the heat treatment is carried out by rolling to a reduction of at least 3, followed by quenching with water at about 830 ° C, passing in the temperature range from 520 to 620 ° C and cooling to room temperature. 9 87471 Use of steel produced according to one of claims 1 to 8 or one of claims 9 to 13 as a weldable structural steel. 10 87471