EP0481575A2 - Process for manufacturing weldable high-tensile steel sheets and use of these sheets - Google Patents

Abstract

A process for manufacturing a thick-walled sheet and the use and application thereof, from steel having a yield strength of 420-500 but especially higher than 500 N/mm<2>, which has a ferritic-perlitic microstructure, high toughness and good weldability, are proposed. A continuous casting ingot having a composition in % by weight of 0.02 to 0.10 % C 0.05 to 0.50 % Si 1.00 to 2.00 % Mn max. 0.02 % P max. 0.01 % S 0.015 to 0.08 % Al max. 0.01 % N 0.30 to 1.60 % Ni 0.20 to 1.60 % Cu 0.04 to 0.10 % V 0.01 to 0.05 % Nb 0.01 to 0.04 % Ti the remainder being iron and unavoidable impurities, is heated to temperatures greater than 1200 DEG C, cooled in air to a surface temperature of less than 1000 DEG C and then thermomechanically rolled without interval between passes at a final rolling temperature of about 750 to 650 DEG C, and the sheet is then cooled in still air or in a stack to less than 200 DEG C and finally, after heating to about 420 to 610 DEG C, again cooled in air to room temperature. <IMAGE>

Description

The invention relates to a method for producing a high-strength weldable sheet and its use according to the preambles of claims 1 and 4.

Such steels are used for all types of welded structures.

Known structural steels of grades StE 460 - 500 with a composition according to DE standard DIN 17 102 have (in% by weight) max. 0.21% C; 0.10 to 0.60% Si; 1.00 to 1.70% Mn; Max. 0.035% P; Max. 0.030% S; Max. 0.3% Cr; Max. 0.70% Cu; Max. 0.10% Mo; Max. 1.00% Ni; Max. 0.22% Nb, Ti, V in combination, balance iron, on. This enables steels with a good weldability to be achieved with a ferritic-pearlitic structure and a yield strength of up to 500 N / mm² by normalizing.

Thermomechanically rolled, easily weldable steels, which can also have a ferritic-pearlitic structure and a yield strength of up to 500 N / mm², e.g. grade StE 480.7 TM, have the following composition (in% by weight) according to DE standard DIN 17172: 0 , 04-0.16% C; 0.55% Si; 1.10 to 1.90% Mn; Max. 0.035% P; Max. 0.025% S; Max. 0.20% V and Nb, balance iron.

Thick-walled sheets made of known steels with yield strength values above 500 N / mm² receive their good ones Strength properties in addition to the alloy additives, in particular Cr, Mo and higher Ni additions, through accelerated cooling with water directly at the rolling heat (Stahlrohranleitung, 10th edition, pp. 79-80, plates XLVII, XLVIII).

In addition, for similar grades, it is known to subject these steels to an austenitizing treatment before they are cooled down with water. In the delivery state, such steels have a structure made of bainite or tempered martensite.
For the accelerated cooling of the steel sheet, complex special water cooling systems with corresponding energy consumption are required in order to specifically cool the material.

The invention is therefore based on the problem of proposing a method for producing a thick-walled sheet from high-strength weldable steel which makes it possible to use the advantages of a ferritic-pearlitic structure of the steel and in which water cooling can be dispensed with and a suitable advantageous one Specify use.

This problem is solved according to the invention by claims 1 and 4.
Advantageous developments of the invention are covered in the subclaims.

A steel produced by this special thermomechanical treatment and hardening at temperatures below the transition point A1 has Yield strengths of more than 500 N / mm² and at the same time ferritic-pearlitic structure. This fine structure gives the steel unexpectedly high toughness values. In extensive tests it was surprisingly found that it is possible to raise structural steels of this type without accelerated cooling with water by means of appropriate hardening to yield strength values of up to approximately 750 N / mm².

It is particularly important that the weldability of the structural steels is maintained. It has turned out completely surprisingly that in the ferritic-pearlitic structure produced in this way, the steel after welding in the area of the heat affected zone does not show the usual hardening and only a very small drop in hardness. In addition to the alloy selection according to the invention, this is apparently due to the cooling of the slab before rolling and the tempering treatment as a combined measure.

The slab can be heated both from room temperature and after hot use to the metallurgically favorable temperature of greater than 1200 ° C. known to the person skilled in the art.

According to the invention, it is particularly important to consciously set the C content lower than the desired strength values according to the previously customary dimensioning. The use of Mo is also avoided and the aim is to reduce Nb as much as possible in order to improve the toughness properties of the sheet produced according to the invention. 0.06-0.10% V is added as a substitute.

Ti is limited to 0.04% in order to positively influence the fine grain structure of the structure in the heat affected zone of welded component edges.

On the other hand, the Cu content is deliberately driven above the usual addition amounts in order to activate the strength-increasing effect of Cu by tempering treatment. The potential strength of the steel produced according to the invention is thus exploited as far as possible.

Small amounts of Ni and Mn are added to increase the toughness.

The synergistic effect of the alloying elements used and the manufacturing process used enables the overall surprising results.

In continuation of the inventive concept, the manufacturing process can also be used for steels with yield strengths of approximately 420-500 N / mm². The alloy additives can be reduced accordingly. Although weldable structural steels of this strength are known, the method according to the invention saves the use of expensive annealing or cooling treatments.

The preferred tempering temperature is 560-600 ° C. In this area, the effect of Cu on the strength values of the steel is optimized. In addition, components in this temperature range usually become low-voltage after welding annealed so that the stress relieving annealing cannot adversely affect the metallurgical effect according to the invention.

Experiments have shown that the hardness curve from the base material via the heat-affected zone to the center of the weld seam is reduced from up to 100% to less than 20% instead of the usual fluctuations if the sheets produced according to the invention are connected to one another by sub-powder welding or other arc welding . The values hardly change even after stress relief annealing.

The sheets produced in accordance with the invention, in particular with thicknesses of greater than 15 mm to 50 mm and higher, can advantageously be used for offshore structures such as oil platforms, pipes and the like, since the high impact strength combined with a high yield strength and a relatively homogeneous hardness curve over the welding zone of components meet the extreme requirements for the swell strength of steels for such structures. With particular economy, the sheets can also be used in commercial vehicle construction such as B. used in mobile cranes or in mining for support purposes.

The invention will be explained in more detail with the aid of exemplary embodiments.

example 1

Two 210 mm thick steel slabs cast from the melt A (Table 1) were cast in the strand and after cooling to room temperature were heated to 1250 ° C. in a pusher furnace after a holding time of 220 min. then cooled in still air until the surface temperature was below 1000 ° C. They were rolled down to a thickness of 67 mm (sheet A 1) or 57 mm (sheet A 2) in the roughing stand VG with initial temperatures (Table 2) of 930 ° C or 920 ° C. In the FG finishing stand at a piercing temperature of 815 ° C and a final rolling temperature of 685 ° C, the sheet A1 received its final thickness of 40 mm. Analogously, the sheet A2 was rolled to a final thickness of 25 mm at a piercing temperature of 820 ° C. A carbon equivalent (according to IIW formula) of C _E = 0.442 can be calculated from the analysis (Table 1), which is very low for a steel of this yield strength class.

A strip of 500 mm width was cut from each of the 2 sheets, divided into 5 sections and annealed in electrically heated laboratory annealing furnaces in the temperature range between 440 and 620 ° C. The individual examinations of the two sheet thicknesses will be discussed below.

1.1 Investigations on sheet A 1

The sheet A 1 was divided into 5 sections Q, R, S, T, U with the dimensions 500 x 400 mm and annealed at 5 tempering temperatures from 480 to 620 ° C. All tempering treatments required an annealing time of 1.5 hours.

• Rundzugproben oberflächennah, quer zur Walzrichtung
• ISO-V-Proben oberflächennah, quer zur Walzrichtung

With this sheet thickness of 40 mm, the round tensile and notch impact bending samples were taken close to the surface (at 1/4 of the sheet thickness):

• Round tensile specimens close to the surface, transverse to the rolling direction
• ISO-V samples close to the surface, transverse to the rolling direction

The results of all tensile tests are shown in Table 3. Table 4 in the upper part provides an overview of the course of the yield strength (R _e ) and tensile strength (R _m ) depending on the tempering temperature.

Up to tempering temperatures of 600 ° C approximately the same yield strength and tensile strength values can be determined. It is remarkable that in the above-mentioned tempering temperature range up to 600 ° C the very high yield strengths with values between 600 and 650 N / mm² for the transverse samples are still associated with good elongation at break values over 24% and very good indentation values over 70%.

A strong drop in the yield strength values and a smaller drop in the tensile strength are then found for the tempering temperature 620 ° C. Here the yield point falls below the target value of 500 N / mm². This is not associated with an increase in elongation at break and constriction, rather these values also decrease at a tempering temperature of 620 ° C.

The notched impact strength-temperature profiles (mean values of several samples) are shown in Table 4, lower area, as a function of the tempering temperature. For the usual sample position in 1/4 of the sheet thickness, ie for 40 mm sheets from near the surface, values of over 200 J / cm² are found for the cross samples even at -40 ° C. The samples left at 480 ° C are at the lower limit of a scattering band 620 ° C annealed samples as expected at the upper limit.

Ground specimens (not shown) were taken over the entire sheet thickness. They consistently showed the appearance of grain lines with coarser grains for all heat treatment conditions. While most of the structure was made up of extremely fine-grained crystallites of sizes 12 to 13, there were occasional lines with grain sizes 7 to 8. The structure consisted largely of acicular ferrite and about pearlite.

1.2 Investigations on sheet metal A 2

• Rundzugproben quer zur Walzrichtung
• Rundzugproben parallel zur Walzrichtung
• ISO-V-Proben quer zur Walzrichtung

The 500 mm long sections were designated V, W, X, Y and Z and tempered at temperatures from 440 to 600 ° C. The glow time was 1 hour. Several samples were taken from each section:

• Round tensile specimens transverse to the rolling direction
• Round tensile specimens parallel to the rolling direction
• ISO-V samples transverse to the rolling direction

All samples had yield strength values that were largely independent of the tempering temperature and were very high (Table 3): between 625 N / mm² and 687 N / mm² for the transverse samples, between 609 and 646 N for the comparatively taken longitudinal samples (not shown) / mm². All tensile strengths of the cross samples gave values around 700 N / mm².

An additional strip was later cut from sheet A 2 and in the hard-rolled condition (without tempering) checked. The result of the tensile tests is also entered in Table 3. Accordingly, the desired minimum yield strength is exceeded on the transverse samples in this state (484 N / mm² were measured on the longitudinal sample - not shown). With 702 N / mm² the tensile strength is at the same level as after the tempering heat treatments.

Since the longitudinal samples had proven to be uncritical, only cross samples were tested. They came from the upper part of the sheet thickness and hardly covered the core area. The a _K -T curves are shown in Table 4, the values in Table 3 can be read.

Despite the high strength values, the notched bar impact specimens also showed extremely high notched bar impact strengths, which were between 239 and 321 J / cm² at the test temperature of -40 ° C. Even at -80 ° C at least 130 J / cm² were measured.

The samples left at 520 ° C were at the lower end of the spread, the highest values were reached by the samples left at 560 ° C and 600 ° C. The samples left hard as rolled were not shown in Table 4.

Longitudinal and cross sections were made from the undeformed heads of the round tensile specimens. Regardless of the inlet temperature used, a line structure of ferrite and some pearlite was found. The grain structure was extremely fine-grained with grain sizes 13 to 14 near the surface and even in the core in the 10 to 13.

Example 2

A steel sheet B 1 of 40 mm thickness was produced from a steel melt B (Table 5) in the same way as in Example 1. The yield strength was 736 N / mm², the tensile strength 882 N / mm² with an elongation at break of 20.2%. The melt B showed random traces of Cr and Mo.

A comparative sheet C 1 of 20 mm thickness from comparative melt C (Table 5) with 0.08% C and higher Nb values of 0.07% and an Mo content of 0.32% had a yield strength of 735 N / mm² and a tensile strength of 857 N / mm² at room temperature. Although the sheet C 1 not produced according to the invention has only half the thickness of the sheet B 1, its values for the impact energy (Table 6) on the ISO-V cross-sample are about 20 to 40% lower than for sheet B 1. This shows clearly the effect of the invention.

Example 3

After the tempering treatment according to the invention, sample sections were cut to length from the sheet A 2 produced according to the invention with a thickness of 25 mm, and these were welded to one another by manual arc welding and UP tandem welding after a V-seam preparation. Immediately after cooling, the samples were subjected to a Vickers hardness test across the weld seam without being subjected to a stress-relieved heat treatment beforehand. Table 7 shows the hardness values for sample A 21. The measured hardness values HV 10 are plotted on the ordinate for the measuring zones of base material (GW), heat affected zone (WEZ) and weld metal. The upper curve in the table shows the Hardness curve on the top of the seam, the lower curve shows the hardness curve on the seam root. The weld seam was created with manual arc welding.
Tables 8 and 10 show in an analogous manner the course of hardness over samples A 22, A 23, which, however, were produced by UP tandem welding.

Typical of the sheets produced according to the invention are unexpectedly small increases in hardness and decreases in hardness in the heat affected zone. The hardness was a maximum of 20% compared to the hardness in the base material (sample A 23, seam root).

A weld sample used for comparison from sheets D1, D2 of 28 mm thickness (Table 9) with X-seam preparation from a water-tempered steel of the type HY80 (steel tube manual, 10th edition, p. 79/80), which was carried out in concurrent tandem -Procedure has been welded, shows the known increase in hardness in the heat affected zone (HAZ) of 50-90% compared to the base material (GW) both on the top of the seam (dashed lines) and on the bottom of the seam (solid line).

Finally, Table 11 shows the notched impact energy measured in the welding area for the three samples A 21, A 22, A 23 at the test temperatures + 20 ° C, -10 ° C, -40 ° C.

As expected, the values for the two samples A 22, A 23 in the transition area (Ü) welding / heat affected zone at the lowest test temperature are less favorable than in the center of the weld seam (MS), but better than was expected according to the prior art.

In the case of sample A 21, which already showed the smallest fluctuations in the hardness profile in Table 7, the analog measured value in the transition range is even better than the comparative values from the weld metal.

Overall, however, the measured values achieved are considerably higher than was to be expected after the melt analysis of the steel.

Claims (5)

Process for the production of a thick-walled steel sheet with a ferritic-pearlitic structure, a yield strength greater than 500 N / mm² with high toughness and good weldability from a slab of the composition cast in weight%

0.04 to 0.10% C
0.25 to 0.50% Si
1.40 to 2.00% Mn
Max. 0.02% P
Max. 0.01% S
0.015 to 0.08% Al
Max. 0.01% N
0.60 to 1.60% Ni
0.60 to 1.60% Cu
0.06 to 0.10% V
0.03 to 0.05% Nb
0.01 to 0.04% Ti

Remainder iron and unavoidable impurities, whereby the slab is heated to temperatures above 1200 ° C, cooled in air to less than 1000 ° C surface temperature, then rolled thermomechanically without a break with a final rolling temperature of approx. 750 to 650 ° C, the sheet is then at rest Air or in a stack is cooled to below 200 ° C and finally, after heating to about 420 to 610 ° C, is again cooled in air to room temperature. A method according to claim 1, characterized by the use of a steel composition (in% by weight)

0.02 to 0.05% C
0.05 to 0.30% Si
1.00 to 1.40% Mn
Max. 0.02% P
Max. 0.01% S
0.015 to 0.08% Al
Max. 0.01% N
0.30 to 0.60% Ni
0.20 to 0.60% Cu
0.04 to 0.06% V
0.01 to 0.03% Nb
0.01 to 0.04% Ti

Remainder iron and unavoidable impurities, for the production of a thick-walled sheet with a yield strength of 420 to 500 N / mm². A method according to claim 1 or 2, characterized by a tempering treatment with heating the sheet to 560 to 600 ° C. Use of a sheet produced according to one of claims 1 to 3 with a thickness of greater than 15 mm for high-strength welded structures for offshore and commercial vehicle construction. Arc welding component made of sheet metal with a thickness greater than 15 mm, consisting of steel with ferritic-pearlitic structure, higher Toughness and good weldability from a slab cast in the strand with the composition (in% by weight)

0.02 to 0.10% C
0.05 to 0.50% Si
1.00 to 2.00% Mn
Max. 0.02% P
Max. 0.01% S
0.015 to 0.08% Al
Max. 0.01% N
0.30 to 1.60% Ni
0.20 to 1.60% Cu
0.04 to 0.10% V
0.01 to 0.05% Nb
0.01 to 0.04% Ti

Remainder iron and unavoidable impurities, whereby the slab is heated to temperatures above 1200 ° C, cooled in air to less than 1000 ° C surface temperature, then rolled thermomechanically without a break with a final rolling temperature of approx. 750 to 650 ° C, the sheet is then at rest Air or in a stack is cooled to below 200 ° C and finally, after heating to around 420 to 610 ° C, is again cooled in air to room temperature, the component having a hardness profile transverse to the weld seam from base material to base material with hardness values, their minima and maxima deviate less than 20%.

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