EP0576107A1 - Use of a steel for the manufacture of constructiontubes - Google Patents

Abstract

The invention relates to the use of a dead-cast air-hardening steel consisting of (in % by weight): 0.15-0.30% of C 0.50 - 0.80% of Si 2.05-3.35% of Mn at most 0.03% of P at most 0.03% of S 0.50-1.00% of Cr at most 0.60% of Mo at most 0.05% of Al 0.01-0.05% of Ti 0.0015-0.0035% of B 0.002-0.015% of N as material for producing construction tubes for mechanically highly stressable construction elements, in particular for door reinforcements in motor vehicle construction, with the proviso that the following relationships are met: Ti (%): N (%) >/= 3.4% Mn (%) + Cr (%) + Mo (%) + Si (%) >/= 3.3%

Description

The invention relates to the use of a steel for the production of construction tubes for mechanically heavy-duty construction elements, in particular for door reinforcements in automobile construction.

To protect against side impact accidents of motor vehicles, reinforcing elements are often used in the vehicle doors for stiffening, which partially absorb the kinetic energy of the impacting vehicle and convert it into plastic deformation. To take on these tasks, the steel pipes used for this must meet comparatively high requirements with regard to strength, flexibility and work capacity. The same applies, for example, to construction tubes for the manufacture of stabilizers or other mechanically stressed components.

Such a construction tubes are usually warmgewalzt` wherein the final rolling temperature between 900 ^o C to 1080 ^o C. The required strength properties can depend on the used steel grade can be adjusted by water hardening. A known steel which is produced by this process contains, for example, 0.18% C, 0.4% Si and 1.14% Mn (balance: iron and usual impurities). The main disadvantage of this process is the additional heat treatment of the steel tube to adjust the mechanical properties. On the one hand, additional heat treatment makes the manufacture of such pipes more expensive. On the other hand, the mechanical properties are altered by thermoforming carried out on the pipe in some areas during further processing, for example to form door reinforcement pipes or in the heat affected zone of weld seams, which may be required for assembly, so that the strength properties can inadvertently decrease significantly compared to those of the initial state.

From DE 37 28 476 Cl and DE 39 35 965 Cl two other steel materials for door reinforcement tubes are known which, due to their chemical composition, obtain their strength properties solely through air cooling from the rolling heat, so that no separate heat treatment is required. However, the use of these steels has other serious disadvantages. For example, both cannot be produced on a large industrial scale using the LD process in a one-step melting process. Because of the high proportion of alloying elements, the chemical composition has to be adjusted in two steps, which inevitably causes corresponding cost increases in the production of primary materials.

The chemical composition (in% by weight) of these two steels is as follows: DE 37 28 476 Cl DE 39 35 965 Cl Max. 0.35% C 0.15-0.25% C Max. 0.50% Si Max. 0.60% Si Max. 1.80% Mn 3.4 - 6.1% Mn Max. 0.030% P Max. 0.030% P Max. 0.030% S Max. 0.030% S 0-1.5% Ni 0-1.0% Ni 1.8 - 2.2% Cr 0-1.0% Cr 0.4-0.7% Mo 0 - 1.0% Mo 0.025 - 0.050% Al Max. 0.005% Al 0-0.15% V Balance iron and usual impurities

In order to set the required mechanical properties of the steel according to DE-37 28 476 Cl, it is necessary to alloy the elements Cr, Ni and Mo in larger quantities. An alloy concept based on these elements is a comparatively expensive solution if only because of the material costs of the alloy elements.

One effect which has a favorable influence on the performance properties of the steel described in DE 39 35 395 Cl is its high tempering resistance. This property prevents a significant reduction in the strength properties in subsequent hot forming or hot-dip galvanizing. On the other hand, it is disadvantageous that the cold formability of this steel as well as that of the steel according to DE 37 28 476 Cl is extremely limited, so that certain pipe dimensions that cannot be produced directly by conventional hot forming cannot be produced from this steel.

Another steel is known from DE-40 32 996 A1 for the production of steel profiles which are to be used as door reinforcements and which have an external corrosion protection by galvanizing. The steel has the following analysis:
0.18-0.25% C
0.30-0.50% Si
1.30 - 2.00% Mn
0.1 - 0.5% Cr
0.1-0.3% Mo
0.02-0.07% Ti
0.002 - 0.007% B
Balance iron and usual impurities
This steel is a water hardener, the mechanical properties of which therefore only have to be adjusted with a corresponding effort by means of a separate heat treatment after hot rolling. Despite the character of a water hardener, it is possible in the steel pipes made therefrom to protect them against corrosion by galvanizing, without the set strength properties being reduced to an unacceptable degree by the heating that occurs.

The object of the invention is to propose an air-hardening steel for use in the production of mechanically heavy-duty structural pipes, in particular for automobile construction, which can be produced by a one-step melting process in LD converters and which can already be used in the hot-rolled state due to its good strength properties , ie does not necessarily have to be subjected to a heat treatment before its use in order to meet the mechanical minimum requirements required, for example, for door booster tubes in terms of tensile strength, yield strength and elongation at break. A secondary object of the invention is to propose such a steel which, moreover, also has significantly improved properties with regard to cold forming.

This object is achieved by a steel with the composition described in the patent claim, the sum of the contents of Mn, Cr, Mo and Si being at least 3.3% and the quantitative ratio Ti: N being set to a value of at least 3.4.

It was surprising that the aim of the invention could be achieved with simple measures. Compared to the known steel according to DE 39 35 965 Cl, the Mn content was considerably reduced and, on the other hand, minimum Cr and Mo contents were prescribed, whereby the character of an air hardener was retained. In addition, B was added to ensure through-hardening and to increase the strength, whereby compliance with the upper limit of 0.0035% is important for the cold formability of the construction pipes made from this steel. The prescribed Si content is also of major importance for achieving the high strength values. Finally, the setting of the minimum ratio Ti: N of 3.4 should be pointed out, the N content being limited to a value between 0.002% and 0.015%.

The steel used according to the invention combines the illustrated positive properties of the already known steels for door reinforcement tubes. At the same time, due to the special chemical composition of the steel described here, metallurgical processes in steel production are simplified. In addition, this steel opens up the possibility of cold forming pipes made from it, so that precision steel pipes can also be produced by cold drawing.

• einstufige Stahlherstellung im LD-Verfahren
• kostengünstige Legierungselemente
• Lufthärtbarkeit
• hohe Anlaßbeständigkeit
• hohe Festigkeitseigenschaften
• hohes Arbeitsaufnahmevermögen.

The following properties can be summarized:

• single-stage steel production using the LD process
• inexpensive alloying elements
• Air hardenability
• high temper resistance
• high strength properties
• high capacity to work.

In addition to door reinforcement tubes, the steel described in accordance with the invention is also suitable for the production of stabilizers, for example, which have hitherto been produced from hardened and tempered and tempered precision steel tubes. They have the task of stiffening the axle bodies of motor vehicles when subjected to torsional loads. For this purpose, the stabilizers must withstand the highest possible number of load cycles in a torsion test with changing loads at a given angle of rotation. Stabilizers can be produced from the steel used according to the invention, which are characterized in that they can be cold drawn after hot rolling.

Another advantageous use of the steel described can be seen with regard to the production of tubes for bicycle frames or for clothes racks, for example, which should be as thin-walled as possible for reasons of weight.

The invention is described in more detail in the following exemplary embodiments.

In a one-step melting process, a steel was used in an LD converter
0.25% C
0.74% Si
2.29% Mn
0.02% P
0.02% S
0.66% Cr
0.25% Mo
0.03% Al
0.046% Ti
0.0029% B
0.008% N
Balance iron and usual impurities
generated and poured into round casting.

R_m =

1400 N/mm²

R_p0,2 =

1000 N/mm²

A₅ =

9 %

Demgegenüber wiesen die erfindungsgemäß hergestellten Rohre folgende Eigenschaften auf:

R_m =

1610 N/mm²

R_p0,2 =

1040 N/mm²

A₅ =

15 %

Dabei ist es von Vorteil, daß der erfindungsgemäß verwendete Stahl kostenintensive Elemente wie beispielsweise Nickel vollständig meidet. Ferner sind auch die Elemente Chrom und Molybdän in nur relativ geringen Umfang enthalten. Sein Charakter als Lufthärter macht bei diesem Stahl eine gesonderte Wärmebehandlung überflüssig und senkt damit die Herstellkosten.The round casting sections were hot-rolled into tubes measuring 25 x 5 mm and, after the last shaping step, were specifically cooled in air. Due to the specially coordinated alloy composition, the pipes already had the properties required for their use as door reinforcement elements when hot-rolled. For example, the following minimum values are required for non-galvanized pipes:

R _m =

1400 N / mm²

R _p0.2 =

1000 N / mm²

A₅ =

9%

In contrast, the tubes produced according to the invention had the following properties:

R _m =

1610 N / mm²

R _p0.2 =

1040 N / mm²

A₅ =

15%

It is advantageous that the steel used according to the invention completely avoids costly elements such as nickel. Furthermore, the elements chromium and molybdenum are only contained to a relatively small extent. Its character as an air hardener makes a separate heat treatment unnecessary for this steel and thus lowers the manufacturing costs.

R_m =

1650 N/mm²

R_p0,2 =

1208 N/mm²

A₅ =

11 %

Ein ähnlicher Effekt konnte auch durch eine Anlaßbehandlung eingestellt werden. Aufgrund der besonderen chemischen Zusammensetzung und gezielt genutzter werkstoffkundlicher Mechanismen stieg die Streckgrenze des Stahls gegenüber dem warmgewalzten Zustand nach einer Anlaßbehandlung bei einer Temperatur von etwa 350^oC an. Es wurden folgende mechanische Eigenschaften erreicht:

R_m =

1428 N/mm²

R_p0,2 =

1236 N/mm²

A₅ =

15 %

In diesem Verfahren zeigt sich bereits das ebenfalls hervorzuhebende ausgezeichnete Anlaßverhalten dieses Stahles. Die mechanischen Kennwerte werden durch eine Anlaßbehandlung sogar noch weniger reduziert als dies z.B. bei dem in der DE 40 32 996 Al beschriebenen Stahl der Fall ist. Aus diesem Grunde eignet sich der erfindungsgemäß verwendete Stahl auch besonders für eine Feuerverzinkung zur Verbesserung des Korrosionsschutzes. Nach einer bis zu 10 min dauernden Verzinkung in einem 450^oC warmen Zinkbad wiesen die Rohre mit der oben genannten Zusammensetzung folgende Werte auf:

R_m =

1262 N/mm²

R_p0,2 =

1128 N/mm²

A₅ =

15 %

Damit wurden die in der DE 40 32 996 Al aufgeführten Mindestwerte für verzinkte Rohre deutlich übertroffen:

R_m =

1100 N/mm²

R_p0,2 =

800 N/mm²

A₅ =

8 %

Es gibt eine Reihe von Anwendungen, bei denen es beispielsweise aufgrund der Rohrabmessungen oder der Querschnittsform nicht möglich ist, ein Rohr im warmgewalzten Zustand einzusetzen. Beispiele für solche Anwendungen sind Türverstärkerrohre mit nicht kreisförmigem Querschnitt oder auch Kleiderständerrohre mit Abmessungen, die auf Warmwalzstraßen nicht darstellbar sind. Zur Herstellung solcher Produkte ist es erforderlich, daß der verwendete Stahl im Kaltziehverfahren weiterverarbeitet werden kann. Diese Möglichkeit ist durch den erfindungsgemäß verwendbaren Stahl gegeben. Durch eine 30-minütige Glühbehandlung bei ca. 700^oC wird die Härte des Stahles so weit reduziert, daß ein Kaltziehen ohne weiteres möglich ist. Dies ist dagegen bei dem Stahl gemäß DE-39 35 965 Cl trotz einer Weichglühung nicht der Fall. Durch die beim Kaltziehen aufgebrachte hohe Verfestigung werden die mechanischen Eigenschaften, die durch die Weichglühung reduziert wurden, wieder stark angehoben, so daß im Anschluß daran die Rohre in verwendungsfähigem Zustand vorliegen. Im Falle von Rohren mit der oben genannten chemischen Zusammensetzung, die von der warmgewalzten Abmessung 33,7 x 5 mm auf die Abmessung 26 x 4 mm kaltgezogen wurden, lagen die mechanischen Kennwerte wie folgt:

R_m =

1049 N/mm²

R_p0,2 =

982 N/mm²

A₅ =

13 %

Durch eine erneute Wärmebehandlung können die Werte im Bedarfsfall wieder auf das Ausgangsniveau für warmgewalzte Rohre zurückgeführt werden.If, in addition to the requirements for the mechanical properties of door reinforcement tubes described above, even higher values are to be set, for example if a particular bending behavior is to be guaranteed in a specified quasi-static bending test, this can be done by a subsequent performed low cold deformation of the pipes can be achieved. After such a treatment has been carried out, the mechanical properties of the pipes, in particular the yield strength, are further improved as a result of the strain hardening, so that even the strictest requirements on the bending behavior can be met. The values of the mechanical properties after cold straightening on the tubes produced according to the invention were:

R _m =

1650 N / mm²

R _p0.2 =

1208 N / mm²

A₅ =

11%

A similar effect could also be achieved by tempering. Owing to the particular chemical composition and specific mechanisms unused on material the yield strength of the steel up against the hot-rolled condition after an annealing treatment at a temperature of from about 350 ^o C. The following mechanical properties were achieved:

R _m =

1428 N / mm²

R _p0.2 =

1236 N / mm²

A₅ =

15%

This process already shows the excellent tempering behavior of this steel. The mechanical characteristics are reduced even less by tempering treatment than is the case, for example, with the steel described in DE 40 32 996 A1. For this reason, the steel used according to the invention is also particularly suitable for hot-dip galvanizing to improve corrosion protection. After galvanizing for up to 10 minutes in a 450 ^o C warm zinc bath, the pipes with the above mentioned Composition the following values:

R _m =

1262 N / mm²

R _p0.2 =

1128 N / mm²

A₅ =

15%

This clearly exceeded the minimum values for galvanized pipes listed in DE 40 32 996 Al:

R _m =

1100 N / mm²

R _p0.2 =

800 N / mm²

A₅ =

8th %

There are a number of applications in which it is not possible, for example due to the pipe dimensions or the cross-sectional shape, to use a pipe in the hot-rolled state. Examples of such applications are door reinforcement tubes with a non-circular cross-section or clothes rack tubes with dimensions that cannot be produced on hot rolling mills. To produce such products, it is necessary that the steel used can be processed further in the cold drawing process. This possibility is given by the steel that can be used according to the invention. A 30-minute annealing treatment at approx. 700 ^o C reduces the hardness of the steel to such an extent that cold drawing is easily possible. In contrast, this is not the case with the steel according to DE-39 35 965 Cl, despite soft annealing. The high strengthening applied during cold drawing increases the mechanical properties, which have been reduced by the soft annealing, so that the pipes are then ready for use. In the case of pipes with the chemical composition mentioned above, which were cold drawn from the hot-rolled dimension 33.7 x 5 mm to the dimension 26 x 4 mm, the mechanical parameters were as follows:

R _m =

1049 N / mm²

R _p0.2 =

982 N / mm²

A₅ =

13%

If necessary, the values can be brought back to the initial level for hot-rolled pipes by a new heat treatment.

Claims (1)

Use of a calmly cast air-hardening steel consisting of (in% by weight):
0.15-0.30% C
0.50-0.80% Si
2.05-3.35% Mn
Max. 0.03% P
Max. 0.03% S
0.50 - 1.00% Cr
Max. 0.60% Mo
Max. 0.05% Al
0.01-0.05% Ti
0.0015 - 0.0035% B
0.002 - 0.015% N
as a material for the production of construction tubes for mechanically heavy-duty construction elements, in particular for door reinforcements in automobile construction, with the proviso that the following relationships are fulfilled:
Ti (%): N (%) ≧ 3.4%
Mn (%) + Cr (%) + Mo (%) + Si (%) ≧ 3.3%