US20090173412A1

US20090173412A1 - High strength, air-hardening steel with excellent shaping properties

Info

Publication number: US20090173412A1
Application number: US11/718,573
Authority: US
Inventors: Joachim Schöttler; Volker Flaxa; Bernd-Michael Peters; Thomas Koch
Original assignee: Salzgitter Flachstahl GmbH
Current assignee: Salzgitter Flachstahl GmbH
Priority date: 2004-11-03
Filing date: 2005-11-03
Publication date: 2009-07-09
Also published as: KR20070073858A; EP1807544B1; WO2006048009A1; EP1807544A1; DE102004053620A1; DE502005002780D1

Abstract

The invention relates to a high strength, air-hardening, temper-resistant steel, which can easily be welded and galvanized and exhibits excellent shaping properties, particularly for the construction of lightweight vehicles. The inventive steel comprises the following elements (contents in mass %): C0.07 to ≦0.15, Al≦0.05, Si 0.15 to ≦0.30, Mn 1.60 to ≦2.10, P≦0.020, S≦0.010, N 0.0030 to ≦0.0150, Cr 0.50 to ≦1.0, Mo 0.30 to ≦0.60, Ti 0.010 to ≦0.050, V 0.12 to ≦0.20, B 0.0015 to ≦0.0040, remainder iron including incidental steel-accompanying elements.

Description

The invention relates to a high strength, air-hardening steel with excellent shaping properties, in particular for the construction of lightweight vehicles according to the preamble of claim 1.
The hotly contested automobile market forces the manufacturer, i.a., to constantly seek solutions for lowering the fleet consumption while still maintaining a highest possible comfort and greatest possible protection of occupants. Weight saving of all vehicle components plays hereby a role, on one hand, as is also, on the other hand, a substantially effective behavior of the individual structures, when exposed to static and dynamic stress during operation as well as in the event of a crash. Suppliers attempt to account for this requirement by reducing the wall thickness of available high strength and super high strength steel while at the same time improving the behavior of the structures during manufacture and operation. Such steels are thus required to satisfy the comparably high demands with respect to strength, ductility, toughness, energy absorption, and workability, for example through cold forming, welding, and/or surface treatment.
As a consequence of the high demands for corrosion protection, the surfaces of these steels must be additionally treated with respective anti-corrosion coats, like, e.g., of zinc, whereby conventional hot dip galvanizing as well as high-temperature galvanizing are used.
Apart from the afore-described general requirements, the following mechanical characteristic values should be reached in the tempered state by way of example:


	R_eland R_p0.2:	700-850[MPa]
	R_m:	800-1000[MPa]
	A₈₀:	≧11 [%], and
	A₅:	≧13 [%]

In the past, mostly conventional steels of relative great metal sheet thickness, water-tempered high strength fine grained steels, polyphase steels, or alternative materials, like aluminum, have been used for this range of application.
The use of conventional steels is hereby accompanied by the drawback of the substantial structure weight. Using super high strength polyphase steels as an alternative has drawbacks, such as poorer welding tendency and malleability as a consequence of the high base hardness. Water-tempered steels are expensive to manufacture and thus oftentimes uneconomically.
For these reasons, air-hardening steel materials have been developed as an alternative which overcome the shortcomings of conventional steels just by air-cooling the steel, following a heat treatment for example, in order to realize the demands on material properties.
When, after cold rolling, the steel is cooled by air, at least in sections, at such a speed as to trigger the air hardening effect, the cold workability can be realized by a subsequent soft annealing process, e.g. in a hood-type annealing furnace, or by homogenizing through annealing. As an alternative, cold workability may also be retained after hot rolling, when slowly cooling a coil that has been wound respectively tight in some circumstances in a special heat-insulated hood.
After cold forming or shaping, air hardening with subsequent tempering may then be realized again by a following heat treatment, for example advantageously by high temperature galvanizing.
The term “cold forming” relates hereby to the following process variants:

a) the direct production of respective structures of hot strip through deep drawing or the like with subsequent optional tempering treatment.
b) The further processing to tubes with respective drawing and annealing processes. The tubes in turn are subsequently shaped and then tempered to parts, e.g. through bending, internal high-pressure forming (IHF) or the like.
c) The further processing of the hot strip to cold strip with integrated (hood) annealing process. The cold strip is then subjected to deep drawing or the like, like in a).

Typical characteristic values for hot-annealed, cold-formable air-hardening steels for hot-formed or cold-formed metal sheets and tubes are listed hereinafter:


	R_eland R_p0.2:	330-500[MPa]
	R_m:	480-620[MPa]
	A₈₀:	≧20 [%], and
	A₅:	≧22 [%]

DE 102 21487 B4 discloses the use of an air-hardening steel material for shaped parts in the construction of lightweight vehicles, containing the primary elements C (0.09-0.13%), Si (0.15-0.30%), Mn (1.10-1.60%), Cr(1.0-1.6%), Mo (0.30-0.60%), and V (0.12-0.25%), remainder iron including incidental accompanying elements.
Although this alloy concept on the basis of Co—Mo—V is able to attain mechanical material properties as well as a good tempering resistance and galvanizing capability, as demanded for the stated field of application, there is the drawback of a relatively high Cr content of 1.0-1.6%, that can cause unwanted chromium-carbide precipitations in the weld seam, in particular when tubes are produced by the predominantly used high frequency induction (HFI) welding process. These precipitations may lead to crack formation in the weld seam and thus to a premature failure of the structure, when the welded tube is further processed by shaping, or when the welded structure is exposed to significant mechanical stress during operation. The relatively high content of chromium also further increases costs.
EP 0 576 107 B1 discloses an air-hardening steel with reduced Cr content for production of seamless, ungalvanized structural tubes, for example as door reinforcements in the manufacture of automobiles. The alloy concept on the basis of Mn—Si—Ti—B includes as primary elements C (0.15-0.30%), Mn (2.05-3.35%), Si (0.50-0.80%), Cr (0.5-1.0%), Mo (max. 0.6%), Ti (0.01-0.05%), B (0.0015-0.0035%), and N (0.002-0.015%), remainder iron and incidental accompanying elements.
This steel, known for the seamless tube production, has the drawback that as a result of the relatively high contents of C and Mn, this alloy concept places constraints on the basic welding capability of the steel, on one hand, and greatly limits the galvanizing capability through hot dipping or high temperature galvanizing as a result of the also high content of Si of up to 0.8%.
In-house test have further shown that the essential tempering resistance of this conventional steel is not ensured in particular because of the absence of vanadium so that the strength decreases significantly below the required values for air-hardened steel, when exposed to higher temperatures, e.g. ≧550° C., as encountered in high temperature galvanizing for example.
As it is generally known, a prerequisite for establishing a sufficient tempering resistance is the formation of a sufficient amount of Cr carbides, Mo carbides and/or V carbides, or carbon nitrides, which prevent slip dislocation at elevated temperatures as a result of precipitation on the grain boundaries. This process is called secondary hardening.
DE 44 46 709 A1 discloses the use of air-hardening steel for structural hollow profiles of seamless, hot formed tubes, for example for door reinforcements.
A steel alloy is hereby used having the primary elements: C (0.17-0.28%), Mn (1.30-2.50%), Si (0.30-0.49%), Cr (≦0.49%), Mo (0.20-0.40%), Ni (0.05-0.19%), Ti (0.02-0.07%), B (0.0015-0.0050%), Nb (0.01-0.10%), V(0.01-0.10%), and N (≦0.015%), remainder iron and incidental accompanying elements. In addition, the total content of V+Nb+Ti may not exceed a value of 0.15%.
This alloy concept with additives of Nb and Ni is expensive for an air-hardening steel to meet the demanded requirements, and welding is problematic because of the relatively high C content. Moreover, this steel has a content of 0.30 to 0.49% of silicon which is critical for galvanizing capability.
Then invention is based on the object to provide a high strength air-hardening steel exhibiting excellent shaping properties, in particular for the construction of lightweight vehicles, by using a different cost-saving alloy concept, and realizing the desired mechanical properties while at the same time ensuring especially the HFI welding capability as well as exhibiting superior general welding, galvanizing, and tempering resistances.
According to the teaching of the invention, this object is attained by steel having the following contents in mass-%:


	C	0.07 to ≦0.15
	Al	≦0.05
	Si	0.15 to ≦0.30
	Mn	1.60 to ≦2.10
	P	≦0.020
	S	≦0.010
	N	0.0030 to ≦0.0150
	Cr	0.50 to ≦1.0
	Mo	0.30 to ≦0.60
	Ti	0.010 to ≦0.050
	V	0.12 to ≦0.20
	B	0.0015 to ≦0.0040,

remainder iron including incidental steel-accompanying elements.

The high strength, air-hardening steel according to the invention for the construction of lightweight vehicles is characterized in that this alloy concept realizes an excellent welding capability during HFI welding, without the undesired chromium-carbide precipitations in the weld seam. Compared to the known air-hardening steel for seamless tubes, there is also a reduced content of C and Mn to ensure a superior general welding capability while at the same time exhibiting excellent shaping properties.
At the same time, the decreased Si content ensures the galvanizing capability of the steel, and the addition of V ensures the tempering resistance.
Extensive laboratory tests have shown that the Cr content that is crucial for the air-hardening effect can be lowered to the non-critical level in order to prevent the precipitation of chromium-carbide during HFI welding, when the air-hardening capability of the steel is simultaneously improved again by a complex alloy concept on the basis of Cr—Mo—Ti—B.
The alloy concept in accordance with the invention is based on the recognition that, in contrast to conventional steel for seamless tubes where nitrogen must completely react with titanium to avoid boron nitride precipitations and to thereby ensure the effectiveness of the added boron, nitrogen fixation may also be realized by other alloy elements such as Cr or Mo.
Establishment of an over-stoichiometric addition of titanium in relation to nitrogen is therefore no longer necessarily required. The addition of vanadium triggers precipitations of vanadium-carbon-nitrides of type V(C,N), when exposed to higher tempering temperatures, and opposes a drop in strength via a secondary hardening.
Tests have been conducted initially with two known air-hardening steel based on different alloy concepts. Alloys on the basis of Cr—Mo—V (variant 1) as well as on the basis of Mn—Si—Ti—B (variants 2 and 3) were established for these comparative steels and their properties were examined.


Melt	C %	Si %	Mn %	N %	Cr %	Mo %	Ti %	V %	B %	Remark

Variant 1	0.15	0.21	1.30	0.0056	1.40	0.58	—	0.19	—	Cr—Mo—V
Variant 2	0.10	0.71	2.16	0.0060	0.69	0.31	0.027	—	0.0034	Mn—Si—Ti—B
Variant 3	0.14	0.53	1.98	0.0060	0.69	0.31	0.027	—	0.0034	Mn—Si—Ti—B
Variant 4	0.10	0.48	1.60	0.0120	0.57	0.56	0.015	0.16	0.0034	V—Ti—B
Variant 5	0.10	0.30	1.92	0.0120	1.15	0.63	0.020	0.14	0.0032	V—Ti—B

Although variant 1 meets the demanded mechanic-technological properties, the high Cr content of 1.4% causes, as already described above, the undesired chromium-carbide precipitations and thus prevents the formation of a high-quality HFI weld seam and thus the production of longitudinally welded tubes.
Variant 2 is used to date only for the seamless tube production in ungalvanized configuration. This steel exhibits in particular a small content of carbide-forming elements, primarily chromium.
Compared to variant 2, variant 3 has lower Si and Mn contents, whereas the C content is slightly increased.
These alloy concepts based on Mn—Si—Ti—B have the drawback that the excessive Si content, although necessary for realizing high strength values, complicates component galvanizing. Furthermore, the material strength significantly deteriorates below the required levels at temperatures of about 550° C. so that the tempering resistance is also not ensured.
The variants 4 and 5, based on a V—Ti—B concept, exhibit in the “soft” state mechanical characteristic values which meet the requirements.
The tempering resistance of these variants can be improved in comparison to the variants 2 and 3 by addition of V as well as by a slightly elevated Mo fraction.
Variant 4 having, compared to the variants 2 and 3, a Mo content which has been increased to 0.56%, still fails to provide sufficient tempering resistance.
In order to improve the tempering resistance, the Mo content of variant 5 has been slightly elevated to 0.63% and the Cr content significantly to 1.15%. In the tempered state, the requirements have been met at tempering temperatures of up to about 550° C., whereas above this tempering temperature a significant strength rise has been determined which does not meet requirements.
On the basis of these recognitions, the alloy concept according to the invention, as described above, has been established, with the following analysis range proven especially advantageous:


	C	0.086 to ≦0.10
	Al	≦0.05
	Si	0.20 to ≦0.30
	Mn	1.80 to ≦2.00
	P	≦0.020
	S	≦0.010
	N	0.0030 to ≦0.0125
	Cr	0.70 to ≦0.80
	Mo	0.40 to ≦0.50
	Ti	0.020 to ≦0.030
	V	0.13 to ≦0.17
	B	0.0025 to ≦0.0035,

remainder iron including incidental steel-accompanying elements.

In a cold strip of 1.5 mm thickness, produced in accordance with this alloy concept, as well as tubes having the dimensions 60×1.5 mm, properties have been determined in soft state as well as also in tempered state, which meet the demanded mechanical requirements, on one hand, as well as requirements with respect to welding and galvanizing capabilities.
The chemical composition of the steel was as follows:


C %	Si %	Mn %	S %	P	N	Cr	Mo	Ti	V	B

0.10	0.25	1.97	0.004	0.011	0.0125	0.72	0.50	0.028	0.15	0.0030

The determined mechanical characteristic values, as determined in the soft state of the cold strip are listed in the following with respect to different tempering temperatures:


	R_elor R_p0.2[MPa]	R_m[MPa]	A₈₀[%]

demanded

State	330-500	480-620	≧20

soft	364	535	28.7
100° C./20 min	360	533	29.1
tempered 150° C.¹⁾	374	532	26.7
250° C.	376	533	27.6
350° C.	434	538	25.7
450° C.	445	538	25.3
550° C.	416	530	26
650° C.	422	523	27.9

¹⁾Tempering time in each case 15 min with subsequent air cooling

Tempering should hereby simulate the heat treatment from the state “soft” during hot galvanizing. The results show that the requirements with respect to the steel properties in the soft state have been met even after galvanizing.
The mechanical characteristic values, as determined in the tempered state of the cold strip, are listed hereinafter in the event of heat treatment comprised of austenitizing for 15 min at 920° C., subsequent air cooling to room temperature, and final tempering to different tempering temperatures:


	R_elor R_p0.2[MPa]	R_m[MPa]	A₈₀[%]

demanded

State	750-850	850-1000	≧11

tempered 350° C.¹⁾	838	998	14.0
tempered 400° C.	817	995	14.8
tempered 450° C.	835	985	15.8
tempered 500° C.	791	971	16.2
tempered 550° C.	764	926	16.8
tempered 600° C.	784	940	17.1
tempered 650° C.	850	962	18.1
tempered 700° C.	780	883	21.0

¹⁾Austenitizing at 920° C. 15 min + air cooling to room temperature + tempering 15 min with subsequent air cooling.

The results show the high tempering resistance of the steel up to temperatures of at least 700° C.
The mechanical characteristic values as determined in the soft state of the tubes are listed hereinafter:


	R_elor R_p0.2[MPa]	R_m[MPa]	A₈₀[%]

demanded

State	330-500	480-620	≧22

soft	337-345	511-512	>37

The mechanical characteristic values as determined in the tempered state of the tubes in the event of a heat treatment, comprised of austenitizing at 930° C., the subsequent air cooling, and tempering to 575° C., are:


	R_elor R_p0.2[MPa]	R_m[MPa]	A₈₀[%]

demanded

State	700-850	800-1000	≧13

tempered	820-830	967-974	13.0

As further tests of the steel according to the invention have shown, this steel is not only advantageous for use in the automobile industry but furthermore in all applications in which enameling in combination with high steel strengths is required. Advantageously, the enameling temperature of the steel of above 900° C. may hereby be used to obtain high steel strengths using air hardening during subsequent cooling. The field of application for such enameled, air-hardened steels may, e.g., include the household appliance industry or the chemical apparatus construction industry.
The advantages of the air-hardening steel according to the invention are listed again hereinafter;

very good cold-forming capability in soft state,
very good welding capability in soft and air-hardened state,
very good HFI welding capability,
easy to coat by means of conventional coating processes, like cathodic dip-coating (CDC), hot-galvanizing, and high temperature galvanizing,
very good enameling capability,
use for welded, statically and dynamically highly stressed structures,
more cost-saving than comparable alloy concepts.

Claims

1. A high strength, air-hardening, temper-resistant steel suitable for welding and galvanizing, comprising the elements (contents in mass-%):

remainder iron including incidental steel-accompanying elements.

2. The steel of claim 1, wherein the steel has a C content of 0.08 to ≦0.10%.

3. The steel of claim 1, wherein the steel has a Si content of 0.20 to ≦0.30%.

4. The steel of claim 1, wherein the steel has a Mn content of 1.80 to ≦2.0%.

5. The steel of claim 1, wherein the steel has a N content of 0.0030 to ≦0.0125%.

6. The steel of claim 5, wherein the steel has a N content of 0.0030 to ≦0.00805%.

7. The steel of claim 1, wherein the steel has Cr content of 0.70 to ≦0.80%.

8. The steel of claim 1, wherein the steel has a Mo content of 0.40 to ≦0.50%.

9. The steel of claim 1, wherein the steel has a Ti content of 0.02 to ≦0.03%.

10. The steel of claim 1, wherein the steel has a V content of 0.13 to ≦0.17%.

11. The steel of claim 1, wherein the steel has a B content of 0.0025 to ≦0.0035%.