DE19610675C1 - Dual phase steel for cold rolled sheet or strip - contg. manganese@, aluminium@ and silicon - Google Patents

Abstract

Cold rolled sheet or strip is made from a steel contg. in wt.%: 0.05-0.3 C; 0.8-3 Mn; 0.4-2.5 Al; 0.01-0.2 Si; balance Fe plus usual impurities. The steel is free from pearlite and has a predominantly ferritic structure with inclusions of martensite and opt. bainite and/or residual austenite. -

Description

The invention relates to a steel with pearlite-free, predominantly ferritic microstructure and a method too its production.

Dual phase steels in short "DP steels" stand out a strong consolidation especially in small plastic strains and a low Yield ratio. Thus also lead small Forming degrees to higher component strength, due to high bake-hardening potential after pre-deformation can be further increased. Under "Bake-Hardening" is Artificial aging as a result of stoving understood that to further increase the Component strength leads. For the automotive industry thus DP steels, especially from the point of view Energy conservation and passive safety contribute for weight-optimized construction. The Processing properties of DP steels are due to the low yield ratio and high Kaltverfestigungsvermögens as very favorable too judge.

Furthermore, the forming process by a compared to other high-strength steels lower springback positively influenced. The conventional higher-strength Steels always occurring loss of ductility  conventional softer steels that are themselves for example, in a decrease in the uniform strain is significantly lower for DP steel.

The structure of conventional DP steels consists of 70 to 90% by volume ferrite, residual martensite. The hard martensite is island-shaped in the soft ferritic matrix stored. In addition to martensite can more carbon-rich transformation structures (bainite) occur. In smaller quantities, especially in one Alloy addition of silicon, which is the carbide formation inhibits, even thermodynamically metastable retained austenite to be available. Metastable retained austenite improves the Forming properties during cold forming.

DP steels can be obtained both by hot rolling with a special rolling strategy as well as by cold rolling subsequent heat treatment are produced. To are necessary for hot strip DP steel analyzes whose Transformation behavior of a strong pre-eutectic Ferrite formation and shifted to longer times Perlite formation is characterized. For cold strip are Alloy compositions make sense, where a high Carbon activity and a shift in the GOS line in the iron-carbon diagram to the right, d. h., too higher carbon levels is observed, so the Carbon enrichment of austenite during annealing in the Two-phase area to favor ferrite-austenite. With increasing carbon activity reduces the too Demix necessary glow time. With rising Carbon content of austenite decreases critical cooling rate. So it can after the annealing in the two-phase area lower  Cooling speeds are applied to a predominantly ferritic-martensitic structure adjust.

The ferrite formation after hot forming can by Silicon be promoted. With manganese can the Perlite formation after hot forming as well suppressed during a continuous annealing become.

When hot rolling known silicon-containing steels comes it is the formation of red scale, associated with danger the scale rolling. As a result, after pickling also Surface inhomogeneities on the strip surface to be available. The red tinder, which is also very high Abspritzdrücken in the hot strip mill not removed also leads to a reduction in Pickling. This is a clearer Productivity drop.

Silicon-containing DP steel is in continuous Hot dip galvanizing lines are not galvanizable, because the zinc wetting the steel very badly. For this reason it is also not possible to use silicon-containing DP steel in the execution galvannealed manufacture. The Temperature cycle of a galvannealing enamel dip finishing would be for Si-alloyed DP steel in principle offer the possibility of metastable To produce retained austenite, by the cold workability is further improved.

The production of DP cold strip of surface finish galvannealed by means of a continuous Hot-dip galvanizing line is also with others for DP cold strip previously known alloy concepts,  including the concept with Si, not reliable possible because the perlite formation under the process conditions most z. Z. existing facilities insufficient is strongly suppressed. The formation of perlite is associated with the loss of Dual-phase steel characteristics.

Known DP steels with predominant ferrite content contain 0.03 to 0.12% C, to 0.8% Si and 0.8 to 1.7% Mn (DE 29 24 340 C2) or 0.02 to 0.2% C, 0.05 to 2.0% Si, 0.5 to 2% Mn, 0.3 to 1.5% Cr and 1% Cu, Ni and Mo (EP 0 072 867 B1). Both steels contain aluminum only in grades resulting from the Calming with aluminum resulted. DP steels this Composition divorce but from the above Reason for the hot dip galvanizing.

Other alloying concepts for cold strip DP steels contain 0.03 to 0.12% C, not more than 0.8% Si and 0.8 to 1.7% Mn (DE 29 24 340 C2). such DP steels generally react very sensitively like DP steels on changes in the annealing parameters, mainly on Changes in the cooling rate in the rapid cooling part. With decreasing cooling rate it comes often to a deterioration of the mechanical Properties, in particular of the yield strengths ratio. Also a steel with 0.08 to 0.20% C, 1.5 to 3.5% Mn, 0.1 to 0.5% Cr and 0.010 to 0.1% Nb (EP 0 501 605 A2) allows the representation of a DP steel as a cold strip, but makes when welding larger due to the increased carbon equivalent Trouble.

From this, the task derives from developing steels, the at least the excellent range of mechanical properties of conventional DP steels have a good weldability and also coated with metal, in particular by Hot dip galvanizing surface treatment can be. To the forming advantages in the production of to be able to use corrosion-protected components is a firm adhesion of the coating, z. As a zinc or Zinc-iron alloy layer, necessary. For operational reasons is also a pronounced insensitivity to annealing parameters fluctuations demanded a uniform Guarantee product quality.

To solve this problem is a steel with (in% by mass):

0.05 to 0.3% carbon
0.8 to 3.0% manganese
0.4 to 2.5% aluminum
0.01 to 0.2% silicon
Rest iron with the melting impurities

proposed.

In addition to these main components, the steel can also contain the following additional elements (in% by mass):

up to 0.05% titanium
to 0.8% chromium
to 0.5% molybdenum
to 0.5% nickel
to 0.05% niobium
to 0.08% phosphorus

The structure is after cold rolling with subsequent Heat treatment in a hot dip galvanizing plant or in a continuous annealing furnace made of a ferritic matrix, in the island-shaped martensite is embedded. Depending on Manufacturing conditions may include shares of Intermediate stage and retained austenite can be set.

In compliance with this procedure are in the with Aluminum alloyed steel has the following minimum mechanical characteristics guaranteed:

_Yield strength (R _p0,2 )  200 N / mm² Tensile strength (R _m )  550 N / mm² Elongation at break (A₈₀)  25% Yield _ratio (R _p0,2 / R _m )  0.7

Aluminum provides in the claimed content range extensive ferrite formation in the annealing between the Conversion temperatures Ac₁ and Ac₃ without Loss of productivity for sure. The formation of perlite is shifted to much longer times so far that they for economically feasible cooling rates is sufficiently suppressed. In case of generation of Cold strip in the "galvannealed" version can be Galvannealing process under usual conditions be carried out, with an improvement of the Phase characteristic possible by setting retained austenite is.

The adhesion of both the zinc layer in galvanized Cold strip as well as the zinc-iron alloy layer at galvannealtem cold strip is made clear by aluminum strengthened.  

Manganese also delays perlite formation. The solid-solution strengthening action increases the strength of the steel. Against the background of the raised Manganese content is a treatment of the melt with Calcium makes sense to stretched manganese sulfides and others To convert sulfides into a globular form, the one Forming less detrimental.

The carbon content should be for strength reasons at least 0.05%. Because of Weldability should not exceed 0.3% C in the steel be included.

The addendum additions have the following effects:

Titanium up to 0.05% leads to an increase in strength Grain refining and precipitation hardening and improves the Cold formability.

Chromium increases the strength and improves the Temper resistance of martensite and thus allows the full exploitation of the Bake-Hardenig potential. More however, 0.8% Cr is not required and would just raise the price.

Molybdenum to 0.5% lowers the critical cooling speed and thus reduces the risk of Training of residual stresses of the third kind, since before the Hot dip galvanizing worked with lower cooling capacity can be. This provides greater security to band waviness due to residual stresses third Art.

Nickel is used to increase strength through Solid solution hardening and lowering of the  Conversion temperatures and the non-diffusion Conversion required cooling rates. Furthermore, nickel is austenite in an amount of up to 0.5% stabilizing.

Niobium acts as a micro-alloying element in quantities up to 0.05% by grain refining and precipitation hardening strengthens and improves through hardenability.

Phosphorus up to 0.08% can increase the strength Solid solution hardening are alloyed.

The steel according to the invention is particularly insensitive against changes in the annealing parameters. A steel of this Composition can be very reliable, d. H. regardless of variations in production conditions, to be produced. He is also very good coat, in particular galvanize. For the precursor Hot strip does not cause the formation of red scale.

The setting of a specific hot strip starting structure is not necessary. Kaltwalztechnisch it is convenient emanating from a ferritic-pearlitic structure, the By reeling at temperatures above 600 ° C is obtained.

After cold rolling with a cold rolling degree ε 40% is the recrystallization of the microstructure between 740 and 850 ° C. From the two-phase area Ferrite austenite subsequently becomes zinc bath temperature cooled. The cooling rates are here between 10 and 50 K / s. The zinc bath temperatures are between 450 and 485 ° C. A slow-cooling down to Temperatures of 650 ° C before the rapid cooling is also permissible and offers the possibility of the  Enrichment of austenite with carbon targeted to control. Even with this slow cooling is not the Danger of perlite formation, because aluminum the Perlite formation shifts to significantly longer times.

For the production of a ferritic-martensitic structure can lower in aluminum alloyed steel Cooling rates are set. This promotes production safety in general and wins with increasing strip thickness further meaning, because increases the necessary cooling capacity with increasing thickness. Increased cooling rates are technical consuming. The heat treatment can take place in one Hot dip galvanizing line also in a continuous furnace be performed.

After galvanizing takes place in a hot dip galvanizing line an immediate cooling or when generating a Cold strip with zinc-iron alloy layer in the Execution "galvannealed" a reheating of the steel at temperatures between 480 and 580 ° C.

The new alloy concept allows the production of a high-strength, good cold-formable, surface-refined, So coated, weldable cold strip in the Finishes "galvanized" and a higher strength, good coldformable, surface - finished cold strip in the Execution "galvannealed" with improved Spot weldability, especially in automated Welding lines is required.

A special feature of the steel according to the invention is his pronounced insensitivity to Glow parameter fluctuations that contribute to a high degree Production safety leads.

Example 1

A steel A according to the invention with 0.073% C, 0.052% Si, 1.44% Mn, 1.27% Al, 0.35% Cr, 0.02% P, and 0.001% S was melted down in a converter steelwork, in one Hot strip mill at a final rolling temperature of 920 ° C hot rolled and at a reel temperature of 680 ° C wound. After pickling with sulfuric acid The cold rolling was done in a five-stand Tandem mill with a cold rolling degree of ε = 75% from 3.04 mm to 0.77 mm.

In a hot-dip galvanizing plant, the steel became 6 K / s heated to 750 ° C and then at 1.2 K / s on 830 ° C further heated. From the two-phase area followed by a slow cooling at 4 K / s to 680 ° C followed by accelerated cooling with 20 K / s to 470 ° C. After passing through the 470 ° C warm Zinc tape was cooled to room temperature at 10 K / s. The steel A was immediately in line with a temperament rolled by 0.8%.

This dual phase steel has after the annealing treatment a ferritic matrix, in which evenly martensite islands are stored. The martensite is located both on the trip points of the ferrite grains as well along the ferrite grain boundaries. The ferrite grain size is around 60 μm². Bainite or other microstructure components are not available.

This steel has the following mechanical properties:

_Yield strength (R _p0,2 ) = 308 N / mm² Tensile strength (R _m ) = 551 N / mm² Elongation at break (A₈₀) = 32.6% Yield _ratio (R _p0,2 / R _m ) = 0.56

Galvanized dual phase steel is quasi-isotropic. The planar Isotropy Δr is -0.02.

example 2

Another inventive steel B with the same Analysis of how steel A was in a converter steelworks melted, in a hot strip mill at a Finish rolling temperature of 880 ° C to hot strip hot rolled, and this was at a reel temperature of 695 ° C wound. After pickling with sulfuric acid Cold rolling in a five-stand tandem mill with a cold rolling degree of ε = 80% from 3.44 mm to 1.04 mm.

In a hot-dip galvanizing plant, the cold strip was with 6 K / s heated to 750 ° C and then at 1.2 K / s on 830 ° C further heated. From the two-phase area followed by a slow cooling at 4 K / s to 720 ° C, followed by accelerated cooling with 20 K / s at 470 ° C. After passing through the 470 ° C warm zinc bath followed by an inductive heating 12 K / s up to the galvannealing temperature of 520 ° C and then a cooling at 10 K / s Room temperature. The galvannealte cold rolled steel strip B was immediately in line with a degree of prevalence of 1.1% cold rolled.

The galvannealte cold-rolled strip has after the annealing treatment a pearlite-free ferritic matrix with a Ferrite grain size of around 60 μm², in the martensite islands are stored evenly. The martensite islands focus on the triple points of the ferrite grains, however, they also occur along the ferrite grain boundaries, associated with traces of bainite, on.  

On samples from the galvanneal cold rolled steel strip B were the mechanical Properties determined.

The galvannealte cold strip is quasi-isotropic with a Δr value = -0.07.

example 3

For comparison, the steel V with the composition according to Table 1 is used. This steel is not reliable hot-dip galvanizable and can not be galvannealed in the embodiment because the pearlite formation is not sufficiently suppressed. The mechanical material properties of this steel are in the scatter band of the properties of DP cold strip. The strength level-related ductility R _m .A₈₀, however, is significantly lower than aluminum-alloyed DP cold-rolled strip, as shown in Table 2.

example 4

Another inventive steel C alloyed with 0.21% C, 1.50% Mn, 1.03% Al was in one Induction furnace melted. The cast block was forged and after mechanical processing hot rolled. The last rolltick took place between 920 and 950 ° C instead. The cooling of the hot strip was carried out on dormant air. After pickling, the hot strip was with a cold rolling degree of ε = 66% to a final thickness of 1 mm cold rolled.

Under ambient atmosphere then a cold strip sample conductively heated at 7 K / s to 740 ° C and then  with 1.2 K / s on 820 ° C further heated. From the Two-phase area was then an accelerated Cool at 35 K / s to 550 ° C followed by a Mild cooling at 4 K / s to a temperature of 450 ° C corresponding to a usual zinc bath temperature. Thereafter, the sample was cooled to a temperature of 7 K / s Heated to 500 ° C, held at 500 ° C for 5 s, then with 35 K / s at 350 ° C and finally at 10 K / s Room temperature cooled. The cycle corresponds to one usual galvannealing process.

The sample heat-treated as galvannealtes cold-rolled strip the steel C according to the invention has after the Annealing treatment a pearlite-free ferritic matrix, in the martensite islands and bainite areas with 8.5% by volume Restaustenit are stored evenly. These embedded phases can be found along the grain boundaries, focus on the triple points of the Ferrite grains. The ferrite grain size is about 70 microns.

This steel according to the invention has the in Table 2 specified mechanical properties.

example 5

A steel D according to the invention, alloyed with 0.21% C, 1.49% Mn, 1.99% Al, was in an induction oven melted. The cast-off block was forged and hot rolled after mechanical working. The last Rolling passes took place between 920 and 950 ° C. The Cooling of the hot strip was done in still air. To the pickling was the hot strip with a cold rolling of ε = 66% to a cold strip with a final thickness of 1 mm cold-rolled.  

Under ambient atmosphere then a cold strip sample conductively heated at 7 K / s to 760 ° C and then with 1.2 K / s on 840 ° C further heated. From the Two-phase area was then an accelerated Cool at 35 K / s to 550 ° C followed by a Mild cooling at 4 K / s to a temperature of 450 ° C, corresponding to a typical zinc bath temperature. Subsequently, the sample was at 7 K / s on a Heated temperature of 500 ° C, held at 500 ° C for 5 s, then with 35 K / s to 350 ° C and finally on Room temperature cooled at 10 K / s. This cycle corresponds to a common galvannealing process.

This steel D according to the invention has after Annealing treatment a pearlite-free ferritic matrix, in the martensite islands and bainite areas with 11% by volume Restaustenit are stored evenly. These embedded phases can be found along the grain boundaries, focus on the triple points of the Ferrite grains. The ferrite grain size is about 80 μm 2. Have samples of the coated cold strip produced in this way mechanical properties, as indicated in Table 2 are.  

Table 1

Chemical composition (in% by mass)

Board 2

strength properties

Claims (6)

1. Cold rolled sheet or strip of steel with (in mass%) 0.05 to 0.3% carbon
0.8 to 3.0% manganese
0.4 to 2.5% aluminum
0.01 to 0.2% silicon,
Remaining iron with impurities caused by melting, a pearlite-free, predominantly ferritic microstructure in which martensite and optionally bainite and / or retained austenite are embedded, the high strength (R _{p 0.2} 200 N / mm 2, R _m 550 N / mm 2) with good ductility ( A₈₀ 25%) and has a hot dip coated surface. 2. cold rolled sheet or strip of a steel of the composition according to claim 1, which additionally (in% by mass) to 0.05% titanium,
up to 0.8% chromium,
to 0.5% molybdenum,
to 0.5% nickel,
up to 0.05% niobium,
to 9.08% phosphorus. 3. cold rolled sheet or strip according to claim 1 or 2 with metallic coated, in particular galvanized surface.   4. cold rolled sheet or strip according to claim 3, characterized in that it galvannealed is. 5. Method for producing a cold-rolled Sheet or strip according to claim 3 of a steel of Composition according to one of Claims 1 or 2, characterized in that hot rolled strip after coiling with a Degree of cold rolling of at least 40%, the cold rolled strip then at a temperature in the range annealed from 740 to 850 ° C recrystallizing and immediately afterwards coated metallically, especially hot-dip galvanized. 6. Method for producing a cold-rolled Sheet or strip according to claim 4 made of a steel of Composition according to one of Claims 1 or 2, characterized in that hot rolled strip after coiling with a Degree of cold rolling of at least 40%, the cold rolled strip at a temperature in the range of 740 annealed to 850 ° C recrystallizing and immediately then subjected to galvanneal treatment becomes.