DE2105218A1 - Galvanised steel - cold-reduced decarburised and heat treated before galvanising - Google Patents

Abstract

Steel with improved deep-drawing properties suitable for car bodies, is produced from steel containing 0.03-0.06% C, which is cold-reduced after usual hot-rolling and pickling etc., subjected to decarburising heat-treatment below 780 degrees C to 0.01% C content; re-heated for short period to temp. above recrystallisation point up to A3- conversion point; galvanised and rolled. Quench hardening is prevented. Pref. C content is 0.005% after decarburising heat-treatment, and pref. decarburising heat-treatment takes place before or after cold reduction, or pref. between a first and second cold-reduction. Pref. rimmed steel is used and decarburising heat-treatment takes place after a 2-step cold-reduction.

Description

Process for the production of a hot-dip galvanized steel for press forming purposes.

The invention relates to a method for producing a hot-dip galvanized one Steel for press deformation purposes, which is characterized in particular by deep-drawability.

Recently, the use of galvanized (galvanized) Sheet steel for vehicle construction, i.e. for the body and chassis, for Prevention of corrosion due to atmospheric conditions, use of salt on the roads to remove ice and the like, greatly increased. These Development is based on the erosion resistance of this galvanized steel much better than that of conventional low-carbon steel is. However, it is known that the seizure deformability of this galvanized steel There are limits. Therefore, some parts cannot be made by compression molding.

Investigations by the inventors have shown that the limits of ireßdeformbarkeit are based on the continuous electroplating process that provides rapid cooling after hot immersion makes it necessary. In this context the familiar forms Armco-Sandzimir process is an example.

Rapid cooling of the running belt with a temperature of around 46,000 on the hot-dip galvanizing stage leads to a quenching hardness of the strip, which is generally due to supersaturated carbon in the steel. This quench hardening leads to the fact that the mechanical properties of the steel, in particular the ductility, be made worse.

Numerous attempts have therefore been made to improve the quench hardness to decrease or cancel. The US steel process in the United States von America and the Dortmund hearing process in West Germany are representative Results of these efforts. The former method is characterized by a Waistband treatment at the heating stage of the belt, the latter based on one Aging treatment after electroplating and winding. However, also in these Cases generated the quenching hardness mentioned. The improvement is accordingly The deep drawability of these steels naturally depends on the respective application limited.

The invention is aimed at avoiding the quench hardness and achieving it directed by deep drawability. It is therefore an object of the invention to provide a method for the production of a hot-dip galvanized, galvanized steel, in which Quench hardening is avoided. In doing so, a method is to be developed that enables the production of a hot-dip galvanized (galvanized) steel that endures severe compression deformation.

The invention is characterized by a decarburization treatment on the cold reduction stage after hot rolling and pickling and to the subsequent introduction into the hot-dip galvanizing container after the renewed heat treatment of the tape. When the tape is treated in this way, excellent Achieve deep drawing properties of galvanized steel. Surpass these properties the properties of known steels for many things.

In the following, for example, preferred embodiments of the Invention explained in more detail with reference to the accompanying drawing.

The single figure shows the relationship between the decarburization temperature and the amount of oxygen in the steel which directly affects the ferrite grain.

In connection with the method according to the invention, a steel with low carbon content (unkilled steel or Al-killed steel) with a carbon content in the range from 0.03 to 0.06%. The usual Methods of making slabs and hot rolling are sufficient for the present invention Steel from. The main feature of the invention lies in the cold reduction stage after the usual pickling treatment. The decarburization process should be at the cold reduction stage be applied. If the warm band is reduced cold in the conventional way, the decarburization heat treatment is carried out before or after this cold reduction. If the warm strip is reduced in two cold stages, the decarburization treatment can be carried out also take place between the two cold rolling stages. The cold reduced and on the above type of decarburized tape is then heated to a higher temperature for a short time warmed up. The temperature is below the A3 point, and the time is about 1 minute. The tape is then continuously cooled to about 460 ° C and hot dip galvanized. The quickly cooled tape is eventually used in the usual way Tempered rolled, the carbon content in the steel after the decarburization treatment is less than 0.01%, preferably 0.005%.

The above method is the reason for the limitation of the carbon range to 0.03 to 0.06% in the following: a carbon content of less than 0.03% can cannot be achieved with the usual steelmaking processes, and a steel with more than 0.06% carbon, it is difficult to effectively decarburize. In particular, an oxygen range of 0.05 to 0.1% oxygen becomes in the steel adhered to.

This oxygen range leads to an improvement in the excellent Compression deformation properties of hot-dipped galvanized steel in connection with the decarburization temperature as will be mentioned later. Less than 0.05% Oxygen leads to a coarsening of BerritAnger, and more than 0.1% oxygen leads to contamination of the steel.

Each of these factors affect the zinc coating layer and the mechanical properties of steel. The accompanying drawing illustrates the relationships between the oxygen content and the decarburization temperature, the has emerged from numerous experiments.

From the drawing it can be seen that the area is uniform Korns with an oxygen content of more than 0.05% and a decarburization temperature less than 7800C. The range of more than 0.06% oxygen and less however, 7500C is to be regarded as preferred. The area less than 0.05% Oxygen and more than 78000 is hardly usable, since in this case it becomes a Coarsening of the grain is coming. Therefore the decarburization temperature is below 7800C limited. Such a decarburization heat treatment is carried out in the usual open-beam heat treatment furnace carried out and can be carried out at any point in time of the above-mentioned Xage reduction stage take place. According to the invention, the cold reduction does not require any particular rolling speed and can be done in the usual way.

After this decarburization heat treatment, the tape becomes continuous to a temperature above the recrystallization point and below of the A3 point of the steel. This ball will have a temperature close to that A3 point preferred as the heating temperature. While the steel already in the previous Stage has been decarburized and the continuous heating of the strip in a very short time Time - about 1 minute - is carried out, the compression deformability is without disadvantage the better, the higher the temperature is in the specified range.

The re-heat treated tape is cooled to about 4600 ° and quickly cooled to room temperature after passing through the Beuerverzink - galvanizing tank has run. This strip is then tempered.

The method according to the invention has numerous advantages in comparison with the conventional, continuous re-galvanizing process. Once they are mechanical properties of the steel according to the invention much better than those of steels manufactured in a known manner. On the other hand, there is resistance to aging significantly improved, since the decarburization heat treatment according to the invention on the Cold reduction stage takes place and the dissolved carbon in the ferrite is very low is. Thirdly, there is no coarsening of the crystal grain despite a heat treatment temperature used near the A3 point. A major improvement in the mechanical Properties is achieved in a simple manner, and the zinc coating layer is in preserved intact condition.

In the following, the invention is to be explained further by means of examples will.

Example ß (decarburization before aging reduction) Steel making furnace: LD converter chemical composition: unkilled steel Tabel IC: 0.04% P: 0.010% N: 0.0018% Mn: 0.33% S: 0.017% O: 0.058% Warm whale conditions after slab formation: Final temperature: 850 ° C Coiling temperature: 595 ° C Thickness of the Tape: 2.3 mm Decarburization conditions after pickling: @ Heat treatment furnace used: Furnace bundle furnace atmosphere: wet hydrogen Treatment temperature: 730 ° C x 18 hours.

Carbon content of steel: 0.002% Cold reduction conditions: Reduction ratio: 65.2% Final thickness: 0.8 mm Continuous hot-dip galvanizing process: Process type used: Armco Sendzimir heat treatment temperature: 8500C x 1 Min.

Hot dip temperature: 460 ° C Tempering rollers: 1.5% The mechanical properties of the above-mentioned steel are summarized in Table II.

Table II Final carbon content: 0.002% yield strength. 17.3 kg / mm2 Elongation at yield point: 0% Tensile strength: 28.7 kg / mm2 Total elongation: 52.3% hardness, HR-30T: 41.8 mean Lankford value, r: 1.67.

A steel compared with Example 1 was replaced by a common one Process made. The manufacturing conditions are the same as in Example 1, except for the decarburization heat treatment. The mechanical properties these steels are shown in Table III.

Table III Final carbon content: 0.04% yield strength: 25.7 kg / mm2 Elongation at yield point: 0.4% tensile strength: 35.4 kg / mm2 total elongation: 41.8% hardness, IIR-30T: 54.5 mean Lankford value: 1.23 The excellent properties of the steel according to the invention result from a comparison of Tables II and III.

Example 2 (time of decarburization as in example 1) Steel making process: LD converter chemical composition: Al-killed steel Table IV C: 0.05% P: 0.011% N: 0.0059% Ha: 0.31% S: 0.014% SolAl: 0.043% according to hot rolling conditions slab formation: final temperature: 860 ° C. coiling temperature: 660 ° C. final thickness: 2.3 mm Decarburization conditions after pickling: Heat treatment furnace used: Open-band furnace atmosphere: wet hydrogen treatment temperature: 7500C x 10 hours

Final Carbon Content: 0.002% Cold Reduction Conditions: Reduction Ratio: 65.2% final thickness: 0.8 mm Continuous hot-dip galvanizing process and tempering rollers: as in example 1.

The mechanical properties of this steel are summarized in Table V.

Table V Final carbon content: 0.002% yield strength: 19.7 kg / mm2 elongation at yield point: 0% tensile strength: 30.4 kg / mm2 total elongation: 50.4% hardness, HR-30T: 43.9 mean Lankford value, r: 1.58 Bin steel compared with example 2 produced by the usual method, i.e. the production conditions were the same as in Example 2 above except for the decarburization heat treatment. The mechanical properties are summarized in Table VI.

Table VI Final carbon content: 0.05% yield strength: 25.0 kg / mm2 elongation at yield point: 0.2% tensile strength: 35.8 kg / mm2 total elongation: 42.0% hardness, HR-30T: 55.8 Mean Lankford value, r: 1.15.

In the above case of the Al-killed steel, the mechanical properties are of the steel according to the invention is also significantly better than that of conventional Stole.

Example 3 (decarburization after cold removal) steel making furnace: LD converter chemical composition: Al-killed steel: Table VII a: 0.05% P: 0.011% N: 0.0048% Mn: 0.31% S: 0.014% Sol, Al: 0.037% according to hot rolling conditions slab formation: final temperature: 8600 ° C., coiling temperature: 540 ° C. final thickness: -3.2 mm Cold reduction conditions after pickling: rolling ratio: 75% final thickness: 0.8 mm decarburization conditions heat treatment furnace used: furnace bundle furnace atmosphere: wet hydrogen holding temperature: 550 ° C x 2 hours

Heating rate to 800 ° C: 50 ° / hour.

Treatment temperature: 8000C x 5 hours

Final carbon content: 0.002% Continuous hot-dip galvanizing process Type used: Armco-Sendzimir reheating temperature: 850 ° C x 30 sec.

J Hot-dip galvanizing temperature: 4600a Tempering rolls: 1.2% The mechanical Properties are shown in Table VIII.

Table VIII Table @ yield point:, 15.9 kg / mm2 X elongation at yield point: 0% 0 tensile strength: 28.6 kg / mm2 total elongation: 49.2% hardness, HR-30T: 38.3 medium Lankfor2 value, r: 2.02 / aging index: 1.2 kg / mm² A compared with example 3 Steel was manufactured in the usual way, i.e. the manufacturing conditions were the same as Example 3 except for the decarburization heat treatment. the mechanical properties are shown in Table IX.

Table IX Yield point: 25.7 kg / mm2 Elongation at yield point: 0.6% Tensile strength: 35.4 kg / mm2 total elongation: 41.3% hardness, HR-30T: 55.2 mean Lankford value, r : 1.24 Aging index: 6.9 kg / mm2 From the above Tables VIII and IX it can be seen that the mechanical properties of the steel according to the invention after the cold reduction decarburized are excellent as those shown in Table V. Steel that has been decarburized before cold reduction.

An example of unkilled steel that has been subjected to cold reduction is decarburized will not be played back. However, the results are the same as in Al-killed steels, as described in Example 3 above.

Example 4 (decarburization between two cold reduction stages) Steel making furnace: LD converter chemical composition: unkilled steel Table X C: 0.04% P: 0.10 N: 0.0018% Mn: 0.33% S: 0.017% 0: 0.058% Hot rolling conditions after slab formation: Final treatment temperature: 8600C Winding temperature: 595 ° C Final thickness: 5.8mm First Kait reduction after pickling: Reduction ratio: 60.3% Final thickness: 2.3 mm Conditions for intermediate decarburization: Heat treatment furnace used: Open-beam furnace Atmosphere: wet hydrogen Treatment temperature: 750 ° C x 10 hours

Final carbon content: 0.002% Second cold reduction conditions: Reduction ratio: 65.2% Final thickness: 0.8 mm Continuous hot-dip galvanizing process: Type used: Armco-Sendzimir rewarming temperature: 8500C x 1 min.

Hot-dip galvanizing temperature: 4600C Tempering Rolls: 1.5% The mechanical properties of this steel are given in Table XI.

Table XI Final Carbon Content: 0.002% Yield Strength: 18.2 kg / mm2 Elongation at yield point: 0% tensile strength: 28.9 kg / mm2 total elongation: 52.1% hardness, HR-30T: 43.7 mean Lankford value, r: 1.72 A steel compared with Example 4 was used manufactured by the usual method under the following conditions: Steel making conditions and chemical composition: as in Example IV.

Hot rolling conditions: Final temperature: 8500C, winding temperature: 595 ° C Final thickness: 2.3 mm Galtr reduction conditions: Reduction ratio: 65.2% Final thickness: 0.8 mm Continuous hot-dip galvanizing process: as in Example IV.

Xempering rollers: 1.5% The mechanical properties of this steel are shown in Table XII.

Table XII Final Carbon Content: 0.04% Yield Strength: 25.7 kg / mm2 Elongation at stress limit: 0.4% Tensile strength: 35.4 kg / mm2 total elongation: 41.8% hardness, HR-30T: 54.5 mean limit requirement value, r: 1.23 The mechanical properties of the steels according to the invention in Example 4 are just as excellent as those of previous examples.

Example 5 (decarburization between two cold reduction stages) Steel making furnace: LD converter chemical composition: Al-killed steel.

Table XIII 0: 0.05% P: 0.011% SolAl-: 0.043 Mn: 0.31% S: 0.014% N: 0.0059% hot rolling conditions after slab formation: Final temperature: 8600C Coiling temperature: 5600C final thickness: 6.0 mm Conditions of the first cold reduction after pickling: Reduction ratio: 61.7% final thickness: 2.3 mm Conditions for intermediate decarburization: same as in the example 4 Conditions of the Second Cold Reduction: Reduction ratio: 65.2% Final temperature: 0.8 mm continuous hot-dip galvanizing process: same as in example 4 Tempering Rolls: 1.5% The mechanical properties of the above steel are shown in Table XIV.

Table XIV Final carbon content: 0.002% yield strength: 17.4 kg / mm2 Elongation at yield point: 0% tensile strength: 28.8 kg / mm2 total elongation: 50.7% hardness, HR-302: 41.8 avg. Lankford value, r: 1.74 A steel compared with Example 5 was made in the usual way. The production conditions were as follows: Steel production conditions and chemical composition: as in example 5.

Hot rolling conditions: Final temperature: 86000 Coiling temperature: 660 C Final thickness: 2.3 mm Cold reduction conditions: the same as in Example 5.

Continuous hot-dip galvanizing process: same as the one above Example.

Tempering Rolls: 1.5% The mechanical properties of the above steel (tnd shown in Table XV.

Table XV Final carbon content: 0.05% yield strength: 25.0 kg / mm² elongation at yield point: 0.2% tensile strength: 35.8 kg / mm2 Total elongation: 42.0 hardness, HR-30T: 55.8 average Lankford value, r: 1.15 The mechanical properties of the Al killed steel of the present invention are excellent as well as those of the unskilled steel of Example 4.

Example 6 (decarburization after the second cold reduction of two cold reduction stages) Steel making furnace: LD converter chemical composition: unkilled steel Table XVI O: 0.06% P: 0; 11 N: 0.0015% Mn: 0.38% S: 0.016% 0: 0.035% Hot rolling conditions after pickling: final temperature: 87000 winding temperature: 6000C final thickness: 6, Ωmm Conditions of the first cold reduction after pickling: Rolling ratio: 61.7% final thickness: 2.3 mm Conditions of the intermediate heat treatment: Furnace used: Dense coil furnace Atmosphere: HNX gas Treatment temperature: 70,000 x 3 hours

Second cold reduction conditions: Reduction ratio: 65.2% Final thickness: 0.8 mm. Conditions of the decarburization treatment: Furnace used: Open-beam furnace The atmosphere: wet hydrogen treatment temperature: 7800C x 10 hrs.

Continuous hot-dip galvanizing process: Type used: Armco-Sendizimir Reheat temp .: 8500C x 30 sec.

Hot-dip galvanized components: 460 ° C Tempering rollers: 1.2% The mechanical properties of the above steel are summarized in Table XVII.

Table XVII yield point: 14.5 kg / mm² elongation at yield point: 0% Tensile strength: 28.1 kg / mm2 total elongation: 54.5% hardness, HR-30T: 34.7 mean Lankford value, r: 1.98 Aging index: 5.3 kg / mm2 A steel that was compared with Example 6, was manufactured in the same way under the following conditions: Bar making furnace: LD converter chemical composition: unkilled steel Table XVIII C: 0.04% P: 0.010% N: 0.0019% Mn: 0.32% S: 0.017% 0: 0.055% Hot rolling conditions after the Slab formation: Final temperature: 8600C Coiling temperature: 6000C Final thickness :. 2.3 mm Cold Reduction Conditions after pickling: Reduction ratio: 75% final thickness: 0.8 mm Continuous hot-dip galvanizing process: as in example 6.

Temper rolls: 1.2%.

The mechanical properties of the above steel are given in Table XIX compiled.

Table XIX yield point: 25.7 kg / mm elongation at yield point: 0.6% Tensile strength: 35.4 kg / mm2 total elongation: 41.3% hardness, HR-30T: 55.2 avg. Lankford value, r: 1.24 aging index: 6.9 kg / mm2 The table above shows that the mechanical Properties of the steel according to the invention are significantly better than those of conventional ones Steels are.

Claims (6)

Claims 1. Process for the production of a hot-dip galvanized steel for press forming purposes, it is noted that one is a steel with a carbon content from 0.03 to 0.06% that the steel is used after the usual flat rolling, hot rolling and cold pickling is reduced by subjecting the steel to a decarburization heat treatment at a temperature of less than 7800C and the carbon content brought to less than 0.01% by reheating the steel for brief zoits to a temperature above the recrystallization point up to the A3 transformation point of steel, that the steel is hot-dip galvanized in the usual way and finally on usual type tempered. 2. The method according to claim 1, characterized in that the carbon content after the decarburization heat treatment is less than 0. Q05 *. 3. The method according to claim t or 2, characterized in that the Decarburization heat treatment is carried out before the cold reduction. 4. The method according to claim t or 2, characterized in that the Decarburization heat treatment is carried out after the cold reduction. 5. The method according to claim 1 or 2, characterized in that the Decarburization heat treatment between a first cold metering and a second Cold reduction is carried out. 6. The method according to claim 1 or 2, characterized in that the Steel is an unkilled steel and the decarburization heat treatment after a two-stage cold reduction is carried out. L e r s e i t e