EP1587963B1 - Ultrahigh strength hot-rolled steel and method of producing bands - Google Patents

Abstract

A very high strength hot rolled steel has the following chemical composition, (by wt %): (a) 0.05 at most C at most 0.1; (b) 0.7 at most Mn at most 1.1; (c) 0.5 at most Cr at most 1.0; (d) 0.05 at most Si at most 0.3; (e) 0.05 at most Ti at most 0.1; (f) Al at most 0.07; (g) S at most 0.03; (h) P at most 0.05; (i) the remainder being iron and production impurities. The steel has a bainite-martensite structure able to contain up to 5% of ferrite.

Description

The present invention relates to a hot-rolled steel with a very high strength, and a method of manufacturing strips of this steel, whose structure is bainito-martensitic and can contain up to 5% of ferrite.

Very high strength steels have been developed in recent years, in particular to meet the specific needs of the automotive industry, which are in particular the reduction of weight and thus the thickness of parts, and the improvement of safety. which goes through the increase of the resistance to fatigue and the shock resistance of the parts. These improvements should not further deteriorate the fitness of the sheets used in the manufacture of parts.

This formability assumes that the steel has a significant elongation A (> 10%) as well as a ratio of the yield strength E on the tensile strength Rm having a low value.

Improved impact resistance of shaped parts can be achieved in different ways and, in particular, by using steels having on the one hand a significant elongation A and on the other hand an E / Rm ratio having a low value, which allows after shaping and thanks to the consolidation capacity of the steel to increase its elastic limit.

The fatigue life of the parts defines their service life according to the stresses undergone, and can be improved by increasing the tensile strength Rm of the steel. But, the increase in strength deteriorates the ability to shape the steel, thus limiting the workable parts, particularly with regard to their thickness.

For the purposes of the present invention, the term "high-strength steel" denotes a steel whose tensile strength Rm is greater than 800 MPa.

There is a first family of very high strength steels, which are steels containing high proportions of carbon (over 0.1%) and manganese (more than 1.2%) and whose structure is entirely martensitic. They have a resistance of more than 1000 MPa, obtained by a quenching heat treatment, but have an elongation A of less than 8%, which prevents any shaping.

A second family of very high strength steels consists of so-called dual phase steels with a structure comprising approximately 10% ferrite and 90% martensite. These steels have a very good formability, but resistance levels not exceeding 800 MPa.

US6364968 discloses a method for producing a very high strength hot rolled steel sheet which comprises preparing and heating a slab of composition 0.05% ≤C≤0.3%, 1.5% ≤Mn≤3, 5%, 0.03% ≤S≤1%, Al≤0.07%, S≤0.05%, P≤0.02%, N≤0.0200%, 0.003% ≤Nb≤0.20% or / and 0.005% ≤Si≤0.20% remains iron and impurities resulting from processing at a temperature <1200 ° C, hot rolling at a temperature> 800 ° C, cooling this sheet with a cooling rate of 20-150 ° C and winding said sheet at a winding temperature of 300-350 ° C and a steel having this composition.

The object of the present invention is to overcome the drawbacks of the steels of the prior art by providing a hot rolled steel with very high strength, suitable for shaping, and having improved fatigue strength and impact resistance.

$$\mathrm{0}\mathrm{,}7\mathrm{\%}\mathrm{\le }\mathrm{Mn}\mathrm{\le }1\mathrm{,}\mathrm{1}\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{5}\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }1\mathrm{,}0\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{\le }\mathrm{0}\mathrm{,}3\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{\le }\mathrm{0}\mathrm{,}\mathrm{1}\mathrm{\%}$$

$$\mathrm{Al}\le 0,07\%$$

$$\mathrm{S}\le 0,03\%$$

$$\mathrm{P}\le 0,05\%$$

le reste étant du fer et des impuretés résultant de l'élaboration,
ledit acier ayant une structure bainito-martensitique pouvant contenir jusqu'à 5% de ferrite.To this end, the invention firstly relates to a hot rolled steel with very high strength, characterized in that its chemical composition comprises, by weight: $$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{VS}\mathrm{\le }\mathrm{0}\mathrm{,}\mathrm{1}\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}7\mathrm{\%}\mathrm{\le }\mathrm{mn}\mathrm{\le }1\mathrm{,}\mathrm{1}\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{5}\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }1\mathrm{,}0\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Yes}\mathrm{\le }\mathrm{0}\mathrm{,}3\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{\le }\mathrm{0}\mathrm{,}\mathrm{1}\mathrm{\%}$$

$$\mathrm{al}\le 0,07\%$$

$$\mathrm{S}\le 0,03\%$$

$$\mathrm{P}\le 0,05\%$$

the rest being iron and impurities resulting from the elaboration,
said steel having a bainito-martensitic structure which can contain up to 5% of ferrite.

$$\mathrm{0}\mathrm{,}8\mathrm{\%}\mathrm{\le }\mathrm{Mn}\mathrm{\le }\mathrm{1}\mathrm{,}0\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}6\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }\mathrm{0}\mathrm{,}9\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}2\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{\le }\mathrm{0}\mathrm{,}3\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{\le }\mathrm{0}\mathrm{,}09\mathrm{\%}$$

$$\mathrm{Al}\le 0,07\%$$

$$\mathrm{S}\le 0,03\%$$

$$\mathrm{P}\le 0,05\%\mathrm{.}$$

le reste étant du fer et des impuretés résultant de l'élaboration.In a preferred embodiment, the chemical composition further comprises, by weight: $$\mathrm{0}\mathrm{,}08\mathrm{\%}\mathrm{\le }\mathrm{VS}\mathrm{\le }\mathrm{0}\mathrm{,}09\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}8\mathrm{\%}\mathrm{\le }\mathrm{mn}\mathrm{\le }\mathrm{1}\mathrm{,}0\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}6\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }\mathrm{0}\mathrm{,}9\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}2\mathrm{\%}\mathrm{\le }\mathrm{Yes}\mathrm{\le }\mathrm{0}\mathrm{,}3\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{\le }\mathrm{0}\mathrm{,}09\mathrm{\%}$$

$$\mathrm{al}\le 0,07\%$$

$$\mathrm{S}\le 0,03\%$$

$$\mathrm{P}\le 0,05\%\mathrm{.}$$

the rest being iron and impurities resulting from the elaboration.

In another preferred embodiment, the structure of the steel according to the invention consists of 70 to 90% of bainite, 10 to 30% of martensite and 0 to 5% of ferrite, and more particularly preferably from 70 to 85% of bainite, from 15 to 30% of martensite and from 0 to 5% of ferrite.

• une résistance à la traction Rm supérieure ou égale à 950 MPa,
• un allongement à la rupture A supérieur ou égal à 10%,
• une limite d'élasticité E supérieure ou égale à 680 MPa,
• un rapport E/Rm inférieur à 0,8.

The steel according to the invention may also comprise the following characteristics, alone or in combination:

• a tensile strength Rm greater than or equal to 950 MPa,
• an elongation at break A greater than or equal to 10%,
• a yield strength E greater than or equal to 680 MPa,
• an E / Rm ratio of less than 0.8.

$$\mathrm{0}\mathrm{,}7\mathrm{\%}\mathrm{\le }\mathrm{Mn}\mathrm{\le }1\mathrm{,}1\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}5\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }1\mathrm{,}0\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{\le }\mathrm{0}\mathrm{,}3\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{\le }\mathrm{0}\mathrm{,}1\mathrm{\%}$$

$$\mathrm{Al}\le 0,07\%$$

$$\mathrm{S}\le 0,03\%$$

$$\mathrm{P}\le 0,05\%$$

le reste étant du fer et des impuretés résultant de l'élaboration,
la température de laminage étant inférieure à 950°C, puis on fait refroidir la bande ainsi obtenue jusqu'à une température inférieure ou égale à 400°C, en maintenant une vitesse de refroidissement supérieure à 50°C/s entre 800 et 700°C, puis on bobine ladite bande à une température de bobinage inférieure ou égale à 250°C.The subject of the invention is also a method for producing a very high-strength hot-rolled steel strip according to the invention, in which a slab is hot-rolled, the composition of which comprises: $$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{VS}\mathrm{\le }\mathrm{0}\mathrm{,}1\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}7\mathrm{\%}\mathrm{\le }\mathrm{mn}\mathrm{\le }1\mathrm{,}1\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}5\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }1\mathrm{,}0\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Yes}\mathrm{\le }\mathrm{0}\mathrm{,}3\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{\le }\mathrm{0}\mathrm{,}1\mathrm{\%}$$

$$\mathrm{al}\le 0,07\%$$

$$\mathrm{S}\le 0,03\%$$

$$\mathrm{P}\le 0,05\%$$

the rest being iron and impurities resulting from the elaboration,
the rolling temperature being below 950 ° C., and the strip thus obtained is cooled to a temperature of less than or equal to 400 ° C., while maintaining a cooling rate greater than 50 ° C./s between 800 and 700 ° C. C, then said coil is wound at a winding temperature of less than or equal to 250 ° C.

$$\mathrm{0}\mathrm{,}8\mathrm{\%}\mathrm{\le }\mathrm{Mn}\mathrm{\le }1\mathrm{,}0\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}6\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }\mathrm{0}\mathrm{,}9\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}2\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{\le }\mathrm{0}\mathrm{,}3\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{\le }\mathrm{0}\mathrm{,}09\mathrm{\%}$$

$$\mathrm{Al}\le 0,07\%$$

$$\mathrm{S}\le 0,03\%$$

$$\mathrm{P}\le 0,05\%$$

le reste étant du fer et des impuretés résultant de l'élaboration.In a preferred embodiment, the composition of the slab is as follows: $$\mathrm{0}\mathrm{,}08\mathrm{\%}\mathrm{\le }\mathrm{VS}\mathrm{\le }\mathrm{0}\mathrm{,}09\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}8\mathrm{\%}\mathrm{\le }\mathrm{mn}\mathrm{\le }1\mathrm{,}0\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}6\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }\mathrm{0}\mathrm{,}9\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}2\mathrm{\%}\mathrm{\le }\mathrm{Yes}\mathrm{\le }\mathrm{0}\mathrm{,}3\mathrm{\%}$$

$$\mathrm{0}\mathrm{,}\mathrm{05}\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{\le }\mathrm{0}\mathrm{,}09\mathrm{\%}$$

$$\mathrm{al}\le 0,07\%$$

$$\mathrm{S}\le 0,03\%$$

$$\mathrm{P}\le 0,05\%$$

the rest being iron and impurities resulting from the elaboration.

In another preferred embodiment, the hot-rolled steel strip is coated with zinc or a zinc alloy, by immersion in a bath of zinc or molten zinc alloy, after winding and after being unreeled, then annealed.

The method according to the invention consists first of all in hot rolling a slab of specific composition, in order to obtain a homogeneous structure. The rolling temperature is less than 950 ° C, and preferably less than 900 ° C.

At the end of rolling, the strip thus obtained is cooled to a temperature of less than or equal to 400 ° C., while maintaining a cooling rate greater than 50 ° C./s between 800 and 700 ° C. This rapid cooling is carried out in such a way that less than 5% of ferrite, which is not desired, is formed because the titanium would precipitate preferentially in this phase. This cooling rate is preferably between 50 ° C / s and 200 ° C / s.

The method then consists in winding the strip at a winding temperature of less than or equal to 250 ° C. We limit the temperature of this stage in order to avoid generating an income from martensite, which would reduce the mechanical strength and raise the elastic limit, resulting in a bad E / Rm ratio.

The composition according to the invention comprises carbon with a content of between 0.05% and 0.100%. This element is essential for obtaining good mechanical characteristics, but must not be present in too large quantities, because it could generate segregations. A carbon content of less than 0.100 makes it possible in particular to have good weldability, and to improve the shaping properties and the endurance limit.

It also includes manganese at a level of between 0.7% and 1.1%. Manganese improves the yield strength of steel while greatly reducing its ductility, for which it limits its content. A content of less than 1.1% also prevents segregation during continuous casting.

The composition also comprises chromium at a level of between 0.50% and 1.0%. A minimum content of 0.50% makes it possible to promote the appearance of bainite in the microstructure. However, its content is limited to 1.0% because a high chromium content would favor the increase of the amount of ferrite formed above 5%, because of its alphagenic character.

The composition also comprises silicon at a content of between 0.05% and 0.3%. It greatly improves the yield strength of steel while reducing its ductility slightly and deteriorating its coating, which is why its content is limited.

The composition also comprises titanium at a content of between 0.05 and 0.1%. This element makes it possible to significantly increase the mechanical characteristics by a phenomenon of precipitation during rolling and cooling. It does not increase the hot hardness because of its moderate content. Its content is limited to 0.1% in order to avoid degrading the impact resistance properties, the hot hardness and the folding ability.

The composition may also comprise phosphorus at a content of less than 0.05%, since, beyond that, it could pose problems of segregation during continuous casting.

The composition also comprises aluminum with a content of less than 0.07%, which occurs during the calming of the steel during its manufacture at the steel mill.

Examples

By way of non-limiting example, and to better illustrate the invention, a steel grade has been developed. Its composition is given in the following table: VS mn Cr Yes Ti S P al AT 0.78 0.95 0.79 0.233 0.094 0,001 0,038 0.048 The rest of the composition consists of iron and unavoidable impurities resulting from the elaboration.

Abbreviations used

Rm:

tensile strength in MPa,

Rp0,2:

yield strength in MPa,

AT :

lengthening, measured in%

From Grade A, three samples were prepared, laminated at 860 ° C, and subjected to different thermomechanical paths. The cooling rates were varied between 800 and 700 ° C, as well as the winding temperature, in order to highlight the structural differences obtained.

The mechanical characteristics of the steels obtained are then measured. The results are summarized in the following table: Trial V _800-700 T bob rm Rp0.2 E / Rm AT% (° C) (° C) (MPa) (MPa) * 1 57 200 995 690 0.7 14 2 42 200 780 635 0.8 14 3 20 400 800 705 0.9 - according to the invention.

The microstructure of test 1, according to the invention, is bainito-martensitic, whereas the microstructure of tests 2 and 3 is ferritic-bainitic.

It is found that a cooling rate between 800 and 700 ° C lower than 50 ° C./s, induces a presence of ferrite in a proportion greater than 5%. The titanium will then precipitate in this ferrite, which no longer allows to obtain the desired level of mechanical characteristics, in particular a high Rm.

Moreover, a winding temperature greater than 250 ° C., associated with a cooling rate between 800 and 700 ° C. of less than 50 ° C./s, increases the yield strength without increasing the mechanical strength. The ratio E / Rm is therefore too high.

Finally, it is found that a cooling rate between 800 and 700 ° C higher than 50 ° C / s, associated with a winding temperature of less than 250 ° C, gives excellent values of mechanical strength and yield strength. . The essentially bainitomensitic structure gives the product a good ratio E / Rm and an elongation greater than 10%.

In addition, the steel according to the invention has a good ability to dip coating in a bath of molten metal, such as zinc or a zinc alloy, or aluminum or an alloy thereof.

Claims (10)

1. Ultrahigh-strength hot-rolled steel, characterized in that its chemical composition comprises, by weight: $$0.05\mathrm{\%}\mathrm{\le }\mathrm{C}\mathrm{\le }0.1\mathrm{\%}$$

   

   $$0.7\mathrm{\%}\mathrm{\le }\mathrm{Mn}\mathrm{\le }1.1\mathrm{\%}$$

   

   $$0.5\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }1.0\mathrm{\%}$$

   

   $$0.05\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{\le }0.3\mathrm{\%}$$

   

   $$0.05\mathrm{\le }\mathrm{Ti}\mathrm{\le }0.1\mathrm{\%}$$

   

   $$\mathrm{Al}\le 0.07\%$$

   

   $$\mathrm{S}\le 0.03\%$$

   

   $$\mathrm{P}\le 0.05\%$$

   

   
   the balance being iron and impurities resulting from the smelting, said steel having a bainite-martensite structure that may contain up to 5% ferrite.
2. Steel according to Claim 1, characterized in that its composition furthermore comprises: $$0.08\mathrm{\%}\mathrm{\le }\mathrm{C}\mathrm{\le }0.09\mathrm{\%}$$

   

   $$0.8\mathrm{\%}\mathrm{\le }\mathrm{Mn}\mathrm{\le }1.0\mathrm{\%}$$

   

   $$0.6\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }0.9\mathrm{\%}$$

   

   $$0.2\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{\le }0.3\mathrm{\%}$$

   

   $$0.05\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{\le }0.09\mathrm{\%}$$

   

   $$\mathrm{Al}\le 0.07\%$$

   

   $$\mathrm{S}\le 0.03\%$$

   

   $$\mathrm{P}\le 0.05\%$$

   

   
   the balance being iron and impurities resulting from the smelting, said steel having a bainite-martensite structure that may contain up to 5% ferrite.
3. Steel according to either of Claims 1 and 2, characterized furthermore in that its structure consists of 70 to 90% bainite, 10 to 30% martensite and 0 to 5% ferrite.
4. Steel according to any one of Claims 1 to 3, characterized in that it has a tensile strength R_m of 950 MPa or higher.
5. Steel according to any one of Claims 1 to 4, characterized in that it has an elongation at break A of 10% or higher.
6. Steel according to any one of Claims 1 to 5, characterized in that it has a yield strength E of 680 MPa or higher.
7. Steel according to any one of Claims 1 to 6, characterized in that it has an E/R_m ratio of less than 0.8.
8. Process for manufacturing a strip of ultrahigh-strength hot-rolled steel according to any one of Claims 1 to 7, characterized in that a slab, whose composition comprises: $$0.05\mathrm{\%}\mathrm{\le }\mathrm{C}\mathrm{\le }0.1\mathrm{\%}$$

   

   $$0.7\mathrm{\%}\mathrm{\le }\mathrm{Mn}\mathrm{\le }1.1\mathrm{\%}$$

   

   $$0.5\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }1.0\mathrm{\%}$$

   

   $$0.05\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{\le }0.3\mathrm{\%}$$

   

   $$0.05\mathrm{\le }\mathrm{Ti}\mathrm{\le }0.1\mathrm{\%}$$

   

   $$\mathrm{Al}\le 0.07\%$$

   

   $$\mathrm{S}\le 0.03\%$$

   

   $$\mathrm{P}\le 0.05\%,$$

   

   
   the balance being iron and impurities resulting from the smelting, is hot-rolled, the rolling temperature being below 950°C, then the strip thus obtained is cooled down to a temperature of 400°C or below, maintaining a cooling rate of greater than 50°C/s between 800 and 700°C, and then said strip is coiled at a coiling temperature of 250°C or below.
9. Manufacturing process according to Claim 8, characterized furthermore in that a slab whose composition comprises: $$0.08\mathrm{\%}\mathrm{\le }\mathrm{C}\mathrm{\le }0.09\mathrm{\%}$$

   

   $$0.8\mathrm{\%}\mathrm{\le }\mathrm{Mn}\mathrm{\le }1.0\mathrm{\%}$$

   

   $$0.6\mathrm{\%}\mathrm{\le }\mathrm{Cr}\mathrm{\le }0.9\mathrm{\%}$$

   

   $$0.2\mathrm{\%}\mathrm{\le }\mathrm{Si}\mathrm{\le }0.3\mathrm{\%}$$

   

   $$0.05\mathrm{\%}\mathrm{\le }\mathrm{Ti}\mathrm{\le }0.09\mathrm{\%}$$

   

   $$\mathrm{Al}\le 0.07\%$$

   

   $$\mathrm{S}\le 0.03\%$$

   

   $$\mathrm{P}\le 0.05\%,$$

   

   
   the balance being iron and impurities resulting from the smelting, is hot-rolled.
10. Manufacturing process according to either of Claims 8 and 9, characterized in that the hot-rolled steel strip is coated with zinc or a zinc alloy, by dipping it into a bath of molten zinc or zinc alloy following said coiling operation and after having been uncoiled, and then annealed.