EP0881306A1 - Ductile steel with high yield strength and process for manufacturing same - Google Patents

Abstract

Multiphase hot-rolled steel exhibiting transformation induced plasticity has a structure comprising ferrite and bainite or a mixture of bainite and martensite, with retained austenite. It contains by weight: 0.05-0.5% carbon, 0.50-2.5% manganese and 0.30-0.80% silicon. Also claimed is preparation of the steel from an ingot of this formula, which is heated at 1150-1300 degrees C for 135-200 minutes, roughly rolled while cooling to between 900 and 1150 degrees C, then finish rolled while cooling to or below the austenite transformation temperature. The resulting steel band is cooled slowly to just above the pearlite formation temperature, then rapidly to below that temperature. It is wound onto a spool below the temperature of bainite formation but above that of martensite formation, thus forming some bainite in the microstructure. Finally it is quenched to stop the bainite formation and prevent precipitation of iron carbide.

Description

The present invention relates to a multiphase steel hot rolled showing an induced plasticity transformation ("TRIP") comprising ferrite, bainite and / or a mixture of bainite and martensite, and residual austenite and whose chemical composition contains carbon, manganese and silicon ,.

High strength steels like steels rephosphorus, microalloyed steels, dualphase steels are widely used for auto parts. Sheets made in such steel require sufficient strength to meet safety automobiles and must also have excellent properties of shaping.

It is also known that the strength and ductility of a multiphase steel can be improved by induced transformation of plasticity ("transformation induced plasticity," TRIP ") of austenite residual.

This phenomenon was first discovered by Zackay et al in steels containing large amounts of nickel and chrome.

However, the presence in these steels of large quantities of such combining elements pose problems for the manufacture of steels under economically profitable conditions.

It should also be noted that a steel containing a significant amount of residual austenite can be obtained by adding of silicon and manganese and by controlled hot rolling producing a multiphase structure in isolated areas of austenite.

The object of the present invention is to provide a steel multiphase of the aforementioned type, however having properties improved strength and ductility particularly suitable for shaping in the automotive industry and this under conditions economically justified.

0,05 à 0,5 % de carbone

0,50 à 2,5 % de manganèse

0,30 à 0,80 % de silicium,
et une faible teneur en niobium, vanadium, zirconium et/ou titane, d'une manière telle à éviter une croissance exagérée des grains d'austénite lors du réchauffement des brammes, le restant étant du fer et des impuretés inévitables.

To this end, the steel according to the invention contains, calculated in% by weight:

0.05 to 0.5% carbon

0.50-2.5% manganese

0.30 to 0.80% of silicon,
and a low content of niobium, vanadium, zirconium and / or titanium, in such a way as to avoid an exaggerated growth of the austenite grains during the heating of the slabs, the remainder being iron and inevitable impurities.

• un réchauffement à une température de 1150°C à 1300°C pendant 135 à 200 minutes,
• un laminage de dégrossissement avec un refroidissement se terminant à une température de 900°C à 1150°C,
• un laminage de finition avec un refroidissement jusqu'à proximité ou en dessous de la température de transformation de l'austénite (A₃),
• un refroidissement lent jusqu'à proximité de la température de formation de perlite (A₁),
• un refroidissement rapide jusqu'en dessous de la température de formation de perlite,
• un bobinage de la tôle, obtenue lors des opérations susdites de laminage, en dessous de la température de départ de formation de bainite et au-dessus de la température de départ de la formation de martensite, de manière à ce que la formation de bainite a lieu dans la tôle enrouleé, et
• une trempe de cette tôle bobinée pour arrêter la formation de bainite et pour éviter le risque de précipitation de carbure de fer.

The invention also relates to a method for manufacturing a multi-phase steel sheet of the aforementioned type. According to this process, a steel ingot corresponding to the above-mentioned chemical composition is first prepared which is then subjected successively to the following operations:

• reheating at a temperature of 1150 ° C to 1300 ° C for 135 to 200 minutes,
• coarse rolling with cooling ending at a temperature of 900 ° C to 1150 ° C,
• finish rolling with cooling to near or below the austenite transformation temperature (A ₃ ),
• slow cooling down to near the perlite formation temperature (A ₁ ),
• rapid cooling to below the perlite formation temperature,
• a winding of the sheet, obtained during the above rolling operations, below the starting temperature of bainite formation and above the starting temperature of martensite formation, so that the bainite formation has place in the rolled sheet, and
• quenching of this coiled sheet to stop the formation of bainite and to avoid the risk of precipitation of iron carbide.

In addition, the invention also relates to a steel multiphase which has a morphology similar to that of steel obtained by the implementation of this process.

It could therefore be a steel whose composition chemical is somewhat different from the one given above and which has been obtained by another method than that described above.

Other details and features of the invention will emerge of the description given below, by way of nonlimiting examples, of some particular embodiments of the steel according to the invention and of the process which can be applied to obtain a steel of the type cited above, with reference to the accompanying graph illustrating this process.

In general, the invention relates to a steel hot-rolled multiphase comprising ferrite, bainite or a mixture of bainite and martensite, and residual austenite and of which the chemical composition contains 0.05% to 0.5% of carbon, 0.50% to 2.5% manganese, 0.30% to 0.80% silicon, low content niobium, vanadium, zirconium and / or titanium, the remainder being iron and traces of unavoidable impurities.

In addition, the steel according to the invention contains preferably less than 0.100% aluminum, less than 0.015% nitrogen, less than 0.300% sulfur, less than 0.100% phosphorus and less than 0.005% boron.

• un réchauffement à une température de 1150°C à 1300°C durant 135 à 200 minutes ;
• un laminage de dégrossissement combiné avec un refroidissement dont la température finale est de 900°C à 1150°C et dont l'épaisseur finale est de 26 mm à 50 mm ;
• un laminage de finition combiné avec un refroidissement jusqu'à proximité ou en dessous de la température de transformation de l'austénite (A3) ;
• un refroidissement lent jusqu'à proximité de la température de formation de perlite (A1);
• un refroidissement rapide jusqu'en dessous de la température de formation de perlite ;
• un bobinage de la tôle obtenue lors des opérations susdites de laminage à une température en dessous de la température de départ de formation de bainite et au-dessus de la température de départ de formation de martensite, de manière à ce que la formation de bainite ait lieu dans la tôle bobinée, et une trempe de cette tôle bobinée pour arrêter la formation de bainite et pour éviter le risque de précipitation de carbure de fer.

As already mentioned above, the invention also relates to a process for the manufacture of a multi-phase steel sheet corresponding to the aforementioned chemical composition, according to which an ingot, having this composition, is subjected successively to the following operations:

• reheating to a temperature of 1150 ° C to 1300 ° C for 135 to 200 minutes;
• coarse rolling combined with cooling, the final temperature of which is from 900 ° C to 1150 ° C and the final thickness of which is from 26 mm to 50 mm;
• finishing rolling combined with cooling to near or below the austenite transformation temperature (A3);
• slow cooling to near the perlite formation temperature (A1);
• rapid cooling to below the perlite formation temperature;
• a winding of the sheet obtained during the above rolling operations at a temperature below the starting temperature of bainite formation and above the starting temperature of martensite formation, so that the bainite formation has place in the coiled sheet, and a quenching of this coiled sheet to stop the formation of bainite and to avoid the risk of precipitation of iron carbide.

The attached graph shows the temperature evolution of the sheet thus treated as a function of the temperature. So, on the ordinate, we give the temperature and, on the abscissa, the time in minutes.

The line segment 1 of this graph represents the slow cooling of the steel during rolling.

The temperature A3 corresponds to the temperature of transformation of the austenite obtained at the end of the aforementioned rolling. The temperature A1 corresponds to the perlite formation temperature at the end of the slow cooling of the sheet on a cooling table ("Runout table"). Slow cooling, represented by line 2, takes place at a rate of 5 K / s to 15 K / s in the presence of air, depending on the thickness sheet metal, steel composition and sheet temperature.

This slow cooling is then followed by a rapid cooling of the order of 50 K / s to 80 K / s by ramps water to a final temperature below the initial bainite formation temperature, as shown by line 3 on the graph.

Then takes place the winding of the sheet combined with a slow air cooling, as shown by line 4, this temperature varying from 250 to 450 ° C for 5 to 120 minutes.

When switching from the cooling table to winding of the sheet, it still loses about 10 K / s.

The winding is carried out at a temperature of below the bainite formation start temperature Bs and above of the martensite flow temperature Ms, so that the bainite formation takes place in the same wound sheet.

This bainite formation, which is a phase containing about 0.2% carbon, also causes carbon diffusion in residual austenite.

After a time of 5 to 120 minutes, depending on the kinetics of bainite, which is itself dependent on the temperature of bainite formation and steel composition, the coiled sheet is subjected to quenching in a liquid medium, in particular in water, which has been represented in the graph by line 5.

In this graph, the hatched area 6 corresponds to the ferrite formation, while the hatched area 7 corresponds to the perlite formation.

Furthermore, since the rolling takes place only at a temperature higher than the transformation temperature of the austenite A ₃ , it is therefore a steel known as "hot rolled".

According to the invention, it has been found that, during the implementation shape, the soft residual austenite is transformed into hard martensite which gives the sheet good resistance against necking and a higher uniform elongation.

By varying the content of the combining elements in the chemical composition of steel, it is possible to influence the properties of the sheet obtained after the above operations.

Thus, the addition of carbon determines the maximum volume of ferrite and allows the martensite Ms starting temperature to be significantly reduced according to the formula: Ms = 539 ° C - 423 * C - 30.4 * Mn - 7.5 * Si (Andrew's formula).

The carbon content thus makes it possible to control the time of start of ferrite formation in a cooling diagram and the kinetics of ferrite and bainite formation.

Manganese increases the area of the formation of bainite in the continuous cooling diagram and in the isothermal maintenance diagram by decreasing the temperature of departure from martensite Ms.

In the ferrite / perlite area of the cooling (above 500 ° C) it forms a "nose" which means that the ferrite and the perlite are vertically separated.

As a result, during the controlled transformation on the cooling table, a large amount of ferrite can be formed without reaching the start time of perlite formation.

Silicon stabilizes carbon in solution in austenite and in bainite by inhibiting precipitation of cementite. This effect is explained by the fact that silicon is relatively poorly soluble in cementite, which requires controlled ejection, by diffusion, of silicon in the transformation front. This causes inhibition of growth of cementite embryos.

Thus, residual austenite can be enriched by diffusion of carbon from a decomposition phase from austenite to ferrite and / or bainite.

The elements, consisting of niobium, vanadium, zirconium and titanium, alone or in combination, are used in low amount to form carbides, nitrides or carbonitrides so to be able to block the growth of grains during the heating of the slab.

Smaller grains lead to a reduced carbon diffusion path and to carbon gradients weaker. This reduces the risk of precipitates forming of cementite.

Aluminum is used to fix nitrogen in solution in forming aluminum nitrides. Aluminum nitrides also have a positive effect by reducing the growth of austenite grains up to a temperature of around 1150 ° C during heating bramma.

Nitrogen and sulfur are impurities whose content must be kept as small as possible. Phosphorus has an effect positive on the strength of the steel, but must also be maintained as small as possible to avoid weakening effects.

In very small quantities, boron is deposited in the joints grain and thus increases ductility. Boron also harms the formation of phases, which are obtained by diffusion, such as ferrite and perlite phases. This therefore requires that the boron content should be kept as low as possible.

The invention also relates to a multiphase steel. obtained, in principle, by any method, but having the same structure and morphology as the multiphase steel obtained directly by the specific process described above.

• un bon rapport entre la limite d'élasticité et la charge de rupture ;
• de bonnes propriétés de formage et un meilleur allongement uniforme comparés à ceux des aciers multiphasés connus;
• un coefficient d'écrouissage élevé lors d'une déformation;
• un freinage contre la striction grâce à la valeur n élevée et un allongement uniforme élevé ;
• des propriétés mécaniques contrôlées par la température de traitement ;
• un durcissement possible de la structure multiphasée par la transformation d'austénite résiduelle en bainite à température élevée dans la tôle d'acier ayant été mise en forme, par exemple dans des procédés d'immersion à chaud, tels que la galvanisation par du zinc et de l'aluminium, ou un traitement chaud subséquent ;
• un durcissement de la structure multiphasée par la transformation d'austénite résiduelle en martensite lors d'un traitement à température basse, tel que dans de l'azote liquide, pour contrôler les propriétés mécaniques de l'acier avant ou après déformation ;
• un bon comportement contre la fatigue mécanique par une combinaison de phases dures formées de bainite et de martensite, et de phases douces, de ferrite et austénite résiduelle ;
• une valeur d'absorption d'énergie élevée pendant la déformation à haute vitesse par suite d'une transformation de phases et un mécanisme de multiplication et dislocation ;
• une bonne soudabilité par suite d'une valeur basse de carbone équivalent.

Among the advantages of the steel according to the invention are:

• a good relationship between the yield strength and the breaking load;
• good forming properties and better uniform elongation compared to those of known multiphase steels;
• a high work hardening coefficient during a deformation;
• braking against necking thanks to the high n value and high uniform elongation;
• mechanical properties controlled by the processing temperature;
• a possible hardening of the multiphase structure by the transformation of residual austenite into bainite at high temperature in the steel sheet having been shaped, for example in hot immersion processes, such as galvanization with zinc and aluminum, or a subsequent hot treatment;
• hardening of the multiphase structure by the transformation of residual austenite into martensite during a treatment at low temperature, such as in liquid nitrogen, to control the mechanical properties of the steel before or after deformation;
• good behavior against mechanical fatigue by a combination of hard phases formed of bainite and martensite, and soft phases, of ferrite and residual austenite;
• a high energy absorption value during high speed deformation as a result of phase transformation and a multiplication and dislocation mechanism;
• good weldability due to a low equivalent carbon value.

By the fact that the silicon content in the steel is less than 0.8% and that this steel is quenched after winding, it is possible to control the amount of residual austenite, as a result of blockage of the bainite formation in the coiled steel, to avoid carbide precipitation, by controlling the enrichment in carbide by blocking the diffusion of carbon, and thus obtaining a steel with higher ductility due to low hardening phases by a reduced Si content. In addition, the following steel the invention allows hot immersion in zinc and aluminum without risk of surface oxidation or adhesion problems. In in addition, the steel according to the invention does not show any surface defect called "The cat's tongue" during hot rolling caused by a high Si content.

In order to further illustrate the purpose of this invention are given below some concrete examples of chemical compositions of a multiphase steel according to the invention and of parameters of the different stages of hot rolling applied to this steel.

EXAMPLE 1

Chemical composition :

0.16% C
1.5% Mn
0.6% If
0.015% Nb

1) Reheating:

temperature: 1280 ° C
holding time: 150 min

2) Coarse rolling:

final temperature: 1100 ° C

3) Finishing lamination:

final temperature: 870 ° C

4) Cooling table:

• first zone: slow cooling of ≤ 20 K / s up to a final temperature of 660 ° C.
• second zone: rapid cooling of ≥ 50 K / s up to a final temperature of 370 ° C.

5) Winding:

winding temperature 350 ° C for 15 min.

6) Tempering:

Water quenching

EXAMPLE 2

Chemical composition :

0.5% C
0.5% Mn
0.8% If
0.020 Ti

1) Reheating:

temperature: 1280 ° C
holding time: 150 min

2) Coarse rolling:

final temperature: 1100 ° C

3) Finishing lamination:

final temperature: 810 ° C

4) Cooling table:

• first zone: slow cooling of ≤ 20 K / s up to a final temperature of 700 ° C.
• second zone: rapid cooling of ≥ 50 K / s up to a final temperature of 320 ° C.

5) Winding:

winding temperature 300 ° C for 20 min.

6) Tempering:

oil quenching

EXAMPLE 3

Chemical composition :

0.05% C
2.5% Mn
0.3% If
0.010% Ti
0.020% V

1) Reheating:

temperature: 1150 ° C
hold time: 155 min

2) Coarse rolling:

final temperature: 960 ° C

3) Finishing rolling: final temperature:

780 ° C

4) Cooling table:

• first zone: slow cooling of ≤ 20 K / s up to a final temperature of 680 ° C.
• second zone: rapid cooling of ≥ 50 K / s up to a final temperature of 320 ° C.

5) Winding:

winding temperature 280 ° C for 60 min

6) Water quenching.

Below are given the mechanical properties obtained by the conventional tensile test of multiphase steel with the "TRIP" effect, which was obtained according to Example 1 during an industrial test on a sheet sample d '' a thickness of 3 mm taken from the axis of the sheet subjected to tension 25/125 (ISO Standard): Yield strength Rp 0.2% (MPa) 544 Breaking load Rm (MPa) 986 Ratio Rp 0.2% Rm (%) 55.2 Bearing elongation (%) 0.0 Uniform elongation (%) 15.3 Total elongation (%) 26.2 Coefficient n 0.160

Claims (10)

Multiphase hot rolled steel showing an induced plasticity transformation ("TRIP") comprising ferrite, bainite or a mixture of bainite and martensite, and residual austenite and whose chemical composition contains carbon, manganese and silicon, this steel being characterized in that it essentially contains, calculated in% by weight: carbon 0.05% to 0.5%, manganese 0.50% to 2.5%, silicon 0.30% to 0.80% Steel according to claim 1, characterized in that it essentially corresponds to the following chemical composition:
titanium, niobium, zirconium and / or vanadium 0.010 to 0; 100% aluminum <0.100% nitrogen ≤ 0.015% sulfur ≤ 0.300 phosphorus ≤ 0.100% boron ≤ 0; 005% the remainder being iron and unavoidable impurities. Process for the production of a multi-phase steel sheet according to claim 1 or 2, characterized in that a steel ingot containing: carbon 0.05% to 0.5%, manganese 0.5% to 2.5%, silicon 0.30% to 0.80% and this ingot is subjected successively to the following operations: reheating at a temperature of 1150 ° C to 1300 ° C for 135 to 200 minutes, coarse rolling with cooling ending at a temperature of 900 ° C to 1150 ° C, finish rolling with cooling to near or below the austenite transformation temperature (A ₃ ), slow cooling down to near the perlite formation temperature (A ₁ ), rapid cooling to below the perlite formation temperature, a winding of the sheet obtained during the above rolling operations below the starting temperature of bainite formation and above the starting temperature of martensite formation, so that bainite formation takes place in the rolled sheet and quenching of this coiled sheet to stop the formation of bainite and to avoid the risk of precipitation of iron carbide. Method according to claim 3, characterized in that we reduce the thickness of the steel ingot to a thickness of 26 to 50 mm during the aforementioned coarse rolling. Process according to either of Claims 3 or 4, characterized in that a finish rolling is applied up to a final temperature from 780 ° to 910 ° C. A method according to any of claims 3 to 5, characterized in that the finishing rolling is followed by a slow cooling at a speed lower than 20 K / s. Method according to any of claims 2 to 6, characterized in that the abovementioned rapid cooling is carried out at a speed greater than 50 K / s, for example up to 80 K / s. A method according to any of claims 3 to 7, characterized in that the sheet is wound up at a temperature from 250 ° C to 450 ° C for 5 to 120 minutes. A method according to any of claims 3 8, characterized in that quenching of the coiled sheet is carried out. Multiphase steel characterized in that it has a morphology which can be obtained by implementing the method according to any one of claims 3 to 9.

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