EP0747495B1 - Niobium containing hot rolled steel sheet with high strength and good deep-drawing properties, and process for its manufacture - Google Patents

Abstract

Steel sheet comprises 0.5-1.5wt.% Mn, 0.01-0.10, pref. 0.010-0.020% Nb, 0.01-0.1% Al and up to 0.12% C, 0.3% Si, 0.1% P, 0.05% S, 1% Cr, also up to 0.05% of Ti not in the form of nitrides, sulphides or oxides. The structure comprises at least 75% of ferrite hardened by precipitation of carbides or carbonitrides of Nb or of Nb and Ti, the remainder comprising at least 10% martensite and possibly bainite and austenite. Also claimed are methods of making the steel sheet by hot-rolling followed by controlled cooling.

Description

The invention relates to the steel industry. More specifically, it concerns the field hot-rolled steel sheets which should have high strength properties and drawability, especially for the automotive industry to form parts of vehicle structures.

In the range of hot rolled flat products whose properties mechanical are obtained by controlled rolling on the band train, there are various categories of steels which have, to varying degrees, mechanical characteristics which can be qualified as high.

Steels with high elastic limit (so-called "HLE steels" or "HSLA") are steels microalloyed with niobium, titanium or vanadium. They have a high elastic limit, the minimum according to the grade can range from approximately 300 MPa to approximately 700 MPa, obtained thanks to a refinement of the ferritic grain and a fine hardening precipitation. However, their formability is limited, especially for the highest grades. They have a high elastic limit / tensile strength (R _e / R _m ) ratio.

The so-called "double phase" or "dual phase" steels have a microstructure composed of ferrite and martensite. Ferritic transformation is favored by rapid cooling of the sheet, from the end of the hot rolling, to a temperature below Ar ₃ , followed by slow cooling in air. The martensitic transformation is then obtained by rapid cooling to a temperature below M _s . For a given resistance level, these steels have excellent formability, but this degrades for resistances greater than 650 MPa, due to the large proportion of martensite that they contain.

The so-called "high strength"("HR") steels have a microstructure composed of ferrite and bainite. Their formability is intermediate between that of steels with high elastic limit and that of double-phase steels, but their weldability is lower than those of these two types of steels. Their resistance is limited to grade R _m = 600 MPa, because otherwise their formability decreases very quickly.

Steels called "very low carbon bainitic structure" ("ULCB") have a extremely fine low carbon bainite microstructure composed of ferrite under form of slats and carbides. To obtain it, the ferritic transformation is inhibited by micro-addition of boron, or even niobium. These steels allow achieve very high resistances, greater than 750 MPa, but with a fairly low formability and ductility.

Finally, TRIP (TRansformation Induced Plasticity) steels have a microstructure composed of ferrite, bainite and residual austenite. They allow reach very high resistances, but their weldability is very low due to their high carbon content.

In order to obtain the best possible compromise between strength, formability and also weldability, we have developed (see document EP 0 548 950) hot-rolled sheet steels whose structure essentially contains ferrite hardened by precipitates of titanium carbide and / or niobium and martensite, or even residual austenite. These steels have the composition, expressed in weight percentages:
C ≤ 0.18%; 0.5 ≤ If ≤ 2.5%; 0.5 ≤ Mn ≤2.5%; P ≤ 0.05%; S ≤ 0.02%; 0.01 ≤ Al ≤ 0.1%; 0.02≤Ti≤0.5% and / or 0.03≤Nb≤1%, with C% ≥0.05 + Ti / 4 + Nb / 8.

These steels do indeed have high strengths (R _m is around 700 MPa) and good formability (R _e / R _m is around 0.65). However, their weldability is not as good as one would like. In addition, their surface appearance is not satisfactory: there is the presence of a category of defects called "tigrage" (or "tiger stripes"). These are scale encrustations that cannot be removed by stripping. These faults restrict the possibilities of using the sheets to manufacture parts intended to remain visible.

The object of the invention is to provide users of rolled steel sheets with hot products with a very good compromise between resistance levels good formability and good weldability, as well as a flawless surface.

• C ≤ 0,12 %;
• 0,5 ≤ Mn ≤ 1,5 %;
• 0 ≤ Si ≤ 0,3 %;
• 0 ≤ P ≤ 0,1%;
• 0 ≤ S ≤ 0,05 %;
• 0,01 ≤ Al ≤ 0,1 %;
• 0 ≤ Cr ≤ 1%;
• 0,01 ≤ Nb ≤ 0,1 %
• 0 ≤ Ti_eff ≤ 0,05 %, Ti_eff étant la teneur en titane non sous forme de nitrures, de sulfures ou d'oxydes;
• reste fer et impuretés inévitables et dont la structure comprend au moins 75 % de ferrite durcie par précipitation de carbures ou de carbonitrures de niobium ou de niobium et de titane, le reste de la structure comprenant au moins 10 % de martensite et éventuellement de la bainite et de l'austénite résiduelle.

To this end, the subject of the invention is a hot-rolled steel sheet of high strength and high drawability, the composition of which, expressed in percentages by weight, is:

• C ≤ 0.12%;
• 0.5 ≤ Mn ≤ 1.5%;
• 0 ≤ If ≤ 0.3%;
• 0 ≤ P ≤ 0.1%;
• 0 ≤ S ≤ 0.05%;
• 0.01 ≤ Al ≤ 0.1%;
• 0 ≤ Cr ≤ 1%;
• 0.01 ≤ Nb ≤ 0.1%
• 0 ≤ Ti _eff ≤ 0.05%, Ti _eff being the titanium content not in the form of nitrides, sulphides or oxides;
• iron and impurities which are unavoidable and whose structure comprises at least 75% of ferrite hardened by precipitation of carbides or carbonitrides of niobium or niobium and titanium, the rest of the structure comprising at least 10% of martensite and possibly of bainite and residual austenite.

A subject of the invention is also methods of manufacturing such sheets, as defined in claims 3 to 6.

As will be understood, the sheets according to the invention are distinguished from those hitherto known for the same uses by their substantially lower content of silicon, their ranges of niobium and titanium contents significantly tightened, and stricter requirements on the distribution of the different phases of the structure. And obtaining the structure, therefore the desired properties for the sheet, involves special conditions during the heat treatment immediately following hot rolling. Their composition and manufacturing method mean that these steels represent, in several respects, a combination of HLE steels and double steels phase.

The invention will be better understood on reading the following description, illustrated by Figure 1 which shows a micrograph of a sheet according to the invention.

To obtain hot-rolled sheets according to the invention, it is first necessary elaborate, then pour in the form of a slab, a steel comprising (all percentages are percentages by weight) a lower carbon content or equal to 0.12%, a manganese content of between 0.5 and 1.5%, a content of silicon less than or equal to 0.3%, a phosphorus content less than or equal to 0.1%, sulfur content less than or equal to 0.05%, aluminum content between 0.01 and 0.1%, a chromium content less than or equal to 1%, a niobium content between 0.01 and 0.10%, and an effective titanium content (we will explain below what this term means) between 0 and 0.05%.

The slab is then hot rolled on a strip train to form a sheet a few mm thick. On leaving the strip train, the sheet undergoes a heat treatment which gives it a microstructure composed at least 75% ferrite and at least 10% martensite. The ferrite is hardened by a precipitation of niobium carbides or carbonitrides, and also carbides or titanium carbonitrides if this element is present significantly. The microstructure may optionally also include bainite and austenite residual.

Limited carbon content keeps steel good weldability, and to obtain the desired proportion of martensite.

• il se place en solution solide;
• en abaissant le point Ar₃, il permet d'abaisser la température de fin de laminage et d'obtenir un grain ferritique fin;
• c'est un élément trempant.

Manganese plays a hardening role because:

• it is placed in solid solution;
• by lowering the point Ar ₃ , it makes it possible to lower the end of rolling temperature and to obtain a fine ferritic grain;
• it is a soaking element.

However, at high contents, it causes the formation of a structure in strips and leads to degradation of fatigue performance and / or formability. he its presence must therefore be limited to the maximum specified content of 1.5%.

Silicon is an alpha-element, which therefore promotes ferritic transformation. It is also hardening in solid solution. However, the invention is based, among other things, on a very significant drop in the silicon content of the steel compared to the prior art illustrated by document EP 0 548 950. The advantage of a significant drop in the content of silicon is that the surface appearance problems encountered on steels of the prior art arise, in fact, from an appearance on the surface of the slab, in the reheating furnace, of oxide Fe ₂ SiO ₄ which forms with FeO oxide a low melting eutectic. This eutectic penetrates into the grain boundaries and promotes the anchoring of the scale, which can therefore only be imperfectly removed during pickling. Another advantage of this lowering of the silicon content is the improvement in the weldability of the steel. The steels of the invention, provided that the other specifications on their composition and method of manufacture are respected, tolerate having only low, or even very low, silicon contents.

Like silicon, phosphorus is alphagene and hardens. But its content should be limited to 0.1%, and may be as low as possible. Indeed, it would likely, at high content, to form a mid-thickness segregation which could cause delamination. Furthermore, it can segregate at grain boundaries, which increases fragility.

Although not strictly speaking necessary, an addition of chromium (limited at 1%) is recommended because it promotes the formation of martensite and the ferritic transformation.

Niobium and titanium are elements of micro-alloy which form carbide and carbonitride precipitates hardening ferrite. Their addition, which for the titanium is only optional, aims to obtain, thanks to this hardening, a level of high resistance.

A great feature of the composition of the steels according to the invention is the presence of niobium, although this element is not usually added when wants to obtain a double phase ferrite-martensite type structure. Indeed, the niobium increases the temperature of non-recrystallization of steel, which results in a strong hardening of the austenite, and can cause a heterogeneity of size of grains. In addition, the precipitation of niobium carbides and carbonitrides slows the ferritic transformation. This is why, to obtain in the presence of niobium a sufficient formation of adequately hardened equiaxed ferrite, it is imperative to respect one of the cooling schemes for hot-rolled sheet which will be described.

Regarding the optional addition of titanium, the hardening effect of the ferrite it provides is however only obtained if the titanium has the possibility of combining with carbon. When adding titanium to the liquid steel bath, account must therefore be taken of the possibilities of titanium oxides, nitrides and sulfides. The significant formation of oxides can be easily avoided by adding aluminum during the deoxidation of the liquid steel. As for the quantities of nitrides and sulphides formed, they depend on the nitrogen and sulfur contents of the liquid steel. If it is not possible, during production and casting, to drastically limit these nitrogen and sulfur contents, a sufficient amount of titanium must be added to the metal bath so that in the solidified metal, after precipitation nitrides and sulfides, the titanium content not in the form of nitrides, sulfides or oxides (and therefore available to form carbides and carbonitrides) is at most 0.05%. It is this content which is called "effective titanium content" and which is shortened to "Ti _eff %". When the steel is deoxidized with aluminum, taking into account the thermodynamic equilibria which are established in the metal in the course of solidification, it can be estimated that, if _total Ti% indicates the total content of titanium steel, Ti eff % = Ti total % - 3.4 x N% - 1.5 x S%.

This addition of titanium can advantageously complement the addition of niobium to reach even higher levels of resistance. But add niobium and titanium in excess of the prescribed quantities is unnecessary, as there would then be a saturation of the hardening effect.

To manufacture the sheets according to the invention, different operating methods can be considered, depending on the level of performance sought and the composition metal.

1) on élabore, et on coule sous forme de brame un acier dont la composition en pourcentages pondéraux est:

• C ≤ 0,12 %;
• 0,5 ≤ Mn ≤ 1,5 %;
• 0 ≤ Si ≤ 0,3 %;
• 0 ≤ P ≤ 0,1%;
• 0 ≤ S ≤ 0,05 %;
• 0,01 ≤ Al ≤ 0,1 %;
• 0 ≤ Cr ≤ 1 %;
• 0,01 ≤ Nb ≤ 0,1 %
• 0 ≤ Ti_eff ≤ 0,05 %, Ti_eff étant la teneur en titane non sous forme de nitrures, de sulfures ou d'oxydes;

2) on lamine à chaud ladite brame sur un train à bandes, avec une température de fin de laminage (TFL) située entre le point Ar₃ de la nuance coulée et 950 °C;

3) à la sortie du train à bandes, on effectue un refroidissement du produit en deux étapes:

• étape 1: refroidissement lent, à l'air, à une vitesse de 2 à 15 °C/s, effectué enttre TFL et une température dite "température de début de trempe" (TDT) située entre 730 °C et le point Ar₁ de la nuance coulée; c'est au cours de ce refroidissement qu'a lieu la transformation ferritique; sa durée ne doit pas, dans le cas général, être inférieure à 8 s pour laisser à la transformation ferritique (dont on rappelle qu'elle est retardée par la présence des carbures et carbonitrures de niobium) de s'effectuer de manière correcte; ce refroidissement ne doit pas, non plus, durer plus de 40 s pour ne pas aboutir à des précipités de trop forte taille qui détérioreraient la résistance à la traction de la tôle;
• étape 2: refroidissement rapide, effectué par exemple par aspersion à l'eau, à une vitesse de 20 à 150 °C/s entre TDT et une température dite "température de fin de refroidissement" (TFR) qui est inférieure ou égale à 300 °C.

According to a first operating mode (No. 1), applicable in a standardized manner to all the steels of the invention, and more particularly to those whose niobium content is between 0.02 and 0.1%, the succession of operations is the following:

1) a steel is produced and poured in the form of a slab, the composition in percentages by weight of which is:

• C ≤ 0.12%;
• 0.5 ≤ Mn ≤ 1.5%;
• 0 ≤ If ≤ 0.3%;
• 0 ≤ P ≤ 0.1%;
• 0 ≤ S ≤ 0.05%;
• 0.01 ≤ Al ≤ 0.1%;
• 0 ≤ Cr ≤ 1%;
• 0.01 ≤ Nb ≤ 0.1%
• 0 ≤ Ti _eff ≤ 0.05%, Ti _eff being the titanium content not in the form of nitrides, sulphides or oxides;

2) said slab is hot rolled on a strip train, with an end of rolling temperature (TFL) situated between the point Ar ₃ of the casting grade and 950 ° C;

3) at the exit of the band train, the product is cooled in two stages:

• step 1: slow cooling, in air, at a speed of 2 to 15 ° C / s, carried out between TFL and a temperature called "tempering start temperature" (TDT) located between 730 ° C and point Ar ₁ casting shade; it is during this cooling that the ferritic transformation takes place; its duration should not, in the general case, be less than 8 s to allow the ferritic transformation (which it is recalled that it is delayed by the presence of niobium carbides and carbonitrides) to be carried out in a correct manner; this cooling must also not last more than 40 s so as not to result in precipitates of too large a size which would deteriorate the tensile strength of the sheet;
• step 2: rapid cooling, carried out for example by spraying with water, at a speed of 20 to 150 ° C / s between TDT and a temperature called "end of cooling temperature" (TFR) which is less than or equal to 300 ° C.

Once these operations have been carried out, the sheet can be wound, either immediately, or after a stay in the air.

• étape 1: refroidissement rapide, à l'eau, à une vitesse de 20 à 150°C/s, commençant moins de 10 s après la fin du laminage à chaud, entre TFL et une température intermédiaire (T_inter) inférieure au point Ar₃ de la nuance; pendant cette opération, l'acier reste dans le domaine austénitique;
• étape 2: refroidissement lent, à l'air, à une vitesse de 2 à 15 °C/s, d'une durée supérieure à 5 s et inférieure à 40 s, entre T_inter et TDT, qui est comprise entre le point Ar₁ de la nuance et 730 °C; la transformation ferritique a lieu au cours de cette étape, et là encore la fixation d'une durée minimale pour le refroidissement a pour but d'assurer le bon déroulement de cette transformation malgré la présence de niobium;
• étape 3: refroidissement rapide, à l'eau, à une vitesse de 20 à 150 °C/s, entre TDT et TFR, cette dernière température étant inférieure ou égale à 300 °C.

According to a second operating mode (No. 2), also applicable to all the steels of the invention in a standardized manner, and in particular to those whose niobium content is between 0.02 and 0.1%, operations 1) and 2) are the same as before. On the other hand, operation 3) no longer comprises two, but three stages of cooling, according to:

• step 1: rapid cooling, with water, at a speed of 20 to 150 ° C / s, starting less than 10 s after the end of the hot rolling, between TFL and an intermediate temperature (T _inter ) lower than the point Ar ₃ of the shade; during this operation, the steel remains in the austenitic domain;
• step 2: slow air cooling at a speed of 2 to 15 ° C / s, lasting longer than 5 s and less than 40 s, between T _inter and TDT, which is between point Ar ₁ of the grade and 730 ° C; the ferritic transformation takes place during this stage, and here again the fixing of a minimum duration for the cooling has the aim of ensuring the good progress of this transformation despite the presence of niobium;
• step 3: rapid cooling, with water, at a speed of 20 to 150 ° C / s, between TDT and TFR, the latter temperature being less than or equal to 300 ° C.

The sheet metal can then be wound, again with or without a previous stay in the air.

• une transformation plus rapide, donc plus complète pour une durée donnée du refroidissement à l'air, qui elle-même peut être limitée par la longueur de la table de refroidissement;
• une taille de grain ferritique plus faible;
• une précipitation de carbures et de carbonitrures de niobium et titane plus fine et durcissante.

In this latter operating mode, the water cooling of step 1 of operation 3) has the function of rapidly bringing the sheet into the ferritic transformation domain. The latter then begins immediately after the water cooling has stopped. It therefore takes place faster and at a lower temperature than in the two-stage operating mode. This is explained by:

• faster transformation, therefore more complete for a given duration of air cooling, which itself can be limited by the length of the cooling table;
• a smaller ferritic grain size;
• a finer and hardening precipitation of niobium and titanium carbides and carbonitrides.

If the steel has a relatively low niobium content, i.e. between 0.01 and 0.02%, setting a minimum duration for the step slow cooling in air of operation 3) of the two operating modes which are just described is no longer imperative, niobium not being sufficiently present to very significantly slow down the ferritic transformation.

It is thus possible to produce a sheet whose guaranteed minimum resistance can be adjusted between 650 and 750 MPa, with a ratio R _e / R _{m of} less than 0.8, a work hardening coefficient of at least 0.13, and a total elongation of at least 15%. The tensile curve does not have an elastic limit plateau, which improves the stamping behavior. Finally, the surface appearance of the pickled product does not show "tigrage". The aims assigned to the invention are therefore achieved.

By way of example, experiments of the invention were carried out on the steel grades cited in Table 1 (the titanium contents are total contents; the effective titanium contents must be calculated as described ): steel grades tested Shade VS % Mn% P% Yes % Cr% NOT % S% Nb% Ti% A (reference) 0.072 0.982 0.040 0.190 0.750 0.0059 0.0021 - - B 0.079 1.210 0.015 0.180 0.021 0.0048 0.0027 0.050 0.010 VS 0.080 0.990 0.040 0.200 0.750 0.0051 0.0020 0.080 0.061

These experiments gave the results recorded in Table 2, where t denotes the duration of the air cooling step during which the ferritic transformation takes place, R _p0.2 denotes the conventional elastic limit at 0.2 % of residual elongation and n the work hardening coefficient, and where the column "cooling mode" refers to the two main operating modes described above: Experimental results Shade Cooling mode TDT (° C) t (s) R _p0.2 (MPa) R _m (MPa) R _p0.2 / R _m not A (reference) # 2 720 15 319 590 0.54 0.20 A (reference) # 2 650 15 308 570 0.54 0.20 B # 1 630 18 439 675 0.65 0.16 B # 2 700 15 449 680 0.66 0.16 B # 2 630 15 445 675 0.66 0.16 VS # 2 720 15 515 765 0.67 0.14 VS # 2 630 15 490 720 0.68 0.15 B # 1 730 6 550 590 0.93 0.12 B # 2 720 3 550 620 0.89 0.12

From these results, it can be seen that the addition of niobium and titanium to the reference steel A in grades B and C makes it possible to very significantly increase the resistance of this steel, in particular when the operating mode No. 2 comprising three-stage cooling is used, while maintaining a _suitable R _p0.2 / R _m ratio. It is also noted, according to the last two tests mentioned, that the addition of niobium is ineffective when the sheet is forced to cool in air too short for the ferritic transformation to be able to be carried out satisfactorily: resistance is not improved compared to the reference, while the ratio R _p0.2 / R _m is even significantly deteriorated. Grade B considered during these two tests is particularly sensitive to this factor because its silicon content is not very high, and its phosphorus content is low, and this does not promote ferritic transformation, therefore the formation of martensite. The hard phase is then formed of bainite and / or perlite.

The micrograph in Figure 1 shows the structure of a steel corresponding to grade B with 0.050% niobium and 0.010% titanium. Sheet metal cooling after hot rolling was carried out according to procedure No. 2. Clear areas are equiaxed ferrite and represent 85% of the structure. The dark beaches are of martensite, and represent practically the entire remainder of the structure.

The steels according to the invention can be used in particular to constitute parts of motor vehicle structures, such as chassis elements, wheel sails, suspension arms, as well as all stamped parts in front have a high resistance to mechanical stress.

Claims (6)

1. Hot-rolled steel sheet with high strength and high drawability, the composition of which, expressed in percentages by weight, is:

   C ≤ 0.12%;

   0.5 ≤ Mn ≤ 1.5%;

   0 ≤ Si ≤ 0.3%;

   0 ≤ P ≤ 0.1%;

   0 ≤ S ≤ 0.05%;

   0.01 ≤ Al ≤ 0.1%;

   0 ≤ Cr ≤ 1%;

   0.01 ≤ Nb ≤ 0.1%;

   0 ≤ Ti_eff ≤ 0.05%, Ti_eff being the content of titanium not in the form of nitrides, sulphides or oxides;

   the balance iron and inevitable impurities; and the structure of which comprises at least 75% ferrite hardened by precipitation of Nb or Nb and Ti carbides or carbonitrides, the remainder of the structure comprising at least 10% martensite and possibly bainite and residual austenite.
2. Steel sheet according to Claim 1, characterized in that its Nb content is between 0.010 and 0.020%.
3. Process for manufacturing a hot-rolled steel sheet with high strength and high drawability, in which:

   a steel whose composition is in accordance with that of the sheet according to Claim 1, is smelted and cast in slab form;

   the said slab is then hot-rolled into sheet form, completing the rolling at a temperature of between the Ar₃ point and 950°C;

   the said sheet is then slow-cooled at a rate of 2 to 15°C/s for a time of between 8 and 40 s down to a temperature of between the Ar₁ point and 730°C;

   the said sheet is then quenched at a rate of 20 to 150°C/s down to a temperature less than or equal to 300°C.
4. Process for manufacturing a hot-rolled steel sheet with high strength and high drawability, in which:

   a steel whose composition is in accordance with that of the sheet according to Claim 1 is smelted and cast in slab form;

   the said slab is then hot-rolled into sheet form, completing the rolling at a temperature of between the Ar₃ point and 950°C;

   less than 10 s after the end of hot rolling, the said sheet is then quenched at a rate of 20 to 150°C/s down to a temperature below the Ar₃ point;

   the said sheet is then slow-cooled at a rate of 2 to 15°C/s for a time of between 5 and 40 s down to a temperature of between the Ar₁ point and 730°C; and

   the said sheet then is quenched at a rate of 20 to 150°C/s down to a temperature less than or equal to 300°C.
5. Process for manufacturing a hot-rolled steel sheet with high strength and high drawability, in which:

   a steel whose composition is in accordance with that of the sheet according to Claim 2 is smelted and cast in slab form;

   the said slab is then hot-rolled into sheet form, completing the rolling at a temperature of between the Ar₃ point and 950°C;

   the said sheet is then slow-cooled at a rate of 2 to 15°C/s for a time of between 5 and 40 s down to a temperature of between the Ar₁ point and 730°C; and

   the said sheet is then quenched at a rate of 20 to 150°C/s down to a temperature less than or equal to 300°C.
6. Process for manufacturing a hot-rolled steel sheet with high strength and high drawability, in which:

   a steel whose composition is in accordance with that of the sheet according to Claim 2 is smelted and cast in slab form;

   the said slab is then hot-rolled into sheet form, completing the rolling at a temperature of between the Ar₃ point and 950°C;

   less than 10 s after the end of hot rolling, the said sheet is then quenched at a rate of 20 to 150°C/s down to a temperature below the Ar₃ point;

   the said sheet is then slow-cooled at a rate of 2 to 15°C/s for a time of less than 40 s down to a temperature of between the Ar₁ point and 730°C; and

   the said sheet is then quenched at a rate of 20 to 150°C/s down to a temperature less than or equal to 300°C.

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