EP0747496A1 - Hot rolled steel sheet with high strength and good deep-drawing properties, containing titanium and process for its manufacturing - Google Patents

Abstract

Steel sheet comprises 0.5-1.5wt.% Mn, 0.01-0.1% Al and up to 0.12% C, 0.3% Si, 0.1% P, 0.05% S, 1% Cr, 0.05% Nb, also 0.03-0.15% 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 Ti or of Ti and Nb, 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 relates to the field of hot-rolled steel sheets which must have high strength and stampability properties, intended in particular for the automotive industry to form parts of vehicle structures.

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

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

Finally, TRIP (TRansformation Induced Plasticity) steels have a microstructure composed of ferrite, bainite and residual austenite. They allow very high resistances to be reached, 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 hot-rolled steel sheets with products which offer a very good compromise between high resistance levels, satisfactory formability and good weldability, as well as an impeccable surface appearance.

• 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 %;
• 0,03 ≤ Ti_eff ≤ 0,15 %, Ti_eff étant la teneur en titane non sous forme de nitrures, de sulfures ou d'oxydes;
• 0 ≤ Nb ≤ 0,05 %;

et en ce que sa structure comprend au moins 75 % de ferrite durcie par précipitation de carbures ou de carbonitrures de Ti ou de Ti et de Nb, 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, characterized in that its composition, expressed in weight percentages, 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%;
• 0.03 ≤ Ti _eff ≤ 0.15%, Ti _eff being the content of titanium not in the form of nitrides, sulphides or oxides;
• 0 ≤ Nb ≤ 0.05%;

and in that its structure comprises at least 75% of ferrite hardened by precipitation of carbides or carbonitrides of Ti or Ti and Nb, the rest of the structure comprising at least 10% martensite and optionally bainite and residual austenite.

The invention also relates to methods of manufacturing such sheets.

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

The invention will be better understood on reading the description which follows, illustrated by FIGS. 1 and 2, which show micrographs of sheets according to the invention.

In order to obtain hot-rolled sheets according to the invention, it is first necessary to prepare, then pour in the form of a slab, a steel comprising (all the percentages are percentages by weight) a carbon content of less than or equal to 0, 12%, a manganese content between 0.5 and 1.5%, a silicon content less than or equal to 0.3%, a phosphorus content less than or equal to 0.1%, a lower sulfur content or equal to 0.05%, an aluminum content of between 0.01 and 0.1%, a chromium content of less than 1%, an effective titanium content (we will explain what this term means below) between 0 , 03 and 0.15% and a niobium content of 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 makes it possible to give it a microstructure composed at least of 75% ferrite and at least 10% martensite. Ferrite is hardened by precipitation of titanium carbides or carbonitrides, and also niobium carbides or carbonitrides if this element is present significantly. The microstructure may optionally also include bainite and residual austenite.

The limited carbon content makes it possible to maintain good weldability of the steel, 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 band structure and leads to the degradation of fatigue performance and / or formability. 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 be 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 to 1%) is advisable, since it promotes the formation of martensite and ferritic transformation.

Titanium is a micro-alloying element which forms precipitates of carbide and carbonitride hardening ferrite. Its addition is intended to obtain, thanks to this hardening, a high level of resistance. However, this effect is only obtained if 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 between 0.03 and 0.15%. It is this content which is called "titanium content efficient "and which is shortened to" Ti _eff % ". When the steel is deoxidized to aluminum, taking into account the thermodynamic equilibria which are established in the metal being solidified, it can be estimated that, if Ti _total % means the total titanium content of the steel, $${\text{Ti}}_{\text{eff}}{\text{\% = Ti}}_{\text{total}}\text{\% - 3.4 x N\% - 1.5 x S\%.}$$

This addition of titanium can advantageously be supplemented by an addition of niobium to achieve even higher resistance levels. However, above a content of 0.05%, niobium makes the sheet more difficult to laminate. On the other hand, adding titanium and niobium beyond the prescribed amounts is useless, as there would then be a saturation of the hardening effect.

To manufacture the sheets according to the invention, different operating methods can be envisaged, depending on the level of performance sought and the composition of the 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%;
  • 0,03 ≤ Ti_eff ≤ 0,15 %, Ti_eff étant la teneur en titane non sous forme de nitrures, de sulfures ou d'oxydes;
  • 0 ≤ Nb ≤ 0,05 %;
• 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; il ne doit pas 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, the succession of operations is as follows:

• 1) a steel is produced and poured in the form of a slab, the composition of which by weight percentages 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%;
  • 0.03 ≤ Ti _eff ≤ 0.15%, Ti _eff being the content of titanium not in the form of nitrides, sulphides or oxides;
  • 0 ≤ Nb ≤ 0.05%;
• 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; it must 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 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;
• 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, operations 1) and 2) are the same as above. 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 less than 40 s, between T _inter and TDT, which is between the point Ar ₁ of the grade and 730 ° C; ferritic transformation takes place during this stage;
• 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 prior 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 titane et niobium plus fine et plus 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;
• finer and more hardening precipitation of titanium and niobium carbides and carbonitrides.

In the case where the steel has a relatively high niobium content, that is to say between 0.020 and 0.050%, obtaining optimum performance for the sheet requires an additional condition. Indeed, the presence of niobium nitrides and carbonitrides slows down the ferritic transformation. It is therefore desirable that the duration of the slow cooling step during which the ferritic transformation takes place is sufficient to ensure that this transformation takes place correctly. For procedure No. 1 which was previously described, we therefore recommend that step 1 last at least 8 s. For procedure No. 2, a minimum duration of step 2 of 5 s is recommended.

It is thus possible to produce a sheet whose guaranteed minimum resistance can be adjusted between 700 and 900 MPa, with a ratio R _e / R _{m of} less than 0.8, a work hardening coefficient of at least 0.12 for the grade the highest, 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 effective titanium contents, calculated from the total titanium content as shown in 'has exposed): Table 1: Steel grades tested Shade VS % Mn% P% Yes % Cr% NOT % S% Ti _eff % Nb% A (reference) 0.072 0.982 0.040 0.190 0.750 0.0059 0.0021 - - B 0.075 0.965 0.040 0.190 0.760 0.0046 0.0025 0.030 - VS 0.072 0.955 0.040 0.180 0.760 0.0046 0.0018 0.060 - D 0.077 0.862 0.043 0.180 0.770 0.0057 0.0025 0.110 - E 0.080 1,200 0.040 0.220 0.750 0.0052 0.0022 0.0120 0.040

These experiments gave the results recorded in table 2, where R _p0,2 designates the conventional elastic limit at 0.2% of residual elongation and n the hardening coefficient, and where the column "cooling mode" refers to the two main operating modes described above: Table 2: Experimental results Shade Cooling mode TDT (° C) R _p0.2 (MPa) R _m (MPa) R _p0.2 / R _m not A (reference) # 2 720 319 590 0.54 0.20 A (reference) # 2 650 308 570 0.54 0.20 B # 1 730 425 685 0.62 0.16 B # 1 660 501 748 0.67 0.15 B # 2 730 511 774 0.66 0.16 B # 2 660 492 745 0.66 0.14 VS # 1 720 475 730 0.65 0.15 VS # 1 650 535 764 0.70 0.15 VS # 2 720 549 820 0.67 0.13 VS # 2 650 528 800 0.66 0.13 D # 1 710 615 848 0.72 0.12 D # 1 620 648 865 0.75 0.12 E # 2 710 595 860 0.69 0.12

From these results, it can be seen that the addition of 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 operating mode No. 2 comprising three-stage cooling is used, while maintaining a _suitable R _p0.2 / R _m ratio. The addition of niobium combined with the addition of titanium (grade E) gives the steel an even higher resistance, without degrading the ratio R _p0.2 / R _m .

The micrograph in Figure 1 shows the structure of a steel corresponding to grade B with 0.030% titanium. The cooling of the sheet after hot rolling was carried out according to procedure No. 2. The clear areas are of equiaxed ferrite and represent 88% of the structure. The dark areas are martensite, and represent almost the entire rest of the structure.

Similarly, Figure 2 shows the structure of a steel corresponding to grade C with 0.060% titanium. The cooling of the sheet after hot rolling was carried out according to procedure No. 2 The equiaxed ferrite there represents 86% 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 linings, suspension arms, as well as all stamped parts which must have a high resistance to mechanical stresses. .

Claims (6)

Hot-rolled steel sheet with high resistance and high drawability, characterized in that its composition, expressed in weight percentages, 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%; - 0.03 ≤ Ti _eff ≤ 0.15%, Ti _eff being the titanium content not in the form of nitrides, sulfides or oxides; - 0 ≤ Nb ≤ 0.05%; and in that its structure comprises at least 75% of ferrite hardened by precipitation of carbides or carbonitrides of Ti or Ti and Nb, the rest of the structure comprising at least 10% of martensite and optionally bainite and l residual austenite. Steel sheet according to claim 1, characterized in that its Nb content is between 0.02 and 0.05%. Method for manufacturing a hot-rolled steel sheet of high strength and high drawability, characterized in that: - A steel is produced and poured in the form of a slab, the composition of which conforms to that of the sheet according to claim 1; - Then hot rolled said slab in the form of sheet metal by completing the rolling at a temperature between point Ar ₃ and 950 ° C; - Then slow cooling is applied to said sheet at a speed of 2 to 15 ° C / s for a period of less than 40 s, to a temperature between point Ar ₁ and 730 ° C; - Then rapid cooling is applied to said sheet at a speed of 20 to 150 ° C / s to a temperature less than or equal to 300 ° C. Method for manufacturing a hot-rolled steel sheet of high strength and high drawability, characterized in that: - A steel is produced and poured in the form of a slab, the composition of which conforms to that of the sheet according to claim 1; - Then hot rolled said slab in the form of sheet metal by completing the rolling at a temperature between point Ar ₃ and 950 ° C; - Then applied to said sheet, less than 10 s after the end of hot rolling, rapid cooling at a speed of 20 to 150 ° C / s to a temperature below the point Ar ₃ ; - Then slow cooling is applied to said sheet at a speed of 2 to 15 ° C / s for a period of less than 40 s, to a temperature between point Ar ₁ and 730 ° C; - Then rapid cooling is applied to said sheet at a speed of 20 to 150 ° C / s to a temperature less than or equal to 300 ° C. Method for manufacturing a hot-rolled steel sheet of high strength and high drawability, characterized in that: - A steel is produced and poured in the form of a slab, the composition of which conforms to that of the sheet according to claim 2; - Then hot rolled said slab in the form of sheet metal by completing the rolling at a temperature between point Ar ₃ and 950 ° C; - Then slow cooling is applied to said sheet at a speed of 2 to 15 ° C / s for a period of between 8 and 40 s, to a temperature between point Ar ₁ and 730 ° C; - Then rapid cooling is applied to said sheet at a speed of 20 to 150 ° C / s to a temperature less than or equal to 300 ° C. Method for manufacturing a hot-rolled steel sheet of high strength and high drawability, characterized in that: - A steel is produced and poured in the form of a slab, the composition of which conforms to that of the sheet according to claim 2; - Then hot rolled said slab in the form of sheet metal by completing the rolling at a temperature between point Ar ₃ and 950 ° C; - Then applied to said sheet, less than 10 s after the end of hot rolling, rapid cooling at a speed of 20 to 150 ° C / s to a temperature below the point Ar ₃ ; - Then slow cooling is applied to said sheet at a speed of 2 to 15 ° C / s for a period of between 5 and 40 s, to a temperature between point Ar ₁ and 730 ° C; - Then rapid cooling is applied to said sheet at a speed of 20 to 150 ° C / s to a temperature less than or equal to 300 ° C.

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