Patent Description:
To produce various equipments such as trailers, trucks, agricultural machines, automotive parts and so on, high strength steel sheets made of DP (dual phase) or TRIP (transformation induced plasticity) steels are used. Some of such steels which are produced on continuous annealing lines, containing for example about <NUM>% C, about <NUM>% Mn and about <NUM>% Si, have a tensile strength of about <NUM> MPa.

In order to reduce the weight of the equipments made of these steels, which is very desirable to improve their energy efficiency, it was proposed to use CMnSi steels containing <NUM>% to <NUM>% C, <NUM>% to <NUM>% Mn, up to <NUM>% Si or Si+AI, such steels being heat treated in order to have a martensitic structure with a significant content of retained austenite or a ferrito-martensitic structure. Such steels are used to produce grades having a tensile strength of more than <NUM> MPa. These sheets are produced on continuous annealing lines and are optionally hot dip coated. The mechanical properties of the sheets depend on the amount of residual austenite which has to be sufficiently high. This requires that the austenite is sufficiently stable. Moreover, in order to perform the treatment on existing lines with a good productivity, it is desirable that the characteristic transformation points of the steel such as Ac<NUM>, Ac<NUM>, Ms and Mf are not too restrictive.

For these reasons, it remains the need to have a steel and a process to manufacture easily high strength steel sheets on continuous heat treatment lines.

A method for manufacturing a high-strength steel sheet according to the invention is defined in claim <NUM>. Preferred embodiments are defined in the dependent method claims.

Preferably, the chemical composition of the steel is such that: <MAT> <MAT> <MAT> <MAT> <MAT> the annealing temperature AT is higher than <NUM>, the quenching temperature QT is less than <NUM> and the structure of the steel contains between <NUM>% and <NUM>% of retained austenite.

In a particular embodiment, the quenching temperature can also be less than <NUM>.

In one embodiment, the overaging temperature PT is between <NUM> and <NUM> and the sheet is maintained at the overaging temperature for a time Pt between <NUM> and <NUM>.

In one embodiment, the chemical composition of the steel is such that: <MAT> <MAT> <MAT> <MAT> <MAT> the annealing temperature AT is higher than <NUM>, the quenching temperature QT is between <NUM> and <NUM>, the overaging temperature PT is between <NUM> and <NUM> and the overaging time Pt is between <NUM> and <NUM>.

In a particular embodiment outside of the claimed invention, the chemical composition of the steel is such that: <MAT> <MAT> <MAT> <MAT> <MAT> and the annealing temperature AT is less than the Ac<NUM> transformation point of the steel, the quenching temperature QT is between <NUM> and <NUM>, the overaging temperature PT is between <NUM> and <NUM>, the overaging time Pt is between <NUM> and <NUM> and preferably between <NUM> and <NUM>, the structure of the steel containing at least <NUM>% of ferrite, at least <NUM>% of martensite and at least <NUM>% of retained austenite.

Preferably the chemical composition of the steel satisfies at least one of the following conditions: <MAT> <MAT> <MAT>.

In one embodiment, the overaging temperature PT is between <NUM> and <NUM> and the sheet is maintained at the overaging temperature for a time Pt between <NUM> and <NUM>. In this case, the holding at the overaging temperature can be made by passing the sheet in a hot dip coating bath. After passing in hot a dip coating bath, the sheet can be further maintained at a temperature between <NUM> and <NUM> to be galvannealed before being cooled down to the ambient temperature.

The annealing, the quenching and the overaging can be made on a continuous heat treatment line such as a continuous annealing line which optionally comprises a hot dip coating section.

The preparation of the sheet by rolling can comprise hot rolling of a slab and optionally cold rolling.

A high-strength steel sheet according to the invention is defined in claim <NUM>. The preferred embodiments are defined in the dependent product claims.

The chemical composition of the steel is preferably such that: <MAT> <MAT> <MAT> <MAT> <MAT>.

Then, the yield strength YS can be higher than <NUM> MPa, the tensile strength TS higher than <NUM> MPa, the uniform elongation UE can be of more than <NUM>% and the total elongation TE of more than <NUM>%.

In one embodiment, the chemical composition of the steel is such that: <MAT> <MAT> <MAT> <MAT> <MAT> and the yield strength YS is higher than <NUM> MPa and the tensile strength TS is higher than <NUM> MPa.

In a particular embodiment outside of the claimed invention, the chemical composition of the steel is such that: <MAT> <MAT> <MAT> <MAT> <MAT> and the structure of the steel comprises at least <NUM>% of ferrite, at least <NUM>% of martensite and at least <NUM>% of retained austenite.

In any case, at least one of the faces of the sheet may comprise a metallic coating or an alloyed metallic coating such as zinc coating or alloyed zinc coating.

The invention will now be described in details and illustrated by examples without introducing limitations.

The steel which is used to manufacture high-strength steel sheets according to the present invention has the following composition:.

The C and Mn contents are such that the carbon-manganese index CMnldex = C x (<NUM> + Mn / <NUM>) is less or equal <NUM> to ensure that the martensite should not be too brittle which is desirable to enable the mechanical cutting in good conditions. In this formula, C and Mn are the contents in weight %.

The remainder is Fe and impurities resulting from the smelting. Such impurities include N, S, P, and residual elements such as Cr, Ni, Mo, Cu, and B.

The N content remains less than <NUM>%, the S content less than <NUM>%, the P content less than <NUM> %, the Cr content less than <NUM>%, the Ni content less than <NUM>%, the Mo content less than <NUM>%, the Cu content less than <NUM>% and the B content less than <NUM>%.

With such steel, hot rolled sheets are produced. These hot rolled sheets have a thickness between <NUM> and <NUM>, for example.

Optionally, the hot rolled sheets are cold rolled in order to obtain cold rolled sheets having a thickness between <NUM> and <NUM>, for example. Those who are skilled in the art know how to produce such hot or cold rolled sheets.

Then the hot or cold rolled sheets are heat treated on a continuous heat treatment line such as continuous annealing line comprising at least a heating zone able to heat the sheet up to an annealing temperature, a soaking zone able to maintain the sheet at the annealing temperature or around this temperature, a cooling zone able to rapidly cool the sheet down to a quenching temperature QT, a reheating zone able to heat the sheet up to an overaging temperature PT and an overaging zone able to maintain the sheet at the overaging temperature or around this temperature for a time Pt. Optionally, the overaging zone can be a hot dip coating zone comprising at least a hot dip coating bath containing a liquid metal such as zinc and optionally an alloying zone.

Such continuous heat treatment line is known to those skilled in the art. The purpose of the heat treatment is to confer to the steel a structure suitable to obtain the desired characteristics of strength and ductility and, possibly, to hot dip the sheet.

It must be noted that contents of microstructural constituents are generally given as a surface fraction based on optical and scanning microscope pictures.

In any case, the annealing temperature AT is higher than the Ac<NUM> transformation point of the steel in order to form enough austenite able to be transformed by quenching and overaging.

If the structure of the sheet before annealing contains ferrite and pearlite and if a significant content of ferrite is desired after quenching and overaging, the annealing temperature must remain less than the Ac<NUM> transformation point of the steel for an embodiment outside of the claimed invention.

The structure before quenching is completely austenitic, the annealing temperature AT is higher than the Ac<NUM> transformation point of the steel, but it remains less than <NUM> in order not to coarsen too much the austenitic grains which is unfavorable for the ductility of the obtained structure.

In any case, it is necessary to maintain the sheet at that annealing temperature at least <NUM> but more than <NUM> is not necessary.

It is desired that during quenching and overaging, the austenite which is formed during the annealing step is transformed at least partially in martensite. The quenching temperature QT must be less than the Ms transformation point of the steel and with a cooling speed enough to obtain a structure just after quenching containing at least martensite. The minimal cooling speed which is the critical martensitic cooling speed depends at least on the chemical composition of the steel and those which are skilled in the art know how to determine it. As it is preferably desired to have a structure containing a significant content of retained austenite, the QT temperature must not be too low and must be chosen according to the desired content of retained austenite. For that reason, the quenching temperature is between <NUM>° which is less than the Ms transformation point, and <NUM> in order to have a sufficient amount of retained austenite. But, the quenching temperature is less than <NUM>° because, when it is higher than this temperature, the amount of retained austenite in too important and this retained austenite can be transformed in fresh martensite after partitioning and cooling to the room temperature, which is detrimental for the ductility. More specifically, it is possible to determine for each chemical composition of the steel an optimal quenching temperature QTop that theoretically achieves an optimum residual austenite content. This optimum quenching temperature can be calculated using a relationship between the chemical composition of the steel and Ms which was newly established by the inventors: <MAT>.

And the Koistinen Marburger relationship: <MAT>.

Those which are skilled in the art know how to make this calculation.

The optimal quenching temperature QTop is not necessarily the quenching temperature QT which is chosen to make actual heat treatments. Preferably, the quenching temperature QT is chosen equal or near to this optimal quenching temperature and less than <NUM> because, when the quenching temperature is too high, after partitioning, the austenite is at least partially transformed in fresh martensite and the obtained structure is very brittle. With the steel according to the present invention, the maximum residual austenite content which is possible to obtain after a full austenitization is between <NUM>% and <NUM>%. As during overaging or after it, some of the residual austenite can be transformed in bainite or in fresh martensite, the structure which is obtained after a full austenitization contains some ferrite or some bainite, the total content of such constituents is less than <NUM>% and preferably less than <NUM>% and the structure contains at least <NUM>% of martensite. With the steel according to the present invention, when the quenching temperature QT is less than <NUM>, the austenite content of the structure is too low, less than about <NUM> % and even can be full martensitic. In this case, the structure which is obtained after partitioning can be very brittle.

When the austenitization is not full i.e. when the annealing temperature is between the Ac<NUM> transformation point and the Ac<NUM> transformation point of the steel, which is outside of the claimed invention, the content of austenite and martensite depends on the content of ferrite after annealing i.e. depending on the annealing temperature. But, preferably, the ferrite content is between <NUM>% and <NUM>%, which is outside of the claimed invention, more preferably higher than <NUM>% and more preferably less than <NUM>%, the martensite content is at least <NUM>% and the retained austenite content is at least <NUM>% and preferably at least <NUM>%.

When the structure contains martensite and retained austenite, the purpose of the overaging is generally to transfer carbon from the martensite to the retained austenite in order to improve the ductility of the martensite and to increase the carbon content of the austenite in order to render possible a TRIP effect, without forming significant amount of bainite and/or of carbides. For this, the overaging temperature PT must be between <NUM> and <NUM> and the overaging time Pt must be at least <NUM> and preferably of more than <NUM> in order that the enrichment of the austenite in carbon is enough. But this time must not be too long and preferably must be not more than <NUM> in order to have no or about no decomposition of the austenite in a structure like bainite. In any case, the overaging temperature PT has to be chosen sufficiently high given the overaging time Pt which depends on the characteristics of the annealing line and on the thickness of the sheet, in order to have enough transfer of carbon from martensite to austenite i.e. enough partitioning.

In a particular embodiment, the overaging temperature PT is equal to the optimal temperature for hot dip coating i.e. between <NUM> and <NUM> and typically about <NUM>. Moreover, the overaging can be made at least partially by the passage of the sheet in the hot dip coating bath. In this case, the overaging temperature is between <NUM> and <NUM>. If the layer of coating is alloyed by heating and maintaining at a temperature between <NUM> and <NUM> for the galvannealing, this treatment will contribute to the overaging of the steel.

More precisely, with a steel having the following composition: <NUM>% ≤ C ≤ <NUM>%, <NUM>% ≤ Mn ≤ <NUM>%, <NUM>% ≤ Si ≤ <NUM>%, <NUM> ≤ Al ≤ <NUM>%, the reminder being Fe and impurities, it is possible to obtain high strength steel sheet having a yield strength YS higher than <NUM> MPa, a tensile strength TS higher than <NUM> MPa and a uniform elongation UE of more than <NUM>% and a total elongation TE of more than <NUM>% if the CMnlndex remains less than <NUM>%. These properties can be obtained if the structure is essentially martensitic with a significant content of retained austenite, preferably containing more than <NUM>% of martensite and more than <NUM>% of retained austenite, the sum of the ferrite and bainite contents remaining less than <NUM>%.

The sheet can be coated or not. When it is coated, it can be galvanized or galvannealed.

To obtain such steel, it is necessary to anneal the sheet at a temperature higher than the Ac<NUM> transformation point of the steel and to quench it down to a temperature less than the Ms transformation point followed by a reheating to the overaging temperature.

Regarding the Ac<NUM> transformation point, it can be noted that for this steel, it is less than about <NUM> when the Al content is less than <NUM>% while it is about <NUM> for the steels generally used to produce sheets of such category. This difference of about <NUM> is very important because it is easier to heat a sheet up to a temperature that must only be higher than <NUM> than to a temperature that must be higher than <NUM>. Heating needs less energy and may be faster. Thus it is possible to have a better productivity, at the same time, the Ac<NUM> and Ac<NUM> points must not be too close because if they are too close, the steel robustness will be impaired since a small annealing temperature change will induce a large modification of phase fractions and consequently unstable mechanical properties.

When the Al content is between <NUM>% and <NUM>%, the Ac<NUM> transformation point can be higher than <NUM>, but the weldability of the steel is improved.

With this steel, it is also possible to obtain sheets having a structure outside of the claimed invention and containing at least <NUM>% of martensite, at least <NUM>% and preferably at least <NUM>% of retained austenite and at least <NUM>% and preferably at least <NUM>% of ferrite. For this, the annealing temperature must be between the Ac<NUM> and Ac<NUM> transformation points, which is outside of the claimed invention, and the quenching temperature must be less than the Ms transformation point. The yield strength can be higher than <NUM> MPa and the total elongation can be of about <NUM>% which is very good for the formability of the sheet. But, the yield strength is only about <NUM> MPa.

With a steel containing <NUM>% to <NUM>% C, <NUM>% to <NUM>% Mn, <NUM>% to <NUM>% Si, less <NUM> ≤ Al ≤ <NUM>% the reminder being Fe and impurities, it is possible to obtain a yield strength higher than <NUM> MPa and a tensile strength higher than <NUM> MPa with a structure consisting of martensite and retained austenite. Due to the high Mn content, the Ac<NUM> and Ms transformation points of this steel are significantly lowered: Ac<NUM> less than <NUM> and Ms less than <NUM>. Moreover Ac3 is lowered if the Al content is less than <NUM>%. In this case, Ac3 could be less than <NUM>. This is useful since heat treatments are easier to realize, i.e. faster annealing and less energy consuming annealing treatments are possible.

Sheets made of steels having the compositions which are reported in table I were produced by hot rolling, the hot rolled sheets having a thickness of <NUM>. The hot rolled sheets were batch annealed at <NUM> for <NUM> hours, then pickled and cold rolled to obtain sheets having a thickness of <NUM>. Then, these sheets were heat treated.

Before heat treatment, an optimal quenching temperature QTop was determined for each composition. This optimal quenching temperature is the temperature at which the quenching has theoretically to be stopped in order to obtain the maximum austenite content in the structure after heat treatment. But, it is not necessarily the QT temperature that is preferable to choose for the actual heat treatment.

Each heat treatment included an annealing at an annealing temperature AT, a quenching down to a quenching temperature QT, an overaging at an overaging temperature PT during an overaging time Pt. The structures and the mechanical properties YS, TS, UE and TE were measured.

The carbon-manganese index CMnlndex, the values of the Ae<NUM>, Ae<NUM> and Ms transformation points of the steels and the optimal quenching temperature QTop are reported in Table I. The Ae<NUM> and Ae<NUM> transformation points are the values at equilibrium and do not depend on the heating speed nor on the holding time at the temperature of transformation contrary to Ac1 and Ac3 which are the heating transformation points. The values of the heating transformation points are always higher than the equilibrium values and depend on the actual conditions of treatment. Those which are skilled in the art know how to determine the values of the transformations points that have to be considered in each specific case. The conditions, the structures and the mechanical properties resulting from the treatments of steels according to the invention or given as comparison are reported in table II and table III. Counter examples corresponding to steels out of the scope of the invention are reported in table IV.

In this table, cast H166 and H167are examples of the invention. The casts H240, H169 and H170 are out of the scope of the invention and are given as comparison.

The examples <NUM> to <NUM> are related to a steel containing <NUM>% C, <NUM>% Mn, <NUM>% Si and <NUM>% Al according to the invention. Example <NUM> corresponds to a treatment of quenching and tempering according to the prior art, the quenching being down to the ambient temperature and the structure being about completely martensitic. For the example <NUM>, the annealing is intercritical, which is outside of the claimed invention.

All the examples <NUM> to <NUM> show that it is possible to obtain a yield strength higher than <NUM> MPa and a tensile strength higher than <NUM> MPa. The examples <NUM>, <NUM>, <NUM>, <NUM>,<NUM>,<NUM> and <NUM> show that with a quenching temperature equal or less than <NUM> and higher or equal to <NUM> and a partitioning (or overaging) at <NUM> for <NUM>, it is possible to obtain a yield strength of more than <NUM> MPa and a tensile strength of more than 1350MPa. But, when the quenching temperature is higher than <NUM> (examples <NUM>, <NUM>, <NUM> and <NUM>), even if the tensile strength is at least <NUM> MPa, the yield strength remains less than <NUM> MPa. The examples <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> show that it is possible to obtain an uniform elongation UE of more than <NUM>% and a total elongation TE of more than <NUM>%. The examples <NUM>, <NUM> and <NUM> for which the total elongation is equal to the uniform elongation are very brittle and show that must remain less than <NUM>. The example <NUM> show that the yield strength and the tensile strength which are obtained with a total quenching are higher than with a partial quenching, but the samples are very brittle.

The examples <NUM> to <NUM> of steel having a high content of aluminum and therefore being more easily weldable can have very good properties, for example, a yield strength of at least <NUM> MPa, a tensile strength of at least <NUM> MPa, a uniform elongation higher than <NUM>% and a total elongation higher than <NUM>% (examples <NUM> and <NUM>). But a comparison with the examples <NUM> to <NUM> show that it is necessary that the annealing temperature remains less than <NUM> in order to not deteriorate the yield strength ot the uniform elongation.

The counter examples <NUM> to <NUM> show that with a steel containing <NUM>% of manganese and having a carbon equivalent Ceq > <NUM> it is possible to obtain high yield strength and high tensile strength (YS > <NUM> MPa and Ts > <NUM> MPa), but all the examples are very brittle. The total elongations are always equal to the uniform elongations and are very low.

The counter examples <NUM> to <NUM> show that with the steel H167 who has a carbon equivalent Ceq of <NUM> is very brittle.

Claim 1:
A method for manufacturing a high-strength steel sheet having a tensile strength of more than <NUM> MPa, a yield strength of more than <NUM> MPa, a uniform elongation UE of at least <NUM>% and a total elongation of at least <NUM>%, made of a steel containing in percent by weight: <MAT> <MAT> <MAT> <MAT>
the remainder being Fe and impurities resulting from the smelting, the impurities including N, S, P and residual elements including Cr, Ni, Mo, Cu, and B, the N content being less than <NUM>%, the S content less than <NUM>%, the P content less than <NUM> %, the Cr content less than <NUM>%, the Ni content less than <NUM>%, the Mo content less than <NUM>%, the Cu content less than <NUM>% and the B content less than <NUM>%,
the composition being such that: <MAT>
C and Mn being the contents in C and Mn in weight %,
the method comprising the steps of:
- annealing a rolled sheet made of said steel by soaking it at an annealing temperature TA higher than the Ac<NUM> transformation point of the steel and higher than the Ac3 transformation point of the steel but less than <NUM>, and maintaining the sheet at the annealing temperature for <NUM> to <NUM>,
- cooling the annealed sheet to a quenching temperature QT between <NUM>° and <NUM>, at a cooling speed sufficient to obtain a structure just after cooling containing martensite and retained austenite, the quenching temperature QT being such that the structure of the steel after heat treatment contains at least <NUM>% of retained austenite and at least <NUM>% of martensite, the sum of the ferrite and bainite contents being less than <NUM>%,
- maintaining the steel sheet at an overaging temperature PT between <NUM> and <NUM> for an overaging time Pt of more than <NUM> and,
- cooling the sheet down to the ambient temperature,
wherein the contents of microstructural elements are given as a surface fraction based on optical and scanning microscope pictures.