RU2803955C1 - Cold-rolled and annealed steel sheet and production method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention is related to cold-rolled and annealed steel sheets used in the automotive industry. The sheet has a composition including, wt.%: 0.060≤C≤0.085, 1.8≤Mn≤2.0, 0.4≤Cr≤0.6, 0.1≤Si≤0.5, 0.010≤Nb≤0.025, 3.42N≤Ti≤0.035, 0≤Mo≤0.030, 0.020≤Al≤0.060, 0.0012≤B≤0.0030, S≤0.005, P≤0.050, 0.002≤N≤0.007, if necessary 0.0005≤Ca≤0.005, the rest is iron and inevitable impurities. The sheet has a microstructure with surface consisting of the following fractions: 34-80% bainite, 10-16% martensite and 10-50% ferrite, while the proportion of the surface of non-recrystallized ferrite relative to the entire structure is less than 30%, and martensite consists of self-tempered martensite and fresh martensite, and the proportion of the surface of self-tempered martensite relative to the entire structure is 4-10%.

EFFECT: sheet has the required level of mechanical properties that are uniform throughout the sheet.

18 cl, 2 dwg, 3 tbl

Description

The invention relates to cold-rolled and annealed steel sheet having high strength, suitable ductility and formability, and a suitable hole expansion ratio. The invention also relates to a method for producing such cold-rolled and annealed steel sheet.

“Dual-phase” steels have received great development, as they combine high strength with high drawability due to their microstructure, in which the hard martensitic or bainite phase is dispersed in a soft ferrite matrix.

In particular, before forming these steels have a relatively low yield strength compared to their tensile strength. As a consequence, these steels exhibit very suitable yield ratio (yield strength/tensile strength ratio) during forming operations.

Their strain hardening properties are very high, resulting in significantly higher yield strength on the parts after forming and good strain distribution in the event of collision. In this way, it is possible to produce the same complex parts as with conventional steels, but with higher mechanical properties, so that the same functional characteristics as conventional steels can be achieved at a lower thickness. Thus, these steels provide an effective response to vehicle weight reduction and safety requirements.

In particular, due to their high energy absorption capacity and fatigue strength, dual-phase steels are particularly well suited for the production of automotive structural and safety parts such as longitudinal members, cross members and reinforcements.

The development of automotive parts having increasingly complex shapes has led to increased demand for steels having very high ductility and formability, especially very high drawability, together with high tensile strength of at least 780 MPa.

To ensure high ductility and high stretchability, a yield strength of at least 350 MPa, but not more than 450 before any pass-pass operation (and not less than 450 MPa and not more than 550 MPa after pass-pass, if performed), is desirable, a total elongation of at least 15 % and a hole expansion ratio HER of at least 35%, in addition to a tensile strength of at least 780, up to 900 MPa.

Tensile strength TS and total elongation TE are measured in accordance with the ISO 6892-1 standard published in October 2009. It should be emphasized that due to differences in measurement methods, in particular due to differences in the geometry of the specimen used, the values of the total TE elongations according to ISO 6892-1 vary greatly and are in particular lower than the total elongation values according to JIS Z 2241.

In addition, temper training increases the yield strength, so that the yield strength of a cold-rolled sheet that has not been temper-trained is not comparable to that of a steel sheet that has been temper-trained.

In this regard, it should be noted that the steel sheet that has been tempered is clearly different and easily distinguishable from the steel sheet that has not been tempered. Indeed, tempering has an impact on the properties of the sheet surface, in particular, it has a clear and generally accepted effect on the hardening and permanent deformations on the sheet surface. In addition, tempering leaves identifiable unique marks on the surface of the sheet in the form of roughness craters that have a distinct shape. These marks can be easily visualized using an electron microscope.

The hole expansion ratio HER is measured according to the ISO 16630:2009 standard. Due to differences in measurement methods, the hole expansion ratio HER values according to ISO 16630:2009 are very different and are not comparable to the hole expansion ratio λ values according to JFS T 1001 (Japan Iron and Steel Federation standard).

The hole expansion coefficient evaluates the extensibility of the edge of a steel hole.

In general, high hole expansion ratio values are associated with high yield ratio values (equal to the yield strength divided by the tensile strength), and therefore, for a given tensile strength, high yield strength values. Indeed, high values of the hole expansion coefficient are due, in particular, to the small difference in strength between the components of the steel microstructure. However, the small difference in strength between the components of the steel microstructure leads to a high yield coefficient.

As a consequence, steel sheets having a yield strength of at least 780 MPa and a high hole expansion ratio will typically have a yield strength greater than 450 MPa and even greater than 500 MPa before any temper rolling, resulting in a yield strength greater than 550 MPa or even above 600 MPa after training. In contrast, steel sheets having a tensile strength of at least 780 MPa and a yield strength of no more than 450 MPa before tempering will have a low hole expansion ratio.

Thus, it remains desirable to produce cold-rolled steel sheet with a tensile strength of 780 - 900 MPa, a yield strength of 350 - 450 MPa before tempering (and 450 - 550 MPa after tempering, if carried out), a total elongation of at least 15% and a coefficient hole distribution of at least 35%.

Thus, one of the objects of the invention is to provide a steel sheet having a tensile strength in the range of 780 - 900 MPa, a yield strength in the range of 350 - 450 MPa before any pass-pass operation (and a yield strength in the range of 450 - 550 MPa after pass-pass, if carried out), a total elongation of at least 15% and a hole expansion ratio of at least 35%, as well as the method of their manufacture.

In addition, as explained in more detail below, the inventors have discovered that adjusting the composition of the steel to obtain these properties is not sufficient because known production methods applied to steel having such a composition result in significant non-uniformity in the mechanical properties in the longitudinal and transverse directions of the sheet.

Therefore, preferably, the invention is further directed to providing a steel sheet having the above properties so that these properties are uniform throughout the sheet, and to a method for producing such a steel sheet.

In addition, on a given production line, the hole expansion ratio usually decreases as the sheet thickness increases. Therefore, the invention is also directed to a method for producing cold-rolled steel sheets having the above mechanical properties, in a wide range of sheet thicknesses of 0.7 - 2.3 mm, for example at least 1.5 mm or at least 2.0 mm.

In view of these objects, the invention relates to cold rolled and annealed steel sheet having a composition including and preferably consisting of, in percentage by weight:

0.060% ≤ C ≤ 0.085%

1.8% ≤ Mn ≤ 2.0%

0.4% ≤ Cr ≤ 0.6%

0.1% ≤ Si ≤ 0.5%

0.010% ≤ Nb ≤ 0.025%

3.42N ≤ Ti ≤ 0.035%

0 ≤ Mo ≤ 0.030%

0.020% ≤ Al ≤ 0.060%

0.0012% ≤ B ≤ 0.0030%

S ≤ 0.005%

P ≤ 0.050%

0.002% ≤ N ≤ 0.007%

and optionally 0.0005% ≤ Ca ≤ 0.005%,

the rest is iron and inevitable impurities resulting from smelting, and the cold-rolled and annealed steel sheet has a microstructure consisting of surface fractions of:

- 34 - 80% bainite,

- 10 - 16% martensite, and

- 10 - 50% ferrite, while the proportion of the surface of non-recrystallized ferrite relative to the entire structure is less than 30%;

martensite consists of self-tempered martensite and fresh martensite, and the proportion of the surface of self-tempered martensite relative to the entire structure is 4 - 10%.

Preferably the bainite is a low carbide bainite containing less than 100 carbides per unit surface area of 100 µm ² .

In one embodiment, the cold rolled and annealed steel sheet is not tempered, wherein the cold rolled and annealed steel sheet has a tensile strength TS of 780 - 900 MPa, a yield strength YS of 350 - 450 MPa, a total elongation TE of at least 15%, and a hole expansion ratio HER, measured according to ISO 16630:2009, of at least 35%.

In another embodiment, the cold-rolled and annealed steel sheet is a tempered sheet having a tensile strength TS ranging from 780 to 900 MPa, a yield strength YS ranging from 450 to 550 MPa, a total elongation TE of at least 15%, and a hole expansion ratio HER , measured in accordance with ISO 16630:2009, at least 35%.

Typically, the cold-rolled and annealed steel sheet has a thickness of 0.7 - 2.3 mm, for example at least 2.0 mm.

Preferably, the cold-rolled and annealed steel sheet has a length in the rolling direction of at least 500 m, and the difference in tensile strength between the regions with the highest tensile strength and the regions with the lowest tensile strength of the cold-rolled and annealed steel sheet is not more than 7% of the limit tensile strength in areas with the highest tensile strength.

In one embodiment, the cold-rolled and annealed steel sheet has a zinc or zinc alloy coating obtained through continuous dip coating.

In another embodiment, the cold rolled and annealed steel sheet includes a vacuum deposition coating of zinc or zinc alloy.

The invention also relates to a method for producing cold-rolled and annealed steel sheet, comprising the following sequential steps:

- preparation of a semi-finished steel product having a composition including and preferably consisting of, in percentage by weight:

0.060% ≤ C ≤ 0.085%

1.8% ≤ Mn ≤ 2.0%

0.4% ≤ Cr ≤ 0.6%

0.1% ≤ Si ≤ 0.5%

0.010% ≤ Nb ≤ 0.025%

3.42N ≤ Ti ≤ 0.035%

0 ≤ Mo ≤ 0.030%

0.020% ≤ Al ≤ 0.060%

0.0012% ≤ B ≤ 0.0030%

S ≤ 0.005%

P ≤ 0.050%

0.002% ≤ N ≤ 0.007%

and optionally 0.0005% ≤ Ca ≤ 0.005%,

the rest is iron and inevitable impurities resulting from smelting,

- heating the said semi-finished product to a temperature T _H1 greater than or equal to 1200°C, then hot rolling the heated semi-finished product with a final rolling temperature T _FRT located between Ar3 and T _NR , where Ar3 represents the temperature at which the austenite transformation begins when cooling the steel and T _NR is the temperature of the absence of recrystallization steel, to produce hot-rolled steel sheets,

- cooling the hot-rolled steel sheet with a first cooling rate V _C1 of at least 10°C/s to the winding temperature T _coil above the temperature of the end of the martensitic transformation of steel M _f and below 500°C, and winding the hot-rolled steel sheet at the winding temperature T _coil to obtain the structure , consisting of bainite and optionally martensite and/or pearlite, with a pearlite surface fraction of less than 15%,

- cold rolling of hot-rolled steel sheet with a cold rolling reduction ratio of at least 40% to obtain cold-rolled steel sheet,

- reheating of a cold-rolled steel sheet to an annealing temperature T _H2 in the range from Ac3-20°C to Ac3+15°C with an average heating rate V _H to an annealing temperature T _H2 in the range 1 - 50°C/s and an average heating rate V _H' between 600°C and Ac1 within 1 - 10°C/s, and holding the cold-rolled steel sheet at annealing temperature T _H2 for an annealing time t _H2 of at least 30 s to obtain a structure including at least 50% austenite,

- cooling of the cold-rolled steel sheet to a temperature T _C in the range of 440 - 480 ° C at a second cooling rate V _C2 in the range of 10 - 50 ° C / s,

- exposure of cold-rolled steel sheet in the temperature range 440 - 480°C for a holding time t _c 20 - 500 s,

- cooling the cold-rolled steel sheet to ambient temperature with a third cooling rate V _C3 of at least 1°C/s.

Preferably, the annealing time _tH2 is no more than 500 s.

In one embodiment, the annealing temperature T _H2 between Ac3 and Ac3+15°C and the second cooling rate V _C2 are 10 - 20°C/s.

Typically, cold-rolled and annealed steel sheets have a microstructure consisting of surface fractions of:

- 34 - 80% bainite,

- 10 - 16% martensite, and

- 10 - 50% ferrite, while the proportion of the surface of non-recrystallized ferrite relative to the entire structure is less than 30%;

martensite consists of self-tempered martensite and fresh martensite, and the proportion of the surface of self-tempered martensite relative to the entire structure is 4 - 10%.

In the embodiment, during said exposure in the temperature range of 440 - 480°C, a cold-rolled steel sheet is coated by immersion in a bath at a temperature lower than or equal to 480°C.

Preferably, the cold-rolled and annealed steel sheet is coated with zinc or zinc alloy.

In another embodiment, after cooling to ambient temperature, the zinc or zinc alloy coating is applied by vacuum deposition.

Preferably, the cold rolling reduction ratio is 40 - 80%.

In the implementation, after cooling to ambient temperature, the steel sheet is subjected to temper rolling with a rolling reduction degree of temper rolling of 0.1 - 0.4%.

The invention will now be described in detail, but without limitation, with reference to the accompanying drawings, of which:

fig. 1 is a micrograph showing the structure of a steel sheet not in accordance with the invention;

fig. 2 is a micrograph showing the structure of a steel sheet in accordance with the invention.

Throughout the description, Ac1 denotes the temperature at which the allotropic transformation begins when heated.

Ac1 can be measured using dilatometry or estimated using the following equation published in "Darstellung der Umwandlungen für technische Anwendungen und Möglichkeiten ihrer Beeinflussung", H.P. Hougardy, Werkstoffkunde Stahl Band 1,198-231, Verlag Stahleisen, Düsseldorf, 1984:

Ac1 = 739 - 22*C – 7*Mn +2*Si +14*Cr+13*Mo- 13*Ni.

In this equation, Ac1 is expressed in degrees Celsius, and C, Mn, Si, Cr, Mo and Ni denote the content of C, Mn, Si, Cr, Mo and Ni in the composition, expressed as percent by weight.

In addition, Ar3 denotes the temperature at which austenite transformation begins when cooled, T _NR denotes the temperature at which steel does not recrystallize, and Ac3 denotes the temperature at which austenite transformation ends when heated.

Ar3 and Ac3 temperatures can be measured using dilatometry or estimated using the renowned Thermo-Calc® software. The non-recrystallization temperature T _NR can be measured using a torsion test.

In addition, _Mf denotes the final temperature of the martensitic transformation, i.e. the temperature at which the transformation of austenite to martensite is completed upon cooling. M _f can be measured by dilatometry.

Further, the content of the element of the chemical composition of steel is expressed in mass percent (or in parts per million, i.e. ppm).

In the chemical composition of steel, carbon plays a role in the formation of microstructure and mechanical properties.

The carbon content is 0.060 - 0.085% to provide a tensile strength of at least 780 MPa, a yield strength of 350 - 450 MPa before temper rolling (and 450 MPa and 550 MPa after temper rolling) and a hole expansion ratio of at least 35%. When the C content is below 0.060%, the tensile strength does not reach 780 MPa. If the C content is higher than 0.085%, too high a percentage of perlite is formed during winding, which leads to a stitch structure that negatively affects the hole expansion ratio. In addition, bainite contains too many carbides such that the yield strength may exceed 450 MPa (before temper rolling) and the total elongation may not reach 15%. Preferably, the C content is less than or equal to 0.075%.

At least 1.8% manganese and at least 0.4% chromium are added to enhance the hardenability of the steel to produce a microstructure containing at least 10% martensite and having a tensile strength of at least 780 MPa.

In particular, the Mn content is at least 1.8% to obtain sufficient hardenability. However, if the Mn content is higher than 2.0%, the stabilization of austenite is excessive and the Ms temperature is too high, so that too high a proportion of martensite will be formed when cooling from the annealing temperature. As a result, the yield strength will exceed 450 MPa (before tempering). In addition, Mn content above 2.0% leads to a stitch structure, which negatively affects the hole expansion ratio. As a result, the hole distribution coefficient does not reach 35%.

Unlike manganese, chromium does not affect the proportion of austenite during annealing. Thus, chromium is added in addition to Mn to further increase the hardenability of the steel, a Cr content of at least 0.4% together with a Mn content of at least 1.8% provides sufficient hardenability to obtain a tensile strength of at least 780 MPa. Indeed, below 0.4% the proportion of self-tempered martensite may be insufficient, while too high a proportion of ferrite may be obtained. Above 0.6% Cr content, the steel's ability to be coated is reduced and the cost of the additive becomes prohibitive. Therefore, the Cr content is no more than 0.6%.

At a content of at least 0.1%, silicon provides ferrite strengthening, which reduces the difference in hardness between the components of the microstructure and increases the hole expansion ratio. Silicon promotes the formation of bainite with a low carbide content, i.e. containing less than 100 carbides per unit surface of 100 microns ² . However, excess Si reduces platability by promoting the formation of oxides that adhere to the surface of the sheet and causes the ferrite to stabilize too much. Therefore, the Si content is no more than 0.5%.

Titanium and niobium are micro-alloying elements used together in accordance with the invention to provide precipitation strengthening and a tensile strength of at least 780 MPa while limiting the martensite fraction to no more than 16%.

At contents between 3.42*N and 0.035% (N denotes the N content of the steel, expressed as a percentage by weight), titanium combines primarily with nitrogen and carbon, precipitating as fine nitrides and/or carbonitrides, allowing control of the austenite grain size . Titanium also has a positive effect on the weldability of steel. If the titanium content is above 0.035%, there is a risk of the formation of coarse titanium nitrides that precipitate in a liquid state, resulting in reduced ductility and early failure during the hole expansion test, thereby reducing the hole expansion ratio.

At this content, titanium ensures complete fixation of nitrogen in the form of nitrides or carbonitrides, so that boron is in free form and can play an effective role in strengthening.

At levels of at least 0.010%, niobium is very effective at forming fine niobium carbonitrides when annealed in a temperature range close to the intercritical transformation range, resulting in precipitation hardening. In addition, Nb refines the austenite grains and thus limits the proportion of pearlite in the hot-rolled sheet after coiling. If the Nb content is below 0.010%, the austenite grain size will be too large, so that the final structure will contain too much self-tempered martensite. As a result, the yield strength will be too high. However, a niobium content above 0.025% excessively delays the ferrite recrystallization during annealing, so that the structure will contain more than 30% unrecrystallized ferrite, which no longer allows achieving the specified hole expansion ratio.

At least 0.0012% boron is added to limit carbon activity to control and limit diffusion phase transformations (pearlitic transformation on cooling) and to form strengthening phases (bainite or martensite) necessary to obtain the required tensile strength. The addition of B also allows the addition of reinforcing elements such as Mn, Mo and Cr to be limited and the cost of the steel grade to be reduced. However, above 0.0030% B, joint segregation with C is possible, which leads to the formation of stitching structures that worsen the expansion ratio of the holes. Therefore, the B content is no more than 0.0030%. Preferably, the B content is at least 0.0015% and/or not more than 0.0025%.

The composition may include up to 0.030% molybdenum as a residual element. Mo retards the precipitation of Nb and Ti during annealing and retards recrystallization and can cause excessive ferrite grain refinement if present in amounts above 0.030%.

Aluminum is a very effective element for deoxidizing steel in the liquid phase during processing. The Al content is at least 0.020% to obtain sufficient deoxidation of the steel. However, the Al content should be no more than 0.060% to avoid an increase in the Ac3 temperature and allow ferrite formation to be controlled during cooling.

A minimum nitrogen content of 0.002% is required to form satisfactory amounts of nitrides and carbonitrides. Nitrogen content is limited to 0.007% to prevent the formation of coarse precipitates of TiN in the liquid state, which tend to reduce ductility and lead to early failure during hole expansion testing, reducing the hole expansion ratio.

Optionally, the steel can be subjected to a sulfide globulization treatment using calcium, which results in improved hole expansion ratio due to MnS globulization. Thus, the steel composition may include at least 0.0005% Ca, up to 0.005%.

The rest of the steel composition is made up of iron and impurities formed as a result of smelting. In this regard, nickel, copper, sulfur and phosphorus are considered residual elements that are unavoidable impurities. Therefore, their content is no more than 0.05% Ni, no more than 0.03% Cu, no more than 0.005% S and no more than 0.050% P.

If the sulfur content is higher than 0.005%, due to the presence of excess sulfides such as MnS, the ductility, in particular the hole expansion ratio, is reduced. Achieving very low sulfur content, i.e. below 0.0001%, is very expensive and does not provide any positive effect. Therefore, the S content is usually greater than or equal to 0.0001%.

However, in the present invention, the sensitivity of the hole expansion ratio with respect to the sulfur content of the steel is reduced so that a hole expansion ratio of at least 35% can be obtained even with a sulfur content higher than 0.001%, which is obtained at a lower cost. Therefore, according to the implementation, the S content is at least 0.001%.

Phosphorus is an element that reduces spot weldability and hot ductility, particularly due to its tendency to segregate at grain boundaries and co-segregate with manganese. For these reasons, its content should be limited to no more than 0.050% and preferably no more than 0.015%. However, achieving very low phosphorus content, i.e. below 0.001% is very expensive. Therefore, the P content is usually greater than or equal to 0.001%.

The microstructure of the cold-rolled and annealed steel sheet according to the invention consists of surface fractions of 34 - 80% bainite, 10 - 16% martensite and 10 - 50% ferrite.

A ferrite proportion of at least 10% contributes to achieving a total elongation of at least 15%.

The ferrite may be composed of intercritical ferrite or may include intercritical ferrite and ferrite formed by cooling during annealing of the cold-rolled steel sheet, as described below. The ferrite created by cooling is hereafter referred to as "converted ferrite". In particular, if the annealing temperature T _H2 in the method of the invention, as described in detail above, is below Ac3, i.e. is between Ac3-20°C and Ac3, ferrite includes intercritical ferrite and may additionally include converted ferrite. In other words, if the annealing temperature T _H2 is lower than Ac3, the ferrite consists of intercritical ferrite or consists of intercritical ferrite and converted ferrite.

On the contrary, if the annealing temperature T _H2 is higher than or equal to Ac3, the ferrite consists of converted ferrite.

"Converted ferrite" is different from intercritical ferrite, which remains in the structure at the end of the annealing stage. In particular, the converted ferrite is enriched in manganese, i.e. has a manganese content higher than the average manganese content of steel, and higher than the manganese content of intercritical ferrite. Thus, intercritical ferrite and converted ferrite can be distinguished by analyzing the micrograph with a FEG-TEM microscope using secondary electrons after metabisulfite etching. In the micrograph, the intercritical ferrite is medium gray in color, while the converted ferrite is dark gray in color due to its higher manganese content.

Some of the ferrite may not be recrystallized. In other words, the ferrite may contain unrecrystallized ferrite. However, the structure must contain (in surface fractions) less than 30% unrecrystallized ferrite. This percentage is expressed relative to the entire structure.

The presence of less than 30% unrecrystallized ferrite is critical to achieving target mechanical properties, especially a hole expansion ratio of at least 35%. Indeed, if the structure includes more than 30% non-recrystallized ferrite, then a stitch structure is obtained, so the hole distribution coefficient will not reach 35%.

Preferably, the surface fraction of unrecrystallized ferrite is no more than 25%, more preferably no more than 20%.

Martensite arises as a result of transformation without diffusion of austenite below the temperature Ms upon cooling. Martensite usually has the shape of islands.

A proportion of martensite of at least 10% is required to obtain a tensile strength of at least 780 MPa. However, due to the high yield strength of martensite, a martensite proportion higher than 16% will result in a yield stress higher than 450 MPa before tempering and higher than 550 MPa after tempering. In addition, the proportion of martensite above 16% will deteriorate the hole expansion ratio. Therefore, the share of martensite is no more than 16%.

Martensite consists of self-tempered martensite and optionally fresh martensite (i.e., untempered and non-self-tempered).

The proportion of self-tempered martensite surface relative to the entire structure is 4 - 10%. In particular, a surface fraction of self-tempered martensite greater than 10% would result in a yield stress greater than 450 MPa before tempering (and greater than 550 MPa after tempering, if carried out).

In addition, the presence of 10 - 16% martensite with a surface fraction of self-tempered martensite in the range of 4 - 10% contributes to achieving a yield strength of at least 350 MPa, but not more than 450 MPa before any tempering and a hole expansion ratio HER of at least 35%.

For self-tempered martensite, the definition refers to the definition given in “Les principles de base de traitement thermique des aciers” by A. Constant and G. Henry, PYC Edition 1986, p.157.

Martensite typically has a C content below 0.75%.

A bainite proportion of at least 34% contributes to achieving a yield strength in the range of 350 - 450 MPa before tempering and a hole expansion coefficient of at least 35%. Indeed, the yield strength of bainite is lower than the yield strength of martensite. In addition, the difference in hardness between bainite and ferrite is small, and bainite, due to the fractionation of martensite islands, helps prevent the formation of a stitch structure and improve the hole distribution coefficient.

If the proportion of bainite is higher than 80%, the structure will not contain at least 10% martensite and at least 10% ferrite, and the tensile strength or total elongation will be too low.

Bainite is formed upon cooling from a completely austenitic or intercritical temperature region above the Ms temperature. Bainite is a combination of bainite laths and cementite particles. Its formation is associated with diffusion over short distances.

Below, a distinction will be made between bainite containing carbides and bainite with a low carbide content.

Low carbide bainite refers to bainite containing less than 100 carbides per unit surface area of 100 µm ² . Low-carbide bainite forms when cooled between 550°C and 450°C.

Unlike low-carbide bainite, carbide-containing bainite always contains more than 100 carbides per unit surface area of 100 square micrometers.

Preferably, the bainite structure consists of bainite with a low carbide content. The presence of only bainite with low carbide content helps to achieve a yield strength of no more than 450 MPa before tempering and a total elongation of at least 15%.

The sheet structure does not include austenite.

These microstructural characteristics are determined, for example, by studying the microstructure using scanning electron microscopy using a field emitter (SEM-FEB method) with a magnification of more than 1200x, coupled to an EBSD (electron backscatter diffraction) detector. The morphology of the laths and grains is then determined by image analysis using known software such as Aphelion®.

The proportion of unrecrystallized ferrite is determined by analyzing the microstructure using scanning electron microscopy after chemical polishing with a solution consisting of hydrofluoric acid and hydrogen peroxide.

Cold rolled and annealed steel sheet typically contains fine titanium and/or niobium carbonitrides. In particular, the surface density of these carbonitrides, the largest dimension of which is less than 5 nm, is preferably lower than or equal to 10 ⁴ /μm ² . Here, the largest size of the carbonitrides refers to the maximum Feret diameter of the carbonitrides.

This surface density can be measured by analyzing the sample using transmission electron microscopy (TEM).

Cold rolled and annealed steel sheet is produced, for example, by a process comprising the following sequential steps.

Steel of the above composition is cast to produce a semi-finished steel product. The steel can be cast as an ingot or continuously as a slab, having a thickness of about 200 mm. At this stage, the intermediate product includes (TiNb)(CN) precipitates.

The semi-finished steel product is reheated to a temperature T _H1 of at least 1200° C. in order to achieve at each point a temperature favorable to the large deformations to which the steel is subjected during rolling. When heated, the (TiNb)(CN) precipitates dissolve.

The semi-finished product is hot rolled in a temperature range in which the steel structure is completely austenitic and the final rolling temperature T _FRT is between the Ar3 temperature and the non-recrystallization temperature T _NR to obtain a hot rolled steel sheet.

If T _FRT is lower than Ar3, ferrite grains are formed at Ar3 before the end of rolling. These grains are strengthened during rolling and the ductility is reduced.

If T _FRT is higher than T _NR , iron borocarbides Fe ₂₃ (BC) ₆ will precipitate at the grain boundaries, thereby interfering with the strengthening effect of B. Indeed, these precipitates will not dissolve in subsequent stages of the manufacturing process.

Typically the final rolling temperature T _FRT is 850 - 930°C.

During hot rolling, fine titanium nitrides are usually released. Their largest size is usually 150 - 200 nm.

The hot-rolled steel product is then cooled at a first cooling rate V _C1 of at least 10°C/s to a winding temperature T _coil below 500°C and coiled.

The first cooling rate V _C1 is at least 10°C/s to avoid the transformation of austenite into ferrite and pearlite during cooling and to avoid partial precipitation of niobium.

The winding temperature T _coil must be below 500°C and above the final martensite transformation temperature _Mf .

Indeed, the inventors have discovered that if the winding temperature T _coil is greater than or equal to 500°C, the mechanical properties of the sheet are not uniform in the longitudinal and transverse directions, and the tensile strength does not reach 780 MPa, and even below 600 MPa at least in some parts leaf.

The inventors investigated this phenomenon and found that it is caused, in particular, by the low Mn content of the steel, which is necessary to obtain a yield strength of no more than 450 MPa before tempering and a hole expansion ratio of at least 35%.

In particular, Mn usually retards the transformation of austenite to bainite and/or martensite during winding. In particular, this applies to steels with a Mn content above 2.0%, which do not require a yield strength of no more than 450 MPa before tempering or no more than 550 MPa after tempering and/or a low hole expansion coefficient.

When the Mn content is reduced to no more than 2.0%, the transformation of austenite to bainite during winding is accelerated, resulting in an increase in the temperature of the sheet during winding, especially in the core and axis region of the coil.

The core of the roll is defined as the portion of the sheet that extends along the longitudinal direction of the sheet from the first end located at 30% of the total length of the sheet to the second end located at 70% of the total length of the sheet. In addition, the axial region is defined as an area centered on the midpoint of the longitudinal axis of the sheet and having a width equal to 60% of the total width of the sheet.

In the core and axis region, the turns are in contact during winding, so that the heat generated during the transformation of austenite to bainite cannot be dissipated to a large extent.

If the winding temperature is greater than or equal to 500°C, this increase in temperature results in the precipitation of borocarbides and coarse carbides of titanium and niobium, thereby suppressing the precipitation strengthening potential of B, Ti and Nb. In addition, the effect of Nb on recrystallization refinement is suppressed, so that ferrite grains become too coarse. In addition, this increase in temperature leads to coalescence of cementite. In particular, cementite does not dissolve completely, so the amount of carbon available for austenite is too small. Consequently, too little austenite is formed in the core and axis region of the coil during winding, resulting in too low a martensite proportion in this region in the final microstructure. As a result of these two effects, the tensile strength in this region of the sheet does not reach 780 MPa.

In addition, if the winding temperature is greater than or equal to 500°C, the mechanical properties of the sheet will not be uniform in either the longitudinal or transverse direction of the sheet.

The authors of the invention found that when winding at temperatures below 500°C, despite the increase in temperature due to the transformation of austenite into bainite, there is no coalescence of cementite and no precipitation of borocarbides or large carbides of titanium and niobium. Consequently, the tensile strength is not reduced, and the mechanical properties of the sheet are uniform in the longitudinal and transverse directions of the sheet.

In addition, winding at temperatures below 500°C makes it possible to limit the proportion of perlite formed during winding, thereby avoiding the formation of a stitch structure, which negatively affects the expansion ratio of holes in subsequent stages of the process.

However, if the winding temperature is below _Mf , the steel will be too hard for cold rolling.

Preferably, the winding temperature is at least 300°C, even more preferably at least 350°C or at least 400°C.

During winding, austenite is converted to bainite and possibly martensite and/or pearlite, so that at the end of winding the structure of the entire sheet consists of bainite and optionally martensite and/or pearlite, with a surface fraction of pearlite less than 15%, without ferrite . In particular, the structure is uniform in the longitudinal and transverse directions of the sheet. Bainite is bainite with low carbide content, i.e. contains less than 100 carbides per unit surface of 100 µm ² .

At this stage, the sheet includes B, Nb and Ti in solid solution. In particular, the Nb content in the solid solution is at least 0.01%.

This microstructure of the hot-rolled sheet after coiling is critical to obtain the required mechanical properties. Indeed, the kinetics of recrystallization in the subsequent annealing stage, which depends on the microstructure of the hot-rolled sheet after winding, has a strong influence on the structure formed during annealing, especially on the size and shape of austenite grains. Especially if the structure of the sheet after winding includes 15% or more pearlite, austenite will mainly occur and grow during annealing in areas of the sheet including pearlite, resulting in a streak structure.

The hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet with a cold-rolling reduction ratio of at least 40%. Below 40%, the strain imparted to the structure is insufficient, resulting in insufficient recrystallization during subsequent annealing and a streak structure.

The reduction rate during cold rolling is usually 40 - 80%.

The cold rolled steel sheet typically has a thickness of 0.7 to 2.3 mm, for example at least 1.5 mm or at least 2.0 mm.

Then, the cold-rolled steel sheet is reheated to an annealing temperature T _H2 between Ac3-20°C and Ac3+15°C.

The average heating rate _VH to the annealing temperature _TH2 is 1 - 50°C/s. In addition, the average heating rate V _H' between 600°C and Ac1 is 1 - 10°C/s.

It should be noted that the average heating rate _VH between 600°C and Ac1 differs from the average heating rate between the beginning of the heating process (for example, room temperature) and Ac1, and also differs from the average heating rate _VH to the annealing temperature _TH2 .

Average heating rates V _H and V _H' are achieved, for example, by heating a cold-rolled sheet in a continuous annealing furnace having several zones through which the sheet passes. In each of these oven zones, oven parameters (e.g. zone temperature, heating power...) are controlled to achieve a certain set heating rate in that zone. This regulation makes it possible to achieve an average heating rate _VH to the annealing temperature in the range of 1 - 50°C/s and an average heating rate _VH' between 600°C and Ac1 in the range of 1 - 10°C/s.

When heated between 600 - Ac1, recrystallization occurs and finely dispersed titanium and niobium carbonitrides are released in the steel. The presence of fine precipitates allows there to still be enough Nb in the solid solution to control the size of the ferrite grains during recrystallization, avoiding too much growth of the ferrite grains.

The inventors have discovered that control of the average heating rate _VH' between 600°C and Ac1 and thus the heating time between 600°C and Ac1, which corresponds to the time between the start of recrystallization and the end of recrystallization, is critical to the kinetics of later phase transformations , in particular during subsequent exposure at the annealing temperature T _H2 .

In particular, controlling the average heating rate between 600°C and Ac1 allows the size and aspect ratio of the ferrite grains produced at Ac1 to be adjusted. Upon subsequent heating from Ac1 to the annealing temperature, austenite grains nucleate at the grain boundaries of recrystallized ferrite. Thus, controlling the average heating rate between 600°C and Ac1 allows one to control the size and redistribution of austenite grains at the end of annealing, as well as the final microstructure.

An average heating rate V _H' below 1°C/s would lead to excessively long heating times between 600°C and Ac1 and, consequently, to excessive growth of ferrite grains and subsequently formed austenite grains. Excessive austenite grain size leads to the formation of too high a proportion of martensite in subsequent stages of the manufacturing process, especially too high a proportion of self-tempered martensite in the final structure. As a result, the yield strength will be too high.

On the contrary, an average heating rate V _H' above 10°C/s would lead to insufficient recrystallization or even no recrystallization of ferrite when heated from 600 to Ac1. As a result, austenite occurs in carbon-rich regions, that is, in bands of pearlite and martensite, so that the final structure has a line structure, which degrades the hole expansion coefficient.

The average heating rate V _H' from 600°C to Ac1, amounting to 1 - 10°C/s, allows at the end of the manufacturing process to obtain steel, the microstructure of which consists of surface fractions of 34 - 80% bainite, 10 - 16% martensite and 10 - 50% ferrite, so that the proportion of the surface of unrecrystallized ferrite in the structure is less than 30%, the proportion of tempered martensite is 4 - 10%.

The annealing temperature T _H2 is from Ac3-20°C to Ac3+15°C to obtain, at the end of exposure at the annealing temperature T _H2 , a structure consisting of at least 50% austenite and optionally ferrite.

If the annealing temperature T _H2 is below Ac3-20°C, the structure may contain too much ferrite and/or not enough bainite and/or self-tempered martensite, and the hole expansion ratio HER will not reach 35%.

If the annealing temperature T _H2 is higher than Ac3+15°C, the austenite grain size will be too large. This excess austenite grain size will result in too high a proportion of bainite and too high a proportion of self-tempered martensite in the final structure, while on cooling an insufficient proportion of ferrite will be created. As a result, the yield strength will be too high and the total elongation will be too low.

The sheet is kept at the annealing temperature T _H2 for an annealing time T _H2 of at least 30 s, preferably no more than 500 s. During this exposure at the annealing temperature T _H2, austenite grains grow and the precipitation of titanium and niobium carbonitrides continues.

If the annealing time t _H2 is less than 30 s, the austenite grains are too fine. As a consequence, the final structure contains an insufficient proportion of martensite and an excess proportion of ferrite, so that a tensile strength of at least 780 MPa is not achieved. If the annealing time t _H2 exceeds 500 s, the niobium and titanium precipitates may coalesce, thereby interfering with the strengthening effect of Nb and Ti, and the austenite grains may be too large. As a result, a yield strength may exceed 450 MPa, a tensile strength of at least 780 MPa may not be achievable, and/or a hole expansion ratio of less than 35% may be obtained.

The sheet is then cooled to a temperature T _C of 440 - 480°C, with a second cooling rate V _C2 of 10 - 50°C/s. During this cooling stage, the austenite is partially converted to bainite and optionally to ferrite.

This cooling may be carried out from temperature T _H2 in one or more stages and may in the latter case involve various cooling modes, such as a cold or boiling water bath, water jets or gas jets.

If the second cooling rate V _C2 is below 10°C/s, the final structure may include an excess proportion of ferrite and will include an insufficient proportion of martensite and/or bainite, so that the tensile strength does not reach 780 MPa and the hole expansion coefficient does not reach 35%.

If the annealing temperature is between Ac3 and Ac3+15°C, the second cooling rate V _C2 is preferably no more than 20°C/s to convert part of the austenite to ferrite so that the final structure includes at least 10% ferrite.

Then the steel sheet is kept in the temperature range 440 - 480°C for a holding time t _C of 20 s - 500 s.

At this stage, partial transformation of the remaining austenite into bainite occurs. If the holding time t _C is less than 20 s, an insufficient proportion of bainite will be formed. If the holding time t _C is more than 500 s, the proportion of bainite will be too large, and the proportion of martensite in the final structure will be insufficient.

Preferably, the holding time t _C is no more than 50 s.

Optionally, during exposure in the temperature range of 440 - 480°C, the steel sheet is coated by immersion in a bath of zinc or zinc alloy at a temperature T _Zn below 480°C.

Optionally, after galvanizing, the steel sheet can be annealed by heating immediately after leaving the zinc or zinc alloy bath to a temperature T _G of 490 - 550 ° C, for a time t _G of typically 10 - 40 s.

Immediately after exposure to the temperature range 440 - 480°C or after galvanizing or after galvanizing and annealing, the sheet is cooled to ambient temperature at a third cooling rate V _C3 equal to at least 1°C/s. During this cooling stage, the remaining austenite is converted to fresh martensite and/or bainite.

Using this manufacturing method, a cold-rolled and annealed steel sheet is obtained, the structure of which consists of surface fractions of 34 - 80% bainite, 10 - 16% martensite and 10 - 50% ferrite. The proportion of non-recrystallized ferrite surface in the structure is less than 30%. Martensite consists of self-tempered martensite and fresh martensite, the proportion of the surface of self-tempered martensite relative to the entire structure is 4 - 10%.

After cooling to room temperature, if galvanizing has not been carried out, the cold-rolled and annealed steel sheet can be coated by vacuum deposition, such as physical vapor deposition (PVD) or jet vapor deposition (JVD).

The inventors have shown that cold-rolled and annealed steel sheet produced by this manufacturing method has a tensile strength of 780 - 900 MPa, a yield strength of 350 - 450 MPa, a total elongation of at least 15% or even at least 18% and a hole expansion ratio HER at least 35%.

A yield strength of 350 - 450 MPa is achieved immediately after cooling to room temperature, without tempering.

In particular, the addition of niobium and titanium to the composition and the precipitation of fine niobium and titanium carbonitrides during the annealing step allows one to obtain a tensile strength of at least 780 MPa with a relatively low martensite fraction not exceeding 16%. Consequently, the yield strength remains no more than 450 MPa, and the difference in hardness between the microstructure components is reduced, so that the hole expansion ratio can exceed 35%.

Optionally, after cooling to room temperature, training is carried out. In this case, the cold rolled and annealed steel sheet has a tensile strength of 780 - 900 MPa, a yield strength of 450 - 550 MPa, a total elongation of at least 15% or even 18% and a hole expansion ratio of at least 35%.

Training, for example, is performed at a compression ratio in the range of 0.1 - 0.4%, for example, in the range of 0.1 - 0.2%.

In addition, these mechanical properties are achieved in a wide range of cold-rolled and annealed steel sheet thicknesses of 0.7 - 2.3 mm. These characteristics are achieved in particular with sheet thicknesses of at least 2.0 and up to 2.3 mm.

In addition, the mechanical properties, in particular the tensile strength, are uniform in the longitudinal and transverse directions of the sheet. Specifically, when considering the entire cold-rolled and annealed steel sheet having a rolling direction length of at least 500 m, the difference in tensile strength between the regions with the highest tensile strength and the regions with the lowest tensile strength in the cold-rolled and annealed steel sheet is no more than 7% of tensile strength in areas of highest tensile strength.

Examples

As examples and comparisons, sheets are made from steels whose composition is shown in Table I, the elements being expressed in mass percentage or ppm (parts per million).

Table I

In this table, "res" means that the corresponding element is present in a trace amount below the lower range defined for that element. Specifically, the residual amount of Ti means that the Ti content is lower than 3.42 N, and the residual amount of B means that the B content is lower than 0.0012%. The underlined values do not correspond to the invention.

Ac3 conversion values are also given in Table I. Ac3 is estimated using Thermo-Calc® software.

Steels having the compositions shown in Table I are cast to produce ingots. The ingots are reheated to a temperature T _H1 of 1250°C, then hot rolled with a final rolling temperature T _FRT between Ar3 and T _NR to produce hot rolled steel sheets.

Hot-rolled steel sheets are cooled at a first cooling rate V _C1 of 30°C/s to a winding temperature T _coil and wound at this temperature T _coil to obtain a structure consisting of bainite and optionally martensite and/or pearlite, the surface fraction of pearlite being less than 15%. For all examples and comparative examples, the winding temperature is above _Mf .

The hot rolled steels are then pickled and cold rolled with a cold rolling reduction ratio of 50% to produce cold rolled sheets with a thickness of 1.4 mm.

The cold-rolled sheets are reheated to an annealing temperature T _H2 at a medium heating rate V _H and at a medium heating rate V _H' between 600° C. and Ac1 to an annealing temperature T _H2 and maintained at an annealing temperature T _H2 for an annealing time t _H2 .

Then the sheets are cooled at the second cooling rate V _C2 to a temperature T _C and maintained at this temperature for a holding time t _C . The sheets are then galvanized by immersion in a zinc bath at a temperature not exceeding 480°C and cooled to room temperature with a third cooling rate V _C3 of at least 1°C/s.

Finally, the sheets are subjected to temper training with a rolling reduction degree of 0.1 - 0.4%.

Processing conditions are shown in Table II.

Table II

In Table II, the underlined values do not correspond to the invention. In Table II, the T _H2 values that are not underlined are such that the structure after annealing includes at least 50% austenite.

The microstructure of the steel sheets thus obtained is determined. The surface fraction of martensite (including tempered martensite and fresh martensite), the surface fraction of bainite, and the surface fraction of low-carbide bainite are quantified after etching with sodium bisulfite. The surface area fraction of fresh martensite is determined quantitatively after etching with the NaOH-NaNO ₃ reagent.

The proportion of ferrite surface is also determined by optical and scanning electron microscopes, where the ferrite phase is identified, and the proportion of non-recrystallized is determined by a scanning electron microscope after chemical polishing with a solution consisting of hydrofluoric acid and hydrogen peroxide.

In addition, the mechanical properties of the sheets are determined.

The measured properties are hole expansion ratio HER, yield strength YS, tensile strength TS, uniform elongation UE and total elongation TE.

Yield strength YS, tensile strength TS, uniform elongation UE and total elongation TE are measured in accordance with ISO 6892-1 published in October 2009. Hole expansion ratio HER is measured in accordance with ISO 16630:2009.

In addition, the difference in ΔTS between the highest and lowest tensile strength regions of the sheets is measured.

The microstructures of steel sheets and their mechanical properties are presented in Table III below.

In Table III, M is the surface fraction of martensite, FM is the surface fraction of fresh martensite, TM is the surface fraction of tempered martensite, B is the surface fraction of bainite, F is the surface fraction of ferrite, the column “UF <30%” indicates whether the surface fraction is unrecrystallized ferrite less than 30%, and LBC/B represents the percentage of bainite that is low carbide bainite.

Steel 1 contains less than 0.4% Cr, which leads to insufficient hardenability, so that the proportion of self-tempered martensite does not reach 4%, and the proportion of ferrite exceeds 50%. An even higher proportion of ferrite is achieved in example 1-a, in which annealing is carried out at temperatures below Ac3-20°C. As a result, the tensile strength does not reach 780 MPa and, in example 1-a, the hole expansion coefficient does not reach 35%.

The composition of steels 2 and 3 also includes less than 0.4% Cr and more than 2.0% Mn. This high Mn content leads to too much stabilization of the austenite, so that upon cooling from the annealing temperature a too high proportion of martensite is formed and a too low proportion of bainite. As a result, the yield strength is too high. In addition, this Mn content above 2.0% results in a stitch structure, so that the hole expansion ratio does not reach 35%.

The composition of steel 4 corresponds to the invention. Example 4-b is obtained by the method according to the invention and has a structure according to the invention, so that the desired mechanical properties are achieved. Fig. 2 illustrates the structure of this example 4-b. In this figure, M stands for martensite, CFB stands for carbide-free bainite, and F stands for ferrite.

In example 4-a, on the contrary, annealing is carried out at a temperature T _H2 below Ac3-20°C, so that the structure does not contain a sufficient amount of self-tempered martensite, and the hole expansion coefficient HER does not reach 35%.

Steel composition 5 includes too much C and Mn and not enough Ti and B. Steel composition 6 includes too much C and Mn, not enough Ti and B, and too little Cr. As a result, samples 5-a, 5-b, 6-a and 6-b do not have the declared structure, in particular, they have too high a ferrite content (ferrite forms during cooling) and too low a bainite content, so that the yield strength is too high, and the hole distribution coefficient does not reach 35%.

The composition of steel 7 also includes too much C and Mn, while the Cr content is too low and the Nb content is too high. Example 7-a includes too much ferrite, too much unrecrystallized ferrite and too little bainite such that the specified yield strength and hole expansion ratio are not achieved.

The composition of steel 8 corresponds to the invention. Examples 8-b, 8-g and 8-h are produced by the method according to the invention and have a structure according to the invention, so that the desired mechanical properties are achieved.

In contrast, in example 8-a the annealing is carried out at a temperature T _H2 below Ac3-20°C, so that the structure does not include enough self-tempered martensite, not enough bainite and too much ferrite. As a result, the HER hole expansion ratio does not reach 35%.

In Examples 8-c, 8-d and 8-e, winding is performed at too high a winding temperature. The resulting structure does not contain enough martensite, self-tempered martensite, not enough bainite, and too much ferrite. As a result, the tensile strength does not reach 780 MPa. In addition, the tensile strength is uneven, the difference ΔTS in tensile strength exceeds 7%.

In example 8-f, annealing is carried out at too low an annealing temperature T _H2 , so that the structure contains too little self-tempered martensite and the hole expansion coefficient does not reach 35%.

In example 8-i, the holding is carried out at too high a temperature after annealing, so that the proportion of self-tempered martensite is too large, the yield strength is higher than 550 MPa and the hole expansion coefficient does not reach 35%.

In example 8-j, the exposure is carried out for a too short exposure time t _c . As a result, the transformation to bainite was incomplete, so that the proportion of self-tempered martensite is too large, the yield strength exceeds 550 MPa, and the hole expansion ratio does not reach 35%.

In example 8-k, heating is carried out at too high a heating rate V _H' to the annealing temperature. As a consequence, the structure contains more than 30% unrecrystallized ferrite, so that the hole expansion ratio does not reach 35% and the yield strength is too high.

The composition of steel 9 includes too much Mo, and in example 9-m the annealing is carried out at too low an annealing temperature, so that the structure of the steel does not correspond to the invention and the specified properties are not achieved.

The composition of steel 10 includes too much C, not enough Cr, Nb and B. As a result, the proportion of martensite is too high, and the hole expansion ratio does not reach 35%. Fig. 1 illustrates the structure of Example 10-a. In this figure, M stands for martensite, CFB stands for carbide-free bainite, and F stands for ferrite. In addition, BC denotes bainite containing carbides.

The composition of steel 11 corresponds to the invention. Example 11-b is carried out using the method according to the invention and has a structure according to the invention, so that the desired mechanical properties are achieved.

In contrast, in example 11-a, annealing is carried out at a temperature T _H2 below Ac3-20°C, so that the structure does not include enough self-tempered martensite, not enough bainite and too much ferrite. As a result, the HER hole expansion ratio does not reach 35%.

In example 11-c, annealing is also performed at a temperature T _H2 below Ac3-20°C, and additionally winding is carried out at a winding temperature that is too high. The structure contains neither enough martensite nor self-tempered martensite, and too much ferrite, so that the tensile strength does not reach 780 MPa. In addition, the tensile strength is uneven, the difference ΔTS in tensile strength exceeds 7%.

The composition of steel 12 includes more than 0.085% C. As a result, even when implementing the method according to the invention, neither the target structure nor the target properties are achieved. Example 12-c again shows that winding at too high a winding temperature results in a difference ΔTS in tensile strength exceeding 7%.

The composition of steel 13 includes too much Mn and not enough Ti and B. As a consequence, even if the method according to the invention is carried out, neither the target structure nor the target properties are achieved. In particular, due to the insufficient content of Ti and B, the proportion of martensite does not reach 10%, so that the tensile strength is less than 780 MPa.

Claims (62)

1. Cold-rolled and annealed steel sheet, having a composition including, wt.%: 0.060 ≤ C ≤ 0.085, 1.8 ≤ Mn ≤ 2.0, 0.4 ≤ Cr ≤ 0.6, 0.1 ≤ Si ≤ 0.5, 0.010 ≤ Nb ≤ 0.025, 3.42N ≤ Ti ≤ 0.035, 0 ≤ Mo ≤ 0.030, 0.020 ≤ Al ≤ 0.060, 0.0012 ≤ V ≤ 0.0030, S ≤ 0.005, P ≤ 0.050, 0.002 ≤ N ≤ 0.007, if necessary 0.0005 ≤ Ca ≤ 0.005, the rest is iron and inevitable impurities resulting from smelting, and the cold-rolled and annealed steel sheet has a microstructure consisting of surface fractions of: - 34-80% bainite, - 10-16% martensite, and - 10-50% ferrite, while the proportion of the surface of non-recrystallized ferrite relative to the entire structure is less than 30%; martensite consists of self-tempered martensite and fresh martensite, and the proportion of the surface of self-tempered martensite relative to the entire structure is 4-10%. 2. The cold rolled and annealed steel sheet of claim 1, wherein said bainite is a low carbide bainite comprising less than 100 carbides per unit surface area of 100 µm ² . 3. Cold-rolled and annealed steel sheet according to claim 1 or 2, which is not subjected to temper training, wherein the cold-rolled and annealed steel sheet has a tensile strength TS in the range of 780-900 MPa, a yield strength YS in the range of 350-450 MPa, total an elongation TE in the range of at least 15%, and a hole expansion ratio HER of at least 35%. 4. Cold rolled and annealed steel sheet according to claim 1 or 2, which is a tempered sheet having a tensile strength TS in the range of 780-900 MPa, a yield strength YS in the range of 450-550 MPa, a total elongation TE of at least 15 %, and a hole expansion ratio HER of at least 35%. 5. Cold-rolled and annealed steel sheet according to any one of paragraphs. 1-4, which has a thickness of 0.7-2.3 mm. 6. Cold-rolled and annealed steel sheet according to claim 5, which has a thickness of at least 2.0 mm. 7. Cold-rolled and annealed steel sheet according to any one of paragraphs. 1-6, which has a length in the rolling direction of at least 500 m, and the difference in tensile strength between the regions with the highest tensile strength and the regions with the lowest tensile strength of the cold-rolled and annealed steel sheet is not more than 7% tensile strength in areas with the highest tensile strength. 8. Cold-rolled and annealed steel sheet according to any one of paragraphs. 1-7, which contains a zinc or zinc alloy coating obtained by continuous dip coating. 9. Cold rolled and annealed steel sheet according to paragraphs. 1-7, which contains a coating of zinc or a zinc alloy obtained by vacuum deposition. 10. A method for producing cold-rolled and annealed steel sheet, including the following sequential stages: - preparation of a semi-finished steel product having a composition including, wt.%: 0.060 ≤ C ≤ 0.085, 1.8 ≤ Mn ≤ 2.0, 0.4 ≤ Cr ≤ 0.6, 0.1 ≤ Si ≤ 0.5, 0.010 ≤ Nb ≤ 0.025, 3.42N ≤ Ti ≤ 0.035, 0 ≤ Mo ≤ 0.030, 0.020 ≤ Al ≤ 0.060, 0.0012 ≤ V ≤ 0.0030, S ≤ 0.005, P ≤ 0.050, 0.002% ≤ N ≤ 0.007, if necessary 0.0005 ≤ Ca ≤ 0.005, the rest is iron and inevitable impurities resulting from smelting, - heating the specified semi-finished product to a temperature T _H1 greater than or equal to 1200°C, then hot rolling the heated semi-finished product with a final rolling temperature T _FRT located between Ar3 and T _NR , where Ar3 represents the temperature at which austenite transformation begins when cooling the steel and T _NR represents the temperature of absence recrystallization of steel to produce hot-rolled steel sheet, - cooling the hot-rolled steel sheet with a first cooling rate V _C1 of at least 10°C/s to a winding temperature T _coil above the end of martensitic transformation temperature M _f of steel and below 500°C, and winding the hot-rolled steel sheet at a winding temperature T _coil to obtain structure consisting of bainite or consisting of bainite and martensite and/or pearlite, with a pearlite surface fraction of less than 15%, - cold rolling of hot-rolled steel sheet with a cold rolling reduction ratio of at least 40% to obtain cold-rolled steel sheet, - reheating of the cold-rolled steel sheet to an annealing temperature T _H2 in the range from Ac3-20°C to Ac3+15°C with an average heating rate _VH to an annealing temperature T _H2 in the range of 1-50°C/s and an average heating rate V _H' between 600°C and Ac1 within 1-10°C/s, and holding the cold-rolled steel sheet at annealing temperature T _H2 for an annealing time t _H2 of at least 30 s, so as to obtain a structure including at least 50% austenite, - cooling of the cold-rolled steel sheet to a temperature T _C in the range of 440-480°C at a second cooling rate V _C2 in the range of 10-50°C/s, - exposure of cold-rolled steel sheet in the temperature range 440-480°C for a holding time t _c 20-500 s, - cooling the cold-rolled steel sheet to ambient temperature with a third cooling rate V _C3 of at least 1°C/s. 11. The method of manufacturing cold-rolled and annealed steel sheet according to claim 10, in which the annealing time t _H2 is no more than 500 s. 12. A method for producing cold-rolled and annealed steel sheet according to claim 10 or 11, in which the annealing temperature T _H2 is between Ac3 and Ac3+15°C, and the second cooling rate V _C2 is 10-20°C/s. 13. A method for producing cold-rolled and annealed steel sheet according to any one of paragraphs. 10-12, in which the cold-rolled and annealed steel sheet has a microstructure consisting in surface fractions of: - 34-80% bainite, - 10-16% martensite, and - 10-50% ferrite, while the proportion of the surface of non-recrystallized ferrite relative to the entire structure is less than 30%; martensite consists of self-tempered martensite and fresh martensite, and the proportion of the surface of self-tempered martensite relative to the entire structure is 4-10%. 14. A method for producing cold-rolled and annealed steel sheet according to any one of paragraphs. 10-13, in which, at the specified exposure in the temperature range of 440-480°C, a cold-rolled steel sheet is coated by hot immersion in a bath at a temperature lower than or equal to 480°C. 15. The method for producing cold-rolled and annealed steel sheet according to claim 14, wherein the cold-rolled and annealed steel sheet is coated with zinc or a zinc alloy. 16. A method for producing cold-rolled and annealed steel sheet according to any one of paragraphs. 10-13, in which, after cooling to ambient temperature, a zinc or zinc alloy coating is applied by vacuum deposition. 17. A method for producing cold-rolled and annealed steel sheet according to any one of paragraphs. 10-16, in which the degree of reduction during cold rolling is 40-80%. 18. A method for producing cold-rolled and annealed steel sheet according to any one of paragraphs. 10-17, in which, after cooling to ambient temperature, the steel sheet is subjected to temper training with a reduction degree of 0.1-0.4%.