Hot-rolled steel sheet and related manufacturing method

Abstract

The present invention relates mainly to a hot-rolled steel sheet with a yield stress of more than 680 MPa at least in the direction transverse to the rolling direction and less than or equal to 840 MPa, a strength of between 780 and 950 MPa, an elongation at break of more than 10% and an expansion-to-cavity ratio (Ac) of greater than or equal to 45%, and its chemical composition comprises the following, expressed by weight:\n0.04% ≤ carbon ≤ 0.08%\n1.2% ≤ manganese ≤ 1.9%\n0.1% ≤ silicon ≤ 0.3%\n0.07% ≤ titanium ≤ 0.125%\n0.05% ≤ molybdenum ≤ 0.35%\n0.15% \u003c chromium ≤ 0.6% when 0.05% ≤ Molybdenum ≤ 0.11%, or\n0.10% ≤ Chromium ≤ 0.6% when 0.11%  Molybdenum ≤ 0.35%\nNiobium ≤ 0.045%\n0.005% ≤ Aluminum ≤ 0.1%\n0.002% ≤ Nitrogen ≤ 0.01%\nSulfur ≤ 0.004%\nPhosphorus \u003c 0.020%\n\nAnd optionally: \n0.001%  Vanadium  0.2%\nThe remainder consists of iron and unavoidable impurities resulting from the process and the microstructure of which is formed by granular bainite with an area percentage of more than 70% and ferrite with an area percentage of less than 20% and the remainder, if any, consists of lower bainite, martensite and retained austenite, the sum of martensite and retained austenite being less than 5%.\nThe invention further relates to a method of manufacturing a sheet of this type.

Description

Hot-rolled steel sheet and related manufacturing methods The present invention relates mainly to hot-rolled steel sheet. This invention further relates to a method that enables the manufacture of such a steel sheet. The need to build lighter vehicles and increase safety has led to the production of high-strength steels. In the past, manufacturing and development were carried out with steels containing additive elements to achieve age hardening. Subsequently, "dual phase" steels were introduced, which had martensite in a ferrite matrix to provide structural toughness. To achieve higher levels of strength along with sheet metal workability, so-called TRIP (transformation ductility) steels were developed, whose microstructure consisted of a ferritic matrix containing bainite and retained austenite, which transformed into martensite under the influence of deformation, for example during stamping operations. To achieve mechanical strength greater than 800 MPa, multiphase steels with a predominantly bainite structure have been proposed. These steels are used in industry and are particularly used in the automotive industry for the manufacture of structural parts. This type of steel is described in EP 2 020 451. In order to achieve an elongation at break of more than 10% and a mechanical strength of more than 800 MPa, the steels described in this document contain, in addition to carbon, manganese and silicon, molybdenum and vanadium. The microstructure of the steels consists essentially of upper bainite (at least 80%) as well as lower bainite, martensite and retained austenite. However, these steels are expensive to produce due to the presence of molybdenum and vanadium. In addition, some automotive parts, such as bumpers and suspension arms, are manufactured by operations that combine different methods of deformation. Some microstructural properties of the steel may be suitable for one type of deformation but less suitable for another. Some parts of the parts must have high yield strength for elongation; others must be suitable for forming a cutting edge. This latter property is measured using the expansion-hole method described in ISO 16630:2009. A type of steel that compensates for these negative points is one that does not contain molybdenum or vanadium and contains titanium and niobium in certain amounts. The latter two elements, among other things, give the steel the desired strength, the necessary hardness and the desired expansion-to-cavity ratio. The steel plates of the present invention are hot rolled because, among other things, this operation allows the precipitation of titanium carbides and makes it possible to create maximum hardness in the steel. It has been found, however, that some steels containing elements which are more oxidizable than iron, such as silicon, manganese, chromium and aluminium, develop surface defects in some sheets after coiling at high temperatures. These defects may be aggravated by subsequent deformation of the sheets. To avoid this, it is necessary either to carry out a rapid cooling of the coils by a supplementary process which involves increased costs, or to carry out the coiling operation at a lower temperature which leads to a reduction in titanium precipitation. Thus, one of the objects of the invention is to provide a plate for which the coiling operation at high temperature does not lead to the production of the above-mentioned surface defects. Another object of the invention is to obtain a steel sheet in an uncoated or galvanized state. The composition and mechanical properties of the steel must be compatible with the limitations and thermal cycles of the continuous hot-dip zinc coating processes. Another object of the present invention is a method for manufacturing a steel sheet that does not require high rolling forces, which allows manufacturing over a wide range of thicknesses, for example between 1.5 and 4.5 mm. Finally, another object of the invention is a hot-rolled steel sheet that is cost-effective to produce and simultaneously exhibits a yield strength greater than 680 MPa, at least in the direction transverse to the rolling direction, equal to or less than 840 MPa, a mechanical strength between 780 MPa and 950 MPa, an elongation at break greater than 10% and an expansion-to-cavity ratio (Ac) greater than or equal to 45%. For this purpose, the plate according to the present invention has the feature that its chemical composition includes the following, expressed in weight percent: 0.04% ≤ Carbon ≤ 0.08% 1.2% ≤ Manganese ≤ 1.9% 0.1% ≤ Silicon ≤ 0.3% 0.07% ≤ Titanium ≤ 0.125% 0.05% ≤ Molybdenum ≤ 0.35% 0.15% < Chromium ≤ 0.6% when 0.05% ≤ Molybdenum ≤ 0.11%, or 0.10% ≤ Chromium ≤ 0.6% when 0.11%  Molybdenum ≤ 0.35% Niobium ≤ 0.045% 0.005% ≤ Aluminum ≤ 0.1% 0.002% ≤ Nitrogen ≤ 0.01% Sulfur ≤ 0.004% Phosphorus < 0.020% And optionally: 0.001%  Vanadium  0.2% The remainder consists of iron and unavoidable impurities resulting from the process and its microstructure is composed of granular bainite with an area percentage of more than 70% and ferrite with an area percentage of less than 20% and the remainder, if present, consists of lower bainite, martensite and retained austenite. The sum of the contents of martensite and retained austenite is less than 5%. The sheet according to the invention may also include the following optional features, taken alone or in any technically possible combination: The chemical composition includes the following, expressed as a percentage by weight: 0.04% ≤ Carbon ≤ 0.08% 1.2% ≤ Manganese ≤ 1.9% 0.1% ≤ Silicon ≤ 0.3% 0.07% ≤ Titanium ≤ 0.125% 0.05% ≤ Molybdenum ≤ 0.25% 0.16% ≤ chromium ≤ 0.55% when 0.05% ≤ molybdenum ≤ 0.11%, or 0.10% ≤ Chromium ≤ 0.55% when 0.11%  Molybdenum ≤ 0.25% Niobium ≤ 0.045% 0.005% ≤ Aluminum ≤ 0.1% 0.002% ≤ Nitrogen ≤ 0.01% Sulfur ≤ 0.004% Phosphorus < 0.020% The remainder consists of iron and unavoidable impurities resulting from the process, - The composition of steel includes the following, expressed as a percentage by weight: 0.27% ≤ chromium ≤ 0.52% when 0.05% ≤ molybdenum ≤ 0.11%, or 0.10% ≤ Chromium ≤ 0.52% when 0.11%  Molybdenum ≤ 0.25% - The composition of steel includes the following, expressed as a percentage by weight: 0.05% ≤ Molybdenum ≤ 0.18%, and 0.16% ≤ chromium ≤ 0.55% when 0.05% ≤ molybdenum ≤ 0.11%, or 0.10% ≤ Chromium ≤ 0.55% when 0.11%  Molybdenum ≤ 0.18% - The chemical composition includes the following, expressed as a percentage by weight: 0.05% ≤ Carbon ≤ 0.07% 1.4% ≤ Manganese ≤ 1.6% 0.15% ≤ Silicon ≤ 0.3% Niobium ≤ 0.04% 0.01% ≤ Aluminum ≤ 0.07% - The chemical composition includes the following, expressed as a percentage by weight: 0.040% ≤ Tieff ≤ 0.095% When Tieff= Ti - 3.42 x N In this case, Ti is the titanium content, expressed by weight. and N is the nitrogen content, expressed by weight. - The steel sheet is coiled and acid-washed. The coiling operation is carried out at a temperature between 525 and 635 degrees Celsius, followed by acid-washed operation, and the depth of surface defects due to oxidation that occur on n oxidation zones i of the coiled sheet, where i is between 1 and n, and n oxidation zones extend over the visible length, which achieves the following: - A maximum depth criterion defined as follows:  8 micrometers where is the maximum depth of a defect due to oxidation in oxidation zone i of this coiled sheet, and - An average depth criterion defined as follows: x li 2.5 μm where is the average depth of defects due to oxidation in oxidation zone i and is the length of oxidation zone i. - Visible length corresponding to defects due to oxidation equal to or greater than 100 μm. - Visible length corresponding to defects due to oxidation equal to or greater than 500 μm. - The sheet is coiled in adjacent turns with a coiling tension of at least 3 metric tons-force. The invention further relates to a method for manufacturing a hot-rolled steel sheet with a yield strength of at least greater than 680 MPa in the direction transverse to the rolling direction and less than or equal to 840 MPa, having a strength between 780 and 950 MPa and an elongation at break greater than 10%, characterized in that it obtains a steel in the form of molten metal comprising the following, expressed as a percentage by weight: 0.04% ≤ Carbon ≤ 0.08% 1.2% ≤ Manganese ≤ 1.9% 0.1% ≤ Silicon ≤ 0.3% 0.07% ≤ Titanium ≤ 0.125% 0.05% ≤ Molybdenum ≤ 0.35% 0.15% < Chromium ≤ 0.6% when 0.05% ≤ Molybdenum ≤ 0.11%, or 0.10% ≤ Chromium ≤ 0.6% when 0.11%  Molybdenum ≤ 0.35% Niobium ≤ 0.045% 0.005% ≤ Aluminum ≤ 0.1% 0.002% ≤ Nitrogen ≤ 0.01% Sulfur ≤ 0.004% Phosphorus < 0.020% And optionally: 0.001% ≤ Vanadium ≤ 0.2% The rest consists of iron and unavoidable impurities. and that a vacuum or SiCa coating is carried out during which, in the second case, the composition additionally includes the following, expressed as a percentage by weight: 0.0005% ≤ Calcium ≤ 0.005%, The amounts of titanium [Ti] and nitrogen [N] dissolved in the molten metal satisfy the following relationship, and the steel is cast to obtain a semi-finished solution cast: (%[Ti]) x (%[N]) < 6.10-4%2 This semi-finished product is optionally reheated to a temperature between 1160 and 1300 degrees Celsius, then This cast and semi-finished product is rolled with a final rolling temperature between 880 and 930 degrees Celsius, and the reduction rate of the penultimate stage is less than 0.25, and the reduction rate of the final stage is less than 0.15, and the sum of these two reduction rates is less than 0.37, and the starting rolling temperature in the penultimate stage is less than 960 degrees Celsius to obtain a hot rolled product, then, This hot rolled product is cooled at a rate between 20 and 150 degrees Celsius to obtain hot rolled steel sheet. The method according to the invention may also include the following features, individually or in technically feasible combinations: - Hot rolled steel sheet is coiled at a temperature between 525 and 635 degrees Celsius. - The composition includes the following elements, expressed as a percentage by weight: 0.04% ≤ Carbon ≤ 0.08% 1.2% ≤ Manganese ≤ 1.9% 0.1% ≤ Silicon ≤ 0.3% 0.07% ≤ Titanium ≤ 0.125% 0.05% ≤ Molybdenum ≤ 0.25% 0.16% ≤ chromium ≤ 0.55% when 0.05% ≤ molybdenum ≤ 0.11%, or 0.10% ≤ Chromium ≤ 0.55% when 0.11%  Molybdenum ≤ 0.25% Niobium ≤ 0.045% 0.005% ≤ Aluminum ≤ 0.1% 0.002% ≤ Nitrogen ≤ 0.01% Sulfur ≤ 0.004% Phosphorus < 0.020% The rest consists of iron and unavoidable impurities. - The cooling rate of the hot rolled product is between 50 and 150 °C / s. - The composition of steel includes the following elements, expressed by weight: 0.27% ≤ chromium ≤ 0.52% when 0.05% ≤ molybdenum ≤ 0.11%, or 0.10% ≤ Chromium ≤ 0.52% when 0.11%  Molybdenum ≤ 0.25% - The composition of steel includes the following elements, expressed by weight: 0.05% ≤ Molybdenum ≤ 0.18%, and 0.16% ≤ chromium ≤ 0.55% when 0.05% ≤ molybdenum ≤ 0.11%, or 0.10% ≤ Chromium ≤ 0.55% when 0.11%  Molybdenum ≤ 0.18% - The composition of steel includes the following elements, expressed by weight: 0.05% ≤ Carbon ≤ 0.08% 1.4% ≤ Manganese ≤ 1.6% 0.15% ≤ Silicon ≤ 0.3% Niobium ≤ 0.04% 0.01% ≤ Aluminum ≤ 0.07% - The sheet is coiled at a temperature between 580 and exactly 630 degrees Celsius. - The sheet is coiled at a temperature between 530 and 600 degrees Celsius, The sheet is acid-washed, the acid-washed sheet is reheated to a temperature between 600 and 750 °C, the reheated and acid-washed sheet is cooled at a rate between 5 and 20 °C / s, and the resulting sheet is coated with zinc in a suitable zinc bath. - The sheet is coiled in adjacent turns with a coiling tension equivalent to 3 metric tons-force. Other features and advantages of the invention will be clearly apparent from the description provided and by way of non-limiting examples with reference to the accompanying drawings. In these drawings: - Figure 1 is a graph showing the results in terms of oxidation in the coil core of sheets according to the invention and sheets of industrial records at a temperature of 590 degrees Celsius, which have different levels of chromium and molybdenum, - Figure 2 is a schematic view of the surface of a sheet, shown in cross section, showing the distribution of surface defects resulting from oxidation on a coiled and pickled sheet, taking into account the definition of the allowable oxidation criterion, Figure 3 is a graph showing the trend of the measured yield strength in the rolling direction as a function of the effective titanium content of sheets according to the invention in which the nitrogen and titanium contents vary, Figure 4 is a graph showing the trend of yield stress in the direction transverse to the rolling direction as a function of the effective titanium content in sheets according to the invention in which the nitrogen and titanium levels vary, Figure 5 is a graph showing the trend of maximum tensile strength in the rolling direction as a function of effective titanium content in sheets according to the invention in which the nitrogen and titanium contents vary. Figure 6 is a graph showing the trend of tensile strength in a direction transverse to the rolling direction as a function of the effective titanium content in sheets according to the invention in which the nitrogen and titanium contents vary. Figure 7 is a scanning electron microscope image showing the surface condition in cross-section of a sheet after pickling, the composition of which is beyond the scope of the present invention and does not meet the oxidation criterion. Figure 8 is a scanning electron microscope image showing the surface condition in cross-section of a sheet after pickling in accordance with the present invention and meeting the oxidation criterion. Figure 9 is an image taken with a scanning electron microscope that shows the surface condition in a cross-section of a sheet after pickling, whose composition is different from the composition of the sheet shown in Figure 8. This case also does not fulfill the oxidation criterion and Figure 10 is an image taken with a scanning electron microscope showing the microstructure of a sheet according to the invention. The inventors have discovered that the surface defects present in some coiled sheets at high temperatures, particularly above 570°C, are mainly located on the surface of the coil core. In this region, the turns are in contact with each other and the partial pressure of oxygen is such that only elements that are more oxidizable than iron, such as silicon, manganese and chromium, are still able to oxidize in contact with oxygen atoms. The iron-oxygen phase diagram at 1 atmosphere shows that the iron oxide wurstite, which forms at high temperatures, is no longer stable above 570°C and decomposes in a thermodynamic equilibrium into two other phases: hematite and magnetite, and one of the products of this reaction is oxygen. The inventors thus realized that the conditions in the coil core were such that the oxygen thus liberated combined with elements that were more oxidizable than iron, especially manganese, silicon, chromium, and aluminum, present on the surface of the sheet. Naturally, the grain boundary of the final microstructure formed a short-circuit of diffusion for these elements compared to uniform diffusion in the matrix. The result was a more pronounced and deeper oxidation at the grain boundary level. During the pickling operation, in order to remove the deposit layer, the oxides that have formed in this way are also removed, making room for defects (discontinuities) that are generally approximately 3 to 5 µm vertical to the sheet surface. Although these defects do not have a significant negative effect on the fatigue performance of a sheet that has not undergone deformation, this is no longer true if the sheet is deformed, especially in the region located at the lower and inner surface of a deformation fold, where the defect depth can reach 25 µm. For a coiling temperature of approximately 590°C, these surface defects naturally occur in the coil core and where the sheet surface is exposed to high temperatures for the longest time, especially above 570°C. Thus, the inventors have found a sheet composition that makes it possible to avoid the formation of intergranular oxidation in the coil core and on the surface of the final microstructure grains after pickling, intergranular oxidation occurring at the grain boundaries of the final microstructure. For this purpose, it was determined that the composition of the sheet should include chromium and molybdenum as specified in specific levels. Surprisingly, the inventors proved that sheets of this type do not show the surface defects mentioned above. According to the invention, the weight percentage of carbon in the sheet is between 0.040% and 0.08%. This range of carbon content allows achieving simultaneously a high elongation at break and a mechanical strength Rm higher than 780 MPa. In addition, the maximum carbon content was determined to be 0.08% by weight, which allows achieving an expansion-cavity ratio Ac% greater than or equal to 45%. Preferably, the carbon content is between 0.05% and 0.07% by weight. According to the invention, the manganese content is between 1.2% and 1.9% by weight. Manganese, when present in this amount, contributes to the strength of the sheet and limits the formation of a central separation band. This element also contributes to achieving an expansion-cavity ratio Ac% greater than or equal to 45%. Preferably, the manganese content is between 1.4% and 1.6% by weight. An aluminum content of between 0.005% and 0.1% ensures deoxidation of the sheet during manufacture. Preferably, the aluminum content is between 0.01% and 0.07%. Titanium is present in the steel sheet of this invention in an amount between 0.07% and 0.125% by weight. Vanadium can be optionally added in an amount between 0.001% and 0.02% by weight. An increase in mechanical strength of up to 250 MPa can be achieved by microstructural refinement and age hardening of the carbonitrides. Furthermore, it is a teaching of the invention that the nitrogen content is between 0.002% and 0.01% by weight. Although the nitrogen content can be extremely low, the range of 0.002% is considered to be such that sheet manufacturing under economically desirable conditions is possible. In the case of niobium, its weight content in the steel composition is less than 0.045%. At a content higher than 0.045% by weight, the recrystallization of austenite is delayed. In this case, the structure will have a significant fraction of elongated grains, which makes it impossible to achieve the specified expansion-cavity ratio Ac%. Preferably, the weight content of niobium is less than 0.04%. The composition according to the invention also contains chromium in an amount between 0.10% and 0.55%. The chromium content at this level allows for an improvement in surface quality. As will be explained below, the chromium content will be defined jointly with the molybdenum content. According to the invention, silicon is present in the chemical composition of the sheet in a weight content of between 0.1 and 0.3%. Silicon delays the precipitation of cementite. In the amounts determined according to the invention, precipitation occurs in very small amounts. That is, in an area concentration of less than 1.5% and in a very thin form. This thinner morphology in the cementite allows for a higher pore-expansion capacity equal to or greater than 45%. Preferably, the weight content of silicon is between 0.15 and 0.3%. The sulfur content of the steel according to the invention should not exceed 0.004% to avoid the formation of sulfides, especially manganese sulfides. The low levels of sulfur and nitrogen present in the steel composition increase its readiness for improved cavity expansion. The phosphorus content of the steel according to the invention is less than 0.020% to improve its cavity expansion and weldability. According to the invention, the composition of the sheet includes chromium and molybdenum in specific concentrations. Reference is made to Tables 1 to 4 as well as Figure 1 for an explanation of the range of chromium and molybdenum content in the sheet composition according to the invention. Tables 1 to 4 show the effect of sheet composition and its production conditions on the yield stress, maximum tensile strength, total elongation at break, cavity expansion, and oxidation index measured in the middle or coil core and strip axis, which concepts of coil core and strip axis will be explained in detail below. The hole-expansion method is described in ISO 16630:2009 as follows: After a hole has been created by cutting in the sheet, a conical tool is used to expand the edges of the hole. It is during this operation that the initial damage occurs near the edges of the hole as it expands and is noticeable. In this case, the damage begins at the second phase particles or at the interface between the different microstructural components of the steel. Thus, the hole-expansion method involves measuring the initial diameter Di of a hole before stamping and then the final diameter Df of the hole following stamping. The measurement is made when cracks are observed that extend through the thickness of the sheet at the edges of the hole and along the entire length. This hole-expansion ability Ac% is then determined according to the following formula: Thus, Ac allows the ability of a steel to withstand stamping on the cutting surface of a hole. According to this method, the final diameter is 10 mm. As previously explained, the purpose of the present invention is to prevent the formation of intergranular oxidation, which is characterized by the creation of discontinuity points on the surface of the coil sheet and acid wash. Therefore, the problem is to obtain a surface where the depth of these defects is sufficiently small so that after the sheet is formed, the increase in the local stress intensity index associated with these defects resulting from forming does not threaten the aging life of the sheet. The inventors have shown that two criteria related to the presence of defects in the coiled sheet must be met in order to achieve the desired fatigue performance. More precisely, these criteria must be met in a region of the coil that is exposed to specific conditions. This region is located in the core of the coil and on the strip axis where the relative oxygen pressure is lower but sufficient to allow elements that are more oxidizable than iron to oxidize. This phenomenon is observed when the sheet is coiled in adjacent turns and at a coiling temperature of at least 3 metric tons-force. The coil core is defined as a region along the length of the coil from which an end region is cut and separated on both sides. The length of each end region is equal to 30% of the total length of the coil. The strip axis is defined in a similar manner and is a region centered in the middle of the strip, in a direction transverse to the rolling direction, and has a width equal to 60% of the strip width. Referring to Figure 2, these two oxidation criteria were measured on a sheet 1 in the middle of the coil and on a strip axis and over an observed length. This observed length was chosen to be representative of the surface state characteristics. The observed length was taken to be 100 μm but could be up to 500 μm or even higher if the aim is to strengthen the requirements in terms of oxidation criteria. The defects due to oxidation 2 are distributed over n oxidation zones Oi of this coiled sheet 1, where i is between 1 and n. Each oxidation zone Oi extends over a length and is distinct from the adjacent zone Oi+1, provided that these two zones Oi, Oi+1 are separated from each other by a zone free from any oxidation defect of at least 3 μm in length. The first criterion [1] The defects of sheet 2 must satisfy the maximum depth criterion, which is:  8 micrometers It follows that in this case the maximum depth of a defect due to oxidation 2 on any oxidation region is Oi. The second criterion [2], which must be fulfilled by defect 2 in sheet 1, is an average depth criterion, which expresses the presence of more or less extensive oxidation zones over the observed length. This second criterion is defined by the following formula: by x li 2.5 μm where the average depth of defects due to oxidation on an oxidation zone is Oi. In Tables 1 to 4 and Figure 1, the surface oxidation results are shown as follows:  Zero or very low oxidation: Criteria [1] and [2] are met  Low oxidation: Criteria are met  Severe oxidation: Criteria are not met. Zero or very low oxidation allows achieving excellent fatigue strength even on parts subjected to high deformation. This means showing an equivalent plastic deformation rate of up to 39%. The equivalent plastic deformation rate at any point in the deformed part is determined based on the principal deformations ε1 and ε2 and by the following formula: Table 1 shows the results obtained for compounds that are not included in the steel framework according to this invention. Table a2 shows the compositions of sheets according to the invention and Table b2 shows the results obtained for the compositions of sheets of Table a2, except in Example 5, where there is no intention to coil and coat them at a temperature of 590°C. Table 3 shows the results obtained for sheet compositions according to the invention, which are also not coated and not coiled at temperatures between 526 and 625 degrees Celsius. Table 4 shows the results obtained for sheet compositions according to the invention, which sheet is galvanized and coiled at a temperature varying between 535 and 585 degrees Celsius. Counterexamples 1 and 11 and Table 1 show that when the chromium and molybdenum contents do not meet the requirements of the invention, the oxidation criteria are not met. Counterexamples 5, 6, 7, and 9 show that in the presence of chromium without molybdenum, oxidation still does not satisfy the criterion. Counterexample 9 also shows that the addition of nickel does not lead to satisfactory results in terms of the oxidation criterion. Conversely, Example 4 shows that in the presence of molybdenum and a very low chromium content, oxidation does not achieve the predefined criterion level. Finally, counterexamples 2, 3, 8 and 11 show that the respective chromium and molybdenum contents should be sufficient. Table B2 shows the results obtained for a sheet composition containing chromium and molybdenum at respective levels between 0.15% and 0.55% for chromium and 0.05% and 0.32% for molybdenum. Table 3 shows the results obtained for a sheet composition containing chromium and molybdenum at respective contents between 0.030% and 0.32% for chromium and between 0.15% and 0.17% for molybdenum. Table 4 shows the results for a sheet composition containing chromium and molybdenum at contents between 0.31% and 0.32% for chromium and between 0.15% and 0.16% for molybdenum. Each of the examples in Tables 2, 3 and 4 fulfills the oxidation criteria as described above. Figure 7 shows the presence of surface defects for sheet 9, which does not meet the oxidation criteria described above, and its composition includes 0.3% chromium and 0.02% molybdenum. Figures 8 and 9 show the surface conditions on two sheets 10 and 11 that meet the oxidation criterion, and their composition includes 0.3% chromium and 0.093% molybdenum in Figure 8 and 0.3% chromium and 0.15% molybdenum in Figure 9. It should be noted that the sheets subject to the results shown in Tables 2 to 4 were coiled in adjacent turns with a minimum coil tension of 3 metric tons-force. Figure 1 shows the test points obtained for the counterexamples and examples at a coiling temperature of 590°C. More precisely, test points 3 correspond to the counterexamples in Table 1, test points a4 correspond to the examples in Tables a2 and b2 where surface oxidation is low, and test points b4 correspond to the examples in Tables a2 and b2 where surface oxidation is zero or very low. It should be noted that the relative overlap of the two test points in molybdenum is 0.10%. A test point 3 is associated with Counterexample 11 where the exact value of the chromium content is 0.150 and a second test point a4 is associated with Example 11 where the exact value of the chromium content is 0.152. Thus, in view of the above information, the teaching of the present invention is that the composition of the sheet according to the invention comprises chromium and molybdenum, the chromium content, when the molybdenum content is between 0.05% and 0.11%, is narrowly higher than 0.15% and less than or equal to 0.6% by weight. Also, the chromium content, when the molybdenum content is narrowly higher than 0.11% and less than or equal to 0.35%, is between 0.10% and 0.6% by weight. The molybdenum content is thus between 0.05% and 0.35%, subject to the chromium content as described above. Preferably, when the molybdenum weight content is between 0.05% and 0.11%, the chromium content is between 0.16% and 0.55% by weight, and when the molybdenum weight content is between 0.11% and 0.25%, the chromium content is between 0.10% and 0.55% by weight. It is more preferred that the chromium weight content is between 0.27% and 0.52% and the molybdenum weight content is between 0.05% and 0.18%. The microstructure of the sheet according to the invention comprises granular bainite. Granular bainite is distinct from upper and lower bainite. For the definition of granular bainite, see the following article: Characterization and quantification of complex bainite microstructures in high and ultra-high strength steels – in the following publication: Materials Science Forum, Vol. 500-501, pp. 387-394; November 2005 According to the aforementioned article, the granular bainite that constitutes the microstructure of the sheet according to the invention has a high proportion of highly heterogeneous adjacent grains and irregular morphology in the grains. The percentage of granular bainite area is higher than 70%. In addition, ferrite is present in a percentage area that does not exceed 20%. The possible excess is formed by lower bainite, martensite and retained austenite, the total content of martensite and retained austenite is and the total content of martensite and retained austenite is less than 5%. Figure 10 shows the microstructure of a sheet according to the invention, which also includes granular bainite 12, islands of martensite and austenite 13 and of ferrite 14. According to the invention, it has been found that a criterion that must be considered in yield stress and maximum tensile strength is a criterion called effective titanium. Assuming that titanium precipitation occurs in the form of nitride and considering the stoichiometric ratio of these two elements in titanium nitride, the effective titanium Tieff represents the amount of excess titanium that is likely to precipitate as carbides. Thus, the effective titanium is defined by the following formula: Tieff= Ti - 3.42 x N Where Ti is the titanium content expressed by weight and N is the nitrogen content expressed by weight. Tables 2 to 4 show the effective titanium indices for each composition tested. Figures 3 to 6 show the results obtained for the elastic limit and ultimate tensile strength, respectively, as a function of the effective titanium content for various compositions in which the titanium and nitrogen pairs are varied. Figures 3 and 5 show these properties in the direction of sheet rolling, and Figures 4 and 6 show these properties in the direction transverse to sheet rolling. In Figures 3 to 6, experimental points 5, a5 are shown by solid circles and correspond to a composition for which the titanium content varies between 0.071% and 0.076% and the nitrogen content varies between 0.0070% and 0.0090%. Experimental points 6, a6 are shown by solid diamonds and correspond to a composition in which the titanium content varies between 0.087% and 0.091% and the nitrogen content varies between 0.0060% and 0.0084%. Experimental points 7, a7 are shown by solid triangles and correspond to a composition in which the titanium content varies between 0.088% and 0.092% and the nitrogen content varies between 0.0073% and 0.0081%. Experimental points 8,a8, shown by solid squares, correspond to a composition in which the titanium content varies between 0.098% and 0.104% and the nitrogen content varies between 0.0048% and 0.0070%. Regarding these images, it is clear that effective titanium should be considered. More precisely, in the rolling direction (Figures 3 and 5), the yield stress and ultimate tensile strength criteria are met for an effective titanium content varying between 0.055% and 0.095%. In the direction transverse to the rolling direction (Figures 4 and 6), the yield stress and ultimate tensile strength characteristics are met for an effective titanium content varying between 0.040% and 0.070%. Thus, the invention teaches that the composition can contain an effective titanium content varying between 0.040% and 0.095%, and preferably between 0.055% and 0.070%, and in this case the criteria are met both in the rolling direction and in the direction transverse thereto. The advantage of including effective titanium is particularly in the ability to take advantage of the high nitrogen content to avoid limiting the content of this element, which is a limiting factor for steel finishing. The method of manufacturing a steel sheet, as described in the previous lines, includes the following steps: Providing steel in molten form with the following composition, expressed as a percentage by weight: 0.04% ≤ Carbon ≤ 0.08% 1.2% ≤ Manganese ≤ 1.9% 0.1% ≤ Silicon ≤ 0.3% 0.07% ≤ Titanium ≤ 0.125% 0.05% ≤ Molybdenum ≤ 0.35% 0.15% < Chromium ≤ 0.6% when 0.05% ≤ Molybdenum ≤ 0.11%, or 0.10% ≤ Chromium ≤ 0.6% when 0.11%  Molybdenum ≤ 0.35% Niobium ≤ 0.045% 0.005% ≤ Aluminum ≤ 0.1% 0.002% ≤ Nitrogen ≤ 0.01% Sulfur ≤ 0.004% Phosphorus < 0.020 Optionally: 0.001% ≤ Vanadium ≤ 0.2% The rest consists of iron and unavoidable impurities. Titanium [Ti] is added to the molten metal containing dissolved nitrogen [N] in such a way that the amounts of titanium [Ti] and nitrogen [N] dissolved in the molten metal satisfy the following relationship: %[Ti] %[N] < 6.10-4%2 #x200fThe molten metal is then either subjected to vacuum polishing or to silicon calcium polishing (#x200f#x200eSiCa#x200e#x200f), in which case, according to the teachings of the invention, the composition has a calcium content by weight of #x200f as follows: 0.0005  Calcium  0.005% Under these conditions, titanium nitrides do not precipitate prematurely and in a crude form in the molten metal, which has the effect of reducing the ability of the cavity to expand. Titanium precipitation occurs at a lower temperature and in the form of carbonitrides that are uniformly distributed. This fine precipitate contributes to hardening and refinement of the microstructure. The steel is then cast to obtain a semi-finished cast product. This is preferably done by continuous casting, and more preferably by casting between cylinders rotating in opposite directions to obtain a semi-finished cast product in the form of thin slabs or strips. Such casting methods reduce the size of the precipitates, which is beneficial for the expansion of the cavity in the product obtained in the final state. The resulting semi-finished product is then reheated to a temperature between 1160 and 1300 °C. At temperatures below 1160 °C, the specified mechanical tensile strength of 780 MPa is not achieved. Naturally, in the case of direct casting of thin plates, the hot rolling step of the semi-finished products, which starts at a temperature above 1160 °C, can be carried out immediately after casting, i.e. without cooling the semi-finished product to ambient temperature and thus there is no need for a reheating step. This semi-finished cast product was then hot rolled at a final rolling temperature between 880 and 930 °C, with a reduction rate in the penultimate stage of less than 0.25 and a reduction rate in the final stage of less than 0.15. The sum of the two reduction rates was less than 0.37 and the starting rolling temperature in the penultimate stage was less than 960 °C to obtain a hot rolled product. Thus, during the final two stages, rolling is carried out at a temperature below the non-recrystallization temperature, which prevents recrystallization of austenite. This requirement is imposed to avoid excessive deformation of austenite during these two final stages. These conditions enable the creation of grains with maximum concentricity to meet the requirements for the expansion-cavity ratio Ac%. After rolling, the hot rolled product is cooled at a rate between 20 and 150 °C / s, preferably between 50 and 150 °C / s, to obtain hot rolled steel sheet. Finally, the resulting sheet is coiled at a temperature between 525 and 635 degrees Celsius. In the case of the manufacture of an uncoated steel and referring to Tables 2 and 3, the coiling temperature will be between 525 and 635 °C in order to make the deposit denser and to obtain the maximum possible hardness, which will allow to obtain a mechanical tensile strength greater than 780 MPa in the longitudinal and transverse directions. According to the results presented in these tables, these coiling temperatures make it possible to obtain a sheet in which the oxidation criterion is fulfilled. Referring to Table 3, it is noteworthy that increasing the coiling temperature (Examples 26 and 28) leads to the generation of defects due to oxidation that does not occur at lower coiling temperatures. However, the sheet composition according to the invention allows the sheet to be coiled at higher temperatures while still meeting the oxidation criterion. In the case of the manufacture of a sheet to be subjected to galvanization, and with reference to Table 4, regardless of whether the desired direction of the properties is in the rolling direction or in the cross direction, and to compensate for the additional precipitation that occurs during the reheating associated with the galvanization operation, the coiling temperature will be between 530 and 600 ° C. According to the results presented in this table, these coiling temperatures make it possible to obtain a sheet in which the oxidation criterion is met. In the second case, the coiled sheet is then acid-pickled according to conventional known methods, then reheated at a temperature between 550 and 750°C. The sheet is then cooled at a rate between 5 and 20°C per second and coated with zinc in a suitable zinc bath. All steel sheets according to the invention are rolled with a reduction rate of less than 0.15 in the penultimate rolling stage and a reduction rate of less than 0.07 in the final rolling stage, in which case the final deformation during these two stages is less than 0.37. Thus, at the end of hot rolling, a less deformed austenite is obtained. As a result, the invention makes it possible to provide steel sheets that have high mechanical tensile properties and are suitable for forming by stamping. The stamped parts made from these sheets have high fatigue strength due to the minimal or absence of surface defects after stamping. Chemical composition (in percent) Carbon Manganese Silicon Aluminum Chromium Molybdenum Niobium Titanium Nickel Phosphorus Sulfur Nitrogen Effective Titanium Counterexample 1 0.049 1.64 0.21 0.03 0 0 0.041 0.112 - - 0.003 0.004 0 .097 Counterexample 2 0.062 1.59 0.24 0.08 0.29 0.005 0.031 0.109 - 0.015 0.002 0.007 0.085 Counterexample 3 0.060 1.58 0.23 0.04 0.29 0.026 0.031 0.114 - 0.015 0.001 0.006 0.093 Counterexample 4 0.069 1.86 0.24 0.03 0.003 0.15 0.024 0.102 - 0.020 0.001 0.005 0.085 Counterexample 5 0.053 1.30 0.21 0.04 0.15 0 0.030 0.105 - 0.014 0.002 0.006 0.084 Counterexample 6 0.054 1.63 0.21 0.04 0.30 0 0.031 0.105 - 0.014 0.002 0.006 0.084 Counterexample 7 0.055 1.65 0.24 0.04 0.61 0 0.031 0.080 - 0.017 0.001 0.006 0.059 Counterexample 8 0.067 1.59 0.24 0.04 0.151 0.10 0.028 0.115 - 0.009 0.001 0.006 0.094 Counterexample 9 0.065 1.61 0.24 0.04 0.33 0 0.031 0.123 0.230 0.013 - 0.008 0.095 Counterexample 10 0.053 1.78 0.22 0.02 0 0 0.030 0.105 - 0.012 0.001 0.006 0.084 Counterexample 11 0.050 1.46 0.24 0.04 0.152 0.05 0.030 0.089 - 0.012 0.002 0.008 NA Coiling temperature (°C) Yield stress Re (Mpa) Maximum tensile strength Rm (Mpa) Total elongation at break (%) Expansion-cavity Ac (ISO method) (%) Oxidation criterion in coil core Oxidation criterion guide Example 1 590 816.5 821 14.8 66.47   Zero or very low oxidation criterion met Example 2 590 785 814 17.2 NA   Slight oxidation: criterion met Example 3 590 810 835 16.8 NA   Severe oxidation: criterion not met Example 4 590 NA NA NA NA  Example 5 590 747 778 17.4 53  Example 6 590 768 797 17.5 49  Example 7 590 NA NA NA NA  Counterexample 8 590 854 877 14.3 NA  Counterexample 9 590 829 849 15.9 NA  Counterexample 10 590 764 786 15.5 72  Counterexample 11 590 703 748 16.5 NA . NA: Not determined –1Exact value: 0.150 –2Exact value: 0.150 Table 1: Test conditions and results for conditions not related to the invention. Chemical composition (in percent) Carbon Manganese Silicon Aluminum Chromium Molybdenum Niobium Titanium Phosphorus Sulfur Nitrogen Titanium Effective Example 1 0.06 1.6 0.2 0.06 0.29 0.09 0.031 0.110 0.015 0.002 0.007 0.086 Example 2 0.06 1.6 0.2 0.04 0.29 0.05 0.034 0.115 0.015 0.001 0.006 0.094 Example 3 0.06 1.6 0.2 0.04 0.29 0.11 0.034 0.111 0.015 0.001 0.006 0.090 Example 4 0.06 1.5 0.2 0.06 0.38 0.15 0.026 0.100 0.017 0.001 0.006 0.078 Example 5 0.07 1.5 0.2 0.04 0.30 0.16 0.030 0.100 0.016 0.001 0.005 0.083 Example 6 0.06 1.5 0.3 0.03 0.41 0.11 0.033 0.093 0.017 0.002 0.009 0.063 Example 7 0.06 1.5 0.3 0.03 0.51 0.11 0.033 0.094 0.017 0.002 0.01 0.059 Example 8 0.06 1.5 0.2 0.05 0.28 0.15 0 0.098 0.017 0.001 0.003 0.087 Example 9 0.080 1.61 0.23 0.04 0.15 0.15 0.028 0.113 0.012 0.001 0.006 0.092 Example 10 0.06 1.5 0.21 0.05 0.47 0.15 0.030 0.074 0.015 0.002 0.008 0.047 Example 11 0.05 1.5 0.24 0.04 0.151 0.10 0.030 0.089 0.012 0.002 0.007 0.065 Example 12 0.05 1.5 0.24 0.04 0.15 0.25 0.030 0.094 0.013 0.002 0.008 0.066 Example 13 0.05 1.5 0.24 0.04 0.30 0.25 0.030 0.092 0.012 0.002 0.008 0.064 Example 14 0.05 1.5 0.25 0.04 0.21 0.06 0.033 0.087 0.012 0.001 - 0.063 Example 15 1 0.05 1.5 0.25 0.04 0.21 0.09 0.033 0.087 0.012 0.001 - 0.063 Example 16 0.05 1.5 0.25 0.04 0.21 0.15 0.032 0.088 0.012 0.001 - 0.064 Example 17 0.05 1.5 0.25 0.04 0.21 0.32 0.033 0.089 0.013 0.001 - 0.065 Example 18 2 0.05 1.5 0.25 0.04 0.25 0.15 0.032 0.088 0.012 0.002 0.008 0.060 Example 19 0.05 1.4 0.25 0.03 0.30 0.20 0.032 0.089 0.013 0.002 0.008 0.061 Example 20 0.05 1.5 0.25 0.04 0.55 0.05 0.030 0.089 0.012 0.002 0.009 0.058 Example 21 0.05 1.5 0.25 0.04 0.54 0.11 0.030 0.087 0.012 0.002 0.008 0.059 Example 22 0.05 1.4 0.24 0.03 0.16 0.20 0.030 0.088 0.013 0.002 0.008 0.060 Example 23 0.05 1.4 0.24 0.03 0.19 0.20 0.030 0.088 0.013 0.002 0.008 0.060 Example 24 0.05 1.4 0.24 0.04 0.39 0.24 0.030 0.087 0.012 0.002 0.008 0.059 Example 25 0.05 1.5 0.24 0.04 0.53 0.26 0.030 0.088 0.012 0.002 0.008 0.060. 1Exact value: 0.152 –2Also includes vanadium = 0.0005% Table a2: Compositions of sheets according to the invention Coiling temperature (°C) Yield stress Re (Mpa) Maximum tensile strength Rm (Mpa) Total elongation at break (%) Expansion-cavity Ac (ISO method) (%) Oxidation criterion in coil core Oxidation criterion guide Example 1 590 808 841 15.8 NA   Zero or very low oxidation criterion met Example 2 590 820 848 15.9 NA   Slight oxidation: criterion met Example 3 590 823 854 15 NA   Severe oxidation: criterion not met Example 4 590 792 832 16.5 58  Example 5 595 810 893 13.3 59  *: Estimated value Example 6 590 766 801 15.6 NA  NA: Not determined Example 7 590 761 798 17.8 NA  Example 8 590 787 818 15.2 71  Example 9 590 823* 854 15.9 NA  Example 10 590 796 834 15.2 56  Example 11 590 711 801* 17.1 NA  Example 12 590 768 809 16.9 NA  Example 13 590 781 825 16.2 NA  Example 14 590 721 807* 17.8 NA  Example 15 590 746 781 17.0 NA  Example 16 590 754 787 16.0 NA  Example 17 590 751 788 16.9 NA  Example 18 590 759 793 19.0 NA  Example 19 590 770 805 17.7 NA  Example 20 590 721 814* 16.9 NA  Example 21 590 744 789 17.6 NA  Example 22 590 757 799 16.5 NA  Example 23 590 764 802 17.5 NA  Example 24 590 796 837 16.5 NA  Example 25 590 760 822 15.8 NA . Table b2: Test conditions and results for sheet compositions according to the invention from Table a2, coiled at 590°C and uncoated Table 3: Test conditions and results for sheet compositions according to the invention and without coating, coiled in the temperature range between 526 and 625 degrees Celsius Chemical composition (in percent) C Mn Si Al Cr Mo Nb Ti PSN Tieff Example 26 0.059 1.54 0.23 0.04 0.31 0.16 0.030 0.093 0.013 0.001 0.007 0.067 Example 27 0.060 1.53 0.23 0.04 0.31 0.15 0.030 0.088 0.012 0.001 0.007 0.063 Example 28 0.065 1.48 0.20 0.04 0.31 0.17 0.029 0.101 0.016 0.001 0.007 0.078 Example 29 0.065 1.50 0.21 0.04 0.30 0.16 0.029 0.102 0.016 0.001 0.005 0.085 Example 30 0.064 1.49 0.20 0.04 0.30 0.16 0.030 0.104 0.016 0.001 0.005 0.087 Example 31 0.057 1.52 0.25 0.04 0.32 0.15 0.032 0.087 0.018 0.001 0.009 0.057 Example 32 0.062 1.46 0.22 0.06 0.32 0.16 0.030 0.074 0.015 0.002 0.008 0.047 Coiling temperature (°C) Yield stress Re (Mpa) Maximum tensile strength Rm (Mpa) Total elongation at break (%) Expansion-cavity Ac (ISO method) (%) Oxidation criterion in coil core Oxidation criterion guide Example 26 615 737 836 22.7 72   Zero or very low oxidation criterion met Example 27 585 695 829 15.2 72   Slight oxidation: criterion met Example 28 625 772 852 18.8 55  Example 29 595 802 876 17.7 53  *: Measurement across the rolling direction Example 30 565 752 857 17.4 53  NA: Undetermined Example 31 535 732 846 15.5 NA  Example 32 526 720* 792* 17.3* 71.3 . Table 4: Test conditions and results for sheet compositions according to the invention coiled in the temperature range between 535 and 585 degrees Celsius Chemical composition (in %) Carbon Manganese Silicon Aluminum Chromium Molybdenum Niobium Titanium Phosphorus Sulfur Nitrogen Titanium Effective Example 33 0.06 1.54 0.23 0.04 0.32 0.16 0.029 0.093 0.011 0.001 0.007 0.067 Example 34 0.06 1.54 0.23 0.04 0.31 0.16 0.029 0.093 0.011 0.001 0.007 0.070 Example 35 0.06 1.53 0.23 0.04 0.31 0.16 0.029 0.093 0.012 0.001 0.007 0.069 Example 36 0.06 1.54 0.23 0.03 0.31 0.15 0.030 0.091 0.012 0.001 0.007 0.065 Coiling temperature (°C) Yield stress Re (Mpa) Maximum tensile strength Rm (Mpa) Total elongation at break (%) Expansion-cavity Ac (ISO method) (%) Oxidation criterion in coil core Oxidation criterion guide Example 33 565 805 839 14.9 63   Zero or very low oxidation criterion met Example 34 535 811 850 13.5 48   Slight oxidation: Meets criterion Example 35 540 790 826 13.6 50   Severe oxidation: Fails criterion Example 36 585 807 862 15.8 NA  NA: Not determined

Claims

ANNEX 2 - AMENDED CLAIMS 1. Hot rolled steel sheet with a thickness between 1.5 and 4.5 millimeters, a yield stress at least greater than 680 MPa in the direction transverse to the rolling direction, and less than or equal to 840 MPa, strength between 780 MPa and 950 MPa, elongation at failure greater than 10% and with a hole expansion ratio (Ac) greater than or equal to 45%, the chemical composition of which consists of, expressed by weight: 0.04% ≤ C ≤ 0.08% 1.2% ≤ Mn ≤ 1.9% 0.1% ≤ Si ≤ 0.3% 0.07% ≤ Ti ≤ 0.125% 0.05% ≤ Mo ≤ 0.35% 0.15% < Cr ≤ 0.6% when 0.05% ≤ Mo≤ 0.11% or 0.10% ≤ Cr ≤ 0.6% when 0.11%  Mo ≤ 0.35% Nb ≤ 0.045% 0.005% ≤ Al ≤ 0.1% 0.002% ≤ N ≤ 0.01% S ≤ 0.004% P < 0.020% and optionally 0.001% ≤ V ≤ 0.2% the remainder consisting of iron and unavoidable impurities resulting from processing, the microstructure of which is constituted by granular bainite, the area percentage of which is greater than 70%, and ferrite, the area percentage of which is less than 20%, with the remainder, if any, consisting of lower bainite, martensite and residual austenite, wherein the sum of the martensite and residual austenite contents is less than 5%.

2. Rolled steel sheet according to Claim 1, characterized in that the chemical composition consists of, expressed by weight: 0.04% ≤ C ≤ 0.08% 1.2% ≤ Mn ≤ 1.9% 0.1% ≤ Si ≤ 0.3% 0.07% ≤ Ti ≤ 0.125% 0.05% ≤ Mo ≤ 0.25% 0.16% ≤ Cr ≤ 0.55% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr ≤ 0.55% when 0.11%  Mo ≤ 0.25% Nb ≤ 0.045% 0.005% ≤ Al ≤ 0.1% 0.002% ≤ N ≤ 0.01% S ≤ 0.004% P < 0.020% the remainder constituted by iron and unavoidable impurities originating from processing.

3. Steel sheet according to any of the Claims 1 and 2, characterized in that the composition of the steel includes, expressed by weight: 0.27% ≤ Cr ≤ 0.52% when 0.05% ≤ Mo ≤ 0.11%, or 0.10% ≤ Cr ≤ 0.52% when 0.11%  Mo ≤ 0.25% 4. Steel sheet according to any of the preceding claims, characterized in that the composition of the steel includes, expressed by weight: 0.05% ≤ Mo ≤ 0.18%, and in that 0.16% ≤ Cr ≤ 0.55% when 0.05% ≤ Mo ≤ 0.11%, or 0.10% ≤ Cr ≤ 0.55% when 0.11% Mo ≤ 0.18% 5. Steel sheet according to any of the preceding claims, characterized in that the composition of the steel includes, expressed by weight: 0.05% ≤ C ≤ 0.07% 1.4% ≤ Mn ≤ 1.6% 0.15% ≤ Si ≤ 0.3% Nb ≤ 0.04% 0.01% ≤ Al ≤ 0.07% 6. Steel sheet according to any of the Claims 1 to 3, characterized in that the chemical composition of the steel includes, expressed by weight: 0.040% ≤ Tieff ≤ 0.095% where Tieff = Ti - 3.42 x N, where Ti is the titanium content expressed by weight N is the nitrogen content expressed by weight.

7. Steel sheet according to any of the preceding claims, characterized in that it is coiled and pickled, the coiling operation being carried out at a temperature between 525°C and 635°C followed by a pickling operation, and the depth of the surface defects due to oxidation distributed over n oxidation zones i of the coiled sheet, where i is between 1 and n, and the n oxidation zones extend over an observed lengthsatisfies : - a first maximum depth criterion defined by  8 micrometers with: maximum depth of a defect due to oxidation in the oxidation zone i of this coiled sheet, and - a second average oxidation criterion defined by x li  2.5 micrometers with : average depth of defects due to oxidation over an oxidation zone i, and : length of the oxidation zone i 8. Steel sheet according to Claim 7, characterized in that the observed length of the defects due to oxidation is greater than or equal to 100 micrometers.

9. Steel sheet according to Claim 8, characterized in that the observed length of the defects due to oxidation is greater than or equal to 500 micrometers.

10. Steel sheet according to any of the preceding claims, characterized in that it is coiled in adjacent turns at a minimum coiling tension of 3 metric tons-force.

11. Method for the fabrication of a hot rolled steel sheet with a thickness between 1.5 and 4.5 millimeters, a yield stress at least greater than 680 MPa in the direction transverse to the rolling direction and less than or equal to 840 MPa, strength between 780 MPa and 950 MPa and elongation at failure greater than 10%, characterized in that a steel having the following composition is obtained in the form of liquid metal, wherein the contents are expressed by weight: 0.04% ≤ C ≤ 0.08% 1.2% ≤ Mn ≤ 1.9% 0.1% ≤ Si ≤ 0.3% 0.07% ≤ Ti ≤ 0.125% 0.05% ≤ Mo ≤ 0.35% 0.15% < Cr≤ 0.6% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr≤ 0.6% when 0.11%  Mo ≤ 0.35% Nb ≤ 0.045% 0.005% ≤ Al ≤ 0.1% 0.002% ≤ N ≤ 0.01% S ≤ 0.004% P < 0.020% and optionally 0.001% ≤ V ≤ 0.2% the remainder consisting of iron and unavoidable impurities, and wherein a vacuum or SiCa treatment is carried out, in this latter case, the composition also includes, the contents expressed by weight 0.0005%  Ca  0.005%, the quantities of titanium [Ti] and nitrogen [N] dissolved in the liquid metal satisfying (%[Ti]) x (%[N]) < 6.10-4 %2 , the steel being cast to obtain a cast semi-finished product, this semi-finished product being optionally reheated to a temperature between 1160°C and 1300°C, then this cast semi-finished product is rolled with an end-of-rolling temperature between 880°C and 930°C, the reduction rate of the penultimate pass being less than 0.25, the reduction rate of the final pass being less than 0.15, the sum of these two rates of reduction being less than 0.37 and the start-of-rolling temperature of the penultimate pass being less than 960°C to obtain a hot-rolled product, then this hot rolled product is cooled at a rate between 20 and 150°C / s to obtain a hot rolled steel sheet, and this hot rolled product is coiled to obtain a hot rolled steel sheet.

12. Method according to Claim 11, characterized in that the hot rolled steel sheet is coiled at a temperature between 525 and 635 °C.

13. Method according to any of the Claims 11 and 12, characterized in that the composition consists of, expressed by weight: 0.04% ≤ C ≤ 0.08% 1.2% ≤ Mn ≤ 1.9% 0.1% ≤ Si ≤ 0.3% 0.07% ≤ Ti ≤ 0.125% 0.05% ≤ Mo≤ 0.25% 0.16% ≤ Cr≤ 0.55% when 0.05% ≤ Mo≤ 0.11% or 0.10% ≤ Cr≤ 0.55% when 0.11%  Mo ≤ 0.25% Nb ≤ 0.045% 0.005% ≤ Al ≤ 0.1% 0.002% ≤ N ≤ 0.01% S ≤ 0.004% P < 0.020% the remainder consisting of iron and unavoidable impurities.

14. Method according to any of the Claims 11 to 13, characterized in that the cooling rate of the hot rolled product is between 50 and 150°C / s.

15. Method according to any of the Claims 11 to 14, characterized in that the composition of the steel includes, expressed by weight: 0.27% ≤ Cr ≤ 0.52% when 0.05% ≤ Mo ≤ 0.11%, or 0.10% ≤ Cr ≤ 0.52% when 0.11%  Mo ≤ 0.25% 16. Method according to any of the Claims 11 to 14, characterized in that the composition of the steel includes, expressed by weight: 0.05% ≤ Mo ≤ 0.18%, and in that 0.16% ≤ Cr ≤ 0.55% when 0.05% ≤ Mo ≤ 0.11%, or 0.10% ≤ Cr ≤ 0.55% when 0.11% Mo ≤ 0.18% 17. Method according to any of the Claims 11 to 16, characterized in that the composition of the steel includes, expressed by weight: 0.05% ≤ C ≤ 0.08% 1.4% ≤ Mn ≤ 1.6% 0.15% ≤ Si ≤ 0.3% Nb ≤ 0.04% 0.01% ≤ Al ≤ 0.07% 18. Method according to any of the Claims 11 to 17, characterized in that the sheet is coiled at a temperature between 580 and strictly 630 °C.

19. Fabrication method for a hot rolled steel sheet according to any of the Claims 10 to 17, characterized in that the sheet is coiled at a temperature between 530 and 600°C, and that this sheet is pickled, then the pickled sheet is reheated to a temperature between 600 and 750°C, then the reheated, pickled sheet is cooled at a rate between 5 and 20°C / s, and that the sheet obtained is then coated with zinc in an appropriate zinc bath.

20. Method for the fabrication of a hot rolled steel sheet according to any of the Claims 10 to 19, characterized in that the sheet is coiled in adjacent turns at a minimum coiling tension of 3 metric tons-force. ادعاها 1- Hot-rolled steel sheet with a thickness between 1.5 and 4.5 mm, with a yield stress greater than 680 MPa at least in the direction diagonal to the rolling direction and less than or equal to 840 MPa, strength between 780 MPa and 950 MPa, elongation at break greater than 10% and with a hole expansion ratio (Ac) greater than or equal to 45%, the chemical composition of which contains, expressed by weight: 0.04% ≤ C ≤ 0.08% 1.2% ≤ Mn ≤ 1.9% 0.1% ≤ Si ≤ 0.3% 0.07% ≤ Ti ≤ 0.125% 0.05% ≤ Mo ≤ 0.35% 0.15% < Cr ≤ 0.6% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr ≤ 0.6% when 0.11%  Mo ≤ 0.35% Nb ≤ 0.045% 0.005% ≤ Al ≤ 0.1% 0.002% ≤ N ≤ 0.01% S ≤ 0.004% P < 0.020% and optionally 0.001% ≤ V ≤ 0.2%. The residue consists of iron and unavoidable impurities resulting from processing, the microstructure of which is made up of granular bainite with an area percentage of more than 70% and ferrite with an area percentage of less than 20%, with the residue, if any, consisting of lower bainite, martensite and retained austenite, in which the sum of the contents of martensite and retained austenite is less than 5%. 2- Rolled steel sheet according to claim 1, characterized in that the chemical composition comprises expressed by weight: 0.04% ≤ C ≤ 0.08% 1.2% ≤ Mn ≤ 1.9% 0.1% ≤ Si ≤ 0.3% 0.07% ≤ Ti ≤ 0.125% 0.05% ≤ Mo ≤ 0.25% 0.16% ≤ Cr ≤ 0.55% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr ≤ 0.55% when 0.11%  Mo ≤ 0.25% Nb ≤ 0.045% 0.005% ≤ Al ≤ 0.1% 0.002% ≤ N ≤ 0.01% S ≤ 0.004% P < 0.020%, the remainder being iron and unavoidable impurities resulting from It is processing. 3- Steel sheet according to any one of claims 1 and 2, wherein the steel composition comprises, expressed by weight: 0.27% ≤ Cr ≤ 0.52% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr ≤ 0.52% when 0.11%  Mo ≤ 0.25%. 4- Steel sheet according to any preceding claim, wherein it is specified that the composition of the steel comprises expressed by weight: 0.05% ≤ Mo ≤ 0.18% and that 0.16% ≤ Cr ≤ 0.55% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr ≤ 0.55% when 0.11%  Mo ≤ 0.18%. 5- Steel sheet according to any of the preceding claims, wherein the steel composition comprises, expressed by weight: 0.05% ≤ C ≤ 0.07% 1.4% ≤ Mn ≤ 1.6% 0.15% ≤ Si ≤ 0.3% Nb ≤ 0.04% 0.01% ≤ Al ≤ 0.07%. 6- Steel sheet according to any one of claims 1 to 3, wherein the chemical composition of the steel comprises, expressed by weight: 0.040% ≤ Tieff ≤ 0.095%, where Tieff = Ti - 3.42 x N where Ti is the titanium content expressed by weight. N is the nitrogen content expressed by weight.

7. A steel sheet according to any one of the preceding claims, characterized in that it is rolled and cleaned, wherein the rolling operation is carried out at a temperature between 525°C and 635°C, followed by a cleaning operation, and the depth of surface defects due to oxidation distributed in n oxidation zones i of the rolled sheet, where i is between 1 and n and the n oxidation zones extend over an observed length. - a first maximum depth criterion defined by  8 μm. With: the maximum depth of a defect due to oxidation in oxidation zone i of the rolled sheet, and - a second average oxidation criterion defined by x li  2.5 μm. With: the average depth of defects due to oxidation in an oxidation zone i and ‌‌li: the length of the oxidation zone i 8. The steel sheet according to claim 7, wherein the observed length of the defects due to oxidation is greater than or equal to 100 micrometers. 9- The steel sheet according to claim 8, wherein the observed length of defects due to oxidation is greater than or equal to 500 micrometers.

10. A steel sheet according to any one of the preceding claims, wherein it is wound in close coils at a minimum winding tension of 3 metric tons. 11- A method for manufacturing a hot-rolled steel sheet with a thickness between 1.5 and 4.5 mm, with a yield stress greater than 680 MPa at least in the direction diagonal to the rolling direction and less than or equal to 840 MPa, a strength between 780 MPa and 950 MPa and an elongation at break greater than 10%, characterized in that a steel having the following composition is obtained in the form of a liquid metal, in which the contents are expressed by weight: 0.04% ≤ C ≤ 0.08% 1.2% ≤ Mn ≤ 1.9% 0.1% ≤ Si ≤ 0.3% 0.07% ≤ Ti ≤ 0.125% 0.05% ≤ Mo ≤ 0.35% 0.15% < Cr≤ 0.6% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr≤ 0.6% when 0.11%  Mo ≤ 0.35% Nb ≤ 0.045% 0.005% ≤ Al ≤ 0.1% 0.002% ≤ N ≤ 0.01% S ≤ 0.004% P < 0.020% and optionally 0.001% ≤ V ≤ 0.2% The remainder consists of iron and unavoidable impurities and where a vacuum or SiCa treatment is carried out, in the latter case the composition also includes contents expressed by weight of 0.0005% ≤ Ca ≤ 0.005% where the amounts of titanium [Ti] and nitrogen [N] dissolved in the liquid metal (%[Ti]) x (%[N]) < 6.10-4%2, in which the steel is formed to obtain a semi-finished shaped product, in which this semi-finished product is optionally reheated to a temperature between 1160°C and 1300°C, then this semi-finished product is rolled with a final rolling temperature between 880°C and 930°C, the reduction in the pre-final pass being less than 0.25.The final pass reduction is less than 0.15, the sum of these two reduction values ​​is less than 0.37, and the starting temperature of the penultimate pass rolling is less than 960°C to obtain a hot rolled product, then the hot rolled product is cooled at a rate between 20 and 150°C / s to obtain a hot rolled steel sheet.

12. The method according to claim 11, wherein the hot rolled steel sheet is coiled at a temperature between 525 and 635 degrees Celsius. 13- The method according to any of claims 11 and 12, wherein the composition comprises expressed by weight: 0.04% ≤ C ≤ 0.08% 1.2% ≤ Mn ≤ 1.9% 0.1% ≤ Si ≤ 0.3% 0.07% ≤ Ti ≤ 0.125% 0.05% ≤ Mo ≤ 0.25% 0.16% ≤ Cr ≤ 0.55% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr ≤ 0.55% when 0.11%  Mo ≤ 0.25% Nb ≤ 0.045% 0.005% ≤ Al ≤ 0.1% 0.002% ≤ N ≤ 0.01% S ≤ 0.004% P < 0.020%. The residue consists of iron and unavoidable impurities.

14. The method according to any one of claims 11 to 13, wherein the cooling rate of the heated rolled product is between 50 and 150 degrees Celsius per second.

15. The method according to any one of claims 11 to 14, wherein the composition of the steel comprises, expressed by weight, 0.27% ≤ Cr ≤ 0.52% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr ≤ 0.52% when 0.11%  Mo ≤ 0.25%.

16. A method according to any one of claims 11 to 14, wherein the steel composition comprises, expressed by weight: 0.05% ≤ Mo ≤ 0.18% and wherein 0.16% ≤ Cr ≤ 0.55% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr ≤ 0.55% when 0.11%  Mo ≤ 0.18%. 17- The method according to any one of claims 11 to 16, wherein the composition of the steel comprises, expressed by weight: 0.05% ≤ C ≤ 0.08% 1.4% ≤ Mn ≤ 1.6% 0.15% ≤ Si ≤ 0.3% Nb ≤ 0.04% 0.01% ≤ Al ≤ 0.07%.

18. The method according to any one of claims 11 to 17, wherein the steel is coiled at a temperature between 580 and exactly 630 degrees.

19. A method of manufacturing a hot-rolled steel sheet according to any one of claims 10 to 17, wherein the sheet is rolled at a temperature between 530 and 600°C and the steel is cleaned, then the cleaned steel is reheated to a temperature between 600 and 750°C, then the cleaned sheet is reheated, cooled to a temperature between 5 and 20°C, and the sheet obtained is then coated with zinc in a suitable zinc bath.

20. A method for manufacturing a hot-rolled steel sheet according to any one of claims 10 to 19, wherein the sheet is wound in close coils at a minimum winding tension of 3 metric tons.

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