The invention relates to a steel substrate having hydrogen embrittlement resistance and a method for manufacturing the same and particularly to a coated steel substrate having good resistance against Hydrogen embrittlement.

High strength steels such as dual phase (DP) steels, advanced high strength steels (AHSS), Ultra high strength steels (UHSS) or Martensitic Steel (MS) are characterized by having a high tensile strength. Because of these properties the use of such steels in the manufacture of automobiles has increased in response to the demands placed on the automotive industry to reduce the weight of motor vehicles without sacrificing passenger safety particularly for structural components such as a pillar, and reinforcing components such as a bumper and an impact beam, are required to further increase the strength thereof.

In addition, the all abovementioned steels to be used in automobiles are also required to be resistant to the occurrence of hydrogen induced delayed fracture which is commonly known as hydrogen embrittlement resistance steel. The hydrogen embrittlement generally refers to as the embrittlement caused by hydrogen generated during processing such electroplating, electrolytic cleaning or during application of end product in a corrosive environment or in atmosphere contents high moisture. This hydrogen diffuses into defective areas, such as dislocations, holes and grain boundaries, in the steel sheet, to embrittle the defective areas and cause deterioration in ductility and rigidity of the steel sheet, and thereby causing fracture under that static or dynamic stress.

Hence the purpose of the present invention is to solve these problems by making available a method and a coated steel substrate that is suitable to be used in automobile industry and that has an Hydrogen Embrittlement ratio of less than 30% and preferably less than 25% and more preferably less than 22%.

In a preferred embodiment, the steel substrate can have: an ultimate tensile strength greater than or equal to 900 MPa and preferably above 980, - a yield strength greater than or equal to 700 MPa and preferably above 800 MPa,

Another object of the present invention is also to make available a method for the manufacturing of these substrates that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts.

The term “coated steel substrate” for the purpose of the present invention includes a hot rolled steel strip, cold rolled steel sheet, flat steel product, tailor welded blank, blank substrate containing one or more from C, Al, Si, and Mn as alloying elements and having a Ni-MoS2 layer thereon.

The present invention remedies the problem of Hydrogen Embrittlement by coating the steel with a layer of Ni-MoS2 having at least 0.3% of MoS2 particles by weight percentage with a thickness of the layer equal to or more than 0.1 micron.

The Ni-MoS2 layer of the present invention is able to withstand welding process so that the Ni-MoS2 layer of the present invention can be welded for manufacturing of automobiles.

The method is specifically explained herein for the appreciation of the invention. The method can be according to the invention can be produced by the method consists of successive steps mentioned herein:

For the purpose of demonstration of the present invention martensitic steel is taken as a preferred embodiment steel which will be manufactured into a cold rolled steel sheet to demonstrate the beneficial effects of the present invention. The use of martensitic steel must not be considered as a limitation of the present invention and method of present invention can be implemented on any steel having any one or more, C, Mn, Al and Si as it is alloying element. A coated steel substrate according to the invention can be produced by any following method. A preferred method consists in providing a semi-finished casting of steel with a chemical composition of the according to the invention. The casting can be done either into ingots or continuously in form of thin slabs or thin strips, i.e. with a thickness ranging from approximately 220mm for slabs up to several tens of millimeters for thin strip.

For example, a slab having the chemical composition of the steel is manufactured by continuous casting wherein the slab optionally underwent the direct soft reduction during the continuous casting process to avoid central segregation and to ensure a ratio of local Carbon to nominal Carbon kept below 1.10. The slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling.

The temperature of the slab, which is subjected to hot rolling, is at least 1000° C and at least 1280°C. It is preferred to have the temperature of the slab more than 1 150° C, as below this temperature excessive load is imposed on a rolling mill and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed Ferrite contained in the structure. Therefore, the temperature of the slab is preferably sufficiently high so that hot rolling can be completed in the temperature range of Ac3 to Ac3+100°C and final rolling temperature remains above Ac3. Reheating at temperatures above 1280°C must be avoided because they are industrially expensive.

A final rolling temperature range from Ac3 to Ac3+100°C is preferred to have a structure that is favorable to recrystallization and rolling. It is necessary to have final rolling pass to be performed at a temperature greater than 850°C, because below this temperature the steel sheet exhibits a significant drop in rollability. The sheet obtained in this manner is then cooled at a cooling rate above 30°C/s to the coiling temperature which below 650°C . Preferably, the cooling rate will be less than or equal to 200° C/s. The hot rolled steel sheet is then coiled at a coiling temperature below 650°C to avoid ovalization and preferably below 625°C to avoid scale formation. The preferred range for such coiling temperature is from 400°C to 625°C. The coiled hot rolled steel sheet is cooled down to room temperature before subjecting it to optional hot band annealing.

The hot rolled steel sheet may be subjected to an optional scale removal step to remove the scale formed during the hot rolling before optional hot band annealing. The hot rolled sheet may then have subjected to an optional Hot Band Annealing at temperatures from 400°C to 750°C for at least 12 hours and not more than 96 hours, the temperature remaining below 750°C to avoid transforming partially the hot-rolled microstructure and, therefore, losing the microstructure homogeneity. Thereafter, an optional scale removal step of this hot rolled steel sheet may performed through, for example, pickling of such sheet. This hot rolled steel sheet is subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction from 35 to 90%. The cold rolled steel sheet is then obtained.

Thereafter the cold rolled steel is sent to continuous annealing cycle for heat treatment which will impart the steel of present invention with requisite properties and microstructure.

In annealing of the cold rolled steel sheet, the cold rolled steel sheet is heated at a heating rate which is greater than 2°C/s and preferably greater than 3°C/s, to a soaking temperature from Ac1 to Ac3+100° C wherein Ac1 and Ac3 for the composite steel sheet is calculated by experimental dilatometer study.

The cold rolled steel sheet is held at the soaking temperature during 10 seconds to 500 seconds to ensure a complete recrystallization of the strongly work hardened initial structure. The cold rolled steel sheet is then cooled at a cooling rate greater than 5°C/s to a temperature less than 550°C and preferably less than 500°C and optionally holding the cold rolled steel sheet during 10 seconds to 1000 seconds from 150°C to 500°C to impart the requisite microstructure to the present invention, then cool the cold rolled steel sheet to obtain cold rolled steel substrate. Then the cold rolled steel substrate is dipped in an acidic pickling solution during 5 seconds to 100seconds at a temperature range from 30°C to 100°C to activate the surface for electroplating.

Ni-MoS2 layer is then coated by electroplating on the surface of the cold rolled steel substrate. Ni-MoS2 layer is made of a Nickel matrix in which the MoS2 particles are embedded. The MoS2 particles must be more than 0.3% by weight percentage of the total coated layer to impart the coated steel substrate with adequate hydrogen embrittlement resistance and preferably 0.4% or more and more preferably equal to or more than 0.5%. In a preferred embodiment, the presence of MoS2 may be restricted to 3% due to the economic reasons.

Ni-MoS2 layer is electroplated by coating an electroplate solution containing NiSO4 and MoS2 wherein the concentration of NiSO4 is from 10Og/l to 500g/l and the concentration of MoS2 is from 1 g/l to 15g/l to obtain a hydrogen embrittlement resistance on the cold rolled steel substrate. The concentration of the MoS2 is kept from 1 g/l to 15g/l because the presence of MoS2 above 15g/l in electroplating process decreases the efficiency of Ni deposition due to enhancement of hydrogen evolution reaction during electroplating. Concentration range of NiSO4 is optimized to obtain enough Ni deposition and embedding the MoS2 particles in the deposited Ni matrix during electroplating. The preferred concentration of the MoS2 is from 2g/l to 14g/l and more preferably from 3g/l to 12g/L The preferred concentration of NiSO4 is from 10Og/l to 400g/l and more preferably from 150g/l to 400g/L

A current density from 15 A/dm ² to 45 A/dm ² is applied during 30 to 300 seconds during electroplating to embed the MoS2 particles with 0.3% or more by weight percentage in the nickel matrix of the Ni-MoS2 layer and to have thickness of at least 0.1 micron for Ni-MoS2 layer. It is preferable to have a layer thickness of more than 0.2 micron and more preferably more than 0.3 micron. If the current density is less than 15 A/dm ² the MoS2 particles with 0.3% or more by weight percentage will not be embedded in the Ni-Matrix, thereby the final layer having Ni-MoS2 will not form. The temperature for electroplating the cold rolled steel substrate is usually maintained from 30°C to 90°C while the pH of the electroplating solution is maintained from 2 to 6. A preferred range for current density during electroplating from 15 A/dm ² to 40 A/dm ² and more preferably from A/dm ² to 38 A/dm ² .The preferred time for electroplating is from 50 to 250 seconds and more preferably from 60 seconds to 200 seconds.

Thereafter, the cold rolled steel substrate is rinsed with any appropriate solvant, like ethanol for instance and dried using, for example, hot air to obtain a coated steel substrate.

The coated steel substrate then may be optionally coated by any of the known industrial processes such as Electro-galvanization, JVD and PVD etc.

Then an optional post batch annealing may be done at a temperature from 150°C to 300°C during 30 minutes to 120 hours.

In a preferred embodiment, the chemical composition of the steel substrate to be used in the method according to the invention is as follows:

Carbon is present in from 0.05% to 0.5%. Carbon is an element necessary for increasing the strength of the Steel of present invention by producing a low- temperature transformation phases such as Martensite, Bainite further Carbon also plays a pivotal role in Austenite stabilization, hence, it is a necessary element for securing Residual Austenite. Therefore, Carbon plays two pivotal roles, one is to increase the strength and another in Retaining Austenite to impart ductility. But Carbon content less than 0.05% will not be able to stabilize Austenite in an adequate amount required by the steel of present invention. On the other hand, at a Carbon content exceeding 0.5%, the steel exhibits poor spot weldability, which limits its application for the automotive parts.

Manganese is present in the steel of present invention from 0.2 % to 5%. This element is gammagenous. The purpose of adding Manganese is essentially to obtain a structure that contains Austenite. Manganese is an element which stabilizes Austenite at room temperature to obtain Residual Austenite. An amount of at least about 0.2% by weight of Manganese is mandatory to provide the strength and hardenability to the Steel of the present invention as well as to stabilize Austenite. Thus, a higher percentage of Manganese is preferred by presented invention such as 2% or more. But when Manganese content is more than 5% it produces adverse effects such as it retards transformation of Austenite during cooling after annealing which retards he formation of other microstructural constituents. In addition, Manganese content of above 5% also deteriorates the weldability of the present steel as well as the ductility targets may not be achieved.

Silicon content of the steel of present invention is from 0.1 % to 2.5%. Silicon is a constituent that can retard the precipitation of carbides during overaging, therefore, due to the presence of Silicon Austenite is stabilized at room temperature. Further due to poor solubility of Silicon in carbide it effectively inhibits or retards the formation of carbides, hence, also promote the formation of low density carbides in Bainitic structure which impart the Steel of present invention with its essential mechanical properties such as tensile strength. However, disproportionate content of Silicon does not produce the mentioned effect and leads to problems such as temper embrittlement. Therefore, the concentration is controlled within an upper limit of 2.5%.

The content of the Aluminum is from 0.01 % to 2%. in the present invention Aluminum removes Oxygen existing in molten steel to prevent Oxygen from forming a gas phase during solidification process. Aluminum also fixes Nitrogen in the steel to form Aluminum nitride so as to reduce the size of the grains. Higher content of Aluminum, above 2%, increases Ac3 point to a high temperature thereby lowering the productivity. Aluminum content from 0.8% to 1 % can be used when high Manganese content is added in order to counterbalance the effect of Manganese on transformation points and Austenite formation evolution with temperature.

Sulfur is not an essential element but may be contained as an impurity in steel and from point of view of the present invention the Sulfur content is preferably as low as possible but is 0.09% or less from the viewpoint of manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese and reduces its beneficial impact on the present invention.

Phosphorus constituent of the Steel of present invention is from 0.002% to 0.09%, Phosphorus reduces the spot weldability and the hot ductility, particularly due to its tendency to segregate at the grain boundaries or co-segregate with Manganese.

For these reasons, its content is limited to 0.09 % and preferably lower than 0.06%.

Nitrogen is limited to 0.09% in order to avoid ageing of material and to minimize the precipitation of Aluminum nitrides during solidification which are detrimental for mechanical properties of the steel.

Chromium content of the composite coil of steel of present invention is from 0% to 1 %. Chromium is an essential element that provide strength and hardening to the steel but when used above 1 % impairs surface finish of steel. Further Chromium content under 1 % coarsen the dispersion pattern of carbide in Bainitic structures, hence, keep the density of Carbide low in Bainite.

Nickel may be added as an optional element in an amount of 0% to 1 % to increase the strength of the steel and to improve its toughness. A minimum of 0.01 % is required to get such effects. However, when its content is above 1 %, Nickel causes ductility deterioration.

Copper may be added as an optional element in an amount of 0% to 1 % to increase the strength of the steel and to improve its corrosion resistance. A minimum of 0.01 % is required to get such effects. However, when its content is above 1 %, it can degrade the surface aspects.

Molybdenum is an optional element that constitutes 0% to 0.5% of the Steel of present invention; Molybdenum plays an effective role in improving hardenability of the steel. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.4%.

Niobium is present in the Steel of present invention from 0% to 0.1% and suitable for forming carbo-nitrides to impart strength of the Steel of present invention by precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbo-nitrides and by retarding the recrystallization during heating process. Thus, finer microstructure formed at the end of the holding temperature and as a consequence after the complete annealing will lead to the hardening of the product. However, Niobium content above 0.1 % is not economically interesting as a saturation effect of its influence is observed this means that additional amount of Niobium does not result in any strength improvement of the product.

Titanium is added to the Steel of present invention from 0 % to 0.1 % same as Niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also forming Titanium-nitrides appearing during solidification of the cast product. The amount of Titanium is so limited to 0.1 % to avoid the formation of coarse Titaniumnitrides detrimental for formability. In case the Titanium content below 0.001 % does not impart any effect on the steel of present invention.

Calcium content in the steel of present invention is from 0.001 % to 0.005%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of the Steel by arresting the detrimental Sulfur content in globular form thereby retarding the harmful effect of Sulfur.

Vanadium is effective in enhancing the strength of steel by forming carbides or carbo-nitrides and the upper limit is 0.1 % from economic points of view. Other elements such as Cerium, Boron, Magnesium or Zirconium can be added individually or in combination in the following proportions: Cerium 1=0.1 %, Boron ^0.003%, Magnesium ^0.010% and Zirconium ^0.010%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. The remainder of the composition of the steel consists of iron and inevitable impurities resulting from processing.

The microstructure of the coated steel substrate may comprise any one or more than one from Residual austenite, martensite, tempered martensite, tempered bainite, ferrite and Bainite. Theses micro-constituents may comprise 90% or more of the microstructure of the coated steel substrate of present invention. In addition to the above-mentioned microstructure, the microstructural components such as pearlite and cementite may also be present in the coated steel substrate but limited upto a maximum of 10%in total.

EXAMPLES

The following tests, examples, figurative exemplification and tables which are 5 presented herein are non-restricting in nature and must be considered for purposes of illustration only and will display the advantageous features of the present invention.

Steel with different compositions is gathered in Table 1 which shows the two example steel composition Steel A and Steel B, wherein the Table 2 showso parameters implemented for the coating NiMoS2. Thereafter Table 3 gathers the microstructures of the steel sheet obtained during the trials and table 4 gathers the result of evaluations of obtained for hydrogen embrittlement and mechanical properties.

Table 1 5

Table 2

Table 2 gathers the coating parameters implemented on steels of table 1 to be0 coated on the steels to become a hydrogen embrittlement resistant steel. The Steel compositions 11 to I6 serve for the manufacture of hydrogen embrittlement resistant steel according to the invention. This table also specifies the reference steel which are designated in table from R1 to R4. Before coating the Steels both Inventive and reference steels were hot rolled with hot rolled finishing temperature of 890°C and5 then coiled at 620°C thereafter cod rolled with a reduction of 60%. The cold rolled steel is annealed at a temperature 880°C and then cooled to room temperature to obtained annealed cold rolled steel sheet which is coated with a coating of NiMoS2 according to the conditions mentioned in table 2 to obtain a hydrogen embrittlement resistant steel.

The table 2 is as follows: I = according to the invention; R = reference; underlined values: not according to the invention.

Table 3 exemplifies the results of the tests conducted for clearly elucidating the inventive feature of the method of the present invention, wherein key parameters of the NiMoS2 layers were determined by measuring with SEM cross section, the Concentration of MoS2 being measured by GDOES method. All trials microstructure was fully martensitic.

Table 3

I = according to the invention; R = reference; underlined values: not according to the invention. Table 4 exemplifies the results of the tests conducted to demonstrate the mechanical properties and the hydrogen embrittlement resistance properties are measured in Hydrogen embrittlement ratio for the inventive and reference steels in accordance with method published in a journal publication titled as “Graphene coating as a protective barrier against hydrogen embrittlement” in the international journal of hydrogen energy of 39(2014) from page number 11810 to 11817.

The results are stipulated herein:

I = according to the invention; R = reference; underlined values: not according to the invention.