Hydrogen embrittlement resistant coated steel

Abstract

To provide a production method of a coated steel base material which is suitable for use in the automobile industry, and has a hydrogen embrittlement ratio less than 30%, and the coated steel base material.SOLUTION: A production method of a coated steel base material comprises the steps of: preparing a steel base material; forming a Ni-MoS2 film layer by electrically plating the steel base material with an electric plating solution having 2 to 6 pH and including 100 g / l to 500 g / l NiSO4 and 1 g / l to 15 g / l MoS2 by applying an electric current with current density of 15 A / dm2 to 45 A / dm2 for a period of 30 to 300 seconds; and washing the steel base material, dehydrating it, and obtaining the coated steel base material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel substrate having resistance to hydrogen embrittlement and a method for producing the same, and more particularly to a coated steel substrate having good resistance to hydrogen embrittlement. [Background technology]

[0002] High-strength steels, such as dual-phase (DP) steels, advanced high-strength steels (AHSS), ultra-high-strength steels (UHSS) or martensitic steels (MS), are characterized by their high tensile strength. Due to these properties, the use of such steels in automobile manufacturing is increasing, especially for structural parts such as pillars, and reinforced parts such as bumpers and impact beams, in response to the demands imposed on the automobile industry to reduce the weight of automobiles without sacrificing passenger safety, which requires further increases in their strength.

[0003] Furthermore, all of the above steels used in automobiles are required to be resistant to delayed fracture caused by hydrogen, commonly known as hydrogen embrittlement-resistant steels. Hydrogen embrittlement generally refers to embrittlement caused by hydrogen generated during processing such as electroplating and electrolytic cleaning, or during application of the final product in a corrosive environment or an atmosphere with a high water content. This hydrogen diffuses into defect areas in the steel sheet, such as dislocations, holes, and grain boundaries, embrittling the defect areas and reducing the ductility and stiffness of the steel sheet, thereby causing fracture under static or dynamic stress. Summary of the Invention [Problem to be solved by the invention]

[0004] It is therefore an object of the present invention to solve these problems by making available a method and a coated steel substrate suitable for use in the automotive industry and having a hydrogen embrittlement ratio of less than 30%, preferably less than 25%, more preferably less than 22%.

[0005] In a preferred embodiment, the steel substrate may have: - an ultimate tensile strength of at least 900 MPa, preferably above 980; - Yield strength of at least 700 MPa, preferably above 800 MPa.

[0006] Another object of the present invention is to make available a method for manufacturing these substrates that is compatible with conventional industrial applications while being stable towards shifts in manufacturing parameters. [Means for solving the problem]

[0007] In the present invention, the term "coated steel substrate" includes hot rolled steel strip, cold rolled steel sheet, flat steel product, tailored blank, blank substrate containing one or more of C, Al, Si, and Mn as alloying elements and having a Ni-MoS2 layer thereon.

[0008] The present invention solves the problem of hydrogen embrittlement by coating steel with a layer of Ni-MoS2 having at least 0.3% by weight of MoS2 particles and a layer thickness of 0.1 microns or more.

[0009] The Ni-MoS2 layer of the present invention can withstand welding processes, such that the Ni-MoS2 layer of the present invention can be welded for automobile manufacturing. DETAILED DESCRIPTION OF THE INVENTION

[0010] This method is specifically described herein for the purpose of understanding the present invention. The method according to the present invention can be produced by a method consisting of the successive steps described herein.

[0011] For purposes of demonstrating the present invention, martensitic steel is considered the embodiment steel that will be manufactured into cold rolled steel sheet to demonstrate the beneficial effects of the present invention. The use of martensitic steel should not be considered a limitation of the present invention, and the method of the present invention can be carried out on any steel having any one or more of C, Mn, Al, and Si as alloying elements.

[0012] The coated steel substrate according to the invention can be produced by any of the following methods. The preferred method consists in providing a semi-finished casting of steel having the chemical composition according to the invention. Casting can be carried out in ingots or continuously in the form of thin slabs or thin strip, i.e., thicknesses ranging from about 220 mm for slabs to up to several tens of mm for thin strip.

[0013] For example, slabs having this steel chemistry are produced by continuous casting, where the slabs are optionally subjected to direct soft reduction during the continuous casting process to avoid center segregation and ensure a local carbon to nominal carbon ratio of less than 1.10. Slabs provided by the continuous casting process can be used directly at high temperature after continuous casting, or they can be first cooled to room temperature and then reheated for hot rolling.

[0014] The temperature of the slab used for hot rolling is at least 1000°C, and preferably at least 1280°C. It is preferable to have the slab temperature exceed 1150°C because, below this temperature, excessive loads are placed on the rolling mill, and furthermore, the temperature of the steel may drop to the ferrite transformation temperature during finish rolling, resulting in the steel being rolled with transformed ferrite contained in the structure. Therefore, the slab temperature is preferably high enough so that hot rolling can be completed in the temperature range of Ac3 to Ac3 + 100°C, and the final rolling temperature remains above Ac3. Reheating at temperatures above 1280°C is industrially expensive and must be avoided.

[0015] A final rolling temperature range of Ac3 to Ac3+100°C is preferred to obtain a structure favorable for recrystallization and rolling. It is necessary to carry out the final rolling pass at a temperature higher than 850°C, because below this temperature the steel sheet shows a significant decrease in rollability. The sheet obtained in this way is cooled to a coiling temperature of less than 650°C at a cooling rate of more than 30°C / s. Preferably, the cooling rate is 200°C / s or less.

[0016] The hot rolled steel sheet is then coiled at a coiling temperature of less than 650°C to avoid ovalization, and preferably less than 625°C to avoid scale formation. A preferred range for such coiling temperatures is 400°C to 625°C. The coiled hot rolled steel sheet is cooled to room temperature before being subjected to an optional hot band annealing.

[0017] The hot-rolled steel sheet may undergo an optional descaling step to remove scale formed during hot rolling before optional hot band annealing. The hot-rolled steel sheet may then be subjected to optional hot band annealing at a temperature of 400°C to 750°C for at least 12 hours and up to 96 hours, with the temperature remaining below 750°C to avoid partially transforming the hot-rolled microstructure and thus losing the homogeneity of the microstructure. An optional descaling step of the hot-rolled steel sheet may then be performed, for example, by pickling the steel sheet. The hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet with a reduction of 35 to 90%. The cold-rolled steel sheet can then be obtained.

[0018] The cold rolled steel is then subjected to a continuous annealing cycle for heat treatment that imparts the properties and microstructure required for the steel of the present invention.

[0019] In the annealing of cold rolled steel sheets, the cold rolled steel sheets are heated to a soaking temperature of Ac1 to Ac3 + 100°C at a heating rate of more than 2°C / s, preferably more than 3°C / s, where Ac1 and Ac3 for the composite steel sheets are calculated by experimental dilatometry studies.

[0020] The cold-rolled steel sheet is held at the soaking temperature for 10 to 500 seconds to ensure complete recrystallization of the initial structure of the severe work hardening. The cold-rolled steel sheet is then cooled at a cooling rate of more than 5°C / s to a temperature below 550°C, preferably below 500°C, and optionally held at 150°C to 500°C for 10 to 1000 seconds to impart the microstructure required for the present invention. The cold-rolled steel sheet is then cooled to obtain the cold-rolled steel sheet substrate.

[0021] The cold rolled steel substrate is then immersed in an acidic pickling solution at a temperature range of 30 to 100°C for a period of 5 to 100 seconds to activate the surface for electroplating.

[0022] The surface of the cold-rolled steel substrate is then coated with a Ni-MoS2 layer by electroplating. The Ni-MoS2 layer is made of a nickel matrix with embedded MoS2 particles. To provide the coated steel substrate with sufficient hydrogen embrittlement resistance, the MoS2 particles must exceed 0.3% by weight of the total coating layer, preferably 0.4% or more, and more preferably 0.5% or more. In a preferred embodiment, the presence of MoS2 may be limited to 3% for economic reasons.

[0023] The Ni-MoS2 layer contains NiSO4 and MoS2, and is electroplated by coating a cold-rolled substrate with an electroplating solution containing NiSO4 at a concentration of 100 g / L to 500 g / L and MoS2 at a concentration of 1 g / L to 15 g / L to achieve hydrogen embrittlement resistance. The MoS2 concentration is maintained between 1 g / L and 15 g / L because the presence of MoS2 in excess of 15 g / L during the electroplating process reduces Ni deposition efficiency due to increased hydrogen evolution during electroplating. The NiSO4 concentration range is optimized to achieve sufficient Ni deposition and embedding of MoS2 particles in the deposited Ni matrix during electroplating. The preferred MoS2 concentration is between 2 g / L and 14 g / L, more preferably between 3 g / L and 12 g / L. The preferred NiSO4 concentration is between 100 g / L and 400 g / L, more preferably between 150 g / L and 400 g / L.

[0024] 15A / dm 2 ~45A / dm 2 is applied for a period of 30 to 300 seconds during electroplating, embedding 0.3 wt. % or more of MoS2 particles in the nickel matrix of the Ni-MoS2 layer, and the Ni-MoS2 layer has a thickness of at least 0.1 microns. It is preferred to have a layer thickness of more than 0.2 microns, more preferably more than 0.3 microns. 2If the concentration is less than 0.3 weight percent, more than 0.3 weight percent of MoS2 particles will not be embedded in the Ni matrix, thereby preventing the formation of a final layer having Ni-MoS2. The temperature at which cold-rolled steel substrates are electroplated is typically maintained between 30 and 90°C, while the pH of the electroplating solution is maintained between 2 and 6. The preferred range of current density during electroplating is 15 A / dm 2 ~40A / dm 2 , more preferably A / dm 2 ~38A / dm 2 The electroplating time is preferably 50 to 250 seconds, and more preferably 60 to 200 seconds.

[0025] The cold rolled steel substrate is then washed with a suitable solvent, such as ethanol, and dried, for example, using hot air, to obtain the coated steel substrate.

[0026] The coated steel substrate can then optionally be coated by any of the known industrial processes such as electrogalvanizing, JVD and PVD.

[0027] An optional post-batch anneal can then be carried out at a temperature of from 150° C. to 300° C. for a period of from 30 minutes to 120 hours.

[0028] In a preferred embodiment, the chemical composition of the steel substrate used in the method according to the invention is as follows:

[0029] Carbon is present in an amount of 0.05% to 0.5%. Carbon is an element necessary for increasing the strength of the steel of the present invention by forming low-temperature transformation phases such as martensite and bainite. Furthermore, carbon also plays an important role in stabilizing austenite, and is therefore an element necessary for ensuring retained austenite. Thus, carbon plays two important roles: one is to increase strength, and the other is to retain austenite to impart ductility. However, if the carbon content is less than 0.05%, it is not possible to stabilize a sufficient amount of austenite required for the steel of the present invention. On the other hand, if the carbon content exceeds 0.5%, the steel will exhibit poor spot weldability, limiting its application to automotive parts.

[0030] Manganese is present in the steel of the present invention at 0.2% to 5%. This element is gamma-generating. The purpose of adding manganese is to obtain a microstructure that primarily contains austenite. Manganese stabilizes austenite at room temperature, resulting in retained austenite. A manganese content of at least about 0.2 wt.% is essential for providing strength and hardenability to the steel of the present invention, as well as stabilizing austenite. Therefore, a manganese content of 2% or more is preferred in the present invention. However, if the manganese content exceeds 5%, manganese has adverse effects, such as delaying the transformation of austenite during cooling after annealing and delaying the formation of other microstructural constituents. Furthermore, if the manganese content exceeds 5%, not only will the ductility target not be achieved, but the weldability of the steel will also be impaired.

[0031] The silicon content of the steel of the present invention is 0.1% to 2.5%. Silicon is a component that can delay the precipitation of carbides during overaging, and therefore, the presence of silicon stabilizes austenite at room temperature. Furthermore, due to the low solubility of silicon in carbides, it effectively inhibits or delays the formation of carbides, thus promoting the formation of low-density carbides in the bainite structure (which impart essential mechanical properties such as tensile strength to the steel of the present invention). However, an unbalanced silicon content does not produce the above effects and leads to problems such as temper embrittlement. Therefore, the concentration is controlled within the upper limit of 2.5%.

[0032] In the present invention, the aluminum content is 0.01% to 2%. Aluminum removes oxygen present in the molten steel and prevents it from forming a gas phase during the solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride, reducing the grain size. A higher aluminum content than 2% raises the Ac3 point, reducing productivity. To counteract the effect of manganese on the transformation point and the onset of austenite formation with temperature, an aluminum content of 0.8% to 1% can be used with a high manganese content.

[0033] Although sulfur is not an essential element, it may be contained as an impurity in steel, and from the viewpoint of the present invention, the sulfur content is preferably as low as possible, but from the viewpoint of production costs, it is 0.09% or less. Furthermore, if a higher sulfur content is present in the steel, sulfur will combine with manganese in particular to form sulfides, thereby reducing its beneficial effect on the present invention.

[0034] The phosphorus constituent of the steel of the present invention is between 0.002% and 0.09%, and phosphorus tends to segregate, particularly at grain boundaries and to co-segregate with manganese, thereby reducing spot weldability and hot rollability. For these reasons, its content is limited to 0.09%, preferably less than 0.06%.

[0035] Nitrogen is limited to 0.09% to avoid deterioration of the material over time and to minimize the precipitation of aluminum nitride during solidification, which is detrimental to the mechanical properties of the steel.

[0036] The chromium content of the steel composite coil of the present invention is 0% to 1%. Chromium is an essential element that provides strength and hardness to the steel, but if used in excess of 1%, it can impair the surface finish of the steel. Furthermore, a chromium content of less than 1% coarsens the carbide distribution pattern in the bainite structure, thereby maintaining a low carbide density in the bainite.

[0037] Nickel can be added as an optional element in amounts between 0% and 1% to increase the strength of steel and improve its toughness. A minimum of 0.01% is needed to achieve such an effect. However, above 1%, nickel causes a deterioration in ductility.

[0038] Copper can be added as an optional element in amounts between 0% and 1% to increase the strength of steel and improve its corrosion resistance. A minimum of 0.01% is required to achieve such an effect. However, if its content exceeds 1%, it may deteriorate the surface morphology.

[0039] Molybdenum is an optional element that constitutes 0% to 0.5% of the steel of the present invention, and it plays an important role in improving the hardenability of the steel. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.4%.

[0040] Niobium is present in the steel of the present invention at 0% to 0.1% and is suitable for forming carbonitrides to impart strength to the steel of the present invention by precipitation hardening. Niobium also influences the size of microstructural components through its precipitation as carbonitrides and by retarding recrystallization during the heating process. Thus, the resulting finer microstructure formed at the end of the holding temperature and after full annealing leads to hardening of the product. However, niobium contents above 0.1% are not economically attractive because a saturation effect of its influence is observed (meaning that additional amounts of niobium do not bring any strength improvement to the product).

[0041] Like niobium, titanium added to the steel of the present invention at 0% to 0.1% contributes to hardening by participating in carbonitrides. However, titanium also forms titanium nitrides that appear during solidification of the cast product. Therefore, the amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitrides that are detrimental to formability. Titanium contents of less than 0.001% have no effect on the steel of the present invention.

[0042] The calcium content of the steel of the present invention is 0.001% to 0.005%. Calcium is added to the steel of the present invention as an optional element, particularly during inclusion treatment. Calcium traps harmful sulfur in a spherical form, suppressing the harmful effects of sulfur, thereby contributing to the refinement of the steel.

[0043] Vanadium is effective in increasing the strength of steel by forming carbides or carbonitrides, and its upper limit is 0.1% from an economical standpoint. Other elements such as cerium, boron, magnesium, or zirconium may be added individually or in combination in the following proportions: cerium ≤ 0.1%, boron ≤ 0.003%, magnesium ≤ 0.010%, and zirconium ≤ 0.010%. Up to the maximum content levels indicated, these elements allow for grain refinement during solidification. The remainder of the steel composition consists of unavoidable impurities resulting from the steel and processing.

[0044] The microstructure of the coated steel substrate may include any one or more of retained austenite, martensite, tempered martensite, tempered bainite, ferrite, and bainite. These microconstituents may comprise 90% or more of the microstructure of the coated steel substrate of the present invention. In addition to the above microstructures, microstructural constituents such as pearlite and cementite may also be present in the coated steel substrate, but are limited to a maximum total of 10%. [Example]

[0045] The following tests, examples, illustrative examples and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, illustrating advantageous features of the present invention.

[0046] The steels with different compositions are summarized in Table 1, showing two example steel compositions, Steel A and Steel B. Table 2 shows the parameters carried out on the coated NiMoS2. Table 3 then summarizes the microstructures of the steel sheets obtained during the tests, and Table 4 summarizes the results of the hydrogen embrittlement and mechanical property evaluations obtained.

[0047] [Table 1]

[0048] Table 2 summarizes the coating parameters carried out on the steels of Table 1 to form hydrogen embrittlement-resistant steels. Steel compositions I1 to I6 are useful for producing hydrogen embrittlement-resistant steels according to the present invention. The table also specifies the reference steels R1 to R4, which are designated in the table. Before coating the steels, both the steels of the present invention and the reference steels were hot-rolled at a hot-rolling finishing temperature of 890°C, then coiled at 620°C, and then cold-rolled with a 60% reduction. The cold-rolled steel sheets were annealed at a temperature of 880°C and then cooled to room temperature to obtain annealed cold-rolled steel sheets coated with a NiMoS2 coating according to the conditions shown in Table 2, resulting in hydrogen embrittlement-resistant steels.

[0049] Table 2 is as follows:

[0050] [Table 2]

[0051] Table 3 illustrates the results of tests carried out to clearly elucidate the inventive features of the method of the present invention, where key parameters of the NiMoS2 layer were measured by SEM cross-section and the MoS2 concentration was measured by GDOES. The microstructure in all tests was fully martensitic.

[0052] [Table 3]

[0053] Table 4 illustrates the results of tests carried out to demonstrate the mechanical properties, in which the hydrogen embrittlement properties are measured in terms of the hydrogen embrittlement rates of the steel of the present invention and the reference steel according to the method published in the international journal Hydrogen Energy, 39 (2014), pages 11810-11817, entitled "Graphene coating as a protective barrier against hydrogen embrittlement".

[0054] The results are defined herein.

[0055] [Table 4] TIFF2025186446000005.tif76166

Claims

1. A method of manufacturing a coated steel substrate comprising the steps of: providing a steel substrate; 15A / dm ２ ~45 A / dm ２ for a period of 30 seconds to 300 seconds to produce a layer of Ni-MoS coating; Thereafter, washing the steel substrate and drying it to obtain a coated steel substrate.

2. The method of claim 1, wherein the pH of the electroplating solution is between 2 and 5.

3. 3. The method according to claim 1, wherein the concentration of NiSO in the electroplating solution is from 100 g / l to 400 g / l.

4. 4. The method according to claim 1, wherein the concentration of MoS2 in the electroplating solution is from 1 g / l to 15 g / l.

5. The method for producing a coated steel substrate according to any one of claims 1 to 4, wherein the steel substrate subjected to the electroplating step is a cold-rolled steel sheet obtained through the following steps: providing a steel semi-finished product; reheating the semi-finished product to a temperature of 1000°C to 1280°C; rolling the semi-finished product in the austenite region so that the hot rolling finishing temperature exceeds 850°C to obtain a hot rolled steel plate; cooling the plate at an average cooling rate of greater than 30°C / s to a coiling temperature of less than 650°C, and coiling the hot-rolled plate; cooling the hot-rolled sheet to room temperature; Optionally, subjecting the hot rolled steel sheet to a descaling step; Optionally, performing annealing on the hot-rolled steel sheet at a temperature of 400°C to 750°C; Optionally, subjecting the hot rolled steel sheet to a descaling step; cold-rolling the hot-rolled steel sheet at a reduction rate of 35% to 90% to obtain a cold-rolled steel sheet; Next, annealing the cold-rolled steel sheet by heating it at a heating rate of more than 2°C / sec to a soaking temperature of Ac1 to Ac3 + 100°C and holding it for a period of 10 seconds to 500 seconds; then cooling the plate at a rate of greater than 5°C / sec to a temperature below 550°C, wherein during said cooling the cold rolled steel plate can optionally be held at a temperature range of 150°C to 500°C for 10 to 1000 seconds, to obtain a cold rolled steel substrate; Next, pickling the cold rolled steel substrate at a temperature ranging from 30°C to 100°C for a period of time ranging from 5 seconds to 100 seconds.

6. 6. A coated steel substrate produced by the method of any one of claims 1 to 5, wherein the Ni-MoS2 layer has a thickness of at least 0.1 microns and contains at least 0.3 weight percent MoS2 particles.

7. The coated steel substrate of claim 6, wherein the Ni-MoS2 layer has a thickness of at least 0.2 microns.

8. The coated steel substrate of claim 6 or 7, wherein the Ni-MoS2 layer comprises at least 0.4 weight percentage of MoS2 particles.

9. A coated steel substrate according to any one of claims 6 to 8, wherein the hydrogen embrittlement rate of such coated steel substrate is less than 30%.

10. The coated steel substrate contains the following elements expressed in weight percentage: 0.05%≦C≦0.5%, 0.2%≦Mn≦5%, 0.1%≦Si≦2.5%, 0.01%≦Al≦2%, 0%≦S≦0.09%, 0.002%≦P≦0.09%, 0%≦N≦0.09%, and any of the following elements: 0%≦Cr≦1%, 0%≦Ni≦1%, 0%≦Cu≦1%, 0%≦Mo≦0.5%, 0%≦Nb≦0.1%, 0%≦Ti≦0.1%, 0%≦V≦0.1%, 0%≦B≦0.003%, 0%≦Mg≦0.010%, 0%≦Zr≦0.010%, 0.001%≦Ca≦0.005%, The coated steel substrate according to any one of claims 6 to 9, wherein the remainder of the composition is cold-rolled steel sheet, which may comprise one or more of the following impurities: iron and unavoidable impurities resulting from processing.

11. The coated steel substrate of any one of claims 6 to 10, having an ultimate tensile strength of 900 MPa or greater and a yield strength of 700 MPa or greater.

12. Use of a coated steel substrate obtainable according to the method of any one of claims 1 to 5 or according to any one of claims 6 to 11 for the manufacture of a structural part for a vehicle.

Images