KR20230107855A - Hydrogen embrittlement-resistant coated steel - Google Patents

Abstract

providing a steel substrate; By applying a current density of 15 A/dm ² to 45 A/dm ² for 30 to 300 seconds, pH is 2 to 6 and 100 g/l to 500 g/l NiSO ₄ and 1 g/l to 15 g/l MoS electroplating the steel substrate with an electroplating solution containing ₂ to produce a Ni-MoS ₂ coating layer; and then rinsing and drying the steel substrate to obtain a coated steel substrate.

Description

Hydrogen embrittlement-resistant coated steel

The present invention relates to a steel substrate having hydrogen embrittlement resistance and a manufacturing method thereof, and particularly to a coated steel substrate having good hydrogen embrittlement resistance.

High-strength steels such as dual phase (DP) steel, advanced high strength steel (AHSS), ultra high strength steel (UHSS), or martensitic steel (MS) are characterized by having high tensile strength. Because of these properties, the use of such steels in the manufacture of automobiles has increased in response to the demand placed by the automotive industry to reduce vehicle weight without sacrificing occupant safety, particularly for structural components such as pillars and bumpers. and reinforcing components such as impact beams are required to further increase their strength.

In addition, all of the above steels to be used in automobiles are also required to be resistant to the occurrence of hydrogen induced delayed fracture, commonly known as hydrogen embrittlement resistant steel. Hydrogen embrittlement generally refers to embrittlement caused by hydrogen generated during processing such as electroplating, electrolytic cleaning or during application of the final product in a corrosive environment or high moisture content of the atmosphere. Such hydrogen diffuses into defective areas of the steel sheet, such as dislocations, holes, and grain boundaries, embrittles the defective areas, and deteriorates ductility and rigidity of the steel sheet, causing fracture under static or dynamic stress.

Accordingly, an object of the present invention is to solve this problem by providing a coated steel substrate and method 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% will be.

In a preferred embodiment, the steel substrate may have:

- ultimate tensile strength of at least 900 MPa, preferably greater than 980 MPa,

- Yield strength of at least 700 MPa, preferably greater than 800 MPa.

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

The term "steel substrate" for the purposes of the present invention refers to a hot-rolled steel strip, a cold-rolled steel sheet, a flat steel product, a tailor welded blank, containing at least one of C, Al, Si and Mn as an alloying element and Ni-MoS2 thereon It includes a blank substrate having layers.

The present invention improves the problem of hydrogen embrittlement by coating the steel with a Ni-MoS2 layer having at least 0.3% by weight of MoS2 particles with a thickness of 0.1 micron or more.

Since the Ni-MoS2 layer of the present invention can withstand the welding process, the Ni-MoS2 layer of the present invention can be welded for automobile manufacturing.

For the purpose of understanding the present invention, the method is specifically described herein. According to the present invention the method can be produced by a method consisting of the successive steps mentioned herein:

For demonstration of the present invention, a martensitic steel is taken as a steel of a preferred embodiment to be produced into a cold-rolled steel sheet, demonstrating the advantageous effects of the present invention. The use of martensitic steels should not be considered a limitation of the present invention, and the method of the present invention can be implemented in any steel having at least one of C, Mn, Al and Si as an alloying element.

The coated steel substrate according to the present invention can be produced by any of the following methods. A preferred method consists in providing a semi-finished casting of a steel having a chemical composition according to the invention. Casting can be done as an ingot or in the form of continuous thin slabs or thin strips, with a thickness from about 220 mm for slabs to several tens of millimeters for thin strips.

For example, a slab with a chemical composition of steel is produced by continuous casting, and the slab is subjected to direct light pressure during the continuous casting process to avoid center segregation and to ensure a ratio of local carbon to nominal carbon maintained below 1.10 ( direct soft reduction). The slabs provided by the continuous casting process can be used directly at elevated temperatures after continuous casting or can be initially cooled to room temperature and then reheated for hot rolling.

The temperature of the slab undergoing hot rolling is at least 1000 °C and at least 1280 °C. It is preferable that the temperature of the slab is higher than 1150° C., but below this temperature, an excessive load is applied to the rolling mill, and the temperature of the steel may also be lowered to the ferrite transformation temperature during finish rolling, whereby the transformed ferrite in the steel is attached to the structure. This is because it will be rolled in a contained state. Therefore, it is preferable that the temperature of the slab is sufficiently high so that hot rolling can be completed in the temperature range of Ac3 to Ac3 + 100° C. and the final rolling temperature is maintained above Ac3. Reheating at temperatures above 1280° C. is industrially expensive and should be avoided.

A final rolling temperature range of Ac3 to Ac3 + 100 DEG C is preferred so as to have a structure favorable to recrystallization and rolling. The final rolling pass needs to be carried out at a temperature higher than Ac3, because below this temperature, the steel sheet exhibits a significant decrease in rollability. Then, the steel 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 will be 200° C./s or less.

And, the hot-rolled steel sheet is 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 of such coiling temperature 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.

The hot rolled steel sheet may be subjected to an optional descaling step to remove scale formed during hot rolling prior to the optional hot band annealing. In addition, the hot-rolled steel sheet may undergo selective hot band annealing at a temperature of 400 ° C to 750 ° C for 12 hours or more and 96 hours or less, and the temperature prevents partial transformation of the hot-rolled microstructure to prevent loss of uniformity of the microstructure. kept below 750°C to avoid Then, an optional descaling step of this hot-rolled steel sheet may be performed, for example by pickling the steel sheet. This hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet with a thickness reduction of 35 to 90%. Then, a cold-rolled steel sheet is obtained.

The cold rolled steel is then sent to a continuous annealing cycle for heat treatment, which will impart the required properties and microstructure to the steel of the present invention.

In the annealing of the cold-rolled steel sheet, the cold-rolled steel sheet is heated at a heating rate of more than 2 ℃/s, preferably more than 3 ℃/s to a soaking temperature of Ac3 to Ac3 + 100 ℃, and for composite steel sheet Ac1 and Ac3 are experimentally expanded It is calculated by system research.

The cold-rolled steel sheet is held at the soaking temperature for 10 to 500 seconds to ensure complete recrystallization of the strongly work-hardened initial structure. Then, the cold-rolled steel sheet is cooled to a temperature of less than 550°C, preferably less than 500°C, at a cooling rate of more than 5°C/s, optionally maintaining the cold-rolled steel sheet at 150°C to 500°C for 10 seconds to 1000 seconds, , After imparting the microstructure required for the present invention, the cold-rolled steel sheet is cooled to obtain a cold-rolled steel substrate.

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

Then, the surface of the cold-rolled steel substrate is coated with a Ni-MoS2 layer by electroplating. The Ni-MoS2 layer consists of a nickel matrix in which MoS2 particles are embedded. The MoS2 particles should be greater than 0.3% by weight, preferably greater than 0.4%, and more preferably greater than 0.5% of the total coated layer in order to impart adequate hydrogen embrittlement resistance to the coated steel substrate. In a preferred embodiment, the presence of MoS2 may be limited to 3% for economic reasons.

The Ni-MoS2 layer is formed by electroplating solution containing NiSO ₄ and MoS ₂ (the concentration of NiSO ₄ is 100 g/l to 500 g/l, and the concentration of MoS ₂ is 1 g/l) to obtain hydrogen embrittlement resistance on the cold-rolled steel substrate. to 15 g/l) is electroplated by coating. The concentration of MoS ₂ is maintained between 1 g/l and 15 g/l, because the presence of more than 15 g/l MoS ₂ in the electroplating process reduces the efficiency of Ni deposition due to the enhancement of the hydrogen evolution reaction during electroplating. Because. The concentration range of NiSO ₄ is optimized to obtain sufficient Ni deposition and to embed the MoS ₂ particles within the deposited Ni matrix during electroplating. The preferred concentration of MoS ₂ is 2 g/l to 14 g/l, more preferably 3 g/l to 12 g/l. The preferred concentration of NiSO ₄ is 100 g/l to 400 g/l, more preferably 150 g/l to 400 g/l.

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

Thereafter, the cold-rolled steel substrate is rinsed with any appropriate solvent such as ethanol, and dried using, for example, hot air to obtain a coated steel substrate.

And, the coated steel substrate may optionally be coated by any known industrial process such as electro-galvanizing, JVD and PVD and the like.

Then, an optional post-batch annealing may be performed at a temperature of 150° C. to 300° C. for 30 minutes to 120 hours.

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

Carbon is present from 0.05% to 0.5%. Carbon is an element required to increase the strength of the steel of the present invention by generating low-temperature transformation phases such as martensite and bainite, and moreover, carbon also plays a pivotal role in stabilizing austenite, and thus an element necessary to secure retained austenite. am. Thus, carbon plays two pivotal roles, one to increase strength and the other to retain austenite and impart ductility. However, a carbon content of less than 0.05% will not be able to stabilize the appropriate amount of austenite required for the steel of this invention. On the other hand, at carbon contents above 0.5%, the steel exhibits poor spot weldability, which limits its application to automotive parts.

Manganese is present at 0.2% to 5% in the steel of the present invention. This element is gammagenous. The purpose of adding manganese is to obtain an essentially austenite-containing structure. Manganese is an element that stabilizes austenite at room temperature to obtain retained austenite. An amount of manganese of at least about 0.2% by weight is essential to stabilize the austenite as well as provide strength and hardenability to the steels of this invention. Accordingly, higher manganese percentages, such as 2% or higher, are preferred in the present invention. However, when the manganese content is greater than 5%, it produces the adverse effect of delaying the transformation of austenite during cooling after annealing, thereby delaying the formation of other microstructural components. Besides, a manganese content of more than 5% also deteriorates the weldability of present steels, and the ductility target may not be achieved.

The silicon content of the steel of the present invention is between 0.1% and 2.5%. Since silicon is a component capable of retarding the precipitation of carbides during overaging, the presence of silicon stabilizes austenite at room temperature. In addition, due to the poor solubility of silicon in carbides, it effectively inhibits or retards carbide formation and thus also promotes the formation of low-density carbides in the binitic structure that impart essential mechanical properties such as tensile strength to the steels of the present invention. do. However, a disproportionate content of silicon does not exhibit the above effect and causes problems such as temper brittleness. Therefore, the concentration is controlled within the upper limit of 2.5%.

The content of aluminum is 0.01% to 2%. In the present invention aluminum scavenges the oxygen present in the molten steel to prevent the oxygen from forming a gas phase during the solidification process. Aluminum also fixes the nitrogen in the steel to form aluminum nitride, reducing the grain size. Higher aluminum content, greater than 2%, increases the Ac3 point to high temperatures, reducing productivity. An aluminum content of 0.8% to 1% may be used when high manganese content is added to balance the effect of manganese on transformation point and austenite formation evolution with temperature.

Sulfur is not an essential element and 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 manufacturing cost, it is 0.09% or less. Moreover, the presence of more sulfur in the steel forms sulphides, especially in combination with manganese, and reduces the beneficial effect in the present invention.

The phosphorus content of the steel of the present invention is 0.002% to 0.09%, and phosphorus tends to segregate particularly at grain boundaries or co-segregate with manganese, thereby reducing spot weldability and high temperature ductility. For this reason, the phosphorus content is limited to 0.09%, preferably less than 0.06%.

Nitrogen is limited to 0.09% to avoid material aging and to minimize the precipitation of aluminum nitride during solidification, which is detrimental to the mechanical properties of the steel.

The chromium content of the composite coil of the present steel is 0% to 1%. Chromium is an essential element that provides strength and hardenability to steel, but when used in excess of 1%, it damages the surface finish of the steel. Moreover, a chromium content of less than 1% coarsens the dispersion pattern of carbides in the bainite structure, thus keeping the density of carbides in the bainite low.

Nickel may be added as an optional component in an amount of 0% to 1% to increase the strength and improve the toughness of the steel. A minimum of 0.01% is required to achieve such an effect. However, if the nickel content is more than 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 improve the corrosion resistance of the steel. A minimum of 0.01% is required to achieve this effect. However, if the copper content is greater than 1%, the surface appearance may deteriorate.

Molybdenum is an optional element constituting 0% to 0.5% of the steel of the present invention, and molybdenum plays an effective role in improving the hardenability of the steel. However, since the addition of molybdenum excessively increases the cost of adding an alloying element, its content is limited to 0.4% for economic reasons.

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 will also affect the size of the microstructured elements through precipitation as carbonitrides and by retarding recrystallization during the heating process. Thus, the finer microstructure formed at the end of the holding temperature and consequently after complete annealing will result in hardening of the product. However, niobium contents greater than 0.1% are economically uninteresting since a saturation effect of that effect is observed, meaning that additional amounts of niobium do not result in any strength enhancement of the product.

Titanium is added to the steel of the present invention at 0% to 0.1%, the same as niobium, and is included in the carbonitride to play a role in hardening. However, it also forms titanium nitrides that appear during solidification of cast products. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitrides detrimental to formability. When the titanium content is less than 0.001%, there is no effect on the steel of the present invention.

The calcium content of the steel of the present invention is between 0.001% and 0.005%. Calcium is added to the steel of the present invention as an optional element, especially during inclusion treatment. Calcium contributes to the refining of steel by trapping the harmful sulfur content in globular form and delaying the harmful effects of sulfur.

Vanadium is effective in improving the strength of steel by forming carbides or carbonitrides, and the upper limit is 0.1% from an economic point of view. 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 indicated maximum content level, these elements allow grain refinement during solidification. The remainder of the composition of the steel consists of iron and unavoidable impurities due to processing.

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 micro-components may comprise more than 90% of the microstructure of the coated steel substrate of the present invention. In addition to the microstructures mentioned above, microstructure components such as perlite and cementite may also be present in the coated steel substrate, but limited to a maximum of 10% in total.

yes

The following tests, examples, figurative examples and tables presented herein are to be considered completely non-limiting and for illustrative purposes only, and will show the advantageous features of the present invention.

Steels with different compositions are collected in Table 1 showing two exemplary steel compositions Steel A and Steel B, and Table 2 shows the parameters implemented for the coating NiMoS ₂ . Then, Table 3 shows the microstructure of the steel sheet obtained during the test, and Table 4 shows the evaluation results obtained for hydrogen embrittlement and mechanical properties.

Table 1

Table 2

Table 2 shows the coating parameters implemented in the steel to be coated on the steel in Table 1 to be a hydrogen embrittlement resistant steel. Steel compositions I1 to I6 are suitable for the production of hydrogen embrittlement resistant steels according to the present invention. This table also shows the reference steels designated as R1 to R4 in the table. Before coating both steels, the invention steel and the reference steel were hot rolled at a hot rolling finishing temperature of 890°C, coiled at 620°C, and then cold rolled at a reduction of 60%. The cold rolled steel was annealed at 880°C and then cooled to room temperature to obtain an annealed cold rolled steel sheet, which was coated with a NiMoS ₂ coating according to the conditions described in Table 2 to obtain a hydrogen embrittlement resistant steel.

Table 2 is as follows:

Table 3 illustrates the results of tests performed to clearly demonstrate the inventive features of the method of the present invention, the key parameters of the NiMoS ₂ layer were determined by measuring with SEM cross-sections, and the concentration of MoS ₂ was determined by the GDOES method. has been measured All test microstructures were fully martensitic.

Table 3

Table 4 exemplifies the results of tests performed to prove the mechanical properties, and the hydrogen embrittlement resistance properties are described in "Graphene coating as a protective barrier against hydrogen embrittlement" on pages 11810 to 11817 of International journal of hydrogen energy of 39 (2014). According to the method published in the journal publication titled, the hydrogen embrittlement ratio is measured for the inventive steel and the reference steel.

Results are defined here:

Claims (12)

A method for manufacturing a coated steel substrate comprising the following steps:
providing a steel substrate;
By applying a current density of 15 A/dm ² to 45 A/dm ² for 30 to 300 seconds, pH is 2 to 6 and 100 g/l to 500 g/l NiSO ₄ and 1 g/l to 15 g/l MoS electroplating the steel substrate with an electroplating solution containing ₂ to produce a Ni-MoS ₂ coating layer;
Then, rinsing and drying the steel substrate to obtain a coated steel substrate. According to claim 1,
The method of manufacturing a coated steel substrate, wherein the pH of the electroplating solution is 2 to 5. According to claim 1 or 2,
wherein the concentration of NiSO ₄ in the electroplating solution is 100 g/l to 400 g/l. According to any one of claims 1 to 3,
A method for producing a coated steel substrate, wherein the concentration of MoS ₂ in the electroplating solution is 1 g/l to 15 g/l. According to any one of claims 1 to 4,
A method for producing a coated steel substrate, wherein the steel substrate submitted to the electroplating step is a cold-rolled steel sheet obtained through the following steps:
providing a semi-finished product of steel;
Reheating the semi-finished product to a temperature of 1000 ° C to 1280 ° C;
rolling the semi-finished product in the austenitic range at a hot rolling finishing temperature higher than 850° C. to obtain a hot-rolled steel sheet;
cooling the steel sheet to a coiling temperature of less than 650° C. at an average cooling rate of more than 30° C./s, and coiling the hot-rolled steel sheet;
cooling the hot-rolled steel sheet to room temperature;
Optionally, performing a scale removal process on the hot-rolled steel sheet;
Optionally, annealing the hot-rolled steel sheet at a temperature of 400° C. to 750° C.;
Optionally, performing a descaling step on the hot-rolled steel sheet;
cold-rolling the hot-rolled steel sheet at a reduction ratio of 35 to 90% to obtain a cold-rolled steel sheet;
then, performing annealing by heating the cold-rolled steel sheet to a soaking temperature of Ac1 to Ac3+100° C. at a heating rate of more than 2° C./s and holding it for 10 seconds to 500 seconds;
Then, cooling the steel sheet to a temperature of less than 550°C at a rate of more than 5°C/s to obtain a cold-rolled steel substrate, wherein during the cooling, the cold-rolled steel sheet is 150°C to 500°C for a time of 10 to 1000 seconds. Obtaining the cold-rolled steel substrate, which can be selectively maintained in a temperature range of
Then, acid pickling the cold-rolled steel substrate at a temperature range of 30° C. to 100° C. for 5 seconds to 100 seconds. A coated steel substrate produced according to the method according to claims 1 to 5,
A coated steel substrate, wherein the Ni-MoS ₂ layer has a thickness of at least 0.1 microns and contains at least 0.3% by weight of MoS ₂ particles. According to claim 6,
wherein the Ni-MoS ₂ layer has a thickness of at least 0.2 microns. According to claim 6 or 7,
wherein the Ni-MoS ₂ layer contains at least 0.4% by weight of MoS ₂ particles. According to any one of claims 6 to 8,
The coated steel substrate, wherein the hydrogen embrittlement ratio of the coated steel substrate is less than 30%. According to any one of claims 6 to 9,
The coated steel substrate contains the following elements, expressed in weight percent:
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%;
It is a cold-rolled steel sheet containing, and the following optional 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%;
may contain one or more of
A coated steel substrate, the remainder of which consists of iron and unavoidable impurities due to processing. According to any one of claims 6 to 10,
wherein the substrate has an ultimate tensile strength of 900 MPa or more and a yield strength of 700 MPa or more. Use of a coated steel substrate obtainable according to the method according to any one of claims 1 to 5 or a coated steel substrate according to any one of claims 6 to 11 for the manufacture of structural parts of vehicles.