Method for manufacturing annealed steel sheets

Abstract

The present invention relates to a method for manufacturing a heat-treated steel sheet in which an annealing process is performed according to a temperature curve T over time t in a furnace containing a hydrogen atmosphere, and a target hydrogen content in the steel sheet is specified at the end of the annealing process step.

Description

Technology Field

[0001] The present invention relates to a method for manufacturing an annealed steel sheet in which process parameters are selected to obtain a specified hydrogen value at the end of any step of the annealing process. Background Technology

[0002] Steel sheets consist of crystal grains in which atoms are arranged within a crystal lattice to form the sheet's structure. The spaces between these atoms are called interstitial sites. Since the arrangement of atoms is not perfectly regular, some arrangement defects may occur, which are linear defects known as dislocations.

[0003] During the heat treatment of steel sheets, hydrogen atoms present in the furnace atmosphere can easily penetrate and be absorbed into the steel. In fact, hydrogen can diffuse into the crystal lattice due to its atomic size being the same as the size of the interstitial sites in the crystal lattice. Hydrogen atoms may also gradually diffuse and be trapped inside defects, such as dislocations.

[0004] The introduction and diffusion of hydrogen within steel sheets is one of the mechanisms causing brittleness in steel sheets, which can lead to, for example, crack formation along grain boundaries and / or dislocation gliding planes.

[0005] Due to hydrogen, steel strips can experience a loss of ductility, also known as hydrogen embrittlement.

[0006] Therefore, the object of the present invention is to provide a method for manufacturing an annealed steel sheet in which process parameters are selected to obtain a specified hydrogen value at the end of any step of the annealing process.

[0007] The object of the present invention is achieved by providing a method according to claim 1. The method may also include the features of any one of claims 2 to 7.

[0008] Below, C L The concentration of hydrogen in the interstitial sites of the crystal lattice of the steel sheet, C T represents the concentration of captured hydrogen in the steel plate. Dislocations are the only capture sites considered in the present invention that are homogeneously distributed within the microstructure. Brief explanation of the drawing

[0009] The present invention will be described and illustrated in detail by embodiments without introducing limitations, with reference to the attached drawings: Figure 1 shows temperature curves T0 and T1, and FIG. 2 illustrates cells used in one embodiment of the method of the present invention for showing the microstructure at point F1 of the temperature curve T1, and Figure 3 shows C of Test 1. L , C T and C total = C L + C T Represents the temporal evolution of, Figure 4 shows C of Test 2. L , C T and C total Represents the temporal evolution of, Figure 5 shows C of Test 3. L , C T and C total Represents the temporal evolution of, Figure 6 shows C of Test 4. L , C T and C total Represents the temporal evolution of, Figure 7 shows C of Test 5. L , C T and C total Represents the temporal evolution of, Figure 8 shows C of Test 6. L , C T and C total It represents the time evolution of. Specific details for implementing the invention

[0010] A method for manufacturing an annealed steel sheet in which process parameters are selected to obtain a specified hydrogen value at the end of any step of the annealing process comprises the following consecutive steps:

[0011] The first step of the method according to the present invention is to obtain a target hydrogen content C in the steel sheet at the end of the annealing process step. total-targeted It defines.

[0012] During this annealing, the steel sheet undergoes at least one heating step and one cooling step according to a thermal path. Generally, the heat treatment can be performed in an oxidizing atmosphere, that is, an atmosphere containing an oxidizing gas such as O2, CH4, CO2, or CO₂. Additionally, the heat treatment can be performed in a neutral atmosphere, that is, an atmosphere containing a neutral gas such as N2, Ar, or He. Finally, the heat treatment can also be performed in a reducing atmosphere, that is, an atmosphere containing a reducing gas such as H2 or HNx.

[0013] In a preferred embodiment, the thermal path may also include at least one isothermal holding step, which generally involves a heating step preceding a cooling step. The cooling step may include isothermal holding referred to as an overaging sub-step followed by a subsequent cooling step. Additionally, a hot-dip coating step in a hot metal bath may be used during this thermal path, and since the metal sheet immersed in this hot metal bath will be maintained at the bath temperature during the holding time in this bath, this is another type of isothermal holding.

[0014] Annealing can be, for example, recrystallization annealing, recovery, or tempering, and these heat treatments may follow:

[0015] - Quenching and tempering,

[0016] - Quenching and partitioning.

[0017] The thermal path according to the present invention corresponds to successive temperatures (T), heating and cooling rates, and the time consumed in each section of the annealing process and optional subsequent heat treatments.

[0018] Temperatures of the thermal path can be measured by sensors during annealing and optional subsequent heat treatment, or through calculations performed using software.

[0019] Next, at least two temperature curves T of the annealing step as a function of time t in a furnace having a hydrogen atmosphere n (n is the number of curves) is defined. The amount of hydrogen in the atmosphere is defined. The atmosphere inside the furnace may include H2, N2 and O2.

[0020] The next step of the method according to the present invention is to determine the microstructure of the steel sheet as a function of the thermal path of the annealing process through calculations performed using software, for example, COMSOL®. The microstructure can also be determined thanks to sensors in the furnace capable of measuring the austenite content in the steel, such as X-CAP®.

[0021] Within the framework of the present invention, the evolution of the microstructure occurs instantaneously only at specific points in response to the phase change at a given temperature.

[0022] Next, the solubility of hydrogen C on the surface of the steel plate H The solubility of hydrogen is the suitability for hydrogen to dissolve in the steel sheet. This solubility depends on the temperature, partial pressure of hydrogen, and the phases present in the microstructure of the steel sheet. This can be calculated through the following equations [1] and [2], which will be explained.

[0023] A temperature T of Ac3 or lower, preferably 280K to 1184K n About:

[0024]

[0025] In the first part of the temperature curve where heating occurs, ferrite is the main structure of the steel sheet. Hydrogen solubility C in ferrite. H is calculated appropriately using Equation [1].

[0026] At the end of the temperature curve where cooling occurs, a portion of the austenite formed on Ac3 may be transformed into bainite and / or martensite depending on the steel composition and cooling rate. In that part of the curve, the solubility of hydrogen C in bainite and martensite H It is the same as in ferrite and can also be appropriately obtained using Equation [1].

[0027] A temperature T greater than Ac3, preferably between 1184K and 1667K n About:

[0028]

[0029] In the middle part of the temperature curve where high-temperature retention can occur, austenite is the major phase of the steel sheet, and the solubility of hydrogen in austenite C H It is calculated appropriately using Equation [2].

[0030] In both Equations [1] and [2], C H is expressed as at%, and p H2 ε is the partial pressure of hydrogen inside the furnace, expressed as Pa. These two equations are derived from the publication of Fromm & Jehn "Hydrogen in Elements", (Bull. Alloys Phase Diagrams, 5(3), 323-326 (1984)).

[0031] Solubility of hydrogen on the steel plate surface C H Determining is necessary as an input for the next step of the method according to the present invention, and the hydrogen volume concentration C at the interstitial sites of the crystal lattice. L and volume concentration C of trapped hydrogen in the steel plate T (Both are iron m 3moles of hydrogen per sugar (mol H / m 3 Fe Expressed as ) is calculated by numerical simulation through the following equations:

[0032]

[0033] Now, the different parameters and constants of both Equations [3] and [4] will be explained.

[0034] D L ε is the diffusion coefficient within the crystal lattice, expressed in m² / s, and depends on the temperature and phases present in the steel sheet at that temperature. This diffusion coefficient represents the suitability of hydrogen to diffuse within the material. The higher the coefficient, the more easily hydrogen diffusion occurs.

[0035] Hydrogen diffusion coefficient D in ferrite, martensite, and bainite L is identical and is calculated through the following formula:

[0036]

[0037] In austenite, the hydrogen diffusion coefficient D L is calculated according to the following formula:

[0038]

[0039] R = 8.314 J / mol·K is the universal gas constant, and T n It is the temperature denoted by K.

[0040] These equations show that at equivalent temperatures, hydrogen diffuses more easily in ferrite, martensite, and bainite than in austenite.

[0041] N L = 5.2x10 29 sites / m 3 N is the bulk density of intrusion sites in the steel plate. In the method according to the present invention, N L It is the same in all phases of the microstructure.

[0042] N Tis sites / m 3 It is the volume density of potentials represented by . The potentials are E B It has an associated capture energy of = 27,000 J / mol. In the method according to the present invention, one potential can capture one or more hydrogen atoms. The volume density N of the potentials T is sites / m 2 The surface density ρ of the potential expressed as dis It is calculated using the following expression:

[0043]

[0044] Here, α is the number of potentials per vector in Burger's equation, representing the ability of potentials to capture H atoms. The higher this coefficient, the more hydrogen atoms the potentials capture. a bcc is a lattice parameter expressed in angstroms in a bcc structure. Within the framework of the present invention, this lattice parameter is the same for ferrite, martensite, and bainite, all of which have a bcc structure. In a preferred embodiment, these parameters may take the following values:

[0045] - α= 7

[0046] - a bcc = 2.87Å

[0047] In the frame of the present invention, hydrogen atoms cannot be trapped by dislocations within the austenite due to their low diffusion coefficient.

[0048] k and p are s, respectively -1 The hydrogen capture and non-capture rates are expressed as such, which correspond to the amounts of captured and non-captured hydrogen atoms, respectively, as a function of time, and are defined by the following equations.

[0049]

[0050] E T = 4 150 J / mol, which is the capture energy that must be provided for a hydrogen atom to be captured, and ED = E T + E B = 31150 J / mol is the energy that must be provided for hydrogen to be uncaptured.

[0051] Constants k0 and p0 are s -1 These are the hydrogen capture and non-capture coefficients expressed as . These represent N in different phases of the microstructure T C together with T and C L It is used as a fitting parameter for the calculation of. Such fitting parameters can be determined by comparing experiments performed on a given steel composition with calculations according to the present invention, and the process is repeated until the experimental and calculated values ​​converge.

[0052] Finally, N A = 6.02 x 10²³ mol -1 is Avogadro's number.

[0053] As described above, some of the calculations performed within the framework of the method of the present invention depend on the phases present in the steel at a given point on the temperature curve. Furthermore, Equation [3] depends on the depth x of the portion of the steel where the calculation is performed. For an accurate assessment of hydrogen trapped in the entire thickness of the steel plate, it is preferable to consider manufacturing the steel plate as a repetition of N cells of 5 μm x 5 μm to simulate at least a portion of the thickness of the steel plate. In a preferred embodiment, half of the thickness of the steel plate is used, and N is calculated through the following formula:

[0054] N = Thickness of steel plate / (2*5μm)

[0055] The remaining half of the steel plate behaves exactly like the first one, and hydrogen diffusion is uniform along the entire length of the steel plate.

[0056] Next, C H All values ​​can be calculated along the temperature curve using Equations [1] and [2]. Then these C HThe values ​​are C for the cells of the first row. L It is used as values.

[0057] Equations [3] and [4] represent C for the total thickness of the steel plate. T and C L It can be applied consecutively to each cell to finally provide a value.

[0058] In the final step of the method according to the present invention, the total hydrogen content C total C at an arbitrary time before being randomly output to the user L and C T It is determined by calculating the sum of.

[0059] Temperature curve T n and C total-targeted C as close to as possible to total By selecting a compositional atmosphere leading to, C total-targeted The temperature curve with the hydrogen content closest to T = T n Further annealed steel sheets are manufactured according to...

[0060] Now, the present invention will be explained by the following examples, which are by no means limiting.

[0061] yes:

[0062] A cold-rolled steel sheet with a thickness of 1 mm is supplied, having a composition of C: 0.07%, Mn: 2.62%, Si: 0.25%, Cr: 0.3%, Al: 0.16%, Mo: 0.091% in weight percent, with the remainder consisting of iron and unavoidable impurities after smelting.

[0063] Then, these steel sheets may undergo a single annealing process. The temperature curve Tn (n=2) as a function of time is illustrated in Fig. 1. Before the steel sheet is cooled to room temperature (RT), the temperature T H Heated to, and atmosphere A at the above temperature H retention time t at H Maintained during, cooling temperature T CCooled to, and the above temperature T C Atmosphere A C retention time t at C It is maintained for a period of time (hereinafter, this stage is referred to as the overaging stage).

[0064] The temperature Ms of this grade is obtained by dilatometry measurement: Ms=240°C.

[0065] To facilitate the subsequent zinc coating step, the lowest possible hydrogen value C at the end of the overaging step total-targeted =0ppm value is obtained.

[0066] The process parameters are summarized in Table 1:

[0067]

[0068] Hydrogen content evaluated according to the present invention

[0069] The microstructure of the steel plate is evaluated at each point of the temperature curves (A1, B1, C1, D1, E1, F1, G1; A2, B2, C2, D2, E2, F2, G2) as shown in Fig. 1.

[0070] Microstructural phase transformations occur instantaneously only at the indicated points. The evaluated microstructures are summarized in Table 2:

[0071]

[0072] T H During the heating step up to, ferrite is dominant until the temperature reaches AC1, where the ferrite begins to transform into austenite. H During the holding phase, starting from points B1 and B2 respectively, the microstructure is prepared as ferrite and austenite.

[0073] During cooling starting at points C1 and C2, a portion of the austenite transforms into bainite. The austenite continues to transform into bainite during the subsequent holding phase, starting at D1 and D2. The microstructure during cooling starting at E1 is identical to the phase between D1 and E1. The microstructure during cooling starting at E2 is identical to the phase between D2 and E2.

[0074] Austenitite is finally transformed into martensite at points F1 and F2, corresponding to the Ms temperature.

[0075] Half of the steel plate thickness is simulated through the repetition of N=100 cells of 5μm x 5μm. The phase percentages in Table 2 are considered through the percentage of the cell surfaces, as shown in Figure 2, which corresponds to the microstructure at point F1: 25% of the cell surfaces represent 25% martensite inside the steel, 45% of the cell surfaces represent 45% bainite, and 30% of the cell surfaces represent 30% ferrite inside the steel plate.

[0076] Next, C H The values ​​are calculated using equations [1] and [2]. Then these C H The values ​​are C for the cells in the first column. L It is used as values.

[0077] N T The capture and non-capture coefficients were fitted using the following protocol. A steel plate having the composition according to Example 1 was heated to a temperature of 850°C in a furnace with an atmosphere of 5% H2 and the remainder N2, held at the temperature for a holding time of 260 seconds, and then quenched. Subsequently, the experimental hydrogen content in each plate was measured through a TDA experiment by heating the steel plate at a heating rate of 1200°C / h.

[0078] After repeating the process several times, the best fitting parameters were selected as follows:

[0079] k0= 10 5 s -1

[0080] p0= 10 2 s -1

[0081] N in ferrite T = 10 24 sites / m 3

[0082] N in Bainite T = 5 10 24 site / m 3

[0083] N in martensite T = 10 25 sites / m 3

[0084] Volume concentration of hydrogen at interstitial sites in the crystal lattice C L and volume concentration of captured hydrogen in steel plates C T It is calculated by considering the microstructure described above through the solution of equations [3] and [4], and for each point where phase transformation occurs along the thermal curves.

[0085] C total (C total = C L +C L ) is then calculated at each of these points and optionally sent to the worker.

[0086] Figures 3, 4, and 5 are C, respectively. L , C T and total hydrogen content C for tests 1, 2, and 3 total It represents the evolution as a function of time.

[0087] Figures 6, 7, and 8 show C for tests 4, 5, and 6, respectively. L , C T and C total It represents the evolution as a function of time.

[0088] The hydrogen content at the end of the overaging stage (corresponding to points E1 and E2 in FIG. 1, t=395s) determined by the method according to the present invention is summarized in the table below:

[0089]

[0090] The method according to the present invention evaluates that the amount of hydrogen in the steel sheet at the end of the overaging stage can be reduced by reducing the amount of hydrogen in the atmosphere during the overaging stage, as can be seen from Tests 1 to 3, where the hydrogen content in Test 3, performed with 1% H2, is lower than the hydrogen content in Test 1, performed with 5% H2. The same applies to Tests 4 to 6, where the hydrogen content in Test 6, performed with 1% H2 during the overaging stage, is lower than in Test 4, performed with 5% H2.

[0091] In addition, the method according to the present invention predicts that maintaining the steel plate at a low cooling temperature, as performed in tests 1 to 3 at Tc = 400°C, reduces the amount of hydrogen compared to the higher temperature in tests 4 to 6.

Claims

Claim 1 A method for manufacturing an annealed steel sheet having a chemical composition, wherein the steel sheet undergoes an annealing process along a thermal path in which the temperature can be measured by sensors in a furnace having a hydrogen-containing atmosphere, wherein the steel sheet comprises grains in which atoms are arranged within a crystal lattice to form a microstructure of steel including dislocations and interstitial sites, and the method comprises the following consecutive steps: - a target hydrogen content C in the steel sheet at any step of the annealing process total-targeted A step defining, - at least two temperature curves of the annealing process as a function of time t T n A step of defining the temperature curve, wherein n is the number of curves; a step of defining the amount of hydrogen in the furnace atmosphere during the annealing process; a step of determining the microstructure of the steel sheet as a function of the thermal path; the microstructure, the temperature T n and hydrogen partial pressure p H2 Solubility of hydrogen C on the surface of the above steel plate as a function of H Step of calculating, - the above temperature T n , hydrogen trapping rate k and hydrogen non-trapping rates p, C H and as a function of the microstructure above, the volume concentration C of captured hydrogen at the dislocation T and volume concentration of hydrogen at the intrusion site C L Step of calculating, - hydrogen content C at any time of the annealing process total = C L +C T Step for calculating hydrogen content C to the user via computer display at any time, - optionally, at any time total Step of printing, - C total-targeted C as close to as possible to total Composition atmosphere reaching and the above temperature curve T n A step of selecting, - a step of providing the chemical composition to the steel plate, - in an atmosphere of the selected composition, as a function of time t, a selected temperature curve T = T n A method for manufacturing an annealed steel plate, comprising the step of annealing the steel plate according to the above. Claim 2 In paragraph 1, C T and C L A method for manufacturing an annealed steel sheet, wherein the thickness is calculated by solving the following formula for at least a portion of the thickness of the steel sheet: D L n is the lattice diffusion coefficient of hydrogen within the crystal lattice, x is the depth inside the steel plate, and N L is the volume density of the intrusion site, and N T is the volume density of dislocations, and N A is Avogadro's constant. Claim 3 In claim 1, the solubility C of the hydrogen above H A method for manufacturing annealed steel sheets calculated through the following formula: - For a temperature T below Ac3, - For temperatures T above Ac3, p H2 is the partial pressure of hydrogen inside the furnace, and C H is displayed as at%. Claim 4 In any one of claims 1 to 3, the microstructure of the steel plate comprises at least one phase selected from ferrite, austenite, martensite, and bainite, and the lattice diffusion coefficient D of hydrogen L A method for manufacturing an annealed steel sheet calculated through the following formula: In ferrite, martensite, and bainite In austenite. Claim 5 A method for manufacturing an annealed steel sheet according to any one of claims 1 to 3, wherein the hydrogen capture rate k and the hydrogen non-capture rate p are calculated through the following formula: Here, k0 and p0 are the capture and non-capture coefficients, and E T is capture energy, and E D is non-captured energy, and R is a general-purpose constant gas. Claim 6 In any one of paragraphs 1 to 3, the volume density N of the potentials through the following formula T Method for manufacturing annealed steel sheets, in which is calculated: ρ dis is the surface density of potentials, α is the number of potentials per Burger vector, and a bcc Is It is a grid parameter. Claim 7 A method for manufacturing an annealed steel sheet according to any one of claims 1 to 3, wherein the annealing process may include different steps of heating, holding, and cooling the steel sheet, the cooling step may include an overaging sub-step, and subsequent and final cooling follows after a reheating step.

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