Method for evaluating hydrogen content in steel sheet

Abstract

The present invention provides a method for evaluating the hydrogen content in a steel sheet during an annealing process, comprising the steps of: estimating the microstructure of the steel sheet according to a temperature curve; and estimating the solubility C of hydrogen. H and the volume concentration of trapped hydrogen in the dislocation, C T and the volume concentration of hydrogen in the interstitial sites of the crystal lattice, C L and calculating the hydrogen content C at each time step of the annealing process. total =C L +C T and the hydrogen content C at each time step. total and outputting the result to a user.

Description

[Technical field]

[0001] The present invention relates to a method for assessing the hydrogen content in a steel sheet while it is being subjected to an annealing process. [Background technology]

[0002] Steel sheets are made up of grains in which atoms are arranged in a crystal lattice to form the steel's microstructure. The spaces between these atoms are called interstitial sites. The arrangement of atoms is not perfectly regular, and some alignment defects can occur, as in the case of dislocations, which are linear defects.

[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 comparable to the size of the interstitial sites of the crystal lattice. Hydrogen atoms can diffuse progressively and become trapped inside defects such as dislocations.

[0004] The introduction and diffusion of hydrogen in steel sheets is one of the mechanisms responsible for the embrittlement of steel sheets and can result in crack formation, for example, along grain boundaries and / or dislocation glide planes.

[0005] Due to hydrogen, the steel strip can suffer a loss of ductility, also known as hydrogen embrittlement. Summary of the Invention [Problem to be solved by the invention]

[0006] It is therefore an object of the present invention to provide a method for evaluating the hydrogen content in a steel sheet undergoing an annealing process and outputting the hydrogen content at any time to a user. [Means for solving the problem]

[0007] The object of the present invention is achieved by providing a method according to claim 1. The method may also comprise the features according to any one of claims 2-6.

[0008] Below, C L indicates the concentration of hydrogen in the interstitial sites of the crystal lattice of the steel sheet, and C T denotes the concentration of trapped hydrogen in the steel sheet. Dislocations are the only trapping sites considered in this invention that are uniformly distributed in the microstructure.

[0009] The invention will now be described in detail with reference to the accompanying drawings, in which: FIG. [Brief description of the drawings]

[0010] [Figure 1] 1 shows the temperature curves for steps P0 and P1. [Diagram 2] 4 shows the temperature curve for step P2. [Diagram 3] 1 shows a cell used in one embodiment of the method of the present invention, depicting a microstructure at point F1 of step P1. [Figure 4] The time evolution of CL, CT and Ctotal = CL + CT of P0 is shown. [Diagram 5] The time evolution of CL, CT and Ctotal of P1 is shown. [Figure 6] The time evolution of CL, CT and Ctotal of P2 is shown. [Figure 7] 4 shows the temperature curve for step P3. [Figure 8] 4 shows the temperature curve for step P4. [Figure 9] The time evolution of CL, CT and Ctotal of P3 is shown. [Figure 10] The time evolution of CL, CT and Ctotal of P4 is shown. [Figure 11] 1 shows a cell used in one embodiment of the method of the present invention, depicting a microstructure at point E3 of step P3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The method according to the invention deals with the assessment of the diffused hydrogen content of a steel sheet subjected to at least one annealing step.

[0012] During this annealing, the steel sheet is subjected to at least one heating step and one cooling step according to a thermal path. Usually, the heat treatment is carried out in an oxidizing atmosphere, i.e., e.g., O 2 , C.H. 4 , CO 2 or CO. They can also be carried out in a neutral atmosphere, i.e., for example, N 2 , Ar or He. Finally, they can also be carried out in a reducing atmosphere, i.e. an atmosphere comprising a reducing gas, for example H2 or HNx.

[0013] In a preferred embodiment, the thermal path may also include at least one isothermal hold step, which is usually preceded by a heating step and followed by a cooling step. The cooling step may include an isothermal hold, called an overaging substep, followed by a subsequent cooling step. A hot dip coating process in a high temperature metal bath may also be used during such a thermal path and is another type of isothermal hold, since the metal sheet immersed in such a high temperature metal bath is maintained at the bath temperature during the hold time in such bath.

[0014] The annealing may be, for example, a recrystallization anneal, a recovery anneal or a tempering anneal, followed by a heat treatment, -Quenching and tempering, -Quenching and separation This can be done.

[0015] The thermal path according to the invention corresponds to the successive temperatures T, the heating and cooling rates, and the time spent in each section of the annealing step and any subsequent heat treatments.

[0016] The temperature of the heat path can be measured during the anneal and any subsequent heat treatments by sensors or by calculations made using software.

[0017] The first step of the method according to the invention is to determine the microstructure of the steel sheet as a function of the thermal path of the annealing process, for example by calculations carried out using software such as COMSOL®. The microstructure can also be determined by a sensor in the furnace capable of measuring the austenite content in the steel, such as X-CAP®.

[0018] In the framework of the present invention, the evolution of the microstructure occurs instantaneously only at a specific point corresponding to the phase change at a given temperature. Then, the solubility of hydrogen at the surface of the steel sheet, C H is calculated. Hydrogen solubility is the ability of hydrogen to dissolve in steel. This solubility depends on temperature, partial pressure of hydrogen, and phases present in the microstructure of the steel. It can be calculated by the following equations [1] and [2]:

[0019] At a temperature T of Ac3 or less, preferably 280K to 1184K,

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[0020] In the first part of the temperature curve, where heating takes place, ferrite is the main structure of the steel sheet. The solubility of hydrogen in ferrite, C H is appropriately calculated using equation [1] above.

[0021] In the last part of the temperature curve where cooling takes place, some of the austenite formed above Ac3 may transform into bainite and / or martensite, depending on the composition of the steel and the cooling rate. In that part of the curve, the solubility of hydrogen in bainite and martensite, C H is assumed to be the same as for ferrite and can be suitably obtained using equation [1] above.

[0022] For temperatures T exceeding Ac3, preferably between 1184K and 1667K,

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[0023] In the central part of the temperature curve where high temperature holding can occur, it is assumed that austenite is the main phase in the steel sheet, and the solubility of hydrogen in austenite, C H is appropriately calculated using equation [2] above.

[0024] In both formulas [1] and [2], C H is expressed in at%, and p H2 is the hydrogen partial pressure in the reactor, expressed in Pa. Both of these formulas are from the publication by Fromm & Jehn, "Hydrogen in Elements," (Bull. Alloys Phase Diagrams, 5(3), 323-326 (1984)).

[0025] Solubility of hydrogen at the surface of the steel sheet C H is necessary as input for the next step of the method according to the invention, determining the volume concentration C of hydrogen in the interstitial sites of the crystal lattice L and the volume concentration of trapped hydrogen in the steel sheet, C T Each is 1m of iron 3 Moles of hydrogen per H / m 3 Fe ) and is calculated using numerical simulation by solving the following equation:

number

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[0026] Here, the different parameters and constants in both equation [3] and equation [4] are explained.

[0027] D L m 2The diffusion coefficient in the crystal lattice, expressed as / s, depends on the temperature and the phases present in the steel at that temperature. This diffusion coefficient indicates how easily hydrogen diffuses into the material. The higher the coefficient, the easier it is for hydrogen to diffuse.

[0028] In ferrite, martensite and bainite, the coefficient of diffusion of hydrogen D L are assumed to be the same and are calculated by the following formula:

number

[0029] In austenite, the coefficient of diffusion of hydrogen D L is calculated according to the following formula:

number

[0030] These equations show that at comparable temperatures, hydrogen diffuses more easily into ferrite, martensite and bainite than into austenite.

[0031] N L =5.2×10 29 Site / m 3 is the volume density of interstitial sites in the steel sheet. L is assumed to be the same for all phases in the microstructure.

[0032] N T is the volume density of dislocation sites / m 3 The rearrangement is represented by E B = 27000 J / mol. In the method according to the invention, one dislocation can trap one or more hydrogen atoms. The volume density of dislocations N T is expressed by the following formula:

number

[0033] In the framework of the present invention, hydrogen atoms cannot be trapped by dislocations in austenite due to their low diffusion coefficient.

[0034] k and p are the hydrogen trapping and detrapping rates, respectively, and s -1 and correspond to the amount of trapped and detrapped hydrogen atoms, respectively, as a function of time and are defined by the following equations:

number

number

[0035] E T = 4150 J / mol is the trapping energy, which is the energy that must be provided for a hydrogen atom to be trapped, and E D =E T +E B = 31150 J / mol is the energy that must be provided for hydrogen to be detrapped.

[0036] The constants k0 and p0 are -1These are the hydrogen trapping and detrapping coefficients, which are the hydrogen trapping and detrapping coefficients for different phases of the microstructure. T With C T and C L Such fitting parameters can be determined by comparison of experiments performed on a given steel composition with the calculations according to the invention, and are repeated until the experimental and calculated values ​​converge.

[0037] Finally, N A = 6.02 × 1023 mol -1 is Avogadro's number.

[0038] As mentioned above, some of the calculations made within the framework of the method of the invention depend on the phases present in the steel at a given point of the temperature curve. Furthermore, equation [3] depends on the depth x of the steel part at which the calculations are made. In order to give an accurate assessment of the trapped hydrogen throughout the thickness of the steel plate, it is preferred to consider the steel plate as consisting of N repeating cells of 5 μm×5 μm to simulate at least a part of the thickness of the steel plate. In a preferred embodiment, half the thickness of the steel plate is used, where N is determined by the following equation: N = thickness of steel plate / (2*5μm) It is calculated as follows.

[0039] The other half of the plate thickness behaves exactly the same as the first thickness, with hydrogen diffusion being uniform over the entire length of the plate.

[0040] Next, C H The value can be calculated using equations [1] and [2] along the temperature curve. Then, such C H The value is the C in the first row of the cell. L Used as a value.

[0041] Equations [3] and [4] are applied to each cell successively to finally obtain the C T and C L can provide a value for

[0042] In the final step of the method according to the invention, the total hydrogen content C total At any point, C L and C. T It is determined by calculating the sum of

[0043] The present invention is illustrated by the following examples which are not intended to be limiting in any way.

[0044] Example 1 Cold rolled steel sheets having the following composition and thickness of 1 mm are supplied: 0.07 wt% C, 2.62 wt% Mn, 0.25 wt% Si, 0.3 wt% Cr, 0.16 wt% Al, 0.091 wt% Mo, with the remainder being iron and unavoidable impurities resulting from smelting.

[0045] Such steel sheets can then be subjected to one of the annealing steps P0, P1 and P2, as described in Fig. 1 and Fig. 2. In all steps, the steel sheets are annealed at a temperature T H 5% hydrogen, the rest N 2 and a holding time t H The temperature is maintained for a period of time.

[0046] Next, in step P0, the steel sheet is cooled to room temperature (RT).

[0047] In process P1, the steel plate is H Temperature T to 465°C 1 and held for 86 seconds. 1 After being maintained at this temperature for 10 min, it is cooled to room temperature at a cooling rate of 10° C. / sec.

[0048] In process P2, the steel plate is first H Temperature T to 600°C 1 It is then cooled at a rate of 2.6°C / s to T 1 Temperature T to 465°C 2The steel plate is cooled at a rate of 40°C / s to 30°C for a holding time of t 2 Over the temperature T 2 The sample is then cooled to room temperature (RT) at a cooling rate of 10° C. / s.

[0049] The temperature Ms at each step is obtained by dilatometry.

[0050] Experiments were carried out using these process parameters to obtain experimental values ​​for hydrogen content. The following specific conditions were applied:

[0051] Table 1: Process parameters [Table 1]

[0052] Table 2: Experimental hydrogen content The experimental hydrogen content is measured by TDA experiments at the end of the process. To obtain the thermal desorption of hydrogen, the steel plate was heated at a heating rate of 1200 °C / h.

[0053] The corresponding diffusible hydrogen contents are summarized in Table 2.

[0054] [Table 2]

[0055] Hydrogen Content Assessed According to the Invention First, as shown in Figures 1 and 2, the microstructure of the steel plate is estimated at each point on the temperature curve (A0, B0, C0, D0, E0; A1, B1, C1, D1, E1, F1, G1; A2, B2, C2, D2, E2, F2, G2, H2).

[0056] The microstructural phase transformations occur instantaneously only at the points indicated. The estimated microstructures are summarized in Table 3.

[0057] Table 3: Estimated microstructures [Table 3]

[0058] T H During the heating step up to point B, ferrite is the predominant phase until the temperature reaches AC1, where ferrite begins to transform into austenite. 0 , B 1 and B. 2 Each starts with T H During the holding step at , the microstructure consists of ferrite and austenite.

[0059] In step P0, during cooling at room temperature, all austenite reaches the Ms0 temperature (point D 0 ) transforms into martensite.

[0060] point C 1 During the first cooling of step P1, which begins with D, a portion of the austenite is transformed into bainite. 1 During the subsequent holding steps starting from E 1 The microstructure during cooling, starting at D 1 and E 1 The austenite reaches the point F, which corresponds to the Ms1 temperature. 1 It finally transforms into martensite.

[0061] In process P2, C 2 During the slow cooling starting at D2, no phase transformation occurs. Part of the austenite transforms to bainite during the subsequent cooling starting at D2. During the subsequent overaging starting at E2, the austenite continues to transform to bainite. 2 The microstructure of E 2 The austenite is at point G 2 It transforms to martensite.

[0062] Half the thickness of the steel plate is simulated by repeating N = 100 cells of 5 μm × 5 μm. The phase percentages in Table 3 are calculated from point F 1The microstructure at 25% of the cell surface is considered through the percentage of the cell surface, as shown in FIG. 3, which corresponds to the microstructure at 25% of the cell surface, 45% of the cell surface represents 45% of the martensite inside the steel, 30% of the cell surface represents 30% of the ferrite inside the steel.

[0063] Then, for all points where the phase transformation occurs, a hydrogen partial pressure p of 5066.5 Pa is applied. H2 Using equations [1] and [2], H Then, such a C H The value is the C in the first row of the cell. L Used as a value.

[0064] Using the following protocol, T The steel plate having the composition according to Example 1 was fitted with 5% H 2 , the rest is N 2 The steel sheets were heated to a temperature of 850°C in a furnace having an atmosphere consisting of 0.01% Cr, 0.01% H, 0.001% Na ...

[0065] After several iterations, the best fitting parameters were selected as follows: k 0 =10 5 s -1 p 0 =10 2 s -1 N in ferrite T =10 24 Site / m 3 N in bainite T =5 10 24 Site / m 3 N in martensite T =10 25 Site / m 3

[0066] Then, the volume concentration of hydrogen in the interstitial sites of the crystal lattice, C L and the volume concentration of trapped hydrogen in the steel sheet, C T is calculated by solving equations [3] and [4] taking into account the microstructure above and for each of the points along the thermal curve where the phase transformation occurs.

[0067] Next, C total (C total =C L +C L ) is calculated for each of these points and transmitted to the operator. Respective curves of all the points are shown in Figures 4 to 6 corresponding to steps P0, P1 and P2.

[0068] For step P0, the final hydrogen content determined by the method according to the invention is 0.36 ppm compared to 0.34 ppm experimentally measured at the end of the annealing step.

[0069] For steps P1 and P2, the final hydrogen content determined by the method according to the invention is 0.09 ppm compared to 0.10 ppm measured experimentally.

[0070] As shown by these examples, the method according to the invention provides good prediction of hydrogen content evolution.

[0071] This shows that the method according to the present invention can accurately evaluate the hydrogen content at any time during the production of the steel plate, since it is output to the user.

[0072] Example 2 A cold rolled steel sheet having a composition of 0.19 wt% C, 3.86 wt% Mn, 1.27 wt% Si, 0.39 wt% Al, 0.2 wt% M0, 0.0235 wt% Nb, 0.0293 wt% Ti, the remainder of the composition being iron and unavoidable impurities resulting from smelting, and having a thickness of 1.2 mm is provided. Such a steel sheet can then be subjected to an annealing step between steps P3 and P4, as described in Figures 7 and 8.

[0073] In all processes, the steel plate is heated to a temperature of 850°C T H and held for 159 seconds. H The steel plate is then cooled at a rate of 2°C / s to T H Temperature T to 600°C 1 After cooling to 170°C T Q is rapidly cooled to

[0074] Next, in process P3, the steel plate is Q and cooled to room temperature (RT).

[0075] Next, in process P4, the steel plate is Q Temperature T to 450°C o and held for 102 seconds. o Over the temperature T o The mixture is then maintained at room temperature.

[0076] The temperature Ms of this grade at each step is obtained by dilatometry.

[0077] Experiments were carried out using these process parameters to obtain experimental values ​​for hydrogen content. The following specific conditions were applied:

[0078] Table 4: Process parameters [Table 4]

[0079] Tests were conducted using these process parameters to obtain experimental values ​​of hydrogen content.

[0080] Table 5: Experimental hydrogen content The experimental hydrogen content is measured by TDA experiments at the end of the process by heating the steel plate at a heating rate of 1200° C. / h.

[0081] The corresponding hydrogen contents are summarized in Table 5. [Table 5]

[0082] Hydrogen Content Assessed According to the Invention First, as shown in Fig. 7 and Fig. 8, the microstructure of the steel sheet is estimated at each point of the temperature curve (A3, B3, C3, D3, E3, F3, G3; A4, B4, C4, D4, E4, F4, G4, H4).

[0083] The microstructural phase transformation occurs instantaneously at the points indicated. The estimated microstructures are summarized in Table 6.

[0084] Table 6: Estimated microstructure [Table 6]

[0085] T H During the heating step up to point B, ferrite is the predominant phase until the temperature reaches AC3, where it transforms to austenite. 3 and point B 4 Each starts with T H During the holding step at , the microstructure consists of 100% austenite.

[0086] In process P3, point C 3 and point D 3 In the microstructure, B 3 All austenite is E 3 At the Ms temperature corresponding to 0.05, the alloy transforms to martensite, and the microstructure remains unchanged until the end of the process.

[0087] In process P4, point C 4 and point D 4 In the microstructure, B 4 The microstructure is the same as that of E. 4 At the Ms temperature, which corresponds to 0.05° C., the alloy transforms to martensite, and the microstructure remains unchanged until the end of the process.

[0088] Half the thickness of the steel plate is simulated by repeating N = 120 cells of 5 μm × 5 μm. The phase percentages in Table 5 are calculated from point E 4 The numerical cell surface percentages are taken into account, as represented in Figure 11, which corresponds to the microstructure at 15%. 15% of the cell surface represents 15% of the austenite inside the steel, and 85% of the cell surface represents 85% of the partitioned martensite.

[0089] Then, for all points where the phase transformation occurs, a hydrogen partial pressure p of 5066.5 Pa is applied. H2 Using equations [1] and [2], H Then, such a C H The value is the C in the first row of the cell. L Used as a value.

[0090] Using the following protocol, T The steel plate having the composition according to Example 2 was fitted with 5% H 2 , the rest is N 2 The steel sheets were heated to a temperature of 850°C in a furnace having an atmosphere consisting of 0.01% Cr, 0.01% H, 0.001% Na ...

[0091] After several iterations, the best fitting parameters were selected as follows: k 0 =10 5 s -1 p 0 =10 2 s -1 N in ferrite T =10 24 Site / m 3 N in bainite T =5 10 24 Site / m 3 N in martensite T =10 25 Site / m 3

[0092] Then, the volume concentration of hydrogen in the interstitial sites of the crystal lattice, C L and the volume concentration of trapped hydrogen in the steel sheet, C T is calculated by solving equations [3] and [4] taking into account the microstructure above and for each of the points along the thermal curve where the phase transformation occurs.

[0093] Next, C total (C total =C L +C L ) is calculated for each of these points and transmitted to the operator. The respective curves collecting all the points are shown in Figures 9 and 10 corresponding to steps P3 and P4.

[0094] As shown by these examples, the method according to the invention predicts the evolution of hydrogen content well.

[0095] point G 3 The calculated hydrogen content at the end of step P3 at (t=430 s) is 0.37 ppm compared to the experimentally measured 0.38 ppm. 4 At (t=590 seconds) the calculated hydrogen content is experimentally measured to be 0.07 ppm.

[0096] The hydrogen content can be output to the user at any point during the annealing process and the entire curve can also be output at the end of the annealing process.

Claims

1. A method for evaluating the hydrogen content in a steel sheet subjected to at least one annealing step according to a thermal path, the temperature of which can be measured by a sensor in a furnace having an atmosphere containing hydrogen, said steel sheet comprising grains in which atoms are arranged in a crystal lattice, thus forming a steel microstructure comprising dislocations and interstitial sites, said method comprising the following successive steps: - determining the microstructure of the steel sheet as a function of the thermal path; - Microstructure, temperature T, and hydrogen partial pressure p Ｈ２ The solubility of hydrogen at the surface of the steel sheet as a function of Ｈ and calculating - volume concentration of trapped hydrogen in the dislocation C Ｔ and the volume concentration of hydrogen in the interstitial sites C Ｌ where T is the temperature, k is the hydrogen trapping rate, p is the hydrogen detrapping rate, and C Ｈ as a function of the microstructure; - Hydrogen content C at any time during the annealing process total = C Ｌ +C Ｔ and calculating - The hydrogen content C at any time via a computer display total to a user; A method comprising:

2. C Ｔ and C Ｌ is expressed by the following formula over at least a portion of the thickness of the steel plate: [0010] [0025] D Ｌ is the lattice diffusion coefficient of hydrogen in the crystal lattice, x is the inner depth of the steel plate, N Ｌ is the volume density of interstitial sites, N Ｔ is the volume density of dislocations, N Ａ is the Avogadro constant, The method for evaluating the hydrogen content in a steel sheet according to claim 1, wherein the hydrogen content is calculated by solving

3. Hydrogen solubility C Ｈ is expressed by the following formula: For temperatures T below Ac3, [0030] For temperatures T exceeding Ac3, [0045] p Ｈ２ is the hydrogen partial pressure in the reactor, and C Ｈ is expressed as atomic percent The method for evaluating a hydrogen content in a steel sheet according to claim 1 or 2, wherein the hydrogen content is calculated by the following formula:

4. The microstructure of the steel sheet contains at least one phase of ferrite, austenite, martensite, and bainite, and has a lattice diffusion coefficient of hydrogen D Ｌ is expressed by the following formula: In ferrite, martensite, and bainite [0050] In austenite [006] The method for evaluating a hydrogen content in a steel sheet according to any one of claims 1 to 3, wherein the hydrogen content is calculated by the following formula.

5. The hydrogen trapping rate k and the hydrogen detrapping rate p are expressed by the following equations: [0070] [0080] k ０ and p ０ is the trapping and detrapping coefficient, E Ｔ is the trap energy, E Ｄ is the detrapping energy, R is the universal gas constant, The method for evaluating a hydrogen content in a steel sheet according to any one of claims 1 to 4, wherein the hydrogen content is calculated by the following formula.

6. Dislocation volume density N Ｔ is expressed by the following formula: [0090] ρ ｄｉｓ is the surface density of dislocations, α is the number of dislocations per Burger vector, a ｂｃｃ is the lattice parameter, The method for evaluating a hydrogen content in a steel sheet according to any one of claims 1 to 5, wherein the hydrogen content is calculated by the following formula.

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