Annealed steel sheet manufacturing method

Abstract

The present invention relates to a method for manufacturing a heat treated steel sheet which is subjected to an annealing step according to a temperature curve T as a function of time t in a furnace with an atmosphere containing hydrogen, wherein a targeted hydrogen content is defined in the steel sheet at the end of the annealing step.

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing annealed steel sheet, in which process parameters are selected to obtain a defined hydrogen value at the end of any step of the 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 structure. 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 producing annealed steel sheet, in which the process parameters are selected to obtain a defined hydrogen value at the end of any step of the annealing process. [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 2-7.

[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] The temperature curves T0 and T1 are shown. [Diagram 2] 1 represents a cell used in one embodiment of the method of the present invention, showing the microstructure at point F1 of temperature curve T1. [Diagram 3] Figure 1 shows the time evolution of CL, CT and Ctotal = CL + CT for Test 1. [Figure 4] 13 shows the time evolution of CL, CT and Ctotal in Test 2. [Diagram 5] 13 shows the time evolution of CL, CT and Ctotal for Test 3. [Figure 6] 13 shows the time evolution of CL, CT and Ctotal for Test 4. [Figure 7] 13 shows the time evolution of CL, CT and Ctotal for test 5. [Figure 8] 13 shows the time evolution of CL, CT and Ctotal for test 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] A method for manufacturing annealed steel sheet is provided, in which process parameters are selected to obtain a defined hydrogen value at the end of any step of the annealing process, said method comprising the successive steps of: The first step of the method according to the invention is to determine the target hydrogen content C in the steel sheet at the end of the annealing process step. total-targeted The purpose of this document is to stipulate the following. During this annealing, the steel sheet is subjected to at least one heating step and one cooling step, following a thermal path. Usually, the heat treatments can be carried out in an oxidizing atmosphere, i.e. an atmosphere comprising an oxidizing gas, for example O2, CH4, CO2 or CO. They can also be carried out in a neutral atmosphere, i.e. an atmosphere comprising a neutral gas, for example N2, 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.

[0012] 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.

[0013] 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 splitting This can be done.

[0014] 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.

[0015] 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.

[0016] Then, in a furnace with a hydrogen atmosphere, at least two temperature curves T of the annealing step as a function of time t are obtained. n(n is the number of curves) is specified. The amount of hydrogen in the atmosphere is specified. The atmosphere in the furnace can include H2, N2, and O2.

[0017] The next 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 specific points corresponding to a phase change at a given temperature.

[0019] Next, 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]:

[0020] Temperature T below Ac3, preferably 280K to 1184K n in the case of,

number

[0021] 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.

[0022] 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 His the same as for ferrite and can be appropriately obtained using equation [1] above.

[0023] Temperature T above Ac3, preferably between 1184K and 1667K n in the case of:

number

[0024] In the central part of the temperature curve where high temperature holding can occur, 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.

[0025] 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)).

[0026] 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 All of the above are iron m 3 Moles of hydrogen per H / m 3 Fe ) and is calculated using numerical simulation by solving the following equation:

number

number

[0027] Here, the different parameters and constants in both equation [3] and equation [4] are explained.

[0028] D L m 2 The 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.

[0029] In ferrite, martensite and bainite, the coefficient of diffusion of hydrogen D L is the same and is calculated by the following formula:

number

[0030] In austenite, the coefficient of diffusion of hydrogen D L is calculated according to the following formula:

number

[0031] These equations show that at comparable temperatures, hydrogen diffuses more easily into ferrite, martensite and bainite than into austenite.

[0032] N L =5.2×10 29 Site / m 3 is the volume density of interstitial sites in the steel sheet. L is the same for all phases in the microstructure.

[0033] 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

[0034] In the framework of the present invention, hydrogen atoms cannot be trapped by dislocations in austenite due to their low diffusion coefficient.

[0035] 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

[0036] 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 =ET +E B = 31150 J / mol is the energy that must be provided for hydrogen to be detrapped.

[0037] The constants k0 and p0 are -1 These 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.

[0038] Finally, N A = 6.02 × 1023 mol -1 is Avogadro's number.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] Equations [3] and [4] are applied to each cell successively to finally obtain the C T and C L can provide a value for

[0043] In the final step of the method according to the invention, the total hydrogen content C total may be optionally printed to the user at any time. L and C. T It is determined by calculating the sum of

[0044] C total C total-targeted Temperature curve T as close as possible to n and the composition atmosphere is selected, and the temperature curve T = T n According to C total-targeted Further, an annealed steel sheet having a hydrogen content closest to the above is produced.

[0045] The present invention is illustrated by the following examples which are not intended to be limiting in any way.

[0046] (Example) 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.

[0047] Such a steel sheet can then be subjected to one annealing step. The temperature curves as a function of time Tn (n=2) are depicted in FIG. 1. The steel sheet is heated to a temperature T H and atmosphere A H Retention time t H and maintained at said temperature for a period of time, and cooled to a temperature T C The temperature is then cooled to ambient temperature A. C Retention time t C Over the temperature T C (hereinafter, this step is called the overaging step).

[0048] The temperature Ms of this grade is obtained by dilatometry: Ms=240°C.

[0049] At the end of the overaging step, the lowest possible hydrogen value C was used to facilitate the subsequent zinc coating step. total-targeted =0 ppm value is required.

[0050] The process parameters are summarized in Table 1.

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

[0052] Hydrogen Content Assessed According to the Invention The microstructure of the steel plate is estimated at each point of the temperature curve (A1, B1, C1, D1, E1, F1, G1; A2, B2, C2, D2, E2, F2, G2) as shown in Figure 1.

[0053] The microstructural phase transformation occurs instantaneously only at the points indicated. The estimated microstructures are summarized in Table 2.

[0054] Table 2: Estimated fine structure [Table 2] TIFF2024545673000012.tif107170

[0055] T H During the heating step up to 1000 K, ferrite is the main phase until the temperature reaches AC1, where ferrite begins to transform into austenite. T starting at points B1 and B2, respectively. H During the holding step at , the microstructure then consists of ferrite and austenite.

[0056] During cooling starting from points C1 and C2, some of the austenite transforms to bainite. The austenite continues to transform to bainite during the subsequent holding steps starting from D1 and D2. The microstructure during cooling starting at E1 is the same as the step between D1 and E1. The microstructure during cooling starting at E2 is the same as the step between D2 and E2.

[0057] The austenite finally transforms to martensite at points F1 and F2, which correspond to the Ms temperature.

[0058] Half the thickness of the steel plate is simulated by N=100 repetitions of cells of 5 μm × 5 μm. The phase percentages in Table 2 are considered through the percentages of the cell surface as shown in Figure 2, which corresponds to the microstructure at point F1. 25% of the cell surface represents 25% of the martensite inside the steel, 45% of the cell surface represents 45% of the bainite, and 30% of the cell surface represents 30% of the ferrite inside the steel.

[0059] Then, using equations [1] and [2], C H Then, such a C H The value is the C in the first row of the cell. L Used as a value.

[0060] Using the following protocol, T and the trapping and detrapping factors were fitted. Steel plates having the composition according to Example 1 were heated to a temperature of 850°C in a furnace with an atmosphere consisting of 5% H2, balance N2, and maintained at said temperature for a holding time of 260 seconds, followed by quenching. The experimental hydrogen content in each steel plate was then measured by TDA experiments by heating the steel plate at a heating rate of 1200°C / h.

[0061] After several iterations, the best fitting parameters were selected as follows: k0=10 5 s -1 p0=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

[0062] 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.

[0063] Next, C total (C total =C L +C L ) is calculated at each of these points and optionally transmitted to the operator.

[0064] Figures 3, 4 and 5 show the C of tests 1, 2 and 3, respectively. L , C T and total hydrogen content C total represents the evolution of as a function of time.

[0065] Figures 6, 7 and 8 show the C for tests 4, 5 and 6, respectively. L , C T and C total represents the evolution of as a function of time.

[0066] The hydrogen contents at the end of the overageing step (corresponding to t=395 s at points E1 and E2 in FIG. 1) determined by the method according to the invention are summarized in the table below. [Table 3]

[0067] It has been evaluated that the method according to the invention can reduce the amount of hydrogen in the steel sheet at the end of the overaging step by reducing the hydrogen content in the atmosphere during this overaging step, as can be seen in tests 1 to 3, where the hydrogen content in test 3, performed with 1% H2 during the overaging step, is lower than in test 1, performed with 5% H2. Similarly for tests 4 to 6, the hydrogen content in test 6, performed with 1% H2 during the overaging step, is lower than in test 4, performed with 5% H2.

[0068] Furthermore, the method according to the invention predicts that by maintaining the steel plate at a low cooling temperature, as performed in tests 1-3 with Tc=400° C., the amount of hydrogen is reduced compared to the higher temperatures in tests 4-6.

Claims

1. A method for the manufacture of an annealed steel sheet having a chemical composition, which is subjected to an annealing process 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 containing grains in which the atoms are arranged in a crystal lattice, thus forming a microstructure of the steel containing dislocations and interstitial sites, said method comprising the following successive steps: The target hydrogen content C in the steel sheet at any step of the annealing process total-targeted and At least two temperature curves T of the annealing process ｎ as a function of time t, where n is the number of curves; defining an amount of hydrogen in the furnace atmosphere during the annealing step; determining the microstructure of the steel sheet as a function of thermal path; Microstructure, temperature T ｎ and hydrogen partial pressure p Ｈ２ The solubility of hydrogen at the surface of the steel sheet as a function of C Ｈ and calculating The volume concentration of trapped hydrogen in the dislocation, C Ｔ and the volume concentration of hydrogen in the interstitial sites C Ｌ At temperature T ｎ , hydrogen trapping rate k and hydrogen detrapping rate p, 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 Optionally, the hydrogen content C can be monitored at any time via a computer display. total to a user; C total-targeted As close as possible to C total The temperature curve T ｎ and selecting a composition atmosphere; providing said chemical composition to a steel sheet; In the selected composition atmosphere, the selected temperature curve T=T ｎ Annealing the steel sheet according to 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 producing an annealed steel sheet according to claim 1, wherein the annealing temperature is calculated by solving the following equation:

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 in at %, The method for producing an annealed steel sheet according to claim 1 or 2, wherein the annealing temperature is calculated by:

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

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 are the trapping and detrapping coefficients, E Ｔ is the trap energy, E Ｄ is the detrapping energy, R is the universal gas constant, The method for producing an annealed steel sheet according to any one of claims 1 to 4, wherein the calculated value is calculated by the above 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 producing an annealed steel sheet according to any one of claims 1 to 5, wherein the calculated value is calculated by

7. The method for producing an annealed steel sheet according to any one of claims 1 to 6, wherein the annealing process may comprise different steps of heating, holding and cooling the steel sheet, the cooling step may comprise an overaging sub-step and a reheating step may be followed by a subsequent final cooling.

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