EP2864505B1 - Method for press hardening of steel - Google Patents

Description

The invention relates to a method for press hardening steel according to the upper handles of claims 1 and 2.

Press hardening of steel is a technique that has been known since the 1970s. In this method, a steel blank with an alloy composition matched to the press hardening is raised to a temperature which enables austenitization, preferably complete austenitization. Complete austenitization usually takes place above the so-called AC ₃ point, which can be read from corresponding multi-substance state diagrams and which also depends in particular on the composition.

After heating and complete austenitizing, such a steel is cooled at a rate which is above the so-called critical hardening rate. This results in a fully martensitic structure, which gives the steel a high hardness, in particular up to 1500 MPa and above.

The strength of such a press-hardened steel is essentially determined by the carbon content, since this determines the martensite hardness.

Other alloy elements in the composition essentially determine the hardenability with the carbon, with certain elements, including in particular boron, influencing the conversion behavior and in particular acting as so-called conversion retarders. These conversion retarders lower the temperature, below which the cooling also exceeds the critical one Hardness speed would no longer be achievable, a fully martensitic structure significantly and can therefore be used in part to favorably influence certain process parameters.

The influence of some alloying elements on hardenability is discussed in " Influence of the Alloying Elements on Phase Transition of High Strength Steels "(Neugebauer et al., Materials Processing Technology, 2011 ) described. Carbon and boron, especially in combination with manganese, molybdenum or chromium, act as retarders for conversion. Simultaneous use of carbon and manganese can lead to an increase in hardness with a comparatively slow cooling rate.

The usual procedure for press hardening is to provide the corresponding press-hardening steel as a sheet, cut a board out of this sheet and either deep-draw this board in the cold state and then heat it up, insert it into a tool and cool it there by all-round contact of the cooling tool or heat the board and to form hot in one tool while cooling at the appropriate speed.

In this method known per se, the cooling rates are determined by the tool or the contact of the press-hardening steel with the tool. A low thermal conductivity, a low heat capacity, the heat transfer, the contact pressure and the percentage contact area, but also the flow temperature of a cooling medium such as water, can influence and in particular reduce the achievable cooling rates.

It is known to optimize the parameters for press hardening using numerical methods. " Determination of Material and Process Characteristics for Hot Stamping Processes of Quenchenable Ultra High Strength Steels with Respect to a FE_based Process Design "(Merklein et al., SAE Int. Materuf., 2018, Vol. 1 (1) p 411 ) describes a numerical process that shows the thermo-mechanical flow behavior of a 22MnB ₅ steel in order to determine the most important parameters for press hardness.

In addition, it has been shown in practice that in the press hardening process, undesired diffusion-controlled conversions can take place at high temperatures (ferrite) due to the transfer of the hot sheet from the furnace to the press and, in particular, to high emissivities (high heat radiation behavior) of the sheet or sheet metal plate.

It was also found that the deep-drawing of these sheets in the hot state accelerates the transformation, so that in this case ferrite and bainite are more likely to be formed before the formation of martensite.

The object of the invention is to provide a method for press hardening of steels which facilitates, improves and makes the process control during press hardening more comprehensible.

The object is achieved with a method having the features of claims 1 and 2.

While only relatively few steels were available, particularly in the early days of press hardening, and thus the system geometry was adapted to these steels, many plants for press hardening steel are now in use. Such existing systems have properties that determine certain process parameters such as temperature, handling time, etc. According to the invention, a simple classification of the press-hardening steel can now be carried out using key figures, so that the classification can be used to estimate whether or not this steel is suitable for press hardening in an existing system or given existing process parameters.

Conversely, certain system parameters can be predetermined for an existing desired steel, and in particular the cooling rate in the tool.

With the given key figures and their ranges, the system parameters can also be taken into account depending on the degree of deformation.

In principle, high degrees of deformation mean that martensite is formed to a lesser extent. In order to achieve a desired martensite content, it can be estimated whether the corresponding hardness is still achieved with a given degree of deformation.

In particular, it can be estimated, for example, whether the steel used is suitable for the indirect press hardening process or also for the direct press hardening process at given cooling rates. In the indirect press hardening process, the steel is formed before the press hardening, so that no pressing takes place during the press hardening, even when heated. Such a process therefore requires a lower press hardness number than For example, a direct press hardening process, in which hot forming is also carried out.

This can reduce the number of attempts until the system can run with the special steel and, in particular, also reduce the scrap. In particular, it can be seen whether a steel is press hardened in the given process parameters in a limit range in which not every component reliably achieves the required properties, so that a large number of possible sources of error can be excluded in a favorable manner from the outset.

According to the invention, a press hardness number is created for this. The press hardness number (PHZ) is a tool with which you can easily estimate from the chemical composition and the cooling rate in the tool whether the desired fully martensitic structure can be achieved. Fully martensitic structure in the sense of this disclosure corresponds to a structure fraction of> 90 vol.%, In particular> 95 vol.% Martensite and residual austenite, remainder ferrite and / or bainite.

The number of press hardnesses can be used to estimate whether the existing tool technology in connection with the sheet thickness (= cooling rate) is sufficient to obtain a complete martensitic structure. For example, it can be determined whether alloy A produces martensite in a tool, but alloy B leads to ferrite and martensite.

In addition, the number of press hardnesses can be used to estimate which alloy is necessary to become fully martensitic at a given degree of deformation.

Figur 1:

eine Tabelle mit mehreren Stahlzusammensetzungen aus denen sich die jeweilige Presshärtezahl ergibt;

Figur 2:

qualitativ die Abhängigkeit der Martensitbildung vom Umformgrad;

Figur 3:

der kritische Umformgrad in Abhängigkeit der Press-härtezahl.

The invention is explained by way of example with reference to a drawing. It shows:

Figure 1:

a table with several steel compositions from which the respective press hardness number results;

Figure 2:

qualitatively the dependence of the formation of martensite on the degree of deformation;

Figure 3:

the critical degree of forming depending on the number of press hardnesses.

From the Figure 1 The press hardness cooling rates measured initially result for the most varied types of steel.

Here P, S, N are only contained as usual unavoidable impurities. V and Ti are not listed separately in the table and are added in the range of <0.5%, in particular <0.2%.

Here, Ti only serves to bind the N, whereby values of Ti / N (in at%) of approximately 3.4 should be sufficient. All other information is in mass%.

From the measured press hardness cooling rates (PHM), two different formulas are necessary according to the invention for determining the theoretical press hardness cooling rate (PHK). A distinction must be made here for steel materials whose dissolved boron content in the starting material is ≥ 5 ppm and the theoretical press hardness cooling rate (PHK) for steel materials whose dissolved boron content in the starting material is <5 ppm.

The formula for the theoretical press hardness cooling rates for material whose boron content in the starting material is ≥ 5 ppm is: $$\mathrm{PHK}\mathrm{K}/\mathrm{s}=1750/{\left(28.5*\mathrm{C.}\mathrm{m}\%+3.5*\mathrm{Si}\mathrm{m}\%+2.3*\mathrm{Mn}\mathrm{m}\%-\mathrm{2nd}*\mathrm{Al}\mathrm{m}\%+\mathrm{4th}*\mathrm{Cr}\mathrm{m}\%+\mathrm{3rd}*\mathrm{Ni}\mathrm{m}\%+\mathrm{25th}*\mathrm{Mon}\mathrm{m}\%-\mathrm{20th}*\mathrm{Nb}\mathrm{m}\%-6.3\right)}^{\wedge }2.7$$

For boron contents dissolved in the starting material <5 ppm, the following formula for the theoretical press hardness cooling rate results: $$\mathrm{PHK}\mathrm{K}/\mathrm{s}=2750/{\left(28.5*\mathrm{C.}\mathrm{m}\%+3.5*\mathrm{Si}\mathrm{m}\%+2.3*\mathrm{Mn}\mathrm{m}\%-\mathrm{2nd}*\mathrm{Al}\mathrm{m}\%+\mathrm{4th}*\mathrm{Cr}\mathrm{m}\%+\mathrm{3rd}*\mathrm{Ni}\mathrm{m}\%+\mathrm{25th}*\mathrm{Mon}\mathrm{m}\%-\mathrm{20th}*\mathrm{Nb}\mathrm{m}\%-7.0\right)}^{\wedge }1.8.$$

All percentages are generally mass percent.

The theoretical press hardness cooling rates can deviate from the measured press hardness cooling rates, since certain safety factors, for example to compensate for the measurement uncertainties, are built in and a sensible generalization has been carried out.

The press hardness number (PHZ) results from these determined formulas or formula values as follows: $$\mathrm{PHZ}=\mathrm{Cooling\; rate}\mathrm{in\; the}\mathrm{Tool}\left(\mathrm{PHW}\right)/\mathrm{theoretical}\mathrm{PH}\_\mathrm{Cooling\; rate}\left(\mathrm{PHK}\right).$$

PHZ < 1:

keine vollständige Härtung durch Martensit gewährleistet

PHZ= 1:

eine unverformte oder vorgeformte Platine kann gehärtet werden = indirekter Prozess

PHZ > 1:

eine Platine kann warm verformt werden bzw. steigende Sicherheit gegen plastische Umformung beim Härten (Warmumformeignung)

According to the invention, this results in the following regularity:

PHZ <1:

no complete hardening guaranteed by martensite

PHZ = 1:

an undeformed or preformed board can be hardened = indirect process

PHZ> 1:

a blank can be thermoformed or increasing security against plastic deformation during hardening (hot stamping suitability)

Out Figure 2 This results qualitatively in the relationship between the critical logarithmic degree of deformation and the hardness, regardless of whether this is measured in% martensite or hardness HV.

The critical logarithmic degree of deformation for the one-dimensional case is as follows: $$\varphi =\ln \left(\frac{l_1}{{\mathrm{<figure-callout\; id="0"\; label="l"\; filenames="imgf0002.png"\; state="\{\{state\}\}">l</figure-callout>}}_0}\right)$$

Out Figure 3 you can see the press hardness number against the critical logarithmic degree of deformation. The hatched area below the straight line from the press hardness number 1 indicates the area in which safe hot forming should be possible. The dashed curves around the straight line indicate possible curves, since this increase does not necessarily have to be linear.

The press hardness number or the theoretical press hardness cooling rate can thus be used to determine a steel material for an existing system which is hardened with sufficient certainty either in an indirect process or even has a high press hardness number such that effective direct press hardening, i.e. Forming in the warm state is possible.

For this purpose, the theoretical press cooling rate (PHK) must be determined according to the formula and the cooling rate (PHW) achievable in continuous operation must be determined for the respective forming tool.

Conversely, for a given desired steel material and the given desired process, i.e. in the direct or indirect process as well as a desired safety factor, the press hardness number can be determined and then the effective cooling rate can be determined in a simple manner by changing the formula given above.

The formal relationships in the theoretical press hardness cooling rate are chosen so that they usually include smaller influencing factors, e.g. different flow temperature of the cooling water for the tool depending on the season are still included.

Claims (2)

1. Method for press hardening of steel, wherein a steel sheet is either pre-formed cold from a hardenable steel alloy, subsequently transferred into a tool that substantially has the contour of the preformed component, and is cooled there following a preceding heating step that effects a complete austenitisation at a speed that lies above the critical hardening speed, so that a quench hardening of the preformed component is realised, or a plate made of a steel with a composition that allows press hardening is heated to a temperature above the austenitisation temperature and subsequently hot-formed in a tool and simultaneously cooled at a speed that lies above the critical hardening speed, so that hardening is brought about, wherein the hardening is effected in that the austenitic structure is transferred into a martensitic structure with > 90 vol. %, in particular > 95 vol. % martensite and residual austenite, residual ferrite and/or bainite, characterised in that
   the press hardness value is calculated for adjusting a desired tool to a given steel type, wherein the press hardness value PHZ is calculated from the equation $$\mathrm{PHZ}\left(\mathrm{press}\mathrm{hardness}\mathrm{value}\right)=\mathrm{cooling}\mathrm{rate}\mathrm{in}\mathrm{the}\mathrm{tool}\mathrm{PHW}/\mathrm{theoretic}\mathrm{PH}\_\mathrm{cooling}\mathrm{rate}\mathrm{PHK}$$

   

   wherein the cooling rate in the tool is predetermined for a desired sheet metal thickness or is measured and the theoretic PH_cooling rate PHK for steel material, the dissolved boron content in its starting material of which is > 5 ppm, is calculated as follows: $$\mathrm{PHK}\mathrm{K}/\mathrm{s}=1750/{\left(28.5\ast \mathrm{C}\mathrm{m}\%+3.5\ast \mathrm{Si}\mathrm{m}\%+2.3\ast \mathrm{Mn}\mathrm{m}\%-2\ast \mathrm{Al}\mathrm{m}\%+4\ast \mathrm{Cr}\mathrm{m}\%+3\ast \mathrm{Ni}\mathrm{m}\%+25\ast \mathrm{Mo}\mathrm{m}\%-20\ast \mathrm{Nb}\mathrm{m}\%-6.3\right)}^{\wedge }2.7$$

   

   and for which the boron content dissolved in the starting material of < 5 ppm results as follows: $$\mathrm{PHK}\mathrm{K}/\mathrm{s}=2750/{\left(28.5\ast \mathrm{C}\mathrm{m}\%+3.5\ast \mathrm{Si}\mathrm{m}\%+2.3\ast \mathrm{Mn}\mathrm{m}\%-2\ast \mathrm{Al}\mathrm{m}\%+4\ast \mathrm{Cr}\mathrm{m}\%+3\ast \mathrm{Ni}\mathrm{m}\%+25\ast \mathrm{Mo}\mathrm{m}\%-20\ast \mathrm{Nb}\mathrm{m}\%-7.0\right)}^{\wedge }1.8,$$

   

   wherein P, S, N are included as unavoidable contamination,
   
   wherein V m% and Ti m% are being alloyed to the same within a range of < 0.5 m%,
   wherein all percentage information is listed in percent by weight,
   wherein the following applies:

   PHZ < 1: no complete hardening through martensite formation guaranteed

   PHZ = 1: a non-deformed or pre-deformed plate can be hardened = indirect process

   PHZ > 1: a plate can be hot-formed in addition to the indirect process or greater resistance against plastic deformation during hardening (hot-forming suitability),

   wherein the reliably realisable cooling rate in the tool (PHW) is measured with a desired steel composition and a desired metal sheet thickness for this metal sheet thickness, and the press hardening value (PHZ) is calculated from the theoretic press hardening cooling rate (PHK) here, wherein the desired steel composition and the given system are suitable for an indirect press hardening process at a press hardening value of 1, and the heat-deformation can be carried out with greater safety as the press hardening value increases to a press hardening value of > 1.
2. Method for press hardening steel, wherein a steel sheet is either preformed cold from a hardenable steel alloy, subsequently transferred into a tool that substantially has the contour of the preformed component, and is cooled there following a preceding heating step that effects a complete austenitisation at a speed that lies above the critical hardening speed, so that a quench hardening of the preformed component is realised, or a plate made of a steel with a composition that allows press hardening is heated to a temperature above the austenitisation temperature and subsequently hot-formed in a tool and simultaneously cooled at a speed that lies above the critical hardening speed, so that hardening is brought about, wherein the hardening is effected in that the austenitic structure is transferred into a martensitic structure with > 90 vol. %, in particular > 95 vol. % martensite and residual austenite, residual ferrite and/or bainite, characterised in that
   the press hardness value is calculated for adjusting the suitable steel alloy to an existing system geometry,
   where the press hardness value PHZ is calculated from the equation $$\mathrm{PHZ}\left(\mathrm{press}\mathrm{hardness}\mathrm{value}\right)=\mathrm{cooling}\mathrm{rate}\mathrm{in}\mathrm{the}\mathrm{tool}\mathrm{PHW}/\mathrm{theoretic}\mathrm{PH}\_\mathrm{cooling}\mathrm{rate}\mathrm{PHK},$$

   

   wherein the cooling rate in the tool is predetermined for a desired sheet metal thickness or is measured and the theoretic PH_cooling rate PHK for steel material, the dissolved boron content in its starting material of which is > 5 ppm, is calculated as follows: $$\mathrm{PHK}\mathrm{K}/\mathrm{s}=1750/{\left(28.5\ast \mathrm{C}\mathrm{m}\%+3.5\ast \mathrm{Si}\mathrm{m}\%+2.3\ast \mathrm{Mn}\mathrm{m}\%-2\ast \mathrm{Al}\mathrm{m}\%+4\ast \mathrm{Cr}\mathrm{m}\%+3\ast \mathrm{Ni}\mathrm{m}\%+25\ast \mathrm{Mo}\mathrm{m}\%-20\ast \mathrm{Nb}\mathrm{m}\%-6.3\right)}^{\wedge }2.7$$

   

   and for which the boron content dissolved in the starting material of < 5 ppm results as follows: $$\mathrm{PHK}\mathrm{K}/\mathrm{s}=2750/{\left(28.5\ast \mathrm{C}\mathrm{m}\%+3.5\ast \mathrm{Si}\mathrm{m}\%+2.3\ast \mathrm{Mn}\mathrm{m}\%-2\ast \mathrm{Al}\mathrm{m}\%+4\ast \mathrm{Cr}\mathrm{m}\%+3\ast \mathrm{Ni}\mathrm{m}\%+25\ast \mathrm{Mo}\mathrm{m}\%-20\ast \mathrm{Nb}\mathrm{m}\%-7.0\right)}^{\wedge }1.8,$$

   

   wherein P, S, N are included as unavoidable contamination,
   wherein V m% and Ti m% are being alloyed to the same within a range of < 0.5 m%,
   wherein all percentage information is listed in percent by weight,
   wherein the following applies:

   PHZ < 1: no complete hardening through martensite formation guaranteed

   PHZ = 1: a non-deformed or pre-deformed plate can be hardened = indirect process

   PHZ > 1: a plate can be hot-formed in addition to the indirect process or greater resistance against plastic deformation during hardening (hot-forming suitability),

   wherein a suitable steel alloy is selected by means of the press hardening value at a known cooling rate in the tool PHW, wherein a steel, the PH_cooling rate of which PHK is set in such a way that at least one press hardening value of 1 is reached for an indirect press hardening process for this, and a press hardening value of > 1, preferably > 2 is reached for a hot-forming process.

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