FI124825B - Process for producing a metal-coated and hot-worked steel component and metal-coated steel strip product - Google Patents

Description

Method for manufacturing a metal coated and hot formed steel component and a metal coated steel strip product

The present invention relates to a method for manufacturing a metal coated and hot-formed steel component by direct hot-forming techniques and especially to a method according to the preamble of the independent claim 1.

The present invention further relates to a metal coated steel strip product for direct hot-forming and especially to such a product according to the preamble of an independent claim 15.

Background of the invention

Hot-forming for producing Automotive steel components is a well established process to produce high strength steel components. As a term, hot-forming includes several terms such as hot stamping or hot press forming. When the metallurgical phase is transformed, additional terms such as die quenching and press hardening are commonly used. Most common practice is the direct hot-forming, in which the shapes of the steel components are formed to the steel sheets at elevated temperatures. In a hot forming process including press hardening, the hardenable steel sheets are hot formed at high temperature and subsequently quenched with cooled press forming dies. Therefore, the steel component gains its shape and mechanical properties in the same press. This is an example of a direct hot-forming process including hardening in the press. Alternatively, the steel sheet can be first cold-formed to a pre-shaped steel component in the first press and subsequently the pre-shaped steel component is heated and hot-formed in the second press. This latter is so called indirect hot-forming process in which usually most of the deformation is carried out in cold-forming press.

In general, this hot-forming technique provides several benefits over cold-forming, for instance, the improved formability of the steel material at the heated state and the reduced spring back effect resulting from the improved dimensional accuracy of the steel components. Also forming at high temperature requires smaller form force that results in better material utilization. In addition, the steel component still includes some deformation capacity even after press hardening. However, this well-known hot-forming process also has some disadvantages such as the oxide layer that will form on the surface of the steel during heating. This iron-oxide must be removed before the coating process and causes the increase in costs. Also bare steel sheet surface can suffer from decarburization during heating that can result in loss of mechanical properties that is unacceptable. These drawbacks have a limited wider use of the hot-forming process in the production of ultra high strength Automotive steel components.

Therefore, in recent years, in order to meet ever increasing demand for lighter structures in the automotive industry, it is established that steel sheets having different metal coatings, such as zinc or zinc based or Al-Si coatings, are suitable for hot forming. It is discovered that the melting of the coating during austenitization is prevented or at least reduced by the mutual diffusion between the coating and the iron of the steel sheet. Those coatings are produced by known techniques such as hot dip galvanization or electroplating and they have the ability to protect the steel surface from decarburization and / or oxidation. Several fresh development efforts have been directed to develop different coatings for hot-forming to meet challenges in both direct and in-direct hot-forming processes. Naturally, in-direct hot-forming, the steel sheet must be formable so that cold press forming in the first press is possible. This means that the cold-rolled steel sheet must be soft annealed prior to being coated. Usually this is done prior to coating in a continuous annealing process at a temperature higher than the recrystallization temperature such as at temperatures 700-900 ° C for the time required for recrystallization. The following patent publications describe a state of prior art. WO2012053636 A1 discloses a steel sheet for direct hot forming that steel sheet can be plated by typical coating methods. The steel sheet comprises equal or more than 50% of the ferrite but equal or less than 30% volume fraction of the non-recrystallized ferrite in the order to avoid that the steel sheet becomes too hard. JP2008156680 A2 discloses a high strength cold-rolled steel sheet for cold-forming process. W012028224 A1 discloses a method for manufacturing a metal coated and hot-formed steel component but discloses a word about cold-rolling step. US5069981 discloses a low-carbon and metal-coated steel strip for a cold-forming process that comprises a non-recrystallized and rolled state.

Welding Is The Most Common Joining Method For Joining These Types Of Steel Components In Automotive Assembly Lines. Especially spot welding is widely used in welding of automotive steel components, for instance because it is suitable and fast method for thin steel sheets and can be completely automated. However, quick and automated process sets several requirements for factors affecting weldability of the steel component. For instance, general weldability of steel material should be as good as possible and lower CEV (carbon equivalent) values would be highly desirable. Typically commonly used hardenable steels such as different boron steel grades 22MnB5, are having quite high CEV values 0.50-0.55. Especially when hardened steel grades provide higher strength than the typical 22MnB5 are hot-formed, there arises a problem that CEV further increases and weakens the weldability, which is undesired without dispute. In addition, while strength increases, ductility typically suffers. To sum up, Improvements in mechanical properties of the metal coated and hot-formed steel component are difficult to obtain without weakening the weldability. In addition, different coating systems suffer from several types of cracking problems.

Object of the invention

The object of the present invention is firstly to provide a metal coated steel strip and secondly to provide a method for manufacturing a metal coated and hot-formed steel component that solves or at least alleviates one or more of the problems of prior art.

The object of the present invention is achieved with the method of manufacturing a metal coated and hot-formed steel component according to the characterizing portion of the claim 1. The object of the present invention is also achieved with a metal coated steel Strip product according to the characterizing portion of claim 15. The preferred embodiments of the present invention are disclosed in the dependent claims 2-14 and 16-24.

Short description of the invention

From the standpoints presented above, the inventories found in the direct press forming, the cold formability is not actually required due to the fact that the forming is carried out at elevated temperatures and that the mechanical properties of the metal coated and hot-formed steel component produced by direct hot-forming process can be affected without weakening weldability by affecting the initial grain structure of the metal coated steel Strip. Therefore, the present invention is based on the initial grain structure of the metal coated steel Strip product prior to heating the sheets for hot-forming. According to the invention, the grain structure of metal coated steel Strip made of hardenable steel alloy is deformed in terms of volume percentages of more than 70% non-recrystallized grains. Recrystallization of said grain structure is at least retarded or even completely avoided or leaved out prior to and during application of the metal coating on the surface of the steel strip.

This kind of steel strip product cannot be used in-direct press hardening, but unexpectedly, fits excellently to direct press forming and provides the ability to affect mechanical properties without weakening weldability. This is because a highly deformed grain structure consists of non-recrystallized grains include a higher number of potential nuclei for austenite (y) grains resulting from finely grained austenite (y) as a result of heating for austenite prior to hot-forming and cooling. When this kind of metal coated steel strip product having a finely grained austenite (y) in the microstructure is hot-formed and cooled, especially hot-formed and press-hardened, it has been found that the metal-coated and hot-formed steel component can include improved mechanical properties due to the finer prior austenite (y) grains. In a case of hot-forming and press hardening, finer packets and blocks of martensite can form from this kind of austenite (y). This provides the ability to lower the alloying level and thus improve the weldability by decreasing CEV without losing the strength. In addition, this kind of steel strip product can provide improved coating characteristics in hot press forming.

Furthermore, the steel Strip product according to the present invention is advantageously utilized in a quick heating and hot-forming cycle, which makes production including different lengths of unit-times more fluent. However, as the initial prior austenite (y) grain size can be small, the steel Strip according to the present invention can be utilized also in typical long hot-forming cycles. This is because the initial austenite (y) grain size is smaller which can at least partially compensate for the grain growth that occurs in the case of prolonged austering times in the furnace.

Brief description of the figures

Figure 1 is a schematic drawing of the steps included in the method according to the invention.

Figure 2 is a diagrammatic drawing of the steps that are included in the method according to the invention.

Figure 3 is a schematic drawing showing some alternative embodiments according to the invention.

Figure 4 is a schematic drawing of the time-temperature showing features of one detailed embodiment according to the invention.

Figure 5 is a schematic drawing of the time-temperature drawing showing the features of the second detail according to the invention.

Figure 6 is showing the steel product 16 according to the invention.

Figure 7 is showing an evolution of grain structure recrystallization when the Tmaxi is varied. Picture is taken by optical microscopy (LePera).

Figure 8 is a picture of a completely non-recrystallized grain structure. Picture is taken by optical microscopy (Nital).

Brief Description Of Reference Numbers And Terms 1 Step 1 For Providing A Steel Strip Made Of Hardenable Steel Alloy 2 Step 2 For Cold Rolling 3 Step 3 For Heating (Optional) 3.1 Step 3.1 For First Annealing (Optional) 4 Step 4 For Applying metal coating 4.1 step 4.1 for hot-dip coating (optional) 4.2 step 4.2 for electroplating (optional) 5 step 5 for blanking 6 step 6 for heating to a temperature higher than Ad 6.1 first heating section of step 6 6.2 second heating section of step 6 7 step 7 for hot-forming 8 step 8 for cooling 9 step 9 for second annealing (optional) 10 deformed grain structure consisting of non-recrystallized grains 10.1 deformed grain structure consisting of non-recrystallized grains 10.2 deformed and possibly recovered grain structure consisting of non-recrystallized grains 10.3 deformed and at least partially recovered grain struc ture composed of non-recrystallized grains 11 step 11 for third annealing (optional) 12 fine grained austenite (y) 13 step 13 for carrying out first preliminary heat treatment (optional) 14 step 14 for carrying out second preliminary heat treatment (optional) 15 step 15 for cooling (optional) 16 metal coated steel strip product for direct hot-forming 17 blank theat time calculated from starting from step 6 to starting from step 7 Tbath temperature the coating metal or metal alloy in the hot-dip coating bath

Tmax maximum temperature during step 13 or 14 Tmaxi maximum temperature during step 3.1 TmaX2 maximum temperature during step 9 Thot-dip temperature of the steel Strip during step 4.1 Trx recrystallization temperature

Ad a temperature in which austenite (y) begins to form during heating

Ac3 a temperature in which Transformation of Ferrite (a) to austenite (y) is completed during heating

Am a temperature in which austenite (γ) to Ferrite (a) is completed during cooling

Ar3 is the temperature in which austenite (γ) begins to transform to ferrite (a) during cooling

Detailed description of the invention

According to the invention, the grain structure of the metal coated steel Strip product is deformed comprising non-recrystallized grains 10.1, 10.2, 10.3. Recrystallization of said grain structure is at least retarded or even completely avoided or leaved out prior to and during application of the metal coating on the surface of the steel strip. This can be done for instance, by controlling the maximum heating temperature Tmax, before beginning of step 6 for austenitization, so that it is lower than the recrystallization temperature of the deformed grain structure comprising the non-recrystallized grains at the specified heating time depending on the line speeds of production facility.

Recrystallization requires a minimum temperature for the Atomic mechanisms to occur. Recrystallization temperature Τα is the temperature at which the grains in the Lattice structure have been re-arranged, leaving the steel free of strain and deformed grains. It is dependent on the heating time and coldrolling reduction rate. Therefore, Trx means the recrystallization temperature of the deformed grain structure comprising non-recrystallized grains prior to heat treatment at issue. This temperature can be calculated, in addition to a few constant material parameters, based on the case-specific heating time and the reduction ratio depending on the production conditions, for instance. However, as deformed grains include the high stored energy state they are thermodynamically unstable. This why, at least partial recovery of the deformed grain structure can occur, but the grain structure of the metal coated steel Strip is still deformed comprising non-recrystallized grains as explained below.

Referring to Figures 1 and 2, there is shown the Essence of the Present Invention for manufacturing a metal coated and hot-formed steel component according to the present invention. At the beginning, the steel strip made of Harden-able steel alloy is provided in step 1. This steel strip can be obtained for instance by hot-rolling and subsequently picking the hot-rolled steel strip. The steel strip includes alloying elements in the amount required for the desired amount of hardening, i.e. desired amount of martensite Transformation, so it is made of hardenable steel alloy. Said steel strip is further subjected to step 2, in which the cold-rolling is carried out. This means that, during step 2, the thickness of the steel strip is reduced. However, even more importantly as stated earlier, this cold-rolling step 2 is essential for the invention especially because it provides the deformed grain structure 10.1 comprising non-recrystallized grains to the steel strip. After said step 2 for cold-rolling, the steel strip is subjected to step 4 for applying the metal coating to the surface of the cold-rolled steel strip having deformed grain structure comprising non-recrystallized grains 10.1, 10.2, 10.3. In other words, said step 4 for applying a metal coating to a surface of a cold-rolled steel Strip having a deformed grain structure comprising non-recrystallized grains 10.1, 10.2, 10.3. Typically and contrary to this invention, especially if the steel strip is tended to be used also in-direct hot forming, as told earlier, this step 4 is carried out for soft-annealed steel strips consisting of recrystallized grains in the microstructure. This step 4 provides the corrosion protection layer to the steel product and protects the steel surface from decarburization and / or oxidation during heating involved in the hot-forming treatment. In Essence, step 4 for coating can be carried out by different methods, few of which are presented later. As can be seen from figs 1 and 3, the metal coated steel strip product according to the present invention is basically obtained after step 4. However, step 4 may be followed by step 9 for second annealing, such as galvanneal type annealing ( ie to form intermetallic compounds comprising iron and coating metal to the coating, shown in Figs. 4 and 5) and subsequent cooling before the steel Strip product is coiled (not shown in the figs). Typically, but not necessarily, steel strip products are delivered from the steel factory in coil form to the site having hot-forming equipments. Also cut to length products, i.e. sheet form is sometimes adequate. Therefore the term steel Strip product includes also steel sheets 17.

Referring again to FIG. 1, the inventive method continues by using the invented metal coated steel strip product 16, which method actually provides for the benefits of the invented metal coated steel strip product. Irrespective of location or delivery format of steel product, after step 4, method comprises step 5 in which steel sheets 17 are blanked from said metal coated steel strip product. This can be done by mechanical cutting equipments, for instance. Thereafter in step 6 the steel sheets 17 are heated to a temperature in which the microstructure comprises austenite (y). This means that the steel sheets 17 are heated to a temperature higher than Aci, preferably higher than Ac3. Thanks to specific step 4, after step 6 there can be obtained fine grained austenite (y) 12 to the microstructure of the steel sheet 17. In other words specific step 4 can be done in order to obtain fine grained austenite (y) 12 to the microstructure of the steel sheet 17 after step 6. Ad is the lower limit because otherwise strength increment cannot be achieved. However, the dual-phase temperature range (Ad to Ac3) can provide, during heating, a dual-phase microstructure having a mixture of fine-grained austenite (y) and elongated ferrite (a) -perlite grains. Ac3 is the preferred lower limit because this way the maximum amount of martensite can be formed in the microstructure and also the effect of the grain Refining can be more effective. Also beneficial equi-axial grain structure has been achieved. However, the temperature at the end of the heating is preferably not more than Ac3 + 50 ° C, preferably not more than Ac3 + 30 ° C. Such as a temperature in the range of 880 to 950 ° C. This is because at higher temperatures, excessive grain growth can occur. If the steel sheet 17 is heated to a temperature higher than Ac3 but lower than Ac3 + 50 ° C, the microstructure consists completely of fine grained austenite (y) 12 can be obtained. It is also possible that step 6 for heating comprises first heating section 6.1 and second heating section 6.2, as shown in figs 4 and 5. After step 6 for heating, heated steel sheet 17 is hot-formed in step 7. This can be carried out for instance in a press forming apparatus, namely a hot-press forming apparatus. It must be noted that said steel sheet is connected to a steel sheet that is not (pre-) shaped before step 6 or step 7. During this step 7, the steel sheet 17 is hot-formed into a steel component, which has at least partially the shapes corresponding to the shapes of the final steel component. After the hot component of the steel component is cooled in step 8. This can be carried out in different ways known in the art depending on the desired mechanical properties of the steel component. During this cooling step, the desired microstructural changes in the steel component take place. Usually after the cooling step, edge trimming and / or some perforations can be made to the steel component. Also removal of oxides on the surface of the steel component is possible prior to painting. Obtained steel component can hold improved mechanical properties.

According to one method the method includes also a step 13 for carrying out the first preliminary heat treatment for the cold-rolled steel Strip. This preliminary heat-treatment step 13 is carried out after step 2 and before step 4. Said step 13 is carried out so that the maximum temperature Tmax during this step 13 is lower than the recrystallization temperature To <of the deformed grain structure including non-recrystallized grains 10.1, 10.2, in other words Tmax <Tn <. This temperature limit provides even though the cold-rolled steel Strip is subjected to heat-treating, the microstructure still maintains the deformed grain structure 10 composed non-recrystallized grains. In other words, annealing below recrystallization temperature Trx will Preserve the deformed grains caused by cold rolling and hence the metal coated steel Strip product preserves also very high yield ratio as described in more detail later in this description.

Preliminary heat-treatment may be needed in order to improve adhesion between the metal coating and the steel surface of the steel strip, prior to step 4 for coating. This is especially the case if step 4 is carried out by hot-dip coating 4.1, as shown in figs 3, 4 and 5. Other reason for preliminary heat-treament can be related to energy efficiently of manufacturing line. However, in other coating processes, such as in the electroplating process, the first preliminary heat treatments are not necessarily carried out, as shown in Figure 3, but the first preliminary heat treatment can be connected to the surface treatments, for instance.

Preferably, said step 4 is carried out by hot-dip coating 4.1 the steel strip in a bath of coating metal. This is because hot dip coating provides excellent coating protection performance. Steel components made of hot-dip coated steel strip, especially hot dip galvanized steel strip, are particularly suitable for end products such as Automotive vehicles. Furthermore, optional step 9 for second annealing, such as galvanneal type annealing, can be more energy efficiently performed due to the initial elevated temperature of the hot dip coating process. In other words, in case of hot-dip coating there was a need for lower temperature increment for second annealing. In addition, hot dip coating may form natural diffusion between the iron and metal of the coating. As shown in Figs. 4 and 5, during this step 4.1 for hot dip coating, the preheated steel is immersed at a specific temperature Thot-dip into a bath of liquid coating at a specific temperature Tbath- Typically the temperature of the coating metal in the hot-dip coating bath is not more than 100 ° C greater than the melting temperature of the coating metal in the bath. The temperature Thot-dip at the moment of immersion of the steel strip into the bath is preferably higher than the temperature Tbath - 100 ° C, more preferably higher than the temperature Tbath, as shown in Figures 4 and 5. In the case of hot-dip coating, this pre-heating is carried out in step 13 for the first preliminary heat treatment, which includes a few embodiments shown below.

According to the first embodiment of the method, shown in FIG. 4, step 13 for the first preliminary heat treatment comprises step 3 for heating the cold rolled steel to a temperature Thot-dip higher than the temperature Tbath-100 ° C, which Tbath is the temperature of the coating metal in the hot-dip coating bath, and carrying out step 4.1 for the hot-dip coating at this Thot-dip temperature. More advantageously, step 3 for heating the cold-rolled steel Strip is carried out to a temperature Thot-dip higher than the temperature Tbath. This way the coating adhesion is improved. This first embodiment is preferred because it excludes excessive heat treatments that can reduce the effect of the present invention due to the recovery and / or recrystallization tendency of the cold-rolled steel Strip at high temperature for a long time. Therefore, in this embodiment shown in Fig. 4, the advantages of hot-dip galvanized steel product according to the present invention can be emphasized. Typical pre-treatments such as surface cleaning and / or other surface treatments are preferably carried out for cold-rolled steel Strip as usual. However, electrolycal cleaning or other methods do not require significant heating are preferred. These pretreatments of the surface improve the coating adhesion.

As shown in Fig. 5, in an alternative second embodiment, step 13 comprises several steps, which first step is a step 3 for heating the cold-rolled steel Strip to a first annealing temperature Tmaxi. Thereafter step 3.1 for first annealing at a temperature of 420 to 800 ° C, more preferably at a temperature of below 420, to 695 ° C, is carried out. First annealing may be needed in order to treat the surface with burners, which is one optional pretreatment method to improve the coating adhesion. Other reason for step 3.1 for first annealing can be related to energy efficiently of manufacturing line, especially if several steel grades are produced on same production line. As can be seen from FIG. 5, step 3.1 is followed by step 15 for cooling the steel to a temperature Thot-dip higher than the temperature Tbath -100 ° C, which Tbath is the temperature of the coating metal in the hot-dip coating bath. and carrying out hot dip coating at this Thot-dip temperature. More advantageously, step 15 for cooling the cold-rolled steel Strip is carried out to a temperature Thot-dip higher than the temperature Tbath. This way the coating adhesion is improved. In other words, in this embodiment, the deformed grain structure after cold rolling can be preserved by lowering the continuous annealing temperature below the recrystallization temperature that can be evaluated for the instance by Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation that is available in literature.

Figure 7 is showing an evolution of grain structure recrystallization when the temperature Tmax of first annealing step 3.1 is varied, that step 3.1 was carried out after step 2 for cold-rolling. Pictures are taken by optical microscopy (LePera). Hardenable steel alloy had the following composition: C: 0.23%, Si: 0.25%, Mn: 1.25%, Al: 0.03%, Cr: 0.2%, Ti: 0.037% and B: 0.003%. As can be seen from experiments when Tmaxi is in the range 420 to 800 ° C in experiments 1 and 2, deformed and at least partially recovered grain structure formed by non-recrystallized grains 10.3 is obtained. As can be seen if Tmaxi is higher than 800 ° C, like in experiments 3 and 4 of Fig. 7, the grain structure consisting of non-recrystallized grains 10 is difficult to be preserved and complete recrystallization very easily, if not inevitable, occurs quickly . Grain structures of experiments 3 and 4 are completely recrystallized.

Microalloying helps to retard the complete recrystallization at temperatures between Ac1 and 800 ° C. However, at temperatures above 800 ° C and with typical carbon content such as around C: 0.25wt-%, austenite (y) becomes dominant / majority phase and the benefits of the present invention are reduced. Steel strips of experiments 1 and 2 were having a yield ratio higher than 0.75 and steel strips of experiments 3 and 4 were having a yield ratio of less than 0.75.

Figure 8 is a picture of a completely non-recrystallized grain structure (Nital) obtained in experiment 5 using a temperature Tmaxi below 695 ° C. Such a steel strip was having a yield ratio higher than 0.80.

It is preferred that said step 7 for hot-forming is started within 3 minutes calculated from starting of step 6 for heating, i.e. theat ^ 3min. This is because this way finer grained austenite (y) 12 is obtained. This way also cycle times the heating door (equipment in step 6) and the forming apparatus (equipment in step 7) are better synchronized with reasonable sized heating ovens. It is not advantageous to oversize these ovens, because in case of an interruption in the forming apparatus, such as tool breakage, all blanks 17 in the door would be quickly scrapped because of the prolonged heating time which results in grain growth and coating failures.

However, as this initial prior austenite (y) grain size can be small, the steel Strip according to the present invention can be utilized also in typical long hot-forming cycles. This is because the initial austenite (y) grain size is smaller which can at least partially compensate for the grain growth that occurs in the case of prolonged austenizing times in the furnace.

It is still more preferred that said step 6 for heating is carried out at an average heating rate higher than 20 ° C / sec, preferably higher than 30 ° C / sec. This can be carried out for instance by induction heating equipment which provides extremely high heating rates. This way, the benefits of the present invention are more advantageously achieved because the opportune time for grain growing and / or dislocations (which provides nucleation sites for austenite (y) grains) is reduced resulting in finer grain size of austenite (y). For these reasons, preferably also step 11 for third annealing, i.e. holding at austenite (y) range, as shown in Figures 4 and 5, is kept short, such as less than 2 minutes or even less than 1 minute. Short holding may be necessary to allow some time for austenite (y) to homogenize, but it should be kept as minimal as possible to avoid grain growth. Therefore, said step 7 for hot-forming can be started within 3 minutes or even within 2 minutes calculated from starting of step 6 for heating, i.e. theat ^ 3 min or even theat ^ 2min. In a case of heating rates higher than 100 ° C / s, Wat ^ 1min can be obtained which is extremely rapid treatment resulting in improvement in properties. High heating rates results in fluent production flow in a method for manufacturing a metal coated and hot-formed steel component with improved mechanical properties.

As shown in Figs. 4 and 5, according to one embodiment, step 6 for heating comprises first heating section 6.1 and second heating section 6.2 and that heating rate for second heating section 6.2 is lower than heating rate for first heating section 6.1. However, the average heating rate of step 6 can be as given above. Lowered heating rate in second heating section 6.2 Improves coating properties during step 6 and 7 for heating and hot-forming while higher heating rate in first heating section 6.1 Suppresses discharge of nucleation sites and supports more dense nucleation of austenite (γ).

As can be seen from Figures 4 and 5, the method comprises but not necessarily, after step 4 and before step 6, also a step 14 for carrying out a second preliminary heat treatment including step 9 for a second annealing at temperature TmaX2 lower than recrystallization temperature of the deformed grain structure comprising non-recrystallized grains 10.1, 10.2, 10.3, such as at a temperature of 450 to 600 ° C, in other words TmaX2 <Trx. However, this step 9 for second annealing can be carried out in any case, in other words irrespective of if or how steps 13 and 4 are realized. In the case of high heating rates during step 6 and low heating times theat, this step 9 is especially preferred since it provides iron pre-alloying in the metal coating of the steel strip, which is particularly advantageous when the benefits of the invention are realized in the step 6 for quick heating revealed above. As explained above, there is not necessarily an overmuch time to wait for suitable iron content to diffuse into the coating metal during step 6 for quick heating or during step 11 for third annealing. However, in order to improve the hot formability and quality of the coating, it may be necessary to have specific intermetallic alloys and / or solid solutions in the metal coating layer during austenitization, especially in the interface between steel and coating because this way problems related to LME (liquid metal embrittlement) can be avoided. For this reason, step 9 for second annealing is highly preferred in case quick heating cycles involved in hot forming the steel product according to the present invention, in other words in the case of small theat.

As can be understood from above, both step 13 for first preliminary heat treatment and / or step 14 for second preliminary heat treatment can cause recovery from deformed grain structure 10.1. However, even though some recovery may occur, deformed and recovered grain structure 10.3 still consists of non-recrystallized grains because recovery does not affect substantially the grain shape. Therefore, in step 6 for heating steel sheet 17 having at least partially recovered and deformed grain structure (10.3) consisting of non-recrystallized grains, can be used. Non-recrystallized grains means grains that have got the shape that they have gained in cold-rolling, such as the elongated grain shape that is distinguishable from the typical soft-annealed equi-axial grain shape of the metal coated steel strip.

In certain production conditions, also partial recrystallization of deformed grain structure may occur. This is because recrystallization can be divided into nucleic-process and growth-processes. This can be the case if the heating time has been long despite the maximum temperatures Tmaxi and / or Tmax2 of preliminary heat treatments 13.14 have been below the recrystallization temperature Trx of the deformed grain structure including non-recrystallized grains. Therefore, in step 6 for heating steel sheet 17 having grain structure comprising, in terms of volume percentages, more than 70% non-recrystallized grains and more preferably more than 95% non-recrystallized grains, can be used. However, most preferably such a sheet of steel is used that holds a completely non-recrystallized grain structure. Recovery and / or partial recrystallization does not necessarily provide any benefits, but can be an unavoidable event in certain production conditions such as certain hot-dip galvanizing lines involving heating furnace.

As said earlier, during step 2, the thickness of the steel strip is reduced. Reduction degree is typically 30-95% but preferably 25-65%. This is because lower reduction rates cause less driving force for recrystallization which can be advantageous especially in case of optional preliminary heat treatment steps 13 and / or 14 are included in the method. However, in other cases, also higher reduction rates can be used.

According to one embodiment, the cooling step 8 for cooling said shaped steel component is carried out at cooling rate sufficient to induce at least partial, preferably complete hardening of the shaped steel component. The cooling rate depends on steel alloy, sheet thickness and degree of hot forming, but can be such as higher than 10 ° C / sec, or higher than 30 ° C / sec. For instance with 22MnB5 cooling rate higher than 30 ° C / s can be required to obtain completely martensitic microstructure. This way ultra high strength of the steel component is assured.

According to one embodiment, step 7 for hot forming is carried out at a temperature in the range 450-850 ° C, preferably 500-700 ° C, because this way the risk of coating failures caused by LME can be reduced. In addition, recrystallization of austenite (y) grains is avoided resulting in fine austenite (y) grains that are elongated during the hot-forming step 7. Lower limits are because otherwise the martensite Transformation can start during the hot-forming. The upper limits are to ensure the above mentioned benefit (s). In this embodiment, the method comprises a pre-cooling step (not shown in figs) by using the accelerated cooling rate to a hot-forming temperature that received the pre-cooling step carried before step 7 for the hot-forming. This way austenite (y) decomposition during pre-cooling can be avoided together with suitable alloying.

It is preferred that step 7 for hot-forming is carried out in a mold of the forming apparatus and that step 8 for cooling is carried out in said mold. This way dimensional accuracy of the steel component is made because the mold prevents dimensional distortions to occur during cooling.

Preferably, said metal coating is a zinc (Zn) based coating. This is because the benefits of the invention are advantageously achieved since Zn enables the use of lower coating temperatures, especially in the hot-dip coating process due to the relatively low melting temperature of Zn. Additionally zinc based coating provides Galvanic corrosion resistance.

As described earlier, the steel strip is made of hardenable steel alloy. Therefore, it may comprise the following alloying: C: 0.08 - 0.45 wt-% Μη + Cr: 0.5 - 3.5 wt-%

Si: <1 wt-%

Al: <0.2 wt-%

Ni: <1.0 wt-%

Cu: <1.0 wt-% B: <0.01 wt-%

Ti: <0.2 wt-%

Nb: <0.2 wt-%

Mo: <1.0 wt-% V: <0.5 wt-%

Ca: <0.01 wt-%, balance being iron and residual contents or unavoidable impurities.

Especially in the case of step 4.1 for hot-dip coating including step 13 and / or 14 for preliminary heat-treatments, in terms of mass percentages, 0.01-0.1 wt-% microalloying with one or more of the alloying element belonging to the group compositions of Titanium (Ti), Vanadium (V), Niobium (Nb) are preferred because microalloying prevents recrystallization during optional steps 13 and / or 14 for carrying out preliminary heat treatments, by increasing recrystallization temperature TIn addition, they also hinder austenite (y) grain growth during steps 6, step 11, and step 7. Therefore, the use of microalloying elements is highly preferred in the case of hot-dip coating.

Niobium is found as a favorite alloying element in the present invention, and therefore such a hardenable steel alloy is used, which contains Nb: 0.005-0.1 wt-%, preferably 0.01-0.08 wt-%, in order to increase the recrystallization temperature Trx and to control the grain size during austenizing.

The reasons and more preferred limits of alloying above are briefly explained later in this description. Next, the product according to the present invention is described in greater detail.

The product according to the present invention is a metal coated steel strip, made of hardenable steel alloy, the product is intended to be used in direct hot-forming, as described earlier. This is because the grain structure of said steel strip is deformed comprising non-recrystallized grains 10, as shown in Fig. 6. Therefore, the metal coated steel strip product holds only very small cold-deformation capacities and can be substantially shaped only by hot-forming. , ie in direct hot-forming process. However, as there is a deformed grain structure comprising non-recrystallized grains 10, there are more nucleation sites for prior austenite (y) in direct hot-forming process and prior austenite (y) grains can be finer, providing the solution or at least alleviation to the problem of prior art. In addition, this kind of steel strip product can provide improved coating characteristics.

The steel strip product is preferably additionally a cold-rolled steel strip, especially such a cold-rolled steel strip that is cold-rolled by using a reduction degree in the range of 30-95%, preferably 25-65%. The degree of reduction means the percentage of reduction of the thickness of the steel strip during cold-rolling. As told earlier cold-rolling was performed in step 2 prior to coating in step 4. Thickness of the steel strip product can be less than 5mm, preferably in the range 0.4 to 2.5mm.

The grain structure of said metal coated steel Strip comprises, in terms of volume percentages, more than 70% non-recrystallized grains and preferably more than 95% non-recrystallized grains. Most preferably the grain structure of the metal coated steel strip is completely non-recrystallized. Also full-hard term may be used. This is because the more non-recrystallized grains, the more nucleation sites. However, the number of nucleation sites can be, in some embodiments, reduced by recovery. Therefore, the grain structure of this metal coated steel strip can be partially or totally recovered 10.3.

As the grain structure of the steel strip is deformed comprising non-recrystallized grains, the yield ratio (Rp0.2 / Rm) of the steel strip product can be more than 0.75, preferably more than 0.80 and most preferably more than 0.90. Typically also yield strength (Rp0.2 or REL) of said steel Strip product is higher than typical value of such product, being preferred over 550MPa, more preferred than 650MPa. As a comparative example, the typical yield strength of a har denable boron steel is 400-450MPa, i.e. lower than 500 MPa. Mentioned strengths are values before step 6 or step 7, i.e. the values of the steel Strip in typical delivery condition from the steel factory. As can be understood from this description, preferably the steel strip product is going to be quenched in the subsequent steps included in the hot-forming process, i.e. it is not quench hardened in delivery condition, i.e. prior to being blanked in step 5. After possible quench hardening in step 8, the yield strength can be more than 900MPa, preferably more than 1100MPa.

Deformed and at least partially recovered grain structure composed of non-recrystallized grains 10.3 can be characterized in a yield ratio greater than 0.80, preferably greater than 0.90 combined with a total elongation higher than 3%.

Preferably, the hardenable steel alloy, from which the steel strip product is made, comprises in terms of weight percentages the following: C: 0.08 - 0.45 wt-%

Mn + Cr: 0.5 - 3.5 wt-%

Si: <1 wt-%

Al: <0.2 wt-%

Ni: <1.0 wt-%

Cu: <1.0 wt-% B: <0.01 wt-%

Ti: <0.2 wt-%

Nb: <0.2 wt-%

Mo: <1.0 wt-% V: <0.5 wt-%

Ca: <0.01%, the balance being iron and residual contents or unavoidable impurities.

Especially in the case of step 4.1 for hot-dip coating including step 13 and / or 14 for preliminary heat-treatments, in terms of mass percentages, 0.01-0.1 wt-% microalloying with one or more of the alloying element belonging to the group compositions of Titanium (Ti), vanadium (V), niobium (Nb) is preferred because microalloying prevents recrystallization during optional steps 13 and / or 14 for carrying out preliminary heat treatments by raising the recrystallization temperature T ^. In addition, they also provide austenite (γ) grain growth during steps 6, step 11, and step 7. Therefore, the use of microalloying elements is highly preferred in the case of hot-dip coating.

Next reasons for given hardenable steel alloy above is explained in short. Carbon C is a basic element to increase hardness of steel and it primary determinants of hardness and tensile strength of steel after hardening quenching. Therefore at least 0.08% carbon is preferred, more preferably the lower limit for carbon is 0.15%. However, if the carbon exceeds 0.45%, the weldability will be weaken too much. Therefore, the upper limit for carbon is preferably 0.3%. Manganese Mn and Chromium Cr are also very effective elements to increase hardness and they are preferred to be used in the range of 0.5-3.5%. However, too high contents may impair ductility and therefore more preferably the sum of Mn and Cr is in the range 0.8-2.0%. Silicon Si is also an element that also increases hardenability. In addition it can be utilized as a deoxidation agent. However, it content is preferred to be limited to below 1% since too high Silicon would cause surface defects. Most preferably, Silicon is alloyed in the range 0.05-0.5%. Aluminum can be alloyed up to 0.2%, but it is more preferably alloyed in the range 0.01-0.06 wt-% to serve as a deoxidation agent. Also Nickel Ni and Copper Cu can be utilized up to 1% for increasing hardness, but their contents are more preferred below 0.5% and below 0.2%, respectively. Boron B is a very effective element to increase hardness with relatively low alloying levels up to 0.01%. The hardenability is advantageously solved by alloying boron in the range 0.0001 - 0.005%. In this case it is necessary to alloy also Titanium Ti 0.01 - 0.1% to protect boron B from Nitrogen N by forming TiN. If Ti is alloyed more than stoichiometric amount in relation to nitrogen (i.e., Ti> 3.4N), excess Titanium hinders also coarsening of austenite (y) grains during step 11 for third annealing, by forming TiC. However, if boron is not alloyed, it is possible to leave out Titanium or also alloy Ti up to 0.2% to gain Precipitation hardening. Niobium Nb can be alloyed up to 0.2%, but it is preferably alloyed within the range 0.005 -0.1%, even more preferably within the range 0.01-0.08 wt-% in order to raise the re-crystallization temperature (Trx) and to control the grain size during austenizing. Niobium form nitrides, carbonitrides, and / or carbides which hinders recrystallization. Also, molybdenum Mo can be alloyed up to 1% to promote martensite formation and to raise the recrystallization temperature. However, since Mo is relatively expensive alloying element, it content is more limited up to 0.2%. Vanadium V can be alloyed up to 0.5% to increase hardenability. However, from the point of view of alloying costs, V up to 0.2% is preferred. Also it can be utilized as a microalloying element in content 0.01-0.1%. Calsium Ca can be included in steel due to possible Calsium treatment included in smelt processing. Such as Ca up to 0.01% or 0.0001 - 0.005%.

Unavoidable impurities can be as nitrogen N, phosphor P, Sulfur S, oxygen O and REM (rare earth metals). Nitrogen N is an element that can bind microalloying elements existing in the steel to nitrides and carbonitrides, thus it can form precipitates. This why, nitrogen content of at least 0.001% can be included in steel. However, nitrogen more than 0.01% would allow nitrides to coarsen, and therefore nitrogen content should be preferably limited to 0.001-0.01%. Phosphor P is usually unavoidably included in steel and can be restricted to 0.06% since higher content can be harmful for ductility. Most advantageously the upper limit for P is 0.02%. However, excessive lowering of P may be disadvantageous economically and therefore lower limit 0.005% may be applied. Therefore P content can be 0.005-0.06%. Sulfur S is usually unavoidably included in the steel and can be restricted to lower contents than 0.02%, preferably to lower contents than 0.01%. However, excessive desulfurization may be disadvantageous economically and therefore a lower limit of 0.0001% may be applied. Therefore S content can be 0.0001-0.02%. Oxygen can exist as an unavoidable element in the steel, but can be restricted to less than 0.005%, more preferably less than 0.002%. This is because it can exist as inclusion that can debilitate properties.

Metal coating referenced in this description and claims related to metal and metal alloy coatings. Similarly zinc based coating is applied to zinc and zinc alloy coatings that are having more than 50% Zn in the metal coating.

As can be understood from earlier sections of this description, it is preferred that the metal coating of the metal coated steel Strip product according to the present invention is a zinc based coating. In other words, the steel strip is galvanized. Even more preferably the steel Strip is hot-dip galvanized.

As a consequence of step 14 for carrying out second preliminary heat treatment including step 9 for second annealing, iron pre-alloying in the metal coating can occur. This means that the iron strip is diffused into the metal coating, thus forming a different type of iron alloyed metal coating compounds, intermetallic alloys and / or solid solutions. For example, Fe-Zn compounds, Fe-Zn intermetallic alloys and / or Fe-Zn solid solutions can form into the coating as a consequence of step 14 including step 9.

Therefore, according to preferred embodiment, the metal coating of the steel is an iron alloyed metal coating, more preferably an iron alloyed zinc coating such as Galvannealed coating. This means that the metal coating of the steel Strip product according to this coverage (Galvannealed coating) can form in terms of weight percentages Fe: 5-50% in addition to Zn. Iron (pre-) alloyed zinc coating provides the ability to shorten the heating cycle in hot-forming thus improving the effects of the present invention.

In other application metal coating of steel strip is aluminum or aluminum-magnesium alloyed zinc coating. This means that the metal coating of the steel strip product according to this can be formed in terms of weight percentages, Al: 1-20% and optionally Mg: 1-5% in addition to Zn.

In yet another embodiment, the metal coating of steel is a galvanized coating comprising terms of mass percentages, Fe: less than 2%, Al: less than 1%, and Zn: higher than 97%.

It will be obvious to a person skilled in the art that, as technology advances, the Inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims (24)

A method for manufacturing a metal coated and hot formed steel component, the method comprising the following steps in a given order: • step 1 to provide a steel strip made of a hardening steel alloy, and • step 2 to cold roll said steel strip to obtain a cold rolled steel strip. recrystallized granules, and • step 4 to deposit a metal coating on the surface of said cold rolled steel strip to obtain a metal coated steel strip product, and • step 5 to extrude said metal coated steel strip product, and • step 6 to heat said steel sheet to , preferably higher than A ^, and • step 7 for hot forming the heated steel sheet (17) to obtain a shaped steel component, and • step 8 for cooling said shaped steel component, characterized in that step 4 to deposit the metal coating on the surface of the cold rolled steel strip is carried out on a steel strip having a further deformed grain structure comprising non-recrystallized granules (10.1, 10.2, 10.3) and that in step 6 a steel plate (17) still having a recrystallized granules (10.1,10.2,10.3). A method for manufacturing a metal-coated, thermoformed steel component according to claim 1, characterized in that the method also comprises the step of performing a first preheating on a cold-rolled steel strip, step 13 after step 2 and before step 4, and wherein said step 13 is performed the temperature (Tmax) during this step 13 is lower than the recrystallization temperature (Tn <) of the modified granular structure comprising the non-recrystallized granules (10.1, 10.2), in other words Tmax <Tn «. Method for producing a metal-coated, thermoformed steel component according to claim 2, characterized in that said step 4 is carried out by hot immersion by coating (4.1) a steel strip in a bath of the coating metal. A method for manufacturing a metal-coated, thermoformed steel component according to claim 3, characterized in that said step 13 comprises the following step: • step 3 to heat a cold rolled steel strip to a temperature (Thot-dip) higher than Tbath - 100 ° C, which Tbatn is bath, and perform step 4.1 for hot immersion coating at this (Thot-dip) temperature. A method for manufacturing a metal-coated, thermoformed steel component according to claim 3, characterized in that said step 13 comprises the following steps in a given order: • step 3 for heating a cold-rolled steel strip to a first annealing temperature (Tmaxi), and • step 3.1 for a first annealing temperature (Tmaxi) ° C to 800 ° C, more preferably at a temperature below Ad, and • Step 15 to cool the steel strip to a temperature (Thot-dip) higher than Tbatb -100 ° C, which Tbath is the temperature of the coating metal in the hot immersion coating bath; at this (Thot-dip) temperature. A process for producing a metal coated and hot formed steel component according to any one of the preceding claims, characterized in that step 7 for hot forming is initiated within 3 minutes of heating from the beginning of step 6, i.e. the thirdest to 3min. A process for producing a metal coated and hot formed steel component according to any one of the preceding claims, characterized in that the average heating rate during step 6 is higher than 20 ° C / sec, preferably higher than 30 ° C / sec and thereafter the heating is carried out by, for example, an induction heater. Process for producing a metal-coated and hot-formed steel component according to any one of claims 1 to 7, in particular claims 6 to 7, characterized in that the process further comprises, after step 4 and before step 6, a second secondary heat treatment comprising step 9 for second annealing at temperature (TmaX2) under a modified granule structure comprising non-recrystallized granules (10.1, 10.2, 10.3), a recrystallization temperature (Tm), such as at a temperature in the range 450 to 600 ° C, i.e. Tmax2 <Trx · Method for producing a metal-coated and hot-formed steel component according to any one of the preceding claims, characterized in that said step 8 for cooling the shaped steel component is carried out at a cooling rate sufficient to cause at least partial, preferably complete, shaped steel component. Method for producing a metal-coated and hot-formed steel component according to any one of the preceding claims, characterized in that said step 7 for hot forming is in the mold of a forming device and said step 8 for cooling is carried out in said mold. Method for producing a metal coated and hot formed steel component according to any one of the preceding claims, characterized in that the metal coating is a Zinc based coating. A process for the production of a metal coated and hot formed steel component according to any one of the preceding claims, characterized in that said hardening steel alloy comprises by weight: C: 0.08 - 0.45 wt-% Mn + Cr: 0.5 - 3.5 wt-% Si: <1 wt-% AI: <0.2 wt-% Ni: <1.0 wt-% Cu: <1.0wt-% B: <0.01 wt-% Ti: <0.2 wt-% Nb: <0.2 wt-% Mo: <1.0 wt-% V : <0.5 wt-% Ca: <0.01wt-%, with the remainder being iron and residual or unavoidable impurities. The process for producing a metal-coated, thermoformed steel component as claimed in claim 12, characterized in that said hardening steel alloy further comprises 0.01 to 0.1 wt% of a microalloy with one or more alloying elements belonging to the group consisting of titanium (Ti), vanadium (V), niobium. (Nb). The process for producing a metal-coated, thermoformed steel component according to claim 13, characterized in that said hardening steel alloy further comprises niobium (Nb) 0.005-0.1 wt%, more preferably niobium (Nb) 0.01-0.08 wt%. A metal-coated steel strip product directly for hot forming (16) made from a hardening steel alloy, characterized in that said steel strip has a granular structure modified to contain more than 70% by volume non-recrystallized granules (10.1,10.2,10.3). Metal coated steel strip for direct hot forming (16) according to claim 15, characterized in that the steel strip yield ratio (Rpo.2 / Rm) is more than 0.75, preferably more than 0.80 and most preferably more than 0.90. Metal coated steel strip product for direct hot forming (16) according to claim 15 or 16, characterized in that the yield strength (Rpo.2 or REi_) of the steel strip is more than 550MPa, preferably more than 650MPa. Metal coated steel strip product for direct hot forming (16) according to any one of the preceding claims, characterized in that the metal structure of the steel coated steel strip comprises more than 95% by volume of non-recrystallized granules, most preferably the metal coated steel strip has a completely crystallized non-recrystallized granular structure. A metal coated steel strip product according to any one of the preceding claims for direct hot forming (16), characterized in that the grain structure of this metal coated steel strip has partially or completely recovered (10.3). A metal coated steel strip product according to any one of the preceding claims for direct hot forming (16), characterized in that said hardening steel alloy comprises by weight: C: 0.08 - 0.45 wt-% Mn + Cr: 0.5 - 3.5 wt-% Si: <1 wt- % AI: <0.2 wt-% Ni: <1.0 wt-% Cu: <1.0wt-% B: <0.01 wt-% Ti: <0.2 wt-% Nb: <0.2 wt-% Mo: <1.0wt-% V: <0.5 wt-% Ca: <0.01wt-%, with the remainder being iron and residual or unavoidable impurities. The metal coated steel strip product for direct hot forming (16) according to claim 20, characterized in that said hardening steel alloy further comprises a weight percentage of 0.01 to 0.1 wt% by one or more alloying elements belonging to the group consisting of titanium (Ti), vanadium (V), (Nb). A metal coated steel strip product for direct hot forming (16) according to claim 21, characterized in that said hardening steel alloy further comprises niobium (Nb) 0.005-0.1wt-%, more preferably niobium (Nb) 0.01-0.08 wt-%. A metal coated steel strip product according to any one of the preceding claims for direct hot forming (16), wherein said metal coating is a Zinc based coating. A metal coated steel strip product for direct hot forming (16) according to claim 23, characterized in that said metal coating is an iron-doped zinc coating, such as a Galvannealed coating.