Patent Publication Number: US-2004047756-A1

Title: Cold rolled and galvanized or galvannealed dual phase high strength steel and method of its production

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates to a method of producing cold rolled annealed and hot dip galvanized and galvannealed dual phase high strength steel sheets having tensile strength of at least about 750 MPa for use in automotive structural and other sheet steel applications, and to steel sheets produced by the method.  
       [0003] 2. Description of the Prior Art  
       [0004] The conflicting demands for lighter weight automobiles with greater crashworthiness has led the steel industry to develop new steels possessing very high strength combined with good ductility. To achieve this, bake hardenable and high strength IF steels have been the most widely used high strength thin sheets. However, their yield and tensile strengths are generally limited to about 320 and 450 MPa, respectively. Known microalloyed steels offer even higher strength by utilizing a precipitation hardening mechanism but their ductility significantly decreases with higher strength. In addition, substantial amounts of microalloying elements must be added to obtain tensile strength levels in excess of 750 MPa, thus making these steels less cost-effective. Dual phase steels, by virtue of their microstructure primarily containing ferrite and martensite, exhibit tensile strengths ranging from 500 to 1000 MPa along with superior ductility as compared with non-dual phase steels.  
       [0005] In order to realize the advantages of the high strength of dual phase steel in automotive applications, good ductility is necessary. The yield ratio (YR) of steel, i.e., the ratio of its yield strength to its tensile strength, should be low. Typical non-dual phase steels have a YR around 75% ± about 2% while dual phase steels typically have a YR of approximately 60%. A lower YR, i.e. lower yield strength and/or higher tensile strengths, are desired to provide the desired formability demanded by the automotive industry.  
       [0006] U.S. Pat. Nos. 4,615,749 and U.S. Pat. No. 4,708,748 discloses a method of making a cold rolled dual phase structural steel sheet asserted to have excellent deep drawability, high ductility, high bake hardenability, and a non-aging property at room temperature. The steel comprises 0.001 to 0.008 wt % of C, not more than 1.0 wt % of Si, 0.05 to 1.8 wt % of Mn, not more than 0.15 wt % of P, 0.01 to 0.1 wt % of Al, 0.002 to 0.05 wt % of Nb, and 0.0005 to 0.050 wt % of B provided that the value of Nb+10% B is in the range of 0.01 to 0.08 wt %, and if necessary, 0.05 to 1.0 wt % of Cr. This steel sheet is hot rolled, cold rolled and continuously annealed between Ac1 and Ac3 temperatures and cooled at an average cooling rate between 0.5 to 20 C./s in a temperature range of from the soaking temperature to 750 C. and subsequently at an average cooling rate of not less than 20 C./s in a temperature range of from 750 C. to not more than 300 C. This patent requires specific processing constraints and the targeted tensile strength levels are lower than 500 MPa.  
       [0007] U.S. Pat. No. 5,180,449 provides a method of manufacturing a galvanized high strength steel sheet with terisile strength of not less than 780 MPa and a yield ratio of not more than 60%. The method comprises preparing a steel slab with a steel composition in the range of 0.08 to 0.2 wt % of C., 1.5 to 3.5 wt % of Mn, 0.01 to 0.1 wt % of Al, 0.01 wt % or less of P, 0.001 wt % or less of S, one or both of 0.01 to 0.1 wt % of Ti and 0.01 to 0.1 wt % of Nb, one or both of 0.1 to 0.5 wt % Cr and 0.0005 to 0.003 wt % of B, and further hot rolling, cold rolling, recrystallization annealing, and galvanizing the steel strip with cooling rates of not less than 5 C./s after recrystallization annealing and between 2 to 50 C./s after galvanizing.  
       [0008] U.S. Pat. No. 5,356,494 discloses a method of producing a dual phase high strength cold rolled steel sheet alleged to have excellent non aging properties at room temperature. The steel sheet comprises 0.001 to 0.025 wt % of C, 0.05 to 1.0 wt % of Si, 0.1 to 2.0 wt % of Mn, 0.001 to 0.2 wt % of Nb, 0.003 to 0.01 wt % of B, 0.005 to 0.1 wt % of Al, not more than 0.1 wt % of P, not more than 0.007 wt % of N, at least one selected from a group consisting of 0.05 to 3.0 wt % of Ni, 0.01 to 2.0 wt % of Mo, and 0.05 to 5.0 wt % of Cu. This steel sheet is hot rolled, cold rolled at a reduction not lower than 60%, and annealed between Ac1 and Ac3 temperatures and cooled at a cooling rate between 5 and 100C./s.  
       [0009] U.S. Pat. No. 5,123,969 discloses a method of manufacturing a bake hardenable cold rolled steel sheet having dual phase structure and comprising of 0.02 to 0.06 wt % of C, 0.6 to 1.04 wt % of Mn, 0.5% or less of Si, 0.1% or less of P, 0.1% or less of Al, 0.01% or less of N, 0.1% or less of Ti, and 50 ppm or less of B. The steel sheet is hot rolled, coiled at temperatures between 560C. to 720C., cold rolled and annealed at temperatures ranging from 780C. to 900C. for less than five minutes, cooled in air to a temperature ranging from 650C. to 750C. followed by cooling to a temperature ranging from 200C. to 400C. at a rate of 50C./s to 400C./s.  
       [0010] Factors responsible for achieving a good balance of high strength and ductility in dual phase steels, available from published literature, include:  
       [0011] (1) The tensile strength of the dual phase steels is linearly dependent upon the amount of martensite and does not depend on the carbon content of martensite.  
       [0012] (2) The dislocation substructure of dual phase steel depends upon the proportion of martensite. With higher martensite contents, the martensite is increasingly of lath variety and greater is the dislocation density of ferrite. The increase in the dislocation density of martensite content arises from the increasing amount of strain that has to be accommodated when the austenite transforms to martensite.  
       [0013] (3) The increase in the strength of the dual phase steel with higher martensite contents is explained as follows: the inherent strength of the ferrite is assumed to be constant; however, the higher the martensite content, the more cold worked, i.e., stronger due to the higher dislocation density, is the ferrite.  
       [0014] (4) Increase in Mn and Cr increases the amount of martensite and thus increases the tensile strength. The yield strength first decreases by virtue of the increase in the density of mobile dislocations, and then increases due to the strengthening caused by the increase in volume fraction of the second phase. Silicon improves the strength and ductility balance, which is believed to be due to its effect on the activity of carbon and the density of dislocations at the ferrite martensite interface.  
       [0015] (5) Increasing the Cr and Mn content increases the hardenability of the austenite pools during intercritical annealing. Increasing C content does not necessarily increase the average hardenability of the austenite pools, since some austenite may form in regions that are not enriched with Cr and Mn. Subsequent slow cooling of these regions can produce pearlite.  
       [0016] (6) The best combination of strength and ductility can be achieved by providing a completely interstitially free matrix devoid of fine precipitates and a 100% conversion of austenite to martensite.  
       [0017] (7) Grain size plays an important role with respect to achieving the best combination of strength and ductility. Elements such as Nb and Ti form carbides and carbonitrides and prevent grain growth during intercritical annealing.  
       SUMMARY OF THE INVENTION  
       [0018] The present invention relates to a method of producing uncoated cold rolled and annealed steel sheet as well as coated, hot dip galvanized or galvannealed, dual phase high strength steel sheet with tensile strength at least about 750 MPa and suitable for use in automotive structural and other sheet steel applications. A study of the prior art available on cold rolled and galvanized and galvannealed dual phase high strength steels highlights important differences between the prior art and the present invention.  
       [0019] It is an object of the present invention is to provide cold rolled and annealed steel sheets and hot dip galvanized and galvannealed dual phase high strength steel sheets having a tensile strength of at least about 750 MPa.  
       [0020] It is another object of the present invention to provide cold rolled and annealed dual phase high strength steel sheets, including hot dip galvanized and galvannealed steel sheets, having a tensile strength of at least about 750 MPa with superior balance of strength and ductility and that can be producing using conventional thermomechanical processing parameters and that is robust to the processing conditions.  
       [0021] It is another object to provide such steel sheets and a method of producing such steel sheets having a low YR and therefore excellent formability.  
       [0022] Another object is to provide such a method and product in which the yield strength can be increased for application where a low YR is not required without requiring a change in steel chemistry.  
       [0023] The object of the present invention is achieved in a cold rolled and annealed steel sheet, and hot -dip galvanized and galvannealed steel sheets having a tensile strength of at least about 750 MPa, comprising  
       [0024] a steel sheet containing 0.07 to 0.2% of C, 0.1 to 0.5% of Si, 1.75 to 3.5% of Mn, 0.05% or less P, 0.01% or less S, 0.15 to 1.0% Cr, 0.02 to 0.10% V,  
       [0025] 0.1% or less of Nb, 0.1% or less of Ti, 0.1% or less Al, 0.01% or less of N, and the balance Fe and incidental impurities;  
       [0026] a steel sheet having a microstructure primarily consisting of a ferrite matrix and martensite; and  
       [0027] a hot dip galvanized or galvannealed layer formed on the steel sheet.  
       [0028] Further, the present invention provides a method for producing a cold rolled and annealed steel sheet and hot dip galvanized or galvannealed steel sheet having a tensile strength of at least about 750 MPa, comprising the steps of:  
       [0029] rough rolling a steel containing 0.07 to 0.2% of C, 0.1 to 0.5% or less of Si, 1.75 to 3.5% of Mn, 0.05% or less P, 0.01% or less S, 0.15 to 1.0% Cr, 0.02 to 0.10% V, 0.1% or less of Nb, 0.1% or less of Ti, 0.1% or less Al, 0.01% or less of N, and the balance Fe and incidental impurities;  
       [0030] finish rolling the rough rolled steel at a temperature not lower than the Ar3 point;  
       [0031] coiling the finish rolled steel at a temperature lower than Ar1 point;  
       [0032] pickling the coiled hot rolled band;  
       [0033] cold rolling the coiled hot rolled band to the desired thickness producing a total reduction not less than 40%;  
       [0034] performing the continuous hot dip galvanizing or galvannealing comprising the steps of:  
       [0035] soaking the cold rolled strip at a temperature between Ac1+20C. and Ac3;  
       [0036] cooling the strip to a zinc bath temperature around 470C. at a conventional cooling rate depending upon the line speed;  
       [0037] hot dip galvanizing or galvannealing the strip at a galvannealing temperature around 520C.; and  
       [0038] cooling the galvanized or galvannealed strip to room temperature at a conventional cooling rate.  
       [0039] The strip can be further subjected to temper rolling or tension leveling to increase yield strength above that which results from the annealing treatment, up to a level approaching that of the tensile strength, if desired for applications not requiring a low YR. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0040] Other features and advantages of the invention will become apparent from the description contained herein below, taken in conjunction with the single drawing FIGURE which is a graph showing the effect on the YR of varying the Cr and V content of the steel. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0041] The improved cold rolled dual phase steel of the invention is a robust product produced using the conventional thermomechanical processing parameters, and hence, exhibiting a uniformity in properties along the length of the coil, and being more cost effective. The chemical composition (in wt %) and the thermomechanical processing parameters are described below. The term “robust” is intended to imply a process in which the parameters such as annealing or rolling temperatures do not require precise control in order to achieve the desired high strength and low YR so that conventional processing and equipment can be employed without adversely affecting the product properties.  
     [0042] C is an austenite-forming element. Higher levels of C help form more austenite during intercritical annealing. The higher the amount of austenite, the higher is the amount of martensite upon cooling the strip at room temperature, assuming other factors are kept constant. The tensile strength of the dual phase material is linearly dependent on the amount of martensite. The higher the amount of martensite, the higher the tensile strength. Thus, increasing C increases tensile strength of the material. When C is below 0.07%, sufficient martensite is not obtained to achieve the minimum tensile strength of at least about 750 MPa. However, increasing C above 0.20% decreases ductility and also deteriorates spot weldability. Hence, C is limited between 0.07% to 0.20% and preferably between 0.08 and 0.16% to maintain the strength-ductility-weldability balance.  
     [0043] Si is a ferrite-forming element that, in amounts above about 0.1%, significantly improves the tensile strength and ductility balance. However, increasing Si above a certain limit adversely affects the coatability of the material and is hence, limited to 0.5% or less. In accordance with the present invention, the amount of Si is therefore held within the range from about 0.1 to about 0.5%, and preferably from about 0.1 to about 0.35%.  
     [0044] Mn is an austenite-forming element. It promotes formation of austenite, which transforms to martensite upon cooling. Higher amount of Mn increases the amount of martensite and hence the tensile strength of the material. A minimum Mn content of 1.75% is required to obtain the minimum tensile strength of at least about 750 MPa. However, Mn above a certain limit, in this case, about 3.5% deteriorates the ductility as well as the strength and ductility balance of the material. Preferably, the Mn content is maintained within the range of about 2.0 to 2.75%.  
     [0045] P is an impurity element and deteriorates ductility of the material and in amounts above 0.05% decreases the spot weldability of the material. A P content of less than 0.05%, and preferably within the range of 0.005 to 0.010, is employed.  
     [0046] S is an impurity element that combines with Mn to form MnS inclusions and reduces the ductility of the material. It also deteriorates the spot weldability and stretch flangeability of the material. The S content is therefore maintained at 0.01% or less, and preferably within the range of 0.002 to 0.008%.  
     [0047] Cr is a ferrite-forming element as well as a hardenability agent. It delays the transformation of austenite to pearlite or bainite during the cooling of the strip after intercritical annealing and thus allows the austenite to transform to martensite at room temperature. Cr, thus, increases the tensile strength of the material. A minimum amount of about 0.15% Cr is necessary to ensure sufficient hardenability to obtain the minimum tensile strength of at least about 750 MPa; however, a Cr content above about 1.0% deteriorates the strength and ductility balance of the material. Preferably the Cr content is within the range of about 0.20% to about 0.55%.  
     [0048] V is also a hardenability agent and helps in transformation of austenite to martensite during cooling after intercritical annealing. The synergistic effect on hardenability produced by a combination of V and Cr helps to produce the product using robust processing conditions. A minimum amount of V of about 0.02% is necessary to obtain sufficient hardenability to achieve the minimum tensile strength, while the higher amount of V above about 0.10% decreases the ductility of the material. Preferably the V content is within the range of about 0.03% to about 0.08% to produce the desired synergistic effect with the Cr content, above.  
     [0049] Nb forms Nb(CN) and NbC precipitates which prevent grain growth during hot rolling and/or intercritical annealing. Finer grain sizes increase the strength of the material. Nb as a solute also acts as a hardenability agent and prevents transformation of austenite to other low temperature products other than martensite. Thus, it is very effective in increasing the strength and may be added when higher tensile strengths around 1000 MPa are desired. However, an addition above about 0.1% decreases the ductility of the material, and the Nb content should therefore not be greater than about 0.1%, and is preferably within the range of about 0.02 to about 0.05%.  
     [0050] Ti combines with N and forms TiN at higher temperatures and thus prevents grain growth at higher temperatures. Finer grain size helps to increase the strength of the material. Like, Nb, Ti may be added when higher tensile strengths are desired; however above about 0.1%, it adversely affects both the ductility and the surface quality of the material. The upper limit of Ti is therefore about 0.1%, and the preferred Ti content is within the range of about 0.02 to 0.05%.  
     [0051] Al is primarily added as a deoxidizer. It also combines with N to form AIN. A high content of N decreases ductility and also causes strain aging behavior. However, if the amount of sol. Al exceeds about 0.1% the ductility is decreased due to higher fraction of AIN. The upper limit of Al is therefore about 0.1%, and the preferred Al content is within the range of about 0.02 to about 0.07%.  
     [0052] N should be maintained as low as practically possible, and limited to no more than about 0.01% and preferably less than about 0.005% because it decreases the ductility of the material.  
     [0053] Other impurities should be restricted to as small a level as possible.  
     [0054] Steel with a composition described above is continuously cast into a slab of desired thickness. The slab is heated to a reheating temperature around 2025° F. (1107° C.). After soaking for about 45 minutes, it is rough rolled and then finish rolled at a temperature above Ar3 to a hot rolled band. The hot rolled band is later cooled to the desired coiling temperature, which is below the Ar1, coiled and cooled to room temperature. Higher coiling temperatures, near Ar1, are not preferred because of the higher scale formation and the problems caused subsequently during pickling.  
     [0055] The as-coiled hot rolled band is cold rolled to the desired thickness. A minimum total reduction of about 40% is necessary to ensure sufficient stored energy of cold work in the material to produce a required grain size after recrystallization during intercritical annealing of the material. Higher cold reductions around 60% to 70% are preferred to obtain a fine ferrite grain size to achieve the best combination of strength and ductility.  
     [0056] The cold rolled strip is annealed in the intercritical region between Ac1+20C. and Ac3 and soaked for a time depending upon the line speed of the strip to ensure the formation of austenite time range. The line speed is a function of the final gage of the material. Higher line speeds are achieved for lighter gage material. If the strip is annealed below the Ac1+20C. temperature, sufficient austenite is not formed at the intercritical temperature for the given line speed and the resulting microstructure does not yield the desired properties due to insufficient martensite. On the other hand, if the annealing temperature increases above Ac3, the ductility deteriorates sharply. The strip is later cooled from the intercritical annealing temperature at a conventional cooling rate of approximately 10C./s depending upon the line speed to the zinc bath temperature of approximately 470C. and galvanized or galvannealed. In the case of galvannealing, the strip passes through a furnace and is heated to a temperature around 520C. The final properties are independent of the cooling rate by virtue of sufficient hardenability in the material. Thus, the required properties are also achieved at cooling rates below 5C./s. The galvanized or galvannealed strip is then cooled to room temperature. An uncoated cold rolled and annealed strip having the desired strength and ductility properties may also be produced by simply cooling the strip from the annealing temperature to room temperature.  
     [0057] The following example further illustrates the present invention:  
     [0058] Steel slabs with chemical compositions given in Table 1 were melted in the form of laboratory ingots. The balance elements not given in Table 1 were Fe and unavoidable impurities. The slabs were hot rolled to a hot band gauge of 2.7 mm at an average finishing temperature of 1650F. (899C.), and coiled at an average coiling temperature of 1050F. (566C.). The hot band was cold rolled to 1.0 mm cold rolled gauge employing a total reduction of 63%. The cold rolled material was later gleeble-annealed simulating a continuous hot-dip galvanizing line at an average soaking temperature of 1544F. (840C.). The strip was later cooled to the zinc bath temperature at a conventional cooling rate of approximately 7C./s to 12C./s depending upon the line speed of the strip and later galvanized. The strip was later cooled to room temperature at a cooling rate of approximately 5C./s. Table 2 indicates the processing conditions of the material. Finally, specimens were tested according to the ASTM-L standard testing procedures. The mechanical properties from these specimens are indicated in Table 3.  
     [0059] Steels A to D having chemical composition in the range of the present invention and steels E and F having chemical composition out of the present invention range were prepared and are shown in Table 1. The steels were hot rolled, pickled, cold rolled, and annealed at conditions shown in Table 2. The processed steels properties are shown in Table 3.  
                                                               TABLE 1                       Steel   C   Mn   Si   P   S   Al   N   Nb   Ti   V   Cr   Class                                                                                    A   0.1   2.0   0.3   0.01   0.01   0.03   0.003   0.0   0.0   0.1   0.5   P       B   0.15   2.25   0.02   0.01   0.01   0.02   0.003   0.025   0.03   0.05   0.2   P       C   0.14   2.30   0.02   0.01   0.01   0.03   0.003   0.03   0.03   0.05   0.45   P       D   0.13   2.30   0.02   0.01   0.01   0.03   0.003   0.025   0.03   0.05   0.7   P       E   0.095   1.75   0.31   0.01   0.01   0.03   0.003   0.0   0.0   0.1   0.88   P       F   0.095   1.65   0.3   0.01   0.01   0.03   0.003   0.0   0.0   0.1   0.5   C                                  
 
     [0060]                                           TABLE 2                           HB               CR               Steel   (mm)   FT (C)   CT (C)   CR (%)   (mm)   AT (C)   (ft/min)                                                                A-D   2.7   899   566   63   1.0   840   330                    
     [0061]                                       TABLE 3                           YS                           Steel   (MPa)   TS (MPa)   YR (%)   EL (%)   n (4-6%)   n (10-15%)                                                            A   403   820   49   17.0   0.179   0.121       B   482   986   49   15.0   0.118   —       C   521   1030   51   13.0   0.102   —       D   548   1036   53   14.0   0.096   —       E   377   791   48   18.0   0.18   0.121       F   342   745   46   18.0   0.21   0.147                    
     [0062] The effect of temper extension on the mechanical properties of this material was also determined with an objective to produce material with different yield strength for a given minimum tensile strength of approximately 1000 MPa. Steel C that was annealed as explained above was further temper rolled at different extensions ranging from 0% to 3%. Table 4 shows the mechanical properties of steel C after different levels of temper extension.  
                                       TABLE 4                           Temper                           Steel   EL (%)   YS (MPa)   TS (MPa)   YR (%)   EL (%)   n (4-6%)                                                            C   0   521   1030   51   13.0   0.102           0.5   654   1036   63   12.0   0.092           1.0   776   1069   73   12.0   0.085           1.5   869   1083   80   10.0   0.079           3.0   925   1087   85   8.0   0.051                  
 
     [0063] The drawing figure shows a plot of the YR for different combinations of Cr and V, with the Cr content, in wt %, on the x-axis and the V content on the y-axis. On this plot, the numbers in % on the body of the graph are the yield ratio YR. As pointed out previously, a lower YR, which implies a lower yield strength and higher tensile strength, is desired because the material can stretch uniformly to higher strains and thus provide better ductility. As is seen in the drawing, in accordance with this invention, there is a defined region in the V-Cr plot where the YR obtained is about 50%. The YR for different combinations of V and Cr are shown and the synergistic effect of the Cr and V within the range described above on the YR is apparent.  
     [0064] The plot is divided into four regions, the first being designated the low yield ratio region which comprises a Cr content between 0.15% and 1.0% and a V content between 0.02% and 0.10%. At V contents below 0.02%, the addition of Cr results in a YR of 60% or greater; however, at V contents above about 0.02%, the YR drops to around 50% with the addition of Cr in amounts above about 0.15%. This decrease in YR is a significant change, enabling the production of superior strength-ductility balance in these sheets.  
     [0065] Region 2 is a high yield ratio region comprising Cr content below about 0.15% and V content below 0.02%. Significantly lower yield ratios, i.e. around 50% are not obtained in this region.  
     [0066] Region 3 comprises a V content of above 0.1% and is not considered economically viable because of the high cost of V as an alloy element.  
     [0067] Region 4 comprises a Cr content above 1.0% which results in a product having poor coatability characteristics.  
     [0068] While preferred embodiments of the invention have been disclosed and described, it is to be understood that the invention is not so limited, but rather that it is intended to include all embodiments which would be apparent to one skilled in the art and which come within the spirit and scope of the invention.