CA1195152A - High strength steel plate and method for manufacturing same - Google Patents
High strength steel plate and method for manufacturing sameInfo
- Publication number
- CA1195152A CA1195152A CA000388173A CA388173A CA1195152A CA 1195152 A CA1195152 A CA 1195152A CA 000388173 A CA000388173 A CA 000388173A CA 388173 A CA388173 A CA 388173A CA 1195152 A CA1195152 A CA 1195152A
- Authority
- CA
- Canada
- Prior art keywords
- steel plate
- set forth
- martensite
- bainite
- phase
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Sheet Steel (AREA)
- Heat Treatment Of Steel (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE:
There are described a high strength steel plate having a low yield ratio and improved strength-elongation balance and stretch flangeability, and a method for producing same, the steel plate containing 0.01 - 0.2% of C, 0.3 - 2.5% of Mn and 0.01 - 1.8% of Si and having a tripple phase of polygonal ferrite, bainite and martensite, the area rates of the bainite and martensite phases being 4 - 45% and 1 - 15%, respectively, and the area rate of the bainite phase being greater than that of the martensite phase.
There are described a high strength steel plate having a low yield ratio and improved strength-elongation balance and stretch flangeability, and a method for producing same, the steel plate containing 0.01 - 0.2% of C, 0.3 - 2.5% of Mn and 0.01 - 1.8% of Si and having a tripple phase of polygonal ferrite, bainite and martensite, the area rates of the bainite and martensite phases being 4 - 45% and 1 - 15%, respectively, and the area rate of the bainite phase being greater than that of the martensite phase.
Description
5~2 BACKGROUND OF THE INVENTION
(1) Field of the Invention This invention relates to a high strenq-th steel plate which is low in yield ratio and improved in strength-elongation balance as well as in stretch flangeability, and a method for manufacturing same. The high strength steel plate of the present invention is particularly suitable for use as a material for the wheel disc and/or wheel rim of a motor vehicle.
~2) Descri~tion of the Prior Art Recently, various attempts are made in order to improve the mileage of motor vehicles, including the reduc-tion of the vehicle body weight which is considered to be most effective. With regard to the reduction of the vehicle body weight, there have thus far been made many proposals concerning ; the use of high strength steel plates and aluminum alloys along with the reductions in size. Among these proposals, the reduc-tion of the vehicle wheel weight is one of the most effective means for improving the mileage, and the possibilities or application of a hi~h strength steel plate to the wheel rim or disc have been a subject of intensive ~tudies. The high strength steel plates which have been proposed for this purpose include the composite structure steel plate tdual phase steel plate of ferrite ~ martensite, which i5 low in yield ratio and has higher elongation as compared with the strength showing excellent properties in formability and shape 1 fixability, However, the steel plate of this sort is inferior in stretch flangeabi.lity so that, if applied to the ~ehicular wheel disc or the like, it gives rise.to problems such as:
(1) the occurrence of cracking at an expanded hole portion in the disc forming operation; or
(1) Field of the Invention This invention relates to a high strenq-th steel plate which is low in yield ratio and improved in strength-elongation balance as well as in stretch flangeability, and a method for manufacturing same. The high strength steel plate of the present invention is particularly suitable for use as a material for the wheel disc and/or wheel rim of a motor vehicle.
~2) Descri~tion of the Prior Art Recently, various attempts are made in order to improve the mileage of motor vehicles, including the reduc-tion of the vehicle body weight which is considered to be most effective. With regard to the reduction of the vehicle body weight, there have thus far been made many proposals concerning ; the use of high strength steel plates and aluminum alloys along with the reductions in size. Among these proposals, the reduc-tion of the vehicle wheel weight is one of the most effective means for improving the mileage, and the possibilities or application of a hi~h strength steel plate to the wheel rim or disc have been a subject of intensive ~tudies. The high strength steel plates which have been proposed for this purpose include the composite structure steel plate tdual phase steel plate of ferrite ~ martensite, which i5 low in yield ratio and has higher elongation as compared with the strength showing excellent properties in formability and shape 1 fixability, However, the steel plate of this sort is inferior in stretch flangeabi.lity so that, if applied to the ~ehicular wheel disc or the like, it gives rise.to problems such as:
(1) the occurrence of cracking at an expanded hole portion in the disc forming operation; or
(2) the occurrence of cracking at an expanded hole por-tion in the fatigue test or in the running test, The present inventors have studiea in detail the :10 relationship between the steel structure and stretch flange-ability for the improvement thereof, and as a result found that a steel plate of a ~errite + bainite structure is super-ior to a dual phase steel plate of ferrite ~ martensite in the stretch. flangea~ility, However, the steel plate of the ferrite ~ ~ainite structure has a drawback that it is inferior in the strength-elongation balance, Further~ the wheel rim requires the resistance weldability in addition to the stretch flangeability which is required by the wheel disc, Another problem which is encountered in applying a high strength steel plate to the wheel rim is the cracking which takes place at a high rate in the roll-forming operation subsequent to the 1ash butt welding, the cracks occurring at a rate as high as about 50%
in the thermally affected zones in the forming stage~ Such a high rate o cracking is detrimental to actual applications, .~'~" , 5~5~:
The present invention aims at rational elimination of the above-men-tioned drawbacks and problems of the prior art. More specifically, ,it is a primary object of the present invention to provide a hiyh stxength steel plate which has a low yield ratio and good strength-elongation balance characteristic to the dual phase steel plate of ferrite ~
martensite along with the excellent stretch flangeability comparable to that of the ferrite + bainite steel, and a ~ethod for manufacturing such high strength steel plates.
It is another object of the present invention to provide a high strength steel plate which, besides the above-mentioned properties, possesses excellent resistance weldability, and a method for manufacturing same.
Still another object of the present inventicn is to provide a high strength steel plate which is particularly suitable for use as a material for wheel discs and/or wheel rims of motor vehicles, and a method for manufacturing same.
According to one aspect of the present invention, the above-mentioned objects are achieved by a high strength steel plate which has low yield ratio and excellent strength-elongation balance and stretch flangeability, the steel plate containing 0O01 - 0.2% of C, 0.3 - 2.5% of Mn and 0.01 - 1.8%
of Si and having a tripple phase structure o polygonal ferrite, bainite and martensite with a bainite areal rate o 5~
1 4-45~ and a martensite areal rate of 1 - 15%, the bainite areal rate being greater than the martensite areal rate.' According to another aspect of the present invention, there is provided a method for producing a high strength steel plate as staked above, which method comprisiny:
(1) subjecting a steel containing 0.01 - 0.02% of C, 0.03 -2.5~ of Mn and 0.01 - 1.8% oE Si to a hot rolling-cooling treatment selected from the group consisting of;
(i) hot rolling the steel at a fininshing roll temperature above Ar3 point, followed by coolin.g the hot rolled steel from the fini-shing rolling temperature to a temperature range between point Ar3 and point Ar~ at an average cooling speed of 3 - 70C/sec, (ii) hot rolling the steel at a finishing rolling temperature above Ar3 pointj followed by - cooliny the hot rolled ~teel from the finish-ing rolling temperature to a temperature range between point Ar3 and point Arl at an average cooling speed of 3 ~ 70C/sec and then by air cooling ar slow cool.ing for 2 -20seconds from the temperature range of ~r3 to Arl, and (iiij hot rolling the steel at a finishing rolling temperature range of Ar3 to Arl, followed by air cooling or slow cooling the hot rolled steel for 2 - 20 ~econds from the temperature range of Ar3 to Arl, thereafter 1 (2) cooling the steel thus subjected to hot rolling-cooli.ng treatment to a temperature below 550C at an average cooling speed not lower than 20C/sec, and
in the thermally affected zones in the forming stage~ Such a high rate o cracking is detrimental to actual applications, .~'~" , 5~5~:
The present invention aims at rational elimination of the above-men-tioned drawbacks and problems of the prior art. More specifically, ,it is a primary object of the present invention to provide a hiyh stxength steel plate which has a low yield ratio and good strength-elongation balance characteristic to the dual phase steel plate of ferrite ~
martensite along with the excellent stretch flangeability comparable to that of the ferrite + bainite steel, and a ~ethod for manufacturing such high strength steel plates.
It is another object of the present invention to provide a high strength steel plate which, besides the above-mentioned properties, possesses excellent resistance weldability, and a method for manufacturing same.
Still another object of the present inventicn is to provide a high strength steel plate which is particularly suitable for use as a material for wheel discs and/or wheel rims of motor vehicles, and a method for manufacturing same.
According to one aspect of the present invention, the above-mentioned objects are achieved by a high strength steel plate which has low yield ratio and excellent strength-elongation balance and stretch flangeability, the steel plate containing 0O01 - 0.2% of C, 0.3 - 2.5% of Mn and 0.01 - 1.8%
of Si and having a tripple phase structure o polygonal ferrite, bainite and martensite with a bainite areal rate o 5~
1 4-45~ and a martensite areal rate of 1 - 15%, the bainite areal rate being greater than the martensite areal rate.' According to another aspect of the present invention, there is provided a method for producing a high strength steel plate as staked above, which method comprisiny:
(1) subjecting a steel containing 0.01 - 0.02% of C, 0.03 -2.5~ of Mn and 0.01 - 1.8% oE Si to a hot rolling-cooling treatment selected from the group consisting of;
(i) hot rolling the steel at a fininshing roll temperature above Ar3 point, followed by coolin.g the hot rolled steel from the fini-shing rolling temperature to a temperature range between point Ar3 and point Ar~ at an average cooling speed of 3 - 70C/sec, (ii) hot rolling the steel at a finishing rolling temperature above Ar3 pointj followed by - cooliny the hot rolled ~teel from the finish-ing rolling temperature to a temperature range between point Ar3 and point Arl at an average cooling speed of 3 ~ 70C/sec and then by air cooling ar slow cool.ing for 2 -20seconds from the temperature range of ~r3 to Arl, and (iiij hot rolling the steel at a finishing rolling temperature range of Ar3 to Arl, followed by air cooling or slow cooling the hot rolled steel for 2 - 20 ~econds from the temperature range of Ar3 to Arl, thereafter 1 (2) cooling the steel thus subjected to hot rolling-cooli.ng treatment to a temperature below 550C at an average cooling speed not lower than 20C/sec, and
(3) taking up the cooled steel.
Further embodiments of the present invention will become apparent from the particular description of the invention which follows.
- BiRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention : and many of the attendant advantages thereof will be readily attained as the same becomes better understood by ~ reference to the following detailed description when taken ; in conjunc*ion with the accompanying drawings, in which ~0 5~
1 E~IGURE 1 is a diagram showing the relationship between tensile s-trength and -total elongation and the relationship between tensile strength and yie].d stress;
FIGURE 2 is a diagrarn showing -the relationship be-tween tensile strength and hole expanding limit;
FIGURE 3 is a diagram showing the relationship between martensite area rate and bainite area rate;
F-rGuRE 4 is a diagram showing the relationship between the bainite area. rate and yield ratio and the relationship between the bainite area rate and the hole expanding limit;
FIGURE 5 is a diagram showing the relationship between the martensite area rate and hole expanding limit and the relationship between the martensite area rate and the yield ratio;
FIGURE 6 iS a dia~ram showing the relationship bet~een the average diameter of martensite particle size and the hole expanding limit;
FIGURE 7 is a diagram showing the hardness distribu-tion in a weld portion after flash-hutt welding; and FIGURE 8 is a diagram conceptionally showing the method of the present invention.
DESCRIP'rION OF PREFERRED EMBODIMENTS
The term "bainite~' as used in the present invention means mainly the bainitic ferrite but includes the bainite which partly contains the so-called accicular fPrrite or a 5~S~
1 a carhide~ The -term "mar-tensite" also includes partly -retained austenite~
It is important in the present invention that the composite structure consists of tripple phases of polygonal ferrite, bainite and martensite. More particularly, as known from FIGURE 1 depicting an embodiment which will be described hereinlater, the yield ra-tio is minimum with the ferrite -~martensite steel and maximum with the ferrite + bainite steel.
The yield ratio is lowered as the polygonal ferrite is intro-duced into the ferrite ~ bainite steel, and it is furtherlowered to a value comparable to that of th`e ferrite ~
martensite steel when a small amount of martensite is intro-duced to for a tripple--phase structure of polygonal ferrite +
bainite + martensite.
Similarly to the yield ratio, the tripple-phase steel of polygonal ferrite + bainite + martensite shows a good balance of strength-elongation which is akin to the value of the ferrite + martensite steel, as shown in FIGURE 1.
With regard to the hole expansibility (the index of the flanging extensibility), the steel of ferrite ~ martensite is the worst and the ferrite + bainite steel is the best as shown ln FIGURE 2. On the other hand, the tripple-phase steel of polygonal ferrite + bainite + martensite shows a high value of hole expansibility approximate to the value of the ferrite + bainite steel.
Turning now to the fatigue strength, the tripple-phase ~ 5~5~
1 steel of polygonal ferrite ~ bainite ~ martensite shows a value similar to the ferrite -~ hainite steel and is superior to the ferrite + martensite steel.
As known from the foregoing date, the tripple-phase steel of polygonal ferrite + bainite + martensite simul-taneously possesses on].y the advantages of the ferrite +
martensite steel and the ferrite bainite steel, and is excellent in a~l of the properties of the strength-elongation balance, stretch flangeability and fatigue strength.
It is also known from these examples that, in the tripple-phase s-teel structure according to the present invention, the area rate of bainite should be in the range of
Further embodiments of the present invention will become apparent from the particular description of the invention which follows.
- BiRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention : and many of the attendant advantages thereof will be readily attained as the same becomes better understood by ~ reference to the following detailed description when taken ; in conjunc*ion with the accompanying drawings, in which ~0 5~
1 E~IGURE 1 is a diagram showing the relationship between tensile s-trength and -total elongation and the relationship between tensile strength and yie].d stress;
FIGURE 2 is a diagrarn showing -the relationship be-tween tensile strength and hole expanding limit;
FIGURE 3 is a diagram showing the relationship between martensite area rate and bainite area rate;
F-rGuRE 4 is a diagram showing the relationship between the bainite area. rate and yield ratio and the relationship between the bainite area rate and the hole expanding limit;
FIGURE 5 is a diagram showing the relationship between the martensite area rate and hole expanding limit and the relationship between the martensite area rate and the yield ratio;
FIGURE 6 iS a dia~ram showing the relationship bet~een the average diameter of martensite particle size and the hole expanding limit;
FIGURE 7 is a diagram showing the hardness distribu-tion in a weld portion after flash-hutt welding; and FIGURE 8 is a diagram conceptionally showing the method of the present invention.
DESCRIP'rION OF PREFERRED EMBODIMENTS
The term "bainite~' as used in the present invention means mainly the bainitic ferrite but includes the bainite which partly contains the so-called accicular fPrrite or a 5~S~
1 a carhide~ The -term "mar-tensite" also includes partly -retained austenite~
It is important in the present invention that the composite structure consists of tripple phases of polygonal ferrite, bainite and martensite. More particularly, as known from FIGURE 1 depicting an embodiment which will be described hereinlater, the yield ra-tio is minimum with the ferrite -~martensite steel and maximum with the ferrite + bainite steel.
The yield ratio is lowered as the polygonal ferrite is intro-duced into the ferrite ~ bainite steel, and it is furtherlowered to a value comparable to that of th`e ferrite ~
martensite steel when a small amount of martensite is intro-duced to for a tripple--phase structure of polygonal ferrite +
bainite + martensite.
Similarly to the yield ratio, the tripple-phase steel of polygonal ferrite + bainite + martensite shows a good balance of strength-elongation which is akin to the value of the ferrite + martensite steel, as shown in FIGURE 1.
With regard to the hole expansibility (the index of the flanging extensibility), the steel of ferrite ~ martensite is the worst and the ferrite + bainite steel is the best as shown ln FIGURE 2. On the other hand, the tripple-phase steel of polygonal ferrite + bainite + martensite shows a high value of hole expansibility approximate to the value of the ferrite + bainite steel.
Turning now to the fatigue strength, the tripple-phase ~ 5~5~
1 steel of polygonal ferrite ~ bainite ~ martensite shows a value similar to the ferrite -~ hainite steel and is superior to the ferrite + martensite steel.
As known from the foregoing date, the tripple-phase steel of polygonal ferrite + bainite + martensite simul-taneously possesses on].y the advantages of the ferrite +
martensite steel and the ferrite bainite steel, and is excellent in a~l of the properties of the strength-elongation balance, stretch flangeability and fatigue strength.
It is also known from these examples that, in the tripple-phase s-teel structure according to the present invention, the area rate of bainite should be in the range of
4 - 45% since a rate over 45% results in a drop in th~ effect of lowering the yield ratio due to introduction of martensite and a rate below 4% makes no dif~erence from the ferrite ~
martensite steel. The area rate of bainite is preferred to be in the range of 6 ~ 35%.
More particularly, referring tc FIGURE 4 which shows the relationship between the bainite area rate and the yield ratio and the relationship between the bainita area rate and the hole expansibility which is the index of the stretch flangeability, plotting the pattern III of Example 4 ~denoted by mark ~ and Example 5 (denoted by mark "O") which will be described hereinlater. A material for ~he wheel disc and/or wheel rim of the motor vehicle is required ~o have a stretch flangeability hîgher than 150%, prefexably higher ~han 160~
martensite steel. The area rate of bainite is preferred to be in the range of 6 ~ 35%.
More particularly, referring tc FIGURE 4 which shows the relationship between the bainite area rate and the yield ratio and the relationship between the bainita area rate and the hole expansibility which is the index of the stretch flangeability, plotting the pattern III of Example 4 ~denoted by mark ~ and Example 5 (denoted by mark "O") which will be described hereinlater. A material for ~he wheel disc and/or wheel rim of the motor vehicle is required ~o have a stretch flangeability hîgher than 150%, prefexably higher ~han 160~
5~
1 in the forming stage, with a yield ratio lower than 0.7 and preferably lower than 0.6. As clear ~rom the experimental data of FIGURE 4, intensive studies conducted by the present inventors revealed the value of the bainite area rate suitable for the yield ratio and the stretch flangeability which are required for the material to be used for the vehicular wheel disc and/or wheel rim. According to FIGURE 4, the bainite area rate should be in the range of 4 - 45%.
FIGURE S plots the relationship between the martensite area rate and the stretch flangeability and the relationship between the martensite area rate and the yield ratio in the specimens 46, 47 and 53 to 57 which will be explained herein-later. Similarly to the bainite area rate mentioned above, the martensite area rate should be held in the range of 1 ~ 15%n As ~lear from FIGURE 5, the yield ratio is increased if the martensite area rate exceeds 1S% and it becomes di~ficult to--attain hole expansibility greater than 150%. On the contrary, with a martensite area rate smaller than 1%, the effect of martensite introduction becomes smaller. The martensite area rate should be in the range which guarantees a hole expansi-bili-ty over 160% and a yield ratio below 0.6, namely, in the range of 1 - 10%.
As seen in FIGUR~ 3 which plo-ts ~he correlation between the martensite area rate and the bainite area rate in relation with the hole expansibility, more particularly, the pattern III of Example 4 (denoted by mark "~") and Example 5 1 (denoted by mark "O") which will appear hereinlater, the bainite area rate in the tripple-phase steel structure according to the present invention should be greater than the martensite area rate in order to secure hole expansibili.ty greater than 150%, in additi.on to the above-defined conditions that the bainite and martensite area rates should be respectively in the ranges of 4 - 45% and 1 - 15~ The range which satisfies these conditions is indicated by hatching in FIGURE 3.
FIGURE 6 is a plot of the relationship between the particle size of the martensite and the hole expansibility, more particularly, a plot of the results of experiments of Table 20 using the composite steel structures of Table 19.
The hole e~pansibility is also dependent on the average dia-meter of the martensite as shown in FIGURE 6. More specifi-cally, this figure shows that the hole expansibility is further improved by making the martensite finer even if the area rates of the bainite and martensite are in the ab~ve~defined ranges, namely, that the hole expansibility becomes greater than 150~ when the average particle size of the martensite is smaller than 6 microns. The hole expansibility is further improved as the grain size is reduced to a value smaller than 5 microns and improved in a greater degree with a value smaller than 4 microns.
It will bè appreciated from the foregoing description that, with the tripple-phase steel structure acc~rding to the present invention t it iS possible to guarantee a 1QW yield s~s~
---ll--l ratio along with excellent strength-elongation balance and stre-tch flangeability by controlling the area rates of the bainite and martensite, and that the stretch flangeability can be further enhanced by making the mar-tensite finer.
As already s-tated hereinbeEore, one problem which is encountered when applying a steel plate to a vehicular wheel rim is the softening of the -thermally influenced por-tions after the flash-butt welding, which occurs conspicuously degree in the ferrite + martensite steel as a result of decomposition of the martensite, the thermally influenced portions giving rise to cracks in the subsequent roll-forming stage in such a degree as to make the application of the ferrite + martensite steel utterly difficult,. The softening of the thermall~ influenced portion, however, is not observed in the steel of the bainite structure, and the problem of cracking in the cold-rolling (roll-forming) subsequent to the welding operation is precluded, In order to overcome the drawback of the ferrite + bainite structure steel, the present inyentors conducted a comprehensi~e study varying the pro-20 Portions of the ferrite + ~ainite + martensite structures in relation with.the chemi.cal components involved, As a result, it ha$ been found that, as shown in FIGUR~ 7, the steel of ; ferrite + martensite structure has an excellent softening resist-ance similarly to the ferrite + bainite structure steel, even in 95~52 - 12 ~
1 the case of spot welding. It has also been revealed that a slightly sof-tened state can be established by letting a stable precipitant like NbC or the like exist in the thermally affected zone or more positively leaving Nb in solid solu-tion of after the hot-rolling for precipitating same in the thermally affected zone, thereby preventing the initiation of cracking from the thermally influenced portion in the succeed-ing roll-forming operation.
The adjustment of the steel structure is attained b~
controlling the cooling condition during and after the hot-rolling or by further controlling the annealing conditions either (either continuous or batch annealing3 in the subsequent stage, in connection with the chemical components which are discûssed hereinafter.
The limitations of the respective chemical components in the present invention are based on the following reasons.
The component C is an element which is essential for maintaining the required streng~h and for forming the low temperature transformation products like bainite and marten-site but its content should be limited since a C-content in excess of 0.2~ will cause a considerable deterioration in ductility and impair the weldability (giving rise to a drop in hardness of butt faces due to decarburization in the butt welding, resulting in a large difference in hardness between the weld line and the adjacent portions). In a case where forming workability is required in particular, its content 1 should desirably be less than 0.09%. The lower limit of the C-content should be 0.01 in order to secure the effects of strengthening the structure and enhancing the hardenability.
The element Mn is necessary for improving the hardena-bility and obtaining the desired structure. The improvemenk in hardenability also contributes to the increase of streng-th and to the enhancement of mechanical properties by stabilizing the y-phase during the y-~ transformation (y-aus-tenite, a=
ferrite) after the hot-rolling. In order to secure these effects, its content should be 0.3~ or more. However, with a Mn content in excess of 2.5%, it causes a welding difficulty and impairs the ductility ~elongation and stretch flangeability) and the weldability in addition to a substantial increase of the cost of the steel plate. Therefore, the upper limit should be placed at 2.5%.
The element Si which is necessary for deoxidation of the molten steel is also very effective as a substitutional solid solu-tion hardening element. Therefore, it is essential in order to obtain a steel plate with high strength and ductility. Besides, it acts advantayeously to the formation of clean polygonal ferrite. In the composite steel structure as of the present invention, it accelera-tes the a-transforma-tion during the y-a transformation after hot rolling and acts to discharge the carbon in solid solution out of ~phase and shift into y-phase. Consequently, it enhances the clean-liness of a--phase and sta~ilizes y-phase by condensing carbon 5~L52 1 thereinto, thus facilitating the formation of a hard phase which contributes to the improvement of the mechanical pro-perties. In order to produce these effects while preventing enbrittling oE the weld (an increase of transition tempera-ture), the lower limit of the Si cont~nt should be placed at 0.01%. On the other hand, its upper limit should be placed at 1.8% to prevent deteriorations of surface condition due to production of oxidation scales.
According to the present invention, the following elements may be included if desired in addition to the above-mentioned elements.
The elements Cr, Cu, Ni and B are useul for improv-ing the hardenability as well as for obtaining a desired structure. The lower limits of their oontents should be placed at a level which is sufficient enough for ~ecuring these effects, while the upper limits should be placed at a level whexe their effects are saturated and uneconomical.
More specifically, the steel plate according to the present invention may contain at least one element selected from the group consisting of 0.1-1.5% of Cr, 0.1 - 0.6~ of Cu, 0.1 - 1.0% of Ni and 0.0005 - 0O01% of B. Further, the element Mo which also serves to improve the hardenability and produce a desired structure similarly to the above-mentioned Cr, Cu, Ni and B may ~lso be included in an amount of 0.01 - 0.2%
for the same reasons.
The elements Nb, V, Ti and Zr which serve to strengthen 9S~5Z
1 the precipitation are necessary not only for increasing the strength but also for ~acilita-ting the forrnation of the bainite structure by imposing an inflwence on the transform-ing structure under coexistence with Mn or the like after hot-rolling. Further, they make the structure finer and serves to improve the stretch flangeability and to prevent drops in hardness, improving not only the fatigue strength of the parent plate but also the fatigue strength of the ~isc as a whole. Moreover, they contribute to produce ~he hardena-bility~-improving effec-t of B to a maximum degree. In order to have these effects, it is necessary to include at least one element selected from the group consisting of 0.01 - 0.16 of Nb, 0.0~ - 0.2~ of V, 0.01 - 0.1% of Ti and 0.02 - 0.2%
of Zr.
In addition, the component Nb particularly has an influence on the transforming behaviors after the hot-rolling and is most effective for the formation of the bainite struc-ture. The elements Ti and Zr are further effective for con-trolling the shape of the sulfide which is harmful to the ductility, and the element V is effective for moderately hardening the center portion of the weld (Hv ~ 25) as compared with that of the parent material.
The rare earth metals (REM), Ca and Mg contribute to the improvement of the ductility, particularly, the stretch flangeability by controlling the shape of the sulfide. The lower limits of their contents should be placed at a level which is suEficient for producing that effect. The lower 1 limit is determined at a value at which the aimed effect becornes saturated or uneconomical or in consideration of the content which inversely impairs the cleanliness. More specifically, the steel plate of the invention may include at least one element selected from the group consisting of 0.005 - 0.1% of a REM, 0.0005 - 0.01~ of Ca and 0.0005 - 0.01% of Mg. However, it is desired that the total additive amount i5 not more than about 0.1% since an excessive additive amount is rather harmful to the cleanliness and lowers the ductility.
Furthermore, Al is added in an amount of 0.005 - 0.6%
to serves as a deoxidi~er at the melting stage. If desired, P may be added in a range which would not cause enbrittling at the grain boundary. Similarly to Si, the element P is a strong hardening component and has an effect of purifying the ferrite, contributing to the improvement of elongation or other properties. In order to have these effects, it should be added in an amount of 0.03 - O~
The element S may be contained in a range which is normally permitted for an impurity element, namely, in a range ]ess than 0.02%. An S-content less than 0.02% can produce the effec~ of improving the formability; especially the stretsh flangeability, and the ductility of the weld to a satisfac-tory degree.
Now, the ba~ic concept of the method according to the present invention is explained with reference to FIGU~E 8.
In FIGURE 8, plotted at ~ and ~ is a method according ~:~9~
1 to the in~ent.ion, in which a steel slab of a predeterrnined composition is, after heat treatment at Tl, subjected to continuous hot rolling, terminat.ing the hot rolling at a temperature higher than the level T2 (corresponding to the point Ar3). The rolled material is then cooled from 'che finishing rolling temperature to a point between temperature levels T2 and T3 (corresponding to the point Arl) at a controlled cooling speed Cl. Thereafter, in I the material is cooled to T4 (take-up temperature~ at the cooling speed 0 C2 and taken up at a tem.perature below T4. In the case of the material is let for a while for air cooling or slow cooling between the just-mentioned temperature levels T2 and T3 and then immediately cooled off to the tempera-ture level T4 (take-up temperature) at a cooling speed of C2 and taken up at a temperature below the level T4.
In another method which is plotted.at ~ of FIGURE 8, a steel slab o~ a predetermined composition is subjected to continuous hot rolling after a heat treatment at the temperature Tl, terminating the hot rolling at a point between the temperature levels T2 and T3. Thereafter, the material undergoes the air cooling or slow cooling, cooling (at cooling speed C2) and take-up (at T4~ i.n ~he same manner as in the above-mentioned method ~
In the method shown in FIGURE 8, the controlled cooling (at C~ to a point between Ar3 and Arl in the pattern the controlled cooling (at Cl) and the temperature and time ~ 5~
1 of the air or slow cooling in the pattern ~ , or the finishing rolling to a point be-tween ~r3 and Arl and the temperature or time of the following air or slow cooling in the pattern are important as a preparatory stage for obtaining a desired composite steel structure. The metallurgical mechanisms which are involved in these three manufacturing patterns are as follows.
The cooling stage from the hot rolliny finishing temperature to the poînt in the range from Ar3 to Arl in the pattern ~ is a region where mostly the polygonal ferrite phase (a) and the austenite phase ~y) coexist, so that carbon in solid solution of ~-phase is condensed into y-phase by employing relatively slow cooling in that stage, thereby stabilizing the ~-phase and improving the ductility through purification of the ~-phase which contains less carbon in solid sollltion~ Thereforej the average cooling speed Cl which is employed for cooling to a temperature range from Ar3 to Ar subsequent to the finish rolling at the tempexatuxe above the point Ar3 should be 3-70C/sec. If the cooling speed Cl exceeds 70C/sec, it becomes diEficult to obtain an amount of the ~-phase of a desired structure and to control the temperature appropri-ately. On the contrary, with a cooling speed lower than 3C/sec, there occurs Eerrite trans~ormation or pearlite transmation, inviting a drop in productivity due to the lengthy cooliny time. Therefore~ the cooling speed Cl is preferred to be in the range of 3-30 CJs~c.
L5;~
1 Similarly to Pattern ~, the aver~ge cooling speed Cl from the rolling finish temperature to the temperature range of Ar3 to Arl in Pattern ~ should be 3-70C/sec and is particularly desired to be 20-70C/sec for the followiny reasons. Namely, al-though the above mentioned metallurgical mechanisms take place by effecting relatively slow cooling in Pattern ~ in cooling the material from the rolling finish temperature to the level between Ar3 and Arl, air cooling or slow cooling in Pattern ~ is e~fected in the temperature range of Ar3 to Arl which is in the vicinity of the ferrite transformation nose, in order to obtain the mechanisms as mentioned above.
Since the air cooling or slow cooling is effected in the vicinity of the ferrite transformation nose, a predetermined amount of ferrite can be ob-tained .in a short time period due to the accelerated ferrite transformation, and carbon in solid solution of the ferrite phase which has ! transformed during the slow cooling is condensed into the austenite phase. As a rPsult, the amount of carbon in solid solution of the ferrite phase is reduced, enhancing the purity and ductility. Namely, as the above-mentioned mechanisms are obtained by the air cooling or slow cooling in the temperature range of Ar3 to Arl, in Pattern ~ , it-is desirable to ~ffect the cooling from the hot rolling finish temperature to the temperature range Ar3 to Arl at as high a speed as possible, in contrast to Pattern ~. On the other hand, the austenite phase with an increased am~unt of carbon is stabilized to facilitate the formation of the low temperature transforma-tion products in the s~sequent cooling stage. The time . .
l period of the air or slow cooling should not be too short in order to obtain the desired amount of ferrite nor too lony to avoid the ferrite transformation in the entire steel structure or the pearlite transforrnation. Further, since it is limited by the length of the run-out table, the time period of the air cooling or slow cooling should be 2 - 20 seconds.
Now, as shown in Pattern ~ , the controlled cooling ~Cl) is not required when the finish rolling is carried out in the temperature range of Ar3 to Arl. In this instance, the fixtish rolling temperature is preferred to be higher than 710C. Although the air cooling or 510w cooling from the finish rolling temperature is effected after termination of the finish rol~ing in Pattern ~ , the metallurgical mechanisms and the time of air cooling or slow cooling are the same as in Pattern ~ .
The cooling at C2 subseguent to the cooling of Patterns ~ to ~ is intended for converting the austenite into a hard low-temperature transformation product and should be e~ected at an average cooling speed (C2) higher than 20C/sec, preferably in the range o~ 30 - 70C/sec. With a lower cooling speed C2, the austenite is transformed into pearlite in part or in its entirety. On the con~rary, a cooling speed higher than the above-defined range results - in a lower strength-ductility balance and a higher yield ratio.
~S^ll 5~
1 The reason why a hard phase is formed at such a relatively low cooling speed C2 is the enhancemen-t of stability of the austenite which takes place in the meantime, so that the transitional stage from the finishing rolliny temperature to the initiation of the C2 cooling is important. Further, the fact that the hard phase is mainly made of the bainite phase permits the cooling at a relatively low speed C~ and contributes to the improvement of the strength-ductility balance:
Thereafter, the steel plate is wound up at a predeter-mined temperature and this take-up temperature (T4) constitutes another important point of the present invention. More particularly, it is desirable to effect the cooling to room temperature in order to form martensite, which however in turn brings about various defects such as a drop in the strength~ductility balance due to the existence of carbon in solid solution remaining in the course of cooling to the tempexature T~ or a rise of the yield ratio, coupled with the above-mentioned shortcomings of the ferrite ~ martensite steel~ Therefore, it is preferred to wind up at A temperature above 300C for obtaining the desired structureO The upper limit of the take-up temperature T4 should be place at 550C
since pearlite transformation takes place at temperatures above 550C unle5s -the alloy elements are added in large quantities.
1 The hot-rolled steel plate which is obtained by the above-described me-thod is of a tripple phase structure con-sisting of polygonal ferri-te, bainite (i.e., the carbide-including bainite and ~ainitic ferrite) and martensite (partly containing retained austenite) and particularly has bainite phase areas at a rate of 4 - 45% and martensite areas at a rate of 1 15%, the area rate of the bainite phase being greater than that of the martensite phase. This steel plate is in any of the properties of yield ratio, strength-elonga-tion balance, stretch flangeability and resistance weldability.
The range of the chemical composition of the steel plate which is intended by the method of the invention, the preferred ranges of the area rates of its bainite and - martensite phases and the particle size of the martensite are same as defined hereinbefore.
Hereafter, the steel structure and its manufacturing method of the invention are illustrated more particularly by way cf a nun~er of examples.
Example 1 The materials of the chemical compositions shown in Table 1 were each melted in a vacuum melter, roughly rolled into a 30 mm thick slab and then into a 4 mm thick plate by 3-pass hot rolling.
The materials which were cooled to room temperature by air cooling subsequent to the hot rolling were heated to a temperature in the range of 7B0 - 950C for 5 - 10 minutes 5~
1 and then cooled under different conditions to ob~,ain specimens of different structures.
The results of the microscopic observatio~ and measurement of the resective specimens are shown in Table 2 along wi-th the results of measurement of the mechanical properties. Wit,h regard to the results of measurement, the relations of the tensile strength with the yield stress, total elongation and hole expansibility are plotted in FIGURES
l~and 2. As stated hereinbefore, the specimen Nos. 1 to 7 which have a steel structure according to the present inven-tion show excellent properties in the yield ratio, strength-~longation balance and stretch flangeability.
Table 3 shows the properties and the results of fatigue test of the specimens which were same as the specimen No. 6 in the chemical composition of the starting material but formed into different structures by varying the, heating and cooling conditions. As seen therefrom, the steel according to the invention shows an excellent property also in the fatigue strength.
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S~ii2 1 Nextly, a steel plate was produced Erom a specimen which was obtained by adding 0.1~ of Ce to Specimen No. 2, hot-rolling, cooling and taking up the specimen under the conditions under the conditions shown in Table 4. The thus obtained steel plate was subjected to a wheel disc forming test of actual scale ~n = 20). Table 5 below shows the results of the m.icroscopic observation and measurement, along with the results of measurement of mechanical properties and the rate of defective wheel discs in forming operation. As seen therefrom, the steel according to the present invention is extremely low in the rate of defective wheel discs.
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1 Example 2:
The materials of the chemical compositions shown in Table 5 were each melted in a vacuum melter, roughly rolled into a 30 ~n thick slab and then into a 4 l~m thick plate by 3-pass hot rollingO
The materials which were cooled to room temperature by air cooling were heated at the temperatures shown in Table 7 for 5 minutes and then cooled under diferent condi-tions to obtain specimens of different structures.
The results of microscopic observation and measure-ment of the thus obtained specimens are shown in Tables 7 and 8, along with the results of measurement o~ mechanical pro-perties. As mentioned hereinbefore, the steel specimens 20 to 22 according to the present invention were invariably low in the yield ratio and excellent in strength-elongation balance and stretch flangeability.
In this Example, it is particularly to be noted that the components which play a main role in the steel structure of the invention are Si and Mn. Namely, in a case where the Si and Mn components are in the above-defined ranges and the ratio o~ Si/Mn is greater than 1, the yield ratio, strength-elongation balance and hole expansibility of the tripple phase steel structure of the invention are improved to a considerable degree. The rea~ons for this phenomenon is not known at the present stage but are considered as ~ollows.
~5~
1 (1) The amount and length of the sulfide inclusions are increased by the addition Si. In view of the larye content of Mn, the ratio of Si/Mn is desired to be greater than 1Ø
(2) The component Si has a higher hardening effect than Mn and lowers the stacking fault energy, delaying the formation of cells. As a result, the cell size becomes finer to permit higher elongation and reduction.
(3) The component Si accelerates the condensation of C from ferrite to y-phase, thereby purifying the ferrite and makiny a large difference in hardness between the ferrite and the low temperature transformation product to permit a high elongation.
.
Table 6 - Chemical Compositions (%) _ _._ _ Spe imen C Si Mn S Other elements _ .
0.06 1.3 1.1 0.007 21 0.06 1.3 1.0 0.006 Nb 0.02, V 0.03, Cr 0.6 22 0.07 1.3 1.0 0.004 Cr 0.85 REM 0.08 Unable to recognize this page.
1 Example 3:
The materials of the chemical compositions shown in Table 9 were each melted in a vacuum melter and roughly rolled into a 30 mm thick slab and then into a 3.4 mm thick steel plate by 3-pass hot rolling.
Further, the materials which were cooled to room temperature by air cooling were quickly hea-ted to 950C and, after retaining that temperature for several minutes, they were cooled under different conditions to obtain specimens of desired st~uctures.
The conditions of -the heat treatment and the results of the measurement of the microscopic structures are shown in Table 1OD
Table 9 ~ Chemical Compositions ¦ P No ¦ C Si Mn S ¦Other elements 23 0.05 0.4 1.5 0.005 Cr 0.8 Inven-24 0.04 0.5 1.6 0.003 Cr 0.5 Ce 0.003 tion Nb 0.02 Unable to recognize this page.
5~
1 A material containing 0.04% of Ce in addition to -the composition of the specimen No. 23 and a material of the sarne composition as the specimen No. 24 were prepared by melting the materials on the spot, followed by blooming and hot rolling, and cooled and wound up under the conditions shown in Table 11. The resulting steel plate specimens were formed into ordinary wheel rims of an actual si~e by flush-butt welding and rolling forming. The microscopic structures, mechanical properties and wheel rim formability of the respec-tive steel plates are shown in Table 12.
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~ s~s~
1 Further, the results of examination of the hardness distribution afker flush-butt welding of these hot-rolled steel plates are shown in FIGURE 7. It will be seen there-from that the softening occurs in a considerable degree in the heat affected zone of the F ~ M steel structure (Specimen No. I) owing to the decomposition of the second phase marten-site.
On the other hand, in the F + B + M steel structure according to the present invention (Specimen Nos. II and III), the softening is leassened conspicuously and, in the presence of NbC, a slight degree of hardening is observed in the heat affected zone instead of softening. Consequently, there is no possibility of the breakage initiating from the heat affected zone in the roll-forming stage of the F + B ~ M
steel structuxe subsequent to the flash-butt welding. This can be confirmed by reference to Table 2 which shows the rate of defective wheel rims.
Example 4:
Slabs of 30mm in thickness were obtained by melting in a high frequency vacuum melter the steel materials of different compositions as shown in Table 13, followed by forgeing and rough rolling. After heating to 1200C, the slabs were finished into 3~2 mm thick steel plates by 3-pass hot rolling employing a variety of tempera~ures above the point Arl and then taken up at different tempera~ures below 600C. Table 14 shows the conditions of hot rolling of these l steel plates along with the results of the observation of microscopic structures. In Table 15, there are sho~n the mechanical proper-ties of the steel plates of Table 14 and the values of tenacity and variations in hardness after flash-butt welding under the following conditions.
Welding Conditions:
Flash margin: 3 mm Flash time: 3 seconds Upset margin: 3 mm Upset time: 2/60 seconds Upset speed: 150 mm/sec Blank size: 30mm(w) x 75mm(Q) x 3.2mm(t) In Table 15, "Y.P./T.S." ~yield ratio) is used as an index for judging ~he Eormability and a lower value means a higher shape fixability or workability. On the other hand, "YPE" (yield poi~t elonyatiDn) indicates the presence or absence o~ wrinkles in those portions which are subjected to tensile stress by working, and the value of the yield point elongation should be as small as possible in order to prevent ~he wrinkling.
The term "TSXEQ" (strength-elongation balance) indicates the balance between the strength and ductility, and a higher value of TSXEQ means a better balance. JThe hole expansibility (~) is an index of the stretch flangeability and a higher value reflects a better stretch flangeability.
1 With regard to the flash butt weldiny, the terrn "vEs"
(upper shelf energy) and "vTrs" ~charpy V-notch transition temperature) are indexes of the weld tenacity~ which is better when higher in the value of vEs and lower in the value of vTrs. The symbolic expression "~Hv" indicates the hardness of the weld bounding portion - the hardness of the parent material, and "aHv" the hardness of the welding heak affected ~one - the hardness of the parent material. The value of "~Hv" should not be too high since otherwise cracking occurs during the roll-forming operation due to a drop in ductility.
On the other hand, a disjoint takes place if the value of '~Hv" is low As seen from Table 15, the steel plates produced by the method of the present invention are-low in the yield ratio with no yield point elongation and have good strength-elongation balance. Besides, the are excellent in the stretch flangeability as well as in the tenacity of the weld, showing a smaller increase of the hardness of the weld bounding portion and a smaler dxop in the hardness of the weld heat affected zone, thus as a whole exhibiting excellent resistance welda-bility.
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5~S~2 1 Example 5:
The s-teel material of the composition shown in Table 16 was melted in a high frequency vacuum melter and hot-rolled in the same manner as in Example 4, varying the cooliny speed and -the taXe-up temperature to obtain intended steel structures. Table 17 below shows the conditions of the hot rolling and the results of the observation of microscopic structures of the hot rolled steel plates, while Table 18 shows their mechanical structures.
Table 16 - Chemical Composition-(wt~) _ _ Steel C Si Mn p S Cr Al Others _ G 0.10 0.2 1.3 0.008 0.005 0.7 0.025 Ce 0.007 .
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5~
1 It will be also obvious from the foregoing results that the steel plates produced by the method of the present invention are low in the vield ratio and conspicuously improved in strength-elongation balance as well as in stretch flangea-bility.
Example 6:
The steel specimens of the compositions shown in Table 19 were prepared, employing the conditions of heat treatments shown also in Table 19. The mlcroscopic structu.res and mechanical properties of the resulting steel plates shown in Table 20. As clear therefrom, the steel plates produced by the method of the invention all show a 150~ or higher hole expanding limit.
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1 in the forming stage, with a yield ratio lower than 0.7 and preferably lower than 0.6. As clear ~rom the experimental data of FIGURE 4, intensive studies conducted by the present inventors revealed the value of the bainite area rate suitable for the yield ratio and the stretch flangeability which are required for the material to be used for the vehicular wheel disc and/or wheel rim. According to FIGURE 4, the bainite area rate should be in the range of 4 - 45%.
FIGURE S plots the relationship between the martensite area rate and the stretch flangeability and the relationship between the martensite area rate and the yield ratio in the specimens 46, 47 and 53 to 57 which will be explained herein-later. Similarly to the bainite area rate mentioned above, the martensite area rate should be held in the range of 1 ~ 15%n As ~lear from FIGURE 5, the yield ratio is increased if the martensite area rate exceeds 1S% and it becomes di~ficult to--attain hole expansibility greater than 150%. On the contrary, with a martensite area rate smaller than 1%, the effect of martensite introduction becomes smaller. The martensite area rate should be in the range which guarantees a hole expansi-bili-ty over 160% and a yield ratio below 0.6, namely, in the range of 1 - 10%.
As seen in FIGUR~ 3 which plo-ts ~he correlation between the martensite area rate and the bainite area rate in relation with the hole expansibility, more particularly, the pattern III of Example 4 (denoted by mark "~") and Example 5 1 (denoted by mark "O") which will appear hereinlater, the bainite area rate in the tripple-phase steel structure according to the present invention should be greater than the martensite area rate in order to secure hole expansibili.ty greater than 150%, in additi.on to the above-defined conditions that the bainite and martensite area rates should be respectively in the ranges of 4 - 45% and 1 - 15~ The range which satisfies these conditions is indicated by hatching in FIGURE 3.
FIGURE 6 is a plot of the relationship between the particle size of the martensite and the hole expansibility, more particularly, a plot of the results of experiments of Table 20 using the composite steel structures of Table 19.
The hole e~pansibility is also dependent on the average dia-meter of the martensite as shown in FIGURE 6. More specifi-cally, this figure shows that the hole expansibility is further improved by making the martensite finer even if the area rates of the bainite and martensite are in the ab~ve~defined ranges, namely, that the hole expansibility becomes greater than 150~ when the average particle size of the martensite is smaller than 6 microns. The hole expansibility is further improved as the grain size is reduced to a value smaller than 5 microns and improved in a greater degree with a value smaller than 4 microns.
It will bè appreciated from the foregoing description that, with the tripple-phase steel structure acc~rding to the present invention t it iS possible to guarantee a 1QW yield s~s~
---ll--l ratio along with excellent strength-elongation balance and stre-tch flangeability by controlling the area rates of the bainite and martensite, and that the stretch flangeability can be further enhanced by making the mar-tensite finer.
As already s-tated hereinbeEore, one problem which is encountered when applying a steel plate to a vehicular wheel rim is the softening of the -thermally influenced por-tions after the flash-butt welding, which occurs conspicuously degree in the ferrite + martensite steel as a result of decomposition of the martensite, the thermally influenced portions giving rise to cracks in the subsequent roll-forming stage in such a degree as to make the application of the ferrite + martensite steel utterly difficult,. The softening of the thermall~ influenced portion, however, is not observed in the steel of the bainite structure, and the problem of cracking in the cold-rolling (roll-forming) subsequent to the welding operation is precluded, In order to overcome the drawback of the ferrite + bainite structure steel, the present inyentors conducted a comprehensi~e study varying the pro-20 Portions of the ferrite + ~ainite + martensite structures in relation with.the chemi.cal components involved, As a result, it ha$ been found that, as shown in FIGUR~ 7, the steel of ; ferrite + martensite structure has an excellent softening resist-ance similarly to the ferrite + bainite structure steel, even in 95~52 - 12 ~
1 the case of spot welding. It has also been revealed that a slightly sof-tened state can be established by letting a stable precipitant like NbC or the like exist in the thermally affected zone or more positively leaving Nb in solid solu-tion of after the hot-rolling for precipitating same in the thermally affected zone, thereby preventing the initiation of cracking from the thermally influenced portion in the succeed-ing roll-forming operation.
The adjustment of the steel structure is attained b~
controlling the cooling condition during and after the hot-rolling or by further controlling the annealing conditions either (either continuous or batch annealing3 in the subsequent stage, in connection with the chemical components which are discûssed hereinafter.
The limitations of the respective chemical components in the present invention are based on the following reasons.
The component C is an element which is essential for maintaining the required streng~h and for forming the low temperature transformation products like bainite and marten-site but its content should be limited since a C-content in excess of 0.2~ will cause a considerable deterioration in ductility and impair the weldability (giving rise to a drop in hardness of butt faces due to decarburization in the butt welding, resulting in a large difference in hardness between the weld line and the adjacent portions). In a case where forming workability is required in particular, its content 1 should desirably be less than 0.09%. The lower limit of the C-content should be 0.01 in order to secure the effects of strengthening the structure and enhancing the hardenability.
The element Mn is necessary for improving the hardena-bility and obtaining the desired structure. The improvemenk in hardenability also contributes to the increase of streng-th and to the enhancement of mechanical properties by stabilizing the y-phase during the y-~ transformation (y-aus-tenite, a=
ferrite) after the hot-rolling. In order to secure these effects, its content should be 0.3~ or more. However, with a Mn content in excess of 2.5%, it causes a welding difficulty and impairs the ductility ~elongation and stretch flangeability) and the weldability in addition to a substantial increase of the cost of the steel plate. Therefore, the upper limit should be placed at 2.5%.
The element Si which is necessary for deoxidation of the molten steel is also very effective as a substitutional solid solu-tion hardening element. Therefore, it is essential in order to obtain a steel plate with high strength and ductility. Besides, it acts advantayeously to the formation of clean polygonal ferrite. In the composite steel structure as of the present invention, it accelera-tes the a-transforma-tion during the y-a transformation after hot rolling and acts to discharge the carbon in solid solution out of ~phase and shift into y-phase. Consequently, it enhances the clean-liness of a--phase and sta~ilizes y-phase by condensing carbon 5~L52 1 thereinto, thus facilitating the formation of a hard phase which contributes to the improvement of the mechanical pro-perties. In order to produce these effects while preventing enbrittling oE the weld (an increase of transition tempera-ture), the lower limit of the Si cont~nt should be placed at 0.01%. On the other hand, its upper limit should be placed at 1.8% to prevent deteriorations of surface condition due to production of oxidation scales.
According to the present invention, the following elements may be included if desired in addition to the above-mentioned elements.
The elements Cr, Cu, Ni and B are useul for improv-ing the hardenability as well as for obtaining a desired structure. The lower limits of their oontents should be placed at a level which is sufficient enough for ~ecuring these effects, while the upper limits should be placed at a level whexe their effects are saturated and uneconomical.
More specifically, the steel plate according to the present invention may contain at least one element selected from the group consisting of 0.1-1.5% of Cr, 0.1 - 0.6~ of Cu, 0.1 - 1.0% of Ni and 0.0005 - 0O01% of B. Further, the element Mo which also serves to improve the hardenability and produce a desired structure similarly to the above-mentioned Cr, Cu, Ni and B may ~lso be included in an amount of 0.01 - 0.2%
for the same reasons.
The elements Nb, V, Ti and Zr which serve to strengthen 9S~5Z
1 the precipitation are necessary not only for increasing the strength but also for ~acilita-ting the forrnation of the bainite structure by imposing an inflwence on the transform-ing structure under coexistence with Mn or the like after hot-rolling. Further, they make the structure finer and serves to improve the stretch flangeability and to prevent drops in hardness, improving not only the fatigue strength of the parent plate but also the fatigue strength of the ~isc as a whole. Moreover, they contribute to produce ~he hardena-bility~-improving effec-t of B to a maximum degree. In order to have these effects, it is necessary to include at least one element selected from the group consisting of 0.01 - 0.16 of Nb, 0.0~ - 0.2~ of V, 0.01 - 0.1% of Ti and 0.02 - 0.2%
of Zr.
In addition, the component Nb particularly has an influence on the transforming behaviors after the hot-rolling and is most effective for the formation of the bainite struc-ture. The elements Ti and Zr are further effective for con-trolling the shape of the sulfide which is harmful to the ductility, and the element V is effective for moderately hardening the center portion of the weld (Hv ~ 25) as compared with that of the parent material.
The rare earth metals (REM), Ca and Mg contribute to the improvement of the ductility, particularly, the stretch flangeability by controlling the shape of the sulfide. The lower limits of their contents should be placed at a level which is suEficient for producing that effect. The lower 1 limit is determined at a value at which the aimed effect becornes saturated or uneconomical or in consideration of the content which inversely impairs the cleanliness. More specifically, the steel plate of the invention may include at least one element selected from the group consisting of 0.005 - 0.1% of a REM, 0.0005 - 0.01~ of Ca and 0.0005 - 0.01% of Mg. However, it is desired that the total additive amount i5 not more than about 0.1% since an excessive additive amount is rather harmful to the cleanliness and lowers the ductility.
Furthermore, Al is added in an amount of 0.005 - 0.6%
to serves as a deoxidi~er at the melting stage. If desired, P may be added in a range which would not cause enbrittling at the grain boundary. Similarly to Si, the element P is a strong hardening component and has an effect of purifying the ferrite, contributing to the improvement of elongation or other properties. In order to have these effects, it should be added in an amount of 0.03 - O~
The element S may be contained in a range which is normally permitted for an impurity element, namely, in a range ]ess than 0.02%. An S-content less than 0.02% can produce the effec~ of improving the formability; especially the stretsh flangeability, and the ductility of the weld to a satisfac-tory degree.
Now, the ba~ic concept of the method according to the present invention is explained with reference to FIGU~E 8.
In FIGURE 8, plotted at ~ and ~ is a method according ~:~9~
1 to the in~ent.ion, in which a steel slab of a predeterrnined composition is, after heat treatment at Tl, subjected to continuous hot rolling, terminat.ing the hot rolling at a temperature higher than the level T2 (corresponding to the point Ar3). The rolled material is then cooled from 'che finishing rolling temperature to a point between temperature levels T2 and T3 (corresponding to the point Arl) at a controlled cooling speed Cl. Thereafter, in I the material is cooled to T4 (take-up temperature~ at the cooling speed 0 C2 and taken up at a tem.perature below T4. In the case of the material is let for a while for air cooling or slow cooling between the just-mentioned temperature levels T2 and T3 and then immediately cooled off to the tempera-ture level T4 (take-up temperature) at a cooling speed of C2 and taken up at a temperature below the level T4.
In another method which is plotted.at ~ of FIGURE 8, a steel slab o~ a predetermined composition is subjected to continuous hot rolling after a heat treatment at the temperature Tl, terminating the hot rolling at a point between the temperature levels T2 and T3. Thereafter, the material undergoes the air cooling or slow cooling, cooling (at cooling speed C2) and take-up (at T4~ i.n ~he same manner as in the above-mentioned method ~
In the method shown in FIGURE 8, the controlled cooling (at C~ to a point between Ar3 and Arl in the pattern the controlled cooling (at Cl) and the temperature and time ~ 5~
1 of the air or slow cooling in the pattern ~ , or the finishing rolling to a point be-tween ~r3 and Arl and the temperature or time of the following air or slow cooling in the pattern are important as a preparatory stage for obtaining a desired composite steel structure. The metallurgical mechanisms which are involved in these three manufacturing patterns are as follows.
The cooling stage from the hot rolliny finishing temperature to the poînt in the range from Ar3 to Arl in the pattern ~ is a region where mostly the polygonal ferrite phase (a) and the austenite phase ~y) coexist, so that carbon in solid solution of ~-phase is condensed into y-phase by employing relatively slow cooling in that stage, thereby stabilizing the ~-phase and improving the ductility through purification of the ~-phase which contains less carbon in solid sollltion~ Thereforej the average cooling speed Cl which is employed for cooling to a temperature range from Ar3 to Ar subsequent to the finish rolling at the tempexatuxe above the point Ar3 should be 3-70C/sec. If the cooling speed Cl exceeds 70C/sec, it becomes diEficult to obtain an amount of the ~-phase of a desired structure and to control the temperature appropri-ately. On the contrary, with a cooling speed lower than 3C/sec, there occurs Eerrite trans~ormation or pearlite transmation, inviting a drop in productivity due to the lengthy cooliny time. Therefore~ the cooling speed Cl is preferred to be in the range of 3-30 CJs~c.
L5;~
1 Similarly to Pattern ~, the aver~ge cooling speed Cl from the rolling finish temperature to the temperature range of Ar3 to Arl in Pattern ~ should be 3-70C/sec and is particularly desired to be 20-70C/sec for the followiny reasons. Namely, al-though the above mentioned metallurgical mechanisms take place by effecting relatively slow cooling in Pattern ~ in cooling the material from the rolling finish temperature to the level between Ar3 and Arl, air cooling or slow cooling in Pattern ~ is e~fected in the temperature range of Ar3 to Arl which is in the vicinity of the ferrite transformation nose, in order to obtain the mechanisms as mentioned above.
Since the air cooling or slow cooling is effected in the vicinity of the ferrite transformation nose, a predetermined amount of ferrite can be ob-tained .in a short time period due to the accelerated ferrite transformation, and carbon in solid solution of the ferrite phase which has ! transformed during the slow cooling is condensed into the austenite phase. As a rPsult, the amount of carbon in solid solution of the ferrite phase is reduced, enhancing the purity and ductility. Namely, as the above-mentioned mechanisms are obtained by the air cooling or slow cooling in the temperature range of Ar3 to Arl, in Pattern ~ , it-is desirable to ~ffect the cooling from the hot rolling finish temperature to the temperature range Ar3 to Arl at as high a speed as possible, in contrast to Pattern ~. On the other hand, the austenite phase with an increased am~unt of carbon is stabilized to facilitate the formation of the low temperature transforma-tion products in the s~sequent cooling stage. The time . .
l period of the air or slow cooling should not be too short in order to obtain the desired amount of ferrite nor too lony to avoid the ferrite transformation in the entire steel structure or the pearlite transforrnation. Further, since it is limited by the length of the run-out table, the time period of the air cooling or slow cooling should be 2 - 20 seconds.
Now, as shown in Pattern ~ , the controlled cooling ~Cl) is not required when the finish rolling is carried out in the temperature range of Ar3 to Arl. In this instance, the fixtish rolling temperature is preferred to be higher than 710C. Although the air cooling or 510w cooling from the finish rolling temperature is effected after termination of the finish rol~ing in Pattern ~ , the metallurgical mechanisms and the time of air cooling or slow cooling are the same as in Pattern ~ .
The cooling at C2 subseguent to the cooling of Patterns ~ to ~ is intended for converting the austenite into a hard low-temperature transformation product and should be e~ected at an average cooling speed (C2) higher than 20C/sec, preferably in the range o~ 30 - 70C/sec. With a lower cooling speed C2, the austenite is transformed into pearlite in part or in its entirety. On the con~rary, a cooling speed higher than the above-defined range results - in a lower strength-ductility balance and a higher yield ratio.
~S^ll 5~
1 The reason why a hard phase is formed at such a relatively low cooling speed C2 is the enhancemen-t of stability of the austenite which takes place in the meantime, so that the transitional stage from the finishing rolliny temperature to the initiation of the C2 cooling is important. Further, the fact that the hard phase is mainly made of the bainite phase permits the cooling at a relatively low speed C~ and contributes to the improvement of the strength-ductility balance:
Thereafter, the steel plate is wound up at a predeter-mined temperature and this take-up temperature (T4) constitutes another important point of the present invention. More particularly, it is desirable to effect the cooling to room temperature in order to form martensite, which however in turn brings about various defects such as a drop in the strength~ductility balance due to the existence of carbon in solid solution remaining in the course of cooling to the tempexature T~ or a rise of the yield ratio, coupled with the above-mentioned shortcomings of the ferrite ~ martensite steel~ Therefore, it is preferred to wind up at A temperature above 300C for obtaining the desired structureO The upper limit of the take-up temperature T4 should be place at 550C
since pearlite transformation takes place at temperatures above 550C unle5s -the alloy elements are added in large quantities.
1 The hot-rolled steel plate which is obtained by the above-described me-thod is of a tripple phase structure con-sisting of polygonal ferri-te, bainite (i.e., the carbide-including bainite and ~ainitic ferrite) and martensite (partly containing retained austenite) and particularly has bainite phase areas at a rate of 4 - 45% and martensite areas at a rate of 1 15%, the area rate of the bainite phase being greater than that of the martensite phase. This steel plate is in any of the properties of yield ratio, strength-elonga-tion balance, stretch flangeability and resistance weldability.
The range of the chemical composition of the steel plate which is intended by the method of the invention, the preferred ranges of the area rates of its bainite and - martensite phases and the particle size of the martensite are same as defined hereinbefore.
Hereafter, the steel structure and its manufacturing method of the invention are illustrated more particularly by way cf a nun~er of examples.
Example 1 The materials of the chemical compositions shown in Table 1 were each melted in a vacuum melter, roughly rolled into a 30 mm thick slab and then into a 4 mm thick plate by 3-pass hot rolling.
The materials which were cooled to room temperature by air cooling subsequent to the hot rolling were heated to a temperature in the range of 7B0 - 950C for 5 - 10 minutes 5~
1 and then cooled under different conditions to ob~,ain specimens of different structures.
The results of the microscopic observatio~ and measurement of the resective specimens are shown in Table 2 along wi-th the results of measurement of the mechanical properties. Wit,h regard to the results of measurement, the relations of the tensile strength with the yield stress, total elongation and hole expansibility are plotted in FIGURES
l~and 2. As stated hereinbefore, the specimen Nos. 1 to 7 which have a steel structure according to the present inven-tion show excellent properties in the yield ratio, strength-~longation balance and stretch flangeability.
Table 3 shows the properties and the results of fatigue test of the specimens which were same as the specimen No. 6 in the chemical composition of the starting material but formed into different structures by varying the, heating and cooling conditions. As seen therefrom, the steel according to the invention shows an excellent property also in the fatigue strength.
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S~ii2 1 Nextly, a steel plate was produced Erom a specimen which was obtained by adding 0.1~ of Ce to Specimen No. 2, hot-rolling, cooling and taking up the specimen under the conditions under the conditions shown in Table 4. The thus obtained steel plate was subjected to a wheel disc forming test of actual scale ~n = 20). Table 5 below shows the results of the m.icroscopic observation and measurement, along with the results of measurement of mechanical properties and the rate of defective wheel discs in forming operation. As seen therefrom, the steel according to the present invention is extremely low in the rate of defective wheel discs.
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1 Example 2:
The materials of the chemical compositions shown in Table 5 were each melted in a vacuum melter, roughly rolled into a 30 ~n thick slab and then into a 4 l~m thick plate by 3-pass hot rollingO
The materials which were cooled to room temperature by air cooling were heated at the temperatures shown in Table 7 for 5 minutes and then cooled under diferent condi-tions to obtain specimens of different structures.
The results of microscopic observation and measure-ment of the thus obtained specimens are shown in Tables 7 and 8, along with the results of measurement o~ mechanical pro-perties. As mentioned hereinbefore, the steel specimens 20 to 22 according to the present invention were invariably low in the yield ratio and excellent in strength-elongation balance and stretch flangeability.
In this Example, it is particularly to be noted that the components which play a main role in the steel structure of the invention are Si and Mn. Namely, in a case where the Si and Mn components are in the above-defined ranges and the ratio o~ Si/Mn is greater than 1, the yield ratio, strength-elongation balance and hole expansibility of the tripple phase steel structure of the invention are improved to a considerable degree. The rea~ons for this phenomenon is not known at the present stage but are considered as ~ollows.
~5~
1 (1) The amount and length of the sulfide inclusions are increased by the addition Si. In view of the larye content of Mn, the ratio of Si/Mn is desired to be greater than 1Ø
(2) The component Si has a higher hardening effect than Mn and lowers the stacking fault energy, delaying the formation of cells. As a result, the cell size becomes finer to permit higher elongation and reduction.
(3) The component Si accelerates the condensation of C from ferrite to y-phase, thereby purifying the ferrite and makiny a large difference in hardness between the ferrite and the low temperature transformation product to permit a high elongation.
.
Table 6 - Chemical Compositions (%) _ _._ _ Spe imen C Si Mn S Other elements _ .
0.06 1.3 1.1 0.007 21 0.06 1.3 1.0 0.006 Nb 0.02, V 0.03, Cr 0.6 22 0.07 1.3 1.0 0.004 Cr 0.85 REM 0.08 Unable to recognize this page.
1 Example 3:
The materials of the chemical compositions shown in Table 9 were each melted in a vacuum melter and roughly rolled into a 30 mm thick slab and then into a 3.4 mm thick steel plate by 3-pass hot rolling.
Further, the materials which were cooled to room temperature by air cooling were quickly hea-ted to 950C and, after retaining that temperature for several minutes, they were cooled under different conditions to obtain specimens of desired st~uctures.
The conditions of -the heat treatment and the results of the measurement of the microscopic structures are shown in Table 1OD
Table 9 ~ Chemical Compositions ¦ P No ¦ C Si Mn S ¦Other elements 23 0.05 0.4 1.5 0.005 Cr 0.8 Inven-24 0.04 0.5 1.6 0.003 Cr 0.5 Ce 0.003 tion Nb 0.02 Unable to recognize this page.
5~
1 A material containing 0.04% of Ce in addition to -the composition of the specimen No. 23 and a material of the sarne composition as the specimen No. 24 were prepared by melting the materials on the spot, followed by blooming and hot rolling, and cooled and wound up under the conditions shown in Table 11. The resulting steel plate specimens were formed into ordinary wheel rims of an actual si~e by flush-butt welding and rolling forming. The microscopic structures, mechanical properties and wheel rim formability of the respec-tive steel plates are shown in Table 12.
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~ s~s~
1 Further, the results of examination of the hardness distribution afker flush-butt welding of these hot-rolled steel plates are shown in FIGURE 7. It will be seen there-from that the softening occurs in a considerable degree in the heat affected zone of the F ~ M steel structure (Specimen No. I) owing to the decomposition of the second phase marten-site.
On the other hand, in the F + B + M steel structure according to the present invention (Specimen Nos. II and III), the softening is leassened conspicuously and, in the presence of NbC, a slight degree of hardening is observed in the heat affected zone instead of softening. Consequently, there is no possibility of the breakage initiating from the heat affected zone in the roll-forming stage of the F + B ~ M
steel structuxe subsequent to the flash-butt welding. This can be confirmed by reference to Table 2 which shows the rate of defective wheel rims.
Example 4:
Slabs of 30mm in thickness were obtained by melting in a high frequency vacuum melter the steel materials of different compositions as shown in Table 13, followed by forgeing and rough rolling. After heating to 1200C, the slabs were finished into 3~2 mm thick steel plates by 3-pass hot rolling employing a variety of tempera~ures above the point Arl and then taken up at different tempera~ures below 600C. Table 14 shows the conditions of hot rolling of these l steel plates along with the results of the observation of microscopic structures. In Table 15, there are sho~n the mechanical proper-ties of the steel plates of Table 14 and the values of tenacity and variations in hardness after flash-butt welding under the following conditions.
Welding Conditions:
Flash margin: 3 mm Flash time: 3 seconds Upset margin: 3 mm Upset time: 2/60 seconds Upset speed: 150 mm/sec Blank size: 30mm(w) x 75mm(Q) x 3.2mm(t) In Table 15, "Y.P./T.S." ~yield ratio) is used as an index for judging ~he Eormability and a lower value means a higher shape fixability or workability. On the other hand, "YPE" (yield poi~t elonyatiDn) indicates the presence or absence o~ wrinkles in those portions which are subjected to tensile stress by working, and the value of the yield point elongation should be as small as possible in order to prevent ~he wrinkling.
The term "TSXEQ" (strength-elongation balance) indicates the balance between the strength and ductility, and a higher value of TSXEQ means a better balance. JThe hole expansibility (~) is an index of the stretch flangeability and a higher value reflects a better stretch flangeability.
1 With regard to the flash butt weldiny, the terrn "vEs"
(upper shelf energy) and "vTrs" ~charpy V-notch transition temperature) are indexes of the weld tenacity~ which is better when higher in the value of vEs and lower in the value of vTrs. The symbolic expression "~Hv" indicates the hardness of the weld bounding portion - the hardness of the parent material, and "aHv" the hardness of the welding heak affected ~one - the hardness of the parent material. The value of "~Hv" should not be too high since otherwise cracking occurs during the roll-forming operation due to a drop in ductility.
On the other hand, a disjoint takes place if the value of '~Hv" is low As seen from Table 15, the steel plates produced by the method of the present invention are-low in the yield ratio with no yield point elongation and have good strength-elongation balance. Besides, the are excellent in the stretch flangeability as well as in the tenacity of the weld, showing a smaller increase of the hardness of the weld bounding portion and a smaler dxop in the hardness of the weld heat affected zone, thus as a whole exhibiting excellent resistance welda-bility.
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5~S~2 1 Example 5:
The s-teel material of the composition shown in Table 16 was melted in a high frequency vacuum melter and hot-rolled in the same manner as in Example 4, varying the cooliny speed and -the taXe-up temperature to obtain intended steel structures. Table 17 below shows the conditions of the hot rolling and the results of the observation of microscopic structures of the hot rolled steel plates, while Table 18 shows their mechanical structures.
Table 16 - Chemical Composition-(wt~) _ _ Steel C Si Mn p S Cr Al Others _ G 0.10 0.2 1.3 0.008 0.005 0.7 0.025 Ce 0.007 .
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5~
1 It will be also obvious from the foregoing results that the steel plates produced by the method of the present invention are low in the vield ratio and conspicuously improved in strength-elongation balance as well as in stretch flangea-bility.
Example 6:
The steel specimens of the compositions shown in Table 19 were prepared, employing the conditions of heat treatments shown also in Table 19. The mlcroscopic structu.res and mechanical properties of the resulting steel plates shown in Table 20. As clear therefrom, the steel plates produced by the method of the invention all show a 150~ or higher hole expanding limit.
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Claims (28)
1. A high strength steel plate having a low yield ratio and improved strength-elongation balance and stretch flangeability, said steel plate comprising 0.01 - 0.2% of C, 0.3 - 2.5% of Mn and 0.01 - 1.8% of Si with the substantial balance iron and inevitable impurities and having a tripple phase of polygonal ferrite, bainite and martensite, the area rates of said bainite and martensite phases being 4 - 45% and 1 - 15%, respectively, and the area rate of said bainite phase being greater than that of martensite phase.
2. A high strength steel plate having a low yield ratio and improved strength-elongation balance and strength flangeability, said steel plate comprising:
0.01 - 0.2% of C, 0.3 - 2.5% of Mn, 0.01 - 1.8% of Si, 0 - 1.5% of Cr, 0 - 0.6% of Cu, 0 - 1% of Ni, 0 - 0.01% of B, 0 - 0.1% of Nb, 0 - 0.2% of V, 0 - 0.1% of Ti, 0 - 0.2% of Zr, 0 - 0.1% of a rare earth metal, 0 - 0.01% of Ca, 0 - 0.01% of Mg, 0 - 0.06% of Al, 0 - 0.1% of P, with the substantial balance comprising iron and inevitable impurities and having a tripple phase of polygonal ferrite, bainite and martensite, the area rates of said bainite and martensite phases being 4 - 45% and 1 - 15%, respectively, and the area rate of said bainite phase being greater than that of martensite phase.
0.01 - 0.2% of C, 0.3 - 2.5% of Mn, 0.01 - 1.8% of Si, 0 - 1.5% of Cr, 0 - 0.6% of Cu, 0 - 1% of Ni, 0 - 0.01% of B, 0 - 0.1% of Nb, 0 - 0.2% of V, 0 - 0.1% of Ti, 0 - 0.2% of Zr, 0 - 0.1% of a rare earth metal, 0 - 0.01% of Ca, 0 - 0.01% of Mg, 0 - 0.06% of Al, 0 - 0.1% of P, with the substantial balance comprising iron and inevitable impurities and having a tripple phase of polygonal ferrite, bainite and martensite, the area rates of said bainite and martensite phases being 4 - 45% and 1 - 15%, respectively, and the area rate of said bainite phase being greater than that of martensite phase.
3. A high strength steel plate as set forth in claim 1 or 2, wherein the area rate of said bainite phase is in the range of 6 - 35%.
4. A high strength steel plate as set forth in claim 1 or 2, wherein the area rate of said martensite phase is in the range of 1 - 10%.
5. A high strength steel plate as set forth in claim 1 or 2, wherein said steel plate is used for a wheel disc of a motor vehicle.
6. A high strength steel plate as set forth in claim 1 or 2, wherein said steel plate is used as a material for a wheel rim of a motor vehicle.
7. A high strength steel plate as set forth in claim 1 or 2, wherein said martensite has a particle size smaller than 6 microns.
8. A high strength steel plate as set forth in claim 2, wherein said steel contains at least one member selected from the group consisting of 0.1 - 1.5% of Cr, 0.1 - 0.6% of Cu, 0.1 - 1% of Ni and 0.0005 - 0.01% of B.
9. A high strength steel plate as set forth in claim 2, wherein said steel contains at least one member selected from the group consisting of 0.01 - 0.1% of Nb, 0.02 - 0.2% of V, 0.01 - 0.1% of Ti and 0.02 - 0.2% of Zr.
10. A high strength steel plate as set forth in claim 2, wherein said steel contains at least one member selected from the group consisting of 0.005 - 0.1% of a rare earth metal, 0.0005 - 0.01% of Ca and 0.0005 - 0.01%
of Mg.
of Mg.
11. A high strength steel plate as set forth in claim 2, wherein said steel contains 0.005 - 0.06% of Al.
12. A high strength steel plate as set forth in claim 2, wherein said steel contains 0.03 - 0.1% of P.
13. A method for producing a high strength steel plate which is low in yield ratio and improved in strength-elongation balance and stretch flangeability and has a tripple phase structure of polygonal ferrite, bainite and martensite, the area rate of said bainite and martensite phases being 4 - 45% and 1 - 15%, respectively, and the area rate of said bainite phase being greater than that of said martensite phase, said method comprising:
(1) subjecting a steel material comprising 0.01 -0.2% of C, 0.3 - 2.5% of Mn and 0.01 - 1.8% of Si with the substantial balance iron and inevitable impurities to a hot rolling-cooling treatement selected from the group consisting of, (i) hot rolling with a finish temperature above point Ar3, followed by cooling from said rolling finish temperature to a temperature range of from point Ar3 to Ar1 at an average cooling speed of 3-70°C/sec, (ii) hot rolling with a finish temperature above point Ar3, followed by cooling from said rolling finish temperature to a temperature range of from point Ar3 to Ar1 at an average cooling speed of 3-70°C/
sec and then by air cooling or slow cooling for 2-20 seconds from said temperature range of Ar3 to Ar1, and (iii) hot rolling with a finish temperature in the range of from point Ar3 to Ar1, follow-ed by air cooling or slow cooling for 2-20 seconds from said temperature range;
(2) further cooling the hot-rolled steel plate to a temperature below 550°C at an average cooling speed higher than 20°C/Sec; and (3) taking up the steel plate.
(1) subjecting a steel material comprising 0.01 -0.2% of C, 0.3 - 2.5% of Mn and 0.01 - 1.8% of Si with the substantial balance iron and inevitable impurities to a hot rolling-cooling treatement selected from the group consisting of, (i) hot rolling with a finish temperature above point Ar3, followed by cooling from said rolling finish temperature to a temperature range of from point Ar3 to Ar1 at an average cooling speed of 3-70°C/sec, (ii) hot rolling with a finish temperature above point Ar3, followed by cooling from said rolling finish temperature to a temperature range of from point Ar3 to Ar1 at an average cooling speed of 3-70°C/
sec and then by air cooling or slow cooling for 2-20 seconds from said temperature range of Ar3 to Ar1, and (iii) hot rolling with a finish temperature in the range of from point Ar3 to Ar1, follow-ed by air cooling or slow cooling for 2-20 seconds from said temperature range;
(2) further cooling the hot-rolled steel plate to a temperature below 550°C at an average cooling speed higher than 20°C/Sec; and (3) taking up the steel plate.
14. A method for producing a high strength steel plate which is low in yield ratio and improved in strength-elongation balance and stretch flangeability and has a tripple phase structure of polygonal ferrite, bainite and martensite, the area rate of said bainite and marten-site phases being 4 - 45% and 1 - 15%, respectively, and the area rate of said bainite phase being greater than that of said martensite phase, said method comprising:
(1) subjecting a steel material comprising:
0.01 - 0.2% of C, 0.3 - 2.5% of Mn, 0.01 - 1.8% of Si, 0 - 1.5% of Cr, 0 - 0.6% of Cu, 0 - 1% of Ni, 0 - 0.01% of B, 0 - 0.1% of Nb, 0 - 0.2% of V, 0 - 0.1% of Ti, 0 - 0.2% of Zr, 0 - 0.1% of a rare earth metal, 0 - 0.01%6 of Ca, 0 - 0.01% of Mg, 0 - 0.06% of Al, 0 - 0.1% of P, with the substantial balance comprising iron and inevitable impurities to a hot rolling-cooling treatement selected from the group consisting of, (i) hot rolling with a finish temperature above point Ar3, followed by cooling from said rolling finish temperature to a temperature range of from point Ar3 to Ar1 at an average cooling speed of 3-70°C/
sec, (ii) hot rolling with a finish temperature above point Ar3, followed by cooling from said rolling finish temperature to a temperature range of from point Ar3 to Ar1 at an average cooling speed of 3 - 70°C/
sec and then by air cooling or slow cooling for 2-20 seconds from said temper-ature range of Ar3 to Ar1, and (iii) hot rolling with a finish temperature in the range of from point Ar3 to Ar1, follow-ed by air cooling or slow cooling for 2-20 seconds from said temperature range;
(2) further cooling the hot-rolled steel plate to a temperature below 550°C at an average cooling speed higher than 20°C/sec; and (3) taking up the steel plate.
(1) subjecting a steel material comprising:
0.01 - 0.2% of C, 0.3 - 2.5% of Mn, 0.01 - 1.8% of Si, 0 - 1.5% of Cr, 0 - 0.6% of Cu, 0 - 1% of Ni, 0 - 0.01% of B, 0 - 0.1% of Nb, 0 - 0.2% of V, 0 - 0.1% of Ti, 0 - 0.2% of Zr, 0 - 0.1% of a rare earth metal, 0 - 0.01%6 of Ca, 0 - 0.01% of Mg, 0 - 0.06% of Al, 0 - 0.1% of P, with the substantial balance comprising iron and inevitable impurities to a hot rolling-cooling treatement selected from the group consisting of, (i) hot rolling with a finish temperature above point Ar3, followed by cooling from said rolling finish temperature to a temperature range of from point Ar3 to Ar1 at an average cooling speed of 3-70°C/
sec, (ii) hot rolling with a finish temperature above point Ar3, followed by cooling from said rolling finish temperature to a temperature range of from point Ar3 to Ar1 at an average cooling speed of 3 - 70°C/
sec and then by air cooling or slow cooling for 2-20 seconds from said temper-ature range of Ar3 to Ar1, and (iii) hot rolling with a finish temperature in the range of from point Ar3 to Ar1, follow-ed by air cooling or slow cooling for 2-20 seconds from said temperature range;
(2) further cooling the hot-rolled steel plate to a temperature below 550°C at an average cooling speed higher than 20°C/sec; and (3) taking up the steel plate.
15. A method as set forth in claim 13 or 14, wherein said steel plate is taken up at a temperature of 300 - 550°C.
16. A method as set forth in claim 13 or 14, wherein said average cooling speed from said hot rolling finish temperature to a temperature in the range of Ar3 to Ar1 in (i) of Step (1) is 3 - 30°C/sec.
17. A method as set forth in claim 13 or 14, wherein said average cooling speed from said hot rolling finish temperature to a temperature in the range of Ar3 to Ar1 in (ii) of Step (1) is 20 - 70°C/sec.
18. A method as set forth in claim 13 or 14, wherein said average cooling speed in Step (2) is 30 - 70°C/sec.
19. A method as set forth in claim 13 or 14, wherein the area rate of said bainite phase is 6 - 35%.
20. A method as set forth in claim 13 or 14, wherein the area rate of said martensite phase is in the range of 1 - 10%.
21. A method as set forth in claim 13 or 14, wherein said steel plate is used as a material for a wheel disc of a motor vehicle.
22. A method as set forth in claim 13 or 14, wherein said steel plate is used as a material for a wheel rim of a motor vehicle.
23. A method as set forth in claim 13 or 14, wherein said martensite has a particle size smaller than 6 microns.
24. A method as set forth in claim 14, wherein said steel contains at least one member selected from the group consisting of 0.1 - 1.5% of Cr, 0.1 - 0.6%
of Cu, 0.1 - 1% of Ni and 0.0005 - 0.01% of B.
of Cu, 0.1 - 1% of Ni and 0.0005 - 0.01% of B.
25. A method as set forth in claim 14, wherein said steel contains at least one member selected from the group consisting of 0.01 - 0.1% of Nb, 0.02 - 0.2%
of V, 0.01 - 0.1% of Ti and 0.02 - 0.2% of Zr.
of V, 0.01 - 0.1% of Ti and 0.02 - 0.2% of Zr.
26. A method as set forth in claim 14, wherein said steel contains at least one member selected from the group consisting of 0.005 - 0.1% of a rare earth metal, 0.0005 - 0.1% of Ca and 0.0005 - 0.01% of Mg.
27. A method as set forth in claim 14, wherein said steel contains 0.005 - 0.06% of Al.
28. A method as set forth in claim 14, wherein said steel contains 0.03 - 0.1% of P.
Applications Claiming Priority (8)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP55-146020 | 1980-10-17 | ||
| JP14602080A JPS5770257A (en) | 1980-10-17 | 1980-10-17 | High strength steel plate |
| JP55-184513 | 1980-12-24 | ||
| JP18451380A JPS57108241A (en) | 1980-12-24 | 1980-12-24 | High strength steel plate with superior formability, especially strength flanging property |
| JP55-187196 | 1980-12-26 | ||
| JP18719680A JPS57110650A (en) | 1980-12-26 | 1980-12-26 | High strength hot rolled steel plate with superior stretch flanging property and resistance weldability |
| JP56-30872 | 1981-03-03 | ||
| JP3087281A JPS57145925A (en) | 1981-03-03 | 1981-03-03 | Production of high strength hot rolled steel plate |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1195152A true CA1195152A (en) | 1985-10-15 |
Family
ID=27459334
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000388173A Expired CA1195152A (en) | 1980-10-17 | 1981-10-16 | High strength steel plate and method for manufacturing same |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA1195152A (en) |
| FR (1) | FR2495189A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2023246096A1 (en) * | 2022-06-22 | 2023-12-28 | 江苏省沙钢钢铁研究院有限公司 | Corrosion-resistant alloy steel bar and preparation method therefor |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS6033338A (en) * | 1983-08-02 | 1985-02-20 | Nissan Motor Co Ltd | Steel to be carburized |
| US4619714A (en) * | 1984-08-06 | 1986-10-28 | The Regents Of The University Of California | Controlled rolling process for dual phase steels and application to rod, wire, sheet and other shapes |
| JP3219820B2 (en) * | 1991-12-27 | 2001-10-15 | 川崎製鉄株式会社 | Low yield ratio high strength hot rolled steel sheet and method for producing the same |
| DE102008004371A1 (en) * | 2008-01-15 | 2009-07-16 | Robert Bosch Gmbh | Component, in particular a motor vehicle component, made of a dual-phase steel |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4072543A (en) * | 1977-01-24 | 1978-02-07 | Amax Inc. | Dual-phase hot-rolled steel strip |
| JPS54163719A (en) * | 1978-06-16 | 1979-12-26 | Nippon Steel Corp | Production of high tensile strength * low yield ratio and high extensibility composite textured steel panel with excellent workability |
| AU527097B2 (en) * | 1979-01-12 | 1983-02-17 | Nippon Steel Corporation | Artifically aged low yield to tensile strength ratio high strength steel sheet |
-
1981
- 1981-10-16 CA CA000388173A patent/CA1195152A/en not_active Expired
- 1981-10-19 FR FR8119609A patent/FR2495189A1/en not_active Withdrawn
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2023246096A1 (en) * | 2022-06-22 | 2023-12-28 | 江苏省沙钢钢铁研究院有限公司 | Corrosion-resistant alloy steel bar and preparation method therefor |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2495189A1 (en) | 1982-06-04 |
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