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Effect of V microalloying on phase transformation in low carbon

VIEWS: 10 PAGES: 4

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      Effect of V microalloying on phase transformation in low
           carbon steels during continuous cooling process
               WANG Kai, WANG Li-jun, CUI Wen-fang, REN Hai-peng, LIU Chun-ming
                      (School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China)


Abstract: The effect of vanadium microalloying on phase transformation in low carbon steels during continuous cooling process
after deformation at temperatures of non-recrystallization of austenite. The experimental results show that the final microstructure
changes from a mixture of ferrite and pearlite to that of ferrite and granular bainite with increase of cooling rate. Compared with
vanadium free steel, vanadium microalloyed steel has higher volume fraction of granular bainite. Addition of vanadium to low
carbon steel made dynamic CCT diagrams move slightly toward the top left corner.
Key words: Vanadium microalloyed low carbon steel; continuous cooling; phase transformation; granular bainite


1    Introduction                                                       diameter    at       1200.C.     Normalizing     treatment
HSLA steels are presently produced mainly through                    (920.Cx30 min) was carried out for the round bars to
thermo-mechanically controlled processing (TMCP)[I].                 avoid the influence of original microstructure on the
Granular bainite microstructure which is beneficial to               phase transformation during deformation at high
properties of HSLA steels[2.3] is achieved through                   temperature. The specimens with diameter of 8mm
controlling the deformation temperature in the region                and height of 5mm for thermomechanical simulation
of non-recrystallization of austenite and accelerating               test were machined from these round bars.
the cooling rate after deformation. Niobium is                             In   order    to    examine     phase   transformation
considered as a microalloying element for effectively                behavior of deformed austenite during continuous
restraining recrystallization of austenite and widely                cooling process, continuous cooling transformation
used in HSLA steels[4.5]. Compared with niobium,                     (CCT) diagrams of deformed austenite had been
vanadium       has faintish effect on restraining                    measured by dilatometry on a Gleeble-15oo
recrystallization of austenite[6]. But, as strong carbide            thermomechanical simulator. The process of dynamic
former, vanadium reduces diffusion rate of carbon in                 continuous cooling transformation tests was shown in
austenite through increasing diffusion activation                    Fig.I. Specimens were heated up to 11oo.C at a rate
energy of carbon, and make austenite steady, which                   of 20.C/s and held for 5min. Then these specimens
may be good for granular bainite transformation                      were cooled to 850.C at a rate of 5.C/s and given a
during continuous cooling process. The purposes of                   compression deformation of 50% reduction with a
the present study are to clarify the effect of vanadium              strain rate of 5s-l. The deformed specimens were
microalloying on continuous cooling transformation.                  cooled to room temperature at rates of 1,5, 10, 15,20
                                                                     and 30.C/s, respectively. The microstructure at the
2    Experimental Procedure                                          center of the deformed specimens was observed by
The test steels were melted using intermediate                       optical microscopy and TEM. The volume fraction of
frequency vacuum induction furnace. The chemical                     granular bainite was measured by image analysis.
compositions of the steels are shown in Table I.
Vanadium free Steel A was used as a reference steel.                 3 Results and discussions
The ingots were forged into round bar of 14mm in                     Microstructures of vanadium free Steel A and


                          Table 1 Chemical compositions of steels used in the present study (mass %)
                      Steel      C      MD        S          P          0         N          V       AI
                       A         0.099     1.036    0.0060     0.0083     0.0026    0.0059                 0.014
                       B         0.10      1.06     0.0047     0.0082     0.0042    0.0085       0.145     0.010

vanadium microalloyed Steel B cooled at different

                                                                                                                                 151
                                                                                                                                                                                                          rates are shown in Fig.2. When cooling rate was
                                                                                                                                                                                                          below S'C/s, the microstructure of both steels was
~          IIOO'C.            SIIlo
t                                                                                                                                                                                                         composed of ferrite and pearlite. Small amount of
;
::
..                                                                                                                                                                                                        granular microstructure could be observed at cooling
!                                                                                                                                                                                                         rate of S'C/s (Fig.2-al and bt). Such granular
                                                                                                                                                                                                          microstructure is composed of martensite-austenite
                                                                                                                                                                                                          (MIA) constituents distributed on ferrite matrix, and
                                                                                                                                                                                                          called as granular bainite[7J. Fig. 3 shows TEM
                                                                                    I-              tempera
                                                                                                          turo                                                                                            observation of granular bainite in vanadium
                                                                                                                                                                                                          microalloyed Steel B cooled at a rate of 20'C/s after
                                                                                                                                                                                                          deformation at 850'C. The volume fraction of pearlite
                                                                                                                                         TlIIC

                                                                                                                                                                                                          decreased and the volume fraction of granular bainite
     Fig. 1 Process of dynamic continuous cooling                                                                                                                                                         increased with increase of cooling rate. When cooling
                 transformation tests                                                                                                                                                                     rates exceeded IS'C/s, little amount of pearlite could
                                                                                                                                                                                                          be observed, and the microstructure were mainly
                                                                                                                                                                                                          composed of ferrite and granular bainite (Fig.2-a3 and
                                                                                                                                                                                                          b3)



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                              Fig.2                     Microstructures of both test steels cooled at different rates after deformation at 8S0'C
                                                                                                  (a) Steel A, (b) Steel B, (1) S'C/s, (2) 1S'C/s, (3) 30'C/s




                                                                                                                                                                                                                                                                                                                                                      152
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                                                                                              Fig. 4 shows the volume fraction of granular bainite
                                                                                              in both test steels cooled at various rates after
                                                                                              deformation          at 850.C.            The volume fraction   of
                                                                                              granular bainite in vanadium microalloyed Steel B
                                                                                              was higher than that in vanadium free Steel A at the
                                                                                              same cooling rate. Therefore, it can be concluded that
                                                                                              vanadium microalloying is beneficial to bainite
                                                                                              transformation during continuous cooling process.
                                                                                              The element beneficial to form M/A constituents can

                    ..            ~~                                                          increase volume fraction of granular bainite.
               ~             - -~            ...    ...
                                                    "              ;11                        Vanadium has a strong affinity with carbon. At the
Fig.3     TEM observation ofM/A constituents in Steel B                                       intermediate  temperature    region    of   bainite
                                                                                              transformation, a long distance diffusion of vanadium
         50                                                                                   is difficult to occur. As a result, diffusion rate of
                         A
                   -- Steel                                                                   carbon is reduced and more carbon kept in austenite.
   ~ 40
                         B
                   -- Steel
   C
                                                                                              Carbon rich austenite has higher stability and finally
   0
   t!                                                                                         forms M/A constituents.
   f!
   -;
   E
         30                                                                                   Through a combination of thermal expansion curves
   '"
   "0
    >                                                                                         with microstructure analysis, dynamic CCT diagrams
   .~ 20
    c:
   '0;
                                                                                              for both test steels could be drawn, shown as Fig.5.
   ..c
    lii                                                                                       It can be seen that the dynamic CCT diagrams of
   :;
   ~ 10
   Cl
                                                                                              vanadium microalloyed Steel B move slightly toward
                                                                                              the top left corner compared to that of vanadium free
                             10         15                20         25       30              Steel A. It suggests that vanadium microalloying
                                      cooling rate, °CIs
                                                                                              accelerate phase transformation of austenite during
 Figo4        Volume fraction of granular                      bainite for both steels        continuous cooling after deformation at region of
              at different   rates after deformation                 at 850.C                 non-recrystallization of austenite. Such effect of
                                                                                              vanadium can be considered as followings. Vanadium



                                         900                                                                                   Steel A
                                                                                                                    -          Steel B
                                         800

                                         700
                                  u
                                  .Q)
                                         600

                                   E 500
                                   S
                                   Q)
                                   0.. 400
                                   E
                                   Q)

                                   E-< 300

                                         200
                                                                                                 30   20 15   10          5            1"C/s
                                         100
                                                                                         10                        100
                                                                                         Time, s

                                                   Fig.5         Effects of V microalloying on dynamic CCT diagrams




                                                                                                                                                              153
                                                                                                     --m   - ---




easily forms vanadium carbide after deformation due       [2] Chaudhury Z A, Sarma D S. Influence of Heat
to strain-induced    precipitation. These carbide         Treatment on Structure and Properties of a HSLA
particles may become nucleating center of phase           Steel [1]. Trans. of the Indian Institute of Metals, 1982,
transformation. Moreover, the precipitation of carbide    35 (2): 119-125.
decreases the amount of carbon dissolved in austenite,    [3] Katsumata M, Ishiyama 0, Inoue T, et al.
which accelerates     the phase    transformation   of    Microstructure and Mechanical Properties of Bainite
austenite.                                                Containing Martensite and Retained Austenite in Low
                                                          Carbon HSLA Steels [J]. Mater. Trans. JIM, 1991,32
4     Conclusions                                         (8): 715-728.
(I)   Addition   of vanadium   to low carbon steel        [4]   Gomez M. Medina S F, Quispe A, et al. Static
accelerates granular bainite transformation and           Recrystallization and Induced Precipitation in a Low
increase volume fraction of granular bainite.             Nb Microalloyed Steel [1]. ISH Inter., 2002, 42 (4):
(2) Vanadium microalloying make dynamic CCT               423-431.
diagrams move slightly toward the top left corner and     [5]   Luton M J, Dorvel R, Petkovic R A. Interaction
elevate starting temperature of transformation during     between      Deformation,    Recrystallization     and
continuous cooling.                                       Precipitation in Niobium Steels [1]. Metall. Trans. A,
                                                          1980, II (3): 411-420.
Acknowledgment                                            [6] Wang K, Wang L J, Ren H P, et al. Dynamic
We are grateful for the financial support from            Recrystallization of Vanadium Microalloyed Low
Vanadium International Technical Committee and            Carbon Steel during High temperature Deformation
Chinese Society for Metals.                               [1]. J. Mater. and Metall., 2005, 4 (I): 51-54 (in
                                                          Chinese).
References                                                [7] Samuel F H. Interrelations of Cooling Rate,
[I] Yakubtsor I A, Boyd J D. Bainite Transformation       Microstructure, and Mechanical Properties in Four
during Continuous Cooling of Low Carbon                   HSLA Steels [1]. Metall. Trans. A, 1984, 15 (l0):
Microalloyed Steel [J]. Mater. Sci. and Tech., 2001, 17   1807-1817
(3): 296-301.




                                                                                                                   154

								
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