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Assignment B3Physical Metallurgy

Dewi Lestari NataliaFakhrul Razan

Ghozali Suprobo

Haposan Edward Daniel

Vincent Irawan

Departement of Metallurgy and MaterialFaculty of Engineering University of Indonesia

Depok

2011

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Assignment B3: Strengthening Mechanism

The Case:Two types of Dual Phase (DP) steel are available: DP500 and DP800. The nominal composition of eachtype is provided in Table 1. The processing of the two steel was as follows: (i) continuously cast to slabs

(ii) reheated in the slab reheating furnace (iii) hot rolled (iv) cold rolled (v) continuous

annealing at 760 oC (vi) subsequently followed by rolling at 300 oC, then quenched into water. TheDP500 was rolled at 0.6 %, while the DP800 was rolled at < 0.2 %. Processes (v) and (vi) is illustrated in

Figure 1. The resulting microstructures are provided in Figure 2.

Table 1: Nominal chemical composition (wt%), Fe

balance.

Steel Grade C Mn Si Nb

DP500 0.08 0.65 0.3 -

DP800 0.13 1.50 0.2 0.015

Figure 1. Process (v) continuous annealing, and

process (vi) rolling at 300oC.

Questions:1. Explain the possible solid solution

strengthening operates in the two alloys. (10

marks)

2. Please note the grain morphology of the steel.Does rolling affect the shape/morphology of

the grain? Will rolling affect the

strengthening of the two steel. Please

elaborate your answer. (15 marks)

3. The main role of Nb is as the grain refiner.Measure the decrease of grain size (in %) by

the addition of Nb. (15 marks)

4. Is the grain refinement (in no. 3) causedONLY by Nb addition? Is there any other

factor that may reduce the grain size inDP800? Please explain. (15 marks)

5. What do you think the white spots revealed inthe ferrite structures in DP500 steel? These

white spots are no longer prominent in

DP800 steel. Will and how the white spotsaffect the strength of the steel? Please

elaborate your answer. (15 marks)

6. Explain the role of martensite in contributingto the strength of the alloy. (10 marks)

7. How do you think the effects of combinationof martensite and ferrite in the DP steel? (10

marks)

8. By analyzing all given information, pleasepredict, which alloy possesses higher

hardness and strength? Please explain. (10

marks).

Figure 2: SEM-pictures of the DP500 and DP800

steel, with the rolling direction (RD) horizontally and

the normal direction (ND) vertically. The martensite

grains appear as light areas and the ferrite grains as

dark gray areas.

760 oC

300 oCrolling

Room temperature

Time

Temperature

(v)

(vi)

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Answer :

5. Kelarutan Si adalah sekitar 0,18 %, sedangkan kadar unsur pada DP500 adalah 0,3 % dan

pada DP800 adalah 0,2 %. Maka dapat kita lihat dari gambar DP500 terdapat butiran putih

yang lebih banyak dari DP800. Butiran putih pada gambar DP500 adalah unsur Si yang jenuh

akibat dari jumlahnya yang melebihi batas kelarutan maksimunnya, sehingga Si terbentuk diluar.Butiran putih inilah yang disebut precipitates. Maka dilihat dari gambarnya, terdapat butiran

putih yang lebih banyak pada DP500 daripada DP800. Oleh karena pada DP500 terdapat

banyak butiran putih yang disebut precipitates, sementara precipitates yang banyak dapat

menghambat pergerakan dislokasi, maka DP500 akan jauh lebih kuat daripada DP800.