钛锆复合脱氧对低碳高合金强度钢的机械性能的影响

The Effect of Ti-Zr Deoxidation on the Mechanical Properties

of High Strength Low Alloy Steels

Weiyu Lu 1,a, Lingdong Meng

2,b , Honghong Wang 1,c, Daoyuan Wang 2,d,

Yongkuan Yao 2,e , Kaiming Wu 1,f

1Institute of Advanced Steels and Welding Technology, Hubei Province Key Laboratory for Systems Science on Metallurgical Processing, Wuhan University of Science and Technology,

Wuhan 430081, China

2Nanjing Iron and Steel Group Company, Nanjing 210035, China

a [email protected], b [email protected], c [email protected],

d [email protected], e [email protected], f [email protected]

Keywords: Microalloying, Microstructure, Deoxidation, Mechanical property, High strength low alloy steel

Abstract. The uniformity of the mechanical properties, especially elongation and impact toughness, were compared between steel A, which was deoxidized with Ti-Zr, and steel B, which was deoxidized with Al. Microstructural observations, energy dispersive X-ray spectroscopy and X-ray diffraction analyses were conducted using an optical microscope, a scanning electron microscope and a transmission electron microscope, respectively. Results showed that sub-micron and nano-sized complex oxides were obtained by the combined deoxidation of Ti-Zr. The stability of the mechanical properties of steel A was better than that of steel B. The elongation and impact toughness of steel A were enhanced relative to those of steel B. This was attributed to spheroidization and the dispersed distribution of MnS inclusions in the matrix of steel A.

Introduction

Microalloyed high strength low alloy (HSLA) steels are essentially low-carbon low-alloy steels that contain small additions of alloying elements such as Nb, V and Ti [1]. These steels have been widely used to fabricate large engineering structures such as oil storage tanks, ships, pressure vessels and buildings because of their combination of good mechanical properties, such as high strength, excellent resistance to brittle fracture and good weldability [2, 3].

Although carbides and nitrides play an important role in precipitation strengthening and particle pinning, oxides have other advantages because they tend to be more stable than carbides and nitrides in steels. This is particularly true when steels undergo high heat input welding. Many studies have been reported on the application of Zr in steels, such as the effects on austenite grain growth control, sulphide shape control, and improving both transverse toughness and weldability [4]. However, very few works have investigated the effects of combined oxidation on the mechanical properties. The present work aims to study the effects of combined oxidation of titanium and zirconium on the mechanical properties of high strength low alloy steels.

Experimental

Two samples of microalloyed HSLA steel plates were taken from a vacuum refined, hot rolled and tempered industrial scale process. The plates were 30 mm thick. The chemical composition of the

steel is listed in Table 1. One sample was deoxidized with Ti-Zr and denoted as Steel A, and the second was deoxidized with Al and denoted as Steel B.

Subsamples from each plate, transverse to the rolling direction with gauge dimensions of 30 mm×10 mm×6 mm, were prepared for tensile tests. The tensile tests were performed at room temperature at an extension rate of 1 mm/min using an universal testing machine. The yield strength was measured at the 0.2 % offset stress. Three tensile samples were tested for each plate and the average value is reported. Transverse Charpy V-notch specimens of the two steels were prepared with the V-notch in the T-L orientation. Impact toughness measurements were then made on three subsamples of each of the two steels at -40 and -60 °C and the average value is reported.

Microstructural observations were taken on a section cut along the center line of each plate parallel to the rolling direction. Mechanically polished samples were etched in a 4% nital solution and submitted to optical microscopy (OM) and scanning electron microscopy (SEM). Then, transmission electron microscopy (TEM) analysis was performed.

Table 1 The chemical composition of the investigated plates (wt.%)

C Mn Si P S

0.04-0.07 1.55-1.68 0.20-0.30 £0.015 £0.002

Nb+Ti Ni+ Cr+ Mo B Ca N

0.06-0.08 0.60-0.80 0.0012 0.0008

-0.0020 -0.0035 £0.006

Results

Microstructures . The optical microstructures of Steel A and B are shown in Fig. 1. The microstructures of the two steels were predominantly bainite with a substantial amount of martensite-retained austenite (M/A). Based on OM and SEM observations, there were no significant differences in the final microstructures of these two steels. The austenite grain size after hot rolling was in the range of 5~10 mm.

a b

Fig. 1 Optical microstructures of Steels A (a) and B (b)

The type and size of oxides. Figure 2 is a TEM micrograph and analysis showing the distribution of inclusions in Steel A. The graph in Fig. 2 illustrates the complexity of the chemical composition of the oxides in Steel A. Results of TEM-EDS analysis reveal that the oxide is a composite oxide formed in the liquid steel. The size of the oxides was usually in the sub-micron or nano-meter range. Figs. 3 and 4 show the size distribution of oxides in Steel A and Steel B, respectively. It can be seen

that there are more small inclusions in Steel A than is the case in Steel B.

Fig. 2 TEM-EDS image of the typical oxide in Steel A

40

35

%

/ 30

n

o

t i

u 25b

r i

t

s i 20 d

y

c

n 15

e

u q

e 10r F

5

0.00.51.01.52.02.53.03.54.0

Size / µm

Fig. 3 The size distribution of oxides in Steel A

40

35

%30

/

n o

i 25t

u

b u

r t 20

s

i

d

y c

n 15e

u

q e 10r F

5

0.00.51.01.52.02.53.03.54.0

Size / µm

Fig. 4 The size distribution of inclusions in Steel B

Dispersed distribution of MnS . Figure 5 shows the fracture surface of a V-notch impacted specimen in a micrograph. It is seen that many fine particles of complex oxides and MnS complex compounds are dispersed in the matrix. It’s likely that MnS tends to form around Zr complex oxides. Clusters of and large-sized MnS were not observed in Steel A, whereas both types of inclusions were formed in Steel B (Fig. 6).

Fig. 5 The fracture surface with inclusions in the dimples, and the EDS spectrum

Fig. 6 Optical micrographs of inclusions in (a) Steel A and (b) Steel B

Mechanical properties. Figure 7 illustrates the distribution of mechanical properties of the two steels. The average yield strength of steel A was 801 MPa with a standard deviation of 28 MPa. Although the average yield strength of Steel B was approximately the same as Steel A (802 MPa), the standard deviation was much higher (66 MPa). Similar findings were obtained with tensile strength. The average tensile strength of Steel A was 845 MPa, and the average tensile strength of Steel B was about 839 MPa. However, the standard deviations were quite different. The standard deviation of the tensile strength of Steel A (15MPa) was much smaller than that of Steel B (50MPa). The elongation and impact toughness of Steels A and B are presented in Fig.7b. The average elongation of Steel A was 35.2% with a standard deviation of 2.4%, and the average for Steel B was 27.1% with a standard deviation of about 2.1%. It is evident that the average elongation of Steel A is much larger than that of Steel B. The average V-notch impact toughness of Steel A was 154.3 J at -60 °C along the plate thickness whereas the averge V-notch impact toughness of Steel B was 136.8 J at -40 °C. It was obvious that the impact toughness of Steel A was more stable than Steel B (Fig. 7b). Generally, the V-notch impact toughness of steel at high temperatures is better than at low temperatures. So, the impact toughness of steel A at -40 °C would likely be better than that of steel

A at –60 °C. These results indicate that Steel A has more stable mechanical properties than Steel B.

a

P M

/

h

t

g

n e r

t S

Number

J

%

/s

/ s

n e

o n

i h

t a g

g u

n t o

l o t

E c

a p

m I

Number

Fig. 7 Distribution of mechanical properties of Steel A and B

(a) Yield strength and tensile strength of Steel A and B

(b) Elongation and impact toughness of Steel A and B

Discussion

Thermodynamic analysis of Ti-Zr combined deoxidation. Ti-Zr combined deoxidation not only results in deoxidation, but also forms complex oxides such as (Ti, Zr) x O y in the solidification process. These complex oxides are stable at higher temperatures. Total oxygen in the melt can be presented by the following equation:

O Total =O Dissolved +O inclusions (1)

The deoxidation reaction in molten steel [5] can be expressed as:

K aMxOy DG 0M M -O

-O =aM aO =exp(-) (2 )

The titanium and zirconium complex oxides often precipitate in the grain boundary (GB) region owing to higher energy. However, they also precipitate in the grain interior. In an equilibrium state where a GB is not moving, the GB segregation results in no net force at the interface. When a GB is moving under a driving force, the concentration profile falls behind the GB position. The resulting break in symmetry between the concentration profile and the GB position brings about an attractive force between them [6]. However, the boundaries are flexible with highly curved particles. Therefore, boundaries may remain in contact longer with the pinning particles than with those influencing their movement. This is a net drag pressure [7]. Indeed, these complex oxides function as fine-grained strengthening particles which are beneficial for the uniform strength, elongation and toughness of steels.

Spheroidization and dispersion of MnS. Clusters of and large-sized MnS inclusions considerably reduce the toughness, ductility and fatigue strength of steels. As a secondary phase in steel, MnS inclusions can cause damage to the continuity of the steel matrix, decreasing the uniformity and thereby exerting a deleterious effect on its mechanical properties. It has been reported, and is

generally known, that cracks may nucleate from inclusions [8]. Inclusions are harder than the surrounding matrix, which can lead to a concentration of stress and strain during matrix deformation. Matrix–inclusion de-cohesion or fracture of inclusion particles can occur, ultimately resulting in the formation of voids [8]. However, when MnS inclusions are small spheres, and are uniformly dispersed in the matrix, they can improve the mechanical properties of steels, especially the transverse impact toughness and fracture resistance [8].

MnS can be formed during solidification in HSLA steels [3]. The MnS precipitations on oxide inclusions are affected by the chemical composition of both the inclusions and the matrix. Oxides, such as the Ti-Zr complex oxides, with high sulfide capacities and low melting temperatures can act as sites for MnS precipitation [9]. MnS precipitates on pre-formed ZrO2 oxide during solidification due to their similarities in plane distances, thus reducing interfacial energy. As a result, the MnS cluster is transformed into small spherical complex inclusion particles [3].

Conclusions

(1) The dispersed oxides in the matrix were identified as titanium and zirconium complex oxides. Most of these oxides are sub-micron and nano-meter in size. These oxides provided nucleation sites for the precipitation of MnS during solidification.

(2) The steel deoxidized with Ti-Zr had more uniform mechanical properties than that deoxidized with Al. Elongation and impact toughness at low temperatures were enhanced, and the variability in the mechanical properties was reduced by the complex deoxidation of Ti-Zr. This was attributed to the spheroidization and dispersed distribution of MnS inclusions in the matrix.

Acknowledgements

Authors gratefully acknowledge the financial support of National Natural Science Foundation of China and Baosteel (No. 50734004).

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Advances in Superalloys

doi:10.4028/www.scientific.net/AMR.146-147

The Effect of Ti-Zr Deoxidation on the Mechanical Properties of High Strength LowAlloy Steels

doi:10.4028/www.scientific.net/AMR.146-147.1878