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High Strength Low Alloy Steels

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TABLE OF CONTENTS

CHAPTER NO.

TITLE

PAGE NO.

ABSTRACT LIST OF TABLES LIST OF FIGURES 1. 1.1 LITERATURE REVIEW INTRODUCTION 1.1.1 Definition 1.1.2 Composition 1.1.3 Classification 1.1.3.1 Weathering Steels 1.1.3.2 Control Rolled Steels 1.1.3.3 Pearlite Reduced Steel 1.1.3.4 Acicular Ferrite Steel 1.1.3.5 Dual Phase Steel 1.1.3.6 Micro-Alloyed Steel 1.1.4 Properties 1.1.5 Specifications 1.1.6 Applications

8 9 9 10 10 10 10 11

11 12 14

1.2

DEVELOPMENT OF HSLA

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1.2.1 Mechanism of Alloying 1.2.2 Effect of Alloying Elements on Properties of HSLA 1.2.2.1 Nitrogen 1.2.2.2 Manganese 1.2.2.3 Silicon 1.2.2.4 Copper 1.2.2.5 Phosphorus

15 16

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1.2.2.6 Chromium 1.2.2.7 Nickel 1.2.2.8 Molybdenum 1.2.2.9 Aluminum 1.2.2.10 Vanadium 1.2.2.11 Titanium 1.2.2.12 Zirconium 1.2.2.13 Boron 1.2.2.14 Calcium

1.3

STRENTHENING MECHANISM OF HSLA 1.3.1 Grain Refinement 1.3.2 Solid Solution Strengthening 1.3.3 Phase Balance Strengthening 1.3.4 Precipitation Strengthening 1.3.5 Work Hardening

21 21 23 25 28 32

1.4

EFFECT OF GRAIN SIZE ON PROPERTIES OF HSLA 1.4.1 Measurement of ASTM Grain Size Number 1.4.1.1 Comparison Method 1.4.1.2 Grain Counting Method 1.4.1.3 Intercept Method 1.4.2 Importance of Grain Size

33 33

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2. 2.1

EXPERIMENTATION SAMPLE PREPARATION 2.1.1 Sectioning 2.1.2 Mounting 2.1.3 Coarse Grinding 2.1.4 Medium and Fine Grinding 2.1.5 Mechanical Polishing 2.1.6 Etching

38 38 38 38 39 39 40 416

2.2

MICROSCOPIC ANALYSIS

42

2.3

HARDNESS TESTING

42

2.4

HEAT TREATMENT

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2.5

RETESTING

43

3.

RESULTS AND DISCUSSION

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CONCLUSION

45

REFRENCES

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ABSTRACT

Structural steels with a ferrite-pearlite microstructure are widely used in civil, mechanical and chemical engineering. Increased working loads, reliability requirements, extreme temperature and chemically aggressive environments lead to an increased demand for high strength, combined with high toughness, in these steels. The use of welding as a joining method requires good weldability, for which the carbon content in a steel composition should be decreased. A decrease in carbon content results in strength decrease, due to a decrease in the amount of the second phase (pearlite).

To overcome this strength drop, additions of titanium, niobium and vanadium microalloying elements, in amounts no more than 0.15 wt%, have been used to provide precipitation strengthening and grain size refinement. Thus, during the last thirty years high strength low alloy (HSLA) structural steels have found widespread use in automotive, construction and pipe-line transportation industries.

The aim of the present project is to study the influence of steel chemistry on grain size as well as to establish a study of effect of grain size on the mechanical properties of High Strength Low Alloy Steels. To bring about the above study, various literatures were surveyed and related information was gathered. An experiment was also conducted on HSLA steel sample in which the sample was subjected to various heat treatments to bring about change in its grain size and hardness testing was done on the heat treated samples so as to establish a relationship between grain size and hardness.

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LIST OF TABLESS. No. 1 2 3 4 5 6 7 8 9 TABLE NAME Typical composition of HSLA steel SAE HSLA Steel Grade composition SAE HSLA Steel Grade Mechanical properties Experimental value for stress and Ky terms in Hall-Petch equation Strengthening coefficient for a no. of solutes Increment of yield stress from the micro alloying element precipitation Effect of grain size on properties of HSLA Composition of HSLA Sample Under Study Results obtained from experimentation Pg. No. 10 12 13 22 24 31 37 38 44

LIST OF FIGURES. No. 1 2 3 4 5 6 7 8 9 10 Fe-Mn Phase Diagram Fe-Si Phase Diagram Fe-Cr Phase Diagram Fe-Ni Phase Diagram Fe-Ti Phase Diagram Strengthening effects of substitutional solute atoms in iron Factors contributing to the strength of C-Mn Steels Effect of pearlite on work hardening rate Influence of carbon content on strength of plain carbon steel Influence of 50% transformation temperature on tensile strength via formation of different steel structures Strengthening contributions of different parameters on yield strength of hot-rolled Addition to strength predicted by Orowan and Ashby-Orowan 12 equations compared with the observed increments of yield strength in micro alloyed steels (vertical lines are experimental data)]9

FIGURE

Pg. No. 16 17 18 19 20 24 25 26 26 28

11

29

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CHAPTER 1: LITERATURE REVIEW1.1: INTRODUCTION1.1.1 Definition High-strength low-alloy steel (HSLA) is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. They have carbon content between 0.050.25% to retain formability and weldability. Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare earth elements, or zirconium. These steel alloys provide increased strength-to-weight ratios over conventional low-carbon steels for only a modest price premium. Because HSLA alloys are stronger, they can be used in thinner sections, making them particularly attractive for transportation-equipment components where weight reduction is important.

1.1.2 Composition Typically, HSLA steels are low-carbon steels with up to 1.5% manganese, strengthened by small additions of elements, such as columbium, copper, vanadium or titanium and sometimes by special rolling and cooling techniques. Improved-formability HSLA steels contain additions such as zirconium, calcium, or rare-earth elements for sulfide-inclusion shape control.

Table 1: Typical Composition of HSLA Steels

Element C Mn Nb V Mo

% Composition 0.06 -0.12 1.4 -1.8 0.02 -0.05 0 - 0.06 0.2-0.35

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1.1.3 Classification 1.1.3.1 Weathering Steel: Steels which have better corrosion resistance. A common example is COR-TEN. 1.1.3.2 Control Rolled Steel: Hot rolled steels which have a highly deformed austenite structure that will transform to a very fine equiaxed ferrite structure upon cooling. 1.1.3.3 Pearlite Reduces Steel: Low carbon content steels which lead to little or no pearlite, but rather a very fine grain ferrite matrix. It is strengthened by precipitation hardening. 1.1.3.4 Acicular Ferrite: These steels are characterized by a very fine high strength acicular ferrite structure, a very low carbon content, and good hardenability. 1.1.3.5 Dual Phase Steel: These steels have a ferritic microstructure that contains small, uniformly distributed sections of martensite. This microstructure gives the steels low yield strength, high rate of work hardening, and good formability. 1.1.3.6 Micro Alloyed Steel: Steels which contain very small additions of niobium, vanadium, and/or titanium to obtain a refined grain size and/or precipitation hardening.

1.1.4 Properties The added elements are intended to alter the microstructure of plain carbon steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite. This eliminates the toughness-reducing effect of a pearlitic volume fraction, yet maintains and even increases the material's strength by precipitation strengthening and by refining the grain size, which in the case of ferrite increases yield strength by 50% for every halving of the mean grain diameter. Its yield strength can be anywhere between 250-590 MPa (35000-85000 psi).

HSLA steels are also more resistant to rust then most carbon steels. Although the material quickly becomes covered with surface rust, this is superficial and rust takes a long time to threaten the integrity of a structure made from the material. HSLA steels usually have densities of around 7800 kg/m3.11

1.1.5 Specification According to the Society of Automobile Engineers, the Standard Steel Grade Compositions for HSLA are as follows:

Table 2: SAE HSLA steel grade compositions % Grade Carbon (max) 0.21 % Manganese (max) 1.35 % Phosphorus (max) 0.04 % Sulfur (max) 0.05 % Silicon (max) 0.90 Niobium or vanadium treated Notes

942X 945A 945C 945X 950A 950B 950C 950D 950X 955X 960X 965X 970X 980X

0.15 0.23 0.22 0.15 0.22 0.25 0.15 0.23 0.25 0.26 0.26 0.26 0.26

1.00 1.40 1.35 1.30 1.30 1.60 1.00 1.35 1.35 1.45 1.45 1.65 1.65

0.04 0.04 0.04 0.04 0.04 0.04 0.15 0.04 0.04 0.04 0.04 0.04 0.04

0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 Niobium or vanadium treated Niobium, vanadium, or nitrogen treated Niobium, vanadium, or nitrogen treated Niobium, vanadium, or nitrogen treated Niobium, vanadium, or nitrogen treated Niobium, vanadium, or nitrogen treated Niobium or vanadium treated

12

Table 3: SAE HSLA steel grade mechanical properties Grade 942X Form Plates, shapes & bars up to 4 in. Sheet & strip Plates, shapes & bars: 945A, C 00.5 in. 0.51.5 in. 1.53 in. 945X Sheet, strip, plates, shapes & bars up to 1.5 in. Sheet & strip Plates, shapes & bars: 950A, B, C, D 00.5 in. 0.51.5 in. 1.53 in. 950X Sheet, strip, plates, shapes & bars up to 1.5 in. Sheet, strip, plates, shapes & bars up to 1.5 in. Sheet, strip, plates, shapes & bars up to 1.5 in. Sheet, strip, plates, shapes & bars up to 0.75 in. Sheet, strip, plates, shapes & bars up to 0.75 in. Sheet, strip & plates up to 0.375 in. 50,000 (345) 45,000 (310) 42,000 (290) 50,

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