Stop figure getting its own page

Question

The figure I've inserted has a whole page to itself, it is in the middle of the page with two large blank gaps above and below. The text on the previous line ends near the bottom of the previous page so I assumed the figure would start at the top of the next page. Here is what I've written:

\begin{figure}
\captionsetup[subfigure]{justification=centering}
\begin{subfigure}{.5\textwidth}
  \centering
  \includegraphics[width=.4\linewidth]{images/a1.png}
  \caption{a}
  \label{subfig1}
\end{subfigure}%
\begin{subfigure}{.5\textwidth}
  \centering
  \includegraphics[width=.4\linewidth]{images/a2.png}
  \caption{b}
  \label{subfig2}
\end{subfigure}
\caption[abc]{abc}
\label{fig1}
\end{figure}

When I write text after \end{figure} it appears on the previous page next to the text I wrote before the figure. I want the figure to appear at the top of the page and any text I write after it to appear below the figure. I've tried \begin{figure}[h] and \begin{figure}[t] but neither works. Picture of whats happening is below, using report document class with no options.

Heres a 'Minimum Working Example' as requested:

\usepackage[utf8]{inputenc}
\usepackage{titlesec}
\usepackage{hyperref}
\usepackage{array}
\usepackage{amsmath,amsthm}
\usepackage{cite}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{float}
\usepackage[parfill]{parskip}
\usepackage{lipsum}
\theoremstyle{definition}
\newtheorem{definition}{Definition}[section]
\newtheorem{theorem}{Theorem}[section]
\begin{document}
\begin{titlepage}
     \centering
       \vspace*{1cm}
   \Huge
   \textbf{Title}

   \vspace{1.5cm}

   \huge
   \textbf{Author}

   \vspace{5cm}

   \Large
   Supervised by Professor

   \vspace{1cm}

   \large
   Department\\
   University


\end{titlepage}
\addcontentsline{toc}{chapter}{Abstract}
\newpage
\chapter*{\centering Abstract}
\tableofcontents
\addcontentsline{toc}{chapter}{Introduction}
\listoffigures
\newpage
\chapter*{Introduction}
\newpage
\chapter{Chapter 1}
\lipsum[66]
\section{Section 1.1}
\lipsum[66]
\lipsum[67]
We can see that the following two properties hold:
\begin{enumerate}
  \item Property 1
  \item Property 2
\end{enumerate}
\section{Section 1.2}
\lipsum[66]
\begin{definition}[Definition 1]
etc.
\end{definition}
Example:\
[ \begin{array}{c|cccc}
    \text{Bit}&2^3  &2^2  &2^1  &2^0\ \hline
     8 &1    &0    &0    &0\
     9 &1    &0    &0    &1\ \hline
     1 &0    &0    &0    &1\ \hline
   \end{array} ]
\lipsum[66]
Theorem below
\begin{theorem}
\label{thm1}
Theorem
\end{theorem}
\begin{proof}
Proof
\begin{flalign}
s &= 0 \oplus s \nonumber \
 &= r \oplus r \oplus s && \text{(By first sentence)} \nonumber \
 &= r \oplus (a_1 \oplus a_2 \oplus \cdots \oplus a_n) \oplus 
(b_1 \oplus b_2 \oplus \cdots \oplus b_n) \nonumber \
 &= r \oplus (a_1 \oplus b_1) \oplus \cdots \oplus (a_n \oplus b_n) \nonumber \
 &= r \oplus a_k \oplus b_k && \text{(Again by first sentence)} \nonumber
\end{flalign}
\lipsum[66]
\begin{flalign}
s &= r \oplus a_k \oplus b_k \nonumber &\
 &= r \oplus a_k \oplus r \oplus a_k \nonumber &\
 &= r \oplus r \oplus a_k \oplus a_k \nonumber &\
 &= 0 \nonumber
\end{flalign}
\lipsum[66]
\end{proof}
\lipsum[66]
\chapter{Chapter 2}
\lipsum[66]
\section{Section 2.1}
\lipsum[66]
\lipsum[66]
\begin{figure}[ht]           %
\captionsetup[subfigure]{justification=centering}
  \centering
\setkeys{Gin}{width=\linewidth}%
\begin{subfigure}{.45\textwidth}% 
  \includegraphics{image.png}
  \caption{a}
  \label{subfig1}
\end{subfigure}%
\hfil
\begin{subfigure}{.45\textwidth}%
  \centering
  \includegraphics{image.png}
  \caption{b}
  \label{subfig2}
\end{subfigure}
\caption[abc]{abc}
\label{fig1}
\end{figure}
\bibliographystyle{plain}
\bibliography{refs}
\end{document}

Answer 1

Inserting your code fragment into simple article document

\documentclass{article}
\usepackage{caption, subcaption}
\usepackage{graphicx}
\usepackage{lipsum}
\begin{document}
\begin{figure}
\captionsetup[subfigure]{justification=centering}
\begin{subfigure}{.5\textwidth}
  \centering
  \includegraphics[width=.4\linewidth]{example-image-duck}
  \caption{a}
  \label{subfig1}
\end{subfigure}%
\begin{subfigure}{.5\textwidth}
  \centering
  \includegraphics[width=.4\linewidth]{example-image-duck}
  \caption{b}
  \label{subfig2}
\end{subfigure}
\caption[abc]{abc}
\label{fig1}
\end{figure}
\lipsum[1]
\end{document}

I get the following result:

So, it is unclear, what you doing in your real document. Apparently you should provide an MWE (Minimal Working Example) which reproduce what you claim in question.

However, your image I would like in the following format:

for which the MWE is:

\documentclass{article}
\usepackage{caption, subcaption}
\usepackage{graphicx}
\usepackage{lipsum}
\begin{document}
\lipsum[66]
    \begin{figure}[ht]           % <---
\captionsetup[subfigure]{justification=centering}
  \centering
\setkeys{Gin}{width=\linewidth}   % <---
\begin{subfigure}{.45\textwidth}  % <---
  \includegraphics{example-image-duck}%{images/a1.png}
  \caption{a}
  \label{subfig1}
\end{subfigure}%
\hfil
\begin{subfigure}{.45\textwidth}  % <---
  \centering
  \includegraphics{example-image-duck}%{images/a1.png}
  \caption{b}
  \label{subfig2}
\end{subfigure}
\caption[abc]{abc}
\label{fig1}
    \end{figure}
\lipsum[1]
\end{document}

Diference in comparison to your code fragment are marked by <---.