自定义粗加号

documentclass{article}
\usepackage{amsmath,tikz}
\begin{document}
\newcommand{\plus}{~
\begin{tikzpicture}[scale=0.1,line width=1pt]
\draw (0,0)--(2,0);
\draw (1,1)--(1,-1);
\end{tikzpicture}~}
$a^2\plus{}b^2=c^2$
\par
\def\pplus{%
  \mathbin{\kern0.045em\rule[0.2em]{0.7em}{0.1em}\kern-0.4em\rule[-0.1em]{0.1em}{0.7em}\kern0.35em}}
$a^2\pplus{}b^2=c^2$
\par
$a^2+b^2=c^2$
\end{document}