.b_btn {
    width: 135px;
    height: 40px;
	position:relative;
}

 .b_link {
    display: block;
    position: absolute;
    left: 0;
    bottom: 0;
    width: 135px;
    color: #fff; 
    font-size: 18px;
    text-align: center;
    border-radius: 6px;
    transition: all 2s ease;
	line-height:1.4;
	z-index:1;
}

 .b_line {
    position: absolute;
    width: 100%;
    height: 10px;
    z-index: -1;
    left: 0;
    bottom: 0;
    background: #ff0000;
    transition: all 1s ease;
}

 .b_link:hover {
    height: 40px;
    line-height: 40px;
}

 .b_link:hover .b_line {
    height: 40px;
}