The process of developed C70S6BY non-quenched and tempered hot-rolled round steel is 80 t top-bottom combined blowing converter-LF+VD refining-240 mm×240 mm bloom continuous casting-Φ60 mm round steel continuous  rolling-slow  cooling-finishing  and  non-destructive  testing process.  The  Al

2

O

3

-CaO-SiO

2

slag  with basicity4～8  has been  used

LF refining    time is 45~60 min; after LF

the[S

]

≤0.010% and [O

]

≤0.001%; sulphur alloying is carried out  after  VD

[S

]

  is  controlled  at  0.063%~0.065%.Before  VD  tapping

100～150m  of  magnesium-containing  cored  wire  is  fed

and  the  yield  of  Mg  is  11%  to17%

and  the  ratio  of magnesium  to  sulphur  content  is  approximately 1.56%.The  superheat  of  the  tundish  liquid  is   15~30  ℃

and  the  casting  spe

ed  is 0.9  m/min. After  normalizing

the  tensile  strength  and yield  strength  of test  steel  are  957  MPa  and  563  MPa.  The material  elongation  and reduction  of area  are  12%  and  21%.After  the  magnesium  modification  treatment

the  percentage  of sulphide  inclusions  with  length-width  ratio  1~2  reaches more than 55%.