Method of manufacturing steel for machine structural use exhibiting excellent free cutting characteristic, cold forging characteristic and post-hardening/tempering fatigue resistance

A method of manufacturing steel for a machine structural use exhibiting excellent free cutting characteristic and cold forging characteristic, for use in hardened/tempered state, said method comprising the steps of:

• selecting steel composed of
• C:
• 0.1 wt % to 1.5 wt %;
• Si:
• 0.5 wt % to 2.0 wt %;
• Mn:
• 0.1 wt % to 2.0 wt %;
• B:
• 0.0003 wt % to 0.0150 wt %;
• Al:
• 0.005 wt % to 0.1 wt %;
• O.ltoreq.0.0030 wt %;
• P.ltoreq.0.020 wt %;
• S.ltoreq.0.035 wt %;
• N:
• 0.0015 wt % to 0.0150 wt %;
• and
• a balance consisting of Fe and unavoidable impurities;
• heating said steel to a temperature level higher than solid-solution temperature for BN and that for AlN;
• hot rolling said steel;
• subjecting said steel to a normalizing process in which said steel is heated to a temperature region from 800.degree. C. to 950.degree. C. and cooled with air;
• heating said steel to a temperature region from 680.degree. C. to 740.degree. C.;
• and
• maintaining said steel at said temperature region for 5 hours or longer.

A method of manufacturing steel for a machine structural use exhibiting excellent free cutting characteristic and cold fogging characteristic, for use in hardened/tempered state, said method comprising the steps of:

• selecting steel composed of
• C:
• 0.1 wt % to 1.5 wt %;
• Si:
• 0.5 wt % to 2.0 wt %;
• Mn:
• 0.1 wt % to 2.0 wt %;
• B:
• 0.0003 wt % to 0.0150 wt %;
• Al:
• 0.005 wt % to 0.1 wt %;
• O.ltoreq.0.0030 wt %;
• P.ltoreq.0.020 wt %;
• S.ltoreq.0.035 wt %;
• N:
• 0.0015 wt % to 0.0150 wt %;
• one or more types of substances selected from a group consisting of
• REM:
• 0.0005 wt % to 0.2 wt %;
• Zr:
• 0.005 wt % to 0.2 wt %;
• Ti:
• 0.005 wt % to 0.05 wt %;
• V:
• 0.05 wt % to 0.5 wt %;
• Nb:
• 0.005 wt % to 0.05 wt %;
• Ni:
• 0.10 wt % to 3.0 wt %;
• Cu:
• 0.1 wt % to 3.0 wt %;
• Co:
• 0.1 wt % to 3.0 wt %;
• and
• Mo:
• 0.1 wt % to 1.0 wt %;
• and
• a balance consisting of Fe and unavoidable impurities;
• heating said steel to a temperature level higher than solid-solution temperature for BN and that for AlN;
• hot rolling said steel;
• heating said steel to a temperature region from 300.degree. C. to 600.degree. C.;
• maintaining said steel at said temperature region for 15 minutes or longer;
• heating said steel to a temperature region from 680.degree. C. to 740.degree. C.;
• and
• maintaining said steel at said temperature region for 5 hours or longer.

A method of manufacturing steel for a machine structural use exhibiting excellent free cutting characteristic and cold forging characteristic, for use in hardened/tempered state, said method comprising the steps of:

• selecting steel composed of
• C:
• 0.1 wt % to 1.5 wt %;
• Si:
• 0.5 wt % to 2.0 wt %;
• Mn:
• 0.1 wt % to 2.0 wt %;
• B:
• 0.0003 wt % to 0.0150 wt %;
• Al:
• 0.005 wt % to 0.1 wt %;
• O.ltoreq.0.0030 wt %;
• P.ltoreq.0.020 wt %;
• S.ltoreq.0.035 wt %;
• N:
• 0.0015 wt % to 0.0150 wt %;
• one or more types of substances selected from a group consisting of
• REM:
• 0.0005 wt % to 0.2 wt %;
• Zr:
• 0.005 wt % to 0.2 wt %;
• Ti:
• 0.005 wt % to 0.05 wt %;
• V:
• 0.05 wt % to 0.5 wt %;
• Nb:
• 0.005 wt % to 0.05 wt %;
• Ni:
• 0.10 wt % to 3.0 wt %;
• Cu:
• 0.1 wt % to 3.0 wt %;
• Co:
• 0.1 wt % to 3.0 wt %;
• Mo:
• 0.1 wt % to 1.0 wt %;
• and
• a balance consisting of Fe and unavoidable impurities;
• heating said steel to a temperature level higher than solid-solution temperature for BN and that for AlN;
• hot rolling said steel;
• subjecting said steel to a normalizing process in which said steel is heated to a temperature region from 800.degree. C. to 950.degree. C. and cooled with air;
• heating said steel to a temperature region from 680.degree. C. to 740.degree. C.;
• and
• maintaining said steel at said temperature region for 5 hours or longer.

A method of manufacturing steel for a machine structural use exhibiting excellent free cutting characteristic and cold forging characteristic for use in hardened/tempered state, said method comprising the steps of:

• selecting steel composed of
• C:
• 0.1 wt % to 1.5 wt %;
• Si:
• 0.5 wt % to 2.0 wt %;
• Mn:
• 0.1 wt % to 2.0 wt %;
• B:
• 0.0003 wt % to 0.0150 wt %;
• Al:
• 0.005 wt % to 0.1 wt %;
• O.ltoreq.0.0030 wt %;
• P.ltoreq.0.020 wt %;
• S.ltoreq.0.035 wt %;
• N:
• 0.0015 wt % to 0.0150 wt %;
• and
• a balance consisting of Fe and unavoidable impurities;
• heating said steel to a temperature level higher than solid-solution temperature for BN and that for AlN;
• hot rolling said steel;
• subjecting said steel to a normalizing process in which said steel is heated to a temperature region from 800.degree. C. to 950.degree. C. and cooled with air;
• heating said steel to a temperature region from 300.degree. C. to 600.degree. C.;
• maintaining said steel at said temperature region for 15 minutes or longer;,
• heating said steel to a temperature region from 680.degree. C. to 740.degree. C.;
• and
• maintaining said steel at said temperature region for 5 hours or longer.

A method of manufacturing steel for a machine structural use exhibiting excellent free cutting characteristic and cold forging characteristic, for use in hardened/tempered state, said method comprising the steps of:

• selecting steel composed of
• C:
• 0.1 wt % to 1.5 wt %;
• Si:
• 0.5 wt % to 2.0 wt %;
• Mn:
• 0.1 wt % to 2.0 wt %;
• B:
• 0.0003 wt % to 0.0150 wt %;
• Al:
• 0.005 wt % to 0.1 wt %;
• O.ltoreq.0.0030 wt %;
• P.ltoreq.0.020 wt %;
• S.ltoreq.0.035 wt %;
• N:
• 0.0015 wt % to 0.0150 wt %;
• one or more types of substances selected from a group consisting of
• REM:
• 0.0005 wt % to 0.2 wt %;
• Zr:
• 0.005 wt % to 0.2 wt %;
• Ti:
• 0.005 wt % to 0.05 wt %;
• V:
• 0.05 wt % to 0.5 wt %;
• Nb:
• 0.005 wt % to 0.05 wt %;
• Ni:
• 0.10 wt % to 3.0 wt %;
• Cu:
• 0.1 wt % to 3.0 wt %;
• Co:
• 0.1 wt % to 3.0 wt %;
• Mo:
• 0.1 wt % to 1.0 wt %;
• and
• a balance consisting of Fe and unavoidable impurities;
• heating said steel to a temperature level higher than solid-solution temperature for BN and that for AlN;
• hot rolling said steel;
• subjecting said steel to a normalizing process in which said steel is heated to a temperature region from 800.degree. C. to 950.degree. C. and cooled with air;
• heating said steel to a temperature region from 300.degree. C. to 600.degree. C.;
• maintaining said steel at said temperature region for 15 minutes or longer;
• heating said steel toga temperature region from 680.degree. C. to 740.degree. C.;
• and
• maintaining said steel at said temperature region for 5 hours or longer.