High toughness low yield ratio, high fatigue strength steel plate and process of producing same

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• hot-rolling a steel to form a steel plate, said steel consisting of:
• 0.02 to 0.5 wt % C,
• 0.02 to 10.0 wt % Mn,
• 0.01 to 1.0 wt % Si,
• 0.1 wt % or less Al,
• at least one element selected from the group consisting of:
• 3.0 wt % or less Mo,
• 10.0 wt % or less Ni,
• 3.0 wt % or less Cr,
• 0.1 wt % or less V,
• 0.1 wt % or less Nb,
• 0.1 wt % or less Ti,
• 0. 003 wt % or less B,
• 10.0 wt % or less Cu,
• 10.0 wt % or less Co, and
• 3.0 wt % or less W, and
• the balance consisting of Fe and unavoidable impurities;
• quench-hardening the steel plate, either immediately or after reheating, after completion of the hot rolling so as to establish a quenched microstructure substantially composed of martensite, bainite or mixture thereof;
• and
• tempering the quench-hardened steel plate by heating at a heating rate of 1.degree. C./sec or more to a temperature of higher than A.sub.C1 and holding at this temperature for a time of not more than 15 min.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• hot-rolling a steel to form a steel plate, said steel consisting of:
• 0.02 to 0.5 wt % C,
• 0.02 to 10.0 wt % Mn,
• 0.01 to 1.0 wt % Si,
• 0.1 wt % or less Al,
• at least one element selected from the group consisting of:
• 10.0 wt % or less Ni,
• 10.0 wt % or less Cu, and
• 10.0 wt % or less Co, and
• the balance consisting of Fe and unavoidable impurities, with the following formula satisfied:
• 2Mn+2.5Ni+1.5 Cu+0.5Co.gtoreq.4 wt %;
• quench-hardening the steel plate either immediately, or after reheating, after completion of the hot rolling so as to establish a quenched microstructure substantially composed of martensite, bainite or mixture thereof;
• and
• tempering the quench-hardened steel plate by heating to a temperature of from higher than A.sub.C1 to the A.sub.C1 +80.degree. C. and holding at this temperature for a time of not more than 30 min.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• hot-rolling a steel to form a steel plate, said steel consisting of:
• 0.02 to 0.5 wt % C,
• 0.02 to 10.0 wt % Mn,
• 0.01 to 1.0 wt % Si,
• 0.1 wt % or less Al,
• at least one element selected from the group consisting of:
• 10.0 wt % or less Ni,
• 10.0 wt % or less Cu, and
• 10.0 wt % or less Co,
• at least one element selected from the group consisting of:
• 3.0 wt % or less Cr,
• 3.0 wt % or less Mo,
• 0.1 wt % or less V,
• 0.1 wt % or less Nb,
• 0.1 wt % or less Ti,
• 0.003 wt % or less B, and
• 3.0 wt % or less W, and
• the balance consisting of Fe and unavoidable impurities, with the following formula being satisfied:
• 2Mn+2.5Ni+1.5 Cu+0.5Co.gtoreq.4 wt %;
• quench-hardening the steel plate, either immediately or after reheating, after completion of the hot rolling so as to establish a quenched microstructure substantially composed of martensite, bainite or mixture thereof;
• and
• tempering the quench-hardened steel plate by heating to a temperature of from higher than A.sub.C1 to the A.sub.C1 +80.degree. C. and holding at this temperature for a time of not more than 30 min.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• hot-rolling a steel to form a steel plate, said steel consisting of:
• 0.02 to 0.5 wt % C,
• 0.02 to 10.0 wt % Mn,
• 0.01 to 1.0 wt % Si,
• 0.1 wt % or less Al,
• at least one element selected from the group consisting of:
• 10.0 wt % or less Ni,
• 10.0 wt % or less Cu, and
• 10.0 wt % or less Co, and
• the balance consisting of Fe and unavoidable impurities, with the following formula being satisfied:
• 2Mn+2.5Ni+1.5 Cu+0.5Co.gtoreq.4 wt %;
• quench-hardening the steel plate, either immediately or after reheating, after completion of the hot rolling so as to establish a quenched microstructure substantially composed of martensite, bainite or mixture thereof;
• and
• tempering the quench-hardened steel plate by heating at a heating rate of 0.3.degree. C./sec or more to a temperature of higher than A.sub.C1 and not higher than A.sub.C1 +20.degree. C. and holding at this temperature for a time of not more than 30 min.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• hot-rolling a steel to form a steel plate, said steel consisting of:
• 0.02 to 0.5 wt % C,
• 0.02 to 10.0 wt % Mn,
• 0.01 to 1.0 wt % Si,
• 0.1 wt % or less Al,
• at least one element selected from the group consisting of:
• 10.0 wt % or less Ni,
• 10.0 wt % or less Cu, and
• 10.0 wt % or less Co,
• at least one element selected from the group consisting of:
• 3.0 wt % or less Cr,
• 3.0 wt % or less Mo,
• 0.1 wt % or less V,
• 0.1 wt % or less Nb,
• 0.1 wt % or less Ti,
• 0.003 wt % or less B, and
• 3.0 wt % or less W, and
• the balance consisting of Fe and unavoidable impurities, with the following formula satisfied:
• 2Mn+2.5Ni+1.5 Cu+0.5Co.gtoreq.4 wt %;
• quench-hardening the steel plate either immediately, or after reheating, after completion of the hot rolling so as to establish a quenched microstructure substantially composed of martensite, bainite or mixture thereof;
• and
• tempering the quench-hardened steel plate by heating at a heating rate of 0.3.degree. C./sec or more to a temperature of higher than A.sub.C1 and not higher than A.sub.C1 +20.degree. C. and holding at this temperature for a time of not more than 30 min.

A steel plate having a high toughness, low yield strength and high fatigue strength, said steel consisting of:

• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• and
• the balance consisting of Fe and unavoidable impurities:
• said steel having a metallographical microstructure substantially composed of ferrite, cementite, and 0.5 to 5% by area of martensite.

A steel plate having a high toughness, low yield strength and high fatigue strength, said steel consisting of:

• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• at least one component selected from the group consisting of the components (a), (b), (c), (d), and (e) which are defined as;
• (a) at least one component for grain refining and precipitation hardening selected from the subgroup consisting of,
• 0.002 to 0.10 wt % Nb, and
• 0.002 to 0.10 wt % Ti,
• (b) at least one component for improving quench-hardenability selected from the subgroup consisting of,
• 0.05 to 3.0 wt % Cu,
• 0.05 to 10.0 wt % Ni,
• 0.05 to 10.0 wt % Cr,
• 0.05 to 3.5 wt % Mo,
• 0.05 to 10.0 wt % Co, and
• 0.05 to 2.0 wt % W,
• (c) 0.002 to 0.10 wt % V as a component for precipitation hardening,
• (d) 0.003 to 0.0025 wt % B as a component for improving quench-hardenability, and
• (e) at least one component for making sulphur harmless selected from the subgroup consisting of,
• 0.002 to 0.10 wt % REM, and
• 0.0003 to 0.0030 wt % Ca
• the balance consisting of Fe and unavoidable impurities:
• said steel having a metallographical microstructure substantially composed of ferrite, cementite, and 0.5 to 5% by area of martensite.

A steel plate having a high toughness, low yield strength and high fatigue strength, said steel consisting of:

• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• and
• the balance consisting of Fe and unavoidable impurities:
• said steel having a metallographical microstructure substantially composed of ferrite, cementite, and 1 to 30% by volume of retained austenite.

A steel plate having a high toughness, low yield strength and high fatigue strength, said steel consisting of:

• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• at least one component selected from the group consisting of the components (a), (b), (c), (d), and (e) which are defined as;
• (a) at least one component for grain refining and precipitation hardening selected from the subgroup consisting of,
• 0.002 to 0.10 wt % Nb, and
• 0.002 to 0.10 wt % Ti,
• (b) at least one component for improving quench-hardenability selected from the subgroup consisting of,
• 0.05 to 3.0 wt % Cu,
• 0.05 to 10.0 wt % Ni,
• 0.05 to 10.0 wt % Cr,
• 0.05 to 3.5 wt % Mo,
• 0.05 to 10.0 wt % Co, and
• 0.05 to 2.0 wt % W,
• (c) 0.002 to 0.10 wt % V as a component for precipitation hardening,
• (d) 0.003 to 0.0025 wt % B as a component for improving quench-hardenability, and
• (e) at least one component for making sulphur harmless selected from the subgroup consisting of,
• 0.002 to 0.10 wt % REM, and
• 0.0003 to 0.0030 wt % Ca
• the balance consisting of Fe and unavoidable impurities:
• said steel having a metallographical microstructure substantially composed of ferrite, cementite, and 0.5 to 5% by area of martensite.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• preparing a hot-rolled steel plate consisting of:
• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• and
• the balance consisting of Fe and unavoidable impurities, said steel plate having a metallographical microstructure substantially composed of ferrite and cementite;
• heating said steel plate sufficiently rapidly to prevent coarsening of said microstructure, to a temperature of A.sub.C1 or higher but sufficiently low to ensure an austenite amount of not more than 30% by volume;
• holding said steel plate at said temperature so that cementite present before, or generated during, said heating is partially transformed into austenite;
• and
• then cooling said steel plate to room temperature at a cooling rate which ensures establishment of a metallographical microstructure substantially composed of ferrite, cementite, and 0.5 to 5% by area of martensite.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• preparing a hot-rolled steel plate consisting of:
• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0. 30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• at least one component selected from the group consisting of the components (a), (b), (c), (d), and (e) which are defined as;
• (a) at least one component for grain refining and precipitation hardening selected from the subgroup consisting of,
• 0.002 to 0.10 wt % Nb, and
• 0.002 to 0.10 wt % Ti,
• (b) at least one component for improving quench-hardenability selected from the subgroup consisting of,
• 0.05 to 3.0 wt % Cu,
• 0.05 to 10.0 wt % Ni,
• 0.05 to 10.0 wt % Cr,
• 0.05 to 3.5 wt % Mo,
• 0.05 to 10.0 wt % Co, and
• 0.05 to 2.0 wt % W,
• (c) 0.002 to 0.10 wt % V as a component for precipitation hardening,
• (d) 0.003 to 0.0025 wt % B as a component for improving quench-hardenability, and
• (e) at least one component for making sulphur harmless selected from the subgroup consisting of,
• 0.002 to 0.10 wt % REM, and
• 0.0003 to 0.0030 wt % Ca;
• and
• the balance consisting of Fe and unavoidable impurities:
• said steel having a metallographical microstructure substantially composed of ferrite and cementite;
• heating said steel plate sufficiently rapidly to prevent coarsening of said microstructure, to a temperature of A.sub.C1 or higher but sufficiently low to ensure an austenite amount of not more than 30% by volume;
• holding said steel plate at said temperature so that cementite present before, or generated during, said heating is partially transformed into austenite;
• and
• then cooling said steel plate to room temperature at a cooling rate which ensures establishment of a metallographical microstructure substantially composed of ferrite, cementite, and 0.5 to 5% by area of martensite.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• preparing a hot-rolled steel plate consisting of:
• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• and
• the balance consisting of Fe and unavoidable impurities, said steel plate having a metallographical microstructure substantially composed of lath-form crystals, such as as-quenched bainite or martensite, that contain dissolved carbon and iron carbides;
• heating said steel plate sufficiently rapidly to prevent coarsening of said microstructure, to a temperature of not lower than A.sub.C1 but sufficiently low to ensure an austenite amount of not more than 30% by volume;
• holding said steel plate at said temperature so that cementite present before, or generated during, said heating is partially transformed into austenite;
• and
• then cooling said steel plate to room temperature at a cooling rate which ensures establishment of a metallographical microstructure substantially composed of ferrite, cementite, and 0.5 to 5% by area of martensite.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• preparing a hot-rolled steel plate consisting of:
• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• at least one component selected from the group consisting of the components (a), (b), (c), (d), and (e) which are defined as;
• (a) at least one component for grain refining and precipitation hardening selected from the subgroup consisting of,
• 0.002 to 0.10 wt % Nb, and
• 0.002 to 0.10 wt % Ti,
• (b) at least one component for improving quench-hardenability selected from the subgroup consisting of,
• 0.05 to 3.0 wt % Cu,
• 0.05 to 10.0 wt % Ni,
• 0.05 to 10.0 wt % Cr,
• 0.05 to 3.5 wt % Mo,
• 0.05 to 10.0 wt % Co, and
• 0.05 to 2.0 wt % W,
• (c) 0.002 to 0.10 wt % V as a component for precipitation hardening,
• (d) 0.003 to 0.0025 wt % B as a component for improving quench-hardenability, and
• (e) at least one component for making sulphur harmless selected from the subgroup consisting of,
• 0.002 to 0.10 wt % REM, and
• 0.0003 to 0.0030 wt % Ca;
• and
• the balance consisting of Fe and unavoidable impurities, said steel plate having a metallographical microstructure substantially composed of lath-form crystals, such as as-quenched bainite or martensite, that contain dissolved carbon and iron carbides;
• heating said steel plate sufficiently rapidly to prevent coarsening of said microstructure, to a temperature of A.sub.C1 or higher but sufficiently low to ensure an austenite amount of not more than 30% by volume;
• holding said steel plate at said temperature so that cementite present before, or generated during, said heating is partially transformed into austenite;
• and
• then cooling said steel plate to room temperature at a cooling rate which ensures establishment of a metallographical microstructure substantially composed of ferrite, cementite, and 0.5 to 5% by area of martensite.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• preparing a hot-rolled steel plate consisting of:
• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• and
• the balance consisting of Fe and unavoidable impurities, said steel plate having a metallographical microstructure substantially composed of ferrite and cementite;
• heating said steel plate sufficiently rapidly to prevent coarsening of said microstructure, to a temperature of not lower than A.sub.C1 but sufficiently low to ensure an austenite amount of not more than 30% by volume;
• holding said steel plate at said temperature so that cementite present before, or generated during, said heating is partially transformed into austenite;
• and
• then cooling said steel plate to room temperature at a cooling rate which ensures establishment of a metallographical microstructure substantially composed of ferrite, cementite, and 1 to 30% by volume of austenite.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• preparing a hot-rolled steel plate consisting of:
• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• at least one component selected from the group consisting of the components (a), (b), (c), (d), and (e) which are defined as;
• (a) at least one component for grain refining and precipitation hardening selected from the subgroup consisting of,
• 0. 002 to 0.10 wt % Nb, and
• 0.002 to 0.10 wt % Ti,
• (b) at least one component for improving quench-hardenability selected from the subgroup consisting of,
• 0.05 to 3.0 wt % Cu,
• 0.05 to 10.0 wt % Ni,
• 0.05 to 10.0 wt % Cr,
• 0.05 to 3.5 wt % Mo,
• 0.05 to 10.0 wt % Co, and
• 0.05 to 2.0 wt % W,
• (c) 0.002 to 0.10 wt % V as a component for precipitation hardening,
• (d) 0.003 to 0.0025 wt % B as a component for improving quench-hardenability, and
• (e) at least one component for making sulphur harmless selected from the subgroup consisting of,
• 0.002 to 0.10 wt % REM, and
• 0.0003 to 0.0030 wt % Ca;
• and
• the balance consisting of Fe and unavoidable impurities, said steel plate having a metallographical microstructure substantially composed of ferrite and cementite;
• heating said steel plate sufficiently rapidly to prevent coarsening of said microstructure, to a temperature of not lower than A.sub.C1 but sufficiently low to ensure an austenite amount of not more than 30% by volume;
• holding said steel plate at said temperature so that cementite present before, or generated during, said heating is partially transformed into austenite;
• and
• then cooling said steel plate to room temperature at a cooling rate which ensures establishment of a metallographical microstructure substantially composed of ferrite, cementite, and 1 to 30% by volume of austenite.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• preparing a hot-rolled steel plate consisting of:
• 0.02 to 0.35 wt % C;
• 0. 02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• and
• the balance consisting of Fe and unavoidable impurities, said steel plate having a metallographical microstructure substantially composed of lath-form crystals, such as as-quenched bainite or martensite, that contain dissolved carbon and iron carbides;
• heating said steel plate sufficiently rapidly to prevent coarsening of said microstructure, to a temperature of A.sub.C1 or higher but sufficiently low to ensure an austenite amount of not more than 30% by volume;
• holding said steel plate at said temperature so that cementite present before, or generated during, said heating is partially transformed into austenite;
• and
• then cooling said steel plate to room temperature at a cooling rate which ensures establishment of a metallographical microstructure substantially composed of ferrite, cementite, and 1 to 30% by volume of austenite.

A process of producing a steel plate having a high toughness, low yield ratio and high fatigue strength, said process comprising the steps of:

• preparing a hot-rolled steel plate consisting of:
• 0.02 to 0.35 wt % C;
• 0.02 to 2.5 wt % Si;
• 0.30 to 3.5 wt % Mn;
• 0.002 to 0.10 wt % Al;
• at least one component selected from the group consisting of the components (a), (b), (c), (d), and (e) which are defined as;
• (a) at least one component for grain refining and precipitation hardening selected from the subgroup consisting of,
• 0.002 to 0.10 wt % Nb, and
• 0.002 to 0.10 wt % Ti,
• (b) at least one component for improving quench-hardenability selected from the subgroup consisting of,
• 0. 05 to 3.0 wt % CU,
• 0.05 to 10.0 wt % Ni,
• 0.05 to 10.0 wt % Cr,
• 0.05 to 3.5 wt % Mo,
• 0.05 to 10.0 wt % Co, and
• 0.05 to 2.0 wt % W,
• (c) 0.002 to 0.10 wt % V as a component for precipitation hardening,
• (d) 0.003 to 0.0025 wt % B as a component for improving quench-hardenability, and
• (e) at least one component for making sulphur harmless selected from the subgroup consisting of,
• 0.002 to 0.10 wt % REM, and
• 0.0003 to 0.0030 wt % Ca;
• and
• the balance consisting of Fe and unavoidable impurities, said steel plate having a metallographical microstructure substantially composed of lath-form crystals, such as as-quenched bainite or martensite, that contain dissolved carbon and iron carbides;
• heating said steel plate sufficiently rapidly to prevent coarsening of said microstructure, to a temperature of A.sub.C1 or higher but sufficiently low to ensure an austenite amount of not more than 30% by volume;
• holding said steel plate at said temperature so that cementite present before, or generated during, said heating is partially transformed into austenite;
• and
• then cooling said steel plate to room temperature at a cooling rate which ensures establishment of a metallographical microstructure substantially composed of ferrite, cementite, and 1 to 30% by volume of austenite.