High strength hot-rolled steel and method for manufacturing high strength hot-rolled steel

Abstract

Described is a hot-rolled steel having a tensile strength of at least 950 MPa and a microstructure that includes bainite at an area ratio of 70% or more; the balance being: martensite at an area ratio of 30% or less, and optionally ferrite at an area ratio of 20% or less. The hot-rolled steel has a chemical composition containing (in mass-%): C: 0.07-0.10, Si: 0.01-0.25, Mn: 1.5-2.0, Cr: 0.5-1.0, Ni: 0.1-0.5, Cu: 0.1-0.3, Mo: 0.01-0.2, Al: 0.01-0.05, Nb: 0.015-0.04, V: 0-0.1, i.e. optionally up to 0.1 mass-% Vanadium, Ti: 0-0.1, whereby the balance is Fe and unavoidable impurities.

Claims

1. A hot-rolled steel having a tensile strength of at least 950 MPa, characterized by: a microstructure comprising, bainite at an area ratio of 70% or more wherein a majority of said bainite is upper bainite; the balance being: martensite at an area ratio of 30% or less, and optionally ferrite at an area ratio of 20% or less, and a chemical composition consisting of, in mass %, C: 0.07-0.10, Si: 0.01-0.25, Mn: 1.5-2.0, Cr: 0.5-1.0, Ni: 0.1-0.5, Cu: 0.1-0.3, Mo: 0.01-0.2, Al: 0.01-0.05, Nb: 0.015-0.04, V: 0-0.1, Ti: 0-0.1, and balance being Fe and unavoidable impurities.

2. The hot-rolled steel according to claim 1, wherein said microstructure comprises islands of martensite in a bainite matrix.

3. The hot-rolled steel according to claim 1, wherein said microstructure comprises martensite at an area ratio of at least 10% to 30% or less.

4. The hot-rolled steel according to claim 1, wherein said microstructure comprises bainite at an area ratio from 70% or more to less than 90%.

5. The hot-rolled steel according to claim 1, wherein said hot-rolled steel has a yield strength of 720-950 MPa.

6. The hot-rolled steel according to claim 1, wherein said hot-rolled steel has an elongation of at least 8%.

7. The hot-rolled steel according to claim 1, wherein said hot-rolled steel has a hole expansion ratio of at least 25%.

8. The hot-rolled steel according to claim 1, wherein said hot-rolled steel has a thickness of 4 mm or less.

9. A vehicle comprising the hot-rolled steel of claim 1.

10. The vehicle of claim 9, wherein the vehicle is an automotive vehicle.

11. A method for manufacturing the hot-rolled steel of claim 1, wherein the method comprises: heating a steel having a chemical composition consisting of, in mass %, C: 0.07-0.10, Si: 0.01-0.25, Mn: 1.5-2.0, Cr: 0.5-1.0, Ni: 0.1-0.5, Cu: 0.1-0.3, Mo: 0.01-0.2, Al: 0.01-0.05, Nb: 0.015-0.04, V: 0-0.1, Ti: 0-0.1, and balance being Fe and unavoidable impurities to a temperature of at least 1250° C., hot-rolling said steel at a finishing rolling temperature of 850-930° C., quenching said steel to a coiling temperature of 450-575° C., coiling said steel at said coiling temperature, cooling said steel, and skin pass rolling said steel.

12. The method according to claim 11, wherein said skin pass rolling step comprises skin pass rolling at a reduction of 0.5-2%.

13. The method according to claim 11, wherein said quenching step comprises quenching said steel at a rate of at least 60° C/s.

14. The method according to claim 11, wherein said cooling step comprises cooling said steel at a cooling rate of 10° C/s or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figures where;

(2) FIG. 1 shows a vehicle that includes at least one component comprising hot-rolled steel according to any of the embodiments of the invention, and

(3) FIG. 2 is a flow chart showing the steps of a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(4) FIG. 1 shows a vehicle 10 that includes at least one component comprising hot-rolled steel according to any of the embodiments of the invention. The vehicle 10 may for example comprise a chassis part, such as an A-pillar 12, that comprises at least one hot-rolled steel sheet having a tensile strength of at least 950 MPa and a thickness of 2-4 mm. The hot-rolled steel has a microstructure comprising bainite at an area ratio of 70% or more; the balance being: martensite at an area ratio of 30% or less, and optionally ferrite at an area ratio of 20% or less, and a chemical composition containing (in mass-%): C: 0.07-0.10, Si: 0.01-0.25, Mn: 1.5-2.0, Cr: 0.5-1.0, Ni: 0.1-0.5, Cu: 0.1-0.3, Mo: 0.01-0.2, Al: 0.01-0.05, Nb: 0.015-0.04, V: 0-0.1, i.e. optionally up to 0.1 mass-% Vanadium, Ti: 0-0.1, whereby the balance is Fe and unavoidable impurities.

(5) For example, the chemical composition of the hot-rolled steel comprises the following in mass-%:

(6) TABLE-US-00003 C 0.09 Si 0.18 Mn 1.80 Cr 0.75 Ni 0.15 Cu 0.15 Mo 0.10 Al 0.035 Nb 0.030 V 0 Ti 0.045 balance Fe and unavoidable impurities.

(7) The hot-rolled steel does not contain any Boron.

(8) The C content is set to be in a range of 0.07 to 0.10 mass-%. In the case where the C content is less than 0.07%, the target tensile strength cannot be achieved. If the C content exceeds 0.10%, weldability, elongation, and consequently the formability of the steel are deteriorated.

(9) Si is a solid-solution strengthening element and is effective in increasing the strength; and therefore, as the Si content is increased, the balance between tensile strength and elongation is improved.

(10) The Mn content is set to be in a range of 1.5 to 2.0 mass-% or 1.7 to 2.0 mass-%. Mn is an effective element in enhancing solid-solution strengthening and hardenability. An excessive addition of Mn causes deterioration of workability due to Mn segregation.

(11) Cr is effective in enhancing hardenability. As the Cr content is increased, the tensile strength of the steel sheet is increased. However, if the Cr content is too large, Cr-based alloy carbides such as Cr.sub.23C.sub.6 are precipitated, and when these carbides are preferentially precipitated in the grain boundaries, press formability is deteriorated. Therefore, the upper limit of the Cr content is set to be 1.0 mass-%.

(12) Ni enhances hardenability of the steel, contributes to the improvement of toughness and prevents hot brittleness. Since Ni is a relatively expensive alloying element, the upper limit of the Ni content is set to be 0.5 mass-%, or 0.3 mass-%.

(13) Cu increases the strength of the steel due to precipitation thereof. Alloying elements such as Ti are bonded to C or N and form alloy carbides; however, Cu is precipitated solely and strengthens the steel material. Steel containing a large amount of Cu may become brittle during hot-rolling. The upper limit of the Cu content is therefore set to be 0.3 mass-%.

(14) Mo is a precipitation strengthening element. However, if the Mo content exceeds 0.2 mass-%, the effect of improving precipitation strengthening is small, and in addition, elongation is deteriorated.

(15) The Al content is set to be in a range of 0.01 to 0.05 mass-%. Al is added as a deoxidizing element so that the amount of dissolved oxygen in a molten steel can be reduced. If the Al content is 0.01 mass-% or more, it is possible to prevent Ti, Nb, Mo, and V from forming alloy oxides with dissolved oxygen.

(16) Nb is a precipitation strengthening element. Nb also delays the rate of recrystallization of austenite during hot-rolling. Therefore, in the case where the Nb content is excessive, workability and elongation are adversely affected. The upper limit of the Nb content is therefore set to be 0.1 mass-%. Nb contributes to making grain sizes finer.

(17) V, an optional element in the hot-rolled steel according to the present invention, is a precipitation strengthening element. However, if the V content exceeds 0.1%, the effect of improving the precipitation strengthening is small, and elongation may deteriorate. A maximum of 0.1 mass-% Vanadium may therefore be added.

(18) The Ti content is set to be in a range of 0 to 0.1 mass-%, or 0.03 to 0.1 mass-%. Ti is a precipitation strengthening element. The steel must be heated to a temperature of at least 1250° C. prior to hot-rolling in order to ensure that this relatively high amount of T is re-dissolved.

(19) It is important that Ti is dissolved before hot-rolling to enable fine precipitates to form during the hot-rolling. The Titanium Carbide (TiC) inclusions in the slabs may be coarse which is not beneficial for strengthening. Therefore, the Ti needs to be dissolved to enable it to form finer TiC inclusions during the hot-rolling, which enables more effective precipitation strengthening. Furthermore, Ti helps to hinder or prevent grain coarsening during the heating step.

(20) The microstructure of the hot-rolled steel may from example comprise bainite at an area ratio of 70-80% and martensite at an area ratio of 10-20%, the remainder being ferrite at an area ratio of 20% or less. Alternatively, the microstructure of the hot-rolled steel may comprise only bainite at an area ratio of 70-90% and martensite at an area ratio of 10-30%. The microstructure may comprise islands of martensite in a bainite matrix. The majority of the bainite in the microstructure of the hot-rolled steel is upper bainite.

(21) The hot-rolled steel has a yield strength of 720-950 MPa and/or an elongation of at least 8% and/or a hole expansion ratio of at least 25%.

(22) FIG. 2 is a flow chart showing the steps of a method for manufacturing hot-rolled steel having a tensile strength of at least 950 MPa and a microstructure comprising bainite at an area ratio of 70% or more; the balance being: martensite at an area ratio of 30% or less and optionally ferrite at an area ratio of 20% or less, and a chemical composition containing (in mass-%): C: 0.07-0.10, Si: 0.01-0.25, Mn: 1.5-2.0, Cr: 0.5-1.0, Ni: 0.1-0.5, Cu: 0.1-0.3, Mo: 0.01-0.2, Al: 0.01-0.05, Nb: 0.015-0.04, V: 0-0.1, i.e. optionally up to 0.1 mass-% Vanadium, Ti: 0.05-0.1, whereby the balance is Fe and unavoidable impurities.

(23) The method comprises the following steps which are carried out in the following order: heating steel having the chemical composition to a temperature of at least 1250° C., hot-rolling the steel at a finishing rolling temperature of 850-930° C., quenching the steel in water for example to a coiling temperature of 450-575° C. or 475-575° C. at a rate of at least 60° C./s, coiling the steel at the coiling temperature, cooling the steel, and skin pass rolling at a reduction of 0.5-2%. During coiling, the cooling rate should be 10° C./s or less, which is achieved by maintaining the steel at the coiling temperature. After coiling the steel may be cooled to room temperature at a cooling rate of 10° C./s or less, over a period of three or four days for example, and then skin pass rolled. The skin pass rolling thereby takes place when the steel is at room temperature or within 5-30° C. of the ambient temperature. Alternatively, there may be one or more additional steps between the coiling step and the skin pass rolling step, such as an annealing step or an acid pickling step.

(24) A method according to an embodiment of the invention produces hot-rolled steel having the tensile strength, microstructure, chemical composition and properties described herein. Such a hot-rolled steel is suitable for use in the automotive or vehicle construction industry, which may result in the manufacture of more light-weight and crash-resistant vehicle components.

Example 1

(25) Hot-rolled steel having the following chemical composition in mass-% was manufactured using a method according to an embodiment of the invention: C 0.09, Si 0.18, Mn 1.80, Cr 0.75, Ni 0.15, Cu 0.15, Mo 0.10, Al 0.035, Nb 0.030, V 0, Ti 0.045, B 0, balance Fe and unavoidable impurities.

(26) The method comprised the following steps: heating steel having said chemical composition to a temperature of 1280° C., hot-rolling said steel at a finishing rolling temperature of 890° C., quenching said steel to a coiling temperature of 525° C. at a cooling rate of 230° C./s, coiling said steel at said coiling temperature of 525° C., cooling said steel to room temperature at a cooling rate of less than 5° C./min, such as 2.5° C./s, whereby a cooling rate of 2.5° C./s may take place on the run-out table of a cooling line, and skin pass rolling at a reduction of 0.5%.

(27) The hot-rolled steel had a yield strength of 836 MPa, a tensile strength of 979 MPa, an elongation of 10% and a hole expansion ratio of 35% which was measured in accordance with the ISO 16630:2009 standard.

Example 2

(28) Hot-rolled steel having the following chemical composition in mass-% was manufactured using a method according to an embodiment of the invention: C 0.088, Si 0.2, Mn 1.78, Cr 0.75, Ni 0.15, Cu 0.15, Mo 0.10, Al 0.038, Nb 0.027, V 0, Ti 0.046, B 0, balance Fe and unavoidable impurities.

(29) The method comprised the following steps: heating steel having said chemical composition to a temperature of 1283° C., hot-rolling said steel at a finishing rolling temperature of 904° C., quenching said steel to a coiling temperature of 530° C. at a cooling rate of 230° C./s, coiling said steel at said coiling temperature of 530° C., cooling said steel to room temperature at a cooling rate of less than 5° C./min, such as 2.5° C./s, whereby a cooling rate of 2.5° C./s may take place on the run-out table of a cooling line, and skin pass rolling at a reduction of 0.5%.

(30) The hot-rolled steel had a yield strength of 854 MPa, a tensile strength of 992 MPa, an elongation of 11% and a hole expansion ratio of 30% which was measured in accordance with the ISO 16630:2009 standard.

Example 3

(31) Hot-rolled steel having the following chemical composition in mass-% was manufactured using a method according to an embodiment of the invention: C 0.082, Si 0.17, Mn 1.8, Cr 0.75, Ni 0.2, Cu 0.2, Mo 0.10, Al 0.035, Nb 0.028, V 0.048, Ti 0, B 0, balance Fe and unavoidable impurities.

(32) The method comprised the following steps: heating steel having said chemical composition to a temperature of 1284° C., hot-rolling said steel at a finishing rolling temperature of 878° C., quenching said steel to a coiling temperature of 519° C. at a cooling rate of 230° C./s, coiling said steel at said coiling temperature of 519° C., cooling said steel to room temperature at a cooling rate of less than 5° C./min, such as 2.5° C./s, whereby a cooling rate of 2.5° C./s may take place on the run-out table of a cooling line, and skin pass rolling at a reduction of 0.5%.

(33) The hot-rolled steel had a yield strength of 852 MPa, a tensile strength of 995 MPa, an elongation of 11% and a hole expansion ratio of 30% which was measured in accordance with the ISO 16630:2009 standard.

(34) Further modifications of the invention within the scope of the claims would be apparent to a skilled person.