HIGH STRENGTH AND HIGH DUCTILITY STEEL PLATE HAVING HYPERFINE CRYSTAL GRAIN STRUCTURE PRODUCED BY SUBJECTING ORDINARY LOW CARBON STEEL TO LOW STRAIN WORKING AND ANNEALING, AND METHOD FOR PRODUCTION THEREOF

Description

[0001] The present invention relates to a high strength and high ductility low carbon steel having a tensile strength of 800MPa or more, an uniform elongation of 5% or more, and an elongation to failure of 20% or more, and possessing an ultrafine crystalline grain ferrite structure of from 100 to 300 nm, and cementites being precipitated in said ultra fine crystalline grain ferrite. The steel is produced by a method comprising (1) subjecting an ordinary low carbon steel or an ordinary low carbon steel added with boron in an amount of 0.01% or less being effective for accelerating martensitic transformation to processing and heat treatment to prepare a steel sheet having coarser austenite crystal grains and then to water-quenching, to provide a steel sheet having a martensite phase in an amount of 90% or more, and (2) subjecting said sheet to a low strain cold-rolling of a total rolling reduction of thickness 20% or more and less than 80%, and to a low temperature annealing at 500°C to 600 °C, and a method for producing said high strength and high ductility low carbon steel.

[0002] In the present invention, the ordinary low carbon steel means as steel whose carbon content is 0.2% or less, manganese content is 1.6% or less, silicon content is 0.5% or less, phosphorous content is 0.05% or less and sulfur content is 0.05% or less. The ordinary low carbon steel added with minute amount (0.01% or less) of boron means the steel produced by adding an effective amount of boron necessary for acceleration of martensitic transformation in an amount of 0.01% or less to the above mentioned ordinary low carbon steel for the purpose of improving the quenching property.

[0003] In the present invention, content % means weight %.

[0004] In recent years, the improvement in usability of vacant space accompanying high-rise building, energy saving requirements for cars or ships and recycling of natural resources are becoming more requisite, and this tendency is also applicable to steel materials. To satisfy the former two requirements, it is necessary to make the strength and ductility of the steel sheet much higher, and in order to improve recycling of natural resources as well, it is necessary to achieve said improvement in making the strength and ductility of the steel sheet much higher by using ordinary low carbon steel and not by adding other alloying elements.

[0005] In order to develop a steel sheet having a high degree of the desired properties, several project teams are established. These project teams are named as, for example, Super Metal Project or Super Steel Project and are aiming to develop a ferrite structure steel having "800MPa" tensile strength, which is two times that of ordinary low carbon steel, having high ductility, and having properties for easy welding as well, by producing ultra fine crystal grains of 1 µm or less in the present "400MPa class composition steel sheet".

[0006] In the concerned technical field, for the improvement in strength by refining the ferrite crystal grains of the steel, it is well known that the relationship of the Hall-Petch equation is realized, that is, yield stress and tensile strength are improved by refining the size of the ferrite crystalline steel and simultaneously the toughness is also improved. However, there is a problem of the elongation falling down in a tension test.

[0007] In CAMP-ISIJ Vol.11 (1998), pp 1031-1034, the following disclosure is reported. In studying to obtain a steel whose strength is improved to 800 MPa grade of 400 MPa grade steel with good weldability as a starting material, the object of the study was to accomplish a grain size of 1µm or less in a ferrite-carbide structure. As the specific measures to accomplish said object, the following process is mentioned. The austenite transforming treatment is carried out on a specimen having 8 mm thickness, namely, after said specimen is heat treated at a temperature of 1000°C for 60sec, cooled down by water so as to obtain a martensite structure, then the biaxial hot rolling is carried out on the specimen at a total rolling reduction of thickness of 90% at 640°C. They reported that the ferrite structure of the obtained steel is characterized to have an equiaxial fine structure, the nominal grain size becomes 0.77µm and Vickers hardness is 245, which corresponds to a tensile strength of 760 MPa. However, in said reference, there is no description reporting the actual measuring procedure for the tensile strength by preparing a test piece for a strength test from the obtained bulk steel, Further, there is no mention concerning elongation. Still more, the steel used in said reference is a steel whose manganese content is increased to 2.03% for the purpose of obtaining the quenching ability, further the rolling of the martensite structure is carried out in hot conditions at 640°C.

[0008] Further, in the development of a steel which satisfies the requirements, such as high strength, high toughness and high ductility, the solid-solution hardening method which adds an alloy element, the precipitation hardening method and the transformation strengthening method are being investigated. However, these methods have a problem of high price because the inclusion of high amounts of alloy element. Further, they have the problem of deteriorating the recycling property. On the other hand, to solve said problems, the strengthening, methods by refining the crystalline grains, which are methods in which no alloy element is added, are investigated and reported. However, since these methods are based on a large strain processing, the problem of - requiring particular processing equipment arises.

[0009] The inventors of the present invention have already investigated the structure and the mechanical properties of a steel sheet obtained by the combination of Accumulative Roll-Bonding (known as ARB) at room temperature and annealing, which is a large strain processing, using a steel sheet whose structure is ferrite-pearlite as a starting material. However, since the structure obtained after large strain processing has a heterogeneous structure in which both a region containing cementite and a region not containing cementite exist, a heterogeneous mixed grain structure whose grain size of ferrite is not uniform is generated in the annealing process. Therefore, the expected high strength and high ductility steel sheet could not be obtained.

[0010] The idea of producing the ultra fine ferrite crystalline grain structure of ordinary low carbon steel from a martensite structure is not a novel one, because said idea is also used by STX-21 Project or Super Metal Project which promotes the development of super steel. However, by this method, the development to accomplish the high strength and high ductility low carbon steel having a tensile strength of 800MPa or more, an uniform elongation of 5% or more, and an elongation to failure of 20% or more has not been realized yet. In particular, the idea to obtain a steel having high strength, high ductility and high toughness is not existing in the concept of these Projects.

[0011] The object of the present invention is to provide a steel sheet having said desired properties and a method of producing a steel sheet having said desired properties without the need for changes in the production plants for conventional steel sheets.

[0012] As mentioned above, the idea of using a steel sheet with a martensite structure as a starting material to realize the ultra fine ferrite crystal grain structure is a well known technique. However, it was considered to be difficult to form a martensite structure overall in the ordinary low carbon steel whose quenching property is not so good in the process of producing said ordinary low carbon steel sheet.

[0013] In order to produce high strength and high ductility low carbon steel having a tensile strength of 800MPa or more, an uniform elongation of 5% or more, and an elongation to failure of 20% or more from a martensite steel as a starting material, as the first step, the present inventors have studied the relationship between martensite steel as a starting material and the properties such as strength or ductility of low carbon steel obtained by a subsequent treatment. We have found out that said high strength and high ductility low carbon steel having the expected strength, elongation and elongation to failure can be obtained from a steel whose martensite phase is 90% or more obtained by making the austenite crystalline grains coarser, and then quenching into water followed by a cold rolling at a total rolling reduction in thickness of 20% or more and less than 80% and by annealing.

[0014] The object of the present invention is accomplished by the combination of said low strain processing and annealing and the specific steel to be provided to said low strain processing and annealing.

[0015] The present invention provides a high strength and high ductility low carbon steel sheet having a tensile strength of 800MPa or more and an uniform elongation of 5% or more and an elongation for break of 20% or more, and possessing an ultra fine crystalline grain ferrite structure of from 100 to 300 nm, and cementites being precipitated in said ultra fine crystalline grain ferrite. The invention further provides a method for producing a high strength and high ductility low carbon steel having a tensile strength of 800MPa or more, an uniform elongation of 5% or more and a elongation for break of 20% or more comprising,
   carrying out a cold rolling at a total rolling of 20% or more and less than 80% and annealing at the temperature of from 500 to 600 °C on a steel product having a martensite phase in an amount of 90% or more obtained by making coarse the size of an austenite crystal grain, which is existing in an ordinary low carbon steel or an ordinary low carbon steel added with boron in an amount of 0.01% or less being effective for accelerating martensitic transformation, to 100 µm or more and then water-quenching. Desirably, the steel is a high strength and high ductility low carbon steel, wherein said steel possesses an ultra fine crystal grain ferrite structure having an average grain diameter of 1.0 µm or less, formed by a low temperature processing and annealing by carrying out a cold rolling at a total rolling reduction of thickness of 20% or more and less than reduction of 80%, and a low temperature annealing at the temperature range between 500°C or more and less than 600 °C.

[0016] Fig. 1 is the optical microscopic (OM) picture showing the structure of the longitudinal-vertical cross sectional view of the ordinary low carbon steel plate (JIS-SS400, 2mm thickness) which is austenitized at 1000 °C for 15 minutes, then quenched into water.

[0017] In the picture, RD indicates the rolling direction and ND indicates normal direction of the sheet.

[0018] Fig. 2 is the optical microscopic picture showing the structure of the longitudinal-vertical cross sectional view of the cold rolled ordinary low carbon steel (JIS-SS400) whose starting structure is a martensite structure.
(a) shows the case of 50% cold rolling and (b) shows the case of 70% rolling. Fig. 3 shows the nominal-stress-nominal-strain curves of the quenched steel of the ordinary low carbon steel (JIS-SS400) and cold rolled steel of various rolling reduction in thickness. In the figure, a is a cold rolled steel at a rolling reduction in thickness of 70%, b is a cold rolled steel at a rolling reduction in thickness of 50%, c is a cold rolled steel at a rolling reduction of thickness of 25%, d is a quenched steel of martensite structure, and e is a steel as received of ferrite-pearlite structure

Fig.4 shows the nominal-stress―nominal-strain curves. In the figure, a is a cold rolled steel at a rolling reduction in thickness of 50% of an ordinary low carbon steel (JIS-SS400) whose starting structure is martensite structure, and 30 minutes annealed steels of it (b; annealed at 400°C, c; annealed at 500°C, d; annealed at 550°C, and e; annealed at 600°C).

Fig.5 shows the relationship between annealing temperature and mechanical properties of a cold rolled and annealed steel at a rolling reduction in thickness of 50% of an ordinary low carbon steel (JIS-SS400) whose starting structure is martensite structure. In the figure, -●- is tensile strength (σ_B), -○- is 0.2% proof stress (σ_0.2), -▲- is elongation of failure (e), and -Δ- is uniform elongation (σ_U).

Fig.6 is the transmission electron microscopic (TEM) picture showing the structure of the longitudinal-vertical cross sectional view of a cold rolled and annealed steel at a rolling reduction of 50% of an ordinary low carbon steel (JIS-SS400) at various annealing temperatures whose starting structure is martensite structure.
The annealing temperatures are (a) 400°C, (b) 500°C, (c) 550°C and (d) 600°C, for 30 minutes.

Fig.7 is the graph showing the comparison of the relationship between tensile strength and elongation to failure (strength-ductility balance) of rolled and annealed steel at a rolling reduction of 50% of an ordinary low carbon steel (JIS-SS400) whose starting structure is a martensite structure at the various annealing temperatures for 30 minutes (○), and that of rolled and annealed steel at a rolling reduction with ARB of 97% of the steel whose starting structure is ferrite-pearlite and annealed at various temperatures for 30 minutes (Δ).

Fig.8 is a JIS 5 test piece for elongation test.

[0019] The present invention will be illustrated in more detail.

A. For the illustration of the present invention, the method for test and apparatus for measurement are illustrated.

1. The shape of a test piece used for the tensile test is 1/5 of the size of JIS 5 test piece (Fig.8) (gauge length 10mm × gauge width 5mm).

2. The specimen for optical microscopic (Nikon Co., Ltd., Opti Photo 100S) and TEM (Hitachi Co., Ltd., H-800) observation is prepared by a well-known method.

B. The important points of the present invention are illustrated with reference to the drawing.

The present invention will be illustrated along with following more specific examples. However, the following examples are mentioned only for easy understanding of the present invention and it is not intended to limit the scope of the present invention.

[0020] Fig.1 is an optical microscopic picture showing the structure of the longitudinal-vertical cross sectional view of a quenched steel which is obtained by using a hot rolled plate having 2mm thickness of the rolled steel material for a general construction use, namely, the steel material containing miner constituents (JIS-SS400) such as C; 0.13%, Si; 0.01%, Mn; 0.37%, P; 0.02%, S; 0.004%, sol. Al; 0.04% as the receiving steel, and austenitization is carried out on said steel at 1000°C for 15 minutes so as to make coarse the size of an austenite crystal grain to 100-200µm size, then water-quenched. This picture shows that the structure is the structure of coarse martensite structure containing about 4% of proeutectoid ferrite.

[0021] Fig.2 is an optical microscopic picture showing the structure of the longitudinal-vertical cross sectional view of a cold rolled steel obtained by cold rolling of the receiving steel of Fig.1 by multi pass cold rolling by a total rolling reduction in thickness of 50% (a) and 70% (b). The proeutectoid ferrite precipitated in prior austenite grains can be observed in black contrast. In general, it is said that the workability of martensite of carbon steel is not so good, However, from Fig.2 it is clearly understood that the low carbon steel martensite, at least the low carbon steel martensite prepared according to the method of the present invention is possible to be cold rolled by reduction of 70% or more.

[0022] Fig.3 shows the nominal-stress-nominal-strain curves by tensile test of quenched steel of Fig.1 and cold rolled steel of Fig.2. For reference, the nominal-stress-nominal-strain curve e of a steel as received having ferrite-pearlite structure is shown by a dotted line. The tensile strength is improved from 410MPa to 1100MPa by quenching (d), further improved to 1340MPa by cold rolling of 25% (c), to 1470MPa by cold rolling of 50% (b) and to 1640MPa by cold rolling of 70% (a). While, elongation to failure is about 10% in the case of quenched steel and about 6% in the case of cold rolled steel. The uniform elongation of the cold rolled steel is 1% or less.

[0023] Fig.4 shows the nominal-stress-nominal-strain curves by tensile test of a cold rolled steel obtained by rolling reduction of 50% of Fig.3 and the annealed steels of it treated at various temperatures for 30 minutes. Although the strength is deteriorated by annealing, the ductility recovers by annealing at 500°C or more, and at the temperature of from 500°C to 550°C, the strength does not deteriorate so much, while the elongation to failure and the uniform elongation are obviously increased. Accordingly, in annealed steel at 550°C (d), the ultra high strength-high ductility steel of 870MPa tensile strength, 710MPa 0.2% proof stress, 21% elongation to failure and 8% uniform elongation is obtained.

[0024] Fig.5 shows the relationship between annealing temperature and tensile strength (-●-), 0.2% proof stress (-○-), elongation to failure (-▲-) and uniform elongation (-Δ-) of cold rolled steel by 50% and the annealed steel thereof. When the annealing temperature exceeds 525°C, elongation to failure and uniform elongation are suddenly recovered, while tensile strength is almost fixed at the temperature between the range of from 500°C to 550°C. This is the reason why the ultra high strength · high ductility steel is obtained.

[0025] Fig.6 is the TEM picture showing the structure of the longitudinal-vertical cross sectional view of a cold rolled and annealed steel at a rolling reduction of 50%. The picture indicates that the structure of 400°C annealed steel (a) is a lamellar structure similar to a heavily rolled steel. In the case of 500°C annealed steel (b), ultra fine equiaxial grains of from 100 to 300nm are observed in broad range. Not shown in the drawing, it already becomes clear from the limited range of vision electron diffraction pattern that these ultra fine equiaxial grains are surrounded by large angle grain boundaries and are not subgrains. The annealed steel at 550°C has also similar ultra fine equiaxial grain structure. However, at the annealing temperature of 600°C, coarser grain whose grain size is grown to several µm and spherically precipitated cementite are observed.

[0026] It is understood that the precipitation of cementite occurs at a higher temperature than 500°C so as to restrict the growth of crystalline grain, and consequently the ultra fine crystalline grain structure of from 100 to 300nm is generated. Further, the work hardening ability necessary for uniform elongation is provided simultaneously.

[0027] As mentioned above, by the use of the low carbon steel of martensite as a starting material, and by low strain processing of 50% rolling reduction and annealing at 550°C, an ultra fine ferrite crystalline grain structure can be obtained. Thus it becomes clear that it is possible to obtain a high strength and high ductility low carbon steel.

[0028] Fig.7 shows strength-ductility balance of 50% cold rolled and annealed steel of martensite which is a steel of the present invention (○) and large strain processed steel (97% cold rolling steel) whose starting structure is ferrite-pearlite structure of conventional art (Δ). As mentioned above, when large strain processing is carried out using ferrite-pearlite structure as a starting structure, the structure obtained by annealing becomes mixed grain structure and desired high strength and high ductility steel can not be obtained. On the contrary, in the case of cold rolled steel and annealed steel of martensite of the present invention, as clearly understood from Fig.7, the strength-ductility balance indicates the experimental point which satisfies the conditions of 800MPa or more tensile strength and 20% or more elongation to failure is obtained.

[0029] As mentioned above, in an ordinary low carbon steel of 0.13C (JIS-SS400), an ultra fine ferrite crystalline grain structure of 100-300nm grain size can be obtained by annealing after 50% cold rolling using the martensite structure of the present invention as a starting structure, and by annealing at 550°C for 30 minutes, a steel which has excellent mechanical properties of 870MPa tensile strength, 21% elongation to failure and 8% uniform elongation is obtained. And it is obvious that the method for production of said steel provides excellent effects, such as good economical advantage from the view point of facility and a satisfaction of social requirement from the view point of the environment and the circulation system of materials.

Claims

1. A high strength and high ductility low carbon steel having a tensile strength of 800MPa or more, an uniform elongation of 5% or more and an elongation for break of 20% or more, and possessing an ultra fine crystalline grain ferrite structure of from 100 to 300 nm, and cementites being precipitated in said ultra fine crystalline grain ferrite.

2. A method for producing a high strength and high ductility low carbon steel having a tensile strength of 800MPa or more, an uniform elongation of 5% or more and a elongation for break of 20% or more comprising,
   carrying out a cold rolling at a total rolling of 20% or more and less than 80% and annealing at the temperature of from 500 to 600 °C on a steel product having a martensite phase in an amount of 90% or more obtained by making coarse the size of an austenite crystal grain, which is existing in an ordinary low carbon steel or an ordinary low carbon steel added with boron in an amount of 0.01 % or less being effective for accelerating martensitic transformation, to 100 µm or more and then water-quenching.

Ansprüche

1. Kohlenstoffarmer Stahl hoher Festigkeit und hoher Duktilität mit einer Zugfestigkeit von 800MPa oder mehr, mit einer gleichmäßigen Dehnung von 5% oder mehr und einer Bruchdehnung von 20% oder mehr, der eine ultrafeine, kristalline, körnige Ferritstruktur von 100 bis 300 nm, und in diesem ultrafeinen, kristallinen, körnigen Ferrit abgelagerte Eisenkarbide aufweist.

2. Verfahren zur Herstellung eines kohlenstoffarmen Stahls hoher Festigkeit und hoher Duktilität mit einer Zugfestigkeit von 800MPa oder mehr, mit einer gleichmäßigen Dehnung von 5% oder mehr und einer Bruchdehnung von 20% oder mehr, umfassend
   das Durchführen einer Kaltwalzung bei einer Gesamtwalzung von 20% oder mehr und weniger als 80% und Tempern bei einer Temperatur von 500 bis 600°C bei einem Stahlprodukt mit einer Martensit-Phase in einer Menge von 90% oder mehr, die erhalten wird durch das Vergröbern der Größe einer Austenit-Kristallkörnung, die vorkommt in einem gewöhnlichen kohlenstoffarmen Stahl oder in einem gewöhnlichen kohlenstoffarmen Stahl, dem Bor in einer Menge von 0,01% oder weniger zugegeben wurde, was wirkungsvoll für die Beschleunigung der Umwandlung in Richtung Martensit ist, auf 100 µm oder mehr, gefolgt von Wasser-Quenchen.

Revendications

1. Acier à faible teneur en carbone, à haute résistance et à ductilité élevée ayant une résistance à la traction égale à 800 MPa ou plus, un allongement uniforme égal à 5 % ou plus et un allongement à la rupture égal à 20 % ou plus, et possédant une structure ferritique à grains cristallins ultra fins de 100 à 300 nm, et des cémentites étant précipitées dans ladite ferrite à grains cristallins ultra fins.

2. Procédé pour produire un acier à faible teneur en carbone, à haute résistance et à ductilité élevée ayant une résistance à la traction égale à 800 MPa ou plus, un allongement uniforme égal à 5 % ou plus et un allongement à la rupture égal à 20 % ou plus comportant,
   la réalisation d'un laminage à froid sur un laminage total de 20 % ou plus et inférieur à 80 % et d'un recuit à la température de 500 à 600°C sur un produit en acier ayant une phase martensitique en quantité de 90 % ou plus obtenue en faisant grossir la taille d'un grain de cristal d'austénite, qui est présent dans un acier ordinaire à faible teneur en carbone ou dans un acier ordinaire à faible teneur en carbone enrichi de bore en une quantité de 0,01 % ou moins efficace pour accélérer la transformation martensitique, jusqu'à 100 µm ou plus et ensuite d'une trempe à l'eau.

Drawing