Process for producing a hot rolled steel sheet with high strength and distinguished formability

Abstract

A hot rolled steel sheet with a high strength and a distinguished formability, and a process for producing the same are disclosed. The steel sheet comprises 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, and has a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. The steel sheet can be produced with a high productivity and without requiring special alloy elements.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to a hot rolled steel sheet with a high ductivility, a high strength and a distinguished formability applicable to automobiles, industrial machinery, etc., and a process for producing the same. The term "sheet" means "sheet" or "plate" in the present specification and claims.

2. Description of the Prior Art

In order to make the automobile steel sheet lighter and ensure safety at collisions, steel sheets with a higher strength have been in keen demand. Steel sheets even with a high strength have been required to have a good formability. That is, a steel sheet must have a high strength and a good formability at the same time.

A dual phase steel composed of a ferrite phase and a martensite phase, which will be hereinafter referred to as "DP steel", has been so far proposed as a hot rolled steel sheet applicable to the fields requiring a high ductility. It is known that the DP steel has a more distinguished strength-ductility balance than a solid solution-intensified steel sheet with a high strength and a precipitation-intensified steel sheet with a high strength. However, there is such a limit to the strength-ductility balance as TS.times.T.El.ltoreq.2,000, where TS represents a tensile strength (kgf/mm.sup.2) and T.El represents a total elongation (%), and thus the DP steel cannot meet more strict requirements.

In order to overcome the limit to the strength-ductility balance, that is, to obtain TS.times.T.El>2,000, it has been proposed to utilize a retained austenite phase. For example, the following processes have been proposed: a process for producing a steel sheet having a retained austenite phase, which comprises hot rolling a steel sheet at a finish temperature of Ar.sub.3 to Ar.sub.3 +50.degree. C., then maintaining the steel sheet at a temperature of 450.degree. C. to 650.degree. C. for 4 to 20 seconds, and then coiling the steel sheet at a temperature of not more than 350.degree. C. [Japanese Patent Application Kokai (Laid-open) No. 60-43425], a process for producing a steel sheet having a retained austenite phase, which comprising rolling a steel sheet at a finish temperature of 850.degree. C. or more with a total draft of 80% or more and under a high reduction with a draft of 60% or more for the last total three passes and a draft of 20% r more for the last pass, and successively cooling the steel sheet down to 300.degree. C. or less at a cooling rate of 50.degree. C./sec. or more [Japanese Patent Application Kokai (Laid-open) No. 60-165,320], etc.

However, the conventional processes requiring the maintenance of a steel sheet at 450.degree. to 650.degree. C. for 4 to 20 seconds during the cooling, the coiling at a low temperature such as not more than 350.degree. C., or the rolling under a high reduction are not operationally preferable with respect to the energy saving and productivity increase. The formability of the steel sheets obtained according to these processes is, for example, TS.times.T.El.ltoreq.2,416 and thus does not always fully satisfy the level required by users. A steel sheet with a higher TS.times.T.El value (desirably more than 2,416) and a process for producing the same with a higher productivity have been in keen demand.

SUMMARY OF THE INVENTION

As a result of extensive tests and researches (in which later-explained Transformation Induced Plasticity phenomenon is utilized, i.e. unstable, high retained austenite is utilized) for obtaining a steel sheet with TS.times.T.El.gtoreq.2,000, which is over the limit of the prior art, the present inventors have found that at least 5% by volume of an austenite phase must be contained, as shown in FIG. 1, directed to steel species A in an Example that follows, and have confirmed that the TS.times.T.El value can be assuredly made to exceed the level of the aforementioned DP steel, i.e. TS.times.T.El.apprxeq.2,000, thereby. Further, the present inventors have found that the increase in TS.times.T.El based on an increase in an amount of retained austenite is greatly based on an increase in uniform elongation, and that if a hot rolled steel sheet contains a retained austenite in an amount of 5% or more, a uniform elongation amount of 20% or more, which is necessary for a hot rolled steel sheet with a high strength and a distinguished formability, can be secured, and further a total elongation amount of 30% or more, which is more preferable, can also be secured in most cases.

The present invention is based on this finding and an object of the present invention is to provide a hot rolled steel sheet with a high strength and a distinguished formability, which contains 5% by volume or more of a retained austenite phase, and also a process for stably, assuredly and economically producing such a steel sheet as above.

The foregoing object of the present invention can be attained by the following means:

(1) A hot rolled steel sheet with a high strength and a distinguished formability, consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and 0.0005 to 0.0100% by weight of Ca, with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities, and having a microstructure composed of ferrite, bainite and retained austenite phase with the ferrite phase being in a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to the polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. (2) A hot rolled steel sheet as described in (1), wherein said steel sheet further contains 0.004 to 0.040% by weight of Al. (3) A hot rolled steel sheet with a high strength and a distinguished formability, consisting of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn, 0.004 to 0.040% by weight of Al and 0.0005 to 0.0100% by weight of Ca, with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities, and having a microstructure composed of ferrite, bainite and retained austenite phase with the ferrite phase being in a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to the polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. (4) A hot rolled steel sheet with a high strength and a distinguished formability, consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and 0.0005 to 0.0100% by weight of Ca, with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities, and having a microstructure composed of ferrite, bainite and retained austenite phase with the ferrite phase being in a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to the polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases, wherein said steel sheet has a uniform elongation of 20% or more. (5) A hot rolled steel sheet as described in (4), wherein said steel sheet further contains 0.004 to 0.040% by weight of Al. (6) A hot rolled steel sheet with a high strength and a distinguished formability, consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and 0.0005 to 0.0100% by weight of Ca, with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities, and having a microstructure composed of ferrite, bainite and retained austenite phase with the ferrite phase being in a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to the polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases, wherein said steel sheet has a uniform elongation of 20% or more and a total elongation of 30% or more. (7) A hot rolled steel sheet as described in (6), wherein said steel sheet further contains 0.0004 to 0.040% by weight of Al. (8) A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises subjecting a steel consisting/essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature is within a range between Ar.sub.3 +50.degree. C. and Ar.sub.3 -50.degree. C., successively cooling the steel down to a desired temperature T within a temperature range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to Ar.sub.1 at a cooling rate of less than 40.degree. C./sec., successively cooling the steel at a cooling rate of 40.degree. C./sec. or more, and coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C. (9) A process as described in (8), wherein cooling is conducted for 3 to 25 seconds to cool said steel within a temperature range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to said desired temperature T or to hold said steel isothermally within said temperature range. (10) A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and one of 0.0005 to 0.0100% by weight of Ca and 0.005 to 0.050% by weight of rare earth metal, with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities, to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature is within a range between Ar.sub.3 +50.degree. C. and Ar.sub.3 -50.degree. C., successively cooling the steel down to a desired temperature T within a range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to Ar.sub.1 at a cooling rate of less than 40.degree. C./sec., successively cooling the steel at a cooling rate of 40.degree. C./sec. or more, and coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C.

The term "rare earth metal" or "REM" hereinafter means at least one of the fifteen metallic metals (elements) (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) following lanthanum through lutetium with atomic numbers 57 through 71. The rare earth metal (REM) is added frequently in the form of a mischmetal which is an alloy of REM and that has a composition comprising 50% of lanthanum, neodymium and the other metal in the same series and 50% of cerium.

(11) A process as described in (10), wherein cooling is conducted for 3 to 25 seconds to cool said steel within a temperature range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to said desired temperature T or to hold said steel isothermally within said temperature range. (12) A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature is within a range between Ar.sub.3 +50.degree. C. and Ar.sub.3 -50.degree. C., setting two desired temperatures T.sub.1 and T.sub.2, wherein T.sub.1 .gtoreq.T.sub.2 within a temperature range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to Ar.sub.1, successively cooling the steel down to the T.sub.1 at a cooling rate of 40.degree. C./sec. or more, successively cooling the steel down to the T.sub.2 at a cooling rate of less than 40.degree. C./sec., further cooling the steel at a cooling rate of 40.degree. C./sec. or more, and coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C. (13) A process as described in (12), wherein cooling is conducted for 3 to 25 seconds to cool said steel within a temperature range from said desired temperature T.sub.1 to said desired temperature T.sub.2 or to hold said steel isothermally within said temperature range. (14) A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and one of 0.0005 to 0.0100% by weight of Ca and 0.005 to 0.050% by weight of rare earth metal, with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities, to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature is within a range between Ar.sub.3 +50.degree. C. and Ar.sub.3 -50.degree. C., setting two desired temperatures T.sub.1 and T.sub.2, wherein T.sub.1 .gtoreq.T.sub.2 within a temperature range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to Ar.sub.1, successively cooling the steel down to the T.sub.1 at a cooling rate of 40.degree. C./sec. or more, successively cooling the steel down to the T.sub.2 at a cooling rate of less than 40.degree. C./sec., further cooling the steel at a cooling rate of 40.degree. C./sec. or more, and coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C. (15) A process as described in (14), wherein cooling is conducted for 3 to 25 seconds to cool said steel within a temperature range from said desired temperature T.sub.1 to said desired temperature T.sub.2 or to hold said steel isothermally within said temperature range. (16) A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature exceeds Ar.sub.3 +50.degree. C., successively cooling the steel down to a desired temperature T within a temperature range from the Ar.sub.3 of the steel to Ar.sub.1 at a cooling rate of less than 40.degree. C./sec., successively cooling the steel at a cooling rate of 40.degree. C./sec. or more, and coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C. (17) A process as described in (16), wherein cooling is conducted for 3 to 25 seconds to cool said steel within a temperature range from the Ar.sub.3 of said steel to said desired temperature T or to hold said steel isothermally within said temperature range. (18) A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises subjecting a steel consisting-essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and one of 0.0005 to 0.0100% by weight of Ca and 0.005 to 0.050% by weight of rare earth metal, with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities, to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature exceeds Ar.sub.3 +50.degree. C. successively cooling the steel down to a desired temperature T within a range from the Ar.sub.3 of the steel to Ar.sub.1 at a cooling rate of less than 40.degree. C./sec., successively cooling the steel at a cooling rate of 40.degree. C./sec. or more, and coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C. (19) A process as described in (18), wherein cooling is conducted for 3 to 25 seconds to cool said steel within a temperature range from the Ar.sub.3 of said steel to said desired temperature T or to hold said steel isothermally within said temperature range. (20) A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature exceeds Ar.sub.3 +50.degree. C., setting two desired temperatures T.sub.1 and T.sub.2, wherein T.sub.1 .gtoreq.T.sub.2 within a temperature range from the Ar.sub.3 of the steel to Ar.sub.1, successively cooling the steel down to the T.sub.1 at a cooling rate of 40.degree. C./sec. or more, successively cooling the steel down to the T.sub.2 at a cooling rate of less than 40.degree. C./sec., further cooling the steel at a cooling rate of 40.degree. C./sec. or more, and coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C. (21) A process as described in (20), wherein cooling is conducted for 3 to 25 seconds to cool said steel within a temperature range from said desired temperature T.sub.1 to said desired temperature T.sub.2 or to hold said steel isothermally within said temperature range. (22) A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and one of 0.0005 to 0.0100% by weight of Ca and 0.005 to 0.050% by weight of rare earth metal, with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities, to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature exceeds Ar.sub.3 +50.degree. C., setting two desired temperatures T.sub.1 and T.sub.2, wherein T.sub.1 .gtoreq.T.sub.2 within a temperature range from the Ar.sub.3 of the steel to Ar.sub.1, successively cooling the steel down to the T.sub.1 at a cooling rate of 40.degree. C./sec. or more, successively cooling the steel down to the T.sub.2 at a cooling rate of less than 40.degree. C./sec., further cooling the steel at a cooling rate of 40.degree. C./sec. or more, and coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C. (23) A process as described in (22), wherein cooling is conducted for 3 to 25 seconds to cool said steel within a temperature range from said desired temperature T.sub.1 to said desired temperature T.sub.2 or to hold said steel isothermally within said temperature range. (24) A process as described in any one of (8) to (23), wherein a hot finish rolling starting temperature of the steel is set to not more than (Ar.sub.3 +100.degree. C.). (25) A process as described in any one of (8) to (23), wherein the steel sheet after the coiling is cooled down to not more than 200.degree. C. at a cooling rate of 30.degree. C./hr. or more. (26) A process as described in any one of (8) to (23), wherein said steel further contains 0.004 to 0.040% by weight of Al. (27) A process as described in any one of (8) to (23), wherein said steel further contains 0.004 to 0.040% by weight of Al and a hot finish rolling starting temperature of the steel is set to not more than (Ar.sub.3 +100.degree. C.). (28) A process as described in any one of (8) to (23), wherein said steel further contains 0.004 to 0.040% by weight of Al and the steel sheet after the coiling is cooled down to not more than 200.degree. C. at a cooling rate of 30.degree. C./hr. or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a relationship between the volume fraction of the retained austenite phase and the TS.times.T.El value.

FIG. 2 is a diagram showing a relationship between the ratio of polygonal ferrite volume fraction V.sub.PF (%) to polygonal ferrite average grain size d.sub.PF (.mu.m) and the TS.times.T.El value.

FIG. 3 is a diagram showing a relationship between the coiling temperature and the volume fraction of the retained austenite phase.

FIG. 4 is a diagram showing a relationship between the coiling temperature and the hole expansion ratio.

FIG. 5 is a diagram showing a relationship between TS and T.El.

FIG. 6 is a temperature pattern diagram showing a relationship among the finish rolling end temperature, the cooling rate 1, T and the cooling rate 2.

FIG. 7 is a temperature pattern diagram showing a relationship among the finish rolling end temperature, the cooling rate 1', T.sub.1, the cooling rate 2', T.sub.2 and the cooling rate 3'.

FIGS. 8-9 illustrate the "uniform elongation" and "total elongation" of the steel sheet, in which, when a test piece of steel sheet is elongated in a tensile test machine [see FIG. 8(a)], first it is uniformly elongated [see FIG. 8(b)], and then a neck portion is formed at a local portion of the test piece [see FIG. 8(c)], and finally it is completely ruptured, and thus, a total elongation is a uniform elongation plus a local elongation (see FIG. 9).

DETAILED DESCRIPTION OF THE INVENTION

The requisite means for achieving the present invention will be explained below. First, the contents of the chemical components of the present steel sheet will be described in detail below:

C is an indispensable element for the intensification of the steel and below 0.15% by weight of C the retained austenite phase that acts to increase the ductility of the present steel cannot be fully obtained, whereas above 0.4% by weight of C the weldability is deteriorated and the steel is embrittled. Thus, 0.15 to 0.4% by weight of C must be added.

Si is effective for the formation and purification of the ferrite phase that contributes to an increase in the ductility with increasing Si content, and is also effective for the enrichment of C into the untransformed austenite phase to obtain a retained austenite phase. Below 0.5% by weight of Si this effect is not fully obtained, whereas above 2% by weight of Si this effect is saturated and the scale properties and the weldability are deteriorated. Thus, 0.5 to 2.0% by weight of Si must be added.

Mn contributes, as is well known, to the retaining of the austenite phase as an austenite-stabilizing element.

Below 0.5% by weight of Mn the effect is not fully obtained, whereas above 2% by weight of Mn the effect is saturated, resulting in adverse effects, such as deterioration of the weldability, etc. Thus, 0.5 to 2.0% by weight of Mn must be added.

Al is preferably added to the steel for deoxidation of the steel, in which case it is added in an amount of 0.004 to 0.040% by weight. Below 0.004% by weight of Al, the desired effect is not fully obtained, whereas above 0.040% by weight of Al the effect is saturated, resulting in an economically adverse effect.

S is a detrimental element to the hole expansibility. Above 0.010% by weight of S the hole expansibility is deteriorated. Thus, the S content must be decreased to not more than 0.010% by weight, and not more than 0.001% by weight of S is preferable.

In order to improve the hole expansibility, it is effective to reduce the S content, thereby reducing the content of sulfide-based inclusions and also to spheriodize the inclusions. For the spheriodization it is effective to add Ca or rare earth metal, which will be hereinafter referred to as "REM". Below 0.0005% by weight of Ca and 0.0050% by weight of REM, the spheroidization effect is not remarkable, whereas above 0.0100% by weight of Ca and 0.050% by weight of REM the spheroidization effect is saturated and the content of the inclusions are rather increased as an adverse effect. Thus, 0.0005 to 0.0100% by weight of Ca or 0.005 to 0.050% by weight of REM must be added.

Cr, V, Nb and Ti are elements which form carbides. Therefore, it is necessary that such an element is not intentionally added to the present steel as a carbide former.

The microstructure of the present steel sheet will be described in detail below.

On the basis of steel species A in the Example that follows, steel sheets were produced according to the present processes described as the means for attaining the object of the present invention, the means being composed of a fundamental idea in which the publicly known TR.I.P. (TRansformation Induced Plasticity) phenomenon is utilized. The TR.I.P. phenomenon means the following: when a steel sheet is subjected to working, a retained austenite is transformed into a martensite so that the steel sheet becomes hardened; and as a result, formation of a constriction, which would be formed at a local portion of the steel sheet by the working, is prevented, so that uniform elongation of the steel sheet is greatly improved and further it becomes hard to cause a rupture of the steel sheet by the working, resulting in the improvement of the total elongation of the steel sheet. The microstructure of the steel sheet which utilizes this TR.I.P. phenomenon is such that austenite, which is unstable for working carried out at ordinary temperature (which is transformed into martensite by being subjected to the working), is retained. In order to concretely establish the above-mentioned means, steel sheets were produced by various manufacturing processes, and also under the conditions approximate to those of the present processes, and such steel sheets were investigated. As a result, the present inventors have found the following facts.

In order to improve the ductility of steel sheets, it is necessary to form 5% by volume or more of a retained austenite phase in the present invention and it is desirable to stabilize the austenite phase through the enrichment of such elements as C, etc. To this effect, it is necessary (1) to form a ferrite phase, thereby promoting the enrichment of such elements as C, etc. into the austenite phase and contributing to the retaining of the austenite phase and (2) to promote the enrichment of such elements as C, etc. into the austenite phase with the progress of bainite phase transformation, thereby contributing to the retaining of the austenite phase.

In order to promote the enrichment of such elements as C, etc. into the austenite phase through the formation of the ferrite phase, thereby contributing to the retaining of the austenite phase, it is necessary to increase the ferrite volume fraction, and to make the ferrite grains finer, because the sites at which the C concentration is highest and the austenite phase is liable to be retained are the boundaries between the ferrite phase and the untransformed austenite phase, and the boundaries can be increased with increasing ferrite volume fraction and decreasing ferrite grain size.

In order at least to obtain TS.times.T.El>2,000 assuredly, it has been found that the ratio V.sub.PF /d.sub.PF, i.e. a ratio of polygonal ferrite volume fraction V.sub.PF (%) to polygonal ferrite grain size d.sub.PF (.mu.m), must be 7 or more, as obvious from FIG. 2 showing the test results obtained under the same conditions as in FIG. 1. Polygonal ferrite volume fraction and polygonal ferrite average grain size are determined on optical microscope pictures. Ferrite grain whose axis ratio (long axis/short axis)=1 to 3, is defined as polygonal ferrite.

Besides the ferrite phase and the retained austenite phase, the remainder must be a bainite phase that contributes to the concentration of such elements as C, etc. into the austenite phase, because C is enriched into the untransformed austenite phase with the progress of the bainite phase transformation, thereby stabilizing the austenite phase, that is, the bainite phase has a good effect upon the retaining of the austenite phase. It is necessary not to form any pearlite phase or martensite phase that reduce the retained austenite phase.

The process of the present invention will be described in detail below:

In order to increase the ferrite volume fraction V.sub.PF, low temperature rolling, rolling under a high pressure, and isothermal holding or slow cooling at a temperature around the nose temperature for the ferrite phase transformation (from Ar.sub.1 to Ar.sub.3) on a cooling table after the finish rolling, where the nose temperature for the ferrite phase transformation means a temperature at which the isothermal ferrite phase transformation starts and ends within a minimum time, are effective steps.

In order to make the ferrite grains finer, that is, to reduce d.sub.PF, low temperature rolling, rolling under a high reduction, rapid cooling around the Ar.sub.3 transformation point and rapid cooling after the ferrite phase transformation to avoid grain growth are effective steps. Thus, processes based on combinations of the former steps with the latter steps can be utilized.

Rolling temperature:

In order to increase the ferrite volume fraction and make the ferrite grains finer, low temperature rolling is effective. At a temperature lower than (Ar.sub.3 -50.degree. C.), the deformed ferrite is increased, deteriorating the ductility, whereas at a temperature higher than (Ar.sub.3 +50.degree. C.) the ferrite phase is not thoroughly formed. Thus, the effective finish rolling end temperature is any temperature within a range between (Ar.sub.3 +50.degree. C.) and (Ar.sub.3 -50.degree. C.). Furthermore, the ferrite formation and the refinement of ferrite grains can be promoted by setting the finish rolling start temperature to a temperature not higher than (Ar.sub.3 +100.degree. C.).

However, the low temperature rolling has operational drawbacks such as an increase in the rolling load, a difficulty in controlling the shape of the sheet, etc. when a thin steel sheet (sheet thickness .ltoreq.2 mm) is rolled, and particularly when a high carbon equivalent material or a high alloy material with a high deformation resistance is rolled. Thus, it is also effective to form the ferrite phase and make the ferrite grains finer by controlling the cooling on a cooling table after the hot finish rolling, as will be described later. In that case, a hot finish rolling end temperature exceeding Ar.sub.3 +50.degree. C. will not increase the aforementioned effect, but must be often employed on operational grounds.

Draft:

The formation of the ferrite phase and the refinement of finer ferrite grains can be promoted by making the total draft 80% or more in the hot finish rolling and a steel sheet with a good formability can be obtained thereby. Thus, the lower limit to the total draft is 80%.

Cooling:

Necessary ferrite formation and C enrichment for retaining the austenite phase are not fully carried out by cooling between Ar.sub.3 and Ar.sub.1 at a cooling rate of 40.degree. C./sec. or more after the hot rolling, and thus a step is carried out to cool or hold isothermally the steel down to T (Ar.sub.1 <T.ltoreq.lower temperature of Ar.sub.3 or the rolling end temperature) at a cooling rate of less than 40.degree. C./sec. along the temperature pattern, as shown in FIG. 6, after the hot rolling. More preferably, it is necessary that cooling is carried out for 3 to 25 seconds to cool the steel within a temperature range from the lower one of the Ar.sub.3 or the rolling end temperature to the temperature T or to hold the steel isothermally within said temperature range. When the cooling or the isothermal holding is carried out for 3 seconds or more, the ferrite formation and C enrichment are more sufficiently carried out. When the time of the cooling or isothermal holding exceeds 25 seconds, the length of the line from a finish rolling mill to a coiling machine becomes remarkably long. Thus, the upper limit to the time is 25 seconds. Incidentally, as means for conducting the cooling at a cooling rate of less than 40.degree. C./sec. or the isothermal holding, there are a heat-holding equipment using electric power, gas, oil and the like, a heat-insulating cover using heat-insulating material and the like, etc. A more desirable cooling pattern is as given in FIG. 7: the ferrite grains formed through the ferrite transformation can be made finer and the growth of grains including the ferrite grains, formed during the hot rolling, can be suppressed by carrying out the cooling down to T.sub.1 (Ar.sub.1 <T<lower one of Ar.sub.3 or the rolling end temperature) at a cooling rate of 40.degree. C./sec. or more after the hot rolling; and after that, the ferrite volume fraction can be increased around the ferrite transformation nose by carrying out the cooling down to T.sub.2 (Ar.sub.1 <T.sub.2 .ltoreq.T.sub.1) at a cooling rate of less than 40.degree. C./sec. or the isothermal holding, more preferably by carrying out the cooling or the isothermal holding within a temperature range from the temperature T.sub.1 to the temperature T.sub.2 for 3 to 25 seconds. In this manner, a steel sheet with a better formability can be obtained.

At a temperature above Ar.sub.3, no ferrite phase is formed even with cooling at a cooling rate of less than 40.degree. C./sec. or conducting the isothermal holding, and a pearlite phase is formed by cooling down to a temperature below Ar.sub.1 at a cooling rate of less than 40.degree. C./sec. or by conducting the isothermal holding at a temperature below Ar.sub.1. Thus, Ar.sub.1 <T.sub.2 .ltoreq.T.sub.1 <(the lower one of Ar.sub.3 or the finish rolling end temperature) is determined.

The successive cooling rate down to the coiling temperature is 40.degree. C./sec. or more from the viewpoint of avoiding formation of a pearlite phase and suppressing the grain growth. In case that the finish rolling end temperature is between not more than the Ar.sub.3 and above the (Ar.sub.3 -50.degree. C.), some deformed ferrite is formed. On the other hand, it is effective in recovering the ductility of the deformed ferrite that the step of cooling at a rate of less than 40.degree. C./sec. is performed within a temperature range from the finish rolling end temperature to more than Ar.sub.1. More preferably, it is effective that the cooling or isothermal holding is conducted for 3 to 25 seconds.

Results of rolling and cooling tests for steel species A that follows while changing the coiling temperature are shown in FIG. 3 and FIG. 4.

When the coiling temperature exceeds 500.degree. C., the bainite transformation excessively proceeds after the coiling, or a pearlite phase is formed, and consequently 5% by volume or more of the retained austenite phase cannot be obtained, as shown in FIG. 3. Thus, the upper limit to the coiling temperature is 500.degree. C. When the coiling temperature is not more than 350.degree. C., martensite is formed to deteriorate the hole expansibility, as shown in FIG. 4. Thus, the lower limit to the coiling temperature is over 350.degree. C.

In order to avoid excessive bainite transformation and retain a larger amount of the austenite phase, it is more effective to cool the steel sheet down to 200.degree. C. or less at a cooling rate of 30.degree. C./hr. or more by dipping in water, mist spraying, etc. after the coiling as shown in FIG. 3.

The present processes based on combinations of the foregoing steps are shown in FIG. 6 and FIG. 7, where the finish rolling end temperature is further classified into two groups, i.e. a lower temperature range (Ar.sub.3 .+-.50.degree. C.) and a higher temperature range {more than (Ar.sub.3 +50.degree. C.)}. Besides the foregoing 4 processes, a process in which the upper limit to the hot finish rolling start temperature is Ar.sub.3 +100.degree. C. or less and a process in which the cooling step after the coiling is limited or a process based on a combination of these two steps are available. Needless to say, a better effect can be obtained by a multiple combination of these process steps.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be described in detail, referring to an Example.

EXAMPLE

Steel sheets having a thickness of 1.4 to 6.0 mm were produced from steel species A to U having chemical components given in Table 1 under the conditions given in Tables 2-4 according to the process pattern given in FIG. 6 or FIG. 7, where the steel species C shows those whose C content is below the lower limit of the present invention, and the steel species F and I show those whose Si content is below the lower limit of the present invention and those whose Mn content is below the lower limit of the present invention, respectively.

The symbols given in Tables 2-4 have the following meanings:

FT.sub.0 : finish rolling start temperature (.degree.C.) FT.sub.7 : finish rolling end temperature (.degree.C.) CT: coiling temperature (.degree.C.) TS: tensile strength (kgf/mm.sup.2) T.El: total elongation (%) .gamma..sub.R : volume fraction of retained austenite (%) V.sub.PF : polygonal ferrite volume fraction (%) d.sub.PF : polygonal ferrite grain size (.mu.m).

In Table 1, the Ar.sub.1 temperature of steel species A was 650.degree. C. and the Ar.sub.3 temperature of this species was 800.degree. C.

The steel species according to the present invention are Nos. 1, 2, 4, 5, 7, 8, 10, 23 to 40, 42, 45, 46, 47, 49, 51, 52, 54, 55, and 57 to 80.

Initially TS.times.T.El.gtoreq.2,000 was aimed at, whereas much better strength-ductility balance such as TS.times.T.El>2,416 was obtained owing to the synergistic effect, as shown in FIG. 5. Particularly, Nos. 61 to 64, and 79 to 80, which are directed to steel species containing Ca, show that the amount of uniform elongation is 20% or more, and the amount of total elongation is 30% or more, and further the fluctuation of TS.times.El is small, so Nos. 61 to 64, 79 and 80 are steel species for working which are excellent especially in terms of a balance of strength and ductility.

In comparative Examples, no good ductility was obtained on the following individual grounds;

Nos. 3 and 56: the C content was too low. Nos. 6 and 50: the Si content was too low. Nos. 9 and 53: the Mn content was too low. No. 11: the total draft was too low at the finish rolling. No. 12: the finish rolling end temperature was too low. No. 13: the temperature T was too high. Nos. 14, 15, 16 and 48: the temperatures T and T.sub.2 were too low. Nos. 17 and 41: the cooling rate 1 was too high. Nos. 18 and 43: the cooling rate 2 was too low. No. 19: the cooling rate 2' was too high. No. 20: the cooling rate 3' was too low. Nos. 21 and 44: the coiling temperature was too high. No. 22: the coiling temperature was too low.

Furthermore, Nos. 26, 29, 33, 37 and 40 are examples of controlling the rolling start temperature and controlling the cooling step after the coiling, and Nos. 65 to 70 are examples of conducting the isothermal holding step in the course of the cooling step.

TABLE 1______________________________________ (wt %)Steel ComponentsSpecies C Si Mn P S Al Ca REM______________________________________A 0.20 1.5 1.5 0.015 0.001 -- -- --B 0.16 1.0 1.2 0.019 0.002 -- -- --C 0.14 1.0 1.2 0.020 0.003 -- -- --D 0.40 1.5 0.80 0.018 0.002 -- -- --E 0.20 0.6 1.80 0.012 0.002 -- -- --F 0.20 0.4 1.80 0.010 0.001 -- -- --G 0.19 2.0 1.0 0.015 0.003 -- -- --H 0.20 1.6 0.6 0.018 0.001 -- -- --I 0.20 1.6 0.4 0.016 0.002 -- -- --J 0.19 0.8 2.0 0.021 0.003 -- -- --K 0.19 1.5 1.5 0.020 0.003 -- -- 0.006L 0.21 1.4 1.6 0.015 0.001 -- 0.003 --M 0.20 1.4 1.5 0.015 0.001 0.028 -- --N 0.16 1.0 1.3 0.019 0.002 0.015 -- --O 0.40 1.5 0.80 0.016 0.002 0.012 -- --P 0.20 0.6 1.80 0.011 0.002 0.027 -- --Q 0.19 2.0 1.1 0.015 0.003 0.028 -- --R 0.20 1.7 0.6 0.018 0.001 0.025 -- --S 0.19 0.8 2.0 0.021 0.002 0.030 -- --T 0.19 1.5 1.6 0.020 0.003 0.022 -- 0.006U 0.21 1.4 1.7 0.015 0.001 0.034 0.003 --______________________________________ TABLE 2__________________________________________________________________________ Steel Total draft at FT.sub.0 FT.sub.7 T T.sub.1 T.sub.2 Cooling rate (.degree.C./s) CT CoolingItem No. species finishing (%) (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) 1 2 1' 2' 3' (.degree.C.) after__________________________________________________________________________ coilingThe invention 1 A 85 890 800 -- 750 655 -- -- 50 20 50 390 Air coolingThe invention 2 B 80 895 830 -- 770 660 -- -- 60 30 55 370 Air coolingComp. Ex. 3 C 80 895 790 -- 750 670 -- -- 55 15 50 450 40.degree.C./hrThe invention 4 D 81 880 825 -- 700 650 -- -- 85 25 80 470 Air coolingThe invention 5 E 85 885 810 -- 755 695 -- -- 70 25 70 370 Air coolingComp. Ex. 6 F 80 900 795 -- 720 670 -- -- 65 20 60 380 40.degree. C./hrThe invention 7 G 85 895 815 -- 735 665 -- -- 80 30 80 375 Air coolingThe invention 8 H 83 870 790 -- 720 665 -- -- 80 30 75 390 Air coolingComp. Ex. 9 I 80 890 805 -- 750 700 -- -- 75 25 65 410 40.degree. C./hrThe invention 10 J 87 880 785 -- 725 675 -- -- 70 20 65 430 Air coolingComp. Ex. 11 A 75 905 855 770 -- -- 30 50 -- -- -- 400 40.degree. C./hrComp. Ex. 12 A 85 895 745 -- 700 655 -- -- 60 20 55 390 40.degree. C./hrComp. Ex. 13 A 80 910 860 810 -- -- 20 60 -- -- -- 415 40.degree. C./hrComp. Ex. 14 A 80 905 865 630 -- -- 15 55 -- -- -- 385 40.degree. C./hrComp. Ex. 15 A 88 910 850 -- 800 630 -- -- 60 20 55 420 40.degree. C./hrComp. Ex. 16 A 85 910 810 -- 700 640 -- -- 85 30 75 400 40.degree. C./hrComp. Ex. 17 A 84 895 860 760 -- -- 45 80 -- -- -- 375 40.degree. C./hrComp. Ex. 18 A 90 890 855 750 -- -- 20 35 -- -- -- 380 35.degree. C./hrComp. Ex. 19 A 91 895 855 -- 720 655 -- -- 85 45 80 390 40.degree. C./hrComp. Ex. 20 A 89 880 815 -- 740 665 -- -- 60 30 35 370 40.degree. C./hrComp. Ex. 21 A 85 905 790 -- 730 660 -- -- 60 25 55 520 Air coolingComp. Ex. 22 A 93 910 785 -- 720 655 -- -- 75 30 70 330 Air coolingThe invention 23 A 87 915 800 750 -- -- 30 65 -- -- -- 400 Air coolingThe invention 24 A 84 895 815 720 -- -- 20 60 -- -- -- 415 Air coolingThe invention 25 A 85 905 840 765 -- -- 25 50 -- -- -- 500 40.degree. C./hrThe invention 26 A 90 895 825 740 -- -- 15 50 -- -- -- 350 35.degree. C./hrThe invention 27 A 85 910 830 -- 740 655 -- -- 50 30 45 385 Air coolingThe invention 28 A 92 905 820 -- 770 690 -- -- 70 35 65 425 40.degree. C./hrThe invention 29 A 93 890 850 -- 765 675 -- -- 55 15 50 465 40.degree. C./hrThe invention 30 A 90 910 855 755 -- -- 35 75 -- -- -- 370 Air coolingThe invention 31 A 90 895 860 770 -- -- 20 45 -- -- -- 470 Air coolingThe invention 32 A 80 905 855 650 -- -- 20 55 -- -- -- 455 40.degree. C./hrThe invention 33 A 85 900 865 800 -- -- 15 50 -- -- -- 395 35.degree. C./hrThe invention 34 A 85 915 860 -- 800 700 -- -- 60 20 55 370 Air coolingThe invention 35 A 90 895 870 -- 750 655 -- -- 65 20 65 390 Air coolingThe invention 36 A 85 905 875 -- 765 680 -- -- 65 20 65 410 40.degree. C./hrThe invention 37 A 80 900 875 -- 770 660 -- -- 55 15 55 415 40.degree.__________________________________________________________________________ C./hrItem No. TS (kgf/mm.sup.2) T.El (%) U.El (%) .sup..gamma. R (%) V.sub.PF /d.sub.PF TS__________________________________________________________________________ .times. T.ElThe invention 1 81 38 26 14 8.8 3078The invention 2 66 41 26 13 7.4 2706Comp. Ex. 3 63 36 21 4 7.2 2268The invention 4 101 31 21 13 8.0 3131The invention 5 79 39 26 13 8.3 3081Comp. Ex. 6 77 29 16 3 7.5 2233The invention 7 75 41 28 14 7.5 3075The invention 8 70 40 27 14 7.7 2800Comp. Ex. 9 68 31 16 4 7.6 2108The invention 10 83 37 25 13 7.9 3071Comp. Ex. 11 82 25 12 3 5.2 2050Comp. Ex. 12 86 22 10 4 8.5 1892Comp. Ex. 13 90 23 12 4 6.5 2070Comp. Ex. 14 79 26 13 3 7.7 2054Comp. Ex. 15 79 27 14 4 6.8 2133Comp. Ex. 16 80 29 17 4 8.0 2320Comp. Ex. 17 88 24 12 2 6.3 2112Comp. Ex. 18 82 26 14 2 8.1 2132Comp. Ex. 19 87 27 15 4 6.2 2349Comp. Ex. 20 79 29 16 4 7.3 2291Comp. Ex. 21 83 28 16 3 7.5 2324Comp. Ex. 22 93 25 14 3 7.6 2325The invention 23 82 35 23 12 7.7 2870The invention 24 81 37 25 13 8.0 2997The invention 25 82 38 26 13 8.1 3116The invention 26 86 37 25 15 8.1 3182The invention 27 85 35 23 14 7.3 2975The invention 28 81 39 27 15 8.1 3159The invention 29 79 41 28 16 8.8 3239The invention 30 84 30 20 6 7.2 2520The invention 31 82 34 22 11 7.4 2788The invention 32 83 35 23 12 8.0 2905The invention 33 82 36 24 14 7.9 2952The invention 34 85 33 21 11 7.7 2805The invention 35 83 35 23 12 7.8 2905The invention 36 84 35 23 13 8.0 2940The invention 37 83 37 25 14 8.1 3071__________________________________________________________________________ Steel Total draft at FT.sub.0 FT.sub.7 T T.sub.1 T.sub.2 Cooling rate (.degree.C./s) CT CoolingItem No. species finishing (%) (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) 1 2 1' 2' 3' (.degree.C.) after__________________________________________________________________________ coilingThe invention 38 A 80 910 865 700 -- -- 20 50 -- -- -- 360 Air coolingThe invention 39 A 82 890 850 690 -- -- 35 45 -- -- -- 370 Air coolingThe invention 40 A 83 890 850 690 -- -- 35 45 -- -- -- 370 40.degree. C./hrComp. Ex. 41 A 85 900 850 -- -- -- 45 45 -- -- -- 370 40.degree. C./hrThe invention 42 A 86 950 870 660 -- -- 15 45 -- -- -- 490 Air coolingComp. Ex. 43 A 90 950 870 680 -- -- 15 35 -- -- -- 490 Air coolingComp. Ex. 44 A 91 950 870 680 -- -- 15 45 -- -- -- 510 Air coolingThe invention 45 A 85 940 860 660 -- -- 20 80 -- -- -- 420 Air coolingThe invention 46 A 90 960 900 720 -- -- 15 70 -- -- -- 430 Air coolingThe invention 47 D 90 890 850 650 -- -- 15 50 -- -- -- 400 Air coolingComp. Ex. 48 D 92 920 850 630 -- -- 15 50 -- -- -- 400 Air coolingThe invention 49 E 95 950 860 680 -- -- 20 60 -- -- -- 390 Air coolingComp. Ex. 50 F 95 900 860 680 -- -- 20 60 -- -- -- 390 Air coolingThe invention 51 G 90 940 850 710 -- -- 10 45 -- -- -- 380 Air coolingThe invention 52 H 82 945 865 690 -- -- 15 55 -- -- -- 400 Air coolingComp. Ex. 53 I 85 920 865 690 -- -- 15 55 -- -- -- 400 Air coolingThe invention 54 J 89 910 860 700 -- -- 15 60 -- -- -- 380 Air coolingThe invention 55 B 88 930 855 700 -- -- 15 60 -- -- -- 400 Air coolingComp. Ex. 56 C 90 930 855 700 -- -- 15 60 -- -- -- 400 Air coolingThe invention 57 K 87 910 810 745 -- -- 30 65 -- -- -- 400 Air coolingThe invention 58 K 86 905 820 -- 745 650 -- -- 50 30 45 385 Air coolingThe invention 59 K 90 915 855 755 -- -- 35 75 -- -- -- 375 Air coolingThe invention 60 K 91 910 860 -- 800 700 -- -- 60 20 50 375 Air coolingThe invention 61 L 92 910 805 740 -- -- 30 60 -- -- -- 395 Air coolingThe invention 62 L 84 920 815 -- 750 655 -- -- 55 30 45 390 Air coolingThe invention 63 L 87 905 855 760 -- -- 35 75 -- -- -- 380 Air coolingThe invention 64 L 85 910 855 -- 800 695 -- -- 60 25 50 385 Air__________________________________________________________________________ coolingItem No. TS (kgf/mm.sup.2) T.El (%) U.El (%) .sup..gamma. R (%) V.sub.PF /d.sub.PF TS__________________________________________________________________________ .times. T.ElThe invention 38 86 31 20 9 7.3 2666The invention 39 81 35 23 11 7.6 2835The invention 40 82 37 25 13 8.6 3034Comp. Ex. 41 86 24 12 3 5.2 2064The invention 42 76 32 20 6 7.1 2432Comp. Ex. 43 75 29 16 4 7.8 2175Comp. Ex. 44 73 27 14 0 7.7 1971The invention 45 77 33 20 7 7.3 2541The invention 46 77 32 20 7 7.2 2464The invention 47 100 28 20 10 7.8 2800Comp. Ex. 48 101 22 12 4 8.0 2222The invention 49 80 31 20 6 7.3 2480Comp. Ex. 50 78 27 14 3 7.2 2106The invention 51 77 32 20 8 7.4 2464The invention 52 70 35 22 6 7.6 2450Comp. Ex. 53 69 31 16 4 7.7 2139The invention 54 84 30 20 7 8.0 2520The invention 55 67 37 22 6 7.9 2479Comp. Ex. 56 64 33 18 3 7.6 2112The invention 57 82 36 24 12 7.7 2952The invention 58 84 36 24 14 7.2 3024The invention 59 83 33 21 6 7.2 2739The invention 60 85 34 22 11 7.7 2890The invention 61 81 37 25 11 7.8 2997The invention 62 85 35 23 13 7.1 2975The invention 63 83 32 20 7 7.2 2656The invention 64 85 34 22 12 7.8 2890__________________________________________________________________________ TABLE 3__________________________________________________________________________ Total draft at FT.sub.0 FT.sub.7 T T.sub.1 T.sub.2 Cooling rate (.degree.C./s)Item No. Steel species finishing (%) (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) 1 2 1' 2' 3'__________________________________________________________________________The invention 65 A 83 910 790 790 -- -- Isothermal 55 -- -- -- holdingThe invention 66 A 85 910 790 790 -- -- Isothermal 60 -- -- -- holdingThe invention 67 A 84 905 790 790 -- -- Isothermal 62 -- -- -- holdingThe invention 68 A 90 925 830 -- 750 750 -- -- 70 Isothermal 70 holdingThe invention 69 A 95 940 865 790 -- -- 35 70 -- -- --The invention 70 A 93 950 870 -- 770 770 -- -- 80 Isothermal 65 holding__________________________________________________________________________ Holding Cooling TSItem No. time (sec.) CT (.degree.C.) after coiling (kgf/mm.sup.2) T.El (%) U.El (%) .sup..gamma. R V.sub.PF /d.sub.PF TS .times.__________________________________________________________________________ T.ElThe invention 65 2 380 Air cooling 80 36 24 12 7.6 2880The invention 66 3 385 Air cooling 80 38 26 13 7.7 3040The invention 67 25 380 Air cooling 81 40 28 15 7.8 3240The invention 68 5 400 Air cooling 81 39 27 14 8.0 3159The invention 69 7 420 Air cooling 85 33 21 12 7.5 2805The invention 70 5 430 Air cooling 82 36 24 13 7.7 2952__________________________________________________________________________ TABLE 4__________________________________________________________________________ Steel Total draft at FT.sub.0 FT.sub.7 T T.sub.1 T.sub.2 Cooling rate (.degree.C./s) CT CoolingItem No. species finishing (%) (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) 1 2 1' 2' 3' (.degree.C.) after__________________________________________________________________________ coilingThe invention 71 M 83 890 805 -- 750 655 -- -- 50 20 50 385 Air coolingThe invention 72 N 81 890 830 -- 770 660 -- -- 60 30 60 365 Air coolingThe invention 73 O 82 880 825 -- 700 655 -- -- 85 25 85 465 Air coolingThe invention 74 P 86 885 810 -- 750 695 -- -- 70 25 70 375 40.degree. C./hrThe invention 75 Q 84 895 810 -- 735 665 -- -- 80 30 80 380 40.degree. C./hrThe invention 76 R 86 870 785 -- 720 665 -- -- 80 30 80 395 Air coolingThe invention 77 S 88 910 860 705 -- -- 15 65 -- -- -- 385 Air coolingThe invention 78 T 88 890 805 745 -- -- 30 60 -- -- -- 410 40.degree. C./hrThe invention 79 U 93 890 805 740 -- -- 30 70 -- -- -- 390 Air coolingThe invention 80 U 85 920 815 -- 750 655 -- -- 55 30 50 390 40.degree.__________________________________________________________________________ C./hrItem No. TS (kgf/mm.sup.2) T.El (%) U.El (%) .sup..gamma. R (%) V.sub.PF /d.sub.PF TS__________________________________________________________________________ .times. T.ElThe invention 71 80 37 26 14 8.8 2960The invention 72 67 40 26 13 7.4 2680The invention 73 102 30 21 13 8.0 3060The invention 74 80 38 26 13 8.3 3040The invention 75 76 40 28 14 7.5 3040The invention 76 71 41 27 14 7.7 2911The invention 77 84 31 21 8 8.0 2604The invention 78 83 35 23 11 7.7 2905The invention 79 82 36 24 10 7.8 2952The invention 80 85 35 25 13 7.3 2975__________________________________________________________________________

As has been described above, a hot rolled steel sheet with a high strength and a particularly distinguished ductility (TS.times.T.El>2,416) can be produced with a high productivity and without requiring special alloy elements according to the present invention, and thus the present invention has a very important industrial significance.

Claims

1. A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises

subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature is within a range between Ar.sub.3 +50.degree. C. and Ar.sub.3 -50.degree. C.,

successively cooling the steel down to a desired temperature T within a temperature range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to Ar.sub.1 at a cooling rate of less than 40.degree. C./sec.,

successively cooling the steel at a cooling rate of 40.degree. C./sec. or more, and

coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C.

2. A process according to claim 1, wherein it is conducted for 3 to 25 seconds to cool said steel within a temperature range from the lower one of the Ar.sub.3 or said rolling end temperature to said desired temperature T or

to hold said steel isothermally within said temperature range.

3. A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises

subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and one of 0.0005 to 0.0100% by weight of Ca and 0.005 to 0.050% by weight of rare earth metal with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature is within a range between Ar.sub.3 +50.degree. C. and Ar.sub.3 -50.degree. C.,

successively cooling the steel down to a desired temperature T within a range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to Ar.sub.1 at a cooling rate of less than 40.degree. C./sec.,

successively cooling the steel at a cooling rate of 40.degree. C./sec. or more, and

coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C.

4. A process according to claim 3, wherein it is conducted for 3 to 25 seconds to cool said steel within a temperature range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to said desired temperature T or

to hold said steel isothermally within said temperature range.

5. A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises

subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature is within a range between Ar.sub.3 +50.degree. C. and Ar.sub.3 -50.degree. C.,

setting two desired temperatures T.sub.1 and T.sub.2, wherein T.sub.1 .gtoreq.T.sub.2 within a temperature range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to Ar.sub.1,

successively cooling the steel down to the T.sub.1 at a cooling rate of 40.degree. C./sec. or more,

successively cooling the steel down to the T.sub.2 at a cooling rate of less than 40.degree. C./sec.,

further cooling the steel at a cooling rate of 40.degree. C./sec. or more, and

coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C.

6. A process according to claim 5, wherein it is conducted for 3 to 25 seconds to cool said steel within a temperature range from said desired temperature T.sub.1 to said desired temperature T.sub.2 or

to hold said steel isothermally within said temperature range.

7. A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises

subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and one of 0.0005 to 0.0100% by weight of Ca and 0.005 to 0.050% by weight of rare earth metal with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature is within a range between Ar.sub.3 +50.degree. C. and Ar.sub.3 -50.degree. C.,

setting two desired temperatures T.sub.1 and T.sub.2, wherein T.sub.1 .gtoreq.T.sub.2 within a temperature range from the lower one of the Ar.sub.3 of said steel or said rolling end temperature to Ar.sub.1,

successively cooling the steel down to the T.sub.1 at a cooling rate of 40.degree. C./sec. or more,

successively cooling the steel down to the T.sub.2 at a cooling rate of less than 40.degree. C./sec.,

further cooling the steel at a cooling rate of 40.degree. C./sec. or more, and

coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C.

8. A process according to claim 7, wherein it is conducted for 3 to 25 seconds to cool said steel within a temperature range from said desired temperature T.sub.1 to said desired temperature T.sub.2 or

to hold said steel isothermally within said temperature range.

9. A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises

subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature exceeds Ar.sub.3 +50.degree. C.,

successively cooling the steel down to a desired temperature T within a temperature range from the Ar.sub.3 of the steel to Ar.sub.1 at a cooling rate of less than 40.degree. C./sec.,

successively cooling the steel at a cooling rate of 40.degree. C./sec. or more, and

coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C.

10. A process according to claim 9, wherein it is conducted for 3 to 25 seconds to cool said steel within a temperature range from the Ar.sub.3 of said steel to said desired temperature T or

to hold said steel isothermally within said temperature range.

11. A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises

subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and one of 0.0005 to 0.0100% by weight of Ca and 0.005 to 0.050% by weight of rare earth metal with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature exceeds Ar.sub.3 +50.degree. C.,

successively cooling the steel down to a desired temperature T within a range from the Ar.sub.3 of the steel to Ar.sub.1 at a cooling rate of less than 40.degree. C./sec.,

successively cooling the steel at a cooling rate of 40.degree. C./sec. or more, and

coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C.

12. A process according to claim 11, wherein it is conducted for 3 to 25 seconds to cool said steel within a temperature range from the Ar.sub.3 of said steel to said desired temperature T or

to hold said steel isothermally within said temperature range.

13. A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises

subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature exceeds Ar.sub.3 +50.degree. C.,

setting two desired temperatures T.sub.1 and T.sub.2, wherein T.sub.1 .gtoreq.T.sub.2 within a temperature range from the Ar.sub.3 of the steel to Ar.sub.1,

successively cooling the steel down to the T.sub.1 at a cooling rate of 40.degree. C./sec. or more,

successively cooling the steel down to the T.sub.2 at a cooling rate of less than 40.degree. C./sec.,

further cooling the steel at a cooling rate of 40.degree. C./sec. or more, and

coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C.

14. A process according to claim 13, wherein it is conducted for 3 to 25 seconds to cool said steel within a temperature range from said desired temperature T.sub.1 to said desired temperature T.sub.2 or

to hold said steel isothermally within said temperature range.

15. A process for producing a hot rolled steel sheet with a high strength and a distinguished formability, which comprises

subjecting a steel consisting essentially of 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, 0.5 to 2.0% by weight of Mn and one of 0.0005 to 0.0100% by weight of Ca and 0.005 to 0.050% by weight of rare earth metal with S being limited to not more than 0.010% by weight and the balance being iron and inevitable impurities to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature exceeds Ar.sub.3 +50.degree. C.,

setting two desired temperatures T.sub.1 and T.sub.2, wherein T.sub.1 .gtoreq.T.sub.2 within a temperature range from the Ar.sub.3 of the steel to Ar.sub.1,

successively cooling the steel down to the T.sub.1 at a cooling rate of 40.degree. C./sec. or more,

successively cooling the steel down to the T.sub.2 at a cooling rate of less than 40.degree. C./sec.,

further cooling the steel at a cooling rate of 40.degree. C./sec. or more, and

coiling the steel at a temperature of from over 350.degree. C. to 500.degree. C.

16. A process according to claim 15, wherein it is conducted for 3 to 25 seconds to cool said steel within a temperature range from said desired temperature T.sub.1 to said desired temperature T.sub.2 or

to hold said steel isothermally within said temperature range.

17. A process according to any one of claims 1 to 16, wherein a hot finish rolling starting temperature of the steel is set to not more than (Ar.sub.3 +100.degree. C.).

18. A process according to any one of claims 1 to 16, wherein the steel sheet after the coiling is cooled down to not more than 200.degree. C. at a cooling rate of 30.degree. C./hr. or more.

19. A process according to any one of claims 1 to 16, wherein said steel further contains 0.004 to 0.040% by weight of Al.

20. A process according to any one of claims 1 to 16, wherein said steel further contains 0.004 to 0.040% by weight of Al and a hot finish rolling starting temperature of the steel is set to not more than (Ar.sub.3 +100.degree. C.).

21. A process according to any one of claims 1 to 16, wherein said steel further contains 0.004 to 0.040% by weight of Al and the steel sheet after the coiling is cooled down to not more than 200.degree. C. at a cooling rate of 30.degree. C./hr. or more.