Steel pipe having high ductility and high strength and process for production thereof

The steel pipe has a structure composed mainly of ferrite or ferrite plus pearlite or ferrite plus cementite. The steel pipe is characterized by grain size not greater than 3 .mu.m, preferably not greater than 1 .mu.m, elongation greater than 20%, tensile strength (TS:MPa) and elongation (E1:%) whose product is greater than 10000, and percent ductile fracture greater than 95%, preferably 100%, measured by Charpy impact test on an actual pipe at -100.degree. C. The structure is characterized by C: 0.005-0.03%, Si: 0.01-3.0%, Mn: 0.01-2.0%, and Al: 0.001-0.10% on a weight basis, and is composed of ferrite or ferrite and a secondary phase, with ferrite grains being not greater than 3 .mu.m and the secondary phase having an areal ratio not more than 30%. A steel pipe stock having the above-mentioned composition is heated at a temperature of (Ac.sub.1 +50.degree. C.) to 400.degree. C. and subsequently reduced at a rolling temperature of (Ac.sub.1 +50.degree. C.) to 400.degree. C. such that the cumulative reduction of diameter is greater than 20%. The reducing is preferably performed such that at least one of rolling passes reduces the diameter by more than 6% per pass. The steel pipe will have high ductility and high strength and will be superior in toughness and stress corrosion cracking resistance, if the content of C, Si, Mn, and other alloying elements is limited low and reducing is performed at the temperature specified above. The resulting steel pipe has good fatigue resistance and is suitable for use as line pipe.

This application is a 371 of PC/JP98/01924 filed Apr. 27, 1998.
 1. Technical Field
 The present invention relates to a steel product which has high strength
 and high ductility and is superior in toughness and resistance to
 collision and impact, particularly a steel product, such as steel pipe,
 wire rod, steel bar, steel section, steel plate, and steel strip, having
 fine crystal grains, and also to a process for production thereof.
 2. Background Art
 Common practice to increase the strength of a steel product is to add an
 alloying element such as Mn and Si, to perform heat treatment such as
 controlled rolling, controlled cooling, quenching, and tempering, and to
 add a precipitation hardening element such as Nb and V. However, what is
 required of steel products is not only high strength but also high
 ductility and high toughness. There has been a demand for a steel product
 which has well-balanced strength and ductility/toughness.
 Making grains finer is one of a few important means to improve both
 strength and ductility/toughness. This is accomplished by performing
 austenite-ferrite transformation from fine austenite while preventing
 austenite grains from becoming coarse, thereby giving fine ferrite grains,
 by working which makes austenite grains finer, thereby giving fine ferrite
 grains, or by utilizing martensite and lower bainite that result from
 quenching and tempering.
 One of these methods in general use for steel production is controlled
 rolling which consists of strengthening in the austenite region and its
 ensuing austenite-ferrite transformation to give rise to fine ferrite
 grains. Another way in practice is to add a trace amount of Nb which
 suppresses the recrystallization of austenite grains, thereby yielding
 finer ferrite grains. Working at a temperature at which austenite does not
 yet recrystallize permits austenite grains to grow, giving rise to the
 deformation zone in grains, and finer ferrite grains occur from this
 deformation zone. A recent practice to obtain finer ferrite grains is
 controlled cooling that is carried out during or after working.
 The above-mentioned methods, however, need rebuilding of the existing
 facilities and considerable remodeling of the current process in the
 production of steel products, such as steel pipes, having improved
 collision and impact resistance required for better automotive safety, an
 ever increasing demand. Therefore, they are economically unfeasible.
 In the meantime, steel products for line pipe need resistance to stress
 corrosion cracking by sulfides, and this object is achieved by hardness
 control through the reduction of impurities or the adjustment of alloying
 elements. In addition, conventional practices to improve fatigue
 resistance include heat treatment, such as thermal refining, induction
 hardening, and carburizing, and addition of a large amount of expensive
 alloying elements such as Ni, Cr, and Mo. The disadvantage of these
 methods is poor weldability and high production cost.
 Steel pipes of small to medium diameter are produced mainly by electric
 resistance welding with high frequency current. The process for their
 production consists of continuously feeding a flat strip steel, making it
 into a pipe stock using a forming roll, heating the opposing edges of the
 pipe stock to a temperature above the melting point of steel by means of
 high frequency current, and butt-welding the heated edges by means of
 squeeze rolls.
 This process, however, has a disadvantage of requiring rolls that conform
 to the dimensions of the desired steel pipe; therefore, it is not suitable
 for multi-product production in small lots.
 In order to address this problem, there has been proposed a new process in,
 for example, Japanese Patent Publication No. 24606/1990. This process
 consists of heating a flat strip steel in a preheating furnace and a
 heating furnace, making the strip steel into a pipe by electric resistance
 welding, heating the pipe to a temperature above the A.sub.3
 transformation point, and rolling the heated pipe by a reducing mill so
 that it has a predetermined outside diameter.
 This process, however, poses problems due to heating above A.sub.3 point.
 Heating gives rise to scale which is bitten during rolling. Heating also
 makes crystal grains coarse, aggravating the ductility, strength, and
 toughness of the resulting steel pipe.
 A cold sizing process has been proposed in Japanese Patent Laid-open No.
 33105/1988. This process is designed to reduce the outside diameter of a
 hollow pipe stock, such as seamless steel pipes and electric welded pipes,
 in the cold state by using a series of reducing mills, each consisting of
 three rolls. The disadvantage of this process is the necessity of a
 large-scale mill to withstand high loads due to cold rolling and the
 necessity of a lubricating facility to prevent rolls from seizing. In
 addition, cold rolling gives rise to working strain, which aggravates
 ductility and toughness.
 It is an object of the present invention, which was completed to address
 the above-mentioned problems, to provide a steel product and a process for
 production thereof, said steel product being superior in ductility,
 strength, toughness, and resistance to collision and impact owing to fine
 ferrite crystal grains.
 DISCLOSURE OF THE INVENTION
 The present inventors carried out extensive studies on a process for
 efficient production of high-strength steel pipes superior in ductility,
 which led to the finding that it is possible to produce desired steel
 pipes with balanced ductility and strength by reducing steel pipes of
 specific composition at a temperature of ferrite recrystallization.
 The present invention is based on the experimental results explained below.
 The experiment was carried out on electric welded steel pipes (42.7 mm in
 dia. and 2.9 mm thick) containing 0.09 wt % C, 0.40 wt % Si, 0.80 wt % Mn,
 and 0.04 wt % Al. After heating at various temperatures ranging from 750
 to 400.degree. C., they were passed through a reducing mill at a rolling
 speed of 200 m/min so that their outside diameter was reduced variously to
 33.2-15.0 mm. The rolled pipes were tested for tensile strength (TS) and
 elongation (El). The relation between elongation and tensile strength is
 shown in FIG. 1 (black dots). Incidentally, white dots in FIG. 1 represent
 the relation between elongation and tensile strength of electric welded
 pipes in various sizes without reducing. Elongation (El) is expressed in
 terms of values calculated from
EQU El=El.sub.0.times.( (a.sub.0 /a))0.4
 (where El.sub.0 is the actually measured elongation, a.sub.0 is 292
 mm.sup.2, and a is the sectional area (mm.sup.2) of the specimen.) This
 converted value was used in consideration of the size effect of the
 specimen.
 It is noted from FIG. 1 that the specimens obtained by reducing after
 heating at 750-400.degree. C. exhibit higher elongation for the same
 strength than electric welded pipes without reducing. In other words, the
 present inventors found that it is possible to produce high-strength steel
 pipes with balanced ductility and strength by reducing steel pipes of
 specific composition at a temperature ranging from 750.degree. C. to
 400.degree. C.
 Moreover, it was found that the above-mentioned steel pipe has fine ferrite
 grains not greater than 3 .mu.m,. In order to examine resistance to
 collision and impact, the present inventors established the relation
 between tensile strength (TS) and ferrite grain size, with the strain rate
 greatly changed over a broad range (2000 s.sup.-1). The results are shown
 in FIG. 2. It is noted from FIG. 2 that the tensile strength remarkably
 increases with the decreasing ferrite grain size not more than 3 .mu.m,
 preferably not more than 1 .mu.m, and this tendency is significant in the
 case of high strain rate as is experienced in deformation by collision and
 impact. In other words, it was found that steel pipes with fine ferrite
 grains are superior in ductile-strength balance and have greatly improved
 resistance to collision and impact.
 The present invention is based on the above-mentioned findings.
 The present invention covers a steel product with high ductility and high
 strength which is characterized in that it has an average grain size lower
 than 3 .mu.m, preferably lower than 1 .mu.m, in the cross section
 perpendicular to its lengthwise direction, that it has a structure
 composed mainly of ferrite or ferrite plus pearlite or ferrite plus
 cementite, and that it has an elongation 20% or more and a product of
 tensile strength (TS in MPa) and elongation (El in %) which is 10000 or
 more.
 The present invention also covers a steel pipe with high ductility and high
 strength which is characterized in that it has an average grain size lower
 than 3 .mu.m, preferably lower than 1 .mu.m, in the cross section
 perpendicular to its lengthwise direction, that it has a structure
 composed mainly of ferrite or ferrite plus pearlite or ferrite plus
 cementite, that it has an elongation greater than 20% and a product of
 tensile strength (TS in MPa) and elongation (El in %) which is 10000 or
 more, and that it has a percent ductile fracture by Charpy impact test of
 95% or more, preferably 100%, in the cross section perpendicular to its
 lengthwise direction.
 The present invention also covers a process for producing a steel product,
 preferably a steel pipe, with high ductility and high strength, said
 process comprising rolling a steel product containing C not more than 0.60
 wt % at a temperature for ferrite recrystallization with a reduction of
 area greater than 20%. Said rolling may be carried out by the aid of
 lubrication.
 The present invention also covers a steel pipe with high ductility and high
 strength characterized in that it has a composition of C 0.005-0.30%, Si
 0.01-3.0%, Mn 0.01-2.0%, Al 0.001-0.10% on a weight basis, with the
 remainder being Fe and unavoidable impurities, and that it has a structure
 of ferrite or a structure of ferrite and a second phase other than ferrite
 not more than 30% in terms of areal ratio, with said ferrite having a
 grain size not greater than 3 .mu.m, preferably not greater than 1 .mu.m.
 In the present invention, the above-mentioned composition may be C
 0.005-0.30%, Si 0.01-3.0%, Mn 0.01-2.0%, Al 0.001-0.10%, and one or more
 selected from Cu not more than 1%, Ni not more than 2%, Cr not more than
 2%, and Mo not more than 1%, with the remainder being Fe and unavoidable
 impurities;
 the above-mentioned composition may be C 0.005-0.30%, Si 0.01-3.0%, Mn
 0.01-2.0%, Al 0.001-0.10%, and one or more selected from Nb not more than
 0.1%, V not more than 0.3%, Ti not more than 0.2%, and B n not more than
 0.004%, with the remainder being Fe and unavoidable impurities;
 or the above-mentioned composition may be C 0.005-0.30%, Si 0.01-3.0%, Mn
 0.01-2.0%, Al 0.001-0.10%, and one or more selected from REM not more than
 0.02% and Ca not more than 0.01%, with the remainder being Fe and
 unavoidable impurities.
 The above-mentioned composition may be C 0.005-0.30%, Si 0.01-3.0%, Mn
 0.01-2.0%, Al 0.001-0.10%, and one or more selected from Cu not more than
 1%, Ni not more than 2%, Cr not more than 2%, and Mo not more than 1% and
 one or more selected from Nb not more than 0.1%, V not more than 0.3%, Ti
 not more than 0.2%, and B not more than 0.004%.
 The above-mentioned composition may be C 0.005-0.30%, Si 0.01-3.0%, Mn
 0.01-2.0%, Al 0.001-0.10%, and one or more selected from Cu not more than
 1%, Ni not more than 2%, Cr not more than 2%, and Mo not more than 1% ,and
 one or more selected from REM not more than 0.02% and Ca not more than
 0.01%, with the remainder being Fe and unavoidable impurities.
 The above-mentioned composition may be C 0.005-0.30%, Si 0.01-3.0%, Mn
 0.01-2.0%, Al 0.001-0.10%,and one or more selected from Nb not more than
 0.1%, V not more than 0.3%, Ti not more than 0.2%, and B not more than
 0.004%, and one or more selected from REM not more than 0.02% and Ca not
 more than 0.01%, with the remainder being Fe and unavoidable impurities.
 Moreover, the above-mentioned composition may be C 0.005-0.30%, Si
 0.01-3.0%, Mn 0.01-2.0%, Al 0.001-0.10%, one or more selected from Cu not
 more than 1%, Ni not more than 2%, Cr not more than 2%, and Mo not more
 than 1%, one or more selected from Nb not more than 0.1%, V not more than
 0.3%, Ti not more than 0.2%, and B not more than 0.004%; and one or more
 selected from REM not more than 0.02% and Ca not more than 0.01%, with the
 remainder being Fe and unavoidable impurities.
 The present invention also covers a process for producing a steel pipe with
 high ductility and high strength, said process comprising heating a pipe
 stock having any of the above-mentioned compositions at (Ac.sub.1
 +50.degree. C.) to 400.degree. C., preferably 750-400.degree. C., and
 reducing the heated pipe stock at (Ac.sub.1 +50.degree. C.) to 400.degree.
 C., preferably 750-400.degree. C., such that the cumulative diameter
 reduction is 20% or more. The rolling is preferably carried out such that
 at least one pass reduces the diameter by 6% or more per pass and the
 cumulative diameter reduction is 60% or more. In addition, the reducing
 mentioned above is preferably carried out by the aid of lubrication.
 The present inventors also found that the above-mentioned process permits
 the production of a steel pipe with high strength and high toughness and
 superior resistance to stress corrosion cracking if the composition of the
 pipe stock is specified in an adequate range. This finding led the present
 inventors to conceive to utilize the process for the production of line
 pipes.
 Line pipes conventionally have the content of impurities, such as S,
 reduced and the hardness controlled by means of alloying elements for
 improvement in resistance to stress corrosion cracking. Such conventional
 methods are limited in strengthening and pose a problem with high
 production cost. Specifying the composition of the pipe stock in an
 adequate range and performing the reduction in the ferrite recrystallizing
 region yield a line pipe with high strength and high toughness, owing to
 dispersed fine ferrite and fine carbide, superior in resistance to stress
 corrosion cracking resistance due to limited alloying elements, leading to
 reduced hardening by welding and less crack generation and propagation.
 Accordingly, the present invention covers a process for producing a steel
 pipe superior in ductility and resistance to collision and impact as well
 as resistance to stress corrosion cracking resistance, said process
 comprising heating a pipe stock at (Ac.sub.1 +50.degree. C.) to
 400.degree. C., preferably 750-400.degree. C., and reducing the heated
 pipe stock at (Ac.sub.1 +50.degree. C.) to 400.degree. C., preferably
 750-400.degree. C., such that the cumulative diameter reduction is 20% or
 more, said pipe stock having a composition of C 0.005-0.10%, Si 0.01-0.5%,
 Mn 0.01-1.8%, Al 0.001-0.10%, one or more selected from Cu not more than
 0.5%, Ni not more than 0.6%, Cr not more than 0.5%, and Mo not more than
 0.5%, and one or more selected from Nb not more than 0.1%, V not more than
 0.1%, Ti not more than 0.1%, and B not more than 0.004%, or further one or
 more selected from REM not more than 0.02% and Ca not more than 0.01%,
 with the remainder being Fe and unavoidable impurities.
 The present inventors also found that the above-mentioned process permits
 the production of a steel pipe with high strength and high toughness and
 superior fatigue resistance if the composition of the pipe stock is
 specified in an adequate range. This finding led the present inventors to
 conceive to utilize the process for the production of steel pipes with
 high fatigue resistance. Specifying the composition of the pipe stock in
 an adequate range and performing the reduction in the ferrite
 recrystallizing region yield a steel pipe with high strength and high
 toughness, owing to dispersed fine ferrite and fine precipitation,
 superior in fatigue resistance due to limited alloying elements, leading
 to reduced hardening by welding and less crack generation and propagation.
 Accordingly, the present invention covers a process for producing a steel
 pipe superior in ductility and strength as well as fatigue resistance,
 said process comprising heating a pipe stock at (Ac.sub.1 +50.degree. C.)
 to 400.degree. C., preferably 750-400.degree. C., and reducing the heated
 pipe stock at (Ac.sub.1 +50.degree. C.) to 400.degree. C., preferably
 750-400.degree. C., such that the cumulative diameter reduction is 20% or
 more, said pipe stock having a composition of C 0.06-0.30%, Si
 0.01-1.5%,Mn 0.01-2.0%, and Al 0.001-0.10%, with the remainder being Fe
 and unavoidable impurities.
 In the present invention, the above-mentioned composition may be C
 0.06-0.30%, Si 0.01-1.5%, Mn 0.01-2.0%,Al 0.001-0.10%, and one or more
 selected from Cu not more than 1.0%, Ni not more than 2.0%, Cr not more
 than 2.0%, and Mo not more than 1.0%, with the remainder being Fe and
 unavoidable impurities; the above-mentioned composition may be C
 0.06-0.30%, Si 0.01-1.5%, Mn 0.01-2.0%, Al 0.001-0.10%, and one or more
 selected from Nb not more than 0.1%, V not more than 0.3%, Ti not more
 than 0.2%, and B not more than 0.004%, with the remainder being Fe and
 unavoidable impurities; or the above-mentioned composition may be C
 0.06-0.30%, Si 0.01-1.5%, Mn 0.01-2.0%, Al 0.001-0.10%, and one or more
 selected from REM not more than 0.02% and Ca not more than 0.01%, with the
 remainder being Fe and unavoidable impurities; the above-mentioned
 composition may be C 0.06-0.30%, Si 0.01-1.5%, Mn 0.01-2.0%, Al
 0.001-0.10%, one or more selected from Cu not more than 1.0%, Ni not more
 than 2.0%, Cr not more than 2.0%, and Mo not more than 1.0%, and one or
 more selected from Nb not more than 0.1%, V not more than 0.3%, Ti not
 more than 0.2%, and B not more than 0.004%, with the remainder being Fe
 and unavoidable impurities; the above-mentioned composition may be C
 0.06-0.30%, Si 0.01-3.0%, Mn 0.01-2.0%, Al 0.001-0.10%, one or more
 selected from Nb not more than 0.1%, V not more than 0.3%, Ti not more
 than 0.2%, and B not more than 0.004%, and one or more selected from REM
 not more than 0.02% and Ca not more than 0.01%, with the remainder being
 Fe and unavoidable impurities; the above-mentioned composition may be C
 0.06-0.30%, Si 0.01-1.5%, Mn 0.01-2.0%, Al 0.001-0.10%, one or more
 selected from Cu not more than 1.0%, Ni not more than 2.0%, and Mo not
 more than 1.0%, Mo not more than 1.0%, and one or more selected from REM
 not more than 0.02% and Ca not more than 0.01%, with the remainder being
 Fe and unavoidable impurities, or the above-mentioned composition may be C
 0.06-0.30%, Si 0.01-1.5%, Mn 0.01-2.0%, Al 0.001-0.10%, one or more
 selected from Cu not more than 1.0%, Ni not more than 2.0%, Cr not more
 than 2.0%, and Mo not more than 1.0%, one or more selected from Nb not
 more than 0.1%, V not more than 0.3%, Ti not more than 0.2%, and B not
 more than 0.004%, and one or more selected from REM not more than 0.02%
 and Ca not more than 0.01%, with the remainder being Fe and unavoidable
 impurities.

BEST MODE FOR CARRYING OUT THE INVENTION
 The following explanation shows the process of producing steel products
 according to the present invention.
 The steel product of the present invention has a structure composed mainly
 of ferrite or ferrite plus pearlite or ferrite plus cementite; therefore,
 it is not specifically restricted in its chemical composition so long as
 it has the structure mentioned above. A preferred composition to give the
 structure of ferrite or ferrite plus pearlite or ferrite plus cementite is
 one which contains C not more than 0.60 wt %, preferably not more than
 0.20 wt %, more preferably not more than 0.10 wt %. Another preferred
 composition is one which contains Si not more than 2.0 wt %, Mn not more
 than 2.0 wt %, Al not more than 0.10 wt %, Cu not more than 1.0 wt %, Ni
 not more than 2.0 wt %, Cr not more than 3.0 wt %, Mo not more than 2.0 wt
 %, Nb not more than 0.1 wt %, V not more than 0.5 wt %, Ti not more than
 0.1 wt %, and B not more than 0.005 wt %. And, the structure may contain,
 in addition to ferrite, pearlite, and cementite, not more than 30 vol % of
 bainite without restriction. Needless to say, the structure composed
 mainly of ferrite plus pearlite or the structure composed mainly of
 ferrite plus cementite may contain a small amount of cementite or
 pearlite, respectively.
 According to the present invention, the steel product is heated to a
 temperature, preferably, 800.degree. C. or lower, and then rolled into a
 desired shape. The heating method is not specifically restricted; however,
 induction heating is desirable because of its high heating speed and its
 ability to suppress the growth of crystal grains. The heating temperature
 is preferably 800.degree. C. or lower at which crystal grains do not
 become coarse, so that the grain size in the raw material is kept not
 greater than 20 .mu.m. This results in fine ferrite grains not greater
 than 3 .mu.m, preferably not greater than 1 .mu.m, after subsequent
 ferrite recrystallization. The lower limit of the heating temperature is
 400.degree. C., preferably 550.degree. C., because with heating under
 400.degree. C., the steel product presents difficulties in rolling due to
 increase in deformation resistance. Consequently, the heating temperature
 for rolling is 400-800.degree. C., preferably 600-700.degree. C. Heating
 is carried out such that the austenitic change is 25% or less.
 The rolling temperature is restricted to a range in which ferrite
 recrystallization takes place. In the present invention, this range is
 preferably 400-750.degree. C., depending on the chemical composition of
 the steel blank used. Rolling at a temperature higher than this range
 gives rise to a two-phase structure of ferrite plus austenite containing a
 large amount of austenite or a single-phase structure of austenite. The
 resulting product does not have the structure composed mainly of ferrite
 or ferrite plus pearlite or ferrite plus cementite. On the other hand,
 rolling at a temperature exceeding 750.degree. C. causes ferrite grains to
 grow remarkably after recrystallization. This is detrimental to the
 desired fine grains not greater than 3 .mu.m, preferably not greater than
 2 .mu.m. Rolling at a temperature lower than 400.degree. C. is difficult
 to carry out due to blue shortness, with decrease in ductility and
 toughness on account of insufficient recrystallization and residual
 deformation strain. Therefore, the rolling temperature is 400-750.degree.
 C., preferably 560-720.degree. C., more preferably 600-700.degree. C. At
 560-720.degree. C., the grain size will be not greater than 1 .mu.m, and
 at 600-700.degree. C., the grain size will be not greater than 0.8 .mu.m,.
 FIG. 3 schematically shows the relation between the grain size and the
 rolling temperature (at the start and end of rolling).
 Rolling is carried out such that the reduction of area is greater than 20%.
 In the present invention, the reduction of area is defined as the value
 calculated by the formula (A.sub.0 -A)/A.times.100, where A.sub.0 is the
 cross sectional area before rolling and A is the cross sectional area
 after rolling. With a reduction of area less than 20%, rolling does not
 make recrystallized grains finer because of insufficient strain. The
 reduction of area is preferably greater than 50%.
 After rolling, the steel product is cooled to room temperature. Cooling may
 be natural air cooling or any of known forced air cooling, water cooling,
 and mist cooling. The latter is desirable to suppress the growth of
 grains. The cooling rate is preferably greater than 1.degree. C./s.
 An appropriate rolling method may be selected according to the shape of the
 stock. For steel pipe stocks, reducing by means of a plurality of grooved
 rolls, called as a reducer, is desirable. Stocks adequate for this process
 include electric resistance welded pipes, forge-welded steel pipes, and
 solid phase pressure-welded steel pipes.
 According to the present invention, rolling is carried out with
 lubrication. Lubricated rolling ensures uniform distribution of strain and
 grain size in the thickness direction. Rolling without lubrication tends
 to cause concentrated strain in the surface and uneven grain size
 distribution in the thickness direction. Ordinary rolling oils, such as
 mineral oil and synthetic ester, may be used for lubricated rolling. They
 are not specifically restricted.
 The above-mentioned process yields a high-toughness, high-ductility steel
 product which has a structure composed mainly of ferrite or ferrite plus
 pearlite or ferrite plus cementite, and which has an average grain size
 not greater than 3 .mu.m, preferably not greater than 1 .mu.m, in the
 cross section perpendicular to the lengthwise direction of the steel
 product. The steel product of the present invention may have a structure
 which contains not more than 30% of bainite in addition to ferrite,
 pearlite, and cementite. The steel product will increase in strength but
 decrease in toughness and ductility if it contains bainite more than
 specified above and martensite.
 With an average grain size in excess of 3 .mu.m, the steel product will
 lose a balance between strength and toughness/ductility; that is, it does
 not meet the requirement that elongation is 20% or more and the product of
 tensile strength (TS: MPa) and elongation (El: %) is 10000 or more. A
 large average grain size leads to brittle cracking that occurs in the
 cross section in the lengthwise direction of the steel pipe during Charpy
 impact test at -100.degree. C. This implies a failure to meet the
 requirement for toughness that the percent ductile fracture is 95% or
 more, preferably 100%. With an average grain size not greater than 3
 .mu.m, preferably not greater than 1 .mu.m, the steel pipe is less
 vulnerable to brittle cracking in the cross section perpendicular to the
 lengthwise direction and is superior in toughness.
 The process of the present invention for producing steel products will be
 described in more detail in the following, with stress placed on steel
 pipes.
 The present invention employs steel pipes as the stock. There are no
 specific restrictions on the process of producing steel pipe stocks.
 Adequate examples include electric resistance welded steel pipes produced
 by electric resistance with high frequency current, solid-phase
 pressure-welded steel pipes produced by pressure welding after heating
 edges to a temperature suitable for solid-phase pressure-welding,
 forge-welded steel pipes, and seamless steel pipes produced by Mannesmann
 piercing rolling.
 The following explains the reason why the chemical composition is
 restricted for the steel pipes as stock and product.
 C: 0.005-0.30%
 C is an element which dissolves in the basic metal to form a solid solution
 or precipitates in the form of carbide in the basic metal, thereby
 increasing the strength of steel. Cementite, martensite, and bainite that
 precipitate in the form of fine grains as the hard secondary phase
 contribute to ductility (uniform elongation). For the desired strength and
 ductility due to cementite that precipitates as the secondary phase, the
 content of C is 0.005% or more, preferably 0.04% or more. C in excess of
 0.30% increases strength so much as to adversely affect ductility.
 Therefore, the content of C is limited to 0.005-0.30%, preferably
 0.04-0.30%. Moreover, the content of C is not more than 0.10% for the
 improvement of line pipe in resistance to stress corrosion cracking. C in
 excess of 0.10% makes the weld zone hard, thereby adversely affecting
 resistance to stress corrosion cracking.
 For the steel pipe to have high fatigue strength and improved fatigue
 resistance characteristics, the content of C is preferably 0.06-0.30%. A
 content less than 0.06% leads to poor fatigue resistance characteristics
 due to strength.
 Si: 0.01-3.0%
 Si is an element that functions as a deoxidizer and also forms a solid
 solution in the basic metal to increase the strength of steel. It produces
 its effect when its content is 0.01%or more, preferably 0.1% or more. With
 a content in excess of 3.0%, it adversely affects ductility. Therefore,
 the content of Si is limited to 0.01-3.0%, preferably 0.1-1.5%.
 Incidentally, the content of Si is not more than 0.5% for line pipes to
 have improve resistance to stress corrosion cracking. Si in excess of 0.5%
 makes the weld zone hard, thereby adversely affecting resistance to stress
 corrosion cracking.
 For the steel pipe to have high fatigue strength and improved fatigue
 resistance characteristics, the content of Si is preferably not more than
 1.5%. A content in excess of 1.5% leads to poor fatigue resistance
 characteristics because it forms inclusions.
 Mn: 0.01-2.0%
 Mn is an element to increase the strength of steel. In the present
 invention, it also causes cementite as the secondary phase to precipitate
 in the form of fine grains and promotes the precipitation of martensite
 and bainite. With an amount less than 0.01%, it does not increase the
 strength, nor does it promote the precipitation of cementite, martensite,
 and bainite. With an amount in excess of 2.0%, it adversely affects
 ductility due to unduly increased excessive strength. Therefore, the
 amount of Mn is limited to 0.01-2.0%. From the standpoint of
 strength-elongation balance, it is 0.2-1.3%, preferably 0.6-1.3%.
 Incidentally, the content of Mn is preferably not more than 1.8% for line
 pipes to have improved resistance to stress corrosion cracking. Mn in
 excess of 1.8% makes the weld zone hard, thereby adversely affecting
 resistance to stress corrosion cracking.
 Al: 0.001-0.10%
 Al helps form fine grains. The content of Al is at least 0.001% for desired
 fine grains. With a content in excess of 0.10%, it increases the amount of
 oxygen-based inclusions, thereby adversely affecting cleanliness.
 Therefore, the content of Al is limited to 0.001-0.10%, preferably
 0.015-0.06%.
 Furthermore, the above-mentioned composition for the steel pipe stock may
 contain additionally one or more of the following alloying elements.
 Cu: not more than 1%, Ni: not more than 2%, Cr: not more than 2%, and Mo:
 not more than 1%.
 These elements improve the hardenability of steel and increase the strength
 of steel. They may be used alone or in combination with one another
 according to need. They lower the transformation point and give rise to
 fine ferrite grains and make the secondary phase fine grains. The content
 of Cu is not more than 1%, preferably 0.1-0.6%, because excessive Cu
 adversely affects hot workability. The content of Ni is not more than 2%,
 preferably 0.1-1.0%, because excessive Ni is wasted without further effect
 of increasing strength and improving toughness. The contents of Cr and Mo
 are not more than 2% and 1%, respectively, preferably 0.1-1.5% and
 0.05-0.5%, respectively; excessive Cr and Mo adversely affect weldability
 and ductility only to be wasted.
 Incidentally, each of the contents of Cu, Ni, Cr, and Mo is not more than
 0.5% for line pipes to have improved resistance to stress corrosion
 cracking. When used in excess of 0.5%, they make the weld zone hard,
 thereby adversely affecting resistance to stress corrosion cracking.
 Nb: not more than 0.1%, V not more than 0.3%, Ti: not more than 0.2%, and
 B: not more than 0.004%.
 These elements precipitate in the form of carbide, nitride, or
 carbonitride, contributing to fine grains and high strength. For steel
 pipes having joints heated at a high temperature, they make grains finer
 during heating and they also function as nuclei for ferrite precipitation
 during cooling, thereby preventing the weld zone from becoming hard. They
 may be used alone or in combination with one another according to need.
 When used excessively, they adversely affect weldability and toughness.
 Therefore, the content of Nb is not more than 0.1%, preferably
 0.005-0.05%; the content of V is not more than 0.3%, preferably 0.05-0.1%;
 the content of Ti is not more than 0.2%, preferably 0.005-0.10%; and the
 content of B is not more than 0.004%, preferably 0.0005-0.002%.
 Incidentally, each content of Ni, V, and Ti is not more than 0.1% for line
 pipes to have improved resistance to stress corrosion cracking. When used
 in excess of 0.1%, they adversely affecting resistance to stress corrosion
 cracking due to precipitation hardening.
 REM: not more than 0.02% and Ca: not more than 0.01%.
 Both REM and Ca adjust the form of inclusions and improve workability. They
 also precipitate in the form of sulfide, oxide or oxysulfide, thereby
 preventing the joints of steel pipe from becoming hard. They may be used
 alone or in combination with one another. When used excessively, they give
 rise to excessive inclusions, which lower cleanliness and adversely affect
 ductility. The content of REM is 0.004-0.02% and the content of Ca is
 0.001-0.01%.
 The above-mentioned composition for the steel pipe stock and steel product
 may additionally contain Fe as a remainder and unavoidable impurities as
 follows.
 Unavoidable impurities are N: not more than 0.010%, O: not more than
 0.006%, P: not more than 0.025%, and S: not more than 0.020%.
 N: not more than 0.010%
 N in an amount up to 0.010% is permissible, which is enough to form fine
 grains in combination with Al; however, excessive N adversely affects
 ductility. The content of N is not more than 0.010%, preferably
 0.002-0.006%.
 O: not more than 0.006%
 O in an amount up to 0.006% is permissible. The content of O is as low as
 possible, because O forms oxides which adversely affect cleanliness.
 P: not more than 0.025%
 P segregates at grain boundaries, thereby adversely affecting toughness.
 The content of P is as low as possible, although up to 0.025% is
 permissible.
 S: not more than 0.020%
 S in an amount up to 0.020% is permissible. The content of S is as low as
 possible, because S forms sulfides which adversely affect cleanliness.
 The following concerns the structure of the steel pipe as the product.
 The steel pipe of the present invention is characterized by its structure
 composed of ferrite grains not larger than 3 .mu.m, preferably not larger
 than 1 .mu.m, so that it is superior in ductility and collision and impact
 resistance. With ferrite grains coarser than 3 .mu.m, the steel pipe will
 not have remarkably improved ductility and collision and impact
 resistance. The ferrite grain size is expressed in terms of average value
 of 200 or more ferrite grains regarded as circles which are observed under
 an optical or electron microscope when the cross section perpendicular to
 the lengthwise direction of the steel pipe is corroded with nitral
 solution.
 In the present invention, the structure composed mainly of ferrite includes
 the one which is composed of ferrite alone without secondary phase and the
 one which is composed of ferrite and a secondary phase other than ferrite.
 The secondary phase other than ferrite includes martensite, bainite, and
 cementite. They may precipitate alone or in combination with one another.
 The secondary phase should have a ratio of area not more than 30%. The
 secondary phase that has precipitated helps elongation to occur evenly at
 the time of deformation, thereby improving the ductility and collision and
 impact resistance of the steel pipe. This effect becomes less significant
 as its ratio of area exceeds 30%. FIG. 4 shows an example of the structure
 of the steel pipe of the present invention.
 The following concerns the process for producing the steel pipe of the
 present invention.
 The process starts with heating the steel pipe stock having the
 above-mentioned composition. The heating temperature is (Ac.sub.1
 +50.degree. C.) to 400.degree. C., preferably 750-400.degree. C. Heating
 beyond the upper limit deteriorates the surface properties and unduly
 increases austenite, resulting is coarse grains. Therefore, the heating
 temperature is not higher than (Ac.sub.1 +50.degree. C.), preferably not
 higher than 750.degree. C. Heating below the lower limit does not provide
 an adequate rolling temperature. Therefore, the heating temperature is
 preferably 400.degree. C. or higher.
 The heated steel pipe stock subsequently undergoes reducing preferably by a
 reducing mill of 3-roll type or 4-roll type or any other types. Continuous
 reducing by a plurality of stands is preferable. The number of stands
 depends on the dimensions of the steel pipe stock and finished steel pipe.
 The rolling temperature for reducing is (Ac.sub.1 +50.degree. C.) to
 400.degree. C., preferably 750-400.degree. C., at which ferrite
 re-crystallization takes place. A rolling temperature beyond the upper
 limit causes ferrite grains to grow excessively after recrystallization,
 thereby decreasing ductility. Therefore, the rolling temperature is not
 higher than (Ac.sub.1 +50.degree. C.), preferably not higher than
 750.degree. C. On the other hand, a rolling temperature below the lower
 limit brings about blue shortness, which leads to brittleness and fracture
 during rolling. A rolling temperature below 400.degree. C. causes such
 troubles as increased deformation resistance, hence difficulties in
 rolling of material, and insufficient recrystallization, hence residual
 strain. Therefore, the rolling temperature for reducing is (Ac.sub.1
 +50.degree. C.) to 400.degree. C., preferably 750-400.degree. C., and more
 preferably 600-700.degree. C.
 Reducing is carried out such that the cumulative diameter reduction is 20%
 or more, which is defined by (A-B)/A.times.100%, where A is the outside
 diameter of the base steel pipe and B is the outside diameter of the
 product pipe. Failing to meet this requirement results in a steel pipe
 poor in ductility because of insufficient action by recrystallization to
 make grains finer. Another problem is a low pipe forming rate and hence
 low productivity. In the present invention, therefore, the cumulative
 diameter reduction is greater than 20%. However, if it exceeds 60%, the
 resulting steel pipe will have high strength and high ductility which are
 well balanced with each other even though the content of the
 above-mentioned alloying elements is low, on account of work hardening,
 leading to increased strength, and finer structure. For this reason, the
 cumulative diameter reduction is preferably 60% or more.
 Reducing is carried out such that at least one of rolling passes
 accomplishes diameter reduction 6% or more per pass. Reducing with a
 diameter reduction smaller than 6% per pass does not produce the effect of
 making crystal grains finer by recrystallization. Reducing with a diameter
 reduction of 6% or more per pass generates heat, hence increases
 temperature, keeping the desired rolling temperature. The diameter
 reduction per pass is preferably 8% or more for dynamic recrystallization
 and finer crystal grains.
 The reducing of steel pipes according to the present invention provides
 biaxial stress, thereby producing a significant effect of making crystal
 grains finer. By contrast, the rolling of steel plates merely provides
 uniaxial stress, with free ends existing in the rolling direction as well
 as the widthwise direction (or the direction perpendicular to the rolling
 direction). Therefore, the rolling in this way is limited in ability to
 make grains finer.
 Also, the reducing of steel pipes according to the present invention is
 preferably carried in the presence of a lubricant. Lubricated rolling
 makes even the strain distribution in the thickness direction and also
 makes even the grain size distribution in the thickness direction. Rolling
 without lubrication concentrates strain in the surface of the material due
 to shear effect, resulting in uneven grain size in the thickness
 direction. Any known rolling oil, such as mineral oil and a mixture of
 mineral oil and synthetic ester, may be used as a lubricant.
 After reducing, the steel pipe is cooled to room temperature. Cooling may
 be natural air cooling or any of known forced air cooling, water cooling,
 and mist cooling to suppress the growth of grains. The cooling rate is
 preferably greater than 10.degree. C./s.
 EXAMPLE 1
 A steel raw material having the chemical composition shown in Table 1 was
 made into flat strip steel of 3.2 mm in thickness by hot rolling. After
 preheating at 600.degree. C., this strip steel was continuously formed
 into an open pipe by means of a plurality of forming rolls. The open pipe
 had its edges preheated to 1000.degree. C. by induction heating, and the
 edges were heated to 1300.degree. C. by induction heating and joined
 together by solid-phase pressure welding using squeeze rolls. Thus there
 was obtained a pipe stock, 31.8 mm in diameter and 3.2 mm in wall
 thickness. With its seam cooled, the pipe stock was induction-heated to
 temperatures shown in Table 2. The heated pipe stock was reduced by means
 of a 3-roll reducing mill to form a product steel pipe having the outside
 diameter shown in Table 2. Incidentally, lubricated rolling with a mixture
 of mineral oil and synthetic ester was performed on the product No. 1-2.
 The product pipe thus obtained was found to have the characteristic
 properties, i.e., structure, grain size, tensile properties, and impact
 properties, as shown in Table 2. Grain size was determined by observing
 the cross section (C) perpendicular to the lengthwise direction of the
 pipe under a microscope (.times.5000) and expressed in terms of an average
 of five or more observations. Tensile properties were measured by using
 JIS No.11 specimens. Incidentally, elongation (El) is expressed in terms
 of values calculated from
EQU El=El.sub.0.times.( (a.sub.0 /a))0.4
 (where El.sub.0 is the actually measured elongation, a.sub.0 is 100
 mm.sup.2, and a is the sectional area (mm.sup.2) of the specimen.) This
 converted value was used in consideration of the size effect of the
 specimen. Impact properties (toughness) was evaluated in terms of percent
 ductile fracture of cross section C at -100.degree. C. measured in Charpy
 impact test with a 2-mm V notch in the lengthwise direction of the pipe.
 It is noted from Table 2 that samples (Nos. 1-1 to 1-3) in examples
 pertaining to the present invention are characterized by a grain size of 2
 .mu.m, or fine grains not greater than 3 .mu.m, and also by high
 elongation and toughness and well-balanced strength and
 toughness/ductility. Sample No. 1-2, which underwent lubricated rolling,
 shows only a little variation in grain size in the thickness direction. In
 contrast, sample Nos. 1-4 and 1-5 (in comparative example) are poor in
 ductility and toughness due to coarse grains. Incidentally, it was found
 that pearlite (P) includes, in addition to the lamellar structure, pseudo
 pearlite which does not form the lamellar structure.
 EXAMPLE 2
 A steel raw material having the chemical composition shown in Table 1 was
 made into flat strip steel of 3.2 mm in thickness by hot rolling. This
 strip steel was continuously formed into an open pipe by means of a
 plurality of forming rolls. The open pipe had its edges preheated above
 the melting point by induction heating, and the edges were butt-welded by
 using squeeze rolls. Thus there was obtained a pipe stock, 31.8 mm in
 diameter and 3.2 mm in wall thickness. With its bead removed by a bead
 cutter, the resulting electric welded pipe was heated again at the
 temperature shown in Table 3 by induction heating. It was reduced by means
 of a 3-roll reducing mill to form a finished pipe having the outside
 diameter shown in Table 3.
 The finished pipe thus obtained was tested for characteristic properties,
 i.e., structure, grain size, tensile properties, and toughness, in the
 same manner as in Example 1. The results are shown in Table 3.
 It is noted from Table 3 that samples (Nos. 2-2, 2-3, 2-5, and 2-7) in
 examples pertaining to the present invention are characterized by fine
 grains not greater than 3 .mu.m and also by high elongation and toughness
 and well-balanced strength and toughness/ductility. Bycontrast, samples
 (Nos. 2-1, 2-4, 2-6, 2-8, and 2-9) in comparative examples are poor in
 ductility and toughness due to coarse grains.
 EXAMPLE 3
 A steel having the composition shown in Table 1 was prepared by using a
 converter, and this steel was made into a billet by the continuous casting
 process. After heating, this billet was made into a seamless pipe of 158
 mm in outside diameter and 8 mm in wall thickness by using a Mannesmann
 mandrel mill. This seamless pipe was heated again to the temperature shown
 in Table 4 by induction heating and then reduced by means of a 3-roll
 reducing mill to form a product pipe having the outside diameter shown in
 Table 4.
 The product pipe thus obtained was tested for characteristic properties in
 the same manner as in Examples 1 and 2. The results are shown in Table 4.
 It is noted from Table 4 that samples (Nos. 3-1, 3-2, 3-4, and 3-5) in
 examples pertaining to the present invention are characterized by fine
 grains not greater than 3 .mu.m and also by high elongation and toughness
 and well-balanced strength and toughness/ductility. By contrast, samples
 (Nos. 3-3 and 3-6) in comparative examples are poor in ductility and
 toughness due to coarse grains.
 EXAMPLE 4
 A base steel pipe having the chemical composition shown in Table 5 was
 heated by induction to a temperature shown in Table 6 and then rolled into
 a finished steel pipe by means of a 3-roll reducing mill under the rolling
 conditions shown in Table 6.
 The base steel pipe in Table 6 is either solid-phase pressure-welded one or
 seamless one. The former was prepared by preheating a 2.6 mm thick
 hot-rolled strip steel to 600.degree. C., continuously forming it into an
 open pipe by means of a plurality of forming rolls, preheating the edges
 of the open pipe to 1000.degree. C. by induction, heating the edges to
 1450.degree. C. below the melting point by induction, and pressure-welding
 the edges by means of a squeeze roll. It is 42.7 mm in diameter and 2.6 mm
 in wall thickness. The seamless pipe was prepared by using a Mannesmann
 mandrel mill from a continuously cast billet (with heating).
 The product pipe thus obtained was tested for tensile properties, collision
 and impact properties, and structure. The results are shown in Table 6.
 Tensile properties were measured by using JIS No.11 specimens.
 Incidentally, elongation (El) is expressed in terms of values calculated
 from
EQU El=El.sub.0.times.( (a.sub.0 /a))0.4
 (where El.sub.0 is the actually measured elongation, a.sub.0 is 292
 mm.sup.2, and a is the sectional area (mm.sup.2) of the specimen.) This
 converted value was used in consideration of the size effect of the
 specimen. Collision and impact properties were evaluated in terms of the
 amount of energy which is absorbed before the amount of strain reaches 30%
 in the stress-strain curve obtained by the high-speed tensile test at a
 strain rate of 2000 s.sup.-1. Incidentally, collision and impact
 properties are a measure of energy required to deform the material when an
 automobile actually collides at a strain rate of 1000-2000 s.sup.-1. The
 larger the amount of this energy, the better the collision and impact
 resistance.
 It is noted from Table 6 that samples (Nos. 4-1 to 4-16 and 4-19 to 4-22)
 in examples pertaining to the present invention have well-balanced
 ductility and strength, with a high tensile strength at a high strain rate
 and a high energy absorption at the time of collision and impact. By
 contrast, samples (Nos. 4-17, 4-18, and 4-23) in comparative examples are
 poor in either ductility or strength, poor in balance between strength and
 ductility, and poor in collision and impact resistance.
 Comparative samples (Nos.4-17 and 4-18), which do not conform to the
 present invention in diameter reduction, have coarse ferrite grains,
 unbalanced strength-ductility, and low energy absorption at the time of
 collision and impact.
 EXAMPLE 5
 A base steel pipe having the chemical composition shown in Table 7 was
 heated by induction to a temperature shown in Table 8 and then rolled into
 a product steel pipe by means of a 3-roll reducing mill under the rolling
 conditions shown in Table 8. Incidentally, the steel pipe stock was
 prepared in the same manner as in Example 4.
 The product steel pipe was tested for tensile properties, collision and
 impact properties, and structure in the same way as in Example 4. The
 results are shown in Table 8.
 It is noted from Table 8 that samples (Nos. 5-1 to 5-3 and 5-7 to 5-10) in
 examples pertaining to the present invention have well-balanced ductility
 and strength, with a high tensile strength at a high strain rate and a
 high energy absorption at the time of collision and impact. By contrast,
 samples (Nos. 5-4 to 5-6) in comparative examples are poor in either
 ductility or strength, poor in balance between strength and ductility, and
 poor in collision and impact resistance.
 The present invention provides a steel pipe having well-balanced ductility
 and strength and good collision and impact properties, unlike the
 conventional technology. This steel pipe is suitable for bulging by
 hydroforming or the like. Bulging will be very easy to perform in the case
 of electric welded pipe or solid-phase pressure-welded pipe with the seam
 cooled, because the hardened seam has the same level of hardness as the
 pipe stock on account of reducing.
 EXAMPLE 6
 A base steel pipe, 110 mm in diameter and 4.5 mm in wall thickness, having
 the chemical composition shown in Table 9 was produced from hot-rolled
 steel plate which had undergone controlled rolling and controlled cooling.
 The base steel pipe was heated by induction to a temperature shown in
 Table 10 and then reduced by using a 3-roll reducing mill under the
 condition shown in Table 10.
 The product steel pipe was tested for tensile properties, collision and
 impact properties, structure, and sulfide stress corrosion cracking
 resistance. The results are shown in Table 10. Tensile properties were
 measured by using JIS No.11 specimens in the same manner as in Example 4.
 Incidentally, elongation (El) is expressed in terms of values calculated
 from
 El=El.sub.0.times.( (a.sub.0 /a))0.4
 (where El.sub.0 is the actually measured elongation, a.sub.0 is 292
 mm.sup.2, and a is the sectional area (mm.sup.2) of the specimen.) This
 converted value was used in consideration of the size effect of the
 specimen.
 Collision and impact properties were evaluated in terms of the amount of
 energy which is absorbed before the amount of strain reaches 30% in the
 stress-strain curve obtained by the high-speed tensile test at a strain
 rate of 2000 s.sup.-1. Incidentally, collision and impact properties are a
 measure of energy required to deform the material when an automobile
 actually collides at a strain rate of 1000-2000 s.sup.-1. The greater the
 amount of this energy, the better the collision and impact resistance.
 Incidentally, the sulfide stress corrosion cracking resistance was
 evaluated by observing whether or not a C-ring test piece shown in FIG. 5
 breaks within 200 hours when it is immersed under a tensile stress
 corresponding to 120% of yield strength in an NACE bath (composed of 0.5%
 acetic acid and 5% sodium chloride, saturated with hydrogen sulfide) at
 25.degree. C. and 1 atm. The C-ring test piece was cut out of the product
 pipe in its circumferential direction. This test was duplicated for each
 sample under the same conditions.
 It is noted from Table 10 that samples (Nos. 6-1 to 6-3, 6-6, 6-8 to 6-10)
 in examples pertaining to the present invention have well-balanced
 ductility and strength, high tensile strength at high strain rate, and
 high energy absorption at the time of collision and impact. They are also
 superior in sulfide stress corrosion cracking resistance, and hence they
 are suitable for use as line pipes. By contrast, samples (Nos. 6-4, 6-5,
 and 6-7) in comparative examples are poor in either ductility or strength,
 poor in balance between strength and ductility, poor in collision and
 impact properties, and poor in sulfide stress corrosion cracking
 resistance as indicated by breakage in the NACE bath.
 Samples (Nos. 6-4 and 6-7) in comparative examples, which were reduced at a
 rolling temperature outside the range specified in the present invention,
 are poor in balance between strength and ductility due to coarse ferrite
 grains, poor in energy absorption at the time of collision and impact, and
 poor in sulfide stress corrosion cracking resistance.
 EXAMPLE 7
 A base steel pipe having the chemical composition shown in Table 11 was
 heated by induction to a temperature shown in Table 12 and then rolled
 into a product steel pipe by means of a 3-roll reducing mill under the
 rolling conditions shown in Table 12. The base steel pipe in this example
 was either electric resistance welded pipe of 110 mm in diameter and 2.0
 mm in wall thickness or seamless steel pipe of 110 mm in diameter and 3.0
 mm in wall thickness. The former was prepared by forming an open pipe from
 hot-rolled strip steel by means of a plurality of forming rolls and then
 welding the edges by induction heating. The latter was prepared by using a
 Mannesmann mandrel mill from a continuously cast billet with heating.
 The product pipe thus obtained was tested for tensile properties, collision
 and impact properties, structure, and fatigue resistance. The results are
 shown in Table 12. Tensile properties and collision and impact properties
 were measured in the same manner as in Example 4. Fatigue strength was
 measured by subjecting the finished pipe as a specimen to cantilever
 reversed fatigue test (at a repeating rate of 20 Hz) in the air.
 It is noted from Table 12 that samples (Nos. 7-1, 7-3, and 7-6 to 7-8) in
 examples have well-balanced ductility and strength, high tensile strength
 at high strain rate, and high energy absorption at the time of collision
 and impact. In addition, they are superior in fatigue resistance. By
 contrast, samples (Nos. 7-2, 7-4, and 7-5) in comparative examples are
 poor in fatigue strength. Sample No. 7-2 did not undergo reducing, sample
 7-5 had a ratio of reduction in diameter which is outside the specified
 range, and sample No. 7-4 was reduced at a temperature outside the
 specified range. Therefore, it is poor in balance between strength and
 ductility due to coarse ferrite grains, poor in energy absorption at the
 time of collision and impact, and poor in fatigue resistance.
 EXPLOITATION IN INDUSTRY
 The present invention provides a high-strength steel product superior in
 toughness and ductility on account of extremely fine grain size not
 greater than 3 .mu.m. Therefore, it will produce a significant industrial
 effect of expanding the application area of steel products. The present
 invention also provides a process for efficient and easy production of
 high-strength steel pipe superior in ductility and impact resistance.
 Therefore, it will produce a significant industrial effect of expanding
 the application area of steel pipe. The present invention permits the
 production of steel pipes for line pipes which need high strength and
 toughness and good stress corrosion cracking resistance. The present
 invention also permits the economical production of high-strength,
 high-ductility steel pipe having good fatigue resistance, with the amount
 of alloying elements reduced.
 TABLE 1
 Steel Chemical composition (wt %)
 No. C SI Mn P S AI N
 A 0.06 0.05 0.35 0.018 0.019 0.028 0.0025
 B 0.08 0.25 1.28 0.007 0.002 0.041 0.0025
 C 0.25 0.20 0.82 0.012 0.007 0.010 0.0028
 D 0.16 0.22 0.75 0.009 0.006 0.031 0.0033
 TABLE 1
 Steel Chemical composition (wt %)
 No. C SI Mn P S AI N
 A 0.06 0.05 0.35 0.018 0.019 0.028 0.0025
 B 0.08 0.25 1.28 0.007 0.002 0.041 0.0025
 C 0.25 0.20 0.82 0.012 0.007 0.010 0.0028
 D 0.16 0.22 0.75 0.009 0.006 0.031 0.0033
 TABLE 3
 Rolling conditions
 O.D. of
 base Temp. at Temp. at
 0.D. of
 temp. Heating starting finishing Reduction
 product
 Sample Steel pipe temp. rolling rolling of area
 pipe
 No. No. (mm) (.degree. C.) (.degree. C.) (.degree. C.)
 (%) (mm)
 2-1 B 31.8 900 770 715 60
 15
 2-2 800 770 715 60
 2-3 700 670 630 60
 2-4 700 520 535 60
 2-5 600 590 605 60
 2-6 520 500 520 60
 2-7 700 660 630 30
 2-8 700 660 650 10
 2-9 --
 31.8
 Characteristics of product pipe
 Percent
 Yield Tensile ductile
 Grain point strength Elonga- fracture by
 Sample size YS, TS, ion El, TS .times. El Charpy
 No. (.mu.m) (MPa) (MPa) (%) (MPa.multidot.%) (%)
 Structure * Note
 2-1 20 430 457 29 13253 30 F + P +
 5% B C. Ex.
 2-2 2 470 500 40 20000 100 F + C +
 15% B Ex.
 2-3 1 523 556 39 21684 100 F + P +
 10% B Ex.
 2-4 6 619 658 13 8554 10 F + P +
 10% B C. Ex.
 2-5 1 581 618 36 22248 100 F + P +
 5% B Ex.
 2-6 6 620 660 14 9240 20 F + P +
 5% B C. Ex.
 2-7 3 502 534 33 17622 100 F + P +
 10% B Ex.
 2-8 9 435 468 30 14040 50 F + P +
 10% B C. Ex.
 2-9 12 424 460 29 13340 40 F + P +
 5% B C. Ex.
 F: ferrite, P: pearlite (including pseudo pearlite), C.: cementite, B:
 Bainite
 Ex.: Example pertaining to the present invention. C. Ex.: Comparative
 Example
 TABLE 4
 Rolling conditions
 O.D. of
 base Temp. at Temp. at
 0.D. of
 temp. Heating starting finishing Reduction
 product
 Sample Steel pipe temp. rolling rolling of area
 pipe
 No. No. (mm) (.degree. C.) (.degree. C.) (.degree. C.)
 (%) (mm)
 3-1 C 158 700 660 657 75
 55.0
 3-2 650 622 613 75
 55.0
 3-3 --
 158.0
 3-4 D 700 690 680 75
 55.0
 3-5 650 635 630 75
 55.0
 3-6 --
 158.0
 Characteristics of product pipe
 Percent
 Yield Tensile ductile
 Grain point strength Elonga- fracture by
 Sample size YS, TS, ion El, TS .times. El Charpy
 No. (.mu.m) (MPa) (MPa) (%) (MPa .multidot. %) (%)
 Structure * Note
 3-1 3 579 658 23 15134 100 F + P
 Ex.
 3-2 3 600 678 24 16272 100 F + P
 Ex.
 3-3 21 401 555 18 9990 10 F + P
 C. Ex.
 3-4 1 500 547 32 17504 100 F + P
 Ex.
 3-5 2 608 627 29 18183 100 F + P
 Ex.
 3-6 9 426 501 27 13527 40 F + P
 C. Ex.
 * F: ferrite, P: pearlite (including pseudo pearlite), C: cementite, B:
 Bainite
 Ex.: Example pertaining to the present invention. C. Ex.: Comparative
 Example
 TABLE 4
 Rolling conditions
 O.D. of
 base Temp. at Temp. at
 0.D. of
 temp. Heating starting finishing Reduction
 product
 Sample Steel pipe temp. rolling rolling of area
 pipe
 No. No. (mm) (.degree. C.) (.degree. C.) (.degree. C.)
 (%) (mm)
 3-1 C 158 700 660 657 75
 55.0
 3-2 650 622 613 75
 55.0
 3-3 --
 158.0
 3-4 D 700 690 680 75
 55.0
 3-5 650 635 630 75
 55.0
 3-6 --
 158.0
 Characteristics of product pipe
 Percent
 Yield Tensile ductile
 Grain point strength Elonga- fracture by
 Sample size YS, TS, ion El, TS .times. El Charpy
 No. (.mu.m) (MPa) (MPa) (%) (MPa .multidot. %) (%)
 Structure * Note
 3-1 3 579 658 23 15134 100 F + P
 Ex.
 3-2 3 600 678 24 16272 100 F + P
 Ex.
 3-3 21 401 555 18 9990 10 F + P
 C. Ex.
 3-4 1 500 547 32 17504 100 F + P
 Ex.
 3-5 2 608 627 29 18183 100 F + P
 Ex.
 3-6 9 426 501 27 13527 40 F + P
 C. Ex.
 * F: ferrite, P: pearlite (including pseudo pearlite), C: cementite, B:
 Bainite
 Ex.: Example pertaining to the present invention. C. Ex.: Comparative
 Example
 TABLE 6 (1)
 Rolling conditions