High toughness and high tensile strength thick steel plate with excellent material homogeneity and production method for same

Abstract

A thick steel plate is provided by heating a continuously-cast slab, hot forging the continuously-cast slab using opposing dies having respective short sides differing such that when a short side length of a die having a shorter one of the short sides is taken to be 1, a short side length of a die having a longer one of the short sides is 1.1 to 3.0, allowing cooling to obtain a steel raw material, reheating the steel raw material, performing hot rolling of the steel raw material including at least two passes carried out, allowing cooling to obtain a thick steel plate, reheating the thick steel plate to at least the Ac.sub.3 temperature and no higher than 1050 C., rapidly cooling the thick steel plate to 350 C. or lower, and tempering the thick steel plate at at least 550 C. and no higher than 700 C.

Claims

1. A thick steel plate having a plate thickness of 100 mm or more, having a chemical composition containing, in mass %, C: 0.08% to 0.20%, Si: 0.40% or less, Mn: 0.5% to 5.0%, P: 0.015% or less, S: 0.0050% or less, Ni: 5.0% or less, Ti: 0.005% to 0.020%, Al: 0.080% or less, N: 0.0070% or less, B: 0.0030% or less, and one or more selected from Cu: 0.50% or less, Cr: 3.0% or less, Mo: 1.50% or less, V: 0.200% or less, and Nb: 0.100% or less, the balance being Fe and incidental impurities, wherein a value Ceq.sup.IIW defined by formula (1) below is 0.55 to 0.80:
Ceq.sup.IIW=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5(1) where each element symbol indicates content, in mass %, of a corresponding element in the chemical composition and is taken to be 0 when the corresponding element is not contained, a mid-thickness part of the steel plate has a yield strength of 500 MPa or more, reduction of area in the mid-thickness part by tension in a plate thickness direction is 40% or more, and the mid-thickness part has a low-temperature toughness at 60 C. of 70 J or more.

2. The thick steel plate of claim 1, wherein the chemical composition further contains, in mass %, one or more selected from Mg: 0.0005% to 0.0100%, Ta: 0.01% to 0.20%, Zr: 0.005% to 0.1%, Y: 0.001% to 0.01%, Ca: 0.0005% to 0.0050%, and REM: 0.0005% to 0.0200%.

3. The thick steel plate of claim 2, wherein in a hardness distribution in the plate thickness direction, a difference HV between average hardness of a plate thickness surface (HVS) and average hardness of the mid-thickness part (HVC), where HV =HVS HVC, is 30 or less.

4. The thick steel plate of claim 1, wherein in a hardness distribution in the plate thickness direction, a difference HV between average hardness of a plate thickness surface (HVS) and average hardness of the mid-thickness part (HVC), where HV=HVSHVC, is 30 or less.

5. A method for producing the thick steel plate of claim 1, comprising heating a continuously-cast slab having the chemical composition containing, in mass %, C: 0.08% to 0.20%, Si: 0.40% or less, Mn: 0.5% to 5.0%, P: 0.015% or less, S: 0.0050% or less, Ni: 5.0% or less, Ti: 0.005% to 0.020%, Al: 0.080% or less, N: 0.0070% or less, B: 0.0030% or less, and one or more selected from Cu: 0.50% or less, Cr: 3.0% or less, Mo: 1.50% or less, V: 0.200% or less, and Nb: 0.100% or less, the balance being Fe and incidental impurities, to at least 1200 C. and no higher than 1350 C., then hot forging the continuously-cast slab under conditions of a temperature of 1000 C. or higher, a strain rate of 3/s or less, and a cumulative working reduction of 15% or more using opposing dies having respective short sides differing such that when a short side length of a die having a shorter one of the short sides is taken to be 1, a short side length of a die having a longer one of the short sides is 1.1 to 3.0, then allowing cooling to obtain a steel raw material, then reheating the steel raw material to at least an Ac.sub.3 temperature and no higher than 1250 C., then performing hot rolling of the steel raw material including at least two passes carried out with a rolling reduction of 4% or more per pass, then allowing cooling to obtain a thick steel plate, then reheating the thick steel plate to at least the Ac.sub.3 temperature and no higher than 1050 C., then rapidly cooling the thick steel plate to 350 C. or lower, and then tempering the thick steel plate at at least 550 C. and no higher 700 C.

6. The method of claim 5, wherein a working reduction ratio from the continuously-cast slab prior to working to the thick steel plate obtained after the hot rolling in production of the high toughness and high tensile strength thick steel plate is 3 or less.

7. The method of claim 5, wherein the chemical composition further contains, in mass %, one or more selected from Mg: 0.0005% to 0.0100%, Ta: 0.01% to 0.20%, Zr: 0.005% to 0.1%, Y: 0.001% to 0.01%, Ca: 0.0005% to 0.0050%, and REM: 0.0005% to 0.0200%.

8. The method of claim 7, wherein a working reduction ratio from the continuously-cast slab prior to working to the thick steel plate obtained after the hot rolling in production of the high toughness and high tensile strength thick steel plate is 3 or less.

9. The method of claim 5, wherein in a hardness distribution in the plate thickness direction, a difference HV between average hardness of a plate thickness surface (HVS) and average hardness of the mid-thickness part (HVC), where HV =HVS HVC, is 30 or less.

10. The method of claim 9, wherein a working reduction ratio from the continuously-cast slab prior to working to the thick steel plate obtained after the hot rolling in production of the high toughness and high tensile strength thick steel plate is 3 or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 illustrates the main points of forging a slab using asymmetrical dies according to this disclosure; and

(3) FIG. 2 compares equivalent plastic strain in a raw material (steel plate) when conventional symmetrical dies (dies having upper/lower symmetry) are used and when asymmetrical dies (dies not having upper/lower symmetry) according to this disclosure are used.

DETAILED DESCRIPTION

(4) The following provides a detailed description of the disclosed techniques.

(5) First, suitable ranges for the steel plate composition will be described. The contents of elements in the steel plate composition, shown in %, are all mass % values.

(6) C: 0.08% to 0.20%

(7) C is an element that is useful for obtaining the strength required for structural-use steel at low-cost. Addition of C in an amount of 0.08% or more is required to obtain this effect. On the other hand, an upper limit of 0.20% is set for the C content because C content exceeding 0.20% causes significant deterioration of base metal toughness and weld toughness. The C content is more preferably 0.08% or more. The C content is more preferably 0.14% or less.

(8) Si: 0.40% or Less

(9) Si is added for deoxidation. However, addition of Si in excess of 0.40% causes significant deterioration of base metal toughness and heat-affected zone toughness. Therefore, the Si content is set as 0.40% or less. The Si content is more preferably 0.05% or more. The Si content is more preferably 0.30% or less. The Si content is even more preferably 0.1% or more and 0.30% or less.

(10) Mn: 0.5% to 5.0%

(11) Mn is added to ensure base metal strength. However, this effect is not sufficiently obtained if less than 0.5% of Mn is added. On the other hand, an upper limit of 5.0% is set for the Mn content because addition of Mn in excess of 5.0% not only causes deterioration of base metal toughness, but also promotes central segregation and increases the scale of slab porosity. The Mn content is more preferably 0.6% or more. The Mn content is more preferably 2.0% or less. The Mn content is even more preferably 0.6% or more and 1.6% or less.

(12) P: 0.015% or Less

(13) The P content is limited to 0.015% or less because P content exceeding 0.015% significantly reduces base metal toughness and heat-affected zone toughness. The P content does not have a specific lower limit and may be 0%.

(14) S: 0.0050% or Less

(15) The S content is limited to 0.0050% or less because S content exceeding 0.0050% significantly reduces base metal toughness and heat-affected zone toughness. The S content does not have a specific lower limit and may be 0%.

(16) Ni: 5.0% or Less

(17) Ni is a useful element for improving steel strength and heat-affected zone toughness. However, an upper limit of 5.0% is set for the Ni content because addition of Ni in excess of 5.0% has a significant negative economical impact. The Ni content is more preferably 0.5% or more. The Ni content is more preferably 4.0% or less.

(18) Ti: 0.005% to 0.020%

(19) Ti forms TiN during heating, effectively inhibits coarsening of austenite, and improves base metal toughness and heat-affected zone toughness. Therefore, the Ti content is 0.005% or more. However, addition of Ti in excess of 0.020% causes coarsening of Ti nitrides and reduces base metal toughness. Therefore, the Ti content is set in a range of 0.005% to 0.020%. The Ti content is more preferably 0.008% or more. The Ti content is more preferably 0.015% or less.

(20) Al: 0.080% or Less

(21) Al is added to sufficiently deoxidize molten steel. However, addition of Al in excess of 0.080% causes a large amount of Al to dissolve in the base metal, leading to a decrease in base metal toughness. Therefore, the Al content is set as 0.080% or less. The Al content is more preferably 0.030% or more and 0.080% or less. The Al content is even more preferably 0.030% or more. The Al content is even more preferably 0.060% or less.

(22) N: 0.0070% or Less

(23) N has an effect of refining structure through formation of nitrides with Ti and the like, and thereby improving base metal toughness and heat-affected zone toughness. However, addition of N in excess of 0.0070% increases the amount of N dissolved in the base metal, leading to a significant decrease in base metal toughness, and also causes formation of coarse nitrides in the heat-affected zone, leading to a decrease in heat-affected zone toughness. Therefore, the N content is set as 0.0070% or less. The N content is more preferably 0.0050% or less. The N content is even more preferably 0.0040% or less. The N content does not have a specific lower limit and may be 0%.

(24) B: 0.0030% or Less

(25) B has an effect of inhibiting ferrite transformation at austenite grain boundaries by segregating at the grain boundaries, and thereby improving quench hardenability. However, addition of B in excess of 0.0030% causes precipitation of B as a carbonitride, leading to poorer quench hardenability and reduced toughness. Therefore, the B content is set as 0.0030% or less. The B content is more preferably 0.0003% or more. The B content is more preferably 0.0030% or less. The B content is even more preferably 0.0005% or more. The B content is even more preferably 0.0020% or less. The B content does not have a specific lower limit and may be 0%.

(26) In addition to the elements described above, one or more selected from Cu, Cr, Mo, V, and Nb are contained in the steel plate composition to further increase strength and/or toughness.

(27) Cu: 0.50% or Less

(28) Cu can improve the strength of steel without loss of toughness. However, addition of Cu in excess of 0.50% causes cracking of the surface of the steel plate during hot working. Therefore, the Cu content is set as 0.50% or less. The Cu content does not have a specific lower limit and may be 0%.

(29) Cr: 3.0% or Less

(30) Cr is an effective element for strengthening the base metal. However, the Cr content is set as 3.0% or less because addition of a large amount of Cr reduces weldability. The Cr content is more preferably 0.1% or more. The Cr content is more preferably 2.0% or less from a viewpoint of production cost.

(31) Mo: 1.50% or Less

(32) Mo is an effective element for strengthening the base metal. However, an upper limit of 1.50% is set for the Mo content because addition of Mo in excess of 1.50% causes precipitation of a hard alloy carbide, leading to an increase in strength and a decrease in toughness. The Mo content is more preferably 0.02% or more. The Mo content is more preferably 0.80% or less.

(33) V: 0.200% or Less

(34) V has an effect of improving base metal strength and/or toughness and effectively reduces the amount of solute N through precipitation as VN. However, addition of V in excess of 0.200% reduces toughness of the steel due to precipitation of hard VC. Therefore, the V content is set as 0.200% or less. The V content is more preferably 0.005% or more. The V content is more preferably 0.100% or less.

(35) Nb: 0.100% or Less

(36) Nb is useful due to an effect of strengthening the base metal. However, an upper limit of 0.100% is set for the Nb content because addition of Nb in excess of 0.100% significantly reduces base metal toughness. The Nb content is more preferably 0.025% or less.

(37) In addition to the basic components described above, one or more selected from Mg, Ta, Zr, Y, Ca, and REM may be contained in the steel plate composition to further enhance material quality.

(38) Mg: 0.0005% to 0.0100%

(39) Mg forms a stable oxide at high temperature and effectively inhibits coarsening of prior (austenite) grains in a heat-affected zone, and is thus an effective element for improving weld toughness. Therefore, the Mg content is preferably 0.0005% or more. However, addition of Mg in excess of 0.0100% increases the amount of inclusions and reduces toughness. Therefore, in a situation in which Mg is added, the Mg content is preferably 0.0100% or less. The Mg content is more preferably 0.0005% or more and 0.0050% or less.

(40) Ta: 0.01% to 0.20%

(41) Ta effectively improves strength when added in an appropriate amount. However, no clear effect is obtained when less than 0.01% of Ta is added. Therefore, the Ta content is preferably 0.01% or more. On the other hand, addition of Ta in excess of 0.20% reduces toughness due to precipitate formation. Therefore, the Ta content is preferably 0.20% or less.

(42) Zr: 0.005% to 0.1%

(43) Zr is an effective element for increasing strength. However, no clear effect is obtained when less than 0.005% of Zr is added. Therefore, the Zr content is preferably 0.005% or more. On the other hand, addition of Zr in excess of 0.1% reduces toughness due to formation of a coarse precipitate. Therefore, the Zr content is preferably 0.1% or less.

(44) Y: 0.001% to 0.01%

(45) Y forms a stable oxide at high temperature and effectively inhibits coarsening of prior grains in a heat-affected zone, and is thus an effective element for improving weld toughness. However, these effects are not obtained if less than 0.001% of Y is added. Therefore, the Y content is preferably 0.001% or more. On the other hand, addition of Y in excess of 0.01% increases the amount of inclusions and reduces toughness. Therefore, the Y content is preferably 0.01% or less.

(46) Ca: 0.0005% to 0.0050%

(47) Ca is a useful element for morphological control of sulfide inclusions. In a situation in which Ca is added, the Ca content is preferably 0.0005% or more in order to display this effect. However, addition of Ca in excess of 0.0050% leads to a decrease in the cleanliness factor and causes deterioration of toughness. Therefore, in a situation in which Ca is added, the Ca content is preferably 0.0050% or less. The Ca content is more preferably 0.0005% or more and 0.0025% or less.

(48) REM: 0.0005% to 0.0200%

(49) REM (rare earth metal) has an effect of improving material quality by forming oxides and sulfides in the steel in the same way as Ca. However, this effect in not obtained unless the REM content is 0.0005% or more. Moreover, this effect reaches saturation when REM is added in excess of 0.0200%. Therefore, in a situation in which REM is added, the REM content is preferably 0.0200% or less. The REM content is more preferably 0.0005% or more. The REM content is more preferably 0.0100% or less.

(50) The basic components and selectable components of the steel plate composition have been described through the above. In addition, it is important that the equivalent carbon content, indicated by Ceq.sup.IIW, is in an appropriate range.

(51) Ceq.sup.IIW (%): 0.55 to 0.80

(52) In the presently disclosed techniques, it is required that appropriate components are added to ensure that the mid-thickness part has a yield strength of 500 MPa or more and good low-temperature toughness at 60 C. It is also required that the composition is adjusted such that Ceq.sup.IIW (%), defined by the following formula (1), is 0.55 to 0.80.
Ceq.sup.IIW=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5(1)

(53) Each element symbol indicates the content, in mass %, of the corresponding element.

(54) By adopting the forging process described below with respect to a thick steel plate having a plate thickness of 100 mm or more and having the chemical composition described above, center porosity in a mid-thickness part of the thick steel plate can be compressed and thus rendered substantially harmless.

(55) Moreover, by subsequently adopting the hot working process described below, strength, ductility, and toughness of the mid-thickness part of the steel plate can be improved, and thus a yield strength in the mid-thickness part of 500 MPa or more, a reduction of area in the mid-thickness part by tension in a plate thickness direction of 40% or more, and a low-temperature toughness at 60 C. in the mid-thickness part of 70 J or more can be achieved.

(56) In the case of a thick steel plate having a plate thickness of 100 mm or more and a yield strength of 500 MPa or more, a hardness distribution in the plate thickness direction of the steel plate is typically high at the surface of the steel plate and falls toward a mid-thickness part of the steel plate. If the composition of the steel plate is inappropriate and quench hardenability is insufficient, a structure of mainly ferrite and upper bainite forms, leading to greater variation in the hardness distribution in the plate thickness direction (i.e., a greater difference between hardness near the surface and hardness of the mid-thickness part), and thus poorer material homogeneity.

(57) Herein, appropriate adjustment of the steel plate composition as described above ensures quench hardenability, resulting in a microstructure that is a martensite and/or bainite structure.

(58) In particular, material homogeneity can be further improved when, in the plate thickness direction hardness distribution, the difference HV between the average hardness of the plate thickness surface (HVS) and the average hardness of the mid-thickness part (HVC), where HV=HVSHVC, is 30 or less.

(59) The average hardness of the plate thickness surface (HVS) and the average hardness of the mid-thickness part (HVC) can be determined, for example, from a cross-section parallel to a longitudinal direction of the steel plate by measuring the hardness at a number of points at a position 2 mm inward from the steel plate surface and a number of points at a mid-thickness position in the cross-section, and then determining an average value for each of these positions.

(60) The following describes production conditions in the presently disclosed techniques.

(61) In the following description, temperatures given in C. refer to the temperature of the mid-thickness part. The presently disclosed production method for a steel plate requires, in particular, that a steel raw material be hot forged under the following conditions to render harmless casting defects such as center porosity in the steel raw material.

(62) I. Hot Forging Conditions of Steel Raw Material

(63) Heating Temperature: 1200 C. to 1350 C.

(64) A steel raw material for a cast steel or slab having the aforementioned composition is subjected to steelmaking and continuous casting by a typically known method, such as using a converter, an electric heating furnace, or a vacuum melting furnace, and is then reheated to at least 1200 C. and no higher than 1350 C. If the reheating temperature is lower than 1200 C., a predetermined cumulative working reduction and temperature lower limit cannot be ensured in hot forging and deformation resistance during the hot forging is high, making it impossible to ensure a sufficient per-pass working reduction. As a result, a larger number of passes are needed, which not only reduces production efficiency, but also makes it impossible to compress casting defects such as center porosity in the steel raw material to render them harmless. Therefore, the slab reheating temperature is set as 1200 C. or higher. An upper limit of 1350 C. is set for the reheating temperature because reheating to a temperature higher than 1350 C. consumes excessive energy and facilitates formation of surface defects due to scale during heating, leading to an increased mending load after hot forging.

(65) Hot forging according to this disclosure is performed using a pair of opposing dies whose long sides lie in the width direction of the continuously-cast slab and whose short sides lie in the traveling direction of the continuously-cast slab. A feature of the hot forging according to this disclosure is that the respective short sides of the opposing dies have different lengths, as illustrated in FIG. 1.

(66) In FIG. 1, reference sign 1 indicates an upper die, reference sign 2 indicates a lower die, and reference sign 3 indicates a slab.

(67) Among the opposing upper and lower dies, when the short side length of the die having a shorter one of the short sides (i.e., the upper die in FIG. 1) is taken to be 1, the short side length of the opposing die having a longer one of the short sides (i.e., the lower die in FIG. 1) is set such as to be from 1.1 times to 3.0 times the short side length of the die having the shorter one of the short sides. As a result, not only can an asymmetrical strain distribution be obtained within the steel material, but also a position of minimum strain application during forging can be set so as not to coincide with a position at which center porosity of the continuously-cast slab occurs. This makes it possible to ensure that the center porosity is rendered harmless.

(68) If the ratio of the longer one of the short sides to the shorter one of the short sides is less than 1.1, the effect of rendering center porosity harmless is not sufficiently achieved. On the other hand, if the ratio of the longer one of the short sides to the shorter one of the short sides exceeds 3.0, the efficiency of hot forging drops significantly. Accordingly, it is important that, with regards to the respective short side lengths of the pair of opposing dies used in the hot forging according to this disclosure, when the shorter one of the short side lengths is taken to be 1, the longer one of the short side lengths is set as 1.1 to 3.0. It should be noted that so long as the respective short side lengths of the opposing dies satisfy the ratio described above, it does not matter whether the die having the shorter one of the short side lengths is located above or below the continuously-cast slab. In other words, the lower die in FIG. 1 may alternatively be the die having the shorter one of the short side lengths.

(69) FIG. 2 compares the equivalent plastic strain in a slab, calculated in a thickness direction of the slab, when hot forging was performed using dies for which the respective short side lengths of the upper and lower dies are the same (conventional dies indicated by white circles in FIG. 2) and when hot forging was performed using dies for which the ratio of short side lengths of a die having a shorter short side and a die having a longer short side is 2.5 (dies according to this disclosure indicated by black circles in FIG. 2). With the exception of the die shape, the hot forging was performed with both pairs of dies under the same conditions of a heating temperature of 1250 C., a working start temperature of 1215 C., a working end temperature of 1050 C., a cumulative working reduction of 16%, a strain rate of 0.1/s, and a maximum working reduction per pass of 8%, and without width direction working.

(70) FIG. 2 clearly illustrates that hot forging using the dies according to this disclosure was more successful in imparting sufficient strain to the center of the slab.

(71) Hot Forging Temperature: 1000 C. or Higher

(72) A forging temperature of lower than 1000 C. in the hot forging raises deformation resistance during the hot forging and thus increases the load on the forging machine, making it impossible to ensure that center porosity is rendered harmless. Therefore, the forging temperature is set as 1000 C. or higher. The forging temperature does not have a specific upper limit but is preferably no higher than approximately 1350 C. in view of production costs.

(73) Cumulative Working Reduction of Hot Forging: 15% or More

(74) If the cumulative working reduction of the hot forging is less than 15%, casting defects such as center porosity in the steel raw material cannot be compressed and rendered harmless. Therefore, the cumulative working reduction is set as 15% or more. Although casting defects can be more effectively rendered harmless with increasing cumulative working reduction, an upper limit of approximately 30% is set for the cumulative working reduction in view of manufacturability. In a situation in which the thickness is increased through hot forging in the width direction of the continuously-cast slab, the cumulative working reduction is measured from the increased thickness.

(75) Particularly in production of a thick steel plate having a plate thickness of 120 mm or more, it is preferable to ensure that at least one pass is performed with a working reduction of 5% or more per pass in the hot forging to ensure that center porosity is rendered harmless. The working reduction per pass is more preferably 7% or more.

(76) Strain Rate of Hot Forging: 3/s or Less

(77) A strain rate exceeding 3/s in the hot forging raises deformation resistance during the hot forging and thus increases the load on the forging machine, making it impossible to ensure that center porosity is rendered harmless. Therefore, the strain rate is set as 3/s or less.

(78) On the other hand, a strain rate of less than 0.01/s lengthens the hot forging time, leading to lower productivity. Therefore, the strain rate is preferably 0.01/s or more. The strain rate is more preferably 0.05/s or more. The strain rate is more preferably 1/s or less.

(79) In the disclosed techniques, the hot forging is followed by hot working to obtain a steel plate of a desired plate thickness and improve strength and toughness of the mid-thickness part.

(80) II. Conditions of Hot Working after Forging

(81) Reheating Temperature of Steel Raw Material after Forging: Ac.sub.3 Temperature to 1250 C.

(82) The steel raw material is reheated to the Ac.sub.3 transformation temperature or higher after the hot forging to homogenize the steel as a single austenite phase. The reheating temperature is required to be at least the Ac.sub.3 temperature and no higher than 1250 C.

(83) Herein, the Ac.sub.3 transformation temperature is taken to be a value calculated according to formula (2), shown below.
Ac.sub.3 ( C.)=937.2-476.5C+56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr+38.1Mo+124.8V+136.3Ti+198.4Al+3315B(2)

(84) Each element symbol in formula (2) indicates the content, in mass %, of the corresponding alloying element in the steel.

(85) Performance of hot rolling including at least two passes carried out with a rolling reduction of 4% or more per pass

(86) In the presently disclosed techniques, the reheating to at least the Ac.sub.3 temperature and no higher than 1250 C. is followed by hot rolling including at least two passes carried out with a rolling reduction of 4% or more per pass. Such rolling allows sufficient working of the mid-thickness part. This can refine structure by promoting recrystallization and can contribute to improving mechanical properties. The number of passes carried out in the hot rolling is preferably 10 or less because mechanical properties improve as the number of passes is reduced.

(87) Conditions of Heat Treatment after Hot Rolling

(88) In the presently disclosed techniques, the steel is allowed to cool after the hot rolling, is then reheated again to at least the Ac.sub.3 temperature and no higher than 1050 C., and is subsequently rapidly cooled from the Ar.sub.3 temperature or higher to 350 C. or lower to improve strength and toughness of the mid-thickness part. The reheating temperature is set as 1050 C. or lower because reheating to a high temperature exceeding 1050 C. significantly reduces base metal toughness due to austenite grain coarsening.

(89) Herein, the Ar.sub.3 transformation temperature is taken to be a value calculated according to formula (3), shown below.
Ar.sub.3 ( C.)=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo(3)

(90) Each element symbol in formula (3) indicates the content, in mass %, of the corresponding alloying element in the steel.

(91) The temperature of the mid-thickness part is determined by simulation calculation or the like based on the plate thickness, surface temperature, cooling conditions, and so forth. For example, the temperature of the mid-thickness part may be determined by calculating a temperature distribution in the plate thickness direction by the finite difference method.

(92) In industry, the method of rapid cooling is normally water cooling. However, a cooling method other than water cooling, such as gas cooling or the like, may be adopted because the cooling rate is preferably as fast as possible.

(93) Tempering Temperature: 550 C. to 700 C.

(94) The rapid cooling is followed by tempering at at least 550 C. and no higher than 700 C. The reason for this is that a tempering temperature of lower than 550 C. does not effectively remove residual stress, whereas a tempering temperature exceeding 700 C. causes precipitation of various carbides and coarsens the structure of the base metal, leading to a significant decrease in strength and toughness. In particular, tempering at a temperature of 600 C. or higher is preferable for adjusting yield strength and improving low-temperature toughness in the tempering step. Tempering at a temperature of 650 C. or higher is more preferable.

(95) In industry, there are instances in which steel is quenched repeatedly to make the steel tougher. While quenching may be performed repeatedly in the disclosed techniques, the final quenching is required to involve heating to at least the Ac.sub.3 temperature and no higher than 1050 C., subsequent rapid cooling to 350 C. or lower, and subsequent tempering at at least 550 C. and no higher than 700 C.

(96) Conventional techniques struggle to achieve the excellent properties described above in a situation in which the working reduction ratio from the slab prior to working is 3 or less. However, according to the presently disclosed techniques, the desired properties can be achieved even in this situation.

(97) By performing quenching and tempering as described above in production of a steel plate according to this disclosure, a steel plate having excellent strength and toughness can be produced.

EXAMPLES

(98) Steels 1-32 shown in Table 1 were produced by steel making to obtain continuously-cast slabs that were then subjected to hot forging and hot rolling under the conditions shown in Table 2. The number of passes of hot rolling was 10 or less. The plate thickness after the hot rolling was in a range of 100 mm to 240 mm. After the hot rolling, quenching and tempering were performed under the conditions shown in Table 3 to produce steel plates indicated as samples 1-44 in Tables 2 and 3. The produced steel plates were tested as follows.

(99) (1) Tensile Test

(100) A round bar tensile test piece (: 12.5 mm, GL: 50 mm) was sampled from a mid-thickness part of each of the steel plates in a direction perpendicular to the rolling direction and was used to measure yield strength (YS) and tensile strength (TS).

(101) (2) Plate Thickness Direction Tensile Test

(102) Three round bar tensile test pieces (10 mm) were collected from each of the steel plates in the plate thickness direction, the reduction of area after fracture was measured, and evaluation was conducted using the smallest value of the three test pieces.

(103) (3) Charpy Impact Test

(104) Three 2 mm V notch Charpy test pieces having a longitudinal direction corresponding to the rolling direction were collected from the mid-thickness part of each of the steel plates, absorbed energy (.sub.VE.sub.60) was measured for each test piece by a Charpy impact test at 60 C., and the average of the three test pieces was calculated.

(105) (4) Hardness Measurement

(106) Test pieces for hardness measurement were collected from the surface and the mid-thickness part of each of the steel plates such that hardness of a cross-section parallel to the longitudinal direction of the steel plate could be measured. Each of the test pieces was embedded and polished. Thereafter, a Vickers hardness meter was used to measure the hardness of three points at a surface position (position 2 mm inward from the surface) and three points at a mid-thickness position (middle position) using a load of 98 N (10 kgf). An average value for each set of three points was calculated as the average hardness of the corresponding position. The hardness difference HV was calculated according to: HV=average hardness of plate thickness surfaceaverage hardness of mid-thickness part.

(107) Results of the tests described above are shown in Table 3.

(108) TABLE-US-00001 TABLE 1 Table 1 Steel Chemical composition (mass %) no. C Si Mn P S Ni Ti Al N B Cu Cr Mo V Nb 1 0.081 0.15 1.6 0.008 0.0009 0.6 0.010 0.043 0.0035 0.0012 0.26 0.8 0.25 0.020 2 0.087 0.10 1.3 0.004 0.0011 0.9 0.009 0.049 0.0030 0.0011 0.22 1.0 0.31 0.025 3 0.105 0.15 1.1 0.007 0.001 1.0 0.007 0.045 0.0033 0.0011 0.24 0.7 0.43 0.041 4 0.115 0.21 1.2 0.006 0.0007 1.5 0.010 0.035 0.0029 0.0010 0.11 0.9 0.35 0.034 5 0.119 0.17 1.2 0.005 0.0009 1.9 0.012 0.041 0.0030 0.0012 0.20 1.0 0.48 0.025 0.012 6 0.123 0.21 1.1 0.005 0.0007 2.1 0.010 0.045 0.0027 0.0010 0.19 0.8 0.44 0.042 7 0.120 0.16 1.1 0.004 0.0006 3.4 0.005 0.066 0.0042 0.0012 0.20 0.4 0.45 0.021 8 0.122 0.19 1.2 0.003 0.0004 2.2 0.011 0.044 0.0031 0.0012 0.15 0.9 0.46 0.035 9 0.125 0.20 1.2 0.005 0.0006 2.1 0.012 0.060 0.0040 0.0010 1.0 0.55 0.045 10 0.115 0.17 1.1 0.005 0.0006 2.4 0.010 0.055 0.0032 0.0012 0.20 0.8 0.50 0.040 11 0.160 0.23 1.5 0.004 0.0005 2.0 0.008 0.048 0.0029 0.0009 0.20 0.8 12 0.179 0.11 0.6 0.003 0.0003 4.2 0.009 0.053 0.0025 0.0008 0.50 13 0.193 0.21 0.9 0.004 0.0009 2.2 0.011 0.050 0.0028 0.0012 1.0 0.015 14 0.125 0.22 1.2 0.006 0.0005 2.0 0.009 0.045 0.0024 0.15 0.7 0.42 0.043 15 0.119 0.24 1.1 0.005 0.0008 1.9 0.012 0.005 0.0025 0.0011 0.21 0.9 0.50 0.045 16 0.120 0.04 0.6 0.003 0.0006 0.1 0.010 0.027 0.0039 0.0009 1.8 0.87 0.090 17 0.123 0.13 1.1 0.003 0.0004 1.8 0.011 0.035 0.0028 0.0012 0.20 0.9 0.50 0.045 18 0.129 0.24 1.2 0.005 0.0012 0.9 0.008 0.004 0.0022 0.0006 0.25 1.0 0.45 19 0.139 0.17 1.3 0.006 0.0009 1.5 0.009 0.004 0.0028 0.0009 0.30 0.6 0.50 0.004 20 0.110 0.28 1.1 0.006 0.001 0.5 0.010 0.040 0.0030 0.0010 0.21 0.7 0.44 0.150 21 0.122 0.21 0.7 0.005 0.0008 1.0 0.009 0.035 0.0028 0.0006 0.25 0.9 0.45 0.060 0.009 22 0.228 0.25 1.3 0.005 0.0009 0.6 0.009 0.043 0.0030 0.0012 0.35 1.1 0.44 0.038 23 0.152 0.56 1.0 0.006 0.0006 0.9 0.010 0.044 0.0032 0.0015 0.17 0.9 0.52 24 0.105 0.40 0.3 0.009 0.0015 1.1 0.009 0.050 0.0030 0.0012 0.22 1.3 0.58 0.035 25 0.136 0.35 1.2 0.019 0.0012 0.5 0.011 0.045 0.0038 0.0009 0.26 1.0 0.52 0.045 26 0.144 0.15 1.3 0.011 0.007 1.2 0.011 0.025 0.0055 0.0006 0.13 1.1 0.44 0.039 27 0.082 0.26 1.6 0.006 0.0005 1.6 0.003 0.050 0.0040 0.0005 0.6 0.35 28 0.093 0.29 1.0 0.005 0.0007 1.5 0.024 0.035 0.0041 0.0008 0.9 0.41 0.010 29 0.122 0.26 1.1 0.006 0.0009 1.5 0.011 0.095 0.0039 0.0006 0.44 1.0 0.44 30 0.120 0.26 1.1 0.007 0.001 2.0 0.006 0.040 0.0085 0.0005 0.33 0.7 0.60 31 0.130 0.26 1.1 0.008 0.0011 2.1 0.008 0.044 0.0030 0.0040 0.26 0.8 0.50 32 0.105 0.17 0.8 0.014 0.0015 1.2 0.012 0.035 0.0030 0.0009 0.17 0.5 0.35 0.020 Steel Chemical composition (mass %) Ac.sub.3 Ar.sub.3 no. Mg Ta Zr Y Ca REM Ceq.sup.IIW C. C. Remarks 1 0.0020 0.62 877 687 Conforming steel 2 0.0110 0.65 873 685 Conforming steel 3 0.0014 0.60 875 686 Conforming steel 4 0.0015 0.68 854 652 Conforming steel 5 0.0021 0.75 843 616 Conforming steel 6 0.0013 0.72 841 617 Conforming steel 7 0.0022 0.72 809 552 Conforming steel 8 0.0019 0.75 838 606 Conforming steel 9 0.0017 0.78 849 605 Conforming steel 10 0.0051 0.74 840 598 Conforming steel 11 0.0023 0.72 798 614 Conforming steel 12 0.66 768 536 Conforming steel 13 0.0018 0.69 793 642 Conforming steel 14 0.0016 0.69 839 619 Conforming steel 15 0.0015 0.73 845 623 Conforming steel 16 0.78 913 723 Conforming steel 17 0.0020 0.0017 0.73 846 627 Conforming steel 18 0.055 0.0013 0.70 854 669 Conforming steel 19 0.007 0.0020 0.70 832 625 Conforming steel 20 0.004 0.0010 0.60 907 711 Conforming steel 21 0.60 877 707 Conforming steel 22 0.82 825 644 Comparative steel 23 0.67 880 675 Comparative steel 24 0.0021 0.63 906 723 Comparative steel 25 0.0097 0.70 885 683 Comparative steel 26 0.0011 0.77 842 641 Comparative steel 27 0.0023 0.65 861 632 Comparative steel 28 0.62 875 672 Comparative steel 29 0.0023 0.72 859 643 Comparative steel 30 0.72 844 610 Comparative steel 31 0.0023 0.73 846 609 Comparative steel 32 0.0016 0.50 871 709 Comparative steel Values for Ceq.sup.IIW, Ac.sub.3, and Ar.sub.3 were calculated according to formulae (1) to (3) in the specification Underlining indicates deviation from the scope of this disclosure

(109) TABLE-US-00002 TABLE 2 Table 2 Hot forging Maximum Cumulative working Slab Heating Working start Working end working Strain reduction Sample Steel thickness temperature temperature temperature reduction rate per pass no. no. (mm) ( C.) ( C.) ( C.) (%) (/s) (%) 1 1 250 1200 1155 1020 20 0.1 10 2 2 250 1270 1160 1120 15 0.1 7 3 3 310 1200 1170 1020 15 0.1 5 4 4 450 1250 1235 1060 15 0.1 10 5 5 310 1270 1245 1120 20 0.1 10 6 6 310 1270 1240 1120 20 0.1 10 7 7 310 1270 1245 1100 20 0.1 10 8 8 310 1200 1165 1050 20 0.1 5 9 9 450 1270 1250 1080 15 0.1 10 10 10 310 1250 1220 1120 20 0.1 7 11 11 310 1250 1215 1150 20 0.1 7 12 12 310 1270 1245 1100 20 0.1 10 13 13 310 1300 1270 1150 20 0.1 10 14 14 250 1200 1160 1050 15 0.1 5 15 15 310 1270 1235 1100 20 0.1 10 16 16 450 1270 1255 1050 15 0.1 10 17 17 310 1200 1165 1050 20 0.1 5 18 18 310 1270 1235 1050 15 0.1 10 19 19 310 1270 1245 1100 20 0.1 10 20 20 250 1200 1135 1050 15 0.1 5 21 21 250 1270 1150 1050 20 0.1 10 22 22 310 1200 1165 1030 15 0.1 5 23 23 250 1200 1145 1050 15 0.1 10 24 24 250 1200 1150 1050 15 0.1 10 25 25 310 1270 1235 1100 20 0.1 10 26 26 310 1270 1240 1100 20 0.1 10 27 27 310 1270 1250 1100 20 0.1 10 28 28 310 1270 1250 1100 20 0.1 10 29 29 310 1270 1245 1100 20 0.1 10 30 30 310 1270 1235 1100 20 0.1 10 31 31 310 1270 1235 1100 20 0.1 10 32 32 310 1270 1250 1100 20 0.1 10 33 5 310 1050 1005 850 15 0.1 3 34 5 310 1200 1165 900 15 0.1 4 35 5 310 1200 1165 1050 7 0.1 4 36 5 310 1200 1170 1050 15 10 8 37 6 310 1250 1215 1050 15 0.1 8 38 6 310 1270 1250 1050 20 0.1 10 39 6 310 1270 1235 1050 20 0.1 5 40 6 310 1270 1260 1050 20 0.1 5 41 6 310 1270 1245 1050 20 0.1 10 42 6 310 1270 1240 1050 20 0.1 5 43 6 310 1270 1235 1050 20 0.1 10 44 6 310 1270 1245 1050 20 0.1 10 Hot forging Hot rolling Working Width Die Reheating Rolling Rolling Plate reduction Sample direction shape temperature reduction conditions thickness ratio from no. working ratio ( C.) (%) (*1) (mm) slab 1 Yes 1.1 1150 55 Conforming 100 2.5 2 No 1.1 1150 39 Conforming 130 1.9 3 No 1.5 1100 51 Conforming 130 2.4 4 Yes 1.5 1200 45 Conforming 210 2.1 5 Yes 1.5 1130 45 Conforming 150 2.1 6 Yes 1.5 1130 32 Conforming 180 1.7 7 Yes 1.5 1170 20 Conforming 210 1.5 8 No 1.5 1130 27 Conforming 180 1.7 9 Yes 2.5 1200 42 Conforming 240 1.9 10 No 1.5 1150 27 Conforming 180 1.7 11 No 1.5 1150 40 Conforming 150 2.1 12 Yes 2.0 1200 32 Conforming 180 1.7 13 Yes 2.0 1200 45 Conforming 150 2.1 14 No 1.5 1130 53 Conforming 100 2.5 15 Yes 1.5 1170 45 Conforming 150 2.1 16 Yes 1.5 1200 50 Conforming 210 2.1 17 No 1.5 1130 40 Conforming 150 2.1 18 Yes 1.5 1170 56 Conforming 130 2.4 19 Yes 1.5 1200 53 Conforming 130 2.4 20 No 1.5 1130 53 Conforming 100 2.5 21 No 1.5 1130 50 Conforming 100 2.5 22 No 1.5 1100 32 Conforming 180 1.7 23 Yes 1.1 1150 58 Conforming 100 2.5 24 Yes 1.1 1150 58 Conforming 100 2.5 25 Yes 1.5 1200 45 Conforming 150 2.1 26 Yes 1.5 1170 45 Conforming 150 2.1 27 Yes 1.5 1200 45 Conforming 150 2.1 28 Yes 1.5 1130 45 Conforming 150 2.1 29 Yes 1.5 1170 45 Conforming 150 2.1 30 Yes 1.5 1200 45 Conforming 150 2.1 31 Yes 1.5 1200 32 Conforming 180 1.7 32 Yes 1.5 1200 32 Conforming 180 1.7 33 No 1.5 1150 43 Conforming 150 2.1 34 Yes 1.5 1150 48 Conforming 150 2.1 35 No 1.5 1150 48 Conforming 150 2.1 36 No 1.5 1100 43 Conforming 150 2.1 37 Yes 1.5 800 48 Conforming 150 2.1 38 Yes 1.5 1150 32 Conforming 180 1.7 39 Yes 1.5 1150 32 Conforming 180 1.7 40 Yes 1.5 1100 32 Conforming 180 1.7 41 Yes 1.5 1100 32 Conforming 180 1.7 42 Yes 1.5 1100 32 Conforming 180 1.7 43 No 1.0 1100 27 Conforming 180 1.7 44 Yes 1.5 1150 32 Non- 180 1.7 conforming (*1) Conforming indicates that at least two passes were carried out with a rolling reduction of 4% or more per pass Underlining indicates deviation from the scope of this disclosure

(110) TABLE-US-00003 TABLE 3 Table 3 Base metal properties Plate thickness Heat treatment conditions of final heat treatment direction Hold- Tem- tension Hardness Reheating ing Cooling end Tempering pering reduction difference Sample Steel temperature time temperature temperature time YS TS .sub.vE.sub.60 of area HV no. no. ( C.) (min) ( C.) ( C.) (min) (MPa) (MPa) (J) (%) () Remarks 1 1 1000 10 150 670 30 517 738 126 60 25 Example 2 2 900 30 100 670 40 539 653 164 70 21 Example 3 3 900 30 100 645 90 542 634 155 75 23 Example 4 4 900 30 100 660 30 535 654 136 65 24 Example 5 5 900 30 150 645 80 547 670 167 70 25 Example 6 6 900 30 100 670 40 613 694 168 70 26 Example 7 7 900 30 100 655 50 608 669 154 60 24 Example 8 8 850 30 100 660 50 567 697 133 60 26 Example 9 9 900 60 100 660 60 576 657 126 70 27 Example 10 10 900 30 200 660 50 571 664 146 75 28 Example 11 11 900 30 100 650 60 556 697 156 60 26 Example 12 12 900 30 100 660 60 568 664 136 75 24 Example 13 13 900 30 150 660 60 550 710 135 65 26 Example 14 14 900 10 100 670 40 572 694 178 65 26 Example 15 15 900 30 150 670 40 565 633 163 65 25 Example 16 16 950 60 100 670 30 590 655 145 55 26 Example 17 17 900 30 150 670 30 525 625 167 55 27 Example 18 18 900 30 100 650 60 568 663 142 60 21 Example 19 19 950 30 100 650 60 557 669 146 65 24 Example 20 20 900 30 150 650 60 552 662 156 60 23 Example 21 21 900 30 100 650 60 614 684 146 65 21 Example 22 22 900 30 100 660 50 621 765 37 45 29 Comparative Example 23 23 900 30 150 650 60 602 669 36 75 24 Comparative Example 24 24 900 10 150 670 30 462 607 36 70 23 Comparative Example 25 25 900 30 150 670 30 527 633 35 65 24 Comparative Example 26 26 900 30 150 670 30 561 680 25 70 26 Comparative Example 27 27 900 30 150 670 30 536 681 26 65 34 Comparative Example 28 28 900 30 150 670 30 555 689 24 65 36 Comparative Example 29 29 900 30 150 650 60 539 704 25 65 24 Comparative Example 30 30 900 30 150 660 60 532 694 32 65 26 Comparative Example 31 31 900 30 100 660 50 526 661 33 60 24 Comparative Example 32 32 900 30 100 660 60 454 542 45 65 45 Comparative Example 33 5 900 30 150 650 60 517 679 105 20 26 Comparative Example 34 5 900 30 150 650 60 539 621 88 15 25 Comparative Example 35 5 900 30 100 660 50 552 681 83 25 24 Comparative Example 36 5 900 30 150 660 50 572 695 91 20 24 Comparative Example 37 6 900 30 100 670 30 579 616 22 45 26 Comparative Example 38 6 1100 10 150 650 60 625 722 32 65 24 Comparative Example 39 6 750 30 100 650 60 463 533 146 60 20 Comparative Example 40 6 900 30 480 650 60 378 576 28 55 24 Comparative Example 41 6 900 30 150 730 30 462 560 170 60 26 Comparative Example 42 6 900 30 150 400 60 596 759 65 55 35 Comparative Example 43 6 900 30 150 650 60 537 702 175 25 26 Comparative Example 44 6 900 30 150 660 60 512 636 26 45 28 Comparative Example Underlining indicates deviation from the scope of this disclosure

(111) It can be seen from Table 3 that for each steel plate obtained in accordance with this disclosure (samples 1-21), YS was 500 MPa or more, TS was 610 MPa or more, base metal toughness (.sub.VE.sub.60) was 70 J or more, reduction of area in the plate thickness direction tensile test was 40% or more, and the hardness difference HV was 30 or less. Accordingly, each of these steel plates had excellent base metal strength, toughness, plate thickness direction tensile properties, and material homogeneity.

(112) In contrast, it can be seen that at least one of these properties was poor in each of samples 22-44 having a chemical composition or production conditions outside of the suitable ranges.

REFERENCE SIGNS LIST

(113) 1 upper die

(114) 2 lower die

(115) 3 slab