SG194424A1

SG194424A1 - Hot rolled sheet steel having excellent formability, and method for producing same

Info

Publication number: SG194424A1
Application number: SG2013054267A
Authority: SG
Inventors: Koichiro Fujita; Kazuhiro Hanazawa; Yasuyuki Takada
Original assignee: Jfe Steel Corp
Priority date: 2009-05-25
Filing date: 2010-05-19
Publication date: 2013-12-30
Also published as: WO2010137598A1; MY153088A; CN102449178B; JP5407552B2; CN102449178A; KR101369098B1; JP2010270383A; KR20120022969A

Abstract

[Title of Irvencion] HOT ROLLED STEEL SHEET WITH EXCELLENT FORMABILITY AND METHOD FOR MANUFACTURING THE SAMEA hot rolled steel sheet having an improved lArl without causing excessive coarse grain formation as well as having aging-resistance and soft/high ductile properties, and a method for manufacturing the same are provided. A hot rolled steel sheet contains, on a percent by mass basis,0.03% to 0.07% of C, 0.1% or less of Si, 0.05% to 0.5% of Mn, 0.03% or less of P, 0.03% or less of S, 0.02% to 0.1% of sol. Al, 0.005% or less of N, less than 0.005% of Nb+Ti, 0.0003% to 0.0020% of B, and the balance being Fe and inevitable impurities. In this steel sheet, the average crystal grain diameter is in a range of 12 to 25 and lArM0.25 and ANg.0 M9e. hold.Nil

Description

DESCRIPTION

[Title of Invention] HOT ROLLED STHEL SHEET WITH EXCELLENT

FORMABILITY AND METHOD FOR MANUFACTURING THE ZAME

[Technical Fleld) 0001)

The present invention relates to a hot rolled steel sheet used for materials for automobiles, home electric appliances, and the like, and more particularly relates to a hot rolled steel sheet having formability suitable for press forming and the like and also being excellent in aglng- resistance and in-plane anisotropy. [Background Art]

[0002]

A hot rolled steel sheet used for automobiles, sleptric goods, and the like ls reguired to have high formabllity, and in particular, development io impart soft properties and high ductility to a hot rolled steel sheet used for deep drawing application, such as a compressor cover, has been aggressively carried out. In addition, as times nevessary for transportation and storage are increased concomitant with recent trend of globalization of manufacturing hases, the stability in material guality, in particular, aging- resistance has also been increasingly considered as

“Pw important. [G003]

Az for a hot rolled steel sheet for deep drawing which is excellent in aging-resistance, Patent Literature 1 has proposed a technigue in which soft/high ductile properties are ensured by defining upper limits of reinforcing elements, such as C and Mn, and the aging-resistance 1s enhanced by adding 0.001% or more of B to fix N in steel. However, according toe the technique described above, since the addition amount of B of the hot rolled steel sheet is as high as 0.001% or more, a problem in that a (Ar! {planar anisotropy of r-value), which is an index indicating the in- plane anisotropy of deep drawing, is increased may arise. [C004] in order to overcome the above problem of a het rolled steel sheet added with B, Patent Literature I has proposed a technique in which finish rolling iz performed at a temperature higher than the Ars point + 20°C or more by specifying the addition amount of B to improve the (Ari. In addition, Although not relating a het rolled steel sheet added with B, a technique has been proposed in Patent

Literature 3 in which in order to improve the [Ari of a hot rolled steel sheet having a specified P content, finish rolling is performed at the Ar; point or more.

However, as disclosed in Non-Patent Literature 1, a hot rolled steel sheet added with B tends to form coarse grains and is greatly dependent on hot roiling conditions,

Therefore, if finish rolling is simply performed at a high temperature as proposed in Patent Literatures 2 and 3, coarse graing are excessively formed, and rough surfaces are generated on rhe steel sheet after press forming. In addition, although the hot rolled steel sheet proposed in

Patent Literature 3 is not added with B, since grain growth properties are enhanced when the content of P is extremely decreased (0.005% or less), ag in the case of the hot rolled steel sheet proposed in Patent Literature Z, the generation of rough surfaces due to excessive cecarse grain formation may cause a problem. [o00e]

On the other hand, Patent Literature 4 has disclosed a technigue in which in order to suppress an excessive growth of grains, Ti and/or Nb is added to a hot rolled steel sheet.

However, Ti and Nb each have significantly high effects of suppressing recryvstalllization and suppressing grain growth.

Hence, in order to improve the (Axl of a hot rolled steel she2et added with Ti and/or No and also to ensure agpropriate grain diameters, the hot rolled steel sheet must be held at a high temperature region for 2a long period of time. In addition, since B lig not added in the technique disclosed in

: we i pe

Patent Literature 4, aging in a low © reglon inevitably cocurs, and hence, a preblem of inferior aging-resistance may also arise. {Citation List] [Patent Literatures] [QC071

PPL 1) Japanese Unexamined Patent Application

Publication No. 5Z2-125411 [PTL 21 Japanese Unexamined Patent Application

Publication No. 2-20%423 [PTL 3] Japanese Unexamined Patent Application

Publication No. 2-209%424 [PTL 4] Japanese Unexamined Patent Application

Publication No. 2-30713 [Non-Patent Literature]

[0008]

NPL 1] Yo Ito et al., "Hot Rolled Steel BZheet ¥FN For

Preas Forming”, KAWASAKI STEEL GIHO, published by Kawasaki

Steel Corp., 1973, Vol. 5, No. 2, pp. 224 to 234 [Summary of Invention] [Technical Problem!

FOG09)

Ag described above, although several techniques for improving the |Ar| of a hot rolled steel sheet for deep drawing have been disclosed, since incidental problems, such w 5 ow ag genevatlion of rough surfaces and degradation of aging- resistance, occur, there has been room for improvement. The present invention was made In consideration of this situation, and an obiesct of the present invention is to provide a hot rolled steel sheet having an improved (Ar without causing excessive coarse grain formation as well as i having excellent formability and aging-resistance, and an advantageous method for manufacturing the same. [Sciution to Problem] [00107 in crder to achieve the above object, the present inventors carried out intensive research on recrystalilzation behavior and grain growth behavior of B- added aluminum killed steel. As a result, a new finding was obtained that when the component composition of steal, finish rolling temperature and reduction rate, cooling conditions after finish roiling, and the like wera controlled, in particular, when a relatively large amount of

P was added as a component, and a step of spontansaous cooling treatment for a zhort periced of time was performed after hot rolling was completed, in a hot rolled steel sheet having aging-resistance and soft/high ductile properties, the [Ar] could be reduced without causing exXCessive Coarse grain formation which would generate rough surfaces and the like,

a 5 [00113

The present invention was made based on the sbove finding, and according to one aspect of the present

Invention, there is provided: {i} a hot rolled steel sheet containing, on a percent by mass basiz, 0.03% wo 0.07% of C, 0.1% or less of Si, 0.05% to 0.5% of Mn,

C.01% to 0.03% of 7, 0.03% or less of 3, 0.02% to 0.1% of sol. Al, 0.00%% or less of HN, less than 0.005% of KL+TL 0.0003% te 0.0020% of B, and the balance being Fe and inevitable impurities, wharein an average crystal grain diameter is in a range of 12 te 25 um, and [Ar{s0.25 and AIZ20 MPa hold. [00121

According to another aspect of the present invention, there is provided: {2} a method for manufacturing a hot rolled steel shaetl comprising the steps of performing a final pass of hot rolling on a steel slab which contains, on a percent by mass basis,

wT

G.03% to 0.07% of 0.1% or less of Si, 0.05%% to D.5% of Mn, 0.01% to 0.03% of Pp, (0.03% or less of 3, 0.02% to 0.1% of sol. AL, 0.005% or less of N, less than 0.005% of Nk+Ti, 0.0003% wo 0.0020% of B, and the balance being Fe and inevitable impurities at a rolling temperature of Ary+30°C or more and a reduction rate of 15% or more; performing spontaneous cooling for 8.5 to 10 seconds after the hot rolling: performing cooling to 740°C cr less at a cooling rate of 20°C/sec or more; and i performing coiling at a temperature in a range of 580°C to

T040°C. iAdvantagsous Effects of Invention] [00131

According to the present invention, sufficient aging- resistance and scft/high ductile properties are inparied to a hot rolled steel sheet, and in addition, the in-plane anisotropy can be reduced while the crystal grain diameter suitable for press forming is maintained. Hence, according to the hot rolled steel sheet of the present invention, a product, such as a compressor cover, which requires highly complicated processing and which is difficult to be formed from a related hot rolled steel sheet can be manufactured without any guality degradation, such as rough surfaces, generated in forming. [Description of Embodiments} [ooid]

Reasons for limiting component compositions and manufacture conditions of the present invention will be described. Although the content of every element in a steel sheet 1g on a "percent by mass” basis, it is simply represented by 7%" unless otherwise particularly noted. (1; Component composition range

Cr 0.03% to 0.07%

When the content of C is high, since a large amount of carbide iz generated, the elongation of a hot rolled steel sheet is reduced, and the formability thereof is degraded: hence, the content of iz set te 0.07% or less. On the other hand, when the content of ¢ is extremely decreased, manufacturing cost is increased, and hence, the lower limit is set to 0.03%.

[0015]

Si: 0.1% or less

If the content of 3i is excessive, since the strength is increased, and the fermability is degraded, the content ig set to 0.1% or less. In the present invention, Si ig an element which degrades the formabillity, and hence the content thereof is preferably decreased. [C016]

Mn: 0.05% Lo 0.5%

Since Mn fixes 8 in the form of Mal and functions to improve hot ductility, the content of Mn must be set to 0.05% or more. On the other hand, since an excessive addition not only hardens steel but also degrades the formability thereof, the upper limit of the content is set to 0.5%. [0C17]

Pr 0.01% wo 0,03%

P is a characteristic element in the present invention.

That is, when the content of FP is small, the grain growth is anhanced, and the probability of generating rough surfaces caused by coarse grain formation may be increased. Hence, in the present invention, at least 0.01% of P is contained.

However, P is a solid-sclution-strengthening element and hardens steel when the content thereof ls excessive, and nence the upper limit is set te 0.03%.

[0018]

S: 0.03% or less 8 1s an element which degrades hot ductility and formability, and hence the content thereof ls preferably decreassd. In addition, although § is fixed in the form of

Mrs in order to improve hot ductility and formablility, the elongation is decreased when the amount of Mnf is emcessive, and hence the upper limit of the content of & is set to 0.03%, [oola: sol, Al: C.02% te 0.1%

Al ig not only effective as a deoxidizing agent but also enhances the aging-resistance by fixing N, which ig not fixed by B, in the form of ALN. As the content of sol. Al, 3.02% or more is required. On the other hand, since excessive addition increases manufacturing cost, the upper limit of sol. Al is set to 0.1%.

[0020]

NM: 0.005% or less

MN is an element of aging a hot rolled steel sheet, and a smaller content thereof is more preferable; however, an excesaive decrease causes a remarkable cost increase. In the present invention, since B and Al are added to fiz N, when the content of ¥ is 0.005% or less, 1lts adverse influence can be ignored, and hence the upper limit 1s set to 0.005%.

[0021]

Np+Ti: less than 0.000%

Since Nb and Ti are each a powerful recrystalliizatiocon suppressing element, if these elements are excessively contained, the reduction of the Ar! is difficult to perform.

In addition, since these elements are each also a powarful grain growth suppressing element, 1f these elements are excessively contained, crystal grains of a hot relled steel sheet are formed to have a smaller grain diameter, and the elongation thereof is decreased. Although a smaller amount of Nbh+Ti ig more preferable from the point described above, when the total content of Nh and Ti 1s lass than 0.005%, the above adverse effect can be ignored: hence, the ugper limit of Nbh+Ti ig set to less than 0.005%. [D022]

B: £.0003% to 0.0020% ¥ is an element to improve the aging-resistance by fixing N. In addition, since B has a function of sppropriately coarsening crystal grains and also has an affect of suppressing a decrease in elongation caused by decreasing the size of crystal grains, the content of B must be €.0003% or more. Cn the other hand, since the [Ari is increased when B is excessively added, the upper limit thereof is set to §.0020%.

In addition, the balance other than the above components includes Fe and inevitable impurities. As the impurities, for example, 0.02% or less of Cu and 0.02% ov less of Ni may be contained.

we 1D = (2) Averages crystal grain diameter

The average crystal grain diameter must be set in a range of 12 to 25 wm. The reascns for this are that when the average crystal grain diameter is less than 1Z um, the vield strength is increased, and the formation is difficult to perform, and whan the average crystal grain diameter is more than 25 um, rough surfaces are generated in press forming. In addition, the average crystal grain diameter is more preferably in a range of 12 to 23 um,

[0024] (3) [Ax [£0.25

When the planar anisotropy [Ari of the average Lankford

Value represented by the following formula (1) is more than 0.25, the yield of draw forming decreases, and hence the

Ar must be set toe 0.25 or lass.

Note

VAT | =] {(rptTop=21an} /2 1 Formula {1}

In this formula, ry represents the Lankford Value in a rolling direction (RD), ry represents the Lankford Values in a direction (TD) perpendicular to the rolling direction, and ry; represents the Lankford Valus in a direction {DD} at 45° to the rolling direction. The average Lankford Value is preferably set te 0.8 or more in terms of reducticn of forming lead. In addition, the steel sheel of the present invention is a hot rolled steel sheet, and the average

Lankford Value 1s approximately 1.0 or Less.

[8025] {4} AILZ0 MPa

An aging index AT indicates, after a hot rolled steel sheet is pulled at an elongation rate of 7.5%, the difference in tensile stress before and after an aging treatment performed at 100° for 30 minutes. In addition, since a yield-elongation phenomenon may ooour in some Cases after the aging treatment, strictly, & lower yield stress after the aging treatment or {.2% proof stress is used.

When the aging indez AT excesds 20 MPa, since the material quality 1s changed during transportation or coil storage, the press forming conditions must be optimized; hence, the product cost is increassd. On the other hand, if the AI is 2C MPa or less, since the change in material guality causes no problems, the AI 1s set to 20 MPa or less and more preferably set to 10 MPa or less. (0026) {5} Manufacturing process

On a steel slab obtained by casting steel prepared to have the component composition ranges described above, a : final pass of hot rolling was performed at a rolling temperature of Ar;+50% or more and a reduction rate of 15% cr more immadiately after casting or after reheating, spontaneous cooling is performed for 0.5 to 10 seconds after the hot volling is completed, cooling 1s subsequently performed to 700° or lesz at a cooling rate of 20% /sec or more, and coiling is then performed at a temperature of 520°C to 700°. Although it 1s not necessary to particularly define the above reheating temperature, reheating must be cerformed under condition which can ensure a finish rolling temperature and is generally performed at a temperature of 1,8B0°C to 1, 300°C.

Foz

In a hot rolled steel sheet added with B as disclosed in the present invention, in order fo reduce the in-plane anlsotrcpy thereof while the crystal grain diameter suitable for press forming is maintained, it is important to optimize the finish rolling temperature and reduction rate, the cociing condition after the finish rolling, and the colliing temperature.

[0028]

The presant inventors investigated the relationship petween the (Ari of a B-containing hot rolled steel sheet having the composition as described above and the rolling temperature of the final pass of het rolling. In this ambodiment, the final pass of hot-rolliing indicates a final pass, that is, a final reducing stand, in finish rolling of hot-rolling, and the rolling temperature (also called a finish rolling temperature) is a temperature at an outlet

Ga 15 i side of this reducing stand. As a result, it was found that the Ar! of the hot rolled steel sheet decreased as the rolling temperature of the final pass was increased from the

Ars point, and at a temperature of Rrs;+30°C or more, the [Ar showed an approximately constant values. Although the reason for this has not been clearly understood, when the rolling temperature of the final pass is in the vicinity of the Ar point, the vgs value of the formula {1} 1s larger than the rs value and the rae value, and as the rolling hemperature of the final pass is increased, the rg value and the rg valus are increased while the ry value 1s decreased, so that the

Ari of the formula (1) decreases; hence, it is estimated that the effect of reducing the Ar] is saturated at a temperature of the Ary point + 50°C.

[0028]

Accordingly, in the present invention, in order to reduce the [Arl, the final pass 1s performed at a high rolling temperature of Ary+h0°C or more. However, the finish roiling temperature is preferably set te 850°C or less in terms of reduction of scale lavers. In addition, in the present invention, after the finish rolling, spontaneous conling is performed for a predetermined time. The reason for this iz that in order to decrease the [Ar], a processed austenite structures formed by finish rolling is recrystallized in an austenite region to randomize the fe ie pas aggregate structure. At this stage, since the recrystallization of the processed austenite structure is not enough when the spontaneous cooling is performed for iesss than 0.5% seconds after the finish rolling, in the present invention, the cooling time 1s set to 0.5 seconds or more.

F00340]

When the finish rolling is performed at 2 high temperatura, and the spontanscus cooling ig then performed for a long period of time as described above, the [Ar] is decreased; however, axcessive coarse grain formation, which is a character of RB-added steel, occurs, and as a result, a press formed product has rough surfaces. In order to solve this problem, ths final pass reduction rate (reduction vate in the final pass) is set to 15% or more and more preferably 20% or more to increase the number of recrystallization nuclei, and the cooling time after the final rolling is further set to 10 seconds or less and prefaracly leas than 5 seconds to suppress the grain growth of austenite grains, sc that the size of crystal grains after the ferrite transformation iz controlled in an appropriate range.

In addition, the above reduction rate is preferably set to less than 25% in order to improve the steel-sheet shape.

RUUCEE

After a predetermined holding {(spontanecus cooling)

= 17 =~ time passes, cooling ig performed to 700° or less abt a cooling rate of 20°C/sec or more, and ceiling is then performed at 580°C wo 700°C. In order to suppress the grain growth during cooling, the cooling rate must be sel Lo 20°C /sec or more. In addition, when the colling temperature is more than 700°C, the surface conditions are degraded by scales generated during coiling, and when the coiling temperature 1s less than 5%0%, since the AL exceeds 20 MPa, the change in material quality occurs during transportation and storage. Therefore, the coiling temperature is set in a range of 594% to T00%C. The coiling temperature 1s preferably €25°C or more. In addition, in order to avoid a large cost increase, in general, the upper limit of the above cooling rate is approximately 500°C/sec. [G0321

For the hot rolled steel sheet of the present invention, properties of a pickied material are not different from those of a black material. In addition, sven when hot dip galvanizing is performed after pilokling or on a black material which is not processed by pickling, no problems ocour. Although conditions for temper rolling are not particularily limited, since the elongation performance of a steel material is degraded when the elongation rats therect is too high, the elongation rate of temper rolling is preferably set to 2% or less.

je. 18 i

P0033]

In addition, the hot rolled steel gheest of the present invention ia a soft hot rolled steel sheet suitable for deep drawing application having a yield stress of approximately 250 MPa or less and a sheet thickness of approximately 2 to & mim. [Examples]

F00347 {Example 1)

After steel having components shown in Table 1 was formad, and a working Formaster test piece having a diameter of & mm and a height of 12 mm was obtained therefrom by cutting, heating to 1,200°C, cooling to 850% at a cooling rate of 10% /sen, compression at 950°C at a strain of 30%, and cooling to 200°C at a cooling rate of 10°%C/sec were performed on the zbove test pilece in this crder. The Ar; point measursd from a thermal sxpansion curve in cooling was 823°C. After this steel was heated to 1200°C, hot rolling was performed under the conditions ghown in Table 2, so that a hot rolled steal sheet having a thickness of 3.2 mm was obtainad.

[0038]

After the hot rolled steel sheet thus obtained was processed by pickling, temper rolling at an elongation rate of 1% was performed, and evaluation of mechanical properties and texture observation were performed. For the evaluation of mechanical properties, a No. 5 test piece of JIS Z 2201 (12098) was obtained along a roiling direction {RD}, and a tensile test was performed in accordance with JIS 2 2241 (1998). [00367 ; For measurement of the Lankford Value, No. 5 test pieces of JIS 4 220% (1998) were obtained along the rolling direction (BRD), a direction perpendicular thereto (TD), and a direction at 45% to the rolling direction (DD), and measurement was performed at a strain of 15%. In addition, scoording toe the formula (1), the [Arl was cbhtained. 0037)

For evaluation of the AIL, after a2 No. 5 test piece of

JIS 72 22¢1 (1298) was c¢biained along the rolling direction {RD}, a heat treatment was performed for 30 minutes at 100°C after a pre-strain of 7.53% was applied, and evaluation was performed by the difference hebween the stress {stress when a pre-strain of 7.5% was applied) before the heat treatment and the yield stress after the healt treatment. [C038]

The steel sheet at a position of 1/4 thickness in a depth direction was etched using a nital sclution, and the average crystal grain diameter was measured In accordance with a cutting method described in JIS G 05bZ2 (1993). The

’ [ER 1 mr by ony Tm Yee Ye ~ results are also shown Table Z.

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_- 23 ow [00411

According to conditions B, ©, G, and H of the examples of the present Invention, thers was cbtainsd a hot rolled steel sheet of soft /high ductile properties having a yield stress YP of 220 MPa or less and an elongation El of 48% or mover sufficient aging~-resistance having an AT of 20 MFa or lesz; and a small anisotropy having a Ar] of $6.85 or less.

In addition, the average crystal grain diameters of the hot rolled steel sheets manufactured under the conditicens B, OC,

G, and H of the examples of the present invention were each within a range of 12 te 2% um, and hence the generation of rough surfaces may not ogour in press forming. Furthermore, it was found that since the coiling Lemperatures under the conditions B, C, and H of the examples of the present invention were each 625% or more, the AI of each of the hot rolled steel sheets thus obtained was 10 MPa or less, and the aging-rezistance was particularly supsrior. [004273

On the other hand, under a condition A in which the spontanecus cooling time was short, conditions B and K in which the rolling temperature of the final pass of the finish rolling was low, and a condition I in which the reduction rate of the final pass of the finish rolling was low, since the recrystallization in the sustenite region did not sufficiently progress, the anilsotropy of each of the hot

- 24 = rolled steel gheets manufactured under these conditions was high, and the [Ari was more than 0.25. In addition, in the hot rolled steel sheets manufactured under a condition I in which the spontaneous cooling time was long and a condition

J in which the cooling rate was low, large and coarse grains ware formed. In addition, in the hot rolled stesl sheet manufactured under a condition FF in which the coiling temperature was less than 590°C, the a1 was more than 220 MPa. [004371 {Example 2}

Steel containing compcensnts shown in Table 3-1 was formed, and the Ar; point was measured in a manner similar to that of Example 1. Subsequently, after the steel was heated to 1,200°C, the final pass of the finish rolling of hot rolling was performed at a reduction rate of 22% and a temperature of 875%, spontaneous cooling was then performed for 4.2 seconds after the finish rolling, cooling was subsaguently performed to 630°C at a cocling rate at 25°C/sec, and coiling was then performed without any additional steps. 00441

The mechanical properties, the Lankford Value, the AT, and the averages crystal grain diameter of each of the obtained hot-rolled stesl sheels were measured by methods similar to those of Example 1. The results are also shown in Table 3-2,

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[6046]

According to steel Nos. 3 and 4 in which the component composition of the present invention was satisfied, there was obtained a hot rolled steel sheet of soft /high ductile properties having a vield stress YP of 220 MPa or lass and an elongaticn Bl of 48% or more; sufficlent aging-resisztance having an AD of Z0 MPa or less; and a small anisotropy having a (Ari of 0.25 or less. In addition, since the average crvstal grain diameter oi the hot rolled steel sheet of steel No. 3 was 20 un or less, the probability of formation of rough surfaces was smaller.

F047

On the other hand, in steel Ng. 2 in which the amount of © was high, the average crystal grain of the hot rolled steel sheet was fine, and the yield strength was increased.

In steel No. 5 in which the amount of P was small, the average crystal grain of the hot rolled steel sheet was large and coarse. In steel No. & in which the amounts of Nb and Ti were large and steel No. 8 in which the amount of B was large, since the recrystallization in the austenite region did not sufficiently progress, the anisotropy of the hott rolled steel sheet was high, and the [Ar] was more than $0.25, In addition, in steel No. 7 in which the amount of B was small, the AL of the hot rolled steel sheet was more than 20 MPa.

Ga 28 id [Industrial Applicablliiuvy] 10048]

According te the present invention, when a B-added hot rolled steel sheet for processing is added with 0.01% to 4.03% of P, is then processed by a final pass at a reduction rate of 15% or more, and is subseguently processed by spontaneous cooling for 0.5 to 10 seconds, a hot rolled steel sheet having, besides aging-resistance and soft/high ductile propertias, an improved (Ar! without causing excessive coarse grain formation can be obtained.

Claims

CLAIMS {Claim 1] A hot relied steel sheet containing, on a percent by masa basis,

0.03% to 0.07% of C,

0.1% or less of 5i,

0.05% to 0.3% of Mn,

3.01% to 0.03% of 2,

0.03% or less of 8,

0.02% to 0.1% of sol. AL,

3.005% or lsss of H, lass than 0.005% of Nh+Ti,

G.0003% to 0.0020% of B, and the balance being Fe and inevitable impurities, wherein an average crystal grain diameter is in a range of 12 to 2& um, and {Az |£0.25 and AIsS20 MPa hold. Claim 2] A method for manufacturing a hot roiled steel sheet comprising the steps of performing a final pass of hot rolling nn a steel slab which contains, on a percent Dy mass basis,

0.03% to 0.07% of C,

0.1% or less of 851i,

30.05% to 0.5% of Mn,

es 10: i

0.01% to 0.03% of F,

C.03% or less cof §,

G.02% to 0.1% of sol. AL,

0.005% or less of N, iozs than 0.005% of Nb+Ti,

0.0003% to 0.0020% of B, and the balance being Fe and inevitable impurities at a rolling temperature of Ar;+30°C or more and a reduction rate of 15% or mere; performing spontaneous ceoling for 8.5 te 10 seconds after the hot rolling: then performing cooling to TGO°C or less at a cooling rate of 20°C/sec or more; and performing coiling at a temperature in a range of 530°C to

ToC.