ES2589640T3 - Hot rolled steel sheet with high elasticity limit and excellent impact energy absorption at low temperature and resistance to softening of the ZAC and method to produce it - Google Patents

Abstract

Hot rolled steel sheet with a high elasticity limit that presents excellent impact energy absorption at low temperature and resistance to softening of the ZAC characterized by comprising,% by mass, C: 0.04 to 0.09%, If: 0.4% or less, Mn: 1.2 to 2.0%, P: 0.1% or less, S: 0.02% or less, Al: 1.0% or less, Nb: 0 , 02 to 0.09%, Ti: 0.02 to 0.07%, N: 0.005% or less and optionally further comprising V: 0.01 to 0.12%, one or more of Cr, Cu, Ni and Mo in a total of 0.02 to 2.0%, B: 0.0003 to 0.005% and one or more of Ca, Mg, La and Ce in a total of 0.0003 to 0.01% and a balance of Fe and unavoidable impurities, where 2.0 <= [% Mn] +8 [% Ti] +12 [% Nb] <= 2.6 and with a metal structure comprising a percentage of the perlite area of 5% or less, a percentage of total martensite and retained austenite area of 0.5% or less and a balance of one or both of ferrite and bainite, with an average grain size of ferrite and bainite of 10 μm or less, with a grain size prome gave carbonitrides of alloys with incoherent interfaces containing Ti and Nb of 20 nm or less, with an elasticity limit of 0.85 or more and with a maximum tensile strength of 600 MPa or more.

Description

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impact, thus the upper limit 5% was made. From the point of view of ensuring folding, 3% or less is the preferable range. Note that the lower limit is not set in particular, but it is more preferable to have a percentage of perlite area close to zero with respect to the energy absorption of the impact.

"Percentage of total martensite and retained austenite area"

In precipitation hardened steel containing Nb and Ti, if the percentage of total area of martensite and retained austenite exceeds 0.5%, fragility breakage at a low temperature easily occurs and, in addition, the impact energy absorption decreases . For this reason, the upper limit of the percentage of total area was 0.5%. Note that the lower limit is not set in particular, but it is more preferable that it has a percentage of total area of martensite and austenite retained close to zero with respect to the energy absorption of the impact.

"Metal structure that presents balance of one or both of ferrite and bainite"

The area percentages of these are not limited in particular, but from the point of view of ensuring folding, the percentage of bainite area is preferably made 10% or more.

"Average grain size of ferrite and bainite"

The average grain size of ferrite and bainite is a correlative factor. If the average particle size is more than 10 µm, even if the average particle size of the carbonitrides of alloys containing Nb and Ti is controlled, sometimes the energy absorption of the low temperature impact cannot be ensured, thus made the upper limit 10 µm. 8 µm or less is a preferable condition that allows impact energy absorption to be more stable. The lower limit is not set in particular, but if the size is less than 2 µm, the production cost increases markedly, so 2 µm is the substantive lower limit.

In the present invention, the metal structure of the steel sheet can be observed based on JIS G 0551 by an optical microscope. The observed surface is obtained by polishing the steel sheet, then attacking it by means of a corrosive solution of Nital.

The percentages of ferrite, bainite, perlite and martensite area can be measured by the point counting method

or image analysis using structural photographs obtained by an optical microscope or scanning electron microscope (SEM). The percentage of retained austenite area is measured by X-ray diffraction.

In the present invention, "bainite" includes upper bainite, lower bainite and granular bainite. In addition, "perlite" includes perlite and pseudo perlite.

The grain size can be measured by observation by means of an optical microscope or by orientation analysis of the crystal by the EBSD method. Here, "grain size" is the average grain size "d" described in JIS G 0551.

"Average particle size of alloy carbonitrides with incoherent interfaces containing Ti and Nb"

The particle size of the carbonitrides of alloys containing Ti and Nb and the corresponding network with the structure of the ferrite or bainite matrix are important factors related to the energy absorption of low temperature impact energy. In general, in precipitation hardened steel, it is known that they cause precipitation of fine alloy carbonitrides with good network correspondence with the matrix structure as fine particles, but for improved low temperature toughness and improved absorption of impact energy, it is important to control the carbonitride particles of alloys with poor network correspondence with the matrix structure. If the average particle size of the carbonitrides of alloys with incoherent interfaces that degrade the network correspondence is more than 20 nm, the energy absorption of the low temperature impact decreases, thus the appropriate range was limited to 20 nm or less. From the point of view of obtaining a better impact energy absorption, 10 nm or less is the most preferable range. The lower limit is not set in particular, but as a size that allows the analysis of the crystal orientation of the precipitate, 2 nm is the substantive lower limit.

Here, "carbonitrides of alloys with incoherent interfaces" means the non-coherent state precipitated in the structure of the ferrite or bainite matrix and joining ferrite and bainite not having the following crystal orientation relationships (Baker-Nutting orientation relationships):

(100) MX // (100) Faith

(010) MX // (011) Faith

(001) MX // (0-11) Fe (Note: -1 is an alternative notation for 1 with a bar above).

Here, M indicates Ti and Nb. The percentages occupied by Ti and Nb are not a problem. In addition, X indicates C and N. The percentages occupied by C and N are not a problem. When V or Mo is added, sometimes M contains V or Mo.

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Note that the orientation of the carbonitride crystal of alloys with incoherent interfaces was analyzed and the average particle size was measured using a transmission type electron microscope (TEM). First, a sample of a steel plate was prepared on a thin film of an extension through which electron beams were passed, TEM was used to analyze the orientation of the crystal between the precipitate and the Fe of the surrounding matrix phase, the average particle size of 20 precipitates was then measured in order from the precipitates of larger diameter in the precipitates that were judged to be incoherent precipitates. Here, the "particle size of a precipitate" is measured as the diameter of the equivalent circle when a circle equivalent to the cross-sectional area of a particle is assumed.

"Elastic limit of 0.85 or more"

If the elasticity limit is less than 0.85, sometimes the energy absorption of the impact at low temperature decreases and the folding decreases. For this reason, the lower limit of the elasticity limit was 0.85.

Note that, in the present invention, rlim / t was used as the criterion for the evaluation of folding. Here, "t" is the thickness of the test piece and rlim is the radius of curvature limit at which no cracks occur in a 90 ° V bending test. A rlim / t of 1.0 or less was considered good folding. 0.5 or less is the most preferable range. The upper limit is not set in particular, but if the value is above 1.1, the folding may decrease, thus 1.1 or less is the most preferable range.

"Maximum tensile strength of 600 MPa or more"

If the maximum tensile strength is less than 600 MPa, the steel sheet does not contribute to a reduction in the weight of the parts of cars, trucks, construction machinery, etc., thus in the present invention, steel sheet is assumed of maximum tensile strength of 600 MPa or more.

Next, the production method will be explained in detail.

Prior to hot rolling, it is necessary to heat the steel plate of the ingredients that are prescribed in the present invention to 1,150 ° C or more to make the carbonitrides of alloys that are present in the steel plate a solid dissolution state. If the heating temperature is less than 1,150 ° C, it becomes difficult to obtain a concentration of a maximum tensile strength 600 MPa or more. In addition, coarse alloy carbonitrides do not dissolve sufficiently and as a result coarse alloy carbonitrides remain, so energy absorption from low temperature impact decreases. For this reason, the heating temperature of the steel plate was limited to 1,150 ° C or more. The upper limit is not set in particular, but if it is above 1,300 ° C, the effect becomes saturated, so this is the substantive upper limit.

The previous heated steel plate is rolled raw to a raw bar. This raw lamination must be completed between 1,000 ° C and 1,080 ° C. If the finishing temperature is less than 1,000 ° C, the raw alloy carbonitrides precipitate in the austenite and decrease the energy absorption of the impact at low temperature, while if it is 1,080 ° C or more, the austenite grains become thicker, it is not possible to obtain an average grain size of ferrite and bainite of 10 µm or less in the transformed structure after finished rolling, cooling and rolling, the low temperature toughness worsens and the impact energy absorption decreases. In addition, in raw rolling performed at 1,150 ° C or less, the retention time between rolling reduction passes is an important parameter that affects the average particle size of the incoherent alloy carbonitrides. In the method of the present invention, the raw lamination is normally performed by lamination 3 to 10 times or so, more preferably lamination 5 to 10 times, but if the maximum retention time t0 between lamination passes made at 1,150 ° C or smaller is 45 s or more, the carbonitrides of alloys become thicker in an extension that affects the energy absorption of the impact. For this reason, the retention time between the rolling reduction passes was limited to 45 seconds. In 30 seconds it is more preferable.

Next, the raw bar is rolled to finish to obtain a rolled material.

The time (t1) after raw rolling ends at the beginning of the finishing rolling is an important parameter that affects the average particle size of the alloy carbonitrides and the grain size of the ferrite and bainite after the transformation. As shown in FIG. 2, the greater the total amount of Ti and Nb, the greater the retention time t1 (arrow mark in the figure) where the absorption of impact energy (vE-40) moves from good (OK) to not good (NB ). The retention time t1 (s) where the absorption shifts from good (OK) to not good (NB) substantially corresponds to 1,000x ([% Ti] + [% Nb]). Thus, if the retention time t1 (s) from after the raw rolling is finished to when the finished rolling begins is 1,000x ([% Ti] + [% Nb]) or more, the coarse alloy carbides precipitate in austenite, the austenite crystal grains become thicker, it is not possible to obtain an average grain size of ferrite and bainite of 10 µm or less in the transformed structure after finished rolling, cooling and rolling, toughness at low temperature it worsens and decreases the absorption of impact energy. 700x ([% Ti] + [% Nb])> tls is the most preferable range. Accordingly, the retention time t1 (s) was defined by the following formula (1):

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Claims (1)

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