JP5589925B2 - High-strength thin steel sheet with excellent elongation and uniform paint bake-hardening performance and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength thin steel sheet excellent in workability and paint bake-hardening performance, which is suitable for structural members such as automobiles that are mainly used after being pressed, and a method for producing the same.

  High press workability and strength are required for steel plates used in automobile body structures. Elongation can be cited as a governing factor for the press workability of high-strength steel sheets. It is known that a retained austenitic steel having a retained austenite in a steel structure placed on a high-strength steel sheet has a very high elongation despite its high strength using the TRIP effect. In this retained austenitic steel, in order to further increase the elongation, for example, in Patent Document 1, uniform elongation is achieved by controlling two types of ferrites (bainitic ferrite and polygonal ferrite) while ensuring a high fraction of retained austenite. A technique for ensuring the above is disclosed. On the other hand, Patent Document 2 discloses a technique for defining the shape of an austenite phase by an aspect ratio for the purpose of securing elongation and shape freezing property. Patent Document 3 states that higher elongation can be secured by optimizing the distribution of the austenite phase. On the other hand, in ultra-low carbon steel, a technique is known in which steel is hardened by using a heat treatment in a baking process after pressing and painting by rubbing some solid solution C. This is a phenomenon that occurs when dislocations entered in the pressing process are fixed by C diffusion occurring in the baking process. Therefore, high strength steel sheets have a large amount of C added, so it is possible to obtain a certain amount of paint baking effect, but hardening by paint baking has a large variation in hardening due to differences in the amount of strain in the press, making it uniform. Since it is difficult to obtain an effect, material processing using this positively has not been performed.

JP 2006-274418 A JP 2007-154283 A JP 2008-56993 A

M. Takahashi: IS3-2007, (2007), 47-50

  The present invention has been made to solve the conventional problems, and is a technique found as a result of intensive studies to obtain uniform baking finish in a retained austenitic steel. It is an object of the present invention to provide a high-strength thin steel sheet having excellent curing performance and a method for producing the same.

  In order to obtain uniform paint bake hardening, the present inventors need to perform paint bake hardening with a small amount of strain. Therefore, the present inventors studied a steel sheet from which this can be obtained, and in a TRIP steel sheet containing extremely unstable austenite. I found that it can be achieved. The reason for this is not clear, but the unstable retained austenite with a low C concentration causes martensitic transformation by slight processing, and local strain is generated in the adjacent ferrite phase by rapid volume change. This strain seems to have been made possible by the occurrence of dislocations in the ferrite phase. Therefore, in order to obtain this effect, an unstable austenite phase distributed extremely uniformly is essential. However, in order to increase the elongation by the TRIP effect, stable retained austenite with a high C concentration is required, and it is difficult to achieve both high-elongation and low-strain baking finish. In order to make this compatible, it is necessary to dispose a highly stable austenite and a low-stability residual austenite phase in one grain.

  In the conventional technique of concentrating C to the retained austenite phase using the bainite transformation of retained austenitic steel, as shown in Non-Patent Document 1, C cannot be enriched beyond the concentration at the T0 point. There was a limit to increasing the stability of the phase. Therefore, as a result of intensive studies, the present inventors have found a technique for controlling the concentration gradient in the austenite phase, making unstable austenite in the grains, and forming an austenite phase having extremely high stability at the grain boundaries, The present invention has been completed.

That is, the high-strength thin steel sheet excellent in elongation and paint bake hardening performance of the present invention is
(1) In mass%,
C: 0.05 or more and 0.35% or less,
Si: 0.05% or more, 2.5% or less,
Mn: 0.6% or more, 3.0% or less,
P: 0.001% or more, 0.1% or less,
S: 0.0002% or more, 0.05% or less,
N: 0.0010% or more, 0.020% or less,
Al: 0.001% or more, 2.0% or less,
In the steel sheet containing the balance iron and inevitable impurities, the metal structure is mainly composed of one or more of ferrite, bainite, and tempered martensite, and contains 3% or more of retained austenite, The average grain size of austenite is 1 μm or more and 8 μm or less, and the central concentration Cgc of the austenite grains and the grain boundary density Cgb of the austenite grains at the interface where the austenite grains are in contact with ferrite, bainite, and martensite are expressed by the following equation (1). A high-strength thin steel sheet excellent in elongation and uniform paint bake-hardening performance, characterized by having 50% or more austenite grains in the range to be satisfied.

Cgb / Cgc ≧ 1.1 (Formula 1)
(2) The high-strength thin steel sheet having excellent elongation and paint bake hardening performance according to (1), wherein the average C concentration in the austenite is 0.7% or more and 1.5% or less.
(3) Total elongation of ferrite, bainite, and tempered martensite is 50% or more in terms of volume fraction with respect to the entire structure. Elongation and uniform coating baking according to (1) or (2) High-strength thin steel sheet with excellent hardening performance.
(4) Furthermore,
% By mass
Mo: 0.02% or more, 0.5% or less,
A high-strength thin steel sheet excellent in elongation and uniform paint bake hardening performance according to any one of (1) to (3).
(5) Furthermore,
% By mass
Nb: 0.003% or more, 0.10% or less,
Ti: 0.003% or more, 0.20% or less,
V: 0.005% or more, 0.10% or less,
Cr: 0.02% or more, 5.0% or less,
W: 0.01% or more, 5.0% or less,
A high-strength thin steel sheet excellent in elongation and uniform paint bake-hardening performance according to any one of (1) to (4), wherein the steel composition contains one or more of the following.
(6) Furthermore,
% By mass
Elongation and uniform coating according to any one of (1) to (5), wherein one or more of Ca, Mg, Zr, and REM are contained in an amount of 0.0005% to 0.05%. High-strength thin steel sheet with excellent bake hardening performance.
(7) Furthermore,
% By mass
Cu: 0.04% or more, 2.0% or less, Ni: 0.02% or more, 1.0% or less, B: 0.0003% or more, 0.007% or less A high-strength thin steel sheet excellent in elongation and uniform paint bake hardening performance according to any one of (1) to (6).
(8) With respect to the cast slab, as it is after casting, or once cooled to 1000 ° C. or lower, and reheated to 1000 ° C. or higher to perform hot rolling, the finishing temperature is 800 ° C. or higher and 980 ° C. It is finished below, and then cooled to a temperature range of 650 ° C. or lower at an average of 10 ° C./second or more and 200 ° C./second or less, wound up in a temperature range of 650 ° C. or less, pickled, and reduced by 30%. Perform the above cold rolling, and perform annealing at an average heating rate in the temperature range of 660 ° C. or higher and 730 ° C. or lower to 2 ° C./second or higher and lower than 15 ° C./second, and the maximum temperature of 750 ° C. or higher and 920 ° C. or lower. After cooling to a temperature range of 150 ° C. or higher and 480 ° C. or lower at an average cooling rate of 0.1 ° C./second or higher and 200 ° C./second or lower, and subsequently in the same temperature range of 350 ° C. or higher and 480 ° C. or lower. After holding for 5 seconds or more and 1000 seconds or less The temperature range from 350 ° C. to 220 ° C. is primarily cooled at a cooling rate of 5 ° C./second to 25 ° C./second, and the temperature range from 120 ° C. to near room temperature is 100 ° C./second or 5 ° C./second. The method for producing a high-strength thin steel sheet excellent in elongation and uniform paint bake-hardening performance according to any one of (1) to (7), wherein secondary cooling is performed at an average cooling rate of seconds or less.
(9) With respect to the cast slab, as it is after casting, or once cooled to 1000 ° C. or lower and then reheated to 1000 ° C. or higher to perform hot rolling, the finishing temperature is 800 ° C. or higher and 980 ° C. It is finished below, and then cooled to a temperature range of 650 ° C. or lower at an average of 10 ° C./second or more and 200 ° C./second or less, wound up in a temperature range of 650 ° C. or less, pickled, and reduced by 30%. Perform the above cold rolling, and perform annealing at an average heating rate in the temperature range of 660 ° C. or higher and 730 ° C. or lower to 2 ° C./second or higher and lower than 15 ° C./second, and the maximum temperature of 750 ° C. or higher and 920 ° C. or lower. After cooling to a temperature range of 350 ° C. or higher and 480 ° C. or lower at an average cooling rate of 0.1 ° C./second or higher and 200 ° C./second or lower, and subsequently maintaining the same temperature range for 5 seconds or longer and 1000 seconds or shorter. After immersing in hot dip galvanizing tank , The temperature range from 350 ° C to 220 ° C is primarily cooled at a cooling rate of 5 ° C / second to 25 ° C / second, and the temperature range from 120 ° C to near room temperature is 100 ° C / second or more or 5 ° C / second. The method for producing a high-strength thin steel sheet excellent in elongation and uniform paint bake hardening performance according to any one of (1) to (7), wherein secondary cooling is performed at the following average cooling rate.
(10) Further, after immersion in a hot dip galvanizing tank , alloying treatment is performed in a range of 400 ° C. or higher and 580 ° C. or lower, and the high strength thin film excellent in elongation and uniform paint bake-hardening performance as described in (9) A method of manufacturing a steel sheet.

  The high-strength thin steel sheet of the present invention can impart bake coating hardening even to low strain areas evenly while ensuring extremely high elongation due to the TRIP effect of retained austenite. It becomes. In addition, the C concentration gradient in the austenite phase, which is the basis of the present invention, is an extremely efficient and effective technique for creating regions of different stability of retained austenite. For the first time, this technology increases elongation and paint bake hardening. It is possible to achieve both levels.

  This effect can be continued if it is possible to create a C concentration gradient in the retained austenite phase after generating the retained austenite. That is, the same effect can be obtained not only in cold-rolled steel sheets and hot-dip galvanized steel sheets but also in hot-rolled steel sheets. In addition, since the effect can be continued without breaking this concentration gradient, it can also be applied to an electroplated steel sheet.

  The present invention is not affected by casting conditions. For example, a special casting-hot rolling method such as a thin slab may be used without being affected by a casting method (continuous casting or ingot casting) or a difference in slab thickness.

It is a figure which shows that the steel plate of this invention has the outstanding elongation compared with comparative steel.

  The high-strength thin steel sheet of the present invention generates retained austenite regions with different stability within the grains in the retained austenitic steel, and creates regions where work-induced transformation occurs at low strain and regions where work-induced transformation does not occur until high strain. As a result of intensive investigations, by controlling the concentration gradient of the retained austenite phase, the region where the stability is enhanced to the limit and the unstable region coexist in the grain, and the strength is high. It was found that high elongation and paint bake-hardening performance can be achieved at a high level.

  The structure is mainly composed of a ferrite phase and a bainite phase and needs to contain 3% or more of a retained austenite phase. When higher strength is desired, martensite may be contained, but when the ferrite phase, bainite phase or tempered martensite phase is not mainly used, the elongation is remarkably reduced. Further, even if pearlite is contained in an amount of 5% or less, the material is not significantly deteriorated.

  The C concentration distribution in the retained austenite phase is one of the most important in the present invention. Each retained austenite grain has a higher C elongation at the phase interface in contact with the ferrite phase, bainite phase, and martensite phase than the concentration at the center, thereby increasing the stability of the retained austenite at the phase boundary and increasing the elongation. To achieve. In addition, paint bake-hardening performance can be obtained by keeping the C concentration in the center low. In order to achieve this, it is necessary that the central concentration Cgc of the retained austenite grains and the grain boundary concentration Cgb of the retained austenite grains are in a range satisfying (Equation 1). In order to secure this effect in the entire structure, it is necessary that the residual austenite grains satisfying (Equation 1) among the entire residual austenite grains be 50% or more.

Cgb / Cgc ≧ 1.1 (Formula 1)
Here, Cgc and Cgb may be any measurement method as long as accuracy is guaranteed under the condition that the decomposition concentration can be accurately obtained. For example, using EPMA attached to FE-SEM, It can be obtained by carefully measuring the C concentration with a pitch of 0.5 μm or less. However, it is currently impossible to measure the local C concentration at the interface. For this reason, the ratio 1.1 shown in (Equation 1) has been studied by the present inventors, and as a result, it has been determined that a sufficient effect can be seen when this value is met at the minimum in normal measurement. Invented.

  The average particle size of the retained austenite needs to be 1 μm or more and 8 μm or less. If the thickness exceeds 8 μm, the dispersion of the retained austenite phase is coarse, paint bake hardening with low strain cannot be obtained, and the TRIP effect cannot be fully exerted due to the size dependence of the stability of the austenite phase. descend. On the other hand, if the thickness is less than 1 μm, it is difficult to obtain a concentration gradient at the phase interface, so 1 μm or more is necessary.

  Similar to the C concentration gradient, the average C concentration of retained austenite greatly contributes to the stability of retained austenite. If the average C concentration is less than 0.7%, the stability of retained austenite becomes extremely low, so that the TRIP effect cannot be obtained effectively and the elongation deteriorates. On the other hand, even if it exceeds 1.5%, the elongation improving effect is not only saturated, but also the cost for producing it increases, so that it is 0.7% or more and 1.5% or less. Further, in order to obtain low strain paint bake-hardening performance uniformly, it is desirable that the C concentration (Cgc) in the central portion of retained austenite be 1.0 or less.

The total structure of ferrite, bainite, and tempered martensite, which are matrix phases, needs to be 50% or more in terms of volume fraction with respect to the entire structure. If it is less than this, the C concentration in the austenite phase cannot be increased, so that it becomes difficult to ensure stability even if a concentration gradient is used, and the elongation deteriorates. On the other hand, if it exceeds 95%, it becomes difficult to secure the necessary fraction of the retained austenite phase, and this causes deterioration in elongation.
The reason for limiting the chemical components of the high-strength thin steel sheet of the present invention will be described below.
C is an essential element from the viewpoint of securing strength and as a basic element for stabilizing austenite. If C is less than 0.05%, the strength is not satisfactory, and residual austenite is not formed. On the other hand, if it exceeds 0.35%, the strength is excessively increased, the ductility is insufficient, and it cannot be used as an industrial material. Moreover, spot weldability is remarkably deteriorated. When high elongation is required, it is desirable to be 0.2% or more. On the other hand, when weldability is required, it is desirable to make it 0.25% or less.

  Si is an element that delays the formation of cementite and is an element that is effective in generating retained austenite, and is usually an element that is added to ensure ductility. However, even if added over 2.5%, the effect is saturated, and embrittlement tends to occur. When hot dip galvanizing properties and ease of chemical conversion treatment are required, 2.0% or less is desirable. On the other hand, if the addition is less than 0.05%, the cementite suppressing effect cannot be obtained. Therefore, 0.05% is set as the lower limit. When the amount of Al added to obtain the same effect as Si is 0.1% or less, addition of 1% or more is desirable.

  Mn is an element that delays the formation of carbides and is advantageous for the formation of retained austenite, in addition to the need for addition from the viewpoint of securing strength. If Mn is less than 0.6%, the strength is not satisfied, and the formation of retained austenite becomes insufficient and the ductility deteriorates. In addition, when the Mn addition amount exceeds 3.0%, the hardenability is improved, so that martensite is generated instead of retained austenite, which easily causes an increase in strength. It cannot be used as an industrial material due to lack of ductility. Therefore, the range of Mn in the present invention is 0.6% or more and 3.0% or less. In terms of material, it is preferably 1.0 to 2.4%.

  P is added according to the strength level required as an element for increasing the strength of the steel sheet. However, if the addition amount is large, segregation to the grain boundary causes deterioration of local ductility. In addition, the weldability is deteriorated. Therefore, the P upper limit is set to 0.1%. On the other hand, if it is less than 0.001%, the deterioration effect of P can be ignored, and if less than this, the cost increases.

  S is an element that degrades local ductility and weldability by generating MnS, and is preferably an element that does not exist in steel. Therefore, the upper limit is made 0.05%. On the other hand, if it is less than 0.0002%, the cost increases.

  Al, like Si, has an effect of promoting the formation of ferrite and is one of important elements that can also suppress cementite. That is, it has the effect of stabilizing the retained austenite. This effect cannot be expected with an Al addition of less than 0.001%. On the other hand, even if Al is added excessively, the above effect is saturated and the steel is embrittled, so 2.0% was made the upper limit. When considering hot dip galvanizing properties, Al degrades this, so it is desirable that the upper limit be 1.8%.

  N is an element inevitably included, but if it is contained in a large amount, it not only deteriorates the aging property but also increases the amount of precipitated AlN and decreases the effect of Al addition, so 0.020% The following contents are preferred. Further, unnecessarily reducing N increases the cost in the steelmaking process, so it is usually preferable to control the N to 0.0010% or more.

  Mo is an element that suppresses the formation of pearlite in the steel, and is an element that is particularly important when the cooling rate during annealing is slow, or when reheating is performed in an alloying treatment of plating or the like. In order to obtain this effect, the minimum addition amount of Mo was set to 0.02%. Below this, the formation of pearlite is not suppressed, and the retained austenite ratio is reduced. On the other hand, excessive addition of Mo may deteriorate ductility and chemical conversion properties, so the upper limit was made 0.5%. In order to obtain a higher strength-ductility balance, the content is preferably 0.3% or less.

  Nb, Ti, V, Cr, and W are elements that generate fine carbides, nitrides, or carbonitrides, and are effective in securing strength. Therefore, one or more of them may be added as necessary. Is possible. In order to achieve this, 0.003% or more of Nb, 0.003% or more of Ti, 0.005% or more of V, 0.02% or more of Cr, and 0.01% or more of W should be added. is necessary. On the other hand, excessive addition increases the strength too much and lowers the ductility, so Nb is 0.10% or less, Ti is 0.20% or less, V is 0.10% or less, and Cr is 5.0%. Hereinafter, W needs to be 5.0% or less.

  The steel can further contain one or more of Ca, Mg, Zr, and REM (rare earth elements) alone or in total from 0.0005% to 0.05%. Ca, Mg, Zr, and REM improve the local ductility and hole expansibility by controlling the shapes of sulfides and oxides. For this purpose, it is necessary to add one or more of these elements alone or in total of 0.0005% or more. However, excessive addition deteriorates workability, so the upper limit was made 0.05%.

  Further, the steel is Cu: 0.04% or more, 2.0% or less, Ni: 0.02% or more, 1.0% or less, B: 0.0003% or more, 0.007% or less. More than seeds can be contained. These elements can delay the transformation and increase the strength of the steel. However, when Cu is less than 0.04%, Ni is less than 0.02%, and B is less than 0.0003%, the hardenability is weak and ferrite is formed at a high temperature. The necessary strength cannot be obtained. On the other hand, addition exceeding this range results in excessive hardenability and slows the transformation of ferrite, bainite, and tempered martensite, and therefore delays C concentration to the retained austenite phase.

In addition to the above elements, steel contains elements inevitably mixed such as Sn and As, and is made of the remaining iron.
Below, the manufacturing method of the high intensity | strength thin steel plate which concerns on this invention is demonstrated.

As a result of intensive studies, the inventors of the present invention have produced a high-strength steel sheet of the present invention by controlling the cooling conditions after promoting C concentration to the austenite phase by bainite transformation or tempering martensite. It was found that concentration gradient control in the phase was possible. Moreover, the stability of the retained austenite phase can be increased by combining with the concentration in the austenite phase before this. In order to achieve this effect, one of the most important things in the present invention is the cooling condition after bainite transformation or tempering martensite. After overaging (OA) treatment, the temperature range from 350 ° C. to 220 ° C. is primarily cooled at an average cooling rate of 5 ° C./second to 25 ° C./second, and further, the temperature range from 120 ° C. to about room temperature is set to 100 ° C. Secondary cooling is performed at an average rate of not less than 5 ° C / second or not more than 5 ° C / second.

The slight transformation that occurs during cooling after OA plays an important role in increasing the C concentration near the grain boundary in austenite. For this reason, in the primary cooling, when the cooling rate in the temperature range of 350 ° C. to 220 ° C. exceeds 30 ° C./second, the transformation does not proceed during this time, and C concentration does not occur in the austenite. In particular, when there is no inconvenience in the line configuration, the cooling rate is desirably 25 ° C./s or less. On the other hand, when the cooling rate in the temperature range from 350 ° C. to 220 ° C. is less than 5 ° C./second, C diffusion in austenite proceeds and the C concentration gradient becomes small.
In addition, C diffusion is further limited in a low temperature range of 120 ° C. or lower, and transformation hardly occurs. For this reason, in the secondary cooling, the steel sheet is cooled at an average cooling rate of 100 ° C./second from 120 ° C. to near room temperature, and the C concentration gradient in the austenite is maintained in the 350 ° C. to 220 ° C. temperature range. Or in secondary cooling, the average cooling rate from 120 degreeC to the normal temperature vicinity is cooled at 5 degrees C / sec or less, and the C density | concentration gradient in an austenite phase is made more remarkable. In the secondary cooling, if it exceeds 5 ° C./second and less than 100 ° C./second, not only transformation does not occur, but also the C concentration at the grain boundary decreases.
The slab before hot rolling is set to 1000 ° C. or higher as it is after continuous casting or by reheating. Below this temperature, the homogenous treatment is insufficient and the strength is reduced.

  Next, the slab is hot-rolled at a finishing temperature of 800 ° C. or higher and 980 ° C. or lower. This is because if the finishing temperature is less than 800 ° C., (α + γ) two-phase region rolling occurs, resulting in a decrease in ductility. If the finishing temperature exceeds 980 ° C., the austenite grain size becomes coarse, the ferrite phase fraction decreases, and the ductility is reduced. This is because of a decrease.

  Then, after cooling at an average temperature of 10 ° C./second or more and 200 ° C./second or less to a temperature range of 650 ° C. or less, winding is performed in a temperature range of 650 ° C. or less. Below this cooling rate and above the coiling temperature, a pearlite phase is generated that consumes C and significantly degrades elongation. When the average cooling rate exceeds 200 ° C./second, the effect of suppressing pearlite is saturated, and the variation in the cooling end point temperature becomes large, making it difficult to ensure a stable material. Therefore, it shall be 200 degrees C / sec or less.

  After pickling, the prototype material can be cold-rolled by 30% or more. If it is less than this, recrystallization and reverse transformation during annealing are suppressed, and elongation decreases.

  Heating up to the annealing temperature is performed at an average heating rate in the temperature range of 660 ° C. or more and 730 ° C. or less of 2 ° C./second or more and 15 ° C./second or less. This temperature range is the range that determines the recrystallization of the steel. If the heating rate exceeds 15 ° C / second, recrystallization does not occur sufficiently, resulting in a decrease in elongation, and C is collected at dislocations in the unrecrystallized ferrite phase. This causes a decrease in bake hardening performance. Further, productivity is poor at 2 ° C./second or less. The maximum temperature during annealing is 750 ° C. or more and 920 ° C. or less. If the temperature is lower than 750 ° C., the recrystallization of the ferrite phase during annealing is delayed, causing a decrease in elongation. On the other hand, at a temperature higher than this, the martensite fraction increases and the elongation deteriorates.

  In cooling after soaking in the annealing process, a faster cooling rate is preferable in order to freeze the structure and efficiently cause bainite transformation. However, the transformation cannot be controlled at less than 0.1 ° C./second. On the other hand, even if it exceeds 200 ° C./second, the effect is saturated, and the temperature controllability of the cooling end point temperature, which is most important for the formation of retained austenite, is significantly deteriorated. For this reason, the cooling rate after annealing is set to 0.1 ° C./second or more and 200 ° C./second or less on average.

  Cooling end point temperature and subsequent holding is an important technique for controlling bainite formation and determining the C concentration of retained austenite. When the cooling end point temperature is less than 150 ° C., a large amount of martensite is generated, the steel strength is excessively increased, and in addition, it becomes difficult to leave austenite, so that the elongation is greatly deteriorated. On the other hand, when it exceeds 480 degreeC, a bainite transformation will become slow, and in addition, the production | generation of cementite will occur during holding | maintenance and the concentration of C in a retained austenite will fall. Therefore, the cooling stop temperature is set to 150 ° C. or higher and 480 ° C. or lower.

  Then, it is kept at a holding temperature of 350 ° C. or higher and 480 ° C. or lower as it is or by heating. If the holding temperature is lower than 350 ° C. or higher than 480 ° C., the bainite transformation is delayed, and if it exceeds 480 ° C., the T0 point (see Non-Patent Document 1) also becomes higher, so that C concentration is sufficiently high. I can't wake up. The longer the holding time, the better in terms of C concentration to retained austenite. If it is less than 5 seconds, the bainite transformation does not occur sufficiently and C concentration becomes insufficient. On the other hand, if it exceeds 1000 seconds, cementite is generated in the austenite phase, and this tends to lower the C concentration. Accordingly, the holding time is 5 seconds or more and 1000 seconds or less.

The present technology can also be applied to a hot-dip galvanized steel sheet. When this technology is applied, it is immersed in a hot-dip galvanizing tank after being held at 350 ° C. to 480 ° C. Moreover, this technique can also perform an alloying process after immersion. At this time, it is desirable to perform alloying treatment of plating in the range of 400 ° C. or higher and 580 ° C. If the temperature is lower than this, alloying becomes insufficient, and if it exceeds this, it becomes an overalloy and the corrosion resistance is remarkably deteriorated.
Hereinafter, the present invention will be described in detail based on examples.

  Steel having the composition shown in Table 1 was manufactured, reheated to 1200 ° C. after cooling and solidification, finish-rolled at 880 ° C., cooled to 550 ° C., cooled to an average cooling rate of 60 ° C./second, and then wound. I took it. Thereafter, this hot-rolled sheet was cold-rolled by 50%. Thereafter, annealing was performed under the conditions shown in Table 2 by continuous annealing. After annealing, 1% skin pass rolling was performed for the purpose of suppressing yield point elongation.

  Tensile properties were evaluated by C direction tension of JIS No. 5 tensile test pieces. The uniform paint bakeability in the present invention can be achieved by obtaining sufficient BH characteristics even at a low strain. Therefore, in the present invention, 170 ° C. × 20 after adding 1% lower than the normal BH pre-strain of 2%. The amount of increase in stress (1% BH) after the treatment equivalent to baking coating was used as an index for uniform coating baking property. Identification of structure, observation of existing position, and measurement of average particle size (average equivalent circle diameter) and occupancy ratio are 500 times to 1000 times by corroding the steel sheet rolling direction cross section or the cross section perpendicular to the rolling direction with the Nital reagent. Was quantified by observation with an optical microscope.

  The volume fraction of retained austenite and its average carbon concentration were determined by X-ray diffraction as described in JP-A-11-193435. That is, the volume fraction Vγ of retained austenite can be calculated from the data obtained using the Mo—Kα ray by the following equation.

Vγ = (2/3) {100 / (0.7 × α (111) / γ (200) +1)} + (1/3) {100 / (0.78 × α (211) / γ (311) +1) }
However, α (211), γ (200), α (211), and γ (311) represent surface strength.

  The carbon concentration Cγ of retained austenite is obtained by calculating the lattice constant (unit: angstrom) from the reflection angles of the (200), (220), and (311) surfaces of austenite by X-ray analysis using Cu-Kα rays. Can be calculated.

Cγ = (Lattice constant-3.572) /0.033
Among the samples A to g which are invention examples, a does not satisfy the C upper limit and b does not satisfy the C lower limit. c does not satisfy the upper limits of S and P. d and e do not satisfy the upper limits of Si and Mn, respectively. f does not satisfy the lower limits of Si and Al. g does not satisfy the upper limits of Al and Mo.

  Of the experimental results in Table 2, A3 has a retention time that is less than the lower limit, and the austenite fraction is out of range. In B3, the annealing temperature exceeds the upper limit, and the austenite grain size and the austenite grain fraction satisfying the formula (1) are out of the range. Regarding D3, the heating rate from 660 ° C. to 730 ° C. during annealing is higher than the upper limit and lower than the lower limit of annealing temperature. F3 has a holding temperature below the lower limit, and both austenite fractions are out of range. In F4, the holding temperature exceeds the upper limit, and the austenite fraction and the average C concentration in the austenite are out of the range. In F5, the cooling stop temperature is lower than the lower limit, and the austenite fraction is out of range. The retention time of H3 is longer than the range, and the retained austenite fraction and the average C concentration in the austenite are out of the range. H4 has a final primary cooling rate outside the range, and austenite grains satisfying the formula (1) are less than 50% of the whole. As for I2, the heating rate from 660 ° C. to 730 ° C. is more than the upper limit. b1 is an austenite fraction and the austenite grain ratio which satisfy | fills Formula (1) is below a range. In c1, the austenite grain ratio satisfying the formula (1) is less than the range. f1 has a low austenite fraction and average C concentration in austenite. In B4 and E3, the final secondary cooling rate is out of the range, and austenite satisfying the formula (1) is out of the range. FIG. 1 shows the elongation of the steel of the present invention together with the comparative steel. The steel sheet of the present invention exhibits a much higher elongation than the comparative steel. In addition, the steel of the present invention has a high BH characteristic exceeding 30 MPa even in BH after 1% pre-strain, and the object of the present invention is achieved.

Claims (10)

% By mass
C: 0.05 or more and 0.35% or less,
Si: 0.05% or more, 2.5% or less,
Mn: 0.6% or more, 3.0% or less,
P: 0.001% or more, 0.1% or less,
S: 0.0002% or more, 0.05% or less,
N: 0.0010% or more, 0.020% or less,
Al: 0.001% or more, 2.0% or less,
A steel composition comprising the balance iron and unavoidable impurities, the metallographic structure containing one or more of ferrite, bainite, tempered martensite, and 3% or more of retained austenite, The average grain size of austenite is 1 μm or more and 8 μm or less, and the central concentration Cgc of the austenite grains and the grain boundary density Cgb of the austenite grains at the interface where the austenite grains are in contact with ferrite, bainite, and tempered martensite are expressed by (Formula 1). A high-strength thin steel sheet excellent in elongation and uniform paint bake-hardening performance, characterized by having 50% or more austenite grains in a range satisfying
Cgb / Cgc ≧ 1.1 (Formula 1)   2. The high strength thin steel sheet having excellent elongation and uniform paint bake hardening performance according to claim 1, wherein an average C concentration in the retained austenite is 0.7% or more and 1.5% or less.   3. The elongation and uniform paint bake hardening according to claim 1, wherein the total structure of the ferrite, the bainite, and the tempered martensite is 50% or more in volume fraction with respect to the entire structure. High strength thin steel plate with excellent performance. further,
% By mass
Mo: 0.02% or more, 0.5% or less,
The high-strength thin steel sheet excellent in elongation and uniform paint bake-hardening performance according to any one of claims 1 to 3. further,
% By mass
Nb: 0.003% or more, 0.10% or less,
Ti: 0.003% or more, 0.20% or less,
V: 0.005% or more, 0.10% or less,
Cr: 0.02% or more, 5.0% or less,
W: 0.01% or more, 5.0% or less,
The high-strength thin steel sheet excellent in elongation and uniform paint bake hardening performance according to any one of claims 1 to 3, wherein the steel composition contains one or more of the following. further,
% By mass
Elongation and uniform coating according to any one of claims 1 to 5, comprising 0.0005% or more and 0.05% or less of one or more of Ca, Mg, Zr, and REM. High-strength thin steel sheet with excellent bake hardening performance. further,
% By mass
Cu: 0.04% or more, 2.0% or less, Ni: 0.02% or more, 1.0% or less, B: 0.0003% or more, 0.007% or less The high-strength thin steel sheet excellent in elongation and uniform paint bake hardening performance according to any one of claims 1 to 6.   For casting slab, as it is after casting, or once cooled to 1000 ° C. or lower, and then reheated to 1000 ° C. or higher to perform hot rolling, the finishing temperature is 800 ° C. or higher and 980 ° C. or lower. After that, after cooling down to a temperature range of 650 ° C. or less at an average of 10 ° C./second or more and 200 ° C./second or less, winding in a temperature range of 650 ° C. or less, pickling, and cooling at a reduction rate of 30% or more After performing an intermediate rolling, annealing is performed with an average heating rate in a temperature range of 660 ° C. or higher and 730 ° C. or lower being 2 ° C./second or higher and lower than 15 ° C./second, and a maximum temperature of 750 ° C. or higher and 920 ° C. or lower Cooling to a temperature range of 150 ° C. or higher and 480 ° C. or lower at a cooling rate of 0.1 ° C./second or higher and 200 ° C./second or lower on average, and subsequently 5 seconds or longer in the same temperature range of 350 ° C. or higher and 480 ° C. or lower After holding for 1000 seconds or less, 35 The temperature range from 0 ° C to 220 ° C is primarily cooled at a cooling rate of 5 ° C / second to 25 ° C / second, and the temperature range from 120 ° C to near room temperature is 100 ° C / second or more or 5 ° C / second. The method for producing a high-strength thin steel sheet excellent in elongation and uniform paint bake hardening performance according to any one of claims 1 to 7, wherein secondary cooling is performed at the following average cooling rate. For casting slab, as it is after casting, or once cooled to 1000 ° C. or lower, and then reheated to 1000 ° C. or higher to perform hot rolling, the finishing temperature is 800 ° C. or higher and 980 ° C. or lower. After that, after cooling down to a temperature range of 650 ° C. or less at an average of 10 ° C./second or more and 200 ° C./second or less, winding in a temperature range of 650 ° C. or less, pickling, and cooling at a reduction rate of 30% or more After performing an intermediate rolling, annealing is performed with an average heating rate in a temperature range of 660 ° C. or higher and 730 ° C. or lower being 2 ° C./second or higher and lower than 15 ° C./second, and a maximum temperature of 750 ° C. or higher and 920 ° C. or lower. It was cooled to a temperature range of 350 ° C. or higher and 480 ° C. or lower at a cooling rate of 0.1 ° C./second or higher and 200 ° C./second or less on average, and subsequently held for 5 seconds or longer and 1000 seconds or shorter in the same temperature range. After that, it was immersed in a hot dip galvanizing bath and 350 The temperature range from ℃ to 220 ℃ is primarily cooled at a cooling rate of 5 ℃ / second or more and 25 ℃ / second or less, and the temperature region from 120 ℃ to near room temperature is 100 ℃ / second or more or 5 ℃ / second or less. The method for producing a high-strength thin steel sheet excellent in elongation and uniform paint bake hardening performance according to any one of claims 1 to 7, wherein secondary cooling is performed at an average cooling rate of.
Furthermore, alloying processing is performed in the range of 400 degreeC or more and 580 degrees C or less after hot-dip galvanization tank immersion, The manufacture of the high strength thin steel plate excellent in elongation and uniform paint bake hardening performance of Claim 9 characterized by the above-mentioned. Method.

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